Maximum Credible Domino Scenario (MCDS) – Fault Trees (FT)

[AbruptSLR]

MAXIMUM CREDIBLE DOMINO SCENARIO (MCDS) – FAULT TREES (FT)

The MCDS-BN thread assumes that, while imperfect, Hansen et al. (2016), DeConto & Pollard (2016) and the CMIP6 'Wolf Pack' models can service as reasonable starting points for developing a Maximum Credible Domino Scenario (which can be thought of as a 'Perfect Storm' scenario) – Domino Effect Analysis using Bayesian Networks (MCDS-BN) for characterizing risks associated with coming climate change; while this MCDS – FT thread uses an approximate (non-rigorous) Maximum Credible Domino Scenario - Domino Fault Tree Analysis (MCDS-FT) to estimate probabilities of the events and feedbacks cited in the MCDS-BN thread (and that the MCDS-REF thread provides selected MCDS references and selected related conversion factors).  In this regard, Hansen et al. (2016), DeConto & Pollard (2016) and the CMIP6 'Wolf Pack' published model projections were all peer-reviewed, which provides some measure of credibility as starting point references.

Climate risk associated with the single MCDS-BN case presented in these three threads is conceptually illustrated in the four attached images and I note that I developed the single MCDS-BN case discussed here to result in a 1% probability of occurrence (estimated in 2020) to be approaching a flip into a Mid-Pliocene climate state by 2100 and a 0.00001% probability of occurrence of flipping into an Equable climate state by 2150; while to fully characterize the climate risk would require many more (an entire suite) of MCDS-BN cases with both lower and higher probabilities of occurrence for the dates in such new maximum credible domino scenarios.  Also, for a fuller understanding of what I am trying to convey, I recommend opening both the MCDS-BN thread and the MCDS-FT thread simultaneously in different windows as you progress through the threads, as I have tried to keep the MCDS-BN thread as simple as practicable while presenting my (AbruptSLR) interpretations of more technical details in this MCDS-FT thread.

It should be noted that the probability, and risk, characterization of the single MCDS-BN case is different than that of the three starting point references, and in many ways is the result of an event-based storyline (see Sillmann et al. 2020) to estimate climate risk for one portion of the probability density function (see the first attached image, and note that the shape of the effective ECS PDF is assumed for illustration purposes only), where deep uncertainty is addressed using AbruptSLR's associated guided Bayesian prior (opinion) of the feedback mechanisms considered in both CMIP6 Wolf Pack projections and by ice-climate feedback mechanisms (particularly those identified by Hansen et al. 2016); which extend and thicken/fatten the right-tail of the associated PDF for ECSeff (note that ECSeff includes both CMIP6 Wolf Pack feedbacks and my reference-based opinion of ice-climate feedbacks).

The second image (by Sutton 2018) illustrates a concept developed by Martin L. Weitzman (see the list of references) that the risk associated with long/fat-tail climate impacts (like those associated with climate sensitivity illustrated) are so high as to require immediate climate action.  However, the second image only illustrates long/fat-tail risk according to CCS documents like CMIP5 and/or AR5; while the third image presents a conceptual illustration (adapted from Sutton 2018) of my opinion of climate risks if the CMIP6 Wolf Pack projections of climate sensitivity were to be evaluated.  However, even CMIP6 Wolf Pack projections do not consider the impacts of major freshwater flux events (including MICI-type events); therefore, the fourth image presents a conceptual illustration (adapted from Sutton 2018) of my opinion of climate risks if both CMIP6 Wolf Pack projections of climate sensitivity and my estimate of MCDS ice-climate feedback mechanisms were to be evaluated (as I have done in this thread).

Sutton, R.T. (2018), "ESD Ideas: a simple proposal to improve the contribution of IPCC WGI to the assessment and communication of climate change risks", Earth Syst. Dynam., 9, 1155–1158, https://doi.org/10.5194/esd-9-1155-2018

 
First image: Representation of ECSeff through about 2050.  Note that under the MCDS-BN case the red PDF shown in panel 'c', slides to the right as more and more positive ice-climate induced feedback mechanisms are tipping into activity.

Image 2 by Sutton 2018

For a discussion of 'a framework for complex climate change risk assessment', see Simpson et al. (2021).

Simpson, N.P., et al. (2021) A framework for complex climate change risk assessment, One Earth, doi:10.1016/j.oneear.2021.03.005
https://www.sciencedirect.com/science/article/pii/S2590332221001792

Furthermore, the MCDS-BN can be thought of either as a stress test with regard potential effective radiative forcing factors (such as freshwater flux, FF and/or high cloud feedback) discounted by CCS (e.g. the MCDS-BN estimates that EEI will be about 5 W/m2 by 2060; which is not considered by CCS; and it is noted that by 2150 the MCDS-BN estimates that ECSeff is about 7.1C but would likely continue to increase as secondary feedbacks are activated; and I note that in the fourth image that climate risk is maximized when ECSeff is about 7.9C sometime well after 2150).  In this regard, Lopez-Molina et al. (2014) presents a discussion of the '… Fault Tree Analysis to Assess the Domino Effect …' methodology used to estimate P (probability) for the event chain timeframes for the MCDS-Bayesian Network assessed in this thread (& discussed in the MCDS-BN thread).

López-Molina, A. et al. (2014), "A Methodology Based on Fault Tree Analysis to Assess the Domino Effect Frequency", Proceedings of the IChemE, Hazards 24, Symposium Series No. 159

I note that the Obama administration used Martin L. Weitzman's analyses (see the second image, by Sutton 2018) to convince numerous world leaders to join the Paris Agreement (who might otherwise not join).  However, I believe that the Paris Agreement is insufficient to prevent an 'Ice Apocalypse', and that it is advisable to immediately make all world leaders/decision-makers aware of the climate risks illustrated in the third and fourth images; which are currently heavily discounted by consensus climate science (CCS) (e.g. AR5 or CMIP5).  Thus, if significant climate action is taken now, some of the worst impacts cited in the MCDS-BN thread can be avoided.

Finally, I note that the MCDS-BN case assumes SSP5-8.5 anthropogenic radiative forcing from 2020 to 2060 (with that from 2050 to 2060 being associated with worldwide wars) and assumes SSP5-2.7 anthropogenic radiative forcing from 2060 to 2150.

[AbruptSLR]

Table of Contents

Maximum Credible Domino Scenario (MCDS) – Fault Trees (FT)       Post 1

Table of Contents                     Reply 1

Introduction                         Replies 2 to 7

Dynamical Statistics and Domino Fault Tree Modeling       Replies 8 to 14

Paleo Calibration                     Replies 15 to 19

Initial Model Conditions                     Replies 20 to 32

Freshwater Flux Breakdown (including a breakdown for change
in SLR) for FT Domino Analysis in the BN Model/Network      Replies 33 to 36

MCDS-FT Analyses (from 2020 to 2150)             Replies 37 to 40


FTs 2020-2030                        Reply 41
X1 = Abrupt Jakobshavn, & Helheim, Grounding Line Retreat;
(BN 2020 to 2030)                      Replies 41 to 43

FTs 2030-2035/36                     Reply 44
X3 = Triggering of MICI Behavior for Thwaites; (2020 to 2035)      Reply 44 to 48
X2 = Beaufort Gyre Reversal/Discharge; (2020 to 2036)         Reply 49 to 50
X4 = Triggering of BSB Ice MICI Discharge; (2035 to 2036)      Reply 51
BN 2036-2040                           Reply 52
X5 = Triggering of PIG MICI Response from the BSB and/or
 Ice Shelf Loss; (2020 to 2040)                      Reply 52
X6 = Triggering of other ASE & Bellingshausen Marine Glaciers from the BSB
and/or loss of its Ice Shelf. (2020 to 2040)               Reply 53
X7 = Abrupt seasonal loss of Arctic Sea Ice (ASI), implies both freshwater hosing
 & albedo loss. (2020 to 2040)                     Reply 54

FTs 2040 – 2050                        Reply 54
X8 = Abrupt seasonal loss of Antarctic Sea Ice (AASI), from rainfall.      Reply 55
X9 = The FRIS collapses due to hydrofracturing from rainfall         Reply 55 -56
X10 = Triggering of Totten MISI retreat.                  Reply 56
X11 = The RIS collapses due to hydrofracturing from rainfall         Reply 57
X12 = Rainfall events for the GrIS                  Reply 58

FTs 2050 – 2060                        Reply 59
X13 = Collapse of Siple Coast Glaciers, West Antarctica            Reply 59
X14 = Collapse of Byrd Glacier, East Antarctica               Reply 59
X15 = Triggering of Wilkes Basin MISI retreat               Reply 60
X16 = Triggering Aurora Subglacial Basins retreat            Reply 61
X17 = Weddell Sea Glaciers                     Reply 61
X18 = 79 North & Zachariae Glacier MISI retreats            Reply 61
X19 = Recovery Glacier                        Reply 62

FTs 2060-2070                           Reply 63
Y20 = Arctic Sea Ice Albedo Flip Feedback               Reply 63
Y21 = Antarctic Sea Ice Albedo Flip                  Reply 63
Y22 = MOC Slowdown/ENSO Positive Feedback               Reply 63
Y23 = Moderate Methane pulse (Thermokarst, Arctic &
Antarctic hydrates, tropical lakes)               Reply 64
Y24 = Amazon dieback                     Reply 64

FTs 2070-2080                        Reply 65
Y25 = West Antarctic Seaways & Circulation            Reply 65
Y26 = Major Permafrost thaw                   Reply 66
Y27 = Boreal forest wildfires                  Reply 66

FTs 2080-2090                        Reply 67
Y28 = Sharp increase in Atmospheric River Rain events         Reply 67
Y29 = Sharp increase in high latitude rainfall            Reply 67
Y30 = Sharp increase in other carbon cycle feedbacks         Reply 67

FTs 2090 -2100                        Reply 68
Y31 = GrIS Albedo Loss Feedback               Reply 68
Y32 = Regional Oceanic Hypoxia States               Reply 68

FTs 2100-2110                        Reply 69
Y33 = Atmospheric flip into a Pliocene-like Pattern         Reply 69
Y34 = Major West Antarctic Seismic / Volcanic event         Reply 70

FTs 2110-2120                        Reply 70
Y35 = Major Pulse of Methane from the East Siberian
Arctic Shelf, & GrIS, hydrates                  Replies 71 to 72
Y36 = Major Change in Cloud feedback               Reply 73

FTs 2120-2130                        Reply 73
Y37 = Atmospheric Flip into a Miocene-type Pattern in the NH      Replies 73 to 74

FTs 2130-2140                        Reply 75
Y38 = Loss of Arctic Winter Sea Ice               Reply 75

FTs 2140-2150                        Reply 75
Y39 = Flip of the atmosphere into an Eocene-like
Equable Atmospheric Pattern in the NH               Replies 75 to 76

Conclusion                         Reply 77

[AbruptSLR]

Introduction

To complement the Introduction in the MCDS-BN thread, I note the CMIP6 Wolf Pack projections consider credibly high values for climate sensitivity resulting from a hypothetical, abrupt doubling of atmospheric CO2 concentration; however, at best they only consider a relatively small fraction of both the current and potential freshwater flux (& do not consider MICI types of fluxes) into the ocean; which, can further accelerate Earth Energy Imbalance, EEI, via various ice-climate positive feedbacks as discussed by Hansen et al. (2016), with one such ice-climate feedback mechanism illustrated by the first image.  While the first image demonstrate how freshwater fluxes slows down the MOC (Meridional Overturning Circulation); it also shows how this ice-climate feedback also increases the upwelling of relatively warm CDW (Circumpolar Deepwater) as highlighted in the second image; which accelerates the basal ice melting of key Antarctic marine ice shelves (tongues) such as shown in the third image for the Thwaites Glacier – Ice Tongue.  Also, it is important to noted that prior to the collapse of a marine glacier's ice shelf (tongue) ice rafted debris (IRD) cannot occur; while after the collapse of such ice shelves (tongues) IRD can/does occur as shown in the fourth image; and as the Southern Ocean seafloor is littered with IRD we can safely conclude that bare marine ice cliffs have been exposed around the perimeter of Antarctica repeated over the past 30 (or so) million years, and could well occur in the future (possibly in the coming decades).

This line of logic illustrates how just a few positive ice-climate feedback mechanisms (this thread will discuss many other positive ice-climate feedback mechanisms) can greatly increase the right-tail risk of abrupt ice mass loss from key marine glaciers in coming decades, which can then, via domino tipping mechanisms, trigger still other positive ice-climate feedback mechanisms leading to a cascade of events leading to possible changes in Earth's climate state in the coming century (see the first posts in both this, and in the MCDS-BN, thread).

[AbruptSLR]

There is no doubt that currently ice mass loss is accelerating as indicated by Slater et al. (2021), and see the first two associated images.  Per Slater et al (2021), over the period from 1994 to 2017, the rate of ice loss accelerated by 65%, the paper found, from 0.8tn tonnes a year in the 1990s to 1.3 trillion tonnes a year by 2017. About half of all the ice lost was from land, which contributes directly to global sea level rises. The ice loss over the study period, from 1994 to 2017, is estimated to have raised sea levels by 35 millimetres.

Slater, T., Lawrence, I. R., Otosaka, I. N., Shepherd, A., Gourmelen, N., Jakob, L., Tepes, P., Gilbert, L., and Nienow, P.: Review article: Earth's ice imbalance, The Cryosphere, 15, 233–246, https://doi.org/10.5194/tc-15-233-2021, 2021.

https://tc.copernicus.org/articles/15/233/2021/

Abstract
We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (58 %) of the ice loss was from the Northern Hemisphere, and the remainder (42 %) was from the Southern Hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 34.6 ± 3.1 mm. The majority of all ice losses were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, these elements of the cryosphere have taken up 3.2 % of the global energy imbalance.
 
Caption for the first image: "Figure 4. Global ice mass change between 1994 and 2017 partitioned into the different floating (blues) and grounded (purples) components. Shaded bars indicate the cumulative mass change and estimated uncertainty for each individual ice component (blues, purples) and their sum (black). The equivalent sea level contribution due to the loss of grounded ice from Antarctica, Greenland and mountain glaciers is shown in the y axis on the right-hand side."
 
Caption for the second image: "Table 1. Average mass change rates (Gt yr−1) of the different global ice components, total floating ice, total grounded ice and global total per decade and over the common period 1994–2017."

While Reply #6 of the MCDS-BN thread, and the freshwater flux sources discussed in Replies 33 to 36 of this thread, present more abrupt freshwater hosing scenarios that can trigger an acceleration of positive ice-climate feedbacks in coming decades; here I note that the observed freshwater hosing indicated by Slater et al. (2021) is sufficient to already be accelerating ice-climate feedbacks as demonstrated by E3SMv1 (which one of the CMIP6 Wolf Pack members & see Hu et al. 2020).

&

With regards to freshwater flux Event X3, Bassis et al. (2021) helps to better define the conditions under which MICI-types of collapses can occur and propagate upstream (see image 3 and the related image 4).

Bassis, J.N., et al. (18 Jun 2021), "Transition to marine ice cliff instability controlled by ice thickness gradients and velocity", Science, Vol. 372, Issue 6548, pp. 1342-1344, DOI: 10.1126/science.abf6271

https://science.sciencemag.org/content/372/6548/1342

Cliff collapse
Tall ice cliffs at the edges of ice sheets can collapse under their own weight in spectacular fashion, a process that can considerably hasten ice sheet mass loss. Bassis et al. used a dynamic ice model to demonstrate that this kind of collapse can be slowed either by upstream thinning of the ice sheet or by the resistive forces from sea ice and calved debris (see the Perspective by Golledge and Lowry). Conversely, when there is upstream ice thickening, a transition to catastrophic collapse can occur.

Abstract
Portions of ice sheets grounded deep beneath sea level can disintegrate if tall ice cliffs at the ice-ocean boundary start to collapse under their own weight. This process, called marine ice cliff instability, could lead to catastrophic retreat of sections of West Antarctica on decadal-to-century time scales. Here we use a model that resolves flow and failure of ice to show that dynamic thinning can slow or stabilize cliff retreat, but when ice thickness increases rapidly upstream from the ice cliff, there is a transition to catastrophic collapse. However, even if vulnerable locations like Thwaites Glacier start to collapse, small resistive forces from sea-ice and calved debris can slow down or arrest retreat, reducing the potential for sustained ice sheet collapse.

Image 4 (from Bassis 2021)

[AbruptSLR]

As a follow-on to my last post (Reply #3), Meehl et al (2020), see images 1 thru 4, also reminds us that aerosol-cloud interactions together cloud feedback are the primary reasons that the high-end CMIP6 models have higher ECS values than those projected by CMIP5.  However, the Meehl et al (2020) fails to point out that E3SMv1's projection of a TCR value of 2.93C is primarily due to freshwater hosing mechanisms that most of the other high-end CMIP6 models (I note that in Meehl et al. 2020 CMIP6 model number 42 is E3SMv1) either downplay or ignore.  Thus, if E3SMv1 is correct about TCR then the world could experience much high GMSTA values in coming decades than currently acknowledged by consensus climate science.

Meehl, G.A. et al. (24 Jun 2020), "Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models", Science Advances, Vol. 6, no. 26, eaba1981, DOI: 10.1126/sciadv.aba1981

https://advances.sciencemag.org/content/6/26/eaba1981

Abstract
For the current generation of earth system models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), the range of equilibrium climate sensitivity (ECS, a hypothetical value of global warming at equilibrium for a doubling of CO2) is 1.8°C to 5.6°C, the largest of any generation of models dating to the 1990s. Meanwhile, the range of transient climate response (TCR, the surface temperature warming around the time of CO2doubling in a 1% per year CO2 increase simulation) for the CMIP6 models of 1.7°C (1.3°C to 3.0°C) is only slightly larger than for the CMIP3 and CMIP5 models. Here we review and synthesize the latest developments in ECS and TCR values in CMIP, compile possible reasons for the current values as supplied by the modeling groups, and highlight future directions. Cloud feedbacks and cloud-aerosol interactions are the most likely contributors to the high values and increased range of ECS in CMIP6 (see the third attached image).

Caption for the first image: "Fig. 1 Historical values of ECS and TCR.
Assessed values of ECS (blue bars) and TCR (red bars), ranges from models of ECS (orange bars), and TCR (green bars; single value from the AR1 is green dot); numbers are individual model values of ECS from CMIP5 and CMIP6 (available on the ESGF as of March 2020). The numbers denoting individual models for CMIP5 are listed in Table 1 and those for CMIP6 in Table 2. Sources for values: AR1: table 3.2a of [IPCC First Assessment Report Ch. 3 (5)]; (ECS, 19 models with variable clouds; TCR, 1 model). AR2/CMIP1: figure 6.4 and table 6.3 of [IPCC Second Assessment Report Ch. 6 (18)] (ECS, 9 models; TCR, 13 models). AR3/CMIP2: table 9.1 of [IPCC Third Assessment Report, Ch. 9 (20)] (ECS, 14 models; TCR, 19 models). AR4/CMIP3: figure 10.25 of [IPCC Fourth Assessment Report Ch. 10 (21)] (ECS and TCR, 19 models). AR5/CMIP5: figure 9.42 and table 9.5 of [IPCC Fifth Assessment Report Ch. 9 (25)] (ECS, 23 models; TCR, 30 models; this differs somewhat from currently available CMIP5 models in the ESGF in Table 1). CMIP6: ECS (37 models) and TCR (37 models), with data available from a total of 39 models on the ESGF in March 2020 (Table 2)."

Caption for the second image: "Fig. 2 ECS as a function of TCR.
(A) From the CMIP5 models in the IPCC AR5 (black line is linear fit); (B) same as (A) except for CMIP6 models (black line is a linear fit). Note that 27 models are plotted for CMIP5 (Table 1) compared to a total of 23 and 30 models that supplied ECS and TCR values, respectively, to the IPCC AR5 used for the ranges in Fig. 1. The greater number of models plotted here denotes those with sufficient available data on the ESGF to perform corresponding ECS and TCR calculations, as defined in the ESMValTool discussed in the text. The R2 values are given in the upper left parts of each panel. The numbers denoting individual models for CMIP5 in (A) are listed in Table 1 and those for CMIP6 in (B) in Table 2.

Caption for the third image: "Fig. 3 ECS calculated for the CMIP6 models in Table 2 using the Gregory method over different time scales.
Using the entire 150-year 4xCO2 experiment (black line), there is an ECS value of 3.7°C; using only the first 20 years (blue dots and blue line), there is an ECS of 3.3°C; and using the last 130 years, there is an ECS of 4.0°C (orange dots and orange line)."

Caption for the fourth image: "Fig. 4 Effective radiative forcing from aerosols versus ECS.
Values supplied by the modeling groups (Table 3); black line is linear fit with R2 of 0.36. The numbers denoting individual models are listed in Table 2."

[AbruptSLR]

Xia et al (2020) indicates that the recovery of the Antarctic Ozone Holes contributes to relatively high Antarctic sea ice extents.  As Antarctic sea ice extent is one of the mechanisms contributing to positive ice-climate feedback, this research increases the calculated probability of an ice apocalypse.

Xia, Y., Hu, Y., Liu, J. et al. Stratospheric Ozone-induced Cloud Radiative Effects on Antarctic Sea Ice. Adv. Atmos. Sci. 37, 505–514 (2020). https://doi.org/10.1007/s00376-019-8251-6

https://link.springer.com/article/10.1007/s00376-019-8251-6

Abstract: "Recent studies demonstrate that the Antarctic Ozone Hole has important influences on Antarctic sea ice. While most of these works have focused on effects associated with atmospheric and oceanic dynamic processes caused by stratospheric ozone changes, here we show that stratospheric ozone-induced cloud radiative effects also play important roles in causing changes in Antarctic sea ice. Our simulations demonstrate that the recovery of the Antarctic Ozone Hole causes decreases in clouds over Southern Hemisphere (SH) high latitudes and increases in clouds over the SH extratropics. The decrease in clouds leads to a reduction in downward infrared radiation, especially in austral autumn. This results in cooling of the Southern Ocean surface and increasing Antarctic sea ice. Surface cooling also involves ice-albedo feedback. Increasing sea ice reflects solar radiation and causes further cooling and more increases in Antarctic sea ice."

&

The pattern effect is an example of a positive feedback mechanism that is made much worse by a rapid cascade of freshwater hosing events as postulated by the MCDS, and in this regard, Xie (2020) (see the first image) indicates that CMIP6 confirms that the pattern effect can result in an increasing climate sensitivity value this century.

Xie, S.-P. (03 March 2020), "Ocean Warming Pattern Effect On Global And Regional Climate Change", AGU Advances, https://doi.org/10.1029/2019AV000130

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019AV000130
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019AV000130

Abstract
Anthropogenic emissions of greenhouse gases cause the planet to warm, and the ocean uptake of anthropogenic heat slows the warming, preventing the climate system from equilibrating with the increasing radiative forcing. The uneven ocean surface warming affects regional changes in tropical rainfall, El Niño, and the global climate sensitivity. Thus, the study of ocean warming patterns bridges the ocean‐atmospheric dynamics community focusing on spatial patterns on one hand and the climate change community with a traditional emphasis on the planetary energy budget and radiative feedback on the other.

Plain Language Summary
The ocean surface warming pattern has emerged as a research frontier in climate science in relation to regional climate change and global climate sensitivity. In the high‐latitudes, observations have largely confirmed the interhemispheric asymmetry in ocean warming climate models predicted between the North Atlantic and Southern Ocean, a pattern resulting from the pole‐to‐pole deep ocean overturning circulation. In the tropics, the ocean warming pattern is closely coupled with atmospheric convection and circulation, modulating tropical cyclone statistics and El Niño influence in faraway regions such as western North America. While climate sensitivity is often considered a constant inherent to a given model, recent studies show that it varies in time as the heat exchange between the surface and deep oceans causes the ocean surface warming pattern to evolve. This warming pattern effect highlights the challenges in estimating climate sensitivity from instrumental observations, which feature evolving radiative forcing (greenhouse gases vs. aerosols) as well as unforced internal variability.


Caption for the first image: "Figure 1. Multimodel ensemble mean change in Coupled Model Intercomparison Project Phase 6 (CMIP6) runs where the atmospheric CO2 concentration is increased 1% per year, calculated as the difference between Years 130–150 (the time of CO2 quadrupling) and Years 1–20. (a) SST (°C). (b) Net surface heat flux (W/m2 , downward positive). (c) Percentage change in precipitation (color shading) and SST change (contours at intervals of 0.5°C) relative to the tropical mean (25°S–25°N), with a spatial correlation of 0.55 in the tropics."

&

Furthermore, Lohmann & Ditlevsen (2021) confirms that the occurrence of rate-induced tipping in a global ocean model gives important evidence that one or more climate sub-systems may tip from being pushed too quickly as a result of global warming; resulting in abrupt irreversible climate change; particularly due to increasing rates of ice melt and its influence on the MOC.  In particular, Lohmann & Ditlevsen find that:

"Using a global ocean model subject to freshwater forcing, we show that a collapse of the Atlantic Meridional Overturning Circulation can indeed be induced even by small-amplitude changes in the forcing, if the rate of change is fast enough."

Lohmann and Ditlevsen (March 2, 2021), "Risk of tipping the overturning circulation due to increasing rates of ice melt", PNAS, 118, (9), e2017989118, https://doi.org/10.1073/pnas.2017989118

https://www.pnas.org/content/118/9/e2017989118

Significance
Ongoing greenhouse gas emissions put elements of the Earth system at risk for crossing critical thresholds (tipping points), leading to abrupt irreversible climate change. Measures for reducing emissions should keep Earth in the safe operating space away from tipping points. Here we show that increasing rates of change of ice melt can induce a collapse of the Atlantic Meridional Overturning Circulation in a global ocean model, while no critical threshold in ice melt is crossed and slower increases to the same level of ice melt do not induce tipping. Moreover, the chaotic dynamics of the climate make such a collapse hard to predict. This shows that the safe operating space of the Earth system might be smaller than previously thought.

Abstract
Central elements of the climate system are at risk for crossing critical thresholds (so-called tipping points) due to future greenhouse gas emissions, leading to an abrupt transition to a qualitatively different climate with potentially catastrophic consequences. Tipping points are often associated with bifurcations, where a previously stable system state loses stability when a system parameter is increased above a well-defined critical value. However, in some cases such transitions can occur even before a parameter threshold is crossed, given that the parameter change is fast enough. It is not known whether this is the case in high-dimensional, complex systems like a state-of-the-art climate model or the real climate system. Using a global ocean model subject to freshwater forcing, we show that a collapse of the Atlantic Meridional Overturning Circulation can indeed be induced even by small-amplitude changes in the forcing, if the rate of change is fast enough. Identifying the location of critical thresholds in climate subsystems by slowly changing system parameters has been a core focus in assessing risks of abrupt climate change. This study suggests that such thresholds might not be relevant in practice, if parameter changes are not slow. Furthermore, we show that due to the chaotic dynamics of complex systems there is no well-defined critical rate of parameter change, which severely limits the predictability of the qualitative long-term behavior. The results show that the safe operating space of elements of the Earth system with respect to future emissions might be smaller than previously thought.

&

Furthermore, Sherwood et al (2014) raises the prospect that an increase in Tropical Ocean SST (say from an abrupt slowing of the MOC) can lead to more tropical atmospheric deep convective mixing that can lead to a positive cloud feedback mechanism (see the second image).
Sherwood, S.C., Bony, S. and Dufresne, J.-L., (2014) "Spread in model climate sensitivity traced to atmospheric convective mixing", Nature; Volume: 505, pp 37–42, doi:10.1038/nature12829

http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html

Abstract: "Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming."
 

Finally, for this post, Olonscheck et al (2020), see the third image, confirms that CMIP5, CMIP6 and observations all indicate that globally SST is increasing at a rapid rate; while the fourth image confirms that the SST over the tropical oceans are increasing faster than the global average.  Such ocean SST patterns contribute directly to the pattern effect; which indicate that climate sensitivity is currently increasing and may increase at an accelerating rate in the coming decades.

Olonscheck, D., Rugenstein, M., & Marotzke, J. (2020). Broad consistency between observed and simulated trends in sea surface temperature patterns. Geophysical Research Letters, 47, e2019GL086773. https://doi.org/ 10.1029/2019GL086773

https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2019GL086773

Abstract
Using seven single-model ensembles and the two multimodel ensembles CMIP5 and CMIP6, we show that observed and simulated trends in sea surface temperature (SST) patterns are globally consistent when accounting for internal variability. Some individual ensemble members simulate trends in large-scale SST patterns that closely resemble the observed ones. Observed regional trends that lie at the outer edge of the models' internal variability range allow two nonexclusive interpretations: (a) Observed trends are unusual realizations of the Earth's possible behavior and/or (b) the models are systematically biased but large internal variability leads to some good matches with the observations. The existing range of multidecadal SST trends is influenced more strongly by large internal variability than by differences in the model formulation or the observational data sets.

Plain Language Summary
Climate model simulations agree well with the observed evolution of the global mean sea surface temperature, but their ability to realistically represent changes in the patterns of sea surface temperatures has been questioned. We show with an unprecedented number of simulations from different models and different initial conditions that the observed and simulated changes in SST patterns are consistent in most regions of the ocean. For each model, a few individual simulations recreate the observed patterns. In some regions, the observed changes may be an extreme realization of the Earth's possible behavior. Alternatively, structural model errors may be hidden by the large range of possible realizations.

Caption for the third image: "Figure 1. Evolution of the annual global mean sea surface temperature (SST) anomaly for 1900–2014 relative to 1961–1990 in CMIP5, CMIP6, seven single-model large ensembles, and three observational estimates. The number of members for each multimodel or single-model ensemble is shown in brackets. For each ensemble, the averaged correlation, r, between each ensemble member and each observational estimate is shown. The range of the correlation between the realizations of each model ensemble and the observational estimates is given in Table S1."

Caption for the fourth image: "Image from the new GHRSST product showing global SST for January 9, 2019. Warmer SST values are indicated in yellow, orange, and red; cooler SSTs are indicated in green and blue. Note that SST values are given in degrees Kelvin (K). PO.DAAC image."

[AbruptSLR]

As a follow-on from my last post, Zhou et al. (2021), see the attached four images, indicates that the current high rate of radiative forcing is contributing to a high effective climate sensitivity, particularly with regard to spatial inhomogeneities in both sea surface temperature (SST) and sea ice change.  Here, I note that the authors do not consider changes in effective radiative forcing associated with abrupt freshwater hosing events; but if a cascade of freshwater hosing events were to occur this research indicates that climate sensitivity could increase rapidly this century.

Zhou, C., Zelinka, M.D., Dessler, A.E. et al. Greater committed warming after accounting for the pattern effect. Nat. Clim. Chang. (2021). https://doi.org/10.1038/s41558-020-00955-x

https://www.nature.com/articles/s41558-020-00955-x

Abstract
Our planet’s energy balance is sensitive to spatial inhomogeneities in sea surface temperature and sea ice changes, but this is typically ignored in climate projections. Here, we show the energy budget during recent decades can be closed by combining changes in effective radiative forcing, linear radiative damping and this pattern effect. The pattern effect is of comparable magnitude but opposite sign to Earth’s net energy imbalance in the 2000s, indicating its importance when predicting the future climate on the basis of observations. After the pattern effect is accounted for, the best-estimate value of committed global warming at present-day forcing rises from 1.31 K (0.99–2.33 K, 5th–95th percentile) to over 2 K, and committed warming in 2100 with constant long-lived forcing increases from 1.32 K (0.94–2.03 K) to over 1.5 K, although the magnitude is sensitive to sea surface temperature dataset. Further constraints on the pattern effect are needed to reduce climate projection uncertainty.

Extract: "Properly accounting for the pattern effect has a major impact on the amount of carbon humans can emit before breaching any particular temperature threshold."
 
Caption for the first image: "Fig. 1 | Attribution of the net TOA fluxes during 1871–2010. a, Time series of effective radiative forcing from IPCC AR5 (red), the linear radiative damping term (−λltΔT, green) and the pattern effect term (blue). b, Time series of reconstructed TOA fluxes. The black line denotes the TOA fluxes reconstructed with equation (4) and the brown line denotes the TOA net flux estimated by ignoring the pattern effect (F – λltΔT). c, Comparison of reconstructed TOA fluxes with observations. The magenta line denotes observed annual TOA net flux from CERES EBAF v.4.0 (ref. 21) and the cyan line denotes net flux observations from merged radiation budget data v.3 (ref. 22), which is calculated from CERES EBAF v.2.8 (ref. 42) and ERBS wide field of view v.3 data43. Thin lines denote values calculated from individual models, while thick lines are model averages."
 
Caption for the second image: "Fig. 2 | Cumulative energy flux into the Earth system during 1961–2010. The magenta and cyan lines are two estimates of the observed changes in the global heat content24,25 (Methods), the black thick line is the net influx calculated from the reconstructed net energy imbalance of Fig. 1 and the brown thick line denotes net energy influx if the pattern effect is zero. The red, green and blue dashed lines denote individual contributors to the net energy influx. Thin lines denote values calculated from individual models, while thick lines are model averages."
 
Caption for the third image: "Fig. 3 | Impact of the pattern effect on equilibrium committed warming with constant forcing. Colours denote the committed warming for a range of Pref and λlt values calculated with equation (9), and the two white contours represent the Paris Agreement thresholds. The black line denotes the relationship of Pref and λlt constrained by equation (. Values printed beside the two black markers denote the committed warming corresponding to Pref = 0 and Pref = −0.63 W m–2, respectively."
 
Caption for the fourth image: "Fig. 4 | Comparison of TOA fluxes reconstructed with CAM5.3 experiments driven by different SST datasets. The TOA fluxes are reconstructed with equation (1), where the climate forcing is from IPCC AR5 and Rfb is from simulations. The black line is the ensemble mean value calculated from three CAM5.3 AMIP-piForcing experiments, which use prescribed AMIPII SST boundary conditions. The blue line is reconstructed with the ensemble mean value of three CAM5.3 HadISST-piForcing experiments, which use prescribed HadISST SST boundary conditions. The correlation coefficients between the time series of observations and reconstructed TOA fluxes, of which the corresponding P values are all below 0.05, are listed in the figure."

See also:

Title: "We've already blown past the warming targets set by the Paris climate agreement, study finds"

https://www.livescience.com/already-too-late-to-meet-paris-agreement-climate-goals.html

Extract: "Dessler told the AP. "It's really the rate of warming that makes climate change so terrible. If we got a few degrees over 100,000 years, that would not be that big a deal. We can deal with that. But a few degrees over 100 years is really bad.""

[AbruptSLR]

The linked article, supported by the associated linked reference, indicates that:

"An overlooked but powerful driver of cloud formation could accelerate the loss of polar sea ice.
…
Global iodine emissions have tripled over the past 70 years, and scientists predict that emissions will continue to accelerate as sea ice melts and surface ozone increases.
…
Researchers have observed in remote areas of Ireland, Greenland and Antarctica that iodine, which is released naturally from melting sea ice, algae and the ocean surface, may also be a significant driver of new particle formation.
…
Based on these results, an increase of molecular iodine could lead to more particles for water vapor to condense onto and spiral into a positive feedback loop."

This newly identified positive cloud aerosol feedback mechanism has not been included in any climate model (including in any CMIP6 model), but could markedly increase polar amplification, and consequently climate sensitivity (possibly even above the CMIP6 Wolf Pack estimates), in coming decades.

Title: "Cloud-Making Aerosol Could Devastate Polar Sea Ice"

https://www.quantamagazine.org/cloud-making-aerosol-could-devastate-polar-sea-ice-20210223/

Extract: "An overlooked but powerful driver of cloud formation could accelerate the loss of polar sea ice.
…
Now, while studying the atmospheric chemistry that produces clouds, researchers have uncovered an unexpectedly potent natural process that seeds their growth. They further suggest that, as the Earth continues to warm from rising levels of greenhouse gases, this process could be a major new mechanism for accelerating the loss of sea ice at the poles — one that no global climate model currently incorporates.
…
The full climate impact of this mechanism still needs to be assessed carefully, but tiny modifications in the behavior of aerosols, which are treated as an input in climate models, can have huge consequences, according to Andrew Gettelman, a senior scientist at the National Center for Atmospheric Research (NCAR) who helps run the organization’s climate models and who was not involved in the study. And one consequence “will definitely be to accelerate melting in the Arctic region,” said Jasper Kirkby, an experimental physicist at CERN who leads the Cosmics Leaving Outdoor Droplets (CLOUD) experiment and a coauthor of the new study.
…
Researchers have observed in remote areas of Ireland, Greenland and Antarctica that iodine, which is released naturally from melting sea ice, algae and the ocean surface, may also be a significant driver of new particle formation. But researchers still wondered how molecular iodine grows into a CCN, and how efficiently it does so, compared with other secondary aerosols. “Even though these particles were known to exist, we weren’t able to link a measured concentration in the atmosphere to a predicted formation of particles,” Kirkby said.
…
The findings are important for understanding the fundamental chemistry in the atmosphere that underlies cloud processes, Kirkby said, but also as a warning sign: Global iodine emissions have tripled over the past 70 years, and scientists predict that emissions will continue to accelerate as sea ice melts and surface ozone increases. Based on these results, an increase of molecular iodine could lead to more particles for water vapor to condense onto and spiral into a positive feedback loop. “The more the ice melts, the more sea surface is exposed, the more iodine is emitted, the more particles are made, the more clouds form, the faster it all goes,” Kirkby said.
…
Clouds generally cool the planet, as the white tops of the clouds reflect sunlight into space. But in polar regions, snowpack has a similar albedo, or reflectivity, as cloud tops, so an increase in clouds would reflect little additional sunlight. Instead, it would trap longwave radiation from the ground, creating a net warming effect.
…
In 2019, NCAR’s model projected a climate sensitivity well above IPCC’s average upper bound and 32% higher than its previous estimate — a warming of 5.3 degrees C (10.1 degrees F) if the global carbon dioxide is doubled — mostly as a result of the way that clouds and their interactions with aerosols are represented in their new model.

Brock remains hopeful that future research into new particle formation will help to chip away at the uncertainty in climate sensitivity. “I think we’re gaining an appreciation for the complexity of these new particle sources,” he said."

See also:

He, X.-C. et al. (05 Feb 2021), "Role of iodine oxoacids in atmospheric aerosol nucleation", Science, Vol. 371, Issue 6529, pp. 589-595, DOI: 10.1126/science.abe0298

https://science.sciencemag.org/content/371/6529/589

Abstract
Iodic acid (HIO3) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO3 particles are rapid, even exceeding sulfuric acid–ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO3− and the sequential addition of HIO3 and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO2) followed by HIO3, showing that HIO2 plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO3, which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere.


Jia et al. (2021) indicates that previous consensus climate science's interpretation of satellite readings significantly underestimated how negative feedback is from aerosol-cloud interactions.  This implies that climate sensitivity is likely higher than such consensus climate scientists previously assumed.

Jia, H., Ma, X., Yu, F. et al. Significant underestimation of radiative forcing by aerosol–cloud interactions derived from satellite-based methods. Nat Commun 12, 3649 (2021). https://doi.org/10.1038/s41467-021-23888-1

https://www.nature.com/articles/s41467-021-23888-1

Abstract: "Satellite-based estimates of radiative forcing by aerosol–cloud interactions (RFaci) are consistently smaller than those from global models, hampering accurate projections of future climate change. Here we show that the discrepancy can be substantially reduced by correcting sampling biases induced by inherent limitations of satellite measurements, which tend to artificially discard the clouds with high cloud fraction. Those missed clouds exert a stronger cooling effect, and are more sensitive to aerosol perturbations. By accounting for the sampling biases, the magnitude of RFaci (from −0.38 to −0.59 W m−2) increases by 55 % globally (133 % over land and 33 % over ocean). Notably, the RFaci further increases to −1.09 W m−2 when switching total aerosol optical depth (AOD) to fine-mode AOD that is a better proxy for CCN than AOD. In contrast to previous weak satellite-based RFaci, the improved one substantially increases (especially over land), resolving a major difference with models."

[AbruptSLR]

Dynamical Statistics and Domino Fault Tree Modeling


One approach to partially address the confusion about how to better understand Ice Apocalypse long-tail risk is to briefly summarize the state-of-the-art of consensus climate science (such as CMIP6 projections) and to identify the caveats, gaps and short-comings of such a consensus gestalt (typically dominated by Frequentist perspectives) in order to better appreciate the probabilities that such long-tailed probabilities can transform first into fat-tailed probabilities and then into likely probabilities and possibly into realty.  While another approach is to invoke the 'Precautionary Principle' to bypass the need to identify specific fault trees of domino events (see the first three images) in order to develop a suite of long/fat-tail 'Ice Apocalypse' scenarios (of which the MCDS-BN discussed here is only one) that would allow decision makers to appreciate how the associated risks are increasing as observations demonstrate that we are progressively locking-in cascades of ice-related tipping points associated with a given scenario within the suite of scenarios envisioned by the Bayesian 'expert' (for the MCDS that means me) responsible for that given Ice Apocalypse scenario [see Lawrence et al (2020)]. 

Next, I provide some discussion about the differences between Bayesian, and Frequentist, Analysis.

In the Bayesian approach one looks for the probability P(model|data), which could be translated to one “assuming” the model and “having” the data. That is, one's model is uncertain, while the data is one's ground truth – the only certain thing we know about reality. In contrast, when one follows the frequentist approach, one is looking for the probability P(data|model), which means that one “assumes” the data and “have” the model. In other words, one is certain about one's model (at least for working purposes) and one has uncertain measurements, i.e. the data, which may or may not perfectly reflect our model (or even reality).

One might easily think that the Bayesian approach makes more sense since one cannot in general know the exact the distribution of a variable inside a population (i.e. the model); and one might say one can only know for sure what one observes through the data that is collected. That’s a good point but be aware that a small number of observations can be misleading (i.e. outliers, extremes, noisy measurements and so on). And although one may have observed some data for sure, most of the time one is not interested in the actual data but in the asymptotic inference that one can safely make based on it.

This is why the proponents of frequentist statistics approach the problem by saying: “ok, it’s true that based on our data, we cannot describe the particular population at a satisfactory level of accuracy. However, we can invent a hypothetical population and test how likely it is that our observed data was obtained from that particular population.” Then they would continue; “Of course, the model that we claim is not an elaborate one but it certainly has well-described characteristics (after all we made it)”. This special population model is called the null hypothesis (see the fourth image for the relationship of the null hypothesis vs Type I and Type II errors).
 

[AbruptSLR]

I provide the following table (the first attached image) of Domino Fault Tree Classes for Probability of Occurrence; and I note that am aiming for a Class 4 (Moderate) qualitative evaluation (of my single MCDS) with a probability of occurrence of 1 in a thousand (10-3) by 2100 based on information available at the time of posting; which implies that a family of such maximum credible domino scenarios (developed by someone else) could quantify a higher probability of occurrence.  Furthermore, the second image shows a table of standard terms used to Define Levels of Confidence for Different Objectives for RSLR (not including adaptive standards, which may vary). Also, I note that the longer we stay on a strong radiative forcing scenario and if we observe future strong positive climate feedback mechanisms, then this probability of occurrence would also increase.

Image 2

Next, I note that Lawrence et al. (2020) indicate that the recommendation by consensus climate scientists Hausfather and Peters to assign a single set of best-estimate probabilities to all future GHG emissions is not an advisable way to assess future climate-change risks.  Lawrence et al. (2020) further note that deep uncertainty needs to be accounted for in any set of climate-change risk assessments; which, to me requires an understanding of tail-risk for my single MCDS case as illustrated by the third attached image.

Lawrence, J., Marjolijn Haasnoot & Robert Lempert (21 APRIL 2020), "Climate change: making decisions in the face of deep uncertainty", Nature (Correspondence), 580, 456, doi: 10.1038/d41586-020-01147-5

https://www.nature.com/articles/d41586-020-01147-5

Extract: "In our view, Zeke Hausfather and Glen Peters’s recommendation to assign a single set of best-estimate probabilities to all future emissions scenarios as a means to assess climate-change risks (Nature 577, 618–620; 2020) could give decision-makers a false sense of certainty, leading to costly adjustments if the world evolves in unanticipated ways.
The Society for Decision Making Under Deep Uncertainty (www.deepuncertainty.org), to which we belong, offers a better strategy. It relies on methods that focus on the implications of alternative scenarios and the extent to which response tactics are shared across a wide range of scenarios. This helps to manage uncertainties — for example, in sea-level rise after 2050 — by identifying long-term options and short-term, flexible actions that can prepare for a range of future emissions.

Bypassing the need to assign probabilities enables decision-makers to better understand the combination of uncertainties that most affect their choices, thereby reducing locked-in choices and decision delays that can arise when using a single scenario."

Finally, for this post, I repeat that I will use fault tree analysis (see the fourth image of a representative fault tree), as discussed in Lopez-Molina et al. (2014).

López-Molina, A. et al. (2014), "A Methodology Based on Fault Tree Analysis to Assess the Domino Effect Frequency", Proceedings of the IChemE, Hazards 24, Symposium Series No. 159
https://www.icheme.org/media/8929/xxiv-paper-34.pdf

[AbruptSLR]

In this post, I note that current climate models/modules of glacier/ice sheet responses to future climate change are all incomplete and that currently no consensus climate, CCS, models account for potential MICI-type of behavior for marine, & marine terminating, glaciers.  Furthermore, all marine, & marine terminating, glaciers are unique and current, and probably future, glacier/ice sheet models will provide projections with varying degree of accuracies for each different glacier/ice sheet.  While CCS uses such consideration to justify their erring on the side of least drama, ESLD, the MCDS methodology, considered here, uses scientifically supported judgement to provide deductively estimated priori scenarios about: anthropogenic input, domino effects consideration and model responses in an effort to better address such deep uncertainty issues. 
Also, while prior probabilities for key marine, & marine terminating, glaciers will be addressed in subsequent sections; here I provide a series of posts discussing two examples of MCDS priors for the 2050 +/- 5yrs that make logical probability propositions for the frequency (probability) of the MCDS domino chain for 2050 +/- 5yrs (see the first and second images) of what may likely (not what must) occur in coming decades based what we currently know; and which can then be updated by future data/analysis into a posterior probability distribution for the MCDS domino cascade for 2050 +/- 5yrs using Bayesian statistical inference (see the third image which shows how the posterior shifts to the right with the passage of time with new observations/measurements/data).  Also, the fourth image show how climate change related damage (impact) increases nonlinearly as the climate stressors (forcings) increase with time.

 
First image of MCDS-BN

Title: "Prior probability"
https://en.wikipedia.org/wiki/Prior_probability
Extract: "In Bayesian statistical inference, a prior probability distribution, often simply called the prior, of an uncertain quantity is the probability distribution that would express one's beliefs about this quantity before some evidence is taken into account.

&

Title: "Posterior probability
https://en.wikipedia.org/wiki/Posterior_probability
Extract: "In Bayesian statistics, the posterior probability of a random event or an uncertain proposition is the conditional probability that is assigned after the relevant evidence or background is taken into account. "Posterior", in this context, means after taking into account the relevant evidence related to the particular case being examined.
The posterior probability distribution is the probability distribution of an unknown quantity, treated as a random variable, conditional on the evidence obtained from an experiment or survey."


L. Mark Berliner, L.M. et al. (01 Nov 2000), "Bayesian Climate Change Assessment", Journal of Climate, pp 3805-3820, DOI: https://doi.org/10.1175/1520-0442(2000)013<3805:BCCA>2.0.CO;2

https://journals.ametsoc.org/view/journals/clim/13/21/1520-0442_2000_013_3805_bcca_2.0.co_2.xml

Abstract
A Bayesian fingerprinting methodology for assessing anthropogenic impacts on climate was developed. This analysis considers the effect of increased CO2 on near-surface temperatures. A spatial CO2fingerprint based on control and forced model output from the National Center for Atmospheric Research Climate System Model was developed. The Bayesian approach is distinguished by several new facets. First, the prior model for the amplitude of the fingerprint is a mixture of two distributions: one reflects prior uncertainty in the anticipated value of the amplitude under the hypothesis of “no climate change.” The second reflects behavior assuming “climate change forced by CO2.” Second, within the Bayesian framework, a new formulation of detection and attribution analyses based on practical significance of impacts rather than traditional statistical significance was presented. Third, since Bayesian analyses can be very sensitive to prior inputs, a robust Bayesian approach, which investigates the ranges of posterior inferences as prior inputs are varied, was used. Following presentation of numerical results that enforce the claim of changes in temperature patterns due to anthropogenic CO2 forcing, the article concludes with a comparative analysis for another CO2 fingerprint and selected discussion.
 
Caption for the third image: "Figure 2 Likelihood function, prior distribution, and posterior distribution of a using our NCAR CSM fingerprint. For each of the time periods (a)–(d): (left) the likelihood (dotted line) and prior distribution components [anthropogenic CO2 forcing (solid line); no anthropogenic impacts (dashed line)], and (right) the posterior mixture distribution"

 
Image 4 – Nonlinear Increase in Damage (Impact) with Increasing Climate Stressor (Forcing) over time.


In balance, Terrer et al. (2021) indicate that consensus climate science (CCS) has overestimated the potential for terrestrial ecosystems to act as a net carbon sink with continued anthropogenic forcing.  This means that effective climate sensitivity will be higher in coming decades than previously assumed by CCS.

Terrer, C., Phillips, R.P., Hungate, B.A. et al. A trade-off between plant and soil carbon storage under elevated CO2. Nature 591, 599–603 (2021). https://doi.org/10.1038/s41586-021-03306-8

https://www.nature.com/articles/s41586-021-03306-8

Abstract: "Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO2) emitted by human activities each year, yet the persistence of this carbon sink depends partly on how plant biomass and soil organic carbon (SOC) stocks respond to future increases in atmospheric CO2. Although plant biomass often increases in elevated CO2 (eCO2) experiments, SOC has been observed to increase, remain unchanged or even decline. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections. Here we synthesized data from 108 eCO2 experiments and found that the effect of eCO2 on SOC stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO2, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases. This trade-off appears to be related to plant nutrient acquisition, in which plants increase their biomass by mining the soil for nutrients, which decreases SOC storage. We found that, overall, SOC stocks increase with eCO2 in grasslands (8 ± 2 per cent) but not in forests (0 ± 2 per cent), even though plant biomass in grasslands increase less (9 ± 3 per cent) than in forests (23 ± 2 per cent). Ecosystem models do not reproduce this trade-off, which implies that projections of SOC may need to be revised."

[AbruptSLR]

Example of the first domino cascade prior contributing to the 2050 +/- 5 yrs MCDS probability:

As a follow-on to my last post, the MCDS Bayesian Network (BN) proposes that major Antarctic ice shelves such as the FRIS (Event X9) and RIS (Event X11) could likely collapse by 2050 +/- 5yrs due to hydrofracturing.  In this regard, Lai et al. (2020) demonstrate numerical that these, and other, ice shelves are indeed vulnerable to hydrofracturing (see the first image); if/when sufficient surface water penetrating into the identified surface fractures.

Lai, CY., Kingslake, J., Wearing, M.G. et al. Vulnerability of Antarctica’s ice shelves to meltwater-driven fracture. Nature 584, 574–578 (2020). https://doi.org/10.1038/s41586-020-2627-8

https://www.nature.com/articles/s41586-020-2627-8

Abstract: "Atmospheric warming threatens to accelerate the retreat of the Antarctic Ice Sheet by increasing surface melting and facilitating ‘hydrofracturing’, where meltwater flows into and enlarges fractures, potentially triggering ice-shelf collapse. The collapse of ice shelves that buttress the ice sheet accelerates ice flow and sea-level rise. However, we do not know if and how much of the buttressing regions of Antarctica’s ice shelves are vulnerable to hydrofracture if inundated with water. Here we provide two lines of evidence suggesting that many buttressing regions are vulnerable. First, we trained a deep convolutional neural network (DCNN) to map the surface expressions of fractures in satellite imagery across all Antarctic ice shelves. Second, we developed a stability diagram of fractures based on linear elastic fracture mechanics to predict where basal and dry surface fractures form under current stress conditions. We find close agreement between the theoretical prediction and the DCNN-mapped fractures, despite limitations associated with detecting fractures in satellite imagery. Finally, we used linear elastic fracture mechanics theory to predict where surface fractures would become unstable if filled with water. Many regions regularly inundated with meltwater today are resilient to hydrofracture—stresses are low enough that all water-filled fractures are stable. Conversely, 60 ± 10 per cent of ice shelves (by area) both buttress upstream ice and are vulnerable to hydrofracture if inundated with water. The DCNN map confirms the presence of fractures in these buttressing regions. Increased surface melting could trigger hydrofracturing if it leads to water inundating the widespread vulnerable regions we identify. These regions are where atmospheric warming may have the largest impact on ice-sheet mass balance."

 
Caption for the first image: "Fig. 4 | Map of ice-shelf vulnerability to hydrofracture. a, Water-filled fractures are unstable in vulnerable areas (red and blue) and stable in resilient regions (yellow and green) unless pre-existing surface fractures of depth di exist. Where stresses are sufficiently compressive, water-filled fractures cannot open (black). c, e, Present-day meltwater on the George VI (c; 4 February 1991, Landsat 5) and Amery (e; 15 and 17 January 2019, Landsat ice shelves predominantly lies in regions resilient to hydrofracture (yellow, green and black in b and d). Blue denotes regions providing weak buttressing13. We find that 60±10% of the Antarctic Ice Shelf area provides buttressing and is vulnerable to hydrofracture (red)."

The next logic step in this example priori is to demonstrate the likelihood of significant quantities of surface water (ice meltwater or rain) on the key Antarctic ice shelves circa 2050 +/- 5yrs.  In this regard, Feng et al (2021) discusses the tropically driven atmospheric teleconnection between the phases of the IPO (interdecadal Pacific Oscillation) and polar amplification on a multidecadal time scale.

Feng, X. et al. (02 Mar 2021), "A Multidecadal-Scale Tropically Driven Global Teleconnection over the Past Millennium and Its Recent Strengthening", Journal of Climate, DOI: https://doi.org/10.1175/JCLI-D-20-0216.1

https://journals.ametsoc.org/view/journals/clim/34/7/JCLI-D-20-0216.1.xml

Abstract
In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this “wavy” pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades.

Second example of domino cascade prior contributing to the 2050 +/- 5 yrs MCDS probability:

Here I propose that an abrupt slowdown of the MOC associated with a series of preceding freshwater hosing events (postulated to be from: Jakobshavn & Heilheim circa 2030 (X1), from Thwaites circa 2035 (X3), from the Beaufort Gyre (X2), from the Byrd Subglacial Basin (X4), from the PIG (X5) and from other ASE & Bellingshausen marine glaciers (X6), circa 2040) will trigger both a great telecommunication of energy from the Tropical Pacific to the SH and of an increase of the number of atmospheric river events from both the Tropical Pacific, and from the Tropical Atlantic, to the SH.
Regarding the coming change in atmospheric river (AR) events circa 2050 (assuming occurrence of X1 thru X6); Espinoza et al (2018), see the third image, which shows projected AR frequencies from CMIP5 without considering any significant freshwater hosing events.


Espinoza, V. et al. (19 April 2018), "Global Analysis of Climate Change Projection Effects on Atmospheric Rivers", Geophysical Research Letters, https://doi.org/10.1029/2017GL076968
 
Caption for the third image: "Figure 2 AR frequency (shading; percent of time steps) and IVT (vectors; kg · m−1 · s−1) for (a) ERA‐Interim reanalysis for the historical period (1979–2002) with six green boxes depicting regions analyzed in Figures S2 and S3, (b) the MMM for the 21 CMIP5 models analyzed in this study for the historical period (1979–2002), (c) RCP4.5 warming scenario (2073–2096), and (d) RCP8.5 warming scenario (2073–2096), (e) the difference between (c) and (b) with six green boxes depicting regions analyzed in Figures S2 and S3, and (f) the difference between (d) and (b). Vector magnitudes are indicated by both their length and their color based on the blue color bar."


See also:

Francis, D. et al. (11 Nov 2020), "On the crucial role of atmospheric rivers in the two major Weddell Polynya events in 1973 and 2017 in Antarctica", Science Advances, Vol. 6, no. 46, eabc2695, DOI: 10.1126/sciadv.abc2695

https://advances.sciencemag.org/content/6/46/eabc2695

Abstract
This study reports the occurrence of intense atmospheric rivers (ARs) during the two large Weddell Polynya events in November 1973 and September 2017 and investigates their role in the opening events via their enhancement of sea ice melt. Few days before the polynya openings, persistent ARs maintained a sustained positive total energy flux at the surface, resulting in sea ice thinning and a decline in sea ice concentration in the Maud Rise region. The ARs were associated with anomalously high amounts of total precipitable water and cloud liquid water content exceeding 3 SDs above the climatological mean. The above-normal integrated water vapor transport (IVT above the 99th climatological percentile), as well as opaque cloud bands, warmed the surface (+10°C in skin and air temperature) via substantial increases (+250 W m−2) in downward longwave radiation and advection of warm air masses, resulting in sea ice melt and inhibited nighttime refreezing.

Extract: "Last, the possibility that winter AR events in the Antarctic may also affect the seasonal evolution of the sea ice environment through the subsequent summer months (and beyond) deserves consideration. In the Arctic, it has been observed that ARs not only affect the surface temperature over the sea ice but also induce warming waves that propagate downward through the sea ice interior to affect its properties and also reduce ice basal growth (. This process has been shown to influence the Arctic sea ice thickness at the onset of seasonal melt in spring, and the probability of similar processes in the relatively thin Antarctic sea ice pack merits investigation. An additional major unknown relates to the possible effects of ARs in increasing regional snowfall/accumulation over the Antarctic sea ice zone—to potentially affect regional sea ice melt/persistence given its insulative properties and its contribution to snow-ice formation (15). We hope that this study will motivate such investigations."

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To provide readers with some feeling about risks and consequences, I note that in the modern global socio-economic world, engineers are required to design systems/features with factors of safety to limit the risk that characteristic demands might exceed characteristic capacities (see the first image), and that the currently engineering approach of using 'resiliency' to address the consequences of the fact that as climate change increases characteristic demands that prior engineered factors of safety are reduced or eliminated.  Furthermore, the second image illustrates how if future climate change forcing were to increase nonlinearly (say due to the domino effect of multiple climate change tipping points being exceeded) that the risk of the failure of a given system would rapidly increase.  Also, the third image illustrates how the safety margin declines with degrading capacity 'R'(say due to the system aging) assuming a constant demand 'S' (which is not likely to be the case with continuing climate change).  Finally, for this reply, the fourth image illustrates that many demand PDFs are not 'normally' distributed and that right-skewed demand PDFs (like the Pareto distribution shown) substantially increase right-tail risks.
 
Caption for the second image: "Demand 'S' vs forcing for a given capacity 'R' (Resistance)"

 
Caption for the third image: "Declining safety margin with degrading capacity 'R' assuming a constant demand 'S'"

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Harries and O'Kane (2021) provide discussion of how dynamic Bayesian networks can be used to evaluate Granger causal relationships in climate reanalyses (see the first and second images).

Harries, D. and Terence J. O’Kane (29 March 2021), "Dynamic Bayesian networks for evaluation of Granger causal relationships in climate reanalyses", JAMES, https://doi.org/10.1029/2020MS002442

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020MS002442

Abstract
We apply a Bayesian structure learning approach to study interactions between global climate modes, illustrating its use as a framework for developing process‐based diagnostics with which to evaluate climate models. Homogeneous dynamic Bayesian network models are constructed for time series of empirical indices diagnosing the activity of major tropical, Northern and Southern Hemisphere modes of climate variability in the NCEP/NCAR and JRA‐55 reanalyses. The resulting probabilistic graphical models are comparable to Granger causal analyses that have recently been advocated. Reversible jump Markov Chain Monte Carlo is employed to provide a quantification of the uncertainty associated with the selection of a single network structure. In general, the models fitted from the NCEP/NCAR reanalysis and the JRA‐55 reanalysis are found to exhibit broad agreement in terms of associations for which there is high posterior confidence. Differences between the two reanalyses are found that involve modes for which known biases are present or that may be attributed to seasonal effects, as well as for features that, while present in point estimates, have low overall posterior mass. We argue that the ability to incorporate such measures of confidence in structural features is a significant advantage provided by the Bayesian approach, as point estimates alone may understate the relevant uncertainties and yield less informative measures of differences between products when network‐based approaches are used for model evaluation.

Plain Language Summary
To produce reliable forecasts and projections, climate models should accurately reproduce the observed behavior of different processes that play a role in Earth’s climate, including the relationships between them. Statistical methods can be used to describe these interactions in models and in observations, which can then be compared to evaluate how well a given model captures the observed relationships. However, networks obtained from estimates of the true historical state of the climate, known as reanalyses, will also be affected by the properties of the systems used to create these estimates, as well as random variability, and hence may have significant uncertainties. Using what are known as Bayesian statistical methods, we estimate the uncertainties associated with particular interactions in two widely used reanalyses. Interactions that are found to be very likely to be present in one reanalysis but not the other are suggested to be due to systematic differences in the two reanalysis systems and need to be kept in mind when these state estimates are used to evaluate climate models. Therefore, it is important to account for the uncertainty associated with each relationship when analyzing state estimates and further employing them to evaluate climate models.

Also, I remind readers that Applegate et al. (2015) uses Greenland surface temperature increase (see the third image) rather than GMSTA. Thus, a 12C temperature increase over Greenland would correspond to something like a 6C GMSTA (assuming a Greenland amplification factor of two, see the second image).  Thefourth image is for RCP 8.5 which indicates that 6C GMSTA is reached circa 2200 (assuming ECS ~ 3C); however, per the Wolf Pack's UKESM1-0-LL for SSP8.5, 6C GMSTA is reached about 2080.  Noting that Applegate et al. (2015) take zero time to be 1950, this implies that 2080 is at 130 years on the third image.

 
Caption for the third image: "Supplemental Materials Figure 1 Modeled ice volume responses to selected Greenland temperature anomalies; shown in the upper right-hand corner of each panel, using the three-dimensional ice sheet model SICOPOLIS (Greve 1997; Greve et al. 2011; sicopolis.greveweb.net). Difference among curves in each panel reflect parametric uncertainty (Applegate et al. 2012). The time axes are truncated to better show the structure of ice volume response to temperature change; all model integrations were run out for 100,000 yr for deltaTgrl < 2 K, and 60,000 yr for deltaTgrl > 2 K.  Open circles indicate the point on each curve corresponding to the e-folding time tau (Fig. 1), diagnosed as the time when individual curves reach Vo-deltaV(1-e-1) (see Eqn. 2). In general, the V(t) curves agree with the conceptual model shown in Figure 1a. m sle, meters of sea level equivalent."

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This reply provides random examples of recent research that could be used to support/calibrate the probabilities of occurrence for various portions of the MCDS-BN.

With regard to FT-32, Sallee et al. (2021) indicates that the upper ocean is becoming stratified six times faster than consensus climate science previously assumed.  If nothing else the high speed of this stratification will increase climate sensitivity via the SST pattern effect:

Sallée, JB., Pellichero, V., Akhoudas, C. et al. Summertime increases in upper-ocean stratification and mixed-layer depth. Nature 591, 592–598 (2021). https://doi.org/10.1038/s41586-021-03303-x

https://www.nature.com/articles/s41586-021-03303-x

Abstract
The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchange between the atmosphere and the oceanic interior. The mixed layer also shapes marine ecosystems by hosting most of the ocean’s primary production and providing the conduit for oxygenation of deep oceanic layers. Despite these important climatic and life-supporting roles, possible changes in the mixed layer during an era of global climate change remain uncertain. Here we use oceanographic observations to show that from 1970 to 2018 the density contrast across the base of the mixed layer increased and that the mixed layer itself became deeper. Using a physically based definition of upper-ocean stability that follows different dynamical regimes across the global ocean, we find that the summertime density contrast increased by 8.9 ± 2.7 per cent per decade (10−6–10−5 per second squared per decade, depending on region), more than six times greater than previous estimates. Whereas prior work has suggested that a thinner mixed layer should accompany a more stratified upper ocean, we find instead that the summertime mixed layer deepened by 2.9 ± 0.5 per cent per decade, or several metres per decade (typically 5–10 metres per decade, depending on region). A detailed mechanistic interpretation is challenging, but the concurrent stratification and deepening of the mixed layer are related to an increase in stability associated with surface warming and high-latitude surface freshening, accompanied by a wind-driven intensification of upper-ocean turbulence. Our findings are based on a complex dataset with incomplete coverage of a vast area. Although our results are robust within a wide range of sensitivity analyses, important uncertainties remain, such as those related to sparse coverage in the early years of the 1970–2018 period. Nonetheless, our work calls for reconsideration of the drivers of ongoing shifts in marine primary production, and reveals stark changes in the world’s upper ocean over the past five decades.
See also:

Title: "Global Warming Is 'Fundamentally' Changing The Structure of Our World's Oceans"

https://www.sciencealert.com/fundamental-changes-to-our-oceans-are-occurring-much-faster-than-we-thought

Climate change has disrupted ocean mixing, a process that helps store away most of the world's excess heat and a significant proportion of CO2.
Water on the surface is warmer – and therefore less dense – than the water below, a contrast that is intensified by climate change.
Global warming is also causing massive amounts of fresh water to flush into the seas from melting ice sheets and glaciers, lowering the salinity of the upper layer and further reducing its density.
This increasing contrast between the density of the ocean layers makes mixing harder, so oxygen, heat and carbon are all less able to penetrate to the deep seas.
"Similar to a layer of water on top of oil, the surface waters in contact with the atmosphere mix less efficiently with the underlying ocean," said lead author Jean-Baptiste Sallee of Sorbonne University and France's CNRS national scientific research center.
He said while scientists were aware that this process was under way, "we here show that this change has occurred at a rate much quicker than previously thought: more than six times quicker."
…
They said increased rainfall and melting of the Greenland ice sheet have increased the freshwater in the upper ocean, disrupting the normal cycle that carries warm, salty surface water northwards from the equator and sends low-salinity deep water back southwards."


With regard to FT1 & FT36, Mackie et al. (2021) shows that increasing tropical SSTs contribute to increasing values of effective ECS, which supports both the concept that ECSeff is already high and that if freshwater flux events slow the MOC and thus increase tropical SSTs that ECSeff would increase still further.

Mackie, A., Helen E. Brindley and Paul I. Palmer (22 March 2021), "Contrasting observed atmospheric responses to tropical SST warming patterns", Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2020JD033564

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JD033564?af=R

Abstract
Equilibrium climate sensitivity (ECS) is a theoretical concept which describes the change in global mean surface temperature that results from a sustained doubling of atmospheric CO2. Current ECS estimates range from ∼1.8–5.6K, reflecting uncertainties in climate feedbacks. The sensitivity of the lower (1000‐700 hPa) and upper (500‐200 hPa) troposphere to changes in spatial patterns of tropical sea surface temperature (SST) have been proposed by recent model studies as key feedbacks controlling climate sensitivity. We examine empirical evidence for these proposed mechanisms using 14 years of satellite data. We examine the response of temperature and humidity profiles, clouds and top‐of‐the‐atmosphere (TOA) radiation to relative warming in tropical ocean regions when there is either strong convection or subsidence. We find warmer SSTs in regions of strong subsidence are coincident with a decrease in lower tropospheric stability (‐0.9±0.4 KK−1) and low cloud cover (∼‐6 %K−1). This leads to a warming associated with the weakening in the shortwave cooling effect of clouds (4.2±1.9 Wm−2K−1), broadly consistent with model calculations. In contrast, warmer SSTs in regions of strong convection are coincident with an increase in upper tropospheric humidity (3.2±1.5 %K−1). In this scenario, the dominant effect is the enhancement of the warming longwave cloud radiative effect (3.8±3.0 Wm−2K−1 ) from an increase in high cloud cover (∼7 %K−1), though changes in the net (longwave and shortwave) effect are not statistically significant (p < 0.003). Our observational evidence supports the existence of mechanisms linking contrasting atmospheric responses to patterns in SST, mechanisms which have been linked to climate sensitivity.

Plain Language Summary
Estimates of how sensitive the Earth's climate is to changes in CO2 vary between climate models. These models are necessary to explore climate projections, but we need to demonstrate that they can accurately describe the real climate system. Recent model studies hypothesize that the location of surface ocean warming may be key to understanding the atmospheric component of climate sensitivity. We examine observational evidence of the extent to which local tropical ocean warming is able to propagate upwards through the atmosphere. We show that the atmospheric response and associated feedbacks are different in contrasting regions. Future patterns in ocean warming may play a key role in determining future climate.

With regard to FT1, Bourdin et al. (2021) concludes that:

"Combining our findings with relative humidity trends in reanalysis data shows a tendency toward Earth becoming more sensitive to forcing over time."

Thus, projected changes in atmospheric relative humidity is yet one more feedback mechanism by which climate sensitivity is likely to increase by in coming decades.

Bourdin, S., L. Kluft and B. Stevens (06 April 2021), "Dependence of Climate Sensitivity on the Given Distribution of Relative Humidity", Geophysical Research Letters,
https://doi.org/10.1029/2021GL092462

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL092462?af=R

Abstract
We study how the vertical distribution of relative humidity (RH) affects climate sensitivity, even if it remains unchanged with warming. Using a radiative‐convective equilibrium model, we show that the climate sensitivity depends on the shape of a fixed vertical distribution of humidity, tending to be higher for atmospheres with higher humidity. We interpret these effects in terms of the effective emission height of water vapor. Differences in the vertical distribution of RH are shown to explain a large part of the 10% to 30% differences in clear‐sky sensitivity seen in climate and storm‐resolving models. The results imply that convective aggregation reduces climate sensitivity, even when the degree of aggregation does not change with warming. Combining our findings with relative humidity trends in reanalysis data shows a tendency toward Earth becoming more sensitive to forcing over time. These trends and their height variation merit further study.

Plain Language Summary
Equilibrium Climate Sensitivity is the change in surface temperature in response to a doubling of atmospheric CO2. We study how the assumed vertical distribution of relative humidity affects this sensitivity. Theoretical considerations show that the more moist an atmosphere is, the more it warms as a response to an increase in CO2. Adding water vapor to the lower troposphere has the counter effect, lowering the sensitivity. We emphasize the importance of climate simulations taking humidity into account, as it is largely responsible for the difference in projections among models without clouds. We note surprising trends in humidity – with substantial drying of the lower troposphere over the ocean – in the last four decades as reported by two reanalyses of meteorological observations. Subject to the accuracy of these reconstructions, there appears to be a change with less moistening than expected, but with moistening/drying profiles which will condition Earth to become more sensitive to forcing over time. We stress the need for a study of observations to more critically evaluate these trends, and know better what models should aim for.


With regard to FT27, Finney et al. (2021) finds that lightening strikes in the Arctic could double by 2100; which, would result in an increase in associated wildfires that would accelerate the thawing of the Arctic permafrost & and increase in the associated GHG emissions.

Finney, D.L. Lightning threatens permafrost. Nat. Clim. Chang. (2021). https://doi.org/10.1038/s41558-021-01016-7

https://www.nature.com/articles/s41558-021-01016-7

Abstract: "Thawing Arctic permafrost, and release of its stored carbon, is a known amplifier of global warming. Now research suggests an increase in Arctic lightning could speed up the permafrost’s demise."

With regard to calibrating FT1, Proud & Bachmeier (2021) discuss the nature of a tropical deep convective atmospheric event, & I note that if such events become more frequent this could lead to more high altitude cloud, and less low altitude cloud, formation.

Proud, S.R. and Scott Bachmeier (22 March 2021), "Record‐Low Cloud Temperatures Associated With a Tropical Deep Convective Event", Geophysical Research Letters, https://doi.org/10.1029/2020GL092261

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL092261

Abstract
Earth‐orbiting satellites have long been used to examine meteorological processes. In the context of severe weather, brightness temperatures (BTs) at infrared wavelengths allow the determination of convective cloud properties. The anvils of cumulonimbus clouds, for example, typically produce BTs close to the tropopause temperature. Particularly severe storms generate overshoots that penetrate the stratosphere and are cooler than the anvil. In this study, we describe clustered storm overshoots in the tropical West Pacific on December 29, 2018 that resulted in the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard NOAA‐20 measuring a temperature of 161.96K (−111.2°C), which is, to our knowledge, the coldest on record. We describe the local meteorological conditions, examine the VIIRS overpass that produced the cold temperature, compare VIIRS with other sensors that observed the region and, finally, analyze the historical context provided by two other satellite instruments to show that such cold temperatures may be becoming more common.

Plain Language Summary
Satellites orbiting the Earth often carry infrared sensors that measure the temperature of the Earth and its atmosphere. When viewing a thunderstorm, tropical cyclone, or other form of severe convective weather these sensors will record a very cold temperature, as the clouds that form these storms are typically at high altitude and are hence well below freezing. In this study, we examine a case in which one sensor, the Visible Infrared Imaging Radiometer Suite, measured an extremely cold temperature that is, to our knowledge, the coldest recorded by satellite. We discuss this measurement, placing it in the historical context, analyzing the storm that produced the temperature and discussing how noise and sensor calibration may affect the measurement.


With regard to calibrating FT1, Gora et al. (2021) discuss the implications of size-dependent tree mortality in tropical forest carbon dynamics.

Gora, E.M. and Esquivel-Muelbert, A. 2021. Implications of size-dependent tree mortality for tropical forest carbon dynamics. Nature Plants. doi: 10.1038/s41477-021-00879-0

https://www.nature.com/articles/s41477-021-00879-0

Abstract: "Tropical forests are mitigating the ongoing climate crisis by absorbing more atmospheric carbon than they emit. However, widespread increases in tree mortality rates are decreasing the ability of tropical forests to assimilate and store carbon. A relatively small number of large trees dominate the contributions of these forests to the global carbon budget, yet we know remarkably little about how these large trees die. Here, we propose a cohesive and empirically informed framework for understanding and investigating size-dependent drivers of tree mortality. This theory-based framework enables us to posit that abiotic drivers of tree mortality—particularly drought, wind and lightning—regulate tropical forest carbon cycling via their disproportionate effects on large trees. As global change is predicted to increase the pressure from abiotic drivers, the associated deaths of large trees could rapidly and lastingly reduce tropical forest biomass stocks. Focused investigations of large tree death are needed to understand how shifting drivers of mortality are restructuring carbon cycling in tropical forests."

See also:

Title: "How Will the Biggest Tropical Trees Respond to Climate Change?"

https://www.webwire.com/ViewPressRel.asp?aId=272071

Extract: "The biggest trees store half of the carbon in mature tropical forests, but they could be at risk of death as a result of climate change—releasing massive amounts of carbon back into the atmosphere.

Evan Gora, STRI Tupper postdoctoral fellow, studies the role of lightning in tropical forests. Adriane Esquivel-Muelbert, lecturer at the University of Birmingham, studies the effects of climate change in the Amazon. The two teamed up to find out what kills big tropical trees. But as they sleuthed through hundreds of papers, they discovered that nearly nothing is known about the biggest trees and how they die because they are extremely rare in field surveys.
….
Hoping to better understand what is happening to big trees, Gora and Esquivel-Muelbert identified three glaring knowledge gaps. First, almost nothing is known about disease, insects and other biological causes of death in big trees. Second, because big trees are often left out of analyses, the relationship between cause of death and size is not clear. And, finally, almost all of the detailed studies of big tropical trees are from a few locations like Manaus in Brazil and Barro Colorado Island in Panama.

To understand how big trees die, there is a trade-off between putting effort into measuring large numbers of trees and measuring them often enough to identify the cause of death. Gora and Esquivel-Muelbert agree that a combination of drone technology and satellite views of the forest will help to find out how these big trees die, but this approach will only work if it is combined with intense, standardized, on-the-ground observations, such as those used by the Smithsonian’s international ForestGEO network of study

[AbruptSLR]

Paleo Calibration of MCDS Probabilities

The paleo record contains numerous examples of abrupt climate change including those associated with freshwater flux events (including MICI events) that generally are associated with domino chains of effects that can lead to transitions in climate state (see the attached image) and that current state of the art climate models cannot replicate. 

Furthermore, I want to emphasize that I do not believe that paleoclimate states represent models that can be used represent stand-ins, or simulations, of where Earth Systems will head in coming decades.  For example, the second, and third, images compare aspects of the Eemian ,and the Holocene, interglacials, and while lessons can be learned from such a comparison, there is no way that the Eemian interglacial simulates our current situation which includes many consideration not present during the Eemian including:
a) An Antarctic ozone hole that is contributing to the dynamic upwelling of warm CDW,
b) GHG concentrations are currently well above any concentrations reached during the Eemian, and which are increasing at a rate several hundred times faster that during conditions that triggered the PETM (see the fourth image) and,
c) Anthropogenic radiative forcing has been on-going since before 1750 and thus many slow response climate feedback mechanisms (with response times on the order of 250-yrs) are already being activated.

Thus, this section of replies is intended to provide background information that could be used to either calibrate future climate models (like CMIP7) and/or to help calibrate expert opinion of the chances that a MCDS-BN scenario might occur in the next hundred years (or so).
 
Caption for the first image: "Representation of Potential Pathways of Potential Climate Equilibrium State vs Global Warming with Time. Illustrated Pathways Are for: A Business as Usual Scenario; The Original (pre-2011) Kyoto Protocol Scenario; and A Potential "Survival Pathway", from Wasdell 2010"


As it is possible that we are collectively heading towards a Super-Interglacial period, beginning in coming decades, I provide the linked collection of references related to Super-Interglacial periods largely based on paleo-findings from Lake El'gygytgyn.

In this regard, I note that MIS 11c was a Super-Interglacial period while MIS 5e was not, and that Wet et al. (2016) states: "Based on brGDGT temperatures from Lake El'gygytgyn (this study and unpublished results), warming in the western Arctic during MIS 31 was matched only by MIS 11 during the Pleistocene."

Also, Wennrich et al. (September 2016) states: "Periods of exceptional warming in the Pleistocene record of Lake El'gygytgyn with dense boreal forests around and peaks of primary production in the lake are assigned to so-called “super-interglacial” periods. The occurrence of these super-interglacials well corresponds to collapses of the West Antarctic Ice Sheet (WAIS) recorded in ice-free periods in the ANDRILL core, which suggests strong intrahemispheric teleconnections presumably driven by changes in the thermocline ocean circulation."

Also, Brigham-Grette et al. (2018) states: "While WAIS may have been gone in MIS 5e, this was not a super interglacial by Arctic standards. This suggests thresholds of sensitivity in the earth system."

Also, Brigham-Grette et al. (2019) states: "When the Bering Strait is closed, models suggest more ocean heat transported into the Arctic, improving the fit between extremely warm Pliocene marine and terrestrial records. Yet the depth of Bering Strait over time is unknown due to glacial isostatic adjustments, dynamic topography and regional tectonics, the last being generally neglected by the paleoclimate community."

Gregory A.de Wet, Isla S. Castañeda, Robert M. DeConto and Julie Brigham-Grette (15 February 2016), "A high-resolution mid-Pleistocene temperature record from Arctic Lake El'gygytgyn: a 50 kyr super interglacial from MIS 33 to MIS 31?", Earth and Planetary Science Letters, Volume 436,  Pages 56-63, https://doi.org/10.1016/j.epsl.2015.12.021

https://www.sciencedirect.com/science/article/abs/pii/S0012821X15007840

Abstract
Previous periods of extreme warmth in Earth's history are of great interest in light of current and predicted anthropogenic warming. Numerous so called “super interglacial” intervals, with summer temperatures significantly warmer than today, have been identified in the 3.6 million year (Ma) sediment record from Lake El'gygytgyn, northeast Russia. To date, however, a high-resolution paleotemperature reconstruction from any of these super interglacials is lacking. Here we present a paleotemperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) from Marine Isotope Stages (MIS) 35 to MIS 29, including super interglacial MIS 31. To investigate this period in detail, samples were analyzed with an unprecedented average sample resolution of 500 yrs from MIS 33 to MIS 30. Our results suggest the entire period currently defined as MIS 33–31 (∼1114–1062 kyr BP) was characterized by generally warm and highly variable conditions at the lake, at times out of phase with Northern Hemisphere summer insolation, and that cold “glacial” conditions during MIS 32 lasted only a few thousand years. Close similarities are seen with coeval records from high southern latitudes, supporting the suggestion that the interval from MIS 33 to MIS 31 was an exceptionally long interglacial (Teitler et al., 2015). Based on brGDGT temperatures from Lake El'gygytgyn (this study and unpublished results), warming in the western Arctic during MIS 31 was matched only by MIS 11 during the Pleistocene.

&

Volker Wennrich et al. (September 2016), "Impact processes, permafrost dynamics, and climate and environmental variability in the terrestrial Arctic as inferred from the unique 3.6 Myr record of Lake El'gygytgyn, Far East Russia – A review", Quaternary Science Reviews, Volume 147, 1, Pages 221-244, https://doi.org/10.1016/j.quascirev.2016.03.019

https://www.sciencedirect.com/science/article/abs/pii/S0277379116300877

Abstract
Lake El'gygytgyn in Far East Russia is a 3.6 Myr old impact crater lake. Located in an area that has never been affected by Cenozoic glaciations nor desiccation, the unique sediment record of the lake represents the longest continuous sediment archive of the terrestrial Arctic. The surrounding crater is the only impact structure on Earth developed in mostly acid volcanic rocks. Recent studies on the impactite, permafrost, and sediment sequences recovered within the framework of the ICDP “El'gygytgyn Drilling Project” and multiple pre-site surveys yielded new insight into the bedrock origin and cratering processes as well as permafrost dynamics and the climate and environmental history of the terrestrial Arctic back to the mid-Pliocene.
Results from the impact rock section recovered during the deep drilling clearly confirm the impact genesis of the El'gygytgyn crater, but indicate an only very reduced fallback impactite sequence without larger coherent melt bodies. Isotope and element data of impact melt samples indicate a F-type asteroid of mixed composition or an ordinary chondrite as the likely impactor. The impact event caused a long-lasting hydrothermal activity in the crater that is assumed to have persisted for c. 300 kyr.

Geochemical and microbial analyses of the permafrost core indicate a subaquatic formation of the lower part during lake-level highstand, but a subaerial genesis of the upper part after a lake-level drop after the Allerød. The isotope signal and ion compositions of ground ice is overprinted by several thaw-freeze cycles due to variations in the talik underneath the lake. Modeling results suggest a modern permafrost thickness in the crater of c. 340 m, and further confirm a pervasive character of the talik below Lake El'gygytgyn.

The lake sediment sequences shed new leight into the Pliocene and Pleistocene climate and environmental evolution of the Arctic. During the mid-Pliocene, significantly warmer and wetter climatic conditions in western Beringia than today enabled dense boreal forests to grow around Lake El'gygytgyn and, in combination with a higher nutrient flux into the lake, promoted primary production. The exceptional warmth during the mid-Pliocene is in accordance with other marine and terrestrial records from the Arctic and indicates a period of enhanced “Arctic amplification”. The favourable conditions during the mid-Pliocene were repeatedly interrupted by climate deteriorations, e.g., during Marine Isotope Stage (MIS) M2, when pollen data and sediment proxies indicate a major cooling and the onset of local permafrost around the lake.

A gradual vegetation change after c. 3.0 Ma points to the onset of a long-term cooling trend during the Late Pliocene that culminated in major temperature drops, first during MIS G6, and later during MIS 104. These cold events coincide with the onset of an intensified Northern Hemisphere (NH) glaciation and the largest extent of the Cordilleran Ice Sheet, respectively.
After the Pliocene/Pleistocene transition, local vegetation and primary production in Lake El'gygtygyn experienced a major change from relatively uniform conditions to a high-amplitude glacial-to-interglacial cyclicity that fluctuated on a dominant 41 kyr obliquity band, but changed to a 100 kyr eccentricity dominance during the Middle Pleistocene transition (MPT) at c. 1.2–0.6 Ma. Periods of exceptional warming in the Pleistocene record of Lake El'gygytgyn with dense boreal forests around and peaks of primary production in the lake are assigned to so-called “super-interglacial” periods. The occurrence of these super-interglacials well corresponds to collapses of the West Antarctic Ice Sheet (WAIS) recorded in ice-free periods in the ANDRILL core, which suggests strong intrahemispheric teleconnections presumably driven by changes in the thermocline ocean circulation.

&

Title: "PP22A-04: Interglacial Intensity with Orbital Pre-conditioning Links Polar Ice Sheet Sensitivity to Warming (Invited)" by Julie Brigham-Grette, Rajarshi Roychowdhury, Robert M Deconto, Isla S. Castañeda and Helen Habicht (2018)

https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/354878

Abstract: "Paleoclimate records of climate change from northeast Arctic Russia (Lake El’gygytgyn) and Antarctica over the past 3-4 Million years provide a new opportunity for understanding the sensitivity of the polar regions to forcings involving natural greenhouse gas variability, changing orbital configurations and associated feedbacks. While geography and transient atmospheric CO2 in excess of preindustrial levels can explain most of the Pliocene warming, the occurrence of Arctic super interglacials without clear pacing documented over the past 2.78 Myrs requires additional explanation. Not all interglacials are alike and the strength of the interglacial might be function of the proxy used for its identification. We hypothesize that numerous super interglacials in the Arctic correspond with extremes in insolation leading to the demise of the WAIS. During MIS 11c, 31, 49, 55, 77, 87, 91, and 93 Milankovitch forcing coinciding with extreme lows in eccentricity and high obliquity likely preconditioned the Earth system to synchronize summer melt intensity and duration to produce bipolar warming. The challenge has been to understand how these high latitude sites are linked with changes in ocean circulation, gateway changes, and heat transport. This preconditioned warming likely led to the demise of the WAIS in the Southern Hemisphere and super interglacials in the Arctic Northern Hemisphere. Diatomite layers in the ANDRILL AND-1B record coincide reasonably well super interglacials (Melles et al, 2012) but unconformities in the AND-1B cores prevent direct correlation. While WAIS may have been gone in MIS 5e, this was not a super interglacial by Arctic standards. This suggests thresholds of sensitivity in the earth system."

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Title: “Beringia as the Arctic Pacific Gateway – Interglacial intensity since the Pliocene and Why its Sea Level History just got Thorny.” by Julie Brigham-Grette and Beth Caissie (2019)

https://sites.uw.edu/amqua50/julie-brigham-grette/

Abstract: The paleoclimate history of Beringia, the largest contiguous land area not covered by continental scale glaciation, provides remarkable archives of the late Cenozoic history of the Arctic Borderlands. Lake El’gygytgyn in western Beringia contains the most continuous record of glacial/interglacial change of the past 3.6 Ma, demonstrating unprecedented evidence for super interglacials and the evolving pace of glacial/interglacial change. Similarly, the International Ocean Discovery Program’s 5 Myr long records in the Bering Sea document the submergence and emergence history of the Pacific Arctic marine gateway revealing how relatively fresher North Pacific waters influenced Atlantic Meridional Overturning Circulation and changes in the production of North Pacific intermediate water production. When the Bering Strait is closed, models suggest more ocean heat transported into the Arctic, improving the fit between extremely warm Pliocene marine and terrestrial records. Yet the depth of Bering Strait over time is unknown due to glacial isostatic adjustments, dynamic topography and regional tectonics, the last being generally neglected by the paleoclimate community. Indeed, paleoseismic evidence shows the Bering Sea has been rotating clockwise for the past 6 Myrs with the active Kobuk fault cutting directly across Bering Strait. Integrating both the land and sea records is now possible.

[AbruptSLR]

Rasmussen et al. (2020) provides paleo-data that indicates that significant portions of the Svalbard-Barents Sea ice sheet (composed of marine glaciers) mostly collapsed virtually instantly (under the right conditions); which, could only have happened due MICI-type of collapses.  This research increases the risk that many of the marine glaciers in the WAIS could undergo similar MICI-types of collapse given continued global warming and increased upwelling of relatively warm mCWD (modified Circumpolar Deepwater), in coming decades.

Rasmussen, T.L. et al, Climate and ocean forcing of ice-sheet dynamics along the Svalbard-Barents Sea ice sheet during the deglaciation ∼20,000–10,000 years BP, Quaternary Science Advances (2020). DOI: 10.1016/j.qsa.2020.100019

https://www.sciencedirect.com/science/article/pii/S2666033420300198?via%3Dihub

Abstract
The last deglaciation, 20,000–10,000 years ago, was a period of global warming and rapidly shrinking ice sheets. It was also climatically unstable and retreats were interrupted by re-advances. Retreat rates and timing relative to climatic changes have therefore been difficult to establish. We here study a suite of 12 marine sediment cores from Storfjorden and Storfjorden Trough, Svalbard. The purpose is to reconstruct retreat patterns and retreat rates of a high northern latitude marine-based ice stream from the Svalbard-Barents Sea Ice Sheet in relation to paleoceanographic and paleoclimatic changes. The study is based on abundance and composition of planktic and benthic foraminiferal assemblages, ice rafted debris (IRD), lithology, and 70 AMS-14C dates. For core 460, we also calculate sea surface and bottom water temperatures and bottom water salinity. The results show that retreat rates of the ice shelf and ice streams of Storfjorden Trough/Storfjorden (‘Storfjorden Ice Stream’) closely followed the deglacial atmospheric and ocean temperature changes. During the start of the Bølling interstadial retreat rates in Storfjorden Trough probably exceeded 2.5 km/year and more than 10,000 km2 of ice disappeared almost instantaneously. A similarly rapid retreat occurred at the start of the Holocene interglacial, when 4500 km2 of ice broke up. Maximum rates during the deglaciation match the fastest modern rates from Antarctica and Greenland. Correlation of data show that the ice streams in several fjords from northern Norway retreated simultaneously with the Storfjorden Ice Stream, indicating that temperature was the most important forcing factor of the Svalbard-Barents Sea Ice Sheet during the deglaciation.

Extract: "The retreat was not uniform over time. About haft of the ice disappeared in two almost instantaneous collapses. The first occurred at the beginning of the Bølling interstadial, when more than 11,000 km2 of ice sheet from Storfjorden Trough and the outer fjord disintegrated. The second collapse took place at the beginning of the early Holocene and comprised about 4500 km2 of ice from the inner fjord system. During both break-ups, the ice front retreated about 100 km. Instant disintegration is the most likely scenario for the early Bølling and early Holocene collapses."

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Furthermore, while Antarctic icebergs behaved much differently during the Pleistocene glacials than they do today; nonetheless, Starr et al. (2021) demonstrates that icebergs from MICI events (implied by the rafted ice debris cited in the study) leading to the glacials can have a profound impact on the AMOC and consequently on climate state.  The associated linked article indicates that the authors believe that modern MICI events would have a different, but still significant, impact on both the AMOC and on climate state, and consequently the encourage future ESMs to evaluate the potential future impacts of such Antarctic iceberg armadas on future climate states (as Hansen et al. 2016 did).

Starr, A. et al. (2021), "Antarctic icebergs reorganize ocean circulation during Pleistocene glacials", Nature, 589, 236–241, DOI: 10.1038/s41586-020-03094-7

https://www.nature.com/articles/s41586-020-03094-7

Abstract
The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.

See also:

Title: "Melting icebergs key to sequence of an ice age, scientists find"

https://phys.org/news/2021-01-icebergs-key-sequence-ice-age.html

Extract: "However, due to the increased global temperatures resulting from anthropogenic CO2 emissions, the researchers suggest the natural rhythm of ice age cycles may be disrupted as the Southern Ocean will likely become too warm for Antarctic icebergs to travel far enough to trigger the changes in ocean circulation required for an ice age to develop.
…
"Likewise as we observe an increase in the mass loss from the Antarctic continent and iceberg activity in the Southern Ocean, resulting from warming associated with current human greenhouse-gas emissions, our study emphasises the importance of understanding iceberg trajectories and melt patterns in developing the most robust predictions of their future impact on ocean circulation and climate," he said."

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Furthermore, many marine terminating glaciers will be subject to MISI-type of behavior in the coming decades, and the linked reference discussions how differences in bed-conditions will change the behavior of such glaciers as they retreat:

Greenwood, S.L. et al. (13 Jan 2021), "Exceptions to bed-controlled ice sheet flow and retreat from glaciated continental margins worldwide", Science Advances, Vol. 7, no. 3, eabb6291, DOI: 10.1126/sciadv.abb6291

https://advances.sciencemag.org/content/7/3/eabb6291

Abstract
Projections of ice sheet behavior hinge on how ice flow velocity evolves and the extent to which marine-based grounding lines are stable. Ice flow and grounding line retreat are variably governed by the coupling between the ice and underlying terrain. We ask to what degree catchment-scale bed characteristics determine ice flow and retreat, drawing on paleo-ice sheet landform imprints from 99 sites on continental shelves worldwide. We find that topographic setting has broadly steered ice flow and that the bed slope favors particular styles of retreat. However, we find exceptions to accepted “rules” of behavior: Regional topographic highs are not always an impediment to fast ice flow, retreat may proceed in a controlled, steady manner on reverse slopes and, unexpectedly, the occurrence of ice streaming is not favored on a particular geological substrate. Furthermore, once grounding line retreat is under way, readvance is rarely observed regardless of regional bed characteristics.

Furthermore, Menounos et al (2017) studies the paleo decay of the Cordilleran ice sheet and finds that it lost most of its ice mass earlier than consensus science previously thought, and it lost much of its ice mass over a relatively short period.  Personally, I am concerned about the impact of rainfall at increasingly high latitudes (with warming) on both the Greenland Ice Sheet, on Arctic permafrost, and on the WAIS:

B. Menounos et al (10 Nov 2017), "Cordilleran Ice Sheet mass loss preceded climate reversals near the Pleistocene Termination", Science, Vol. 358, Issue 6364, pp. 781-784, DOI: 10.1126/science.aan3001

http://science.sciencemag.org/content/358/6364/781

Abstract: "The Cordilleran Ice Sheet (CIS) once covered an area comparable to that of Greenland. Previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost Canada as late as 12.5 thousand years ago (ka). New data indicate that substantial areas throughout westernmost Canada were ice free prior to 12.5 ka and some as early as 14.0 ka, with implications for climate dynamics and the timing of meltwater discharge to the Pacific and Arctic oceans. Early Bølling-Allerød warmth halved the mass of the CIS in as little as 500 years, causing 2.5 to 3.0 meters of sea-level rise. Dozens of cirque and valley glaciers, along with the southern margin of the CIS, advanced into recently deglaciated regions during the Bølling-Allerød and Younger Dryas."

Disappearance of an ice sheet

The Cordilleran Ice Sheet is thought to have covered westernmost Canada until about 13,000 years ago, even though the warming and sea level rise of the last deglaciation had begun more than a thousand years earlier. This out-of-phase behavior has puzzled glaciologists because it is not clear what mechanisms could account for it. Menounos et al. report measurements of the ages of cirque and valley glaciers that show that much of western Canada was ice-free as early as 14,000 years ago—a finding that better agrees with the record of global ice volume (see the Perspective by Marcott and Shakun). Previous reconstructions seem not to have adequately reflected the complexity of ice sheet decay.

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Furthermore, Tian et al (2020) discusses Deglacial-Holocene Svalbard paleoceanography and evidence of meltwater pulse 1B (see also the attached images 1 thru 3); which could be used to help calibrate CMIP7 models w.r.t. the impacts of freshwater flux events.

Tian, S.Y., Moriaki Yasuhara, Yuanyuan Hong, Huai-Hsuan M. Huang, Hokuto Iwatani, Wing-Tung Ruby Chiu, Briony Mamo, Hisayo Okahashi, Tine L. Rasmussen (2020) "Deglacial–Holocene Svalbard paleoceanography and evidence of meltwater pulse 1B", Quaternary Science Reviews, 233: 106237 DOI: 10.1016/j.quascirev.2020.106237.

https://www.sciencedirect.com/science/article/abs/pii/S0277379119309485?via%3Dihub

Abstract: "Better understanding of deglacial meltwater pulses (MWPs) is imperative for future predictions of human-induced warming and abrupt sea-level change because of their potential for catastrophic damage. However, our knowledge of the second largest meltwater pulse MWP-1B that occurred shortly after the start of the Holocene interglacial remains very limited. Here, we studied fossil ostracods as paleoenvironmental indicators of water depth, salinity, and temperature in two marine sediment cores from Storfjorden, Svalbard margin (the Arctic Ocean), to investigate near-field (i.e. areas located beneath continental ice sheets at the Last Glacial Maximum) evidence of MWP-1B. The depositional environment changed from a cold bathyal environment to a warmer bathyal environment at ∼11,300 yr BP indicating incursion of warm Atlantic water into the Nordic seas, and eventually to a cold neritic environment by ∼11,000 yr BP because of melting of the Svalbard-Barents Sea ice sheet and resultant isostatic rebound. This process corresponds to rapid relative sea-level fall of 40–80 m of MWP-1B from ∼11,300 to 11,000 yr BP."
 

[AbruptSLR]

Boers et al. (2018) build on earlier research on Dansgaard–Oeschger (D-O) cycles [including that from Bassis et al. (2017) for Henrich events] to propose a conceptual model emphasizing ocean circulation (AMOC), ice shelf and sea ice and bipolar seesaw interactions to better explain D-O cycles.  Furthermore, many of the conceptual ice-ocean interactions are relevant to the current instability of the WAIS (especially if the AMOC is abruptly modified by a reversal of the Beaufort Gyre in coming decades).

Boers, N. et al. (November 20, 2018), "Ocean circulation, ice shelf, and sea ice interactions explain Dansgaard–Oeschger cycles", PNAS, 115, (47), E11005-E11014; https://doi.org/10.1073/pnas.1802573115

https://www.pnas.org/content/115/47/E11005

Significance
Paleoclimatic proxy records from Greenland ice cores show that the last glacial interval was punctuated by abrupt climatic transitions called Dansgaard–Oeschger (DO) events. These events are characterized by temperature increases over Greenland of up to 15°C within a few decades. The cause of these transitions and their out-of-phase relationship with corresponding records from Antarctica remains unclear. Based on earlier hypotheses, we propose a model focusing on interactions between ice shelves, sea ice, and ocean currents to explain DO events in Greenland and their Antarctic counterparts. Our model reproduces the main features of the observations. Our study provides a potential explanation of DO events and could help assess more accurately the risk of abrupt climatic transitions in the future.

Abstract
The last glacial interval experienced abrupt climatic changes called Dansgaard–Oeschger (DO) events. These events manifest themselves as rapid increases followed by slow decreases of oxygen isotope ratios in Greenland ice core records. Despite promising advances, a comprehensive theory of the DO cycles, with their repeated ups and downs of isotope ratios, is still lacking. Here, based on earlier hypotheses, we introduce a dynamical model that explains the DO variability by rapid retreat and slow regrowth of thick ice shelves and thin sea ice in conjunction with changing subsurface water temperatures due to insulation by the ice cover. Our model successfully reproduces observed features of the records, such as the sawtooth shape of the DO cycles, waiting times between DO events across the last glacial, and the shifted antiphase relationship between Greenland and Antarctic ice cores. Our results show that these features can be obtained via internal feedbacks alone. Warming subsurface waters could have also contributed to the triggering of Heinrich events. Our model thus offers a unified framework for explaining major features of multimillennial climate variability during glacial intervals.

Extract: "Corresponding δ18O time series obtained from Antarctic ice cores show an antiphase relationship with the temporal evolution in Greenland, with gradual increases during Greenland stadials and gradual cooling during Greenland interstadials (10–12) (compare with Fig. 1). A recent study based on a high-resolution Antarctic ice core estimates a delay of roughly 200 y between DO events in Greenland and the onset of cooling in Antarctica as well as between the return to stadial conditions in Greenland and the onset of warming in Antarctica. From the sign and magnitude of this delay, an oceanic north-to-south transmission of the climatic signal has been inferred (12).

Since the DO events are outstanding examples of abrupt and dramatic climate transitions in the past, a better understanding of the underlying mechanisms is urgently needed to better assess the risk of abrupt climatic transitions in the future.
…
Variations in marginal ice sheets, ice shelves, and sea ice cover near Greenland and other North Atlantic basin coasts, possibly in concert with AMOC changes, have also been proposed to explain the observed DO cycles (23, 25–29). Model results suggest that Nordic Sea sea ice retreat can increase winter temperatures by 10°10° C (30). Furthermore, it has been shown that freshwater pulses induced by iceberg discharges can trigger DO-type oscillations via coherence resonance (31). The latter study also provides a possible explanation for the suggested relationship between DO events and Heinrich events (32–34), which are characterized by massive iceberg discharges into the Labrador Sea. These discharges are evident as pronounced bands of ice-rafted debris in marine sediment cores (34). The Heinrich events themselves might have been triggered by warming subsurface waters in the northern North Atlantic during stadial conditions (35–38). Heinrich-type iceberg calving occurs during the cooler stadials, possibly acting as a feedback stabilizing the stadial conditions (39, 40). It should be noted here that—in addition to the Heinrich events, during which icebergs were mainly discharged into the Labrador Sea—there is empirical evidence for substantial iceberg discharges at several other locations around the northern North Atlantic (41) and in particular, into the Denmark Straight and Icelandic Sea (23, 42).
…
To our knowledge, studies focusing on sea ice or ice shelf variability to explain the DO events do not account for the antiphase coupling between Greenland and Antarctic temperatures (compare with Fig. 1 and ref. 12), and they do not account for the fact that, even in high northern latitudes, subsurface water temperatures are in phase with the temperature evolution observed in Antarctica (49). A possible explanation for these couplings is that reductions in North Atlantic subsurface water temperatures at the DO onset lead to a switch of the AMOC from its weak mode to its strong mode. Recent observational evidence of an approximately 200-y lag between Greenland and Antarctic temperatures, suggesting an oceanic north-to-south propagation of the climatic signal (12), supports this hypothesis. Note that, in such a setting, changes in AMOC strength would not be the cause but rather, a consequence of the DO events.

In this paper, we test the hypothesis that Greenland ice shelves and sea ice interact with the AMOC to produce the observed DO cycles and the shifted antiphase relationship between the two hemispheres.
…
A key ingredient of our model is that heat transported northward by the AMOC accumulates below ice shelves and sea ice and eventually, removes the ice cover. So far, we only suggested this mechanism for a hypothesized Greenland ice shelf to trigger DO events; it could, however, apply—at least during some Greenland stadials—also for the Laurentide and Fennoscandian ice sheets or the ice shelves attached to either one of them. The ice sheets themselves could have been destabilized directly (60) or by the removal of the attached ice shelves. The resulting massive iceberg discharges would then cause the pronounced bands of ice-rafted debris in marine sediments that mark the Heinrich events (32–36, 38, 41).
…
The model proposed herein reproduces the key features associated with DO cycles during the last glacial interval, including the sawtooth shape of the DO oscillations in Greenland—with its two-step cooling from interstadials to stadials—as well as the correct waiting times between the DO events across the last glacial and the shifted antiphase relationship with Antarctica. No external forcing was included in our model to trigger the DO events, implying that these oscillations can be produced by internal feedbacks alone.
…
38. Bassis JN, Petersen SV, Mac Cathles L (2017) Heinrich events triggered by ocean forcing and modulated by isostatic adjustment. Nature 542:332–334.

Caption for the first image: "Fig. 1. Variability of the last glacial interval as expressed by oxygen isotope ratios (δ18O). Blue indicates Greenland δ18O data obtained from the NGRIP (3) at a regular sampling rate of 20 y (13). Orange indicates Antarctic δ18O data from the WAIS ice core (12). As in ref. 12, the layer-counted NGRIP chronology GICC05 (14, 15) is rescaled by a factor of 1.0063, because the layer-counted WAIS divide deep ice core chronology (WD2014) (16), on which the WAIS δ18Oδ18O record is shown, is synchronized to this rescaled chronology. The δ18O values are commonly interpreted as a proxy for atmospheric temperatures at the location of the ice core, with higher values indicating warmer temperatures. The training period for our model is the interval from 59 to 23 ky b2k, which roughly corresponds to Marine Isotope Stage 3 (MIS3). DO events are indicated by vertical magenta lines, Heinrich stadials are marked by grey shading, and MISs are indicated at the top of the figure. The thin vertical dashed lines indicate time steps in intervals of 20 ky. Inset shows the geographical locations of the NGRIP and WAIS sites and a sketch of the oceanic circulation, with warmer surface flow in red and colder bottom flow in blue."

Caption for the second image: "Fig. 2. Schematic diagram of typical stadial and interstadial conditions during the last glacial interval together with the relevant observables included in the proposed model: δ18O in Greenland cores (IGIG), atmospheric temperature in Greenland (TGTG), subsurface water temperatures in the northern North Atlantic (TNAWTNAW), extent of ice cover close to Greenland (C), AMOC strength (ψ), and δ18O in Antarctic cores (IAIA); details are in the text. The blue arrows indicate the distinct atmospheric paths of δ18O from the evaporative source in the Atlantic Ocean to the ice core site in northern Greenland. Small and large fonts correspond to low and high values, respectively, of the observables. Inset shows the feedback mechanisms involved; here, solid arrows indicate a positive or enhancing influence, while dashed arrows indicate a negative or damping influence."
 

[AbruptSLR]

The linked reference (& associate linked article) provides evidence of the synchronous timing of abrupt climate changes around the globe during Dansgaard-Oeschger (DO) events during the last glacial period.  To me this clearly demonstrates that freshwater hosing events can trigger multiple abrupt changes; which were stabilized during the last glacial period, but which could abruptly trigger a transition into a new/higher climate state given our current unprecedented rate of climate change:

Corrick, E.C. et al. (21 Aug 2020), "Synchronous timing of abrupt climate changes during the last glacial period", Science, Vol. 369, Issue 6506, pp. 963-969, DOI: 10.1126/science.aay5538

https://science.sciencemag.org/content/369/6506/963.abstract

Abstract: "Abrupt climate changes during the last glacial period have been detected in a global array of palaeoclimate records, but our understanding of their absolute timing and regional synchrony is incomplete. Our compilation of 63 published, independently dated speleothem records shows that abrupt warmings in Greenland were associated with synchronous climate changes across the Asian Monsoon, South American Monsoon, and European-Mediterranean regions that occurred within decades. Together with the demonstration of bipolar synchrony in atmospheric response, this provides independent evidence of synchronous high-latitude–to-tropical coupling of climate changes during these abrupt warmings. Our results provide a globally coherent framework with which to validate model simulations of abrupt climate change and to constrain ice-core chronologies."

See also:

Title: "Abrupt global climate change events occurred synchronously during last glacial period"

https://eurekalert.org/pub_releases/2020-08/aaft-agc081720.php

Extract: "The abrupt climate warming events that occurred in Greenland during the last glacial period occurred very close in time to other rapid climate change events seen in paleoclimate records from lower latitudes, according to a new study, which reveals a near-synchronous teleconnection of climate events spanning Earth's hemispheres. The new high-resolution paleoclimate chronology, which was derived from thin layers of sedimentary cave rocks from around the world, provides a framework to improve climate change models and constrain ice-core chronologies. This is important in the context of considering future abrupt climate change around the globe. Climate records from Greenland ice cores spanning the last glacial cycle (115,000 to 11,700 years ago) reveal a series of abrupt climate fluctuations between warm and cold conditions. These oscillations, also known as Dansgaard-Oeschger (DO) events, are characterized by an abrupt transition to a period of rapid warming, which is followed by a more gradual, and then abrupt, return to a cooler climate state. The oscillations occur quasi-periodically on a centennial- to millennial-scale. Outside of the Arctic, similar abrupt climate change events during the last glacial have also been identified in a host of other paleoclimate records from far-away regions across the globe."


I note that ice-rafted debris (IRD) indicates the occurrence of past MICI occurrences, and McKay et al. (2020) discusses one effort to quantify IRD in the Wilkes Land and Ross Sea region of Antarctica.

McKay, R.M. Albot, Olga B; Dunbar, Gavin B; Lee, Jae Il; Lee, Min Kyung; Yoo, Kyu-Cheul; Kim, S; Turton, Nikita A; Levy, Richard H (2020): Ice rafted debris proxies for sediment cores RS15-LC42, RS15-LC48, IODP Site 318-U1361 and ODP Site 118-1165. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.920653

https://doi.pangaea.de/10.1594/PANGAEA.920653

Abstract: "Quantification of ice-rafted debris (IRD) abundances in deep-sea records using three independent methodologies of obtaining IRD abundances and how different approaches will affect determinations of mass accumulation rates (MARs). The three methodologies for this cross comparison of methods include: counting clasts >2 mm in x-radiograph images; the sieved weight percentage of the medium-to-coarse sand fraction (250 μm-2 mm); and volumetric estimates of the >125 μm sand fraction using Laser diffraction Particle Size Analysis (LPSA) methods to determine particle size. The data are collected from the Wilkes Land and Ross Sea region of Antarctica, using cores RS15-LC42,RS15-LC48, IODP sites 1361 and ODP site 1165."

See also:

McKay, Robert M; Albot, Olga B; Dunbar, Gavin B; Lee, Jae Il; Lee, Min Kyung; Yoo, Kyu-Cheul; Kim, Sunghan; Turton, Nikita A; Levy, Richard H (submitted): A comparison of methods for identifying and quantifying Ice Rafted Debris on the Antarctic margin. Paleoceanography and Paleoclimatology

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Zheng et al (2020) uses chaos theory to investigate the maximum likelihood climate change for global warming under the influence of greenhouse effects and Lévy fluctuations (see the three attached images).

Zheng, Y., Yang, F., Duan, J., Sun, X., Fu, L., Kurths, J. (2020): The maximum likelihood climate change for global warming under the influence of greenhouse effect and Lévy noise. - Chaos, 30, 1, 013132.
https://doi.org/10.1063/1.5129003

Abstract: "An abrupt climatic transition could be triggered by a single extreme event, and an α-stable non-Gaussian Lévy noise is regarded as a type of noise to generate such extreme events. In contrast with the classic Gaussian noise, a comprehensive approach of the most probable transition path for systems under α-stable Lévy noise is still lacking. We develop here a probabilistic framework, based on the nonlocal Fokker-Planck equation, to investigate the maximum likelihood climate change for an energy balance system under the influence of greenhouse effect and Lévy fluctuations. We find that a period of the cold climate state can be interrupted by a sharp shift to the warmer one due to larger noise jumps with low frequency. Additionally, the climate change for warming 1.5°C under an enhanced greenhouse effect generates a steplike growth process. These results provide important insights into the underlying mechanisms of abrupt climate transitions triggered by a Lévy process."

See also:

Title: "The maximum likelihood climate change for global warming under the influence of greenhouse effect and Levy noise" Zheng, Y. et al. (2019), arXiv:1909.08693v1

https://arxiv.org/pdf/1909.08693.pdf

 
Caption for the first image: "Figure 1: The maximum likelihood transition path. (A) The conditional probability density function PA(x, t) for a scalar Ornstein-Uhlenbeck process (see Supplementary Materials S2.). (B) The numerical simulation of the maximum likelihood transition path is compared with the analytical solution"

Caption for the second image: "Figure 4: The effect of -stable Levy noise intensities on the maximum likelihood transition path for bifurcation greenhouse factor. Transition from the current state 290K to the stable state 304.7K of desert heat bifurcation h = 0.49 with (A) = 0.01. (B) = 0.1. (C) = 1. (D) = 5. (E) Transition from the stable state 226.5K of deep-frozed bifurcation f = 0.68846 to the current state 290K."

Caption for the third image: "The maximum likelihood transition from the current temperature state to the warmer one for global warming, under the influence of Levy noise and the greenhouse effect. (y-axis is temperature in Kelvin; x-axis is time in undetermined segments)
Credit: Yayun Zheng

Furthermore, Zheng et al (2020) indicates that in 2020 the global mean surface temperature (GMST) in kelvin is 290K (or a GMSTA of about 1.5C) and that GMST should be close to 295K (GMSTA ~ 6.5C),with alpha = 0.5, for an abrupt change in climate state, and I note that per the MCDS this may occur around 2025 just before a MCDS project flip up to a Miocene climate state

[AbruptSLR]

While Aguiar et al. (2021) only compares the freshwater flux/forcing for the 8.2 ka event with modern meltwater from the GIS (without considering other potential modern freshwater flux sources such as from the Beaufort Gyre, WAIS, abrupt Arctic Sea Ice loss or from a discharge of freshwater currently accumulating in the Arctic Ocean from the recent acceleration of river discharge into the Arctic Ocean); this reference does make me wonder whether the Holocene climatic optimum, that roughly peaked about 8 ka (see the attached image), was at least partially due to the 8.2 event.  If so this would provide some measure of supporting paleo-evidence that a freshwater flux pulse can lead to a sharp increase in GMSTA.

Aguiar, W., Meissner, K.J., Montenegro, A. et al. Magnitude of the 8.2 ka event freshwater forcing based on stable isotope modelling and comparison to future Greenland melting. Sci Rep 11, 5473 (2021). https://doi.org/10.1038/s41598-021-84709-5

https://www.nature.com/articles/s41598-021-84709-5

Abstract: "The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of the poleward oceanic heat transport. Although this event has been considered a possible analogue for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a comparison difficult. In this study, we compare sea surface temperatures and oxygen isotope ratios from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea level rise equivalent (SLR) released over a thousand years, with a short intensification over ~ 130 years (an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting rates under the high emission scenario RCP8.5."

Extract: "However, the cold event 8.2 kiloyears before present (8.2 ka event hereafter) differs from previous cold events due to its short, century-long duration. The 8.2 ka event also took place in the current interglacial period under boundary conditions that were closer to pre-industrial conditions than earlier cold events.
…
Freshwater forcing of the hybrid scenarios. In addition to this first set of simulations, which are based on earlier geological reconstructions and described in “Freshwater forcing for the simulations based on earlier reconstructions” section, we also integrated additional sensitivity simulations. Twenty-four experiments were performed based on the uncertainty ranges of the Peltier, Li et al., and Carlson et al. estimates (Table 1). The 7.5 m in SLR equivalent estimated by Peltier was not fully released into the Labrador Sea. In turn, Li et al. estimated the date of the meltwater outburst within 8.245±0.065 ka and their flux estimate has a 0.06 Sv uncertainty. Additionally, the Canadian continental basin routing event from Carlson et al. likely contributed to an enhancement of freshwater flow to the Labrador Sea of 0.13 Sv lasting up to 300 years. Together, these result in potential freshwater fluxes varying between 0.046 and 0.26 Sv and lasting between 200 and 1000 years. With these experiments, called “hybrid”, we test a more complex meltwater flux scenario, based on a background freshwater forcing over a longer time period, a rerouting event and a shorter pulse, more intensive, drainage event. Both the magnitude of the meltwater fluxes (Part A), and their duration (Part B) are tested. Finally, a 2.5 SV freshwater flow to the Labrador Sea was added at year 8.47 ka in all simulations in order to simulate the Lake Agassiz outburst. The exact date of the Lake Agassiz collapse is uncertain due to uncertainties on reservoir ages of marine cores, which precludes further exploration of the date of the collapse in the simulations in this study

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Muschitiello et al. (2019) concludes that:

"Given recent evidence that the current AMOC has been slowing down for several decades, our findings broach the question as to whether the current decline in deep-water circulation may herald a new phase of abrupt change."

Muschitiello, F., D’Andrea, W.J., Schmittner, A. et al. Deep-water circulation changes lead North Atlantic climate during deglaciation. Nat Commun 10, 1272 (2019). https://doi.org/10.1038/s41467-019-09237-3

https://www.nature.com/articles/s41467-019-09237-3

Abstract: "Constraining the response time of the climate system to changes in North Atlantic Deep Water (NADW) formation is fundamental to improving climate and Atlantic Meridional Overturning Circulation predictability. Here we report a new synchronization of terrestrial, marine, and ice-core records, which allows the first quantitative determination of the response time of North Atlantic climate to changes in high-latitude NADW formation rate during the last deglaciation. Using a continuous record of deep water ventilation from the Nordic Seas, we identify a ∼400-year lead of changes in high-latitude NADW formation ahead of abrupt climate changes recorded in Greenland ice cores at the onset and end of the Younger Dryas stadial, which likely occurred in response to gradual changes in temperature- and wind-driven freshwater transport. We suggest that variations in Nordic Seas deep-water circulation are precursors to abrupt climate changes and that future model studies should address this phasing."

Extract: "In conclusion, our results suggest that gradual changes in high-latitude NADW formation are precursors of rapid climate shifts in the North Atlantic and emphasize the central role of ocean circulation in abrupt climate change, as well as its sensitivity to atmosphere and cryosphere dynamics. Given recent evidence that the current AMOC has been slowing down for several decades, our findings broach the question as to whether the current decline in deep-water circulation may herald a new phase of abrupt change."

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Myers et al. (2021) find that projected reductions of low altitude marine clouds with continued global warming should result in a positive feedback for more climate change.

Myers, T.A., et al. (13 May 2021) “Observational constraints on low cloud feedback reduce uncertainty of climate sensitivity”, Nature Climate Change, DOI: 10.1038/s41558-021-01039-0

https://www.nature.com/articles/s41558-021-01039-0

Abstract: "Marine low clouds strongly cool the planet. How this cooling effect will respond to climate change is a leading source of uncertainty in climate sensitivity, the planetary warming resulting from CO2 doubling. Here, we observationally constrain this low cloud feedback at a near-global scale. Satellite observations are used to estimate the sensitivity of low clouds to interannual meteorological perturbations. Combined with model predictions of meteorological changes under greenhouse warming, this permits quantification of spatially resolved cloud feedbacks. We predict positive feedbacks from midlatitude low clouds and eastern ocean stratocumulus, nearly unchanged trade cumulus and a near-global marine low cloud feedback of 0.19 ± 0.12 W m−2 K−1 (90% confidence). These constraints imply a moderate climate sensitivity (~3 K). Despite improved midlatitude cloud feedback simulation by several current-generation climate models, their erroneously positive trade cumulus feedbacks produce unrealistically high climate sensitivities. Conversely, models simulating erroneously weak low cloud feedbacks produce unrealistically low climate sensitivities."

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Initial Model Conditions

This section of replies provides references to relatively recent research findings that are relevant to current climate conditions that future climate models (that consider freshwater flux events) should consider in establishing their initial conditions.  Furthermore, this information was used to help calibrate the probabilities that I assigned to the MCDS-BN.

Deng et al. (2021) confirms that there has been a close relationship between polar motion and climate change in the recent past, which, implies that there will also be such a close relationship in the future.

Deng, S., et al. (22 March 2021), "Polar Drift in the 1990s Explained by Terrestrial Water Storage Changes", Geophysical Research Letters, https://doi.org/10.1029/2020GL092114

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL092114

Abstract
Secular polar drift underwent a directional change in the 1990s, but the underlying mechanism remains unclear. In this study, polar motion observations are compared with geophysical excitations from the atmosphere, oceans, solid Earth, and terrestrial water storage (TWS) during the period of 1981–2020 to determine major drivers. When contributions from the atmosphere, oceans, and solid Earth are removed, the residual dominates the change in the 1990s. The contribution of TWS to the residual is quantified by comparing the hydrological excitations from modeled TWS changes in two different scenarios. One scenario assumes that the TWS change is stationary over the entire study period, and another scenario corrects the stationary result with actual glacier mass change. The accelerated ice melting over major glacial areas drives the polar drift toward 26°E for 3.28 mas/yr after the 1990s. The findings offer a clue for studying past climate‐driven polar motion.

Plain Language Summary
The Earth's pole, the point where the Earth's rotational axis intersects its crust in the Northern Hemisphere, drifted in a new eastward direction in the 1990s, as observed by space geodetic observations. Generally, polar motion is caused by changes in the hydrosphere, atmosphere, oceans, or solid Earth. However, short‐term observational records of key information in the hydrosphere (i.e., changes in terrestrial water storage) limit a better understanding of new polar drift in the 1990s. This study introduces a novel approach to quantify the contribution from changes in terrestrial water storage by comparing its drift path under two different scenarios. One scenario assumes that the terrestrial water storage change throughout the entire study period (1981–2020) is similar to that observed recently (2002–2020). The second scenario assumes that it changed from observed glacier ice melting. Only the latter scenario, along with the atmosphere, oceans, and solid Earth, agrees with the polar motion during the period of 1981–2020. The accelerated terrestrial water storage decline resulting from glacial ice melting is thus the main driver of the rapid polar drift toward the east after the 1990s. This new finding indicates that a close relationship existed between polar motion and climate change in the past.

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Maasakkers et al. (2021) indicates that anthropogenic methane emissions from North America are about twice that previously reported by the US EPA.

Maasakkers, J. D., Jacob, D. J., Sulprizio, M. P., Scarpelli, T. R., Nesser, H., Sheng, J., Zhang, Y., Lu, X., Bloom, A. A., Bowman, K. W., Worden, J. R., and Parker, R. J.: 2010–2015 North American methane emissions, sectoral contributions, and trends: a high-resolution inversion of GOSAT observations of atmospheric methane, Atmos. Chem. Phys., 21, 4339–4356, https://doi.org/10.5194/acp-21-4339-2021, 2021.

https://acp.copernicus.org/articles/21/4339/2021/

Abstract: "We use 2010–2015 Greenhouse Gases Observing Satellite (GOSAT) observations of atmospheric methane columns over North America in a high-resolution inversion of methane emissions, including contributions from different sectors and their trends over the period. The inversion involves an analytical solution to the Bayesian optimization problem for a Gaussian mixture model (GMM) of the emission field with up to 0.5∘×0.625∘ resolution in concentrated source regions. The analytical solution provides a closed-form characterization of the information content from the inversion and facilitates the construction of a large ensemble of solutions exploring the effect of different uncertainties and assumptions in the inverse analysis. Prior estimates for the inversion include a gridded version of the Environmental Protection Agency (EPA) Inventory of US Greenhouse Gas Emissions and Sinks (GHGI) and the WetCHARTs model ensemble for wetlands. Our best estimate for mean 2010–2015 US anthropogenic emissions is 30.6 (range: 29.4–31.3) Tg a−1, slightly higher than the gridded EPA inventory (28.7 (26.4–36.2) Tg a−1). The main discrepancy is for the oil and gas production sectors, where we find higher emissions than the GHGI by 35 % and 22 %, respectively. The most recent version of the EPA GHGI revises downward its estimate of emissions from oil production, and we find that these are lower than our estimate by a factor of 2. Our best estimate of US wetland emissions is 10.2 (5.6–11.1) Tg a−1, on the low end of the prior WetCHARTs inventory uncertainty range (14.2 (3.3–32.4) Tg a−1), which calls for better understanding of these emissions. We find an increasing trend in US anthropogenic emissions over 2010–2015 of 0.4 % a−1, lower than previous GOSAT-based estimates but opposite to the decrease reported by the EPA GHGI. Most of this increase appears driven by unconventional oil and gas production in the eastern US. We also find that oil and gas production emissions in Mexico are higher than in the nationally reported inventory, though there is evidence for a 2010–2015 decrease in emissions from offshore oil production."

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The posts in this sub-section address initial model conditions that likely will impact both ERF and ECSeff.  Furthermore, I begin with a modern interpretation of the paleoclimate determined Zealandia Switch mechanism as discussed in Denton et al. (2021).  As stated in the associated linked article:

"The mechanism, dubbed the Zealandia Switch, relates to the general position of the Southern Hemisphere westerly wind belt — the strongest wind system on Earth — and the continental platforms of the southwest Pacific Ocean, and their control on ocean currents. Shifts in the latitude of the westerly winds affects the strength of the subtropical oceanic gyres and, in turn, influences the release of energy from the tropical ocean waters, the planet’s “heat engine.” Tropical heat spreads rapidly through the atmosphere and ocean to the polar regions of both hemispheres, acting as the planet’s thermostat.
…
Together with interhemispheric paleoclimate records and with the results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres,” according to the researchers, whose work was funded by the Comer Family Foundation, the Quesada Family Foundation, the National Science Foundation and the New Zealand government."

Thus, this Zealand Switch conceptual model supports the MCDS-BN concept linking heat from the Tropical Pacific to poleward telecommunication of this tropical energy as regulated by both the Southern Hemisphere westerly winds and bipolar seesaw mechanisms, leading to abrupt climate change via a domino effect.

Denton, G.H. et al. The Zealandia Switch: Ice age climate shifts viewed from Southern Hemisphere moraines. Quaternary Science Reviews, 2021 DOI: 10.1016/j.quascirev.2020.106771

https://www.sciencedirect.com/science/article/abs/pii/S0277379120307332

Abstract
Two fundamental questions about the ice-age climate system await satisfactory resolution. First, if summer solar radiation intensity truly controls the orbital signature of the last glacial cycle, then why were major climatic shifts, including the last termination, globally synchronous? Second, what caused the millennial-scale climate oscillations superimposed on this cycle? We address these questions from a Southern Hemisphere perspective focused on mid-latitude mountain ice fields. We put particular emphasis on the last glacial termination, which involved both orbital-scale and millennial-scale climate elements and has generally well-resolved chronological control.

Sustained retreat of mountain glaciers, documented by detailed mapping and chronology of glacial landforms in the Southern Alps and southern Andes, marked the termination of the last ice age, beginning ∼18 kyrs ago and involved a change from glacial to near-interglacial atmospheric temperature within a millennium or two. A rapid poleward shift of the Subtropical Front, delineating the northern margin of the Southern Ocean, ∼18 kyrs ago implies a concurrent poleward shift of the austral westerlies and leads us to hypothesize a southern origin for the dominant phase of the last glacial termination. Together with interhemispheric paleoclimate records and with results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast, and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres. We posit that a shift in the Southern Ocean circulation and austral westerly wind system, tied to southern orbital forcing, caused this global warming episode by affecting the tropical heat engine and hence global climate.

Central to this hypothesis, dubbed the ‘Zealandia Switch’, is the location of the Australia and Zealandia continents relative to Southern Hemisphere oceanic and atmospheric circulation. Coupled ocean-atmosphere climate modeling shows that the locus of the austral westerlies, whether in a more equatorward position representing a glacial-mode climate or in a poleward-shifted position marking interglacial-mode climate, has profound effects on oceanic and associated atmospheric linkages between the tropical Pacific and the Southern Ocean. Shifts in the austral westerlies have global climatic consequences, especially through resulting changes in the greenhouse gas content of the atmosphere and altered heat flux from the tropical Pacific into the Northern and Southern Hemispheres. We suggest that the last glacial termination was a global warming episode that led to extreme seasonality in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. This fresh-water influx resulted in widespread North Atlantic sea ice that caused very cold boreal winters, thus amplifying the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. We further suggest that muted manifestations of the Zealandia Switch mechanism were responsible for smaller, recurring millennial-scale climate oscillations within the last glacial cycle.

See also:

Title: "Zealandia Switch may be the missing link in understanding ice age climates"

https://umaine.edu/news/blog/2021/03/12/zealandia-switch-may-be-the-missing-link-in-understanding-ice-age-climates/

Extract: "The mechanism, dubbed the Zealandia Switch, relates to the general position of the Southern Hemisphere westerly wind belt — the strongest wind system on Earth — and the continental platforms of the southwest Pacific Ocean, and their control on ocean currents. Shifts in the latitude of the westerly winds affects the strength of the subtropical oceanic gyres and, in turn, influences the release of energy from the tropical ocean waters, the planet’s “heat engine.” Tropical heat spreads rapidly through the atmosphere and ocean to the polar regions of both hemispheres, acting as the planet’s thermostat.
…
Deep insights into the climate dynamics come from co-author Joellen Russell, climate scientist at the University of Arizona and Thomas R. Brown Distinguished Chair of Integrative Science. Following on her longstanding efforts at modeling the climatic modulation of the westerly winds, she evaluated simulations done as part of the Southern Ocean Model Intercomparison Project, part of the Southern Ocean Carbon and Climate Observations and Modeling initiative. The modeling showed the changes to the southern wind systems have profound consequences for the global heat budget, as monitored by glacier systems.
The “switch” takes its name from Zealandia, a largely submerged continental platform about a third of the size of Australia, with the islands of New Zealand being the largest emergent parts. Zealandia presents a physical impediment to ocean current flow. When the westerly wind belt is farther north, the southward flow of warm ocean water from the tropical Pacific is directed north of the New Zealand landmass (glacial mode). With the wind belt farther south, warm ocean water extends to the south of New Zealand (interglacial mode). Computer modelling shows that global climate effects arise from the latitude at which the westerlies are circulating. A southward shift of the southern westerlies invigorates water circulation in the South Pacific and Southern oceans, and warms the surface ocean waters across much of the globe.
…
Adding weight to the Zealandia Switch hypothesis is that the Southern Hemisphere westerlies regulate the exchange of carbon dioxide and heat between the ocean and atmosphere, and, thus, exert a further influence on global climate.
“Together with interhemispheric paleoclimate records and with the results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres,” according to the researchers, whose work was funded by the Comer Family Foundation, the Quesada Family Foundation, the National Science Foundation and the New Zealand government.
The last glacial termination was a global warming episode that led to extreme seasonality (winter vs. summer conditions) in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. Summer warming led to freshwater influx, resulting in widespread North Atlantic sea ice that caused very cold northern winters and amplified the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. Although this has created an impression of differing temperature responses between the polar hemispheres, the so-called “bipolar seesaw,” the researchers suggest this is due to contrasting interregional effects of global warming or cooling. A succession of short-lived, abrupt, episodes of cold northern winters during the last ice age are suggested to have been caused by temporary shifts of the Zealandia Switch mechanism.
The southward shift of the Southern Hemisphere westerlies at the termination of the last ice age was accompanied by gradual but sustained release of carbon dioxide from the Southern Ocean, which may have helped to lock the climate system into a warm interglacial mode.
The researchers suggest that the introduction of fossil CO2 into the atmosphere may be reawakening the same dynamics that ended the last ice age, potentially propelling the climate system into a new mode.
“The mapping and dating of mid-latitude Southern Hemisphere mountain-glacier moraines leads us to the view that the latitude and strength of the austral westerlies, and their effect on the tropical/subtropical ocean, particularly in the region spanning the Indo-Pacific Warm Pool and Tasman Sea through to the Southern Ocean, provides an explanation for driving orbital-scale global shifts between glacial and interglacial climatic modes, via the Zealandia Switch mechanism,” the research team wrote. “Such behavior of the ocean-atmosphere system may be operative in today’s warming world, introducing a distinctly nonlinear mechanism for accelerating global warming due to atmospheric CO2 rise."

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Perren et al. (2020) discusses centennial timescale paleodata of Southern Hemisphere westerly winds (SHW) taken from Marion Island (see the attached image), that can be used to help to better calibrate CMIP7 models.

Perren, B.B., Hodgson, D.A., Roberts, S.J. et al. Southward migration of the Southern Hemisphere westerly winds corresponds with warming climate over centennial timescales. Commun Earth Environ 1, 58 (2020). https://doi.org/10.1038/s43247-020-00059-6

https://www.nature.com/articles/s43247-020-00059-6

Abstract: "Recent changes in the strength and location of the Southern Hemisphere westerly winds (SHW) have been linked to continental droughts and wildfires, changes in the Southern Ocean carbon sink, sea ice extent, ocean circulation, and ice shelf stability. Despite their critical role, our ability to predict their impacts under future climates is limited by a lack of data on SHW behaviour over centennial timescales. Here, we present a 700-year record of changes in SHW intensity from sub-Antarctic Marion Island using diatom and geochemical proxies and compare it with paleoclimate records and recent instrumental data. During cool periods, such as the Little Ice Age (c. 1400–1870 CE), the winds weakened and shifted towards the equator, and during warm periods they intensified and migrated poleward. These results imply that changes in the latitudinal temperature gradient drive century-scale SHW migrations, and that intensification of impacts can be anticipated in the coming century."

Extract: "Recent intensification and southward migration of the Southern Hemisphere westerly winds (SHW) have been implicated in a number of important physical changes in the southern high latitudes. These include: (1) changes to the Southern Ocean CO2 sink through alterations in ocean mixing; (2) the loss of ice shelves fringing the West Antarctic Ice Sheet and Antarctic Peninsula ice sheets through enhanced basal melting; (3) changes to Antarctic sea ice extent; (4) enhanced Agulhas leakage, and (5) warming along the Antarctic Peninsula. Many of these changes have far-reaching implications for global climate, ocean circulation, and sea level rise.
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Our data also confirm modelling results, which identify latitudinal shifts in the SHW over time with increased future warming and suggest that these relationships have persisted over centennial to millennial timescales. The SHW-temperature relationship has important feedbacks on the climate system, not only on continental drought but also on atmospheric CO2 concentrations, and Antarctic Ice Sheet stability. Changes in the strength and location of the SHW have been linked to weakening of the Southern Ocean carbon sink both on multimillennial and recent instrumental timescales. Hence, further strengthening of the winds from increasing temperatures will likely exert a positive feedback through the accumulation of more CO2 in the atmosphere. The SHW also influence ocean circulation, driving warmer water masses towards Antarctica leading to increased basal melting of ice shelves and ice sheet mass loss. Future poleward-shifted and strengthened SHW will exacerbate this process, making already unstable ice shelves and the ice sheets they buttress increasingly susceptible to melting."

 
Caption for the first image: "Fig. 1 Map showing location of Marion Island and study site. a Map showing location of Marion Island and the core belt of the Southern Hemisphere westerly winds. Arrows mark the direction of surface winds (see also Supplementary Fig. 3). Shaded annual sea surface-level (10 m) mean wind speeds are based on NOAA blended high resolution (0.25 degree grid) vector data downloaded from (https://www.ncdc.noaa.gov/data-access/marineocean-data/ blended-global/blended-sea-winds), b the location of the lake coring site at La Grange Kop on Marion Island (satellite imagery courtesy of NASA Visible Earth, https://visibleearth.nasa.gov/). Numbered sites indicate places referred to in the text: (I) Verlorenvlei, in the winter rainfall zone (WRZ) of South Africa, (II) Îles Kerguelen, (III) West Antarctic Ice Sheet (WAIS)."

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Goyal et al (2021) confirms that CMIP6 does a better job of projecting changes in westerly winds over the Southern Ocean, than did CMIP5, and concludes that only moderate increases in westerly wind velocities and also moderate poleward shifts, in coming decades.  This is important for reasons including that too high of increases in projected westerly wind velocities over the Southern Ocean would reduce the projected associated increase in the upwelling of warm CDW.  I also, note that changes in SH westerly wind patterns are only one reason that the upwelling of warm CDW has been observed to increase in recent decades; with a progressive freshening of the Southern Ocean's surface water near Antarctica likely being of even more importance.

Goyal, R. et al. (20 January 2021), "Historical and projected changes in the Southern Hemisphere surface westerlies", Geophysical Research Letters, https://doi.org/10.1029/2020GL090849

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL090849?af=R

Abstract: "The Southern Hemisphere (SH) surface westerlies fundamentally control regional patterns of air temperature, storm tracks, and precipitation while also regulating ocean circulation, heat transport and carbon uptake. Wind-forced ocean perturbation experiments commonly apply idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude and wind intensification factors of between 10% and 300%. In addition, changes in winds are often prescribed ad hoc as a zonally uniform anomaly that neglects important regional and seasonal differences. Here we quantify historical and projected SH westerly wind changes based on examination of CMIP5, CMIP6, and reanalysis data. We find a significant reduction in the location bias of the CMIP6 ensemble and an associated reduction in the projected poleward shift compared to CMIP5. Under a high emission scenario, we find a projected end of 21st Century ensemble mean wind increase of ∼10% and a poleward shift of ∼0.8° latitude, although there are important seasonal and regional variations."

Plain Language Summary
The westerly winds in the Southern Hemisphere have increased in speed and shifted towards Antarctica over the last few decades, with model projections suggesting further poleward intensification in the future. Changes in the westerly winds are of great importance because they control the Southern Ocean circulation, ocean carbon uptake, and ocean heat transport, with ramifications for global climate. To understand the impacts of changes in the westerlies on the Southern Ocean, ocean model simulations are often run by artificially increasing and/or shifting winds towards Antarctica to understand the ocean impact of future changes in the winds. However, there is no consistency in the way these changes are incorporated, with large variations in the applied shift and strengthening. In this study, we quantify recent observed and projected changes in the surface westerlies, aiming to provide guidance as to what wind perturbations should be applied in ocean models. We further show that the latest generation of coupled climate models show a reduction in the equatorward bias in the location of westerly winds as compared to the previous generation of models.

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Doddridge et al. (2020) discusses both reasons why current Antarctic Sea Ice extends are currently relatively high; and why we might expect relatively rapid sea ice declines in the future due to changes in both wind patterns and ocean heat uptake/content:

Doddridge, E.W. et al. (2020), "Southern Ocean heat storage, reemergence, and winter sea ice decline induced by summertime winds", J. Climate 1–47, https://doi.org/10.1175/JCLI-D-20-0322.1

https://journals.ametsoc.org/jcli/article-abstract/doi/10.1175/JCLI-D-20-0322.1/355537/Southern-Ocean-heat-storage-reemergence-and-winter?redirectedFrom=fulltext

Abstract: "The observational record shows a substantial 40-year upward trend in summertime westerly winds over the Southern Ocean, as characterised by the Southern Annular Mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downwards, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season."

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The linked articles (& associated linked references) indicate that anthropogenic GHG emissions is causing the mesosphere above Antarctica to cool at rates up to 10 times faster than the rate of increase of GMSTA.  Furthermore, this research identifies a new natural cycle not previously identified in the Antarctic upper atmosphere; which is a four-year cycle that the authored called the Quasi-Quadrennial Oscillation (QQO), which caused observed mesosphere temperatures to vary by 3-4 degrees C.  As it is not likely that the CMIP projections to date account for this QQO cycle in their models future generations (CMIP7) of ESM projects should be adjusted to simulate this signal which plausibly arises from an ocean–atmosphere response, and appears to have a signature in Antarctic sea ice extent.  Lastly, I note that the current ozone hole over Antarctica also serves to cool the upper atmosphere over Antarctica; so as the ozone hole heals itself this GHG driven cooling of the mesosphere over Antarctica will likely maintain current conditions that have causes the westerly winds over the Southern Ocean to accelerate since the 1970s; which will likely at least maintain the current upwelling volumes of warm CDW that has been accelerating ocean related ice melting around the Antarctic coastline since the 1970s.


Title: "Carbon Emissions Are Chilling The Atmosphere 90 Km Above Antarctica"

https://www.sciencealert.com/carbon-emissions-are-chilling-the-atmosphere-90km-above-antarctica-at-the-edge-of-space

While the linked reference does not consider the impacts of meltwater from the AIS; nevertheless its analysis represents an improvement on earlier studies with regard to the effects of buoyancy and wind forcing on the Southern Ocean:

Jia-Rui Shi, Lynne D. Talley, Shang-Ping Xie, Wei Liu and Sarah T. Gille (2020), "Effects of Buoyancy and Wind Forcing on Southern Ocean Climate Change", J. Climate, 1–53, https://doi.org/10.1175/JCLI-D-19-0877.1

https://journals.ametsoc.org/jcli/article/doi/10.1175/JCLI-D-19-0877.1/353484/Effects-of-Buoyancy-and-Wind-Forcing-on-Southern?searchresult=1

Abstract
Observations show that since the 1950s, the Southern Ocean has stored a large amount of anthropogenic heat and has freshened at the surface. These patterns can be attributed to two components of surface forcing: poleward-intensified westerly winds and increased buoyancy flux from freshwater and heat. Here we separate the effects of these two forcing components by using a novel partial-coupling technique. We show that buoyancy forcing dominates the overall response in the temperature and salinity structure of the Southern Ocean. Wind stress change results in changes in subsurface temperature and salinity that are closely related to intensified  . As an important result, we show that buoyancy and wind forcing result in opposing changes in salinity: the wind-induced surface salinity increase due to upwelling of saltier subsurface water offsets surface freshening due to amplification of the global hydrological cycle. Buoyancy and wind forcing further lead to different vertical structures of Antarctic Circumpolar Current (ACC) transport change; buoyancy forcing causes an ACC transport increase (3.1±1.6 Sv; 1Sv ≡ 106m3s−1) by increasing the meridional density gradient across the ACC in the upper 2000m, while the wind-induced response is more barotropic, with the whole column transport increased by 8.7±2.3 Sv. While previous research focused on the wind effect on ACC intensity, we show that surface horizontal current acceleration within the ACC is dominated by buoyancy forcing. These results shed light on how the Southern Ocean might change under global warming, contributing to more reliable future projections.

The linked reference finds that with lags of about a decade oscillations of the Southern Ocean Westerly winds (commonly associated with variations in SAM) can drive variations in the MOC.  This represents a climate risk as these variations in MOC might temporarily be super imposed on freshwater hosing events that tend to slow the MOC.

Woo Geun Cheon & Jong-Seong Kug (2020), "The Role of Oscillating Southern Hemisphere Westerly Winds: Global Ocean Circulation", Journal of Climate, https://doi.org/10.1175/JCLI-D-19-0364.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-19-0364.1


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the linked reference (see also the linked associated article) marshals a large amount of data to verify that the Antarctic ozone hole is not only directly related to accelerating, and southward shifting, the westerly winds over the Southern Ocean; but that ozone depletion has been a significant/driving factor in multiple changes in the climate in the Southern Hemisphere in recent decades.  Thus, I wonder whether Steig underestimates the extent of the influence of the Antarctic ozone hole; which clearly extends all the way from the South Pole to the equatorial Pacific, and may very well have acted to accelerate the telecommunication of energy from the Tropical Pacific to the West Antarctic region.

Barnes, P.W., Williamson, C.E., Lucas, R.M. et al. Ozone depletion, ultraviolet radiation, climate change and prospects for a sustainable future. Nat Sustain 2, 569–579 (2019) doi:10.1038/s41893-019-0314-2

https://www.nature.com/articles/s41893-019-0314-2

Abstract: "Changes in stratospheric ozone and climate over the past 40-plus years have altered the solar ultraviolet (UV) radiation conditions at the Earth’s surface. Ozone depletion has also contributed to climate change across the Southern Hemisphere. These changes are interacting in complex ways to affect human health, food and water security, and ecosystem services. Many adverse effects of high UV exposure have been avoided thanks to the Montreal Protocol with its Amendments and Adjustments, which have effectively controlled the production and use of ozone-depleting substances. This international treaty has also played an important role in mitigating climate change. Climate change is modifying UV exposure and affecting how people and ecosystems respond to UV; these effects will become more pronounced in the future. The interactions between stratospheric ozone, climate and UV radiation will therefore shift over time; however, the Montreal Protocol will continue to have far-reaching benefits for human well-being and environmental sustainability."

See also:

Title: "Ozone depletion driving climate change in Southern Hemisphere"

https://phys.org/news/2019-06-ozone-depletion-climate-southern-hemisphere.html

Extract: ""It is now clear that ozone depletion is directly contributing to climate change across the Southern Hemisphere," Professor Robinson said.

"Ozone is a greenhouse gas, so the ozone hole has kept Antarctica cooler, pulling the westerly wind jet that circles the continent closer and tighter to Antarctica. This has increased the speed of the wind, making Antarctica cooler and drier, pulling other Southern Hemisphere weather zones further south."

The Southern Annular Mode describes the north-south movement of the wind belt that circles the Southern Hemisphere. Analysis of ice cores shows these winds are the furthest south they have been for a thousand years.

As climate zones have shifted southwards, rainfall patterns, sea-surface temperatures and ocean currents across large areas of the southern hemisphere have also shifted, impacting terrestrial and aquatic ecosystems in Australia, New Zealand, Antarctica, South America, Africa and the Southern Ocean.

"We are seeing changes across the Southern Hemisphere, from the pole to the tropics," Professor Robinson said. "Some areas are getting more rain and some have become drier, which has a huge effect on plants and animals, including on agriculture."

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If iodine in the atmosphere (from the ocean) slows the recovery of ozone hole over Antarctica, then the westerly wind velocities many remain high over the Southern Ocean for some time to come:

Title: "Iodine may slow ozone layer recovery"

https://phys.org/news/2020-01-iodine-ozone-layer-recovery.html

Extract: "A new paper quantifying small levels of iodine in Earth's stratosphere could help explain why some of the planet's protective ozone layer isn't healing as fast as expected.
…
"The ozone layer is starting to show early signs of recovery in the upper stratosphere, but ozone in the lower stratosphere continues to decline for unclear reasons," said Rainer Volkamer, a CIRES Fellow, CU Boulder professor of chemistry and corresponding author of the new assessment.

"Before now, the decline was thought to be due to changes in how air mixes between the troposphere and stratosphere. Our measurements show there is also a chemical explanation, due to iodine from oceans. What I find exciting is that iodine changes ozone by just enough to provide a plausible explanation for why ozone in the lower stratosphere continues to decline.""

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If the linked reference's finding that part of the E3SM version 1 projected high value of TCR is due to a projected slowing of the AMOC; then this may well be because E3SM version 1 did a better job of evaluating the influence of freshwater hosing from glacier meltwater; then this implies that ice-climate feedbacks may likely have a much higher impact on increasing climate sensitivity than consensus climate science is currently acknowledging.

Aixue Hu et al. (17 April 2020), "Role of AMOC in transient climate response to greenhouse gas forcing in two coupled models", Journal of Climate, https://doi.org/10.1175/JCLI-D-19-1027.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-19-1027.1?af=R

Abstract
As the greenhouse gas concentrations increase, a warmer climate is expected. However, numerous internal climate processes can modulate the primary radiative warming response of the climate system to rising greenhouse gas forcing. Here the particular internal climate process that we focus on is the Atlantic Meridional Overturning Circulation (AMOC) – an important global scale feature of ocean circulation that serves to transport heat and other scalars, and we address the question of how the mean strength of AMOC can modulate the transient climate response. While the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting a faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise the stronger AMOC in CESM2 by permitting a slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial timescale.

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The linked reference indicates that CMIP5 likely underestimated how negative aerosol forcing has been and indicates that the mean values of aerosol forcing and of TCR from CMIP6 are closer to matching the observed regional land temperatures.  If so, this indicates that the relatively high mean climate sensitivity values (both TCR and ECS) determined in CMIP6 are more likely than so far acknowledged by consensus climate documents like AR5.  I note that while the TCR value of 2.0K determined by this reference is relatively high, this value does not consider the impacts of freshwater hosing on TCR and that when E3SMv1 did so it projected a value of 2.95K for TCR.

Shen, Z., Yi Ming and Isaac M. Held (05 Aug 2020), "Using the fast impact of anthropogenic aerosols on regional land temperature to constrain aerosol forcing", Science Advances, Vol. 6, no. 32, eabb5297, DOI: 10.1126/sciadv.abb5297

https://advances.sciencemag.org/content/6/32/eabb5297.full

Abstract
Anthropogenic aerosols have been postulated to have a cooling effect on climate, but its magnitude remains uncertain. Using atmospheric general circulation model simulations, we separate the land temperature response into a fast response to radiative forcings and a slow response to changing oceanic conditions and find that the former accounts for about one fifth of the observed warming of the Northern Hemisphere land during summer and autumn since the 1960s. While small, this fast response can be constrained by observations. Spatially varying aerosol effects can be detected on the regional scale, specifically warming over Europe and cooling over Asia. These results provide empirical evidence for the important role of aerosols in setting regional land temperature trends and point to an emergent constraint that suggests strong global aerosol forcing and high transient climate response.

Extract: "One can further estimate the global aerosol forcing at −1.4 ± 0.7 W m−2 by subtracting the nonaerosol forcings (2.9 ± 0.2 W m−2 in the three models) from the inferred total forcing (1.5 ± 0.7 W m−2). This value is appreciably stronger than the best AR5 estimate (−0.9 W m−2) but well within the 90% confidence interval (−0.1 to −1.9 W m−2). It is also within the 68% confidence interval of −0.65 to −1.60 W m−2 provided by (27). The best estimate is at the lowest end of the Coupled Model Intercomparison Project Phase 6 (CMIP6) aerosol ERF range (−0.63 to −1.37 W m−2) (28).

The transient climate response (TCR; defined as the surface temperature change in response to a 1% per year increase of CO2 at the time of doubling), a quantity crucial for near-term climate projection, can be calculated from the historical warming (δT, 0.80 K) (29) and ERF (F) as F2XδT/F, where F2X is the ERF of CO2 doubling (3.8 W m−2) (29). At a historical forcing of 1.5 W m−2 as estimated here, the implied TCR is 2.0 K. This is at the higher end of the AR5 likely range of 1 to 2.5 K (30) but is close to the median TCR of 1.95 K based on CMIP6 models (31)."

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I provide the following linked reference that indicates that GHG emissions from future thermokarst lakes in northern permafrost regions represents a meaningful climate risk:

Cristian Estop‐Aragonés et al. (02 September 2020), "Assessing the Potential for Mobilization of Old Soil Carbon after Permafrost Thaw: A Synthesis of 14C Measurements from the Northern Permafrost Region", Global Biogeochemical Cycles, https://doi.org/10.1029/2020GB006672

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GB006672?af=R

Abstract
The magnitude of future emissions of greenhouse gases from the northern permafrost region depend crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (14C) to quantify the release of this “old” SOC as CO2 or CH4 to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled ~1,900 14C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil 14C‐CO2 emissions generally had a modern (post‐1950s) signature, but that well‐drained, oxic soils had increased CO2 emissions derived from older sources following recent thaw. The age of CO2 and CH4 emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously‐frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously‐frozen SOC as CO2, CH4, and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of “old” sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future 14C studies in the permafrost region.

The linked reference confirms that CMIP6 does a better job of projecting changes in westerly winds over the Southern Ocean, than did CMIP5, and concludes that only moderate increases in westerly wind velocities and also moderate poleward shifts, in coming decades.  This is important for reasons including that too high of increases in projected westerly wind velocities over the Southern Ocean would reduce the projected associated increase in the upwelling of warm CDW.  I also, note that changes in SH westerly wind patterns are only one reason that the upwelling of warm CDW has been observed to increase in recent decades; with a progressive freshening of the Southern Ocean's surface water near Antarctica likely being of even more importance.

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The linked reference indicates that numerous CMIP6 models (including Wolf Pack models) project higher effective radiative forcing (ERF) for methane (in particular) than previously estimated by consensus climate science (CCS) projections (see the attached images).

Thornhill, G. D., Collins, W. J., Kramer, R. J., Olivié, D., Skeie, R. B., O'Connor, F. M., Abraham, N. L., Checa-Garcia, R., Bauer, S. E., Deushi, M., Emmons, L. K., Forster, P. M., Horowitz, L. W., Johnson, B., Keeble, J., Lamarque, J.-F., Michou, M., Mills, M. J., Mulcahy, J. P., Myhre, G., Nabat, P., Naik, V., Oshima, N., Schulz, M., Smith, C. J., Takemura, T., Tilmes, S., Wu, T., Zeng, G., and Zhang, J.: Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison, Atmos. Chem. Phys., 21, 853–874, https://doi.org/10.5194/acp-21-853-2021, 2021.

https://acp.copernicus.org/articles/21/853/2021/

Abstract
This paper quantifies the pre-industrial (1850) to present-day (2014) effective radiative forcing (ERF) of anthropogenic emissions of NOX, volatile organic compounds (VOCs; including CO), SO2, NH3, black carbon, organic carbon, and concentrations of methane, N2O and ozone-depleting halocarbons, using CMIP6 models. Concentration and emission changes of reactive species can cause multiple changes in the composition of radiatively active species: tropospheric ozone, stratospheric ozone, stratospheric water vapour, secondary inorganic and organic aerosol, and methane. Where possible we break down the ERFs from each emitted species into the contributions from the composition changes. The ERFs are calculated for each of the models that participated in the AerChemMIP experiments as part of the CMIP6 project, where the relevant model output was available.
The 1850 to 2014 multi-model mean ERFs (± standard deviations) are −1.03 ± 0.37 W m−2 for SO2 emissions, −0.25 ± 0.09 W m−2 for organic carbon (OC), 0.15 ± 0.17 W m−2 for black carbon (BC) and −0.07 ± 0.01 W m−2 for NH3. For the combined aerosols (in the piClim-aer experiment) it is −1.01 ± 0.25 W m−2. The multi-model means for the reactive well-mixed greenhouse gases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 W m−2 for methane (CH4), 0.26 ± 0.07 W m−2 for nitrous oxide (N2O) and 0.12 ± 0.2 W m−2 for ozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogen oxides (NOx), volatile organic compounds and both together (O3) lead to ERFs of 0.14 ± 0.13, 0.09 ± 0.14 and 0.20 ± 0.07 W m−2 respectively. The differences in ERFs calculated for the different models reflect differences in the complexity of their aerosol and chemistry schemes, especially in the case of methane where tropospheric chemistry captures increased forcing from ozone production.
Extract: "The experimental set-up and diagnostics in CMIP6 have allowed us for the first time to calculate the effective radiative forcing (ERF) for present-day reactive gas and aerosol concentrations and emissions in a range of Earth system models. Quantifying the forcing in these models is an essential step to understanding their climate responses.
…
We find that the ERF from well-mixed greenhouse gases (methane, nitrous oxide and halocarbons) has significant contributions through their effects on ozone, aerosols and clouds, which vary strongly across Earth system models. This indicates that Earth system processes need to be taken into account when understanding the contribution WMGHGs have made to present climate and when projecting the climate effects of different WMGHG scenarios."
 
Caption for the first image: "Figure 8 Estimated SARF from the greenhouse gas changes (WMGHGs and ozone), using radiative efficiencies for the WMGHGs and kernel calculations for ozone (see text). Hatched bars show decreases in ozone SARF. Symbols show the modelled ERF, SARF and ERFcs,af (estimate of greenhouse gas clear-sky ERF). Uncertainties on the bars are due to uncertainties in radiative efficiencies. Uncertainties on the symbols are errors in the mean due to interannual variability in the model diagnostic."
 
Caption for the second image: "Figure 9 Changes in methane lifetime (%), for each experiment. Uncertainties for individual models are errors on the mean from interannual variability. Uncertainties for the multi-model mean are standard deviations across models."

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Recent research has found that the oceans have recently been absorbing 10% more CO2 than projected by CMIP models; while the linked reference finds carbon absorbed by the oceans now inhibit future marine CO2 uptake through reductions to the buffering capacity of surface seawater; which amplifies TCR.  This is not good news:

K. B. Rodgers et al. (02 September 2020), "Re‐emergence of anthropogenic carbon into the ocean’s mixed layer strongly amplifies transient climate sensitivity", Geophysical Research Letters, https://doi.org/10.1029/2020GL089275

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089275?af=R

Abstract
A positive marine chemistry‐climate feedback was originally proposed by Revelle and Suess (1957), whereby the invasion flux of anthropogenic carbon into the ocean serves to inhibit future marine CO2 uptake through reductions to the buffering capacity of surface seawater. Here we use an ocean‐circulation‐carbon‐cycle model to identify an upper limit on the impact of re‐emergence of anthropogenic carbon into the ocean’s mixed layer on the cumulative airborne fraction of CO2 in the atmosphere. We find under an RCP8.5 emissions pathway (with steady circulation) that the cumulative airborne fraction of CO2 has a seven‐fold reduction by 2100 when the CO2 buffering capacity of surface seawater is maintained at pre‐industrial levels. Our results indicate that the effect of re‐emergence of anthropogenic carbon into the mixed layer on the buffering capacity of CO2 amplifies the transient climate sensitivity of the Earth system.

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The linked reference concludes with regard to the terrestrial biosphere that:

"Under business-as-usual emissions, this divergence elicits a near halving of the land sink strength by as early as 2040."

Such a quick drop in the terrestrial biosphere carbon sink will make it virtually assured that we will stay on a SSP5-8.5 radiative pathway in the coming decades.

Duffy K.A. el al. (13 Jan 2021), "How close are we to the temperature tipping point of the terrestrial biosphere?," Science Advances ,Vol. 7, no. 3, eaay1052, DOI: 10.1126/sciadv.aay1052

https://advances.sciencemag.org/content/7/3/eaay1052

Abstract
The temperature dependence of global photosynthesis and respiration determine land carbon sink strength. While the land sink currently mitigates ~30% of anthropogenic carbon emissions, it is unclear whether this ecosystem service will persist and, more specifically, what hard temperature limits, if any, regulate carbon uptake. Here, we use the largest continuous carbon flux monitoring network to construct the first observationally derived temperature response curves for global land carbon uptake. We show that the mean temperature of the warmest quarter (3-month period) passed the thermal maximum for photosynthesis during the past decade. At higher temperatures, respiration rates continue to rise in contrast to sharply declining rates of photosynthesis. Under business-as-usual emissions, this divergence elicits a near halving of the land sink strength by as early as 2040.

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Regarding Arctic Amplification (AA), the linked reference concludes that:

"Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes."

This indicates that AA's contribution to abrupt climate change would occur quickly (within months) of a perturbation (in my opinion including a significant freshwater hosing event).

Michael Previdi et al. (25 August 2020), "Arctic Amplification: a Rapid Response to Radiative Forcing", Geophysical Research Letters, https://doi.org/10.1029/2020GL089933

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089933?af=R

Abstract
Arctic amplification (AA) of surface warming is a prominent feature of anthropogenic climate change with important implications for human and natural systems. Despite its importance, the underlying causes of AA are not fully understood. Here, analyzing coupled climate model simulations, we show that AA develops rapidly (within the first few months) following an instantaneous quadrupling of atmospheric CO2. This rapid AA response ‐ which occurs before any significant loss of Arctic sea ice ‐ is produced by a positive lapse rate feedback over the Arctic. Sea ice loss is therefore not needed to produce polar‐amplified warming, although it contributes significantly to this warming after the first few months. Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes.

Plain Language Summary
Climate warming is greater in the Arctic than at lower latitudes, a phenomenon known as Arctic Amplification. Despite its importance for humans and ecosystems, the causes of Arctic Amplification are not fully understood. Sea ice loss has long been thought to be a primary cause. However, we show here that Arctic Amplification happens faster than sea ice loss in climate models when atmospheric CO2 is increased. This indicates that atmospheric processes alone are capable of causing Arctic Amplification.

Key Points
•   Arctic Amplification develops rapidly (within a few months) in climate models forced with abrupt CO2 quadrupling
•   This rapid response is produced by a positive lapse rate feedback over the Arctic
•   The response occurs before Arctic sea ice loss becomes significant

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The linked reference indicates that the climate forcing from global aviation is higher than previously assumed by consensus climate science:

Lee, D. S. et al. (2020) The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, doi:10.1016/j.atmosenv.2020.117834

https://www.sciencedirect.com/science/article/pii/S1352231020305689
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The linked reference discusses the importance and uncertainties associated with air-sea-ice fluxes in the Southern Ocean.

Sebastiaan Swart et al. (31 July 2019), "Constraining Southern Ocean Air-Sea-Ice Fluxes Through Enhanced Observations", Front. Mar. Sci., | https://doi.org/10.3389/fmars.2019.00421

https://www.frontiersin.org/articles/10.3389/fmars.2019.00421/full

Abstract
Air-sea and air-sea-ice fluxes in the Southern Ocean play a critical role in global climate through their impact on the overturning circulation and oceanic heat and carbon uptake. The challenging conditions in the Southern Ocean have led to sparse spatial and temporal coverage of observations. This has led to a “knowledge gap” that increases uncertainty in atmosphere and ocean dynamics and boundary-layer thermodynamic processes, impeding improvements in weather and climate models. Improvements will require both process-based research to understand the mechanisms governing air-sea exchange and a significant expansion of the observing system. This will improve flux parameterizations and reduce uncertainty associated with bulk formulae and satellite observations. Improved estimates spanning the full Southern Ocean will need to take advantage of ships, surface moorings, and the growing capabilities of autonomous platforms with robust and miniaturized sensors. A key challenge is to identify observing system sampling requirements. This requires models, Observing System Simulation Experiments (OSSEs), and assessments of the specific spatial-temporal accuracy and resolution required for priority science and assessment of observational uncertainties of the mean state and direct flux measurements. Year-round, high-quality, quasi-continuous in situ flux measurements and observations of extreme events are needed to validate, improve and characterize uncertainties in blended reanalysis products and satellite data as well as to improve parameterizations. Building a robust observing system will require community consensus on observational methodologies, observational priorities, and effective strategies for data management and discovery.

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The linked May 2020 reference makes it clear that air-sea heat flux is a major consideration in the rate of high-latitude ocean heat uptake; which is a relatively quick process and which is affected by the presence of both sea ice and relatively fresh surface waters; both of which can trap OHC within the Southern Ocean by limiting heat flux into the air.

Kewei Lyu; Xuebin Zhang; John A. Church and Quran Wu (2020), "Processes Responsible for the Southern Hemisphere Ocean Heat Uptake and Redistribution under Anthropogenic Warming", J. Climate, 33 (9): 3787–3807, https://doi.org/10.1175/JCLI-D-19-0478.1

https://journals.ametsoc.org/jcli/article/33/9/3787/344998/Processes-Responsible-for-the-Southern-Hemisphere

Abstract
The Southern Hemisphere oceans absorb most of the excess heat stored in the climate system due to anthropogenic warming. By analyzing future climate projections from a large ensemble of the CMIP5 models under a high emission scenario (RCP8.5), we investigate how the atmospheric forcing and ocean circulation determine heat uptake and redistribution in the Southern Hemisphere oceans. About two-thirds of the net surface heat gain in the high-latitude Southern Ocean is redistributed northward, leading to enhanced and deep-reaching warming at middle latitudes near the boundary between the subtropical gyres and the Antarctic Circumpolar Current. The projected magnitudes of the ocean warming are closely related to the magnitudes of the wind and gyre boundary poleward shifts across the models. For those models with the simulated gyre boundary biased equatorward, the latitude where the projected ocean warming peaks is also located farther equatorward and a larger poleward shift of the gyre boundary is projected. In a theoretical framework, the subsurface ocean changes are explored using three distinctive processes on the temperature–salinity diagram: pure heave, pure warming, and pure freshening. The enhanced middle-latitude warming and the deepening of isopycnals are attributed to the pure heave and pure warming processes, likely related to the wind-driven heat convergence and the accumulation of extra surface heat uptake by the background ocean circulation, respectively. The equatorward and downward subductions of the surface heat and freshwater input at high latitudes (i.e., pure warming and pure freshening processes) result in cooling and freshening spiciness changes on density surfaces within the Subantarctic Mode Water and Antarctic Intermediate Water.

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Menary et al. (2020) provides more input for future climate models (e.g. CMIP7) to better model AMOC behavior.

Menary, M.B, Laura C. Jackson and M. Susan Lozier (09 September 2020), "Reconciling the Relationship Between the AMOC and Labrador Sea in OSNAP Observations and Climate Models", Geophysical Research Letters, https://doi.org/10.1029/2020GL089793

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089793?af=R

Abstract
The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate variability. Our understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high‐profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence the credibility of the AMOC representation in climate models. Here, we attempt to reconcile these viewpoints by making the first direct comparison between OSNAP and a coupled climate model. The model compares well to the observations, demonstrating a more prominent role for overturning in the eastern than western subpolar gyre. Density anomalies generated by surface forcing in the Irminger Sea propagate into the Labrador Sea, where they dominate the density variability. Thus, the Labrador Sea may not be the origin of AMOC variability despite correlations with densities there.

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Zarakas et al. (2020) finds that when evaluating the influence of plant physiology with increasing CO2 levels, that TCR (full) has a mean value in CMIP5 & CMIP6 that is about 6.1% higher than the TCR estimated using only radiative forcing (see the first attached image & caption) & that this deviation increases with higher values of GMSTA.  Thus consensus science is likely underestimating the risks of global warming in the coming decades:

Claire M. Zarakas et al. (2020), "Plant Physiology Increases the Magnitude and Spread of the Transient Climate Response to CO2 in CMIP6 Earth System Models
J. Climate, 33, (19), 8561–8578, https://doi.org/10.1175/JCLI-D-20-0078.1

https://journals.ametsoc.org/jcli/article/33/19/8561/353455/Plant-Physiology-Increases-the-Magnitude-and

Abstract: "Increasing concentrations of CO2 in the atmosphere influence climate both through CO2’s role as a greenhouse gas and through its impact on plants. Plants respond to atmospheric CO2 concentrations in several ways that can alter surface energy and water fluxes and thus surface climate, including changes in stomatal conductance, water use, and canopy leaf area. These plant physiological responses are already embedded in most Earth system models, and a robust literature demonstrates that they can affect global-scale temperature. However, the physiological contribution to transient warming has yet to be assessed systematically in Earth system models. Here this gap is addressed using carbon cycle simulations from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) to isolate the radiative and physiological contributions to the transient climate response (TCR), which is defined as the change in globally averaged near-surface air temperature during the 20-yr window centered on the time of CO2 doubling relative to preindustrial CO2 concentrations. In CMIP6 models, the physiological effect contributes 0.12°C (σ: 0.09°C; range: 0.02°–0.29°C) of warming to the TCR, corresponding to 6.1% of the full TCR (σ: 3.8%; range: 1.4%–13.9%). Moreover, variation in the physiological contribution to the TCR across models contributes disproportionately more to the intermodel spread of TCR estimates than it does to the mean. The largest contribution of plant physiology to CO2-forced warming—and the intermodel spread in warming—occurs over land, especially in forested regions."

 
Caption for the first image: "The relationship between TCRRAD (RAD-PI) and TCRFULL (FULL-PI). The gray 1:1 line is where we would expect all models to be if the TCR were entirely caused by the radiative effects of CO2. The added warming from the physiological effect is the vertical distance between the gray 1:1 line and each point. Marker types indicate CMIP phase (CMIP5: circles; CMIP6: triangles) and colors indicate modeling center. Crosses demarcate the CMIP6 (solid) and CMIP5 (dashed) multimodel means, and the width of each cross corresponds to two times the ensemble mean standard deviation in global mean near-surface temperature from the preindustrial control."

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Morrison et al. (2020) provides more data to help future climate models (e.g. CMIP7) to better project the impacts of the poleward advection of warm CDW.

A. K. Morrison et al. (01 May 2020), "Warm Circumpolar Deep Water transport toward Antarctica driven by local dense water export in canyons", Science Advances, Vol. 6, no. 18, eaav2516, DOI: 10.1126/sciadv.aav2516

https://advances.sciencemag.org/content/6/18/eaav2516

Abstract
Poleward transport of warm Circumpolar Deep Water (CDW) has been linked to melting of Antarctic ice shelves. However, even the steady-state spatial distribution and mechanisms of CDW transport remain poorly understood. Using a global, eddying ocean model, we explore the relationship between the cross-slope transports of CDW and descending Dense Shelf Water (DSW). We find large spatial variability in CDW heat and volume transport around Antarctica, with substantially enhanced flow where DSW descends in canyons. The CDW and DSW transports are highly spatially correlated within ~20 km and temporally correlated on subdaily time scales. Focusing on the Ross Sea, we show that the relationship is driven by pulses of overflowing DSW lowering sea surface height, leading to net onshore CDW transport. The majority of simulated onshore CDW transport is concentrated in cold-water regions, rather than warm-water regions, with potential implications for ice-ocean interactions and global sea level rise.

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The linked reference (& associated article) indicates that observed data shows that warm CDW is rising toward the ocean’s surface at the rate of about 130 feet per decade"; which, is three to ten times faster that consensus climate science previously assumed.  As indicated by the second attached image, this is bad news for the stability not only of ASE marine glaciers but also for marine glaciers in the Wilkes Basin and the Aurora Basin in the EAIS.

Auger, M., Morrow, R., Kestenare, E. et al. Southern Ocean in-situ temperature trends over 25 years emerge from interannual variability. Nat Commun 12, 514 (2021). https://doi.org/10.1038/s41467-020-20781-1

https://www.nature.com/articles/s41467-020-20781-1

Abstract
Despite playing a major role in global ocean heat storage, the Southern Ocean remains the most sparsely measured region of the global ocean. Here, a unique 25-year temperature time-series of the upper 800 m, repeated several times a year across the Southern Ocean, allows us to document the long-term change within water-masses and how it compares to the interannual variability. Three regions stand out as having strong trends that dominate over interannual variability: warming of the subantarctic waters (0.29 ± 0.09 °C per decade); cooling of the near-surface subpolar waters (−0.07 ± 0.04 °C per decade); and warming of the subsurface subpolar deep waters (0.04 ± 0.01 °C per decade). Although this subsurface warming of subpolar deep waters is small, it is the most robust long-term trend of our section, being in a region with weak interannual variability. This robust warming is associated with a large shoaling of the maximum temperature core in the subpolar deep water (39 ± 09 m per decade), which has been significantly underestimated by a factor of 3 to 10 in past studies. We find temperature changes of comparable magnitude to those reported in Amundsen–Bellingshausen Seas, which calls for a reconsideration of current ocean changes with important consequences for our understanding of future Antarctic ice-sheet mass loss.
 

See also:

Title: "Southern Ocean waters are warming faster than thought, threatening Antarctic ice"

https://www.washingtonpost.com/weather/2021/01/21/southern-ocean-warming-antarctica/

Extract: "Now a new study, published Thursday in the journal Nature Communications, finds that beneath the surface layer of waters circling Antarctica, the seas are warming much more rapidly than previously known. Furthermore, the study concludes, this relatively warm water is rising toward the surface over time, at a rate three to 10 times what was previously estimated.

This means that there is a greater potential for the waters of the Southern Ocean, which are absorbing vast quantities of added heat and carbon dioxide from the atmosphere as a result of human activities, may soon help destabilize parts of the Antarctic Ice Sheet.
…
The researchers found that warming under the sea surface within waters near Antarctica stands out from naturally occurring trends, with temperatures increasing at a rate of about 0.072 degrees Fahrenheit (0.04 Celsius) per decade. At the same time, the relatively warm water — usually located under a colder layer — is rising toward the ocean’s surface at the rate of about 130 feet per decade. While the temperature change within waters that move from west to east around Antarctica may appear small, the study indicates it is a “radical” change from its average state and is enough to threaten ice stability where glaciers empty into the sea via fragile floating ice shelves.
…
“This study shows the threat of subsurface water warming, that can affect Antarctic ice cap all around Antarctica. Our hope for future studies is a better understanding of the Southern Ocean models. Ocean models are affected by the lack of observation in the region, and a better representation of the Southern Ocean by the models would be an important step forward,” Auger wrote.
…
However, he called the findings about warming and rising waters “frightening” compared with findings from his own work just a few years ago, saying, if correct, these waters could have a “potentially imminent impact on several Antarctic glaciers.”"

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Zhu & Liu (2020) provides more observational evidence that the AMOC is indeed currently slowing down.

Zhu, C., Liu, Z. Weakening Atlantic overturning circulation causes South Atlantic salinity pile-up. Nat. Clim. Chang. (2020). https://doi.org/10.1038/s41558-020-0897-7

https://www.nature.com/articles/s41558-020-0897-7

Abstract
The Atlantic Meridional Overturning Circulation (AMOC) is an active component of the Earth’s climate system and its response to global warming is of critical importance to society. Climate models have shown an AMOC slowdown under anthropogenic warming since the industrial revolution, but this slowdown has been difficult to detect in the short observational record because of substantial interdecadal climate variability. This has led to the indirect detection of the slowdown from longer-term fingerprints such as the subpolar North Atlantic ‘warming hole’. However, these fingerprints, which exhibit some uncertainties, are all local indicators of AMOC slowdown around the subpolar North Atlantic. Here we show observational and modelling evidence of a remote indicator of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a ‘salinity pile-up’ remotely in the South Atlantic. This evidence is consistent with the AMOC slowdown under anthropogenic warming and, furthermore, suggests that this weakening has likely occurred all the way into the South Atlantic.

The linked reference discusses regional dynamic sea level simulations from CMIP5 and CMIP6; neither of which ensemble has interactive ice sheet modules, so I am presenting this as background information rather than as information relevant to a possible 'Ice Apocalypse'.

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Lyu et al. (2020) analyzes the regional dynamic sea levels simulated by both CMIP5 & CMIP6 (including the influences of the AMOC & see the three attached images) and discusses how future climate model projections (e.g. CMIP7) can be improved.

Lyu, K., Xuebin Zhang, and John A. Church (01 Aug 2020), "Regional Dynamic Sea Level Simulated in the CMIP5 and CMIP6 Models: Mean Biases, Future Projections, and Their Linkages", Journal of Climate, DOI: https://doi.org/10.1175/JCLI-D-19-1029.1

https://journals.ametsoc.org/view/journals/clim/33/15/JCLI-D-19-1029.1.xml

Abstract: "The ocean dynamic sea level (DSL) is an important component of regional sea level projections. In this study, we analyze mean states and future projections of the DSL from the global coupled climate models participating in phase 5 and phase 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively). Despite persistent biases relative to observations, both CMIP5 and CMIP6 simulate the mean sea level reasonably well. The equatorward bias of the Southern Hemisphere westerly wind stress is reduced from CMIP5 to CMIP6, which improves the simulated mean sea level in the Southern Ocean. The CMIP5 and CMIP6 DSL projections exhibit very similar features and intermodel uncertainties. With several models having a notably high climate sensitivity, CMIP6 projects larger DSL changes in the North Atlantic and Arctic associated with a larger weakening of the Atlantic meridional overturning circulation (AMOC). We further identify linkages between model mean states and future projections by looking for their intermodel relationships. The common cold-tongue bias leads to an underestimation of DSL rise in the western tropical Pacific. Models with their simulated midlatitude westerly winds located more equatorward tend to project larger DSL changes in the Southern Ocean and North Pacific. In contrast, a more equatorward location of the North Atlantic westerly winds or a weaker AMOC under current climatology is associated with a smaller weakening of the AMOC and weaker DSL changes in the North Atlantic and coastal Arctic. Our study provides useful emergent constraints for DSL projections and highlights the importance of reducing model mean-state biases for future projections."

Extract: "In addition, models analysed here still don’t have an interactive ice sheet  module and thus the DSL responses to the freshwater discharge from glaciers and ice sheets are not included, which is a gap potentially to be filled by making use of the simulations from the Ice Sheet Model Intercomparison Project (ISMIP6; Nowicki et al. 2016)."
 
Caption for first image: "Figure 1. (a) Observed mean ocean dynamic topography over 1992–2012; differences of the (b) CMIP6 and (c) CMIP5 multi-model averaged mean sea level over 1986–2005 from the observed mean dynamic topography; (d) differences between the mean sea level from CMIP6 and CMIP5. (e-h) similar as (a-d) but for mean sea surface zonal wind stress from QuikSCAT observations (1999–2009) and climate model simulations (1986–2005). Stippling in lower panels indicates where the difference between CMIP6 and CMIP5 is statistically significant at 886 the 95% confidence level based on the two-sample t-test."
 
Caption for the second image: "Figure 9. Regional DSL projections (m) under high-emission scenarios from four modelling groups. (Left) CMIP5 RCP8.5; (Right) CMIP6 SSP5-8.5. The equilibrium climate sensitivity (ECS, K) for each model is given after the model name in the subtitles."

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Holliday et al. (2020) discuss how ocean circulations caused the largest freshening event for the past 120 years in the eastern subpolar North Atlantic.

Holliday, N.P., Bersch, M., Berx, B. et al. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nat Commun 11, 585 (2020). https://doi.org/10.1038/s41467-020-14474-y

https://www.nature.com/articles/s41467-020-14474-y

Abstract
The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic.

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Rye et al. (2020) quantifies the influence of the observed Antarctic Melt Anomaly (AAMA) that was missing from all CMIP5 projections and most CMIP6 projections, see the three associated images; but which was largely accounted for Hansen et al. (2016):

Rye, C.D., J.Marshall, M. Kelley, G. Russell, L.S. Nazarenko, Y. Kostov, G.A. Schmidt, and J. Hansen, 2020: Antarctic Glacial Melt as a Driver of Recent Southern Ocean Climate Trends, Geophysical Research Letters 47, 11, doi:10.1029/2019GL086892.

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL086892

Abstract
Recent trends in Southern Ocean (SO) climate—of surface cooling, freshening, and sea ice expansion—are not captured in historical climate simulations. Here we demonstrate that the addition of a plausible increase in Antarctic meltwater to a coupled climate model can produce a closer match to a wide range of climate trends. We use an ensemble of simulations of the Goddard Institute for Space Studies Earth system model to compute “climate response functions” (CRFs) for the addition of meltwater. These imply a cooling and freshening of the SO, an expansion of sea ice, and an increase in steric height, all consistent with observations since 1992. The CRF framework allows one to compare the efficacy of Antarctic meltwater as a driver of SO climate trends, relative to greenhouse gas and surface wind forcing. The meltwater CRFs presented here strongly suggest that interactive Antarctic ice melt should be included in climate models.
Plain Language Summary
Climate models do not capture recent Southern Ocean (SO) climate trends of surface cooling, freshening, and sea ice expansion. Here we demonstrate that including a realistic increase in Antarctic meltwater can improve a model's representation of SO trends. We use an ensemble of simulations of the Goddard Institute for Space Studies Earth system model. Model results suggest that Antarctic meltwater drives a cooling and freshening of the SO and an expansion of winter sea ice, all consistent with observations. Results suggest that a better representation of Antarctic ice melt should be included in climate models.

Extract: "CMIP5 earth system models (ESM) do not explicitly represent the increase in Antarctic glacial melt (AAMA) over recent decades. The Antarctic grounded ice sheet mass loss has increased to perhaps 250 Gt/yr in 2017 (IMBIE 2018). The thinning and retreat of floating ice shelves is thought to have also contributed as much as 280 Gt/yr in recent years (2003-2015; Paolo 2015). Furthermore, a series of large ice-shelf retreats not included in the above estimates has contributed an additional flux of perhaps 210 Gt/yr over the period 1988 to 2008 (Shepherd et al., 2010).
…
Finally, Rye et al., (2014) highlights an anomalous trend in Antarctic Subpolar Sea Surface Height (SSH) and finds that an AAMA of around 430 Gt/yr is sufficient to drive a steric height increase consistent with observations."
 
Caption for the first image: "Figure 3| Modeled response to a 200 Gt yr-1 step change in AMMA. Decadal trends calculated over 30 year model runs from a 20-member ensemble in (a) SST, (b) SSS (c) zonal-average potential temperature (d) zonalaverage salinity (e) Interior temperature, averaged between 500 and 3000 m depth. (f) SSH. Red and Green contours denote the winter Sea Ice extent in the control run and after 30 years of perturbation experiment respectively."
 
Caption for the second image: "Figure 4| Linear convolution projections of Southern Ocean SST. ModelE Southern Ocean SST CRFs for: (a) 200 Gt/yr step change in AAMA, (b) a 1-standard deviation step-change in the Southern Annular mode (Doddridge 2019). Grey area: CMIP-5 multi-model spread. (c) Double CO2 forcing. In all plots: Grey lines: Individual ensemble members. Black lines: Ensemble means. Blue. Green and Red lines: Exponential fits. (d) Observed forcing histories. Red line: Green House Gases (GHG; Butler et al., 1999). Blue line: Combined AAMA (IMBIE 2018; Paolo et al., 2015). Green line: Southern Annular Mode (SAM; Marshall 2003). (e) The convolution of CRFs (a-c) with forcing histories (d). Red line: GHGs. Green line: SAM (wind). Blue line: AAMA. Black line: combined response. The purple dashed line and markers: observed Southern Ocean SST cooling (HadSST; Kennedy et al., 2019). (f) Summary of SST trends. Red bars: GHG. Green bars: SAM (Wind). Blue bar: AAMA. Grey bars: Combined forcing. For GHG, Wind and Total, the left-side bar shows convolution results for ModelE and the right-side bar shows results for the CMIP-5 multi-model mean derived from Kostov et al., (2018) and Doddridge et al., (2019). For AAMA and Total, the full bars denote convolutions for the time histories of grounded Antarctic mass loss anomaly; the shaded bars denote convolutions for the combined time history of grounded ice and floating ice shelves mass loss anomaly. The purple line: observed Southern Ocean cooling. Black whiskers: standard deviation."
 
Caption for the third image: "Figure 5| Southern Ocean Climate Response Functions. ModelE CRFs for a 200 Gt/yr step change in Antarctic glacial melt. Grey lines: individual ensemble members. Black line: Ensemble mean. Red lines: Exponential or linear fit to ensemble mean. (a) Sea Surface Salinity averaged over 55 to 70 S. (b) Winter Sea Ice Extent. (c) Antarctic Subpolar Sea Ice Surface Height averaged between the continent and 70 S. (d-f) convolutions of CRF’s a-c with AAMA forcing shown in Figure 4d. d. Response of Southern Ocean SSS. e. Response of Southern Ocean SIE f. Response of Antarctic Subpolar Sea SSH."

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Stephen et al (2017) provides observations that cloud characteristics are trending toward increasingly net positive cloud feedback in the future.

Stephens, G. et. al. (2017), "CloudSat and CALIPSO within the A-Train: Ten years of actively observing the Earth system", BAMS, https://doi.org/10.1175/BAMS-D-16-0324.1

http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-16-0324.1?utm_content=bufferebbb9&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer
or
http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-16-0324.1

Abstract: "The more than 10 years of observations jointly collected by CloudSat and CALIPSO satellites has resulted in new ways of looking at aerosol, clouds, and precipitation and new discoveries about processes that connect them.

One of the most successful demonstrations of an integrated approach to observe Earth from multiple perspectives is the A-Train satellite constellation (e.g. Stephens et al., 2002). The science enabled by this constellation flourished with the introduction of the two active sensors carried by the NASA CloudSat and the NASA/CNES Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites that were launched together on April 28th, 2006. These two missions have provided a 10-year demonstration of coordinated formation flying that made it possible to develop integrated products and that offered new insights on key atmospheric processes. The progress achieved over this decade of observations, summarized in this paper, clearly demonstrate the fundamental importance of the vertical structure of clouds and aerosol for understanding the influences of the larger scale atmospheric circulation on aerosol, the hydrological cycle, the cloud-scale physics and on the formation of the major storm systems of Earth. The research also underscored inherent ambiguities in radiance data in describing cloud properties and how these active systems have greatly enhanced passive observation. It is now clear that monitoring the vertical structure of clouds and aerosol is essential and a climate data record is now being constructed. These pioneering efforts are to be continued with EarthCARE mission planned for launch in 2019."

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Chang et al. (2020) discusses the findings of high-resolution ESM projections including the finding that"

"An emerging prominent feature of the high‐resolution pre‐industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation."

Such interactions between Antarctic sea ice patterns and ENSO patterns are largely missing from CMIP5 & CMIP6 projections; but could have significant impacts on the effective climate sensitivity in coming decades.

Chang, P. et al. (18 November 2020), "An Unprecedented Set of High‐Resolution Earth System Simulations for Understanding Multiscale Interactions in Climate Variability and Change", JAMES, https://doi.org/10.1029/2020MS002298

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020MS002298

Abstract
We present an unprecedented set of high‐resolution climate simulations, consisting of a 500‐year pre‐industrial control simulation and a 250‐year historical and future climate simulation from 1850 to 2100. A high‐resolution configuration of the Community Earth System Model version 1.3 (CESM1.3) is used for the simulations with a nominal horizontal resolution of 0.25° for the atmosphere and land models and 0.1° for the ocean and sea‐ice models. At these resolutions, the model permits tropical cyclones and ocean mesoscale eddies, allowing interactions between these synoptic and mesoscale phenomena with large‐scale circulations. An overview of the results from these simulations is provided with a focus on model drift, mean climate, internal modes of variability, representation of the historical and future climates, and extreme events. Comparisons are made to solutions from an identical set of simulations using the standard resolution (nominal 1°) CESM1.3 and to available observations for the historical period to address some key scientific questions concerning the impact and benefit of increasing model horizontal resolution in climate simulations. An emerging prominent feature of the high‐resolution pre‐industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation. Overall, high‐resolution simulations show significant improvements in representing global mean temperature changes, seasonal cycle of sea‐surface temperature and mixed layer depth, extreme events and in relationships between extreme events and climate modes.

Plain Language Summary
Although the current generation of climate models has demonstrated high fidelity in simulating and projecting global temperature change, these models show large uncertainties when it comes to questions concerning how rising global temperatures will impact local weather conditions. This is because the resolution (~100 km) at which the majority of climate models simulate the climate is not fine enough to resolve these small‐scale regional features. Conducting long‐term (multi‐centuries) high‐resolution (~10 km) climate simulations has been a great challenge for the research community due to the extremely high computational demands. Through international collaboration, we are able to address this challenge by delivering an unprecedented set of multi‐century high‐resolution climate simulations using the Community Earth System Model (CESM), capable of directly representing tropical cyclones and extreme rainfall events. In this paper, we give an overall assessment of the value and benefit of the high‐resolution CESM climate simulations by making a direct comparison to an identical set of low‐resolution CESM simulations. We showcase some of the major improvements of the high‐resolution CESM in simulating global mean temperature changes, seasonal cycle of sea‐surface temperature, and extreme events, such as tropical cyclones and relationships between tropical cyclones and El Niño‐Southern Oscillation.

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Wood et al. (2020) provides discussion of the role of SST patterns in the SH on circumpolar atmospheric jet stream response to GHG forcing.  Better modeling of this role in CMIP7 models would be important to better determine the amounts and patterns of future upwelling of warm CDW along the Antarctic coastline.

Wood T. et al (23 December 2020), "Role of sea surface temperature patterns for the Southern Hemisphere jet stream response to CO2 forcing", Environmental Research Letters, Volume 16, Number 1, https://doi.org/10.1088/1748-9326/abce27

https://iopscience.iop.org/article/10.1088/1748-9326/abce27

Abstract
The Southern Hemisphere (SH) eddy-driven jet stream has been shown to move poleward in climate models in response to greenhouse gas forcing, but the magnitude of the shift is uncertain. Here we address the fact that the latest Coupled Model Intercomparison Project phase 6 (CMIP6) models simulate, on average, a smaller jet shift in response to an abrupt quadrupling in CO2 than the predecessor models (Coupled Model Intercomparison Project phase 5 (CMIP5)), despite producing larger global average surface warming. We focus on the response in the first decade when the majority of the long-term jet shift occurs and when the difference between CMIP5 and CMIP6 models emerges. We hypothesise the smaller poleward jet shift is related to the weaker increase in the meridional sea surface temperature (SST) gradient across the southern extratropics in CMIP6 models. We impose the multi-model mean SST patterns alongside a quadrupling in CO2 in an intermediate complexity general circulation model (IGCM4) and show that many of the regional and seasonal differences in lower tropospheric zonal winds between CMIP5 and CMIP6 models are reproduced by prescribing the SST patterns. The main exception is in austral summer when the imposed SST patterns and CO2 increase in IGCM4 produce weaker differences in zonal wind response compared to those simulated by CMIP5/6 models. Further IGCM4 experiments that prescribe only SH extratropical SSTs simulate a weaker jet shift for CMIP6 SSTs than for CMIP5, comparable to the full experiment. The results demonstrate that SH SST patterns are an important source of uncertainty for the shift of the midlatitude circulation in response to CO2 forcing. The study also provides an alternative explanation than was proposed by Curtis et al (2020 Environ. Res. Lett. 15 64011), who suggested model improvements in jet biases could account for the smaller jet shift in CMIP6 models in the extended austral winter season.

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Sommer et al (2020) discuss observed accelerated glacier mass loss in the Russian Arctic from 2010 to 2017, and I note that such ice mass loss can contribute to the advection of relatively warm North Atlantic water deeper into the Arctic Ocean.

Sommer, C., Seehaus, T., Glazovsky, A., and Braun, M. H.: Brief communication: Accelerated glacier mass loss in the Russian Arctic (2010–2017), The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2020-358, in review, 2020.

https://tc.copernicus.org/preprints/tc-2020-358/

Abstract. Glaciers in the Russian High Arctic have been subject to extensive warming due to global climate change, yet their contribution to sea level rise has been relatively small over the past decades. Here we show surface elevation change measurements and geodetic mass balances of 93 % of all glacierized areas of Novaya Zemlya, Severnaya Zemlya and Franz Josef Land using interferometric synthetic aperture radar measurements taken between 2010 and 2017. We calculate an overall mass loss 10 rate of −23 ± 5 Gt a−1, corresponding to a sea level rise contribution of 0.06 ± 0.01 mm a−1. Compared to measurements prior to 2010, mass loss of glaciers on the Russian archipelagos has doubled in recent years.

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Zhou et al. (2021) indicates that consideration of the pattern effect leads to higher committed warming for our current anthropogenic radiative forcing; which can be thought of as implying a current higher effective climate sensitivity (if one chooses to ignore the pattern effect), particularly with regard to spatial inhomogeneities in both sea surface temperature (SST) and sea ice change.  Here, I note that the authors do not consider changes in effective radiative forcing associated with abrupt freshwater hosing events.

Zhou, C., Zelinka, M.D., Dessler, A.E. et al. Greater committed warming after accounting for the pattern effect. Nat. Clim. Chang. (2021). https://doi.org/10.1038/s41558-020-00955-x

https://www.nature.com/articles/s41558-020-00955-x

Abstract
Our planet’s energy balance is sensitive to spatial inhomogeneities in sea surface temperature and sea ice changes, but this is typically ignored in climate projections. Here, we show the energy budget during recent decades can be closed by combining changes in effective radiative forcing, linear radiative damping and this pattern effect. The pattern effect is of comparable magnitude but opposite sign to Earth’s net energy imbalance in the 2000s, indicating its importance when predicting the future climate on the basis of observations. After the pattern effect is accounted for, the best-estimate value of committed global warming at present-day forcing rises from 1.31 K (0.99–2.33 K, 5th–95th percentile) to over 2 K, and committed warming in 2100 with constant long-lived forcing increases from 1.32 K (0.94–2.03 K) to over 1.5 K, although the magnitude is sensitive to sea surface temperature dataset. Further constraints on the pattern effect are needed to reduce climate projection uncertainty.

Extract: "Properly accounting for the pattern effect has a major impact on the amount of carbon humans can emit before breaching any particular temperature threshold."
 
Caption for the first image: "Fig. 1 | Attribution of the net TOA fluxes during 1871–2010. a, Time series of effective radiative forcing from IPCC AR5 (red), the linear radiative damping term (−λltΔT, green) and the pattern effect term (blue). b, Time series of reconstructed TOA fluxes. The black line denotes the TOA fluxes reconstructed with equation (4) and the brown line denotes the TOA net flux estimated by ignoring the pattern effect (F – λltΔT). c, Comparison of reconstructed TOA fluxes with observations. The magenta line denotes observed annual TOA net flux from CERES EBAF v.4.0 (ref. 21) and the cyan line denotes net flux observations from merged radiation budget data v.3 (ref. 22), which is calculated from CERES EBAF v.2.8 (ref. 42) and ERBS wide field of view v.3 data43. Thin lines denote values calculated from individual models, while thick lines are model averages."
 
Caption for the second image: "Fig. 2 | Cumulative energy flux into the Earth system during 1961–2010. The magenta and cyan lines are two estimates of the observed changes in the global heat content24,25 (Methods), the black thick line is the net influx calculated from the reconstructed net energy imbalance of Fig. 1 and the brown thick line denotes net energy influx if the pattern effect is zero. The red, green and blue dashed lines denote individual contributors to the net energy influx. Thin lines denote values calculated from individual models, while thick lines are model averages."
 
Caption for the third image: "Fig. 3 | Impact of the pattern effect on equilibrium committed warming with constant forcing. Colours denote the committed warming for a range of Pref and λlt values calculated with equation (9), and the two white contours represent the Paris Agreement thresholds. The black line denotes the relationship of Pref and λlt constrained by equation (. Values printed beside the two black markers denote the committed warming corresponding to Pref = 0 and Pref = −0.63 W m–2, respectively."
 
Caption for the fourth image: "Fig. 4 | Comparison of TOA fluxes reconstructed with CAM5.3 experiments driven by different SST datasets. The TOA fluxes are reconstructed with equation (1), where the climate forcing is from IPCC AR5 and Rfb is from simulations. The black line is the ensemble mean value calculated from three CAM5.3 AMIP-piForcing experiments, which use prescribed AMIPII SST boundary conditions. The blue line is reconstructed with the ensemble mean value of three CAM5.3 HadISST-piForcing experiments, which use prescribed HadISST SST boundary conditions. The correlation coefficients between the time series of observations and reconstructed TOA fluxes, of which the corresponding P values are all below 0.05, are listed in the figure."

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Dessler reminds us that consensus climate scientists adhere to strict definitions (of which there are many) of ECS all of which exclude the influence of the pattern effect (while I include the influence of the pattern effect within my estimates of ECSeff) as indicated by image 1.  The first linked article indicates that while using a mean consensus climate science value of about 3.1C together with the pattern effect, that we have already committed to a GMSTA value of well over 2C (to about 2.8C) for our current GHG concentrations unless geoengineering, or negative emission, measures are engaged (I note that the MCDS-BN assumes that a global war circa 2050-2060 would render any geoengineering, or negative emission, measures ineffective, and the second linked article provides other arguments against relying on either of these two conceptual measures).

Title: "We've already blown past the warming targets set by the Paris climate agreement, study finds"

https://www.livescience.com/already-too-late-to-meet-paris-agreement-climate-goals.html

Extract: "Dessler told the AP. "It's really the rate of warming that makes climate change so terrible. If we got a few degrees over 100,000 years, that would not be that big a deal. We can deal with that. But a few degrees over 100 years is really bad.""

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Title: "A dangerous distraction: Increasing climate risk with solar geoengineering"

https://thehill.com/opinion/energy-environment/559329-a-dangerous-distraction-increasing-climate-risk-with-solar



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Furthermore, the third linked article provides more background discussion about the relationship between the Earth's Energy Imbalance (EEI), see the second image, and consensus climate science's definition of ECS.

Title: "Measuring Earth's energy imbalance"

https://skepticalscience.com/Measuring-Earths-energy-imbalance.html

Extract: "Climate sensitivity is an expression of how much global temperature changes for a given radiative forcing. The general consensus of peer reviewed estimates of climate sensitivity (both modelled and empirically determined) tend to cluster around a global warming of 3 ± 1°C for doubled CO2.

As the ERBE and CERES satellites measure the net energy imbalance, this data can be combined with temperature records to place constraints on climate sensitivity. Because the ERBE satellite record covers only 15 years, it doesn't encompass slower feedback processes such as receding Arctic sea ice. Hence the data provides only a weak upper bound of climate sensitivity (a maximum of around 10°C warming for doubled CO2). However, the analysis rules out climate sensitivities lower than 2°C. This finding is consistent with the general consensus estimate of climate sensitivity (in addition, the author Dan Murphy informs me he's currently doing follow-up work to calculate a more precise lower bound)."
 

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With regard to a coming tipping point in carbon storage in tundra regions invaded by shrubs, Clemmensen et al (2021) conclude that:

"Advancement of the forest boundary would promote an ectomycorrhizal fungal community with the capacity to release the large organic N reserves and favour tree growth, potentially leading to a strong, positive plant–soil feedback that accelerates tree expansion into former tundra and underpins a positive feedback of tundra to climate warming."

This implies that consensus climate science projections of carbon emissions from tundra regions err on the side of least drama:

Clemmensen, K.E. et al. (22 March 2021), "A tipping point in carbon storage when forest expands into tundra is related to mycorrhizal recycling of nitrogen", Ecology Letters, https://doi.org/10.1111/ele.13735

https://onlinelibrary.wiley.com/doi/10.1111/ele.13735?af=R

Abstract
Tundra ecosystems are global belowground sinks for atmospheric CO2. Ongoing warming‐induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long‐term carbon (C) and nitrogen (N) stocks, we studied small‐scale soil depth profiles of fungal communities and C–N dynamics across a subarctic‐alpine forest‐heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree‐associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic‐alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

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Zhou et al. (2021) provides more attribution insights into methane emissions from 2010 to 2018 than I am prepared to cover in this post, so I will only note that the third image indicates that prior atmosphere methane concentration calculations assumed that methane was being oxidized at a higher rate than was actually occurring due the observed trend of decreasing OH concentration in the atmosphere (which indicating the prospect of higher climate sensitivity in the future). While the fourth image shows that global methane emissions from livestock, rice and wastewater have all been previously underestimated; likely due to a growing trend toward richer diets in developing countries (a trend that is likely to continue for some decades to come).

Zhang, Y., Jacob, D. J., Lu, X., Maasakkers, J. D., Scarpelli, T. R., Sheng, J.-X., Shen, L., Qu, Z., Sulprizio, M. P., Chang, J., Bloom, A. A., Ma, S., Worden, J., Parker, R. J., and Boesch, H.: Attribution of the accelerating increase in atmospheric methane during 2010–2018 by inverse analysis of GOSAT observations, Atmos. Chem. Phys., 21, 3643–3666, https://doi.org/10.5194/acp-21-3643-2021, 2021.

https://acp.copernicus.org/articles/21/3643/2021/acp-21-3643-2021.html

Abstract: "We conduct a global inverse analysis of 2010–2018 GOSAT observations to better understand the factors controlling atmospheric methane and its accelerating increase over the 2010–2018 period. The inversion optimizes anthropogenic methane emissions and their 2010–2018 trends on a 4∘×5∘ grid, monthly regional wetland emissions, and annual hemispheric concentrations of tropospheric OH (the main sink of methane). We use an analytical solution to the Bayesian optimization problem that provides closed-form estimates of error covariances and information content for the solution. We verify our inversion results with independent methane observations from the TCCON and NOAA networks. Our inversion successfully reproduces the interannual variability of the methane growth rate inferred from NOAA background sites. We find that prior estimates of fuel-related emissions reported by individual countries to the United Nations are too high for China (coal) and Russia (oil and gas) and too low for Venezuela (oil and gas) and the US (oil and gas). We show large 2010–2018 increases in anthropogenic methane emissions over South Asia, tropical Africa, and Brazil, coincident with rapidly growing livestock populations in these regions. We do not find a significant trend in anthropogenic emissions over regions with high rates of production or use of fossil methane, including the US, Russia, and Europe. Our results indicate that the peak methane growth rates in 2014–2015 are driven by low OH concentrations (2014) and high fire emissions (2015), while strong emissions from tropical (Amazon and tropical Africa) and boreal (Eurasia) wetlands combined with increasing anthropogenic emissions drive high growth rates in 2016–2018. Our best estimate is that OH did not contribute significantly to the 2010–2018 methane trend other than the 2014 spike, though error correlation with global anthropogenic emissions limits confidence in this result."

Caption for third image: "Figure 13 Methane loss frequency and lifetime against oxidation by tropospheric OH for 2010–2018. Values are annual means with error standard deviations. The loss frequency (k) is as calculated by Eq. (1) and the lifetime (τ) is the inverse. The prior estimate from Wecht et al. (2014) assumes no 2010–2018 trend in OH concentrations; the slight variability seen in the figure is due to temperature."

 
Caption for fourth image: "Figure 15 The 2010–2018 global methane anthropogenic emissions and emission trends partitioned by individual sectors. Posterior estimates are from our inversion of GOSAT data. Prior estimates for anthropogenic emission trends are zero. Error bars in (b) show posterior error standard deviations for emission trends. Posterior error standard deviations for mean emissions are small and are thus not shown in (a)."

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Freshwater Flux Breakdown (including a breakdown for change in SLR) for FT Domino Analysis in the BN Model/Network

The first two images summarize the estimated findings of the freshwater flux FT domino analysis/assessment from 2020 thru 2060.  I note that the estimated change in sea level rise shown on the MCDS-BN diagram of 5m by 2060 actual covers a period thru 2065 and includes allowances for snow melt and permafrost degradation; and also I note that after 2060 the majority of the increase in sea level rise up to 6m by 2150 is assumed to come from both surface melting of the GrIS and from MISI ice mass loss from marine and marine-terminating glaciers.

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The third image comes from AR5-Chapter 13, and show both the catchment basins for key GrIS marine-terminating glaciers and consensus climate science estimates of the contributions from this basins thru 2100 (which the MCDS-BN assumes that these consensus climate science values ESLD).  The fourth image (also from AR5-Chapter 4) summarizes some useful current parameters for ice/snow both on land and in the ocean.

WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT REPORT CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS - Chapter 13 – Sea Level Change

http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter13.pdf


WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT REPORT CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS - Chapter 4: Observations: Cryosphere, 2013

http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter04.pdf

 
Caption for the third image: "FAQ 13.2, Figure 1: Illustrative synthesis of projected changes in SMB and outflow by 2100 for (a) Greenland and (b) Antarctic ice sheets. Colours shown on the maps refer to projected SMB change between 1990–2009 and 2079–2098 using the RACMO2 regional climate model under scenarios A1B (Antarctic) and RCP4.5 (Greenland) AOGCMs. For Greenland, average equilibrium line altitudes during both these time periods are shown in purple and green, respectively. Ice perimeter and grounding lines are shown as black lines, as are ice sheet sectors. For Greenland, results of flow-line modelling for four major outlet glaciers are shown as inserts, while for Antarctica the coloured rings reflect projected change in outflow on the basis of a statistical extrapolation of observed trends. The outer and inner radius of each ring indicate the 17th and 83rd percentiles, respectively (scale in upper right); red refers to mass loss (sea level rise) while blue refers to mass gain (sea level fall). Finally, total sea level contribution is shown for each ice sheet (insert located above maps) with light grey referring to SMB (model experiment used to generate the SMB map is shown as a dashed line) and dark grey to outflow. All projections refer to the 17–83% probability range across all scenarios. Data provided by F. M. Nick, C. M. Little, J. H. van Angelen, S. R. M. Ligtenberg, J. T. M. Lenaerts and M. R. van den Broeke."

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Rignot et al. (2021) indicates that two thirds of the ice mass loss from the Humboldt Gletscher glacier in North Greenland is due to undercutting of grounded ice by warm ocean currents, '… which is a physical process not included in most ice sheet models.'  This indicates that most consensus science projections of ice mass loss from the GrIS (see the first image) with continued global warming err on the side of least drama.

Rignot, E. et al. (10 March 2021), "Retreat of Humboldt Gletscher, North Greenland, driven by undercutting from a warmer ocean", Geophysical Research Letters, https://doi.org/10.1029/2020GL091342

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL091342?af=R

Abstract
Humboldt Gletscher is a 100‐km wide, slow‐moving glacier in north Greenland which holds a 19‐cm global sea level equivalent. Humboldt has been the fourth largest contributor to sea level rise since 1972 but the cause of its mass loss has not been elucidated. Multi‐beam echo sounding data collected in 2019 indicate a seabed 200 m deeper than previously known. Conductivity temperature depth (CTD) data reveal the presence of warm water of Atlantic origin at 0°C at the glacier front and a warming of the ocean waters by 0.9 ± 0.1°C since 1962. Using an ocean model, we reconstruct grounded ice undercutting by the ocean, combine it with calculated retreat caused by ice thinning to floatation, and are able to fully explain the observed retreat. Two thirds of the retreat is caused by undercutting of grounded ice, which is a physical process not included in most ice sheet models.

Plain Language Summary
Humboldt Gletscher is the widest glacier in Greenland, slow moving, and terminating in shallow waters in its southern half, but grounded 200 m deeper than previously known in its northern half, with a submarine trough extending more than 100 km inland. The glacier has been retreating at 0.6 km/yr and contributing significantly to sea level rise. We attribute the retreat to undercutting of grounded ice by warmer ocean waters combined with a retreat caused by ice thinning to floatation sooner due to glacier speed up. The glacier, which hosts an ice volume equivalent to a 19‐cm global sea level, will remain a major contributor to sea level rise this Century.

 
Image 1 from AR5

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The second image shows the observed image mass loss from Antarctic ice shelves between 2003 and 2008; which reminds us both that the introduction of freshwater fluxes into the Southern Ocean has been on-going for many decades already, and that not all freshwater fluxes contribute to sea level rise but that they do contribute to ice-climate feedback mechanisms like the observed slowing of the MOC.

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The third and fourth images (for the AIS and Thwaites Catchment bedmap elevations); which illustrate how much glacier ice currently exists below sea level and thus are susceptible to dynamic ice mass loss, potentially from either MISI and/or MICI types of ice mass loss modes, even without surface ice mass loss from melting.

Caption for the third and fourth images: Thwaites Glacier, West Antarctica. (A) The bed elevation of the Thwaites Glacier catchment (black square) in the context of the marine West Antarctic Ice Sheet. (B) The bed elevation for only the Thwaites Glacier catchment (gray boundary).

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Schild et al (2021) provides measurements of iceberg melt rates in the subpolar North Atlantic in order to gain a better understanding of how the associated freshwater fluxes impact regional ocean current circulation patterns (which can also impact global ocean circulation patterns).

Schild, K.M. et al. (20 January 2021), "Measurements of iceberg melt rates using high‐resolution GPS and iceberg surface scans", Geophysical Research Letters, https://doi.org/10.1029/2020GL089765

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089765

Abstract
Increasing freshwater input to the subpolar North Atlantic through iceberg melting can influence fjord‐scale to basin‐scale ocean circulation. However, the magnitude, timing, and distribution of this freshwater have been challenging to quantify due to minimal direct observations of subsurface iceberg geometry and melt rates. Here we present novel in situ methods capturing iceberg change at high‐temporal and ‐spatial resolution using four high‐precision GPS units deployed on two large icebergs (>500 m length). In combination with measurements of surface and subsurface geometry, we calculate iceberg melt rates between 0.10–0.27 m/d over the 9‐day survey. These melt rates are lower than those proposed in previous studies, likely due to using individual subsurface iceberg geometries in calculations. In combining these new measurements of iceberg geometry and melt rate with the broad spatial coverage of remote sensing, we can better predict the impact of increasing freshwater injection from the Greenland Ice Sheet.

Plain Language Summary
The acceleration of Greenland glaciers has led to an increase of icebergs discharged in nearby waters. As icebergs melt, they release freshwater into salty ocean waters, impacting local circulation. In order to understand how global circulation will change in the future, we need accurate iceberg melt rates. To do this, we use measurements of mass loss from on‐iceberg GPS units, and 3D iceberg geometry constructed from aerial drone and subsurface sonar data. We found melt rates smaller than previous studies and strong evidence for variable overall melt rates with different keel depths and over time. This study is the first of its kind to calculate melt rates using exact iceberg geometry. To better predict iceberg impacts, future iceberg studies should take these geometry results into account.

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As dropstones from ice rafted debris are associated with iceberg calving due to ice cliff failures, the linked reference (and associated YouTube video) indicates that icebergs from ice cliff failures were occurring in the WAIS during the Pliocene and that there likely was a seaway connecting the Amundsen Sea to the Weddell Sea.  This suggest that the WAIS is less stable than many consensus climate scientists assume.

Siddoway, C., Thomson, S., Hemming, S., Buchband, H., Quigley, C., Furlong, H., Hilderman, R., Robinson, D., Watkins, C., Cox, S., and Licht, K. and the IODP Expedition 379 Scientists and Expedition 382 Scientists: U-Pb zircon geochronology of dropstones and IRD in the Amundsen Sea, applied to the question of bedrock provenance and Miocene-Pliocene ice sheet extent in West Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9151, https://doi.org/10.5194/egusphere-egu21-9151, 2021.

https://meetingorganizer.copernicus.org/EGU21/EGU21-9151.html

Summary: "IODP Expedition 379 to the Amundsen Sea continental rise recovered latest Miocene-Holocene sediments from two sites on a drift in water depths >3900m. Sediments are dominated by clay and silty clay with coarser-grained intervals and ice-rafted detritus (IRD) (Gohl et al. 2021, doi:10.14379/iodp.proc.379.2021). Cobble-sized dropstones appear as fall-in, in cores recovered from sediments >5.3 Ma.  We consider that abundant IRD and the sparse dropstones melted out of icebergs formed due to Antarctic ice-sheet calving events. We are using petrological and age characteristics of the clasts from the Exp379 sites to fingerprint their bedrock provenance. The results may aid in reconstruction of past changes in icesheet extent and extend knowledge of subglacial bedrock.
Mapped onshore geology shows pronounced distinctions in bedrock age between tectonic provinces of West or East Antarctica (e.g. Cox et al. 2020, doi:10.21420/7SH7-6K05; Jordan et al. 2020, doi.org/10.1038/s43017-019-0013-6). This allows us to use geochronology and thermochronology of rock clasts and minerals for tracing their provenance, and ascertain whether IRD deposited at IODP379 drillsites originated from proximal or distal Antarctic sources. We here report zircon and apatite U-Pb dates from four sand samples and five dropstones taken from latest Miocene, early Pliocene, and Plio-Pleistocene-boundary sediments. Additional Hf isotope data, and apatite fission track and 40Ar/39Ar Kfeldspar ages for some of the same samples help to strengthen provenance interpretations.
The study revealed three distinct zircon age populations at ca. 100, 175, and 250 Ma. Using Kolmogorov-Smirnov (K-S) statistical tests to compare our new igneous and detrital zircon (DZ) U-Pb results with previously published data, we found strong similarities to West Antarctic bedrock, but low correspondence to prospective sources in East Antarctica, implying a role for icebergs calved from the West Antarctic Ice Sheet (WAIS). The ~100 Ma age resembles plutonic ages from Marie Byrd Land and islands in Pine Island Bay.  The ~250 and 175 Ma populations match published mineral dates from shelf sediments in the eastern Amundsen Sea Embayment as well as granite ages from the Antarctic Peninsula and the Ellsworth-Whitmore Mountains (EWM). The different derivation of coarse sediment sources requires changes in iceberg origin through the latest Miocene, early Pliocene, and Plio/Pleistocene, likely the result of changes in WAIS extent.
One unique dropstone recovered from Exp379 Site U1533B is green quartz arenite, which yielded mostly 500-625 Ma detrital zircons. In visual appearance and dominant U-Pb age population, it resembles a sandstone dropstone recovered from Exp382 Site U1536 in the Scotia Sea (Hemming et al. 2020, https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/357276). K-S tests yield high values (P ≥ 0.6), suggesting a common provenance for both dropstones recovered from late Miocene to Pliocene sediments, despite the 3270 km distance separating the sites. Comparisons to published data, in progress, narrow the group of potential on-land sources to exposures in the EWM or isolated ranges at far south latitudes in the Antarctic interior.  If both dropstones originated from the same source area, they could signify dramatic shifts in the WAIS grounding line position, and the possibility of the periodic opening of a seaway connecting the Amundsen and Weddell Seas."

See also the first presentation in the linked video:
Title: " #vEGU21 - Press Conference 2: Scientific sleuthing: Geoforensics and fingerprinting"



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Per Felikson et al. (2020), the current dynamical ice mass loss from the Jakobshavn Glacier is about 25Gt/year and as 100 Gt of ice mass loss ~ 0.28mm of eustatic SLR, this corresponds to about 0.07mm of eustatic SLR/year.  However, in 2012 7.4 cubic km (6.78 Gt) of ice calved in 75-minutes.

Felikson, D., Ginny Catania, Timothy C. Bartholomaus, Mathieu Morlighem and Brice P. Y. Noël (11 December 2020), “Steep glacier bed knickpoints mitigate inland thinning in Greenland”, Geophysical Research Letters, DOI: 10.1029/2020GL090112.

For those who are interested in better understanding the consensus climate science estimates of sea-level rise contributions from the GrIS this century, I provide the following information from Goelzer et al (2020) and see the associated first image.

Goelzer, H., Nowicki, S., Payne, A., Larour, E., Seroussi, H., Lipscomb, W. H., Gregory, J., Abe-Ouchi, A., Shepherd, A., Simon, E., Agosta, C., Alexander, P., Aschwanden, A., Barthel, A., Calov, R., Chambers, C., Choi, Y., Cuzzone, J., Dumas, C., Edwards, T., Felikson, D., Fettweis, X., Golledge, N. R., Greve, R., Humbert, A., Huybrechts, P., Le clec'h, S., Lee, V., Leguy, G., Little, C., Lowry, D. P., Morlighem, M., Nias, I., Quiquet, A., Rückamp, M., Schlegel, N.-J., Slater, D. A., Smith, R. S., Straneo, F., Tarasov, L., van de Wal, R., and van den Broeke, M.: The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6, The Cryosphere, 14, 3071–3096, https://doi.org/10.5194/tc-14-3071-2020, 2020.

https://tc.copernicus.org/articles/14/3071/2020/

Abstract
The Greenland ice sheet is one of the largest contributors to global mean sea-level rise today and is expected to continue to lose mass as the Arctic continues to warm. The two predominant mass loss mechanisms are increased surface meltwater run-off and mass loss associated with the retreat of marine-terminating outlet glaciers. In this paper we use a large ensemble of Greenland ice sheet models forced by output from a representative subset of the Coupled Model Intercomparison Project (CMIP5) global climate models to project ice sheet changes and sea-level rise contributions over the 21st century. The simulations are part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We estimate the sea-level contribution together with uncertainties due to future climate forcing, ice sheet model formulations and ocean forcing for the two greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results indicate that the Greenland ice sheet will continue to lose mass in both scenarios until 2100, with contributions of 90±50 and 32±17 mm to sea-level rise for RCP8.5 and RCP2.6, respectively. The largest mass loss is expected from the south-west of Greenland, which is governed by surface mass balance changes, continuing what is already observed today. Because the contributions are calculated against an unforced control experiment, these numbers do not include any committed mass loss, i.e. mass loss that would occur over the coming century if the climate forcing remained constant. Under RCP8.5 forcing, ice sheet model uncertainty explains an ensemble spread of 40 mm, while climate model uncertainty and ocean forcing uncertainty account for a spread of 36 and 19 mm, respectively. Apart from those formally derived uncertainty ranges, the largest gap in our knowledge is about the physical understanding and implementation of the calving process, i.e. the interaction of the ice sheet with the ocean.
 
Caption for the first and second images: "Figure 7. Ensemble sea-level projections. (a) ISM ensemble mean projections for the core experiments (solid) and extended experiments (dashed). The background shading gives the model spread for the two MIROC5 scenarios and is omitted for the other GCMs for clarity but indicated by the bars on the right-hand side. (b) Model specific results for MIROC5-RCP8.5. The colour scheme is the same as in previous figures. The dashed line is the result of applying the atmosphere and ocean forcing to the present-day ice sheet without any dynamical response (NOISM)."

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For those who are interested in better understanding the consensus climate science estimates of sea-level rise contributions from the AIS this century, I provide the following information from Seroussi et al (2020) and see the associated second and third images.

Seroussi, H., Nowicki, S., Payne, A. J., Goelzer, H., Lipscomb, W. H., Abe-Ouchi, A., Agosta, C., Albrecht, T., Asay-Davis, X., Barthel, A., Calov, R., Cullather, R., Dumas, C., Galton-Fenzi, B. K., Gladstone, R., Golledge, N. R., Gregory, J. M., Greve, R., Hattermann, T., Hoffman, M. J., Humbert, A., Huybrechts, P., Jourdain, N. C., Kleiner, T., Larour, E., Leguy, G. R., Lowry, D. P., Little, C. M., Morlighem, M., Pattyn, F., Pelle, T., Price, S. F., Quiquet, A., Reese, R., Schlegel, N.-J., Shepherd, A., Simon, E., Smith, R. S., Straneo, F., Sun, S., Trusel, L. D., Van Breedam, J., van de Wal, R. S. W., Winkelmann, R., Zhao, C., Zhang, T., and Zwinger, T.: ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century, The Cryosphere, 14, 3033–3070, https://doi.org/10.5194/tc-14-3033-2020, 2020.

https://tc.copernicus.org/articles/14/3033/2020/

Abstract
Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and assess the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimates of the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes, forcings employed and initial states of ice sheet models. This study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015–2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between −7.8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between −6.1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28 mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica.
 
Caption for the third and fourth images: "Figure 9. Impact of RCP scenario on projected evolution of ice volume above floatation for the NorESM1-M (a) and IPSL (b) models. Red and blue curves show mean evolution for RCP 8.5 and RCP 2.6, respectively, and the shaded background shows the standard deviation."

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Kopec et al. (2020) indicate that sublimation will play an increasing important role in the GrIS surface mass balance; which will accelerate future ice mass loss from the GrIS.

Kopec, B. G., Akers, P. D., Klein, E. S., and Welker, J. M.: Significant water vapor fluxes from the Greenland Ice Sheet detected through water vapor isotopic (δ18O, δD, deuterium excess) measurements, The Cryosphere Discuss., https://doi.org/10.5194/tc-2020-276, in review, 2020.

https://tc.copernicus.org/preprints/tc-2020-276/

Abstract. The summer of 2019 was marked by an extensive early onset of surface melt and record volume losses of the Greenland Ice Sheet (GrIS), which is part of a larger trend of increasing melt over time. Given the growing spatial extent of melt, the flux of water vapor from the ice to the atmosphere is becoming an increasingly important component of the GrIS mass balance that merits investigation and quantification. We examine the isotopic composition of water vapor from Thule Air Base, NW Greenland, particularly the deuterium excess (d-excess), to quantify the magnitude of GrIS vapor fluxes. To do this, we observe only water vapor transported off the ice sheet (i.e., when easterly winds occur) and during the active melt season. We find that the GrIS-derived water vapor d-excess values are controlled by two main factors: 1) the d-excess of the sublimating vapor, which is determined, in part, by the relative humidity and wind speed above the ice sheet, and 2) the proportion of sublimation- vs. marine-sourced moisture. Here, the GrIS melt extent serves as a proxy for the sublimation source and the North Atlantic Oscillation provides a measure of the meridional transport of marine moisture. We demonstrate that sublimation contributes ~20 % of the water vapor transported from the GrIS during the melt season. Sublimation is thus an important component of GrIS mass balance and the regional hydrologic cycle, and this flux will become more important in the coming years as further warming continues GrIS negative mass balance trends.

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Gorodetskaya et al. (2014) discusses the role of atmospheric rivers on observed anomalous snow accumulations in East Antarctica; and I note that with continued global warm precipitation from such atmospheric river events may fall as rain in coast regions of the Antarctic.

Gorodetskaya, I. V.  et al. (14 August 2014), "The role of atmospheric rivers in anomalous snow accumulation in East Antarctica", Geophysical Research Letters, https://doi.org/10.1002/2014GL060881

https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2014GL060881

Abstract
Recent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here we combine in situ accumulation measurements and radar‐derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the European Centre for Medium‐range Weather Forecasts Interim reanalysis to investigate moisture transport patterns responsible for these events. In particular, two high‐accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV‐based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four and five ARs reaching the coastal DML that contributed 74–80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB.

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Haine et al. (2020) indicates that the observed freshening of the Arctic Ocean is linked to anthropogenic climate change; and the first attached image provides a convenient summary of freshwater flux sources have been observed contributing to this freshening of the Arctic Ocean.

Haine, T. W. N. (09 November 2020), "Arctic Ocean Freshening Linked to Anthropogenic Climate Change: All Hands on Deck", Geophysical Research Letters, https://doi.org/10.1029/2020GL090678

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL090678

Abstract
Arctic Ocean freshwater storage increased since the mid‐1990s, but the cause was unknown. Jahn and Laiho (2020, https://doi.org/10.1029/2020GL088854) use ensemble runs of a coupled climate model to suggest that the observed increase is anthropogenic. The paper quantifies when the anthropogenic signals should emerge from the noise of natural variability. This result contextualizes research on the Arctic Ocean freshwater system and sketches an unprecedented opportunity. Future work should elucidate mechanisms, seek to observe the anthropogenic freshwater changes, and investigate the impacts on biogeochemistry and the North Atlantic Ocean circulation.

Plain Language Summary
The Arctic is a region of clear man‐made climate change. Changes in the Arctic Ocean salinity and currents have been seen, but the cause was unknown. A new paper shows that the changes are probably due to man‐made climate change. The reason is they only occur in a climate model with man‐made climate forcing. This is an important result because it helps scientists focus their research into how the changes work. It also points to a valuable opportunity to watch the Arctic Ocean respond to man‐made climate change. There might be important future impacts on North Atlantic oceanography and North Atlantic climate that scientists can now look for.

Caption for the first image: "Figure 1 Climate model projections and observations of the Arctic Ocean freshwater cycle. The left and right subplots show the principal time series of freshwater (FW) inflows and outflows (km3 yr−1 relative to a salinity of 34.80; positive poleward). The middle subplots show the freshwater storage in the Arctic Ocean as sea ice (solid, top) and liquid (bottom) freshwater (km3 relative to 34.80). Results from the Community Earth System Model (CESM) control (gray), large ensemble (LE, purple), and low warming (LW, green) experiments are shown in each case, adapted from Jahn and Laiho's (2020) Figure 2. The subplots show the times when the models show emergence of a forced, anthropogenic signal (meaning the time of first permanent departure from the ±3.5 σ envelope of control variability, where σ is the standard deviation; horizontal and vertical lines). The observations synthesized by Haine et al. (2015) are plotted in red (with updates from de Steur et al., 2018, Woodgate, 2018, and Spreen et al., 2020; the liquid storage data are adjusted to match the Jahn & Laiho, 2020 Arctic Ocean control volume by excluding Baffin Bay). For estimates and discussion of the uncertainty in the observations, see Haine et al. (2015). The basemap shows the liquid freshwater content, which is the vertically integrated salinity anomaly relative to 34.80, based on Haine et al.'s (2015) Figure 6."

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Jahn & Laiho (2020) discuss forced changes in Arctic freshwater budget in the early 21st Century.

Jahn, A. and Rory Laiho (27 July 2020), "Forced Changes in the Arctic Freshwater Budget Emerge in the Early 21st Century", Geophysical Research Letters, https://doi.org/10.1029/2020GL088854

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL088854

Abstract
Arctic liquid freshwater (FW) storage has shown a large increase over the past decades, posing the question: Is the Arctic FW budget already showing clear signs of anthropogenic climate change, or are the observed changes the result of multidecadal variability? We show that the observed change in liquid and solid Arctic FW storage is likely already driven by the changing climate, based on ensemble simulations from a state‐of‐the‐art climate model. Generally, the emergence of forced changes in Arctic FW fluxes occurs earlier for oceanic fluxes than for atmospheric or land fluxes. Nares Strait liquid FW flux is the first flux to show emergence outside the range of background variability, with this change potentially already occurring. Other FW fluxes have likely started to shift but have not yet emerged into a completely different regime. Future emissions reductions have the potential to avoid the emergence of some FW fluxes beyond the background variability.

Plain Language Summary
The surface waters of the Arctic Ocean are fresher than the rest of the world oceans, due to the input of large amounts of river runoff. The very fresh surface ocean affects the ocean circulation and climate not just in the Arctic Ocean but also at lower latitudes, especially in the North Atlantic. The last two decades have seen a freshening of the surface Arctic Ocean, for reasons that are currently unknown. Here we demonstrate that this freshening is likely already driven by climate change. Furthermore, we find that due to manmade climate change, Arctic freshwater fluxes to the North Atlantic are also likely to soon start showing signs of change beyond the range of the variability we have observed in the past. The information provided here about the expected timing of the emergence of climate change signals will allow us to monitor upcoming changes in real time, to better understand how changes in the Arctic Ocean can impact climate worldwide.

Note: The Arctic Mediterranean consists of the Arctic Ocean and Nordic Seas

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The second and third images illustrate DeConto & Pollard's estimates of sea level raise contributions, and total freshwater fluxes, from the AIS this century, and I note that the MCDS-BN freshwater flux estimates assume that DeConto & Pollard's estimates occur several decades sooner than indicated in the third image for forcing with ice-climate feedback.
 
See also:

Li, H., Fedorov, A.V. Persistent freshening of the Arctic Ocean and changes in the North Atlantic salinity caused by Arctic sea ice decline. Clim Dyn (2021). https://doi.org/10.1007/s00382-021-05850-5

https://link.springer.com/article/10.1007/s00382-021-05850-5
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Zanowski, H., Alexandra Jahn and Marika M. Holland (20 March 2021), "Arctic Ocean freshwater in CMIP6 Ensembles: Declining Sea Ice, Increasing Ocean Storage and Export", Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2020JC016930

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JC016930?af=R

Abstract
The Arctic has undergone dramatic changes in sea ice cover and the hydrologic cycle, both of which strongly impact the freshwater storage in, and export from, the Arctic Ocean. Here we analyze Arctic freshwater storage and fluxes in seven climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) and assess their performance over the historical period (1980‐2000) and in two future emissions scenarios, SSP1‐2.6 and SSP5‐8.5. Similar to CMIP5, substantial differences exist between the models’ Arctic mean states and the magnitude of their 21st century storage and flux changes. In the historical simulation, most models disagree with observations over 1980‐2000. In both future scenarios the models show an increase in liquid freshwater storage and a reduction in solid storage and fluxes through the major Arctic gateways (Bering Strait, Fram Strait, Davis Strait, and the Barents Sea Opening) that is typically larger for SSP5‐8.5 than SSP1‐2.6. The liquid fluxes are driven by both volume and salinity changes, with models exhibiting a change in sign (relative to 1980‐2000) of the freshwater flux through the Barents Sea Opening by mid‐century, little change in the Bering Strait flux, and increased export from the remaining straits by the end of the 21st century. In the straits west of Greenland (Nares, Barrow, and Davis straits), the models disagree on the behavior of the liquid freshwater export in the early‐to‐mid 21st century due to differences in the magnitude and timing of a simulated decrease in the volume flux.

Plain Language Summary
The Arctic Ocean has changed dramatically due to melting sea ice and increasing river water input and rain‐ and snowfall. Keeping track of these sources of freshwater helps us understand how the Arctic Ocean is changing and how it will change in the future. In this study we use several state‐of‐the‐art climate models to understand these freshwater changes by calculating the amount of freshwater stored in, and transported in and out of, the Arctic Ocean. We first compare the models’ freshwater values to observations and then determine how these values have changed at the end of the 21st century. We find that most models do not agree well with observations, and large differences in the size of the freshwater storage and transport also exist between them. Despite these differences, all models show that freshwater stored in and transported by sea ice decreases strongly by the end of the 21st century, while freshwater stored in the Arctic Ocean increases as well as freshwater transported out of the Arctic Ocean in most places. These changes indicate that by the end of this century, the Arctic Ocean will be very different than it is today.

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MCDS-FT Analyses (from 2020 to 2150)

I begin this series of posts by pointing out that consensus climate models are incomplete and that anyone relying on the projected probabilities of occurrences and associated confidence levels of Earth System responses are effectively denying the relatively high levels of right-tail risks addressed in this MCDS-FT, and the associated MCDS-BN, threads.  For instance many/most consensus climate scientists exclude the pattern effect from contributing to their definition of ECS (which is one reason that I report ECSeff instead), while several ice-climate feedback mechanisms (that are not fully accounted for in consensus climate models) can serve to amplify the pattern effect this century (thus increasing ECSeff and potentially flipping the atmosphere into a higher climate state that hysteresis would likely maintain for several millennia even with zero anthropogenic radiative forcing).  Examples of such ice-climate feedback mechanisms that can amplify the pattern effect include:
1.   Freshwater fluxes introduced into the North Atlantic and/or the Southern Ocean would work to slow the MOC; which would serve to increase tropical ocean SSTs (and the associated evaporation) that would likely increase the net positive cloud feedbacks, and the atmospheric telecommunication of tropical energy to higher latitudes where it would increase polar amplification.
2.   Freshwater fluxes contributing to the North Atlantic cool spot (or cool blob) can advect more warm water from the Gulf Stream deeper into the Arctic Ocean near the Barents Sea, thus increasing Arctic Amplification and increasing the risk of methane hydrate decomposition along key Arctic Ocean continental slopes, including those along the ESAS.
3.   Freshwater fluxes into the Southern Ocean can accelerate the upwelling of warm CDW and its advection to the grounding lines of key marine glaciers that can lead to MICI-types of collapses and a cascade of associated tipping points.

Furthermore, I emphasize that most of the ice-climate feedback mechanisms are not primarily driven by radiative forcing (as assumed by consensus climate science definitions of ECS) but rather by forcings such as gravity (in the case of MICI-type collapses) and existing ocean heat content (OHC) particularly in the Southern Ocean and Arctic Ocean.

In order to limit repetition; here I will try not to discuss many of the Earth System domino effect responses discussed in the MCDS-BN Overview 2020-2150 posts (in the MCDS-BN thread).

In this regard, I note that the first image (from Hansen et al. 2016) shows the freshwater fluxes from Antarctic ice shelves as reported by Rignot et al. (2013); which indicates that significant levels of freshwater fluxes are already feeding into the Southern Ocean and are already contributing to several ice-climate feedbacks (including an associated reduction in buttressing from Antarctic ice shelves/tongues against the associated marine glaciers like the PIG and the Thwaites Glacier, see the second image).  Furthermore, the third image illustrates the importance of the subglacial hydrological systems w.r.t. the stability of the WAIS, sediment studies in the Amundsen Sea Sector have confirmed that MICI-type of icebergs have calved into the ASE in paleo times and the MCDS-BN assumes that the subglacial hydrological system beneath at least the Thwaites Glacier may play a key role in triggering a MICI-type collapse mechanism in the Thwaites Gateway circa 2035/36 +/- 5yrs.  Finally, for this post, the fourth image from Pollard et al. (2018) indicates that the WAIS (& portions of the EAIS) can collapse very rapidly once triggered and that the presence of an ice mélange would do little to slow the rate of collapse of marine glaciers once a MICI-type mechanism has been triggered.

Pollard, D., Robert M. DeConto, Richard B. Alley (13 March 2018), "A continuum model (PSUMEL1) of ice mélange and its role during retreat of the Antarctic Ice Sheet", Geosci. Model Devel., 11, 5149-5172., https://doi.org/10.5194/gmd-2018-28
https://www.geosci-model-dev-discuss.net/gmd-2018-28/gmd-2018-28.pdf
 
Image 4 from Pollard et al. (2018) regarding potential AIS contributions

In the linked abstract of a presentation made at the December 2019 AGU Convention, Richard Alley et al. point out that ice-shelf stability is one of the most important considerations for determining whether future ice-cliff failure mechanisms may occur with continued global warming, and I note that CMIP6 models make very primitive assumptions about ice-shelf stability that do not appear to be what is currently happening seaward of the PIG and Thwaites Glacier.  Furthermore, Richard Alley et al.  support Hansen et al. (2016)'s observation that a cooling of the ocean surface (as is currently happening in the Southern Ocean) can direct more warm deep ocean water towards the bases of key ice shelves such as the PIIS, TEIS and the Thwaites Ice Tongue; which reduces their stability.  Richard Alley et al. also point out that ice-rafted debris (IRD) cannot occur with ice shelves intact, thus the factor than numerous IRD fields in both the Southern Ocean and the North Atlantic mean the ice-cliff failure mechanism likely occurred in paleo times under conditions similar to our current situation.

C51A-08 - “Then new problems came, from above and below…”: Heinrich Events and the future of West Antarctic ice (Invited)

https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/587813

Abstract
Heinrich Events (HE), in particular H2, record ice-shelf loss and resulting ice-flow acceleration in a cold surface climate, with implications for the future of sea-level change. Essentially all ice shelves buttress ice inflow, and experience basal melting near the grounding line that reduces or eliminates ice-rafted debris (IRD) before calving. This understanding indicates that IRD pulses in cold climates record ice-shelf loss and not just faster flow with intact ice shelves. Ice-shelf loss has been observed recently in response to atmospheric warming (Larsen B), but also in response to oceanic warming (Jakobshavn) with shelf thinning and flow acceleration causing marginal rifting leading to shelf calving (Joughin et al. 2008 JGR), perhaps preconditioned by marginal troughs (K.E. Alley et al. 2019 SciAdv). As shown by Marcott et al. (2011 PNAS), surface cooling preceding HE allowed warm water to access the grounding zone of the Hudson Strait ice stream, and the IRD pulses then record the ice-shelf loss, accompanied by faster flow in response to loss of buttressing. H2 occurred at a minimum in atmospheric temperatures. Future warming could reach Larsen B-type conditions in West Antarctica. But, if ice-sheet mass loss stabilizes the water column and causes surface cooling but subsurface warming, similar or even greater instability may result, as confirmed by H2. Proper understanding of these processes implemented in models is essential for accurate projections.

Second, the linked article makes it clear that the ice-rafted debris (IRD) already found on the seafloor of the Southern Ocean supported the concept that MICI-types of ice mass loss may be in our future and it discusses efforts to obtain more extensive such evidence (see image).

Title: "Antarctica’s iceberg graveyard could reveal the ice sheet’s future"

https://www.sciencenews.org/article/antarctica-iceberg-graveyard-climate-change

Extract: "“By looking at material carried by icebergs that calved off of the continent, we should be able to infer which sectors of the ice sheet were most unstable in the past,” Raymo says. “We can correlate the age and mineralogy of the ice-rafted debris to the bedrock in the section of Antarctica from which the bergs originated.”

Icebergs breaking off from the edges of Antarctica’s ice sheet tend to stay close to the continent, floating counterclockwise around the continent. But when the bergs reach the Weddell Sea, on the eastern side of the peninsula, they are shunted northward through a region known as Iceberg Alley toward warmer waters in the Scotia Sea.
…
But Antarctica may have played a larger role than once thought. In a study published in Nature in 2014, Kuhn, Weber and other colleagues reported that ice-rafted debris from that time period, as recorded in relatively short sediment cores from Iceberg Alley, often occurred in large pulses lasting a few centuries to millennia. Those data suggested that the southernmost continent was shedding lots of bergs much more quickly during those times than once thought.
…
“The existing [ice core] record from Iceberg Alley taught us Antarctica lost ice through a threshold reaction,” Weber says. That means that when the continent reached a certain transition point, there was sudden and massive ice loss rather than just a slow, gradual melt.

“We have rather firm evidence that this threshold is passed once the ice sheet loses contact with the underlying ocean floor,” he says, adding that at that point, the shedding of ice becomes self-sustaining, and can go on for centuries. “With mounting evidence of recent ice-mass loss in many sectors of West Antarctica of a similar fashion, we need to be concerned that a new ice-mass loss event is already underway, and there is no stopping it.”"

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The first and second images are from Vaughan et al. (2011) and indicate (which has been supported by subsequently paleo-biological evidence) that as recently as MIS 5e (the Eemian) that seaways likely existed where the current WAIS exists.  Such evidence suggests that the WAIS is susceptible to MICI-type of collapse under relatively mild climate states (like MIS 5e).  Also, I note that if the WAIS does collapse in the coming decades that many of the feedback mechanisms (including local cloud feedback mechanisms) that have driven Arctic Amplification in recent years to be about three to four times higher than the GMSTA would likely be activated for West Antarctica.

Vaughan, D.G., David K. A. Barnes, Peter T. Fretwell & Robert G. Bingham (07 October 2011), "Potential seaways across West Antarctica", Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2011GC003688

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011GC003688

Abstract: "The West Antarctic ice sheet (WAIS) has long been considered vulnerable to rapid retreat and today parts are rapidly losing ice. Projection of future change in WAIS is, however, hampered by our poor understanding of past changes, especially during interglacial periods that could be analogs for the future, but which undoubtedly provide an opportunity for testing predictive models. We consider how ice-loss would open seaways across WAIS; these would likely alter Southern Ocean circulation and climate, and would broadly define the de-glacial state, but they may also have left evidence of their existence in the coastal seas they once connected. We show the most likely routes for such seaways, and that a direct seaway between Weddell and Ross seas, which did not pass through the Amundsen Sea sector, is unlikely. Continued ice-loss at present rates would open seaways between Amundsen and Weddell seas (A-W), and Amundsen and Bellingshausen seas (A-B), in around one thousand years. This timescale indicates potential future vulnerability, but also suggests seaways may have opened in recent interglacial periods. We attempt to test this hypothesis using contemporary bryozoan species assemblages around Antarctica, concluding that anomalously high similarity in assemblages in the Weddell and Amundsen seas supports recent migration through A-W. Other authors have suggested opening of seaways last occurred during Marine Isotope Stage 7a (209 ka BP), but we conclude that opening could have occurred in MIS 5e (100 ka BP) when Antarctica was warmer than present and likely contributed to global sea levels higher than today."

The third image illustrates an atmospheric river event in Dronning Maud Land in May 2009; which was representative of a series of such events, between 2009 and 2011, that contributed to heavy snowfall that lead to a temporary increase in Antarctic surface mass balance; which temporarily caused some consensus climate scientists to assume that ice mass loss from the AIS was not as high of a risk as subsequent research has demonstrated.  In fact, many consensus climate scientists assume that the projected increase in precipitation after 2100 in Antarctica will fall as snow (thus reducing net ice mass loss after 2100); however, the MCDS-FT assessment is that after about 2090 (Event Y29) that most of this projected precipitation will fall as rain in most of the Antarctic coastal regions.

 
Caption for the first image: "Map of Antarctica (with inset of West Antarctica) showing the thickness of ice that would need to be removed before flotation would occur, calculated assuming an ice-density of 910 kg m−3, seawater-density of 1030 kg m−3, a satellite-derived ice-surface elevation model [Bamber et al., 2009a] and sub-glacial bed elevation [Le Brocq et al., 2010] supplemented with unpublished data collected inland of Eltanin Bay in 2009/10. Elevations referenced to the EIGEN-GL04c geoid and current sea level. The labeled sections, defining our hypothesized seaways, were chosen as the routes requiring least ice loss. The location of Up-B is shown according to the position given by Whillans et al. [1987]."

 
Caption for the second image: "Vertically exaggerated cross-sections along candidate seaways shown in Figure 1. The elevation of the current ice surface and bed, and flotation level calculated assuming ice density of 910 kg m−3 and seawater density of 1030 kg m−3 are shown. The portion of ice that would need to be removed for flotation to occur is indicated by hatching. The bed elevation achieved after full Airy isostatic adjustment is indicated by the dashed line, assuming upper-mantle density of the 3320 kg m−3 [see Ranalli, 1995, p. 132]."

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NASA's Oceans Melting Greenland (OMG) program has demonstrated that relatively warm ocean currents have been working to accelerate ice mass loss from key GrIS marine-terminating glaciers (see the first image), and the MCDS-FT assessment assumes that ice-climate feedbacks will significantly increase such ice mass loss from such glaciers as N79 (Event X18) as early as 2060.  Furthermore, the second image illustrates that the average age of Arctic Sea Ice has rapidly decreased in recent decades, and the MCDS-FT assessment assumes that Arctic will be seasonally ice free (< 1 million sq km) circa the boreal summer of 2040 (Event X7) and will be ice free (< 1 million sq km) year round circa 2070 (Event Y20).  The third image illustrates how quickly the calving rate for Antarctic marine glaciers can increase as a function of water depth and glacier freeboard; while the fourth image illustrates how much of the AIS is subject to either MICI and/or MISI types of potential future ice mass loss.

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For ease of reference I provide the MCDS-BN in the first attached image and a table of key MCDS parameters from 2020 to 2100 in image 2.

Again I recommend that readers look at the MCDS-BN Overview posts in the MCDS-BN thread, about other ice-climate feedback mechanisms and Earth System responses, I close out this post by noting that the third image shows how much annual ice mass change occurred in different regions of the global in 2020 (all of which is currently contributing to ice climate feedback mechanisms, while the fourth image shows a perspective view of warm and cool ocean currents in the North Atlantic which can be influenced by freshwater fluxes into the North Atlantic, which can impact for the MOC and Arctic Amplification.

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FT for BN: 2020 to 2030
FT1 and Associated Probability of Occurrence for X1 (Abrupt Retreat of the Jakobshavn, and Helheim Area, Glacier Grounding Line circa 2030):

The relatively simple first image taken from the MCDS-BN, where the FT1 assessment indicates that the combination of initial Earth System conditions/phasing indicates a high probability of a multi-year long pulse of about 0.2-Sverdrup discharge of freshwater due to dynamic calving as both the Jakobshavn and Helheim grounding lines rapidly retreat (Event X1) over relatively deep basins within their respective fjords, circa 2030 +/- 5yrs.  The timing of this assessment is significantly based both on the current grounding line locations, current surface ice melting trends and trending in relatively warm coastal ocean currents that are likely to soften the respective ice mélanges with the likely coming period of positive AMO.  I further note that this freshwater discharge pulse event (X1) is assumed to serve as a trigger to tip Events X2, X3 and X4; leading eventually to the full cascade of assumed MCDS-BN events culminating in meaningful risks of potential climate state transitions into Pliocene-like, Miocene-like and potentially Eocene-like climate states circa 2150.

In order to put into perspective some the possible future climate states that ice-climate feedbacks could trigger, I present the second image (from Westerhold et al. 2020) showing CMIP5 GMSTA (with a 1961-1990 baseline) projections for the RCP families of forcing scenarios vs paleo climate states divided into Hothouse, Warmhouse, Coolhouse, Icehouse and Modern climate state conditions.  This image also shows that the EAIS began to form about 33 MYA, that the WAIS began to form about 14 MYA and that the GrIS began to form about 2 MYA at the start of the 'Icehouse' climate state.

Westerhold, T. et al. (11 Sep 2020), "An astronomically dated record of Earth’s climate and its predictability over the last 66 million years", Science, Vol. 369, Issue 6509, pp. 1383-1387, DOI: 10.1126/science.aba6853


FT1 and Associated Probability of Occurrence for X1 (Abrupt Retreat of the Jakobshavn, and Helheim Area, Glacier Grounding Line circa 2030):

Fault Tree Analysis 1 (FT1) holds a special position in the MCDS Domino Effect Analysis using a Bayesian Network (for a more general discussion see the MCDS-BN thread), in that it is not only the first domino to potentially fall, to trigger a subsequent domino wave; but it also serves to define the initial boundary that marks the end of Earth System responses that reasonably match those assumed by consensus climate science.  To date freshwater fluxes have not been sufficient to drive Earth Systems (like the MOC) beyond the uncertainties associated with natural variability; but I note that both Helheim, and Jakobshavn, glaciers are currently experiencing significant grounding line retreat due to cliff failures, and they both contribute icebergs that feed into the subpolar gyre and subpolar North Atlantic (which directly influence the AMOC as well as the Beaufort Gyre). The third attached image shows that both the Jakobshavn (labelled2) bedmap and the Helheim (labelled 1) bedmap are the two most significant areas for potential contribution of icebergs to cool the subpolar North Atlantic from Southern Greenland by 2030 (see the Greenland folder for the current location of these grounding lines) are at, or near, regions of bed with negative slopes that indicate that icebergs calvings for both of these glaciers are likely to accelerate from the next boreal summer thru about 2030.

Finally, the fourth image from the following linked reference shows the complex relationship between the marine glaciers in both the western and eastern South Greenland, iceberg calving, the local oceanic currents, the subpolar gyre and consequently the AMOC.  This relationship could both limit flushing of freshwater from the Beaufort Gyre, BG, into the North Atlantic (thus allowing the BG to accumulate still more freshwater) and could cause westerly winds around Antarctica to accelerate due to the bipolar seesaw; both of which would contribute to Hansen's ice-climate feedback:

Camilla S. Andresen et al. (2017), "Exceptional 20th century glaciological regime of a major SE Greenland outlet glacier", Scientific Reports 7, Article number: 13626, doi:10.1038/s41598-017-13246-x

http://www.nature.com/articles/s41598-017-13246-x?WT.feed_name=subjects_climate-sciences

Abstract: "The early 2000s accelerated ice-mass loss from large outlet glaciers in W and SE Greenland has been linked to warming of the subpolar North Atlantic. To investigate the uniqueness of this event, we extend the record of glacier and ocean changes back 1700 years by analyzing a sediment core from Sermilik Fjord near Helheim Glacier in SE Greenland. We show that multidecadal to centennial increases in alkenone-inferred Atlantic Water SSTs on the shelf occurred at times of reduced solar activity during the Little Ice Age, when the subpolar gyre weakened and shifted westward promoted by atmospheric blocking events. Helheim Glacier responded to many of these episodes with increased calving, but despite earlier multidecadal warming episodes matching the 20th century high SSTs in magnitude, the glacier behaved differently during the 20th century. We suggest the presence of a floating ice tongue since at least 300 AD lasting until 1900 AD followed by elevated 20th century glacier calving due to the loss of the tongue. We attribute this regime shift to 20th century unprecedented low sea-ice occurrence in the East Greenland Current and conclude that properties of this current are important for the stability of the present ice tongues in NE Greenland."

Extract: "Our results imply that model predictions of dynamic loss from ice streams in North Greenland need to account for threshold effects from sea-ice to better predict the future evolution of Greenland coastal glaciers in a warming North Atlantic Ocean."

Caption for the fourth attached image:"Map of the North Atlantic region showing the major surface ocean currents (colour of arrows indicate temperature; red = warm, blue = cold, yellow = mixture) and location of sites referred to in Fig. 3a–f. HG = Helheim Glacier; KG = Kangerdlugssuaq Glacier; JI = Jakobshavn Isbræ. Magenta circles show location of sediment cores discussed in the text and shown on Figs 2 and 3. The magenta box delineates the extent of the inset map. Bathymetric data are from IBCAO v3. Terrestrial topographic data are from the ETOPO1 Global Relief model and the GIMP surface digital elevation model. The inset map of Sermilik Fjord shows the location of sediment core ER07 and the local bathymetry."

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Boers et al. (2021) discusses evidence for an ice-climate positive feedback on western Greenland marine terminating glaciers where thinning of the glacier (associated with both surface ice melting and the acceleration of ice velocities) results in a reduction in surface altitude (see image 1) that allows for more warm air to contact the ice surface that accelerates more surface ice melting in a positive feedback loop.  Furthermore, this research suggests that marine terminating glaciers like Jakobshavn are rapidly approaching a tipping point.

Boers, Niklas and Martin Rypdal (May 25, 2021), "Critical slowing down suggests that the western Greenland Ice Sheet is close to a tipping point", PNAS, 118, (21), e2024192118; https://doi.org/10.1073/pnas.2024192118

https://www.pnas.org/content/118/21/e2024192118

The Greenland Ice Sheet (GrIS) is a potentially unstable component of the Earth system and may exhibit a critical transition under ongoing global warming. Mass reductions of the GrIS have substantial impacts on global sea level and the speed of the Atlantic Meridional Overturning Circulation, due to the additional freshwater caused by increased meltwater runoff into the northern Atlantic. The stability of the GrIS depends crucially on the positive melt-elevation feedback (MEF), by which melt rates increase as the overall ice sheet height decreases under rising temperatures. Melting rates across Greenland have accelerated nonlinearly in recent decades, and models predict a critical temperature threshold beyond which the current ice sheet state is not maintainable. Here, we investigate long-term melt rate and ice sheet height reconstructions from the central-western GrIS in combination with model simulations to quantify the stability of this part of the GrIS. We reveal significant early-warning signals (EWS) indicating that the central-western GrIS is close to a critical transition. By relating the statistical EWS to underlying physical processes, our results suggest that the MEF plays a dominant role in the observed, ongoing destabilization of the central-western GrIS. Our results suggest substantial further GrIS mass loss in the near future and call for urgent, observation-constrained stability assessments of other parts of the GrIS.

See also:

Title: "Greenland ice sheet on brink of major tipping point, says study"

https://www.theguardian.com/environment/2021/may/17/greenland-ice-sheet-on-brink-of-major-tipping-point-says-study

Extract: "The new analysis detected the warning signals of a tipping point in a 140-year record of ice-sheet height and melting rates in the Jakobshavn basin, one of the five biggest basins in Greenland and the fastest-melting. The prime suspect for a surge in melting is a vicious circle in which melting reduces the height of the ice sheet, exposing it to the warmer air found at lower altitudes, which causes further melting."

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Mallalieu et al (2021) provides observations of an increase in the number and area of ice-marginal lakes situated along the south-western margin of the GrIS since the 1980s, and discusses evidence that this increasing in ice-marginal lakes is associated with enhanced recession of the south-western marginal GrIS.

Mallalieu, J. et al. (8 May 2021), "Ice-marginal lakes associated with enhanced recession of the Greenland Ice Sheet" Global and Planetary Change, 103503, https://doi.org/10.1016/j.gloplacha.2021.103503

https://www.sciencedirect.com/science/article/abs/pii/S0921818121000886?dgcid=rss_sd_all

Abstract
There has been a progressive increase in the number and area of ice-marginal lakes situated along the south-western margin of the Greenland Ice Sheet (GrIS) since the 1980s. The increased prevalence of ice-marginal lakes is notable because of their capacity to enhance mass loss and ice-margin recession through a number of thermo-mechanical controls. Although such effects have been extensively documented at alpine glaciers, an understanding of how ice-marginal lakes impact the dynamics of the GrIS has been limited by a sparsity of observational records. This study employs the Landsat archive to conduct a multi-decadal, regional-scale statistical analysis of ice-margin advance and recession along a ~ 5000 km length of the south-western margin of the GrIS, incorporating its terrestrial, lacustrine and marine ice-margins. We reveal an extended and accelerating phase of ice-margin recession in south-west Greenland from 1992 onwards, irrespective of margin type, but also observe considerable heterogeneity in the behaviour of the different ice-marginal environments. Marine ice-margins exhibited the greatest magnitude and variability in ice-margin change, however lacustrine termini were notable for a progressive increase in ice-margin recession rates from 1987 to 2015, which increasingly outpaced those measured at terrestrial ice-margins. Furthermore, significant correlations were identified between lake parameters and rates of lacustrine ice-margin recession, including lake area, latitude, altitude and the length of the lake – ice-margin interface. These results suggest that ice-marginal lakes have become increasingly important drivers of ice-margin recession and thus mass loss at the GrIS, however further research is needed to better parameterise the causal connections between ice-marginal lake evolution and enhanced ice-margin recession. More widely, a detailed understanding of the impacts of ice-marginal lakes on ice-margin dynamics across Greenland is increasingly necessary to accurately forecast the response of the ice sheet to enhanced ice-marginal lake prevalence and thus refine projections of recession, mass loss and sea level rise.

Caption for image 1: AR5 Chapter 13 FAQ Fig 1 2013

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Felikson et al. (2020) discusses the nature of steep glacier bed knickpoints and their relation to the timing and extent of inland ice thinning in Greenland (see the second image)

Felikson, Ginny Catania, Timothy C. Bartholomaus, Mathieu Morlighem and Brice P. Y. Noël (11 December 2020), “Steep glacier bed knickpoints mitigate inland thinning in Greenland” by Denis, Geophysical Research Letters.
DOI: 10.1029/2020GL090112

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL090112

Abstract
Greenland's outlet glaciers have been a leading source of mass loss and accompanying sea‐level rise from the Greenland Ice Sheet over the last 25 years. The dynamic component of outlet glacier mass loss depends on both the ice flux through the terminus and the inland extent of glacier thinning, initiated at the ice‐ocean interface. Here, we find limits to the inland spread of thinning that initiates at glacier termini for 141 ocean‐terminating outlet glaciers around the Greenland Ice Sheet. Inland diffusion of thinning is limited by steep reaches of bed topography that we call “knickpoints.” We show that knickpoints exist beneath the majority of outlet glaciers but they are less steep in regions of gentle bed topography, giving glaciers in gentle bed topography the potential to contribute to ongoing and future mass loss from the Greenland Ice Sheet by allowing the diffusion of thinning far into the ice sheet interior.

Plain Language Summary
Fast‐flowing outlet glaciers around the edge of the Greenland Ice Sheet transport ice from the ice sheet interior to the ocean. In response to a warming climate, many of these glaciers have retreated, causing thinning at the edge of the ice sheet that spreads into the interior. Depending on the shape of each outlet glacier, thinning can either spread far into the interior or stall where the glacier flows over particularly steep bedrock beneath the glacier. By investigating the shapes of 141 outlet glaciers around Greenland, we find that steep bedrock features, which we call “knickpoints,” can effectively stall the inland spread of thinning in regions where the bedrock beneath the ice sheet is mountainous. On the other hand, in regions of more gentle topography, these knickpoints are either not present or are less steep and cannot stall thinning from spreading far into the ice sheet interior. This means that numerous small glaciers flowing over the gentle topography of Northwest Greenland may allow thinning to spread to the center of the ice sheet. Because of this, these smaller glaciers may play as big of a role in future sea‐level rise as the more well‐known and well‐studied larger glaciers.
…
Of particular concern is a group of glaciers along the northwest coast of the GrIS, between Upernavik Isstrom C and Cornell Gletscher, where 9 of 12 neighboring glaciers have the potential to lead to long term, diffusive thinning >250 km into the interior of the ice sheet over a ~140-km-wide region.  This provides a mechanistic explanation for why the northwest sector of the GrIS is the only region experiencing an on-going increase in observed discharge."

Extract: "A glacier's potential for dynamic ice mass loss is governed by both the inland extent of thinning resulting from terminus retreat and the glacier's ice flux.  A glacier with a far inland thinning limit has the potential for large dynamic mass loss because thinning can draw down a larger part of the ice sheet, whereas a glacier with high ice flux has the potential for large dynamic mass loss because a perturbation to its flux can lead to larger ice discharge than a glacier with small ice flux.  In other words, a 1% perturbation to the flux of a high ice flux glacier yields larger discharge and, thus, more dynamic mass loss than for a low ice flux glacier.

 
Caption for the second image: "Figure 3. Glacier thinning limits and potential for dynamic mass loss of 141 GrIS outlet glaciers. (a) Distances from ice sheet margin to thinning limits plotted against ice fluxes for glaciers in regions of gentle (circles) and mountainous (squares) bed topography. Purple markers indicate a group of glaciers with thinning limits >200 km from the ice margin; yellow markers indicate a group of glaciers with > 5 km3/year ice flux.  White x's inside purple markers indicate 9 glaciers in NW Greenland, discussed in the text. (b) Flowlines for each glacier drawn from the terminus to the predicted glacier thinning limit and colored according to groupings shown in a, shown on top of Greenland bed topography. Regions of mountainous bed topography (red coastlines) and gentle bed topography (blue coastlines) shown.  Upernavik Isstrom C (UPR-C), Cornell Glestcher (COR), Humboldt Gletcher (HUM), Kangerlussuaq Gletscher (KAN), Helheimgletscher (HEL) and Jakobshavn Isbrae (JAK) referenced in the text are labeled in both panels."

Also see:

Title: "NASA Finds What a Glacier’s Slope Reveals About Future Greenland Ice Sheet Thinning"

https://scitechdaily.com/nasa-finds-what-a-glaciers-slope-reveals-about-future-greenland-ice-sheet-thinning/

Extract: "The research, which was published December 11th in Geophysical Research Letters, analyzed 141 outlet glaciers on the Greenland Ice Sheet to predict how far into the interior thinning may spread along their flow lines, starting from the ocean edge.

“What we discovered is some glaciers flow over these steep drops in the bed, and some don’t,” said lead author Denis Felikson with NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the Universities Space Research Association (USRA). “For the glaciers that do have that steep drop in the bed, thinning can’t make its way past those drops.” Borrowing a term from geomorphology – the study of Earth’s physical features – they coined these steep drop features “knickpoints.”
…
Over the gentle topography of the northwest coast of Greenland, nine of twelve neighboring glaciers are predicted to thin more than 250 km (155.3 miles) into the interior of the ice sheet, over a ~140-km (86.9 mile) wide region. The northwest sector of the ice sheet is also the only region experiencing an ongoing increase in ice discharge over the last couple decades, and Felikson predicts that it will continue to do so given the characteristics of these glaciers."

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Catania et al (2019) discusses various mechanisms that may drive future ice mass loss from marine-terminating glaciers in the GrIS, including the influence of ocean currents (see the third image).
Catania, G.A.  et al (10 December 2019), "Future Evolution of Greenland's Marine‐Terminating Outlet Glaciers", JGR Earth Surface, https://doi.org/10.1029/2018JF004873

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JF004873

Abstract
Mass loss from the Greenland ice sheet (GrIS) has increased over the last two decades in response to changes in global climate, motivating the scientific community to question how the GrIS will contribute to sea‐level rise on timescales that are relevant to coastal communities. Observations also indicate that the impact of a melting GrIS extends beyond sea‐level rise, including changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Unfortunately, despite the rapid growth of interest in GrIS mass loss and its impacts, we still lack the ability to confidently predict the rate of future mass loss and the full impacts of this mass loss on the globe. Uncertainty in GrIS mass loss projections in part stems from the nonlinear response of the ice sheet to climate forcing, with many processes at play that influence how mass is lost. This is particularly true for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice‐atmosphere, ice‐ocean, and ice‐bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe, making them difficult to understand and thus missing in prognostic ice sheet/climate models. For example, recent (beginning in the late 1990s) mass loss via outlet glaciers has been attributed primarily to changing ice‐ocean interactions, driven by both oceanic and atmospheric warming, but the exact mechanisms controlling the onset of glacier retreat and the processes that regulate the amount of retreat remain uncertain. Here we review the progress in understanding GrIS outlet glacier sensitivity to climate change, how mass loss has changed over time, and how our understanding has evolved as observational capacity expanded. Although many processes are far better understood than they were even a decade ago, fundamental gaps in our understanding of certain processes remain. These gaps impede our ability to understand past changes in dynamics and to make more accurate mass loss projections under future climate change. As such, there is a pressing need for (1) improved, long‐term observations at the ice‐ocean and ice‐bed boundaries, (2) more observationally constrained numerical ice flow models that are coupled to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community.

Plain Language Summary
Increasing mass loss from the Greenland ice sheet (GrIS) in response to changes in global climate has motivated the scientific community to understand how much sea level rise will happen in the coming decades. Observations now indicate that the impact of a melting GrIS are more widespread than just sea‐level rise and include changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Major uncertainties still hamper accurate predictions of these impacts, particularly for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice‐atmosphere, ice‐ocean, and ice‐bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe. Here we review the scientific progress in understanding how GrIS outlet glaciers respond to climate and how our understanding has changed over time as observations have increased. We conclude with recommendations for (1) improved, long‐term observations at the ice‐ocean and ice‐bed boundaries, (2) more observationally‐constrained ice flow models that are linked to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community.

Extract: "Most GrIS outlet glaciers have bedrock margins and deep central troughs carved through erosion, similar to their marine‐terminating mountain‐glacier counterparts in Alaska and the Antarctic Peninsula. However, some outlet glaciers in Greenland have fast surface flow speeds that reach far into the ice sheet interior (e.g., Jakobshavn Isbrae and the Northeast Greenland Ice Stream) with similarities to the large ice streams draining the Antarctic ice sheet. For these glaciers, lateral boundaries of fast flow are constrained by slow‐moving ice in the interior and bedrock towards the ice sheet margin.
…
The GrIS contains  280 fast‐flowing (>100 m/year) marine‐terminating glaciers (Figure 1) with a high degree of heterogeneity across a range of parameters (Mankoff et al., 2019). Ice discharge from these glaciers is  500 Gt/year total, with large variations in individual glacier ice discharge associated with interglacier differences in width (1–30 km), thickness (  100–2,000 m), terminus basal conditions (grounded, partially floating, fully floating), terminus conditions (open water, mélange—a granular matrix of icebergs, bergy bits, and sea ice—or ice shelf presence), basal substrate (bedrock, sediments, water), and other topographic controls (Enderlin et al., 2014; King et al., 2018; Mankoff et al., 2019).

 Caption for the third image: "Figure 1 The Greenland Ice Sheet situated in the Arctic showing ocean currents. The size of ocean current arrows indicates water mass; color of arrows indicates heat transport. Surface ice flow speed from (Joughin et al., 2010) highlights the numerous fast‐moving outlet glaciers around the periphery of the ice sheet that drain ice from the interior. Purple triangles indicate outlet glaciers identified with flow rates above 50 m/year. Bathymetry data from the GEBCO Grid (GEBCO Compilation Group, 2019) show the deep troughs created on the sea floor from the extent of past ice streams to the continental slope."

Also, I note that per Felikson et al. (2020), the current dynamical ice mass loss from the Jakobshavn Glacier is about 25Gt/year and as 100 Gt of ice mass loss ~ 0.28mm of eustatic SLR, this corresponds to about 0.07mm of eustatic SLR/year.  However, in 2012 7.4 cubic km (6.78 Gt) of ice calved in 75-minutes; which illustrates just how quickly ice mass loss from both Jakobshavn and Helheim could accelerate circa 2030.

See also

Title: "Jakobshavn Glacier"
https://en.wikipedia.org/wiki/Jakobshavn_Glacier

Extract: "In the 2012 documentary entitled Chasing Ice by cinematographer Jeff Orlowski, nature photographer James Balog and his Extreme Ice Survey (EIS) team, there is a 75-minute segment showing the Jakobshavn Glacier calving. Two EIS videographers waited several weeks in a small tent overlooking the glacier, and were finally able to witness 7.4 cubic kilometres (1.8 cu mi) of ice crashing off the glacier."

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The first attached image shows that in August 2013 to August 2015 timeframe the retreat of the Jakobshavn calving face came very close to the sill of a retrograde bed slope shown on this first image, that once crossed would likely accelerate image mass loss from Jakobshavn due to ice cliff failures.  However, the second image from Joughin et al. (2020) shows that since 2013 the Jakobshavn calving face has advanced downstream until 2019 apparently due a correlation with the AMO [see the third image, and I note that so far 2021 has been a very active calving season for Jakobshavn].

Given that the current warm phase of the AMO that started in 2002 and may, on average, last about 25 to 40 years this means that it is likely that the Jakobshavn calving face will cross the retrograde bed sill (see the first image) sometime between 2027 and 2042; and if so such an event would work to increase the probability of the an MICI-type of collapse of the ice in the Thwaites gateway (via the bipolar seesaw mechanism) in the 2030 to 2040 timeframe.  Finally, for this post, I provide the fourth image of typical mechanisms that contribute to ice-cliff failures for Jakobshavn and I note that these mechanisms are markedly different than those likely to trigger a MICI-type of ice cliff failures in the Thwaites gateway.

Joughin, I., et al (2020): "A decade of variability on Jakobshavn Isbræ: ocean temperatures pace speed through influence on mélange rigidity", The Cryosphere, 14, 211–227, https://doi.org/10.5194/tc-14-211-2020

See also"

Title: "GREENLAND’S DYING ICE

https://vis.sciencemag.org/greenlands-dying-ice/

Extract: "As befits its mythical name, the domain of the Norse god of the dead, Helheim is truly an arbiter of Greenland's fate. The glacier is one of the ice sheet's primary drains, sliding into the sea at 8 kilometers per year and accounting for 4% of the ice sheet's annual mass loss. Its towering front, as tall as the Statue of Liberty, measures 6 kilometers across. Sea ice shed from the glacier chokes the fjord for tens of kilometers. The glacier's terminus has behaved erratically over the past 15 years, first retreating by 5 kilometers from 2002 to 2005 and then advancing and stabilizing for nearly 10 years. Then, in 2014, a more severe retreat began, sending the terminus 2 kilometers beyond its previous low. Meanwhile, the glacier has thinned by more than 100 meters, leaving a telltale "bathtub ring" high on the rock around the fjord.
Some two-thirds of Greenland's ice loss comes not as meltwater, but as chunks of ice that detach, or calve, from its 300 outlet glaciers—fast-moving rivers of ice that end in long fjords. Those narrow channels, hundreds of meters deep, "are the bottlenecks," Straneo says. They also are a fateful meeting place, where a glacier's calving front encounters currents of increasingly warm ocean water. "These tiny systems are the connection between the ice sheet and the ocean," she says.
Straneo's past work showed that warm Atlantic water is penetrating Sermilik Fjord, which researchers once thought was dominated by Arctic waters. Here, it meets cold meltwater draining through channels beneath the ice. Straneo believes the emerging freshwater, buoyant because of its low salinity, mixes with the warm water and forms a plume that wells up against the glacier's front, causing more melting and fracturing. It's like the ice in your glass of whiskey, she says. "If you just put it in and don't stir, it lasts a long time. If you stir it, it melts really quickly."

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King, M.D., Howat, I.M., Candela, S.G. et al. Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat. Commun Earth Environ 1, 1 (2020).

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FTs for BN: 2030 to 2040
X3 = Triggering of MICI Behavior for Thwaites; (2020 to 2035)
X2 = Beaufort Gyre Reversal/Discharge; (2020 to 2036)
X4 = Triggering of BSB Ice MICI Discharge; (2035 to 2036)

The first image taken from the MCDS-BN largely consists of:
a) three complex FT arrows representing all initiation global conditions assuming abrupt grounding retreats for both Jakobshavn and Helheim, Glaciers circa 2030 +/- 5 yrs.
b) three freshwater flux events X3, X2 and X4 from 2035 to 2036 all +/- 5 yrs (where it is assumed that event X3 precedes and triggers events X2 and X4, but it noted that event X2 could precede event X3 without introducing a significant change in the values for ECSeff, ERF, P, or FF show on the MCDS-BN).
c) Collectively, the three events X3, X2 and X4 are assigned a 75% probability of occurrence, by 2036 +/- 5 yrs, for reasons including:
•   Event X1 (and possibly X2) will have slowed the MOC by 2035/36 thus telecommunicating energy from the Tropical Pacific directly to the Bellingshausen, Amundsen and Ross, Sea Sectors; which, will increase the probability of a Super El Nino, stronger local cyclones, and a strong ASL in this timeframe.
•   The subglacial lakes beneath the Thwaites Glacier will likely be fully recharged by this time; while will likely lead to a major discharge of basal meltwater at the base of the Thwaites Ice Tongue in this timeframe.
•   Richard Alley has noted that currently the Thwaites grounding line at the base of the Thwaites Ice Tongue is retreating towards the BSB at a rate of about 1km per year and thus this local grounding line will likely reach the negative bed slope leading to the BSB circa 2035/36.
•   Wild et al. (2021) has estimated that the Thwaites Eastern Ice Shelf will become unpinned circa 2035; which could lead to the complete collapse of the TEIS by 2036.
•   Computer models project a significant increase in the advection of warm CDW into the ASE circa 2035.
•   Acceleration of the ice velocities for the SW Tributary Glacier (near the Pine Island Bay) may likely reduce the restraint currently provided along the eastern shear margin of Thwaites Glacier circa 2035.

FT3 and the Associated Probability of Occurrence for X3 (Triggering of MICI-type Collapse for Thwaites Glacier by 2035-36)

Wild et al. (2021) projects that the Thwaites Eastern Ice Shelf (TEIS) will become unpinned circa 2030 and that thereafter the Thwaites Ice Tongue will become unstable (see images 2 thru 4 and the extract).

Wild, C. T., Alley, K. E., Muto, A., Truffer, M., Scambos, T. A., and Pettit, E. C.: Weakening of the pinning point buttressing Thwaites Glacier, West Antarctica, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-130, in review, 2021.

https://tc.copernicus.org/preprints/tc-2021-130/
Abstract. The Thwaites Eastern Ice Shelf continues to buttresses a significant portion of Thwaites Glacier through contact with a pinning point 40 km offshore of the present grounding line. Predicting future rates of Thwaites Glacier’s contribution to sea-level rise depends on the evolution of this pinning point and the resultant change in the ice-shelf stress field since the break-up of the Thwaites Western Glacier Tongue in 2009. Here we use Landsat-8 feature tracking of ice velocity in combination with model perturbation experiments to show how past changes in flow velocity have been governed in large part by changes in lateral shear and pinning point interactions with the Thwaites Western Glacier Tongue. We then use recent satellite altimetry data from ICESat-2 to show that Thwaites Glacier’s grounding line has continued to retreat rapidly; in particular, the grounded area of the pinning point is greatly reduced from earlier mappings in 2014, and grounded ice elevations continuing to decrease. This loss has created two pinned areas with ice flow now funneled between them. If current rates of surface lowering persist, the entire Thwaites Eastern Ice Shelf will unpin from the seafloor in less than a decade, despite our finding from airborne radar data that the seafloor underneath the pinning point is about 200 m shallower than previously reported. Advection of relatively thin and mechanically damaged ice onto the remaining portions of the pinning point and feedback mechanisms involving basal melting, may further accelerate the unpinning. As a result, ice discharge will likely increase along a 45 km stretch of the grounding line that is currently buttressed by the Thwaites Eastern Ice Shelf.

Extract: "Since the break-up of the Western Glacier Tongue in 2009, the ice flow on the Eastern Ice Shelf has slowed down, rotated counter-clockwise, and now funnels through a bathymetric saddle between the two remaining portions of Thwaites pinning point. Model simulations using ISSM reproduce this counter-clockwise rotation of ice flow with the removal of Western Glacier Tongue from the stress balance and attribute the satellite-observed ice funneling to weakening of the pinning point and opening of the saddle. Given current rates of surface lowering from ICESat-2 laser altimetry data in this area, in combination with the advection of thinner and mechanically damaged ice upstream, Thwaites pinning point will reach flotation within less than one decade with implications for the stability of the Eastern Ice Shelf and thus the whole Thwaites Glacier.

Compared to other proposed scenarios of ice-shelf break-up in the near future, such as cracking through the central part of the Eastern Ice Shelf or failure along rifts within the narrow shear zone upstream of the pinning point, our analysis strongly supports that unpinning of the Eastern Ice Shelf from the seafloor ridge and subsequent loss of its structural integrity as a short-range mechanism for break-up. We conclude that unpinning within the next decade is possibly the most likely scenario of regional destabilization, followed by break-up similar to the Western Glacier Tongue. Following other ice shelf disintegration events (e.g., Scambos et al., 2004; Rack and Rott, 2004) we expect increased ice discharge along a 45-km stretch of the grounding line thereafter."

 
Caption for second image: "Figure 1. Data sets assembled for the Eastern Ice Shelf and the Western Glacier Tongue study area overlain on a Sentinel 1 SAR image from October 2019: (a) Reference Elevation Model of Antarctica (REMA, Howat et al., 2019); (b) BedMachine version 2 ice thickness (Morlighem, 2020); (c) gravity-derived bathymetry (Jordan et al., 2020); (d) MEaSUREs ice surface velocity field before break-up of Western Glacier Tongue (Rignot et al., 2017). Past grounding lines: yellow is approximately 2004 (Bindschadler et al., 2008); black is 2011 from MEaSUREs, dashed black is 2017 from InSAR (Milillo et al., 2019). We define the green (2014) and red (2020) grounding lines from REMA and ICESat-2 data, respectively. The white outline shows the spatial extent of field operations in the 2019/20 Antarctic season. Airborne surveys from 2009 and 2019 (panel a) are shown as white dashed lines. The red star in the inset in panel (a) marks the location of Thwaites Glacier In the Amundsen Sea Embayment. The two yellow stars in panel (b) mark the locations of GPS and barometric pressure records used for tidal analysis (AMIGOS stations). The red and blue arrows in panel (c) indicate the pathways of warm Circumpolar Deep Water and cold ice-shelf melt water (Wåhlin et al., 2021). Coordinates in an Antarctic polar stereographic projection (EPSG:3031)."

 
Caption for the third image: "Figure 3. Regional changes: Height above flotation from (a) REMA data in 2014 and (b) ICESat-2 surface altimetry data in 2019/20 overlain on a Sentinel 1 SAR image from October 2019. Airborne surveys from 2009 and 2019 (panels a and b) are shown as white dashed lines. Red circles in panel (b) show rates of grounding-line retreat since the last assessment in 2017 (Milillo et al., 2019). Roman numerals in panel (c) refer to areas of interest discussed in the text and are overlain on the Landsat Image Mosaic of Antarctica (Bindschadler et al., 2008). Panel (d) shows histograms of grounding-line retreat rates along the (light blue) profiles in panel (c) in comparison to (dashed black) the InSAR-derived retreat rates between 2011 to 2017 from Milillo et al. (2019). The white rectangle (panels a and b) shows the spatial extent of Figure 5. The white outline shows the area of field operations in the 2019/20 Antarctic season."

 
Caption for the fourth image: "Figure 9. The effect of sustained ungrounding between the two remaining portions of Thwaites pinning point on regional ice dynamics, as modeled with ISSM. The black line shows the 2011 grounding line. (a) The red line shows the spatial extent of ungrounding and is guided by our analysis of height above flotation from ICESat-2. Note the funneling of ice over the bathymetric saddle and the increase in ice flow velocities on the Eastern Ice Shelf and further upstream. (b) Modelled acceleration following the entire ungrounding of the Eastern Ice Shelf from the pinning point."

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1. My proposed location of the first MICI-type of ice cliff immediately upstream of the Big Ear subglacial cavity (see the first image) is currently stable not only due to the buttressing from the confined icebergs floating over the top of the Big Ear subglacial cavity, but also due to buttressing from the Thwaites Eastern Ice Shelf; which results in a particularly high surface elevation relief that would resist ice creep after the constrained icebergs float away; which would maintain sufficient ice cliff freeboard to initiate a slumping type of ice cliff failure likely between 2030 & 2040.

2. The high ice surface relief discusses in item 1, provides the vertical pressure on the bed to form an ice plug that resists jökulhlaup events until a substantial subglacial hydrostatic pressure has built-up upstream, that when released (likely between 2030 and 2040) will likely flush the ice mélange downstream of the ice cliff face out into the ASE.

3. Virtually all ice sheet models assume that the Thwaites Ice Tongue consists of solid ice, and not a fractured mass of constrained icebergs that are likely to float away prior to 2030.

Also, I suspect that the zone marked "correlated anomalies' on the second and third images occurs due to an the assumed presence of a reservoir of basal subglacial meltwater upstream of the assumed ice plug; and the fourth image shows how relatively warm mCDW entering the Big Ear subglacial cavity contributes to a continuing retreat of the grounding line.

Jordan, T. A., Porter, D., Tinto, K., Millan, R., Muto, A., Hogan, K., Larter, R. D., Graham, A. G. C., and Paden, J. D.: New gravity-derived bathymetry for the Thwaites, Crosson and Dotson ice shelves revealing two ice shelf populations, The Cryosphere Discuss., https://doi.org/10.5194/tc-2019-294, in review, 2020.

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Malczyk et al. (2020) indicates that previous models underestimated basal melting under the region of Thwaites Glacier by nearly 150%, and that the subglacial lake system that feeds discharge into the Thwaites Glacier gateway is recharging; which could lead to another major subglacial lake drainage event such as happened in 2013-2014 (see the first two images).  Such a future major subglacial lake drainage event might contribute to a possible MICI-type of event in the Thwaites Glacier gateway circa 2030 to 2040.

Malczyk, G. et al. (09 November 2020), "Repeat Subglacial Lake Drainage and Filling Beneath Thwaites Glacier", Geophysical Research Letters, https://doi.org/10.1029/2020GL089658

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL089658

Abstract
Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and their impact on ice sheet mass balance. Here we use high‐resolution altimetry measurements from 2010 to 2019 to show that a lake system under the Thwaites Glacier undertook a large episode of activity in 2017, only four years after the system underwent a substantial drainage event. Our observations suggest significant modifications of the drainage system between the two events, with 2017 experiencing greater upstream discharge, faster lake‐to‐lake connectivity, and the transfer of water within a closed system. Measured rates of lake recharge during the inter‐drainage period are 137% larger than modeled estimates, suggesting processes that drive subglacial meltwater production, such as geothermal heat flux or basal friction, are currently underestimated.

Plain Language Summary
Antarctic subglacial lakes can play an important role in ice sheet dynamics. When subglacial lakes drain, they release large amounts of water that interact with the subglacial drainage system. Here we show lakes draining only four years after a previous drainage event. Our results suggest that lake activity increases the efficiency of the subglacial drainage network. Rates of lake recharge indicate that basal melt‐water production is significantly higher than previously thought.

Extract: "It appears that the most upstream lake, Thw170, was first to activate in early April 2013, draining until early January 2014 with a total volume loss of 0.45 ± 0.03 km3. Second in the procession was Thw124, draining from mid‐May 2013 until May 2014 with a total water loss of 3.83 ± 0.11 km3. Thw142 activated from early July 2013 draining until October 2013 with an average volume loss of 0.55 ± 0.03 km3. Last in succession was Thw70 which drained from mid‐August 2013 until May 2014 with a total water loss of 0.90 ± 0.06 km3."
 
Caption for the first image: "Mean elevation and volume change of subglacial lakes relative to July 2010. Elevation and volume changes from 2010 until 2017 were derived assuming 2013 lake sizes, while 2017 to 2020 changes were derived using 2017 lake sizes. Vertical dashed lines represent the onset and termination of lake activity. (a) Mean elevation change within feature boundaries. (b) Mean volume change within feature boundaries. (c) Derivative of volume change for each lake. Black dashed line represents total discharge within the subglacial system."
 
Caption for the second image: "Rates of surface elevation change for the Thwaites lake region from January 2014 to August 2019. Location of the lake region is illustrated by the map insert. Black dashed lines represent lake boundaries during the 2013 event as described in Smith et al. (2017). White dashed lines represent lake boundaries during the 2017 drainage event. Navy lines represent theoretical drainage routes derived by applying a D8 routing algorithm to a hydro‐potential map of the region."

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Boehme & Rosso (2021), discusses how relatively warm mCDW can advect towards the grounding lines of key Amundsen Sea Embayment (ASE) marine glaciers.

Boehme, L. & Isabella Rosso (06 February 2021), "Classifying Oceanographic Structures in the Amundsen Sea, Antarctica", Geophysical Research Letters, https://doi.org/10.1029/2020GL089412

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089412

Abstract
The remote and often ice‐covered Amundsen Sea Embayment in Antarctica is important for transporting relatively warm modified Circumpolar Deep Water (mCDW) to the Western Antarctic Ice Sheet, potentially accelerating its thinning and contribution to sea level rise. To investigate potential pathways and variability of mCDW, 3809 CTD profiles (instrumented seal and ship‐based data) are classified using a machine learning approach (Profile Classification Model). Five vertical regimes are identified, and areas of larger variability highlighted. Three spatial regimes are captured: Off‐Shelf, Eastern and Central Troughs. The on‐shelf profiles further show a separation between cold and warm modes. The variability is higher north of Burke Island and at the southern end of the Eastern Trough, which reflects the convergence of different mCDW pathways between the Eastern and the Central Trough. Finally, a clear but variable clockwise circulation is identified in Pine Island Bay.

Plain Language Summary
The glaciers of West Antarctica are melting and could change sea level by about 30 cm or more by 2100. Pine Island and Thwaites Glaciers, both flowing into the Amundsen Sea, are currently retreating partly driven by warm water melting the glaciers from below. This water sits only a few degrees above freezing, but can still accelerate the melt and sea level rise. This warm water comes from the deeper Southern Ocean further north and travels through deep channels onto the shallower Amundsen Sea shelf following deeper troughs until reaching the glaciers further south. Understanding the pathways and the variability of this warm water on its way south is of high importance. Here, we use a machine learning method to identify different pathways from a large number of observations.

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Bevan et al. (2021) indicates that the ice height above floatation over the Thwaites Glacier cavity has been decreasing from 2017 to 2021, as the relatively high ice velocity is causing the glacial ice in this area to thin, as indicated by images 3 & 4.

Bevan, S. L., Luckman, A. J., Benn, D. I., Adusumilli, S., and Crawford, A.: Brief Communication: Thwaites Glacier cavity evolution, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-66, in review, 2021.

https://tc.copernicus.org/preprints/tc-2021-66/

Abstract. Between 2014 and 2017, ocean melt eroded a large cavity beneath and along the western margin of the fast-flowing core of Thwaites Glacier. Here we show that from 2017 to the end of 2020 the cavity persisted but did not expand. This behaviour, of melt concentrated at the grounding line within confined sub-shelf cavities, fits with prior observations and modelling studies. We also show that acceleration and thinning of Thwaites Glacier grounded ice continue, with an increase in speed of 400 ma−1 and a thinning rate of 1.5 ma−1, between 2012 and 2020.
 
Caption for the third image: " Figure 2. Ice surface elevation and hydrostatic thickness extracted from a) the along-flow profile, and b) the across-flow profile marked in Fig. 1b. Where the ice base assuming hydrostatic thickness lies below the bedrock elevation this should be interpreted as a scaled height-above-floatation where the scaling factor is (ρw − ρi)/ρw. Vertical dashed lines mark the intersections of the two profiles. Coloured arrows indicate grounding line locations for 2011 and 2016/2017 coloured according to year."
 
Caption for the fourth image: "Figure A1. a) Mean surface speeds for January 2021 based on feature tracking Sentinel-1 data. b) Change in surface speed from January 2012 based on feature tracking TerraSAR-X data to January 2021 speeds. The white star marks the velocity extraction point."

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On the first image from Milillo et al. (2019) I superimposed another view of my proposed location of the ice-cliff in the Thwaites gateway that may trigger a MICI-type of collapse of the BSB. There are different bed slope pathways leading down into the BSB where the upstream ice thickness gradient is between -0.04 and -0.05; which is an indication of a high probability that an MICI-type of failure may occur in this area circa 2030 to 2040.

Milillo, P. et al. (30 Jan 2019), "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica", Science Advances,Vol. 5, no. 1, eaau3433, DOI: 10.1126/sciadv.aau3433

https://advances.sciencemag.org/content/5/1/eaau3433

 
Fig. 1 Thwaites Glacier, West Antarctica.
(A) Map of Antarctica with Thwaites Glacier (red box). (B) Shaded-relief bed topography (blue) with 50-m contour levels (white) (16), grounding lines color-coded from 1992 to 2017, and retreat rates for 1992–2011 (green circle) versus 2011–2017 (red circle) in kilometer per year. Thick yellow arrows indicate CDW pathways (32). White boxes indicate outline of figs. S1 and S2 (C) DInSAR data for 11 to 12 and 27 to 28 April 2016, with grounding lines in 2011, 2016, and 2017 showing vertical displacement, dz, in 17-mm increments color-coded from purple to green, yellow, red, and purple again. Points A to F are used in Fig. 2. (D) Height of the ice surface above flotation, hf, in meters. (E) Change in ice surface elevation, dh, between decimal years 2013.5 and 2016.66 color-coded from red (lowering) to blue (rising). (F) Ice surface speed in 2016–2017 color-coded from brown (low) to green, purple, and red (greater than 2.5 km/year), with contour levels of 200 m/year in dotted black.

I repost the second image from the ITGC TARSAN project (showing a 2011 ice face); in order to emphasize that we are currently essentially have the conditions postulated for the third perturbation experiment with a pinned Thwaites Eastern Ice Shelf and essentially no Thwaites Ice Tongue; while sometime between 2030 and 2040 we are likely to abruptly transition to the second perturbation experiment with the Thwaites Eastern Ice Shelf unpinned and no Thwaites Ice Tongue (at all); which would abruptly end the current buttressing action of the Thwaites Eastern Ice Shelf on the ice plug that the MICI-type future ice cliff location that I propose (see the first image).

I repost the third image in order to remind readers that between January 2012 and January 2013 the ice surface elevation over the Little Ear subglacial cavity dropped by over 6m (& I believe that this occurred abruptly in September 2012 with a collapse of the roof of the Little Ear subglacial cavity); which (in my opinion) lead to an temporary abrupt loss of the buttressing from the Thwaites Eastern Ice Shelf on my proposed ice plug location (see the first image); which (in my opinion) lead to a jökulhlaup event that flushed a series of icebergs out from the base area of the Thwaites Ice Tongue from September 2012 until early 2014.  I repost this image to remind readers that the Big Ear subglacial cavity did not exist in 2012; and that by 2030 to 2040 it is likely that another jökulhlaup event is likely to occur that will flush all icebergs in front of my proposed MICI-type ice cliff location, thus abruptly exposing the bare ice cliff to a MICI-type failure before any significant ice creep could occur.

Also, the fourth image shows that MICI-types of ice cliff failures result in icebergs with relatively shallow drafts that are not likely to become grounded as they exist the ASE.

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Again, I cite that the decade from 2030 to 2040 as being the most likely decade for initiating an MICI-type of collapse from the base of the Thwaites Ice Tongue down into the Byrd Subglacial Basin for many reasons, including the following:

1. The last super El Nino occurred in the 2015-16 season and such events typically happen roughly every 20-years which would be the 2035-36 season; which is in the middle of the 2030-2040 range; and super El Nino event both advect more warm CDW into the ASE than average and they also increase the likelihood of surface ice melt at the base of the Thwaites Ice Tongue.

2. I believe that the last jökulhlaup event to occur in the Thwaites Gateway occurred in 2012; and it takes about 20 to 25 years to recharge the subglacial lakes associated with such events; which would most likely be between 2032 and 2037; and such an event would likely displace any retaining partially constrained icebergs that might be buttressing the MICI-ice cliff that I have proposed (see my prior posts on this location).

3. The icebergs currently floating above the 'Big Ear' subglacial cavity, are constrained by immediately downstream grounded icebergs with about 3 to 6m of ice height above floatation hf; and at the current rate of basal ice melt in this area and the current rate of thinning of the Thwaites Ice Tongue; I estimate that these partially grounded icebergs will be free to float away in the 2030 to 2040 timeframe; which could likely lead to the proposed future MICI-type of ice cliff at the base of the Thwaites Ice Tongue being abruptly exposed (without time to creep).

4. I estimate that due to basal ice melt that the Thwaites Eastern Ice Shelf will likely become ungrounded in the 2030-2040 timeframe; which in my opinion would relieve a buttressing action from the western base of the Thwaites Eastern Ice Shelf against the proposed ice plug just upstream of the 'Big Ear' subglacial cavity.

5. The first attached image of a computer model projection of CDW temperatures around Antarctica; shows a marked increase in this local CDW temperature in the 2030 to 2040 timeframe; which would increase basal ice melting beneath both the Thwaites Eastern Ice Shelf and the Thwaites Ice Tongue as well as within the Big Ear subglacial cavity.

6. The buttressing action of its ice shelf on the Pine Island SWT Glacier is rapidly deteriorating and I estimate that by 2030-2040 this buttressing action will be eliminated which would decrease the associated restraint on the Thwaites Eastern Shear Margin (see the second image).

7. I am concerned that the Beaufort Gyre will reverse itself sometime in the next twenty years; which in my opinion would increase the likelihood of an MICI-type of collapse of the Thwaite Glacier in the 2030-2040 timeframe; due to the bipolar seesaw mechanism.
 
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Jenkins et al. (2018) provides evidence of how ice mass loss from marine glaciers in the Amundsen Sea sector has been significantly impacted by the ENSO cycle (see the third and fourth images); which is one reason that I believe that a MICI-type of collapse could be triggered in the Thwaites gateway circa 2035/36 +/- 5 yrs as that is when I expect another super El Nino event to occur.

Jenkins, A. et al. (2018), "West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability", Nature Geoscience, volume 11, pages733–738, DOI: https://doi.org/10.1038/s41561-018-0207-4

https://www.nature.com/articles/s41561-018-0207-4

Abstract: "Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high."
 
Image 4
Caption for the fourth image: "Fig. 5 Multi-decadal history of ocean forcing and outlet glacier response in the eastern Amundsen Sea. Time series of glacier outflow changes (righthand axis, with one standard deviation error bars) and ocean forcing (red, warm conditions; blue, cool conditions) as documented here (darker shading) and as inferred (lighter shading) from central tropical Pacific sea surface temperatures (left-hand axis, both normalized). Shaded boxes (outlined and colour-coded by glacier) indicate the range of estimated times for the initiation of the most recent phase of rapid thinning at the grounding lines, while boxes without outlines are inferred times of initial and final detachment of Pine Island Glacier from a submarine ridge."

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While, it would be nice if I were wrong about the fragility of the ice constraints at the base of the Thwaites Ice Tongue; however, the linked open access reference provides recent evidence that the stability of the ice in this area is becoming less and less stable with each passing year.

Wåhlin, A.K., et al. (09 Apr 2021), "Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica", Science Advances, Vol. 7, no. 15, eabd7254, DOI: 10.1126/sciadv.abd7254

https://advances.sciencemag.org/content/7/15/eabd7254

Abstract: "Thwaites Glacier is the most rapidly changing outlet of the West Antarctic Ice Sheet and adds large uncertainty to 21st century sea-level rise predictions. Here, we present the first direct observations of ocean temperature, salinity, and oxygen beneath Thwaites Ice Shelf front, collected by an autonomous underwater vehicle. On the basis of these data, pathways and modification of water flowing into the cavity are identified. Deep water underneath the central ice shelf derives from a previously underestimated eastern branch of warm water entering the cavity from Pine Island Bay. Inflow of warm and outflow of melt-enriched waters are identified in two seafloor troughs to the north. Spatial property gradients highlight a previously unknown convergence zone in one trough, where different water masses meet and mix. Our observations show warm water impinging from all sides on pinning points critical to ice-shelf stability, a scenario that may lead to unpinning and retreat."

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Alley et al. (2021) concludes that:

"This evidence of weakening at a rapid pace suggests that the TEIS is likely to fully destabilize in the next few decades, leading to further acceleration of Thwaites Glacier."

I note that any such near-term full destabilization of the TEIS should increase the risk of an MICI-type of collapse of Thwaites Glacier initiating in the gateway near the base of the TWIT (Thwaites Western Ice Tongue) in the coming decades.

Alley, K.E. et al. (2021), "Two decades of dynamic change and progressive destabilization on the Thwaites Eastern Ice Shelf", The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-76

https://tc.copernicus.org/preprints/tc-2021-76/

Abstract. The Thwaites Eastern Ice Shelf (TEIS) buttresses the eastern grounded portion of Thwaites Glacier through contact with a pinning point at its seaward limit. Loss of this ice shelf will promote further acceleration of Thwaites Glacier. Understanding the dynamic controls and structural integrity of the TEIS is therefore important to estimating Thwaites' future sea-level contribution. We present a ~20-year record of change on the TEIS that reveals the dynamic controls governing the ice shelf's past behavior and ongoing evolution. We derived ice velocities from MODIS and Sentinel-1 image data using feature tracking and speckle tracking, respectively, and combined these records with ITS_LIVE and GOLIVE velocity products from Landsat 7 and 8. In addition, we estimated surface lowering and basal melt rates using the REMA DEM in comparison to ICESat and ICESat-2 altimetry. Early in the record, TEIS flow dynamics were strongly controlled by the neighboring Thwaites Western Ice Tongue (TWIT). Flow patterns on the TEIS changed following the disintegration of the TWIT in ~2008, with a new divergence in ice flow developing around the pinning point at its seaward limit. Simultaneously, the TEIS developed new rifting that extends from the shear zone upstream of the ice rise and increased strain concentration within this shear zone. As these horizontal changes occurred, sustained thinning driven by basal melt reduced ice thickness, particularly near the grounding line and in the shear zone area upstream of the pinning point. This evidence of weakening at a rapid pace suggests that the TEIS is likely to fully destabilize in the next few decades, leading to further acceleration of Thwaites Glacier.

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Deb et al. (2018) indicates that the projected increase (with continued global warming) of more frequent strong El Nino events combined with the projected increase in positive SAM, will significantly increase ice mass loss from the ASE, which will increase the risk of a collapse of the WAIS:

Deb, P., A. Orr, D. H. Bromwich, J. P. Nicolas, J. Turner, and J. S. Hosking, 2018: Summer drivers of atmospheric variability affecting ice shelf thinning in the Amundsen Sea Embayment, West Antarctica. Geophy. Res. Lett., 45. doi: 10.1029/2018GL077092.

http://polarmet.osu.edu/PMG_publications/deb_bromwich_grl_2018.pdf

Abstract:  "Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Niño episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Niño episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves."

Extract: "Our study suggests that ASE ice shelves could experience an intensification of melt in the future from both above and below as a result of both regional and large-scale atmospheric changes, potentially increasing the risk of their disintegration, which in turn could potentially trigger a collapse of the West Antarctic ice sheet (DeConto & Pollard, 2016). To better understand this threat will require further detailed investigation of the impacts of ENSO, the polarity of the SAM, and the depth/location of the ASL on ASE ice shelves. Also necessary is improving the reliability of future projections, such as ENSO and its teleconnections, as well as the response of the SAM to recovery of the Antarctic ozone hole and increased greenhouse gas emissions (Polvani, Waugh, et al., 2011)."

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FT2 and the Associated Probability of Occurrence for X2 (Triggering of a Temporary Reversal of the Beaufort Gyre)

Armitage et al. (2020) indicates that although the freshwater content of the Beaufort Gyre has increased to about 8,000 cubic km since the 1990s, subsequent to about 2007 this freshwater content has remained relatively stable in response to sea ice loss (since 2007), see the first three images.  This recent relative stability of the freshwater content of the Beaufort Gyre is one reason why the MCBS-BN assumes that Event X1 is required to trigger Event X2, by altering the flow of warm ocean water from the Gulf Stream into the Barents Sea as indicated in the fourth image.

Armitage, T.W.K., Manucharyan, G.E., Petty, A.A., Thompson, A.F., Kwok, R., Enhanced eddy activity in the Beaufort Gyre in response to sea ice loss. Nat Commun 11, 761 (2020). https://doi.org/10.1038/s41467-020-14449-z

https://www.nature.com/articles/s41467-020-14449-z

Abstract: "The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years. The mechanisms proposed to explain the stabilization involve either mesoscale eddy activity that opposes Ekman pumping or the reduction of Ekman pumping due to reduced sea ice–ocean surface stress. However, the relative importance of these mechanisms is unclear. Here, we present observational estimates of the Beaufort Gyre mechanical energy budget and show that energy dissipation and freshwater content stabilization by eddies increased in the late-2000s. The loss of sea ice and acceleration of ocean currents after 2007 resulted in enhanced mechanical energy input but without corresponding increases in potential energy storage. To balance the energy surplus, eddy dissipation and its role in gyre stabilization must have increased after 2007. Our results imply that declining Arctic sea ice will lead to an increasingly energetic Beaufort Gyre with eddies playing a greater role in its stabilization."

 
Caption for the first image: "a Before and b after 2007, including the wind work, W (comprised of atmosphere-ocean, Wao, and ice–ocean, Wio, components), available potential energy (APE), and eddy dissipation, Weddy. The atmosphere and ocean circulations are illustrated by ua and ug, respectively. The size of the arrows/vectors represents their relative strength. The loss of sea ice after 2007 led to increased wind energy input to the BG, increased APE, and increased energy dissipation and freshwater stabilization by eddies."

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Title: "Arctic ice melt changing major ocean current"

https://earthsky.org/earth/arctic-ice-melt-changing-major-ocean-current-beaufort-gyre

Extract: "But the since the 1990s, the gyre has accumulated a large amount of fresh water – 1,920 cubic miles (8,000 cubic km) – or almost twice the volume of Lake Michigan. The new study found that the cause of this gain in freshwater concentration is the loss of sea ice in summer and autumn. This decades-long decline of the Arctic’s summertime sea ice cover has left the Beaufort Gyre more exposed to the wind, which spins the gyre faster and traps the fresh water in its current."