Ice Apocalypse - Multiple Meters Sea Level Rise

[AbruptSLR]

Re 744.

Have you seen the news in #64 here.

https://forum.arctic-sea-ice.net/index.php/topic,2449.msg191933.html

"The plant assemblages indicate that there was an abrupt and major shift in the vegetation from wet, cold conditions at Pilauco to warm, dry conditions," Kennett said. According to him, the atmospheric zonal climatic belts shifted "like a seesaw," with a synergistic mechanism, bringing warming to the Southern Hemisphere even as the Northern Hemisphere experienced cooling and expanding sea ice.

They could be related?

kassy,

Thanks for the link, as I had not seen that article before; however the Younger Dryas was from approximately 12,900 to about 11,700 years BP; so the partial WAIS discussed in 11,500BP in Reply #744, may, or may not, be related.

Best,
ASLR

[AbruptSLR]

CMIP6 is already being conducted with at best consideration of MISI types of ice mass loss from both the GIS & AIS, and completely without consideration of MICI types of ice mass losses, therefore, a discussion of the implications of the growing number of consensus climate scientists who are pointing to the findings of Edwards et al. (2019) as justification for ignoring MICI types of ice mass loss in their model projections this century may be academic.  Nevertheless, here I point out that Edwards et al. (2019) used a statistical emulator of ice sheet mass loss over the past 1 million years to indicate that most likely MISI behavior could account for the paleo record; however, this finding is a far different matter than concluding that MICI types of behavior can be discounted in model projections for the rest of this century.

First, as SSP5-Baseline indicates that we may well be at Mid-Pliocene (3.3 Ma–3 Ma) conditions by (or before) 2040, the Edwards et al. (2019) emulation of the past 1Ma are likely not relevant; while Pollard, DeConto & Alley (2018)'s evaluation of MICI under Mid-Pliocene conditions are more relevant.

Second, we are approaching Mid-Pliocene conditions thousands of times faster than occurred during the Mid-Pliocene; which Pollard, DeConto & Alley (2018) took into account by abruptly introducing Mid-Pliocene conditions onto modern AIS conditions in their model.

Third, both Edwards et al (2019) and Pollard, DeConto & Alley (2018) use AIS boundary & starting conditions for the modern AIS that are less aggressive than what is currently observed in 2019 w.r.t. such factors as: a) the large subglacial cavity at the base of the Thwaites Ice Tongue; b) the loss of ice shelf buttressing of the Southwest Tributary Glacier in Pine Island Bay; and c) the amount of ice-climate feedback mechanisms that are already being activated (including MOC slowing, shifting of the ENSO towards more frequent El Nino events & increased advection of warm CDW towards the grounding lines of key AIS marine glaciers).

I understand that CMIP6 is already pushing the computational capacity of the participation ESMs; I think that it is wasteful of research funds to be investigating scenarios with little chance of occurring, and also I note that currently the cost of using Exascale computers is dropping by half about every 3 months.  Therefore, I believe that rather than likely committing a Type 2 error (a false negative, i.e. assuming that MICI ice mass loss will not occur this century when it actually might occur after 2040); it would be better for consensus climate scientists to work harder (possibly by using Exascale computing resources) on including MICI mechanisms in at least some of their scenarios so as to at least evaluate right-tail risks.

Finally, I provide the attached images related to use of the scientific method (without comment), as I believe that the proper use of this method would not result in MICI behavior being discounted in on-going ESM modeling efforts.

Edit: I forgot to mention that the emulator used by Edwards et al. (2019) did not include the impact of the Antarctic ozone holes (as this did not occur in paleo-times); which this ozone hole has accelerated ice mass loss from Antarctica and projections of the impact of continuing GHG emissions on the westerly winds over the Southern Ocean, indicate that as the ozone hole heals the regional impact of increasing GHG atmospheric concentrations will keep these regional westerly winds in an optimal zone for promoting ice mass loss for at least the next few decades.

[AbruptSLR]

The linked open access commentary provides a convenient summary of the nature and likely impacts of Arctic Amplification:

Twila A. Moon et al. (07 March 2019), "The expanding footprint of rapid Arctic change", Earth's Future, https://doi.org/10.1029/2018EF001088

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018EF001088

Abstract
Arctic land ice is melting, sea ice is decreasing, and permafrost is thawing. Changes in these Arctic elements are interconnected, and most interactions accelerate the rate of change. The changes affect infrastructure, economics, and cultures of people inside and outside of the Arctic, including in temperate and tropical regions, through sea level rise, worsening storm and hurricane impacts, and enhanced warming. Coastal communities worldwide are already experiencing more regular flooding, drinking water contamination, and coastal erosion. We describe and summarize the nature of change for Arctic permafrost, land ice, and sea ice, and its influences on lower latitudes, particularly the United States. We emphasize that impacts will worsen in the future unless individuals, businesses, communities, and policy makers proactively engage in mitigation and adaptation activities to reduce the effects of Arctic changes and safeguard people and society.

[AbruptSLR]

We shouldn't forget that shrubs thrive (relative to trees as explained in the linked reference) in relatively cold regions, and there continued expansion into the tundra decreases albedo & thus increases Arctic Amplification:

Treml et al. (2019), "Differences in growth between shrubs and trees: How does the stature of woody plants influence their ability to thrive in cold regions?", Agricultural and Forest Meteorology, https://doi.org/10.1016/j.agrformet.2019.02.036

https://www.sciencedirect.com/science/article/pii/S0168192319300954

Abstract
Shrubs can be found far above or beyond cold tree limits. However, the mechanisms shrubs employ to thrive at sites not allowing the development of trees remain poorly understood. We hypothesize that shrubs are advantaged over trees thanks to: (i) their low stature reflected in a better thermal environment; (ii) differences in temperature thresholds of wood formation; and (iii) a shorter period of wood formation in the slender stems of shrubs with narrow cells compared to tree stems with large cells. We studied wood formation of Picea abies (trees) and Pinus mugo (shrubs) growing on the same site in the treeline ecotone of the Krkonoše Mts in the Czech Republic. We measured air temperature near tree (shrub) tops, stem temperature and soil temperature in the root zone. In addition, we determined the number of cells in individual phases of wood phenology. We then computed the duration of individual wood-phenology phases and temperature thresholds for the onset of wood formation. Our results show that in the growing season, shrubs experience higher amplitude of air and stem temperatures compared to trees. Mean growing season air and stem temperatures are similar between the two growth forms whereas mean soil temperatures are lower for shrubs because their dense canopies shade the ground. Temperature thresholds for wood formation are either similar (3 °C for soil temperature, onset of cell division) or greater by 1.2–2.6 K (onset of cell enlargement) for P. mugo shrubs than for P. abies trees, depending on the temperature metrics considered. Although we found ambiguous differences in the onset of wood formation, this was completed earlier in P. mugo than in P. abies, leading to a generally shorter growing period of shrubs (103 days) than trees (125 days). In conclusion, the main advantage of shrubs over trees resides in the earlier completion of wood formation and thus a shorter growing season. Trees with wide cells at stem base require more time for cell differentiation and maturation than shrubs with narrow cells. Other differences are either of lesser importance (the ambient thermal environment) or probably species-specific (temperature thresholds for wood formation).

Edit: I also not that the continuing expansion of digging creatures (like ground squirrels, etc.) into the tundra (with continued warming) accelerates permafrost degradation and thus also contributes to increasing Arctic Amplification.

[AbruptSLR]

I do not have time to comment adequately about the information provided by the WAIS Divide ice core project, but perhaps come of the extracts below from the linked open access reference provides some idea of the sensitivity of the WAIS over the time period to 68 ka:

Kendrick Taylor (10 September 2016), "Introduction to special section on the WAIS Divide Special Issue of Paleoceanography", Paleoceanography and Paleoclimatology,  https://doi.org/10.1002/2016PA002995

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016PA002995

Extract: "The high temporal resolution of the atmospheric carbon dioxide record revealed three abrupt increases in carbon dioxide at the times of major climate events (Heinrich Stadial 1, the Bølling warming, and the termination of the Younger Dryas). These enigmatic features of the deglaciation have spurred a discussion of the possible sources of carbon capable of responding to climate on the centennial scale [Bauska et al., 2016]. The high temporal resolution also allowed detailed records of late Holocene variability in the concentration [Ahn et al., 2012] and isotopic composition [Bauska et al., 2015] of atmospheric carbon dioxide, revealing multidecadal changes in land carbon storage.

The detailed atmospheric methane record defined new modes of variability, including sharp increases during some of the cold Greenland “Heinrich” stadials. This led to a hypothesis that extreme southward migration of the Intertropical Convergence Zone during the Greenland stadials activated Southern Hemisphere methane sources [Rhodes et al., 2015]. During Heinrich Stadial 1, this possible Southern Hemisphere source of methane is directly associated with an abrupt rise in atmospheric carbon dioxide. Additional work constrained the anthropogenic contribution to atmospheric methane during the late Holocene [Mitchell et al., 2013], possibly including preindustrial human influence [Mischler et al., 2009] and other details of methane during the Holocene period [Sowers, 2010]."

Edit, note that per the linked reference a significant discharge of icebergs can cause an abrupt increase in tropical methane production:

Rhodes, R. H., E. J. Brook, J. C. H. Chiang, T. Blunier, O. J. Maselli, J. M. McConnell, D. Romanini, and J. P. Severinghaus (2015), Enhanced tropical methane production in response to iceberg discharge in the North Atlantic, Science, 348(6238), 1016–1019, doi:10.1126/science.1262005.

http://science.sciencemag.org/content/348/6238/1016

The tropical impact of iceberg armadas
The massive discharges of icebergs from the Greenland ice sheet during the Last Glacial Period are called Heinrich events. But did Heinrich events cause abrupt climate change, or were they a product of it? Methane levels represent a proxy for climate, because methane production increases mostly due to wetter conditions in the tropics. Rhodes et al. report a highly resolved record of atmospheric methane concentrations, derived from an ice core from Antarctica. Methane levels varied—i.e., the tropical climate changed—in response to cooling in the Northern Hemisphere caused by Heinrich events.

Abstract
The causal mechanisms responsible for the abrupt climate changes of the Last Glacial Period remain unclear. One major difficulty is dating ice-rafted debris deposits associated with Heinrich events: Extensive iceberg influxes into the North Atlantic Ocean linked to global impacts on climate and biogeochemistry. In a new ice core record of atmospheric methane with ultrahigh temporal resolution, we find abrupt methane increases within Heinrich stadials 1, 2, 4, and 5 that, uniquely, have no counterparts in Greenland temperature proxies. Using a heuristic model of tropical rainfall distribution, we propose that Hudson Strait Heinrich events caused rainfall intensification over Southern Hemisphere land areas, thereby producing excess methane in tropical wetlands. Our findings suggest that the climatic impacts of Heinrich events persisted for 740 to 1520 years.

Edit 2, the following link leads to the table of content of all articles in the special issue of Paleoceanography and Paleoclimatology on the WAIS Divide ice core project:

https://agupubs.onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)1944-9186.WAISDIV

Edit 3, in particular see also:

Rachael H. Rhodes  Edward J. Brook  Joseph R. McConnell  Thomas Blunier  Louise C. Sime  Xavier Faïn  Robert Mulvaney (14 March 2017), "Atmospheric methane variability: Centennial‐scale signals in the Last Glacial Period", Global Biogeochemical Cycles, https://doi.org/10.1002/2016GB005570

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2016GB005570

Abstract
In order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh‐resolution CH4 record of the West Antarctic Ice Sheet Divide ice core 67.2–9.8 ka and find novel, atmospheric CH4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80–500 year range, but we find that age‐scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn‐based signal smoothing processes. Within interstadial and stadial periods, the peak‐to‐peak signal amplitudes vary in proportion to the underlying millennial‐scale oscillations in CH4 concentration—the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low‐latitude wetland CH4 emissions.

Plain Language Summary
Using a new method to measure methane concentrations of ancient air trapped in ice cores, we have detected variability in atmospheric methane concentration on centennial time scales in the Last Glacial Period for the first time. We know these signals represent past changes in atmospheric methane because they appear in several ice core records. We propose that changes in methane emissions from tropical wetlands are responsible. How this new variability might be related to similar signals found in the late Holocene ice core records and the instrumental record of atmospheric methane is an open question.

Edit 4, see also:

Title: "How complexity science can quickly detect climate record anomalies", 2018

https://www.santafe.edu/news-center/news/how-complexity-science-can-quickly-detect-anomalies-climate-records

Extract: "In previous cores, Garland notes that decades, even centuries, were aggregated into a single point. The WAIS data, by contrast, sometimes gives more than forty data points per year. But as scientists move to analyze the data at shorter time scales, even small anomalies can be problematic.

“As fine-grained data becomes available, fine-grained analyses can be performed,” Garland notes. “But it also makes the analysis susceptible to fine-grained anomalies.”

To quickly identify which anomalies require further investigation, the team uses information theoretic techniques to measure how much complexity appears at each point in the time sequence. A sudden spike in the complexity could mean that there was either a major, unexpected climate event, like a super volcano, or that there was an issue in the data or the data processing pipeline."

[AbruptSLR]

As noted previously in this thread E3SMv1 has a reported mean ECS of 5.3C (see the attached image & associated caption), and now JAMES has a paper with all of the details (which are too numerous for me to address here), and I note that Andrew Dessler reports that several other CMIP6 models also have values of ECS greater than 5C.  If so this increases the calculated probability of a MICI type of WAIS collapse beginning circa 2040:

Jean‐Christophe Golaz et al. (15 March 2019), "The DOE E3SM coupled model version 1: Overview and evaluation at standard resolution", JAMES, https://doi.org/10.1029/2018MS001603

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018MS001603#.XIv0M0dysFU.twitter
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https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018MS001603

Abstract
This work documents the first version of the U.S. Department of Energy (DOE) new Energy Exascale Earth System Model (E3SMv1). We focus on the standard resolution of the fully‐coupled physical model designed to address DOE mission‐relevant water cycle questions. Its components include atmosphere and land (110km grid spacing), ocean and sea ice (60km in the mid‐latitudes and 30km at the equator and poles), and river transport (55km) models. This base configuration will also serve as a foundation for additional configurations exploring higher horizontal resolution as well as augmented capabilities in the form of biogeochemistry and cryosphere configurations.

The performance of E3SMv1 is evaluated by means of a standard set of Coupled Model Intercomparison Project Phase 6 Diagnosis, Evaluation, and Characterization of Klima (CMIP6 DECK) simulations consisting of a long pre‐industrial control, historical simulations (ensembles of fully coupled and prescribed SSTs) as well as idealized CO2 forcing simulations. The model performs well overall with biases typical of other CMIP‐class models, although the simulated Atlantic Meridional Overturning Circulation is weaker than many CMIP‐class models. While the E3SMv1 historical ensemble captures the bulk of the observed warming between pre‐industrial (1850) and present‐day, the trajectory of the warming diverges from observations in the second half of the 20th century with a period of delayed warming followed by an excessive warming trend. Using a two‐layer energy balance model, we attribute this divergence to the model's strong aerosol‐related effective radiative forcing (ERFari+aci = ‐1.65 W m‐2) and high equilibrium climate sensitivity (ECS = 5.3 K).

Plain Language Summary
The United States Department of Energy funded the development of a new state‐of‐the‐art Earth system model for research and applications relevant to its mission. The Energy Exascale Earth System Model version 1 (E3SMv1) consists of five interacting components for the global atmosphere, land surface, ocean, sea ice and rivers. Three of these components (ocean, sea ice and river) are new and have not been coupled into an Earth system model previously. The atmosphere and land surface components were created by extending existing components part of the Community Earth System Model, Version 1.

E3SMv1's capabilities are demonstrated by performing a set of standardized simulation experiments described by the Coupled Model Intercomparison Project Phase 6 Diagnosis, Evaluation, and Characterization of Klima (CMIP6 DECK) protocol at standard horizontal spatial resolution of approximately 1 degree latitude and longitude. The model reproduces global and regional climate features well compared to observations. Simulated warming between 1850 and 2015 matches observations, but the model is too cold by about 0.5°C between 1960 and 1990 and later warms at a rate greater than observed. A thermodynamic analysis of the model's response to greenhouse gas and aerosol radiative affects may explain the reasons for the discrepancy.

Caption: "Figure 27. Time evolution of annual global mean air surface temperature anomalies for the idealized CO₂ forcing simulations abrupt-4xCO2 (red), 1pctCO2 (blue) and the control simulation (piControl; green). Solid lines are fits obtained with a two-layer energy balance model (discussed in sub-section 6.3). Also depicted are estimates of ECS and TCR."

[AbruptSLR]

I have previously noted that earlier consensus climate science reports like AR5 have made liberal use of caveats to exclude several right-tailed feedback mechanisms from the confidence ranges that they report for various climate change parameters (see also my comments in at least Replies #707 & #751).  Furthermore, in at least Reply #719 I recommended that consensus science generate families of Maximum-credible Climate-risk Scenarios (MCSs) in order to better assess right-tail climate risks.

In this post I briefly note consensus science only formally cites two limit states (i.e. the aspirational 1.5C GMSTA target and the 'Well below' 2C GMSTA limit by 2100).  To my Civil Engineering way of thinking the 1.5C target can be associated with an 'Operations' limit state where the proper function of our socioeconomic systems begin to fail; while the 'Well below' 2C limit can be associated where significant human life safety becomes threatened.

However, also to my Civil Engineering way of thinking, modern civil society depends on numerous other limit states (in civil engineering designs there are several other limit states such as: 'Progressive Collapse', 'Fatigue' and 'Durability limit states), that it would be good for consensus science to formally specify acceptable limit states for this century such as: a) Sea Level Rise and the associated rate of Sea Level Rise; b) Tropical Pacific SSTA, and c) Cascades of tipping points leading to abrupt changes in climate state.  Also, I note that in limit state design engineers assign partial load factors of safety for each separate forcing according to its uncertainty and partial resistance factors of safety to reflex the fragility of each separate system component according to its uncertainty.  Furthermore, I note that all civil engineering designs to adapt to climate change use the incomplete forcing from consensus science reports like AR5/CMIP5 and use load and resistance safety factors that were empirically established for quasi-static climate change, and that little or no attempts have been made to develop such factors of safety appropriate to maintain civil society under conditions of rapid/dynamic climate change.

Finally, I provide the linked UC Berkeley article and associated images on how the real process of science works; which per the attached images includes community feedback and societal expectations of benefits from scientific work.  In this sense the real consensus climate science process is politicized by community resistance to change and by societal expectation that science bring benefits ('good science') rather than reporting increasing risks/dangers ('bad science'):

Title: "The real process of science"

https://undsci.berkeley.edu/article/howscienceworks_02

Extract: "At first this process might seem overwhelming. Even within the scope of a single investigation, science may involve many different people engaged in all sorts of different activities in different orders and at different points in time — it is simply much more dynamic, flexible, unpredictable, and rich than many textbooks represent it as."

[AbruptSLR]

In my last post I suggested that consensus science formally set limits for socioeconomically acceptable levels of changes in Tropical Pacific SST, as this region of the Earth is critical from driving climate change.  In this regard, the linked reference provides discussions of how this regions manages to influence so much of the Earth's climate and also notes that during El Nino events (which are projected to become more frequent with continued warming) heat is extracted from the Tropical Pacific ocean water via increase evaporation, and I note that water vapor is a key driver of climate change:

Lijing Cheng (14 March 2019), "Evolution of ocean heat content related to ENSO", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0607.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0607.1

Abstract
As the strongest inter-annual perturbation to the climate system, the El Niño-Southern Oscillation (ENSO) dominates the year-to-year variability of the ocean energy budget. Here we combine ocean observations, re-analyses, and surface flux data with Earth system model simulations to obtain estimates of the different terms affecting the redistribution of energy in the Earth system during ENSO events, including exchanges between ocean and atmosphere, among different ocean basins, lateral and vertical rearrangements. This comprehensive inventory allows better understanding of the regional and global evolution of ocean heat related to ENSO, and provides observational metrics to benchmark performance of climate models. Results confirm that there is a strong negative ocean heat content tendency (OHCT) in the tropical Pacific Ocean during El Niño, mainly through enhanced air-sea heat fluxes (Q) into the atmosphere driven by high sea surface temperatures. As well as this diabatic component, there is an adiabatic redistribution of heat both laterally and vertically (0-100m and 100-300m) in the tropical Pacific and Indian oceans that dominates the local OHCT. Heat is also transported and discharged from 20oS-5oN into off-equatorial regions within 5-20oN during and after El Niño. OHCT and Q changes outside of the tropical Pacific Ocean indicates the ENSO-driven atmospheric teleconnections and changes of ocean heat transport (i.e. Indonesian Throughflow). The tropical Atlantic and Indian oceans warm during El Niño, partly offsetting the tropical Pacific cooling for the tropical oceans as a whole. While there are distinct regional OHCT changes, many compensate each other resulting in a weak but robust net global ocean cooling during and after El Niño.

[AbruptSLR]

Ed Hawkins provides insight on how to correctly interpret various different climate parameters at 'Climate Lab Book'"

https://www.climate-lab-book.ac.uk/author/ehawkins/

See also:

Title: "What does ‘mean’ actually mean?"

https://www.climate-lab-book.ac.uk/2018/what-does-mean-actually-mean/#more-5569
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See also:

Kevin Cowtan et al. (20 July 2018), Statistical analysis of coverage error in simple global temperature estimators", Dynamics and Statistics of the Climate System, Volume 3, Issue 1, dzy003, https://doi.org/10.1093/climsys/dzy003

https://academic.oup.com/climatesystem/article/3/1/dzy003/5056434

Abstract

Background
Global mean surface temperature is widely used in the climate literature as a measure of the impact of human activity on the climate system. While the concept of a spatial average is simple, the estimation of that average from spatially incomplete data is not. Correlation between nearby map grid cells means that missing data cannot simply be ignored. Estimators that (often implicitly) assume uncorrelated observations can be biased when naively applied to the observed data, and in particular, the commonly used area weighted average is a biased estimator under these circumstances. Some surface temperature products use different forms of infilling or imputation to estimate temperatures for regions distant from the nearest observation, however the impacts of such methods on estimation of the global mean are not necessarily obvious or themselves unbiased. This issue was addressed in the 1970s by Ruvim Kagan, however his work has not been widely adopted, possibly due to its complexity and dependence on subjective choices in estimating the dependence between geographically proximate observations.

Objectives
The aim of this work is to present a simple estimator for global mean surface temperature from spatially incomplete data which retains many of the benefits of the work of Kagan, while being fully specified by two equations and a single parameter. The main purpose of the simplified estimator is to better explain to users of temperature data the problems associated with estimating an unbiased global mean from spatially incomplete data, however the estimator may also be useful for problems with specific requirements for reproducibility and performance.

Methods
The new estimator is based on generalized least squares, and uses the correlation matrix of the observations to weight each observation in accordance with the independent information it contributes. It can be implemented in fewer than 20 lines of computer code. The performance of the estimator is evaluated for different levels of observational coverage using reanalysis data with artificial noise.

Results
For recent decades the generalized least squares estimator mitigates most of the error associated with the use of a naive area weighted average. The improvement arises from the fact that coverage bias in the historical temperature record does not arise from an absolute shortage of observations (at least for recent decades), but rather from spatial heterogeneity in the distribution of observations, with some regions being relatively undersampled and others oversampled. The estimator addresses this problem by reducing the weight of the oversampled regions, in contrast to some existing global temperature datasets which extrapolate temperatures into the unobserved regions. The results are almost identical to the use of kriging (Gaussian process interpolation) to impute missing data to global coverage, followed by an area weighted average of the resulting field. However, the new formulation allows direct diagnosis of the contribution of individual observations and sources of error.

Conclusions
More sophisticated solutions to the problem of missing data in global temperature estimation already exist. However the simple estimator presented here, and the error analysis that it enables, demonstrate why such solutions are necessary. The 2013 Fifth Assessment Report of the Intergovernmental Panel on Climate Change discussed a slowdown in warming for the period 1998-2012, quoting the trend in the HadCRUT4 historical temperature dataset from the United Kingdom Meteorological Office in collaboration with the Climatic Research Unit of the University of East Anglia, along with other records. Use of the new estimator for global mean surface temperature would have reduced the apparent slowdown in warming of the early 21st century by one third in the spatially incomplete HadCRUT4 product.
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Title: "Reconciling estimates of climate sensitivity"

https://www.climate-lab-book.ac.uk/2016/reconciling-estimates-of-climate-sensitivity/#more-4441
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Ed Hawkins Twitter

https://twitter.com/ed_hawkins?ref_src=twsrc%5Egoogle%7Ctwcamp%5Eserp%7Ctwgr%5Eauthor

[AbruptSLR]

I think that the linked 2018 YouTube video by Jim White is worth watching (if you have a spare 1.15 hours), as he provides discussions of: how Earth Systems work, how people impact those systems and what are some abrupt change risks):

Title: "Climate Change: Dr Jim White (January 2018)"



See also:

https://nas-sites.org/americasclimatechoices/other-reports-on-climate-change/abrupt-impacts-of-climate-change/

[AbruptSLR]

An Ice Apocalypse includes an albedo flip associated with a seasonally sea ice free Arctic Ocean, and the linked reference indicates how such a future Arctic sea ice free season will grow faster in the autumn than in the spring:

Lebrun, M., Vancoppenolle, M., Madec, G., and Massonnet, F.: Arctic sea-ice-free season projected to extend into autumn, The Cryosphere, 13, 79-96, https://doi.org/10.5194/tc-13-79-2019, 2019.

https://www.the-cryosphere.net/13/79/2019/

Abstract
The recent Arctic sea ice reduction comes with an increase in the ice-free season duration, with comparable contributions of earlier ice retreat and later advance. CMIP5 models all project that the trend towards later advance should progressively exceed and ultimately double the trend towards earlier retreat, causing the ice-free season to shift into autumn. We show that such a shift is a basic feature of the thermodynamic response of seasonal ice to warming. The detailed analysis of an idealised thermodynamic ice–ocean model stresses the role of two seasonal amplifying feedbacks. The summer feedback generates a 1.6-day-later advance in response to a 1-day-earlier retreat. The underlying physics are the property of the upper ocean to absorb solar radiation more efficiently than it can release heat right before ice advance. The winter feedback is comparatively weak, prompting a 0.3-day-earlier retreat in response to a 1-day shift towards later advance. This is because a shorter growth season implies thinner ice, which subsequently melts away faster. However, the winter feedback is dampened by the relatively long ice growth period and by the inverse relationship between ice growth rate and thickness. At inter-annual timescales, the thermodynamic response of ice seasonality to warming is obscured by inter-annual variability. Nevertheless, in the long term, because all feedback mechanisms relate to basic and stable elements of the Arctic climate system, there is little inter-model uncertainty on the projected long-term shift into autumn of the ice-free season.

[AbruptSLR]

The linked open access reference is relevant to projecting the future stability of Arctic sea ice and the underlying ocean stratification and warming of the inflowing waters:

H. Sadatzki, T. M. Dokken, S. M. P. Berben, F. Muschitiello, R. Stein, K. Fahl, L. Menviel, A. Timmermann, E. Jansen, Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles. Sci. Adv. 5, eaau6174 (2019).

http://advances.sciencemag.org/content/5/3/eaau6174

Abstract
The last glacial period was marked by pronounced millennial-scale variability in ocean circulation and global climate. Shifts in sea ice cover within the Nordic Seas are believed to have amplified the glacial climate variability in northern high latitudes and contributed to abrupt, high-amplitude temperature changes over Greenland. We present unprecedented empirical evidence that resolves the nature, timing, and role of sea ice fluctuations for abrupt ocean and climate change 32 to 40 thousand years ago, using biomarker sea ice reconstructions from the southern Norwegian Sea. Our results document that initial sea ice reductions at the core site preceded the major reinvigoration of convective deep-water formation in the Nordic Seas and abrupt Greenland warming; sea ice expansions preceded the buildup of a deep oceanic heat reservoir. Our findings suggest that the sea ice variability shaped regime shifts between surface stratification and deep convection in the Nordic Seas during abrupt climate changes.

See also: "New PhD thesis solves mysteries around large abrupt climate changes"

https://www.bjerknes.uib.no/en/article/news/new-phd-thesis-solves-mysteries-around-large-abrupt-climate-changes


Extract: "Henrik Sadatzki defends on Friday 15.02.2019 his thesis for the PhD degree at the University of Bergen. The thesis is entitled: “Sea ice variability in the Nordic Seas over Dansgaard–Oeschger climate cycles during the last glacial – A biomarker approach”.

[AbruptSLR]

The first two (associated) linked sources indicate that due to global warming intense El Nino events will become more frequent (estimated to occur about every 10 years); while the third linked source indicates that there is a good chance that there will be an intense El Nino event in the 2019-2020 season; which would be only four years after the 2015-16 intense El Nino event.  This is a clear indication that the likely range for ECS is higher than AR5 indicates; and this adds to the probability that the WAIS may initiate a MICI type of collapse circa 2040:

Wenju Cai et al. (2018), "Increased variability of eastern Pacific El Nino under greenhouse warming", Nature 564, 201-206, https://doi.org/10.1038/s41586-018-0776-9

https://www.nature.com/articles/s41586-018-0776-9?WT.feed_name=subjects_ocean-sciences

Extract: "An increase in SST variance implies an increase in the number of 'strong' EP-El Nino events (corresponding to large SST anomalies) and associated extreme weather events."

See also:

Title: "El Nino events to become 'stronger' and more intense, study finds"

https://www.smh.com.au/environment/climate-change/el-nino-events-to-become-stronger-and-more-intense-study-finds-20181212-p50lrv.html

"They are stronger and more frequent," Dr Cai said, adding the likelihood of intense El Nino events as measured by sea-surface temperatures will increase from about one every 15 years now to every 10 years on average during this century.
…
Big El Ninos of recent decades include 1982-83, 1997-98 and 2015-16."

Also, particularly see:

Title: "'Monster' El Nino a chance later this year, pointing to extended dry times"

https://www.smh.com.au/environment/climate-change/monster-el-nino-a-chance-later-this-year-pointing-to-extended-dry-times-20190315-p514hi.html

Extract: "The prospect of a big El Nino later this year was raised at an international conference of climate scientists in Chile earlier this month.

They considered parallel years, such as 2014 when a near-El Nino was reached before conditions revived a year later, creating one of the three most powerful such events in the past half century.

"There is more heat now below the surface waiting to be tapped than there was in early 2015," said Michael McPhaden, a senior scientist with US National Atmospheric and Oceanic Administration who attended the Chilean event.

"If westerly wind bursts of sufficient amplitude, duration and zonal extent develop along the equator in the next couple of months, 2019-20 could be very exciting," he said.
…
"While it's not a slam dunk that El Nino is going to persist, I think that the odds have certainly increased over one to two months ago," Phil Klotzbach, a research scientist at Colorado State University, said. " We've had a big build up of heat in the eastern and central Pacific."
…
Cai Wenju, a senior CSIRO scientist who has published widely on the El Nino Southern Oscillation climate pattern, said the chance of El Nino returning is high.

A return of westerlies by about June to halt the easterly tradewinds “could spark the fire and there’s a lot of fuel", Dr Cai said.

“If it’s similar to 2015, the impact this time will be big," he said."

[AbruptSLR]

I have made a point that I do not think that the WAIS will begin a MICI type of collapse until about 2040 (note that SSP5-Baseline projects a GMSTA of about 2C, relative to 1986-2005, by 2040); however, that do not mean that I do not think that we will activate numerous nonlinear positive self-reinforcing feedback mechanisms between now and then, which not only increases the probability of a MICI type of WAIS collapse this century, but also increases the probably that we cross a tipping point for abrupt climate change well before 2040 as cited in the two linked sources and illustrated by the attached image of abrupt regional feedback mechanisms.

Title: "Climate report understates threat"

https://thebulletin.org/2018/10/climate-report-understates-threat/

Extract: "The UN’s Intergovernmental Panel on Climate Change’s Special Report on Global Warming of 1.5 degrees Celsius, released on Monday, is a major advance over previous efforts to alert world leaders and citizens to the growing climate risk. But the report, dire as it is, misses a key point: Self-reinforcing feedbacks and tipping points—the wildcards of the climate system—could cause the climate to destabilize even further.
…
To put it bluntly, there is a significant risk of self-reinforcing climate feedback loops pushing the planet into chaos beyond human control.
…
These cascading feedbacks include the loss of the Arctic’s sea ice, which could disappear entirely in summer in the next 15 years. The ice serves as a shield, reflecting heat back into the atmosphere, but is increasingly being melted into water that absorbs heat instead. Losing the ice would tremendously increase the Arctic’s warming, which is already at least twice the global average rate. This, in turn, would accelerate the collapse of permafrost, releasing its ancient stores of methane, a super climate pollutant 30 times more potent in causing warming than carbon dioxide.

By largely ignoring such feedbacks, the IPCC report fails to adequately warn leaders about the cluster of six similar climate tipping points that could be crossed between today’s temperature and an increase to 1.5 degrees—let alone nearly another dozen tipping points between 1.5 and 2 degrees."

See also:

Drijfhout et al. (2015), "Catalogue of abrupt shifts in Intergovernmental Panel on Climate Change climate models", PNAS https://doi.org/10.1073/pnas.1511451112

https://www.pnas.org/content/112/43/E5777

Abstract: "Abrupt transitions of regional climate in response to the gradual rise in atmospheric greenhouse gas concentrations are notoriously difficult to foresee. However, such events could be particularly challenging in view of the capacity required for society and ecosystems to adapt to them. We present, to our knowledge, the first systematic screening of the massive climate model ensemble informing the recent Intergovernmental Panel on Climate Change report, and reveal evidence of 37 forced regional abrupt changes in the ocean, sea ice, snow cover, permafrost, and terrestrial biosphere that arise after a certain global temperature increase. Eighteen out of 37 events occur for global warming levels of less than 2°, a threshold sometimes presented as a safe limit. Although most models predict one or more such events, any specific occurrence typically appears in only a few models. We find no compelling evidence for a general relation between the overall number of abrupt shifts and the level of global warming. However, we do note that abrupt changes in ocean circulation occur more often for moderate warming (less than 2°), whereas over land they occur more often for warming larger than 2°. Using a basic proportion test, however, we find that the number of abrupt shifts identified in Representative Concentration Pathway (RCP) 8.5 scenarios is significantly larger than in other scenarios of lower radiative forcing. This suggests the potential for a gradual trend of destabilization of the climate with respect to such shifts, due to increasing global mean temperature change."


Caption for the attached image: "Abrupt shifts as a function of global temperature increase. Shown are the number of abrupt climate changes occurring in the CMIP5 database for different intervals of warming relative to the preindustrial climate."

[AbruptSLR]

Here is an example of how a proposed geoengineering measure could lead to unintended consequences:

Gürses, Ö., Kolatschek, V., Wang, Q., and Rodehacke, C. B.: Brief communication: A submarine wall protecting the Amundsen Sea intensifies melting of neighboring ice shelves, The Cryosphere Discuss., https://doi.org/10.5194/tc-2019-32, in review, 2019.

https://www.the-cryosphere-discuss.net/tc-2019-32/

Abstract. Disintegration of ice shelves in the Amundsen Sea has the potential to cause sea level rise by inducing an acceleration of grounded ice streams. Moore et al. (2018) proposed that using a submarine wall to block the penetration of warm water into the ice shelf cavities could reduce this risk. We use a global sea ice-ocean model to show that a wall shielding the Amundsen Sea below 350 m depth successfully suppresses the inflow of warm water and reduces ice shelf melting. However, the warm water gets redirected towards neighboring ice shelves, which reduces the effectiveness of the wall.

[gerontocrat]

Here is an example of how a proposed geoengineering measure could lead to unintended consequences:

Moore et al. (2018) proposed that using a submarine wall to block the penetration of warm water into the ice shelf cavities could reduce this risk. We use a global sea ice-ocean model to show that a wall shielding the Amundsen Sea below 350 m depth successfully suppresses the inflow of warm water and reduces ice shelf melting. However, the warm water gets redirected towards neighboring ice shelves, which reduces the effectiveness of the wall.

I recommend Part 3 of John Wyndhm's SF novel - "The Kraken Wakes" especially to those who think we can fix stuff with massive infrastructure and other engineering projects.

[Shared Humanity]

We are not yet certain what the impacts of our current geoengineering project over the last 200 years will look like and we have been studying this fairly intently for the better part of 40 years. There is no way we can predict fully the effects of future projects. We should expect unintended, unanticipated and quite painful consequences of our hubris in approaching climate change as an engineering problem to be solved.

Never the less, given our refusal to halt the current project, we should expect many increasingly frantic projects, implemented far to quickly in the very near future.

May the Goddess help us.

[AbruptSLR]

We are not yet certain what the impacts of our current geoengineering project over the last 200 years will look like and we have been studying this fairly intently for the better part of 40 years. There is no way we can predict fully the effects of future projects. We should expect unintended, unanticipated and quite painful consequences of our hubris in approaching climate change as an engineering problem to be solved.
...

In addition to the abrupt feedback mechanisms identified by CMIP5 (see Reply #763), I not that CMIP5 does not consider many ice-climate feedback mechanism (some of which are being included in CMIP6 that has several ESMs with ECS over 5C).  Therefore, all of the findings earlier studies of geoengineering are likely irrelevant, and if the WAIS begins a MICI type of collapse circa 2040, then likely CMIP6, 7 & 8 findings may well associate be of no use in assessing geoengineering impacts (nevertheless policy makers assume that they will be able to at least implement BECCS).

[AbruptSLR]

The linked article indicates that the USA and Saudi Arabia recently blocked a U.N. resolution to make the UNEA (U.N. Environment Assembly) the governing body for regulating any future potential implementation of geoengineering.  Apparently, the USA and Saudi Arabia did not want a U.N. body to limit/regulate the impacts of geoengineering on the overall environment and on small countries:

Title: "U.S. Blocks U.N. Resolution on Geoengineering"

https://www.scientificamerican.com/article/u-s-blocks-u-n-resolution-on-geoengineering/

[gerontocrat]

The linked article indicates that the USA and Saudi Arabia recently blocked a U.N. resolution to make the UNEA (U.N. Environment Assembly) the governing body for regulating any future potential implementation of geoengineering.  Apparently, the USA and Saudi Arabia did not want a U.N. body to limit/regulate the impacts of geoengineering on the overall environment and on small countries:

Title: "U.S. Blocks U.N. Resolution on Geoengineering"

https://www.scientificamerican.com/article/u-s-blocks-u-n-resolution-on-geoengineering/

Here we have the USA - in search of energy dominance based on oil and gas as official Government policy, and Saudi Arabia - facing bankruptcy if/when demand for the black stuff collapses.

So not a surprise if they want unfettered freedom to try geo-engineering by any means to capture CO2 so fossil fuel production can not only continue unabated but increase. It won't work but.......

The lunatics have taken over the asylum - the brakes on the Trumpistan roller-coaster have failed.

[AbruptSLR]

While the laws of nature will be followed no matter what, it is the judicial system that determines what is legal fact with regard to human actions (e.g. see the linked article about goverments suing regarding professionally irresponsible behavior and or fraud by the fossil fuel industry); and consensus climate science evaluations play a major role with regard to what judges and juries determine to be legally liable facts about anthropogenic radiative forcing.

Consensus science has helped to establish a legal expectation that staying well below a GMSTA of 2C (relative to pre-industrial) is acceptable; however, James Hansen has stated that establishing an acceptable limit of 350ppm of atmospheric CO₂ concentrations would be advisable.  If consensus science would follow the Precautionary Principle, then it would be easier for youth groups to be successful in their lawsuits; and Greta Thunberg's youth movement might then file multiple lawsuits against governments for failing to adequately safeguard their collective futures (including lawsuits to block irresponsible implementation of geoengineering):

Title: "DC moves closer to climate lawsuit against Exxon"

https://thehill.com/policy/energy-environment/434590-dc-building-team-for-exxon-climate-challenge

Extract: "The D.C. government is beefing up its legal team ahead of an anticipated legal challenge against Exxon.
…
If D.C. moves forward with a lawsuit against Exxon, it will be joining a handful of other states and municipalities looking into how the oil and gas giant may have failed to publicize science it had linking emissions to global warming.
…
New York sued Exxon in October for allegedly engaging in "a longstanding fraudulent scheme.""

Edit: To be clear, if youth groups were successful in hundreds of lawsuits against hundreds of governments worldwide, then we would likely see these governments enact progressive carbon tax programs around the world.

Edit2, With regards to Hansen's 350ppm acceptable limit for atmospheric CO2 concentrations, the first attached image shows that we past that limit around 1987; and thus I assume that Hansen also meant that atmospheric CH4 concentrations should have an acceptable limit around 1685ppb where they were around 1987 as indicated by the second attached image.

[AbruptSLR]

I have made a point that I do not think that the WAIS will begin a MICI type of collapse until about 2040 (note that SSP5-Baseline projects a GMSTA of about 2C, relative to 1986-2005, by 2040); however, that do not mean that I do not think that we will activate numerous nonlinear positive self-reinforcing feedback mechanisms between now and then, which not only increases the probability of a MICI type of WAIS collapse this century, but also increases the probably that we cross a tipping point for abrupt climate change well before 2040 as cited in the two linked sources and illustrated by the attached image of abrupt regional feedback mechanisms.

Title: "Climate report understates threat"

https://thebulletin.org/2018/10/climate-report-understates-threat/

...

I would like to note that most of the abrupt regional feedback mechanisms identified in quote above came from CMIP5; however, if CMIP6 has several ESMs with ECS greater than 5C, then CMIP6 will most likely identify many more abrupt feedback mechanisms including several related to ice-climate feedback mechanisms (omitted in CMIP5) even though CMIP6 only considers MISI types of ice sheet collapse mechanisms and omits consideration of MICI type collapse mechanisms.

Furthermore, I note CMIP5 many very well have underestimated the negative feedback from both natural and anthropogenic aerosols, which means that if rainforests (especially the Amazon) degrades rapidly and/or anthropogenic aerosol emissions are rapidly decreased in the coming decades, we are likely to cross many more tipping point thresholds earlier than identified in CMIP5.

[AbruptSLR]

While the linked reference does not consider a MICI type of WAIS collapse beginning circa 2040, it does provide valuable insights into transient responses of both atmospheric and oceanic heat transports up to about 2040.  In this regards, the attached image indicates that as the AMOC slows down it will transport more ocean heat content into the North Atlantic which could contribute to a regional ice albedo flip around Greenland, that could in turn significantly increase surface ice melting in Greenland circa 2040, which would be reinforced by the bipolar seesaw if a MICI type of WAIS collapse begins around 2040:

He, C., Liu, Z., & Hu, A. (2019). The transient response of atmospheric and oceanic heat transports to anthropogenic warming. Nature Climate Change. doi:10.1038/s41558-018-0387-3

https://www.nature.com/articles/s41558-018-0387-3

Abstract: "Model projections of the near-future response to anthropogenic warming show compensation between meridional heat transports by the atmosphere (AHT) and ocean (OHT) that are largely symmetric about the equator, the causes of which remain unclear. Here, using both the Coupled Model Intercomparison Project Phase 5 archive and Community Climate System Model version 4 simulations forced with Representative Concentration Pathway 8.5 to 2600, we show that this transient compensation—specifically during the initial stage of warming—is caused by combined changes in both atmospheric and oceanic circulations. In particular, it is caused by a southward OHT associated with a weakened Atlantic Meridional Overturning Circulation, a northward apparent OHT associated with an ocean heat storage maximum around the Southern Ocean, and a symmetric coupled response of the Hadley and Subtropical cells in the Indo-Pacific basin. It is further shown that the true advective OHT differs from the flux-inferred OHT in the initial warming due to the inhomogeneous responses of ocean heat storage. These results provide new insights to further our understanding of future heat transport responses, and thereby global climatic processes such as the redistribution of ocean heat."

Edit, the above comments assume that we follow at least a RCP 8.5 pathway thru at least 2035.

[Sleepy]

Nice mini-thread by Glen Peters here on emission scenarios:
https://twitter.com/Peters_Glen/status/1107923815508701184
For fossil CO₂ emissions, we started to move onto track, but that quickly changed…

[AbruptSLR]

The linked reference (& associated article), how Emergent Constraints (ECs), see the two attached panels and the associated caption, can be used to improve ESM projections"

Hall et al. (2019), "Progressing emergent constraints on future climate change", Nature Climate Change (2019). DOI: 10.1038/s41558-019-0436-6 , https://www.nature.com/articles/s41558-019-0436-6

https://www.nature.com/articles/s41558-019-0436-6

Abstract: "In recent years, an evaluation technique for Earth System Models (ESMs) has arisen—emergent constraints (ECs)—which rely on strong statistical relationships between aspects of current climate and future change across an ESM ensemble. Combining the EC relationship with observations could reduce uncertainty surrounding future change. Here, we articulate a framework to assess ECs, and provide indicators whereby a proposed EC may move from a strong statistical relationship to confirmation. The primary indicators are verified mechanisms and out-of-sample testing. Confirmed ECs have the potential to improve ESMs by focusing attention on the variables most relevant to climate projections. Looking forward, there may be undiscovered ECs for extremes and teleconnections, and ECs may help identify climate system tipping points."

Caption for attached images: "Fig. 2 | emergent relationships for two ECs relating to physical and biogeochemical components of the climate system. a,b, An EC for snow-albedo feedbacks is shown in a, while b shows an EC for carbon loss from tropical land. In both cases the observational constraint is shown as a vertical bar. In a the y-axis is the strength of springtime snow-albedo feedback strength over Northern Hemisphere land masses, as measured in the climate change context (% albedo change per unit warming), while the x-axis is the snow-albedo feedback strength associated with springtime warming in the current climate’s seasonal cycle (same units). In b, the y-axis is the sensitivity of tropical land carbon losses to anthropogenic warming (Pg C K–1), and the x-axis is the sensitivity of interannual CO2 growth rate to inter-annual warming anomalies (Pg C yr–1 K–1). The models in a are from the CMIP3 and CMIP5 ensembles. The models in b are the C4MIP (black symbols) and CMIP5 (coloured asterisks) models. Panel b is adapted from ref. 21, Wiley."

Title: "Uncertain projections help to reveal the truth about future climate change"

https://phys.org/news/2019-03-uncertain-reveal-truth-future-climate.html

Extract: "Overall the tone of the study is a very positive about emergent constraints which enable the ensemble of climate models being developed worldwide, to be more than the sum of the parts.

Professor Chris Huntingford, study co-author based at the UK Centre for Ecology and Hydrology summarised this shared perspective: "An enormous amount of effort has gone into developing climate models by research groups around the world. Unfortunately, there remain significant differences between their projections.

"This uncertainty has to be reduced to help policymakers plan. At present, the only game in town to aid uncertainty removal is that of Emergent Constraints.""

[AbruptSLR]

Just to support James Hansen's recommendation to limit atmospheric CO₂ concentrations to 350ppm, the linked article indicate that it is likely that we have already triggered a Sixth Mass Extinction, which is not only morally wrong, and which threatens our collective food supplies in the coming decades; but also will likely act as a positive feedback mechanism for accelerating more warming:

Title: "The sixth mass extinction, explained"

https://theweek.com/articles/823904/sixth-mass-extinction-explained

Extract: ""We are sleepwalking toward the edge of a cliff," said Mike Barrett, executive director at WWF.
…
Potentially enormous. The loss of species can have catastrophic effects on the food chain on which humanity depends. Ocean reefs, which sustain more than 25 percent of marine life, have declined by 50 percent already — and could be lost altogether by 2050. This is almost certainly contributing to the decline of global marine life, down — on average — by 50 percent since 1970, according to the WWF. Insects pollinate crops humans eat. "This is far more than just being about losing the wonders of nature, desperately sad though that is," the WWF's Barrett said. "This is actually now jeopardizing the future of people. Nature is not ‘nice to have' — it is our life-support system.""
&

Title: "Earth’s Sixth Mass Extinction Has Begun, New Study Confirms"

https://www.iflscience.com/plants-and-animals/earth-s-sixth-mass-extinction-has-begun-new-study-confirms/

Extract: "We are currently witnessing the start of a mass extinction event the likes of which have not been seen on Earth for at least 65 million years. This is the alarming finding of a new study published in the journal Science Advances."

[AbruptSLR]

The linked reference indicates that the negative impacts of climate change on food production this century will be so severe (resulting in very high food prices), that it will not be economically viable to use aggressive forest carbon sequestration to help mitigation carbon emissions.  To me this emphasizes the need to immediately implement aggressively progressive carbon tax plans worldwide, in order to quickly reduce carbon emissions:

Luis Moisés Peña-Lévano, Farzad Taheripour & Wallace E. Tyner (19 March 2019), "Climate Change Interactions with Agriculture, Forestry Sequestration, and Food Security", Environmental and Resource Economics, pp 1–23, https://doi.org/10.1007/s10640-019-00339-6

https://link.springer.com/article/10.1007%2Fs10640-019-00339-6

Abstract: "Climate change can negatively affect crop productivity decreasing food production in many regions across the world. Literature suggests forest carbon sequestration (FCS) is a good alternative to mitigate climate change due to its ability to sequester carbon at low cost. Nevertheless, FCS subsidies have not been addressed together with impacts on food security and climate change reduced crop yields. In our multidisciplinary work, we collected the crop yield shocks from global circulation—crop modeling. We also developed a new version of a computable general equilibrium model for the economic analysis. Thus, we evaluate the global economic impacts of using carbon taxes and FCS to achieve 50% emission reductions. We find that implementing an aggressive FCS incentive can cause substantial increases in food prices because of land competition between forest and crop production. Without climate induced yield reductions, FCS is attractive, but not with the yield reductions. With the climate induced yield shocks, food price increases are huge—so large that it is clear this approach could not be adopted in the real world. The results cry out for investment in agricultural research on climate adaptation. Our findings suggest economic well-being falls more without mitigation than with 50% emission reductions."

[AbruptSLR]

Don't forget this climate impacts of air conditioning; when wondering whether to support aggressively progressive carbon tax programs worldwide:

Title: "Air conditioning is threatening our ability to tackle climate change. Here's what we need to do"

https://www.weforum.org/agenda/2019/01/why-keeping-ourselves-cool-doesnt-have-to-mean-heating-the-planet/

Extract: "As the latest round of global climate talks, COP24, came to a close in Kratowice, in Poland’s coal country, much of our climate discourse has taken on a note of abject despair. The National Academy of Sciences tells us that current rates of warming, comparable only to the effects of a “meteorite impact”, will take us back to a climate that predates the evolution of modern humans. An article in Nature warns us of a “Nonlinear rise in Greenland runoff in response to post-industrial Arctic warming” (or, in non-geek-speak, that the melting of the world’s ice sheets has gone into overdrive).

The University of Washington draws parallels between our situation today and the “great dying”, the mass extinction event 252 million years ago that wiped out 96% of ocean life. The IPCC’s latest report warns of the most damaging effects from climate change - severe food shortages, wildfires, mass die-offs of coral reefs, climate-induced conflict - coming a lot sooner and at a lower temperature threshold than previously thought.

Meanwhile there has been an increasing amount of attention paid to the massive climate risk posed by one mundane and increasingly ubiquitous household technology: the air conditioner. Researchers at Lawrence Berkeley National Laboratory, the International Energy Agency (IEA), and Rocky Mountain Institute (RMI) have concluded that room air conditioners alone - the typical window and split units used in most homes - are set to account for over 130 gigatons (GT) of CO2 emissions between now and 2050."

[AbruptSLR]

With a hat-tip to vox_mundi, the linked reference uses statistics to demonstrate that AR5 errs on the side of least drama, and that the impacts of climate change are coming fasters than conveyed by AR5.  I recommend that scientists stop focusing on providing information to policy matters, and instead provide transparent evaluations of climate risks that can be used in courts around the world to sue governments and other parties that are acting inappropriately w.r.t. these true climate risks (including ice-climate feedback mechanisms):

Salvador Herrando-Perez et al. (18 March 2019), "Statistical Language Backs Conservatism in Climate-Change Assessments", BioScience, Volume 69, Issue 3, Pages 209–219, https://doi.org/10.1093/biosci/biz004

https://academic.oup.com/bioscience/article-abstract/69/3/209/5382637?redirectedFrom=fulltext

Abstract: "The scientific evidence for anthropogenic climate change is empirically settled, but communicating it to nonscientific audiences remains challenging. To be explicit about the state of knowledge on climate science, the Intergovernmental Panel on Climate Change (IPCC) has adopted a vocabulary that ranks climate findings through certainty-calibrated qualifiers of confidence and likelihood. In this article, we quantified the occurrence of knowns and unknowns about “The Physical Science Basis” of the IPCC's Fifth Assessment Report by counting the frequency of calibrated qualifiers. We found that the tone of the IPCC's probabilistic language is remarkably conservative (mean confidence is medium, and mean likelihood is 66%–100% or 0–33%), and emanates from the IPCC recommendations themselves, complexity of climate research, and exposure to politically motivated debates. Leveraging communication of uncertainty with overwhelming scientific consensus about anthropogenic climate change should be one element of a wider reform, whereby the creation of an IPCC outreach working group could enhance the transmission of climate science to the panel's audiences."

Extract: "In the scientific court, one of the most urgent actions we scientists can currently take to address the climate crisis is to communicate unequivocally to nonscientists the state of knowledge. We should frame and underline what the data tell us that we know for certain about climate change, and be transparent about those areas that remain uncertain and by how much."

[AbruptSLR]

The linked reference & associated article, indicate that while the ocean is currently absorbing about 30% of anthropogenic CO₂ emissions, as the MOC slows this absorption rate may very likely decrease.  Furthermore, I note that one major consequence of ice-climate feedback is to slow the MOC, particularly if the WAIS were to collapse following an abrupt MICI type of pathway beginning circa 2040 (which would thus likely thereafter rapidly decrease the amount of CO2 absorbed by the oceans):

Nicolas Gruber et al. (15 Mar 2019), "The oceanic sink for anthropogenic CO2 from 1994 to 2007", Science, Vol. 363, Issue 6432, pp. 1193-1199, DOI: 10.1126/science.aau5153

http://science.sciencemag.org/content/363/6432/1193

The state of ocean CO2 uptake
The ocean is an important sink for anthropogenic CO2 and has absorbed roughly 30% of our emissions between the beginning of the industrial revolution and the mid-1990s. This effect is an important moderator of climate change, but can we count on it to remain as strong in the future? Gruber et al. calculated the ocean uptake of anthropogenic CO2 for the interval from 1994 to 2007, which continued as expected. They also observed clear regional deviations from this pattern, suggesting that there is no guarantee that uptake will remain as robust with time.
Science, this issue p. 1193

Abstract
We quantify the oceanic sink for anthropogenic carbon dioxide (CO2) over the period 1994 to 2007 by using observations from the global repeat hydrography program and contrasting them to observations from the 1990s. Using a linear regression–based method, we find a global increase in the anthropogenic CO2 inventory of 34 ± 4 petagrams of carbon (Pg C) between 1994 and 2007. This is equivalent to an average uptake rate of 2.6 ± 0.3 Pg C year−1 and represents 31 ± 4% of the global anthropogenic CO2 emissions over this period. Although this global ocean sink estimate is consistent with the expectation of the ocean uptake having increased in proportion to the rise in atmospheric CO2, substantial regional differences in storage rate are found, likely owing to climate variability–driven changes in ocean circulation.

See also:
Title: "Oceanic carbon uptake could falter"

https://climatenewsnetwork.net/oceanic-carbon-uptake-could-falter/

Extract: "It also confirms that although, on average, the high seas are responding to atmospheric change as expected, different ocean basins can vary: the North Atlantic actually absorbed 20% less CO2 than expected between 1994 and 2007, probably thanks to the slowing of the North Atlantic Meridional Overturning Circulation at the time.

And, the researchers say, the acidification of the oceans is on the increase, to depths of 3000 metres. The next step is to understand a little better the interplay between ocean, atmosphere and human emissions of greenhouse gases.

“We learned that the marine sink does not just respond to the increase in atmospheric CO2,” said Nicolas Gruber of the Swiss Federal Institute of Technology, always known as ETH Zurich, who led the study.

“Its substantial sensitivity to climate variations suggests a significant potential for feedbacks with the ongoing change in climate.” "

[AbruptSLR]

While water supply for human use will rapidly become a problem (with continued global warming), Africa will likely be hit particularly hard due to its projected rapid population growth (see the attached image).  Certainly ice-climate feedbacks will make this existing problem worse, with continued global warming:

Title: "World Water Development Report 2019 - Leaving No One Behind"

https://en.unesco.org/water-security/wwap/wwdr/2019
&
https://unesdoc.unesco.org/ark:/48223/pf0000367276

Extract: "Billions still lack safe water and sanitation facilities, and people are being left behind for reasons related to their gender, ethnicity, culture and/or socioeconomic status, among others. Exclusion, discrimination, entrenched power asymmetries, poverty and material inequalities are among the main obstacles to fulfilling the human rights to water and sanitation and achieving the water-related goals of the 2030 Agenda for Sustainable Development.

The wealthy generally receive high levels of service at very low price, while the poor often pay a much higher price for services of similar or lesser quality."

[AbruptSLR]

The linked websites indicate that the IPCC is not ignoring ocean and cryosphere interactions for future climate change projections, and that a promising Special Report on this topic will be issued in September 2019.  Unfortunately, I strongly suspect that this special report will not address MICI risks, and will likely discount the speed and intensity of many of the coming ice-climate feedback mechanisms:

Title: "The Ocean and Cryosphere in a Changing Climate"

https://www.ipcc.ch/report/srocc/

Extract: "During its 45th Session (Guadalajara, Mexico, 28 – 31 March 2017), the IPCC Panel approved the outline of the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). The report will be finalized in September 2019."

See also:

Title: "Decision IPCC/XLV-2. Sixth Assessment Report (AR6) Products, Outline of the Special Report on climate change and oceans and the cryosphere"

https://www.ipcc.ch/site/assets/uploads/2018/04/Decision_Outline_SR_Oceans.pdf

Extract: "Chapter 1: Framing and Context of the Report (~15 pages)
• Integrated storyline of the report, chapter narrative, chapter sequence and their linkages (including coverage of extremes and abrupt change and irreversible changes)
• Definition of ocean and cryosphere and their components
• Observing capacities, progress and limitations (e.g., time series and spatial coverage)
• Assessment methodologies, including indigenous and community knowledge, risk, including cascading risks, and applications of detection and attribution
• Role of ocean and cryosphere in the climate system, including characteristics, ocean heat content in Earth’s energy budget, key feedbacks and time scales
• Implications of climate-related ocean and cryosphere change for resources, natural systems (e.g., change and loss of habitat, extinctions), human systems (e.g., psychological, social, political, cultural and economic aspects), and vulnerability assessments, adaptation limits, and residual risks
• Solutions, including policy options and governance, and linkages of this report to relevant institutional and policy contexts (e.g., UNFCCC, Paris Agreement and SDGs, Sendai Framework)
• Treatment of vulnerabilities and marginalized areas and people (e.g., gender) in this report
• Scenarios and time frames considered in this report
• Treatment of uncertainty
…
Chapter 3: Polar Regions (~50 pages)
• Changes in atmospheric and ocean circulation that influence polar regions, including climate feedbacks and teleconnections and paleo perspectives
• Greenland and Antarctic ice sheets and ice shelves, Arctic and Antarctic glaciers, mass change, physics of dynamical instability and accelerated ice discharge; consequences for ocean circulation and biogeochemistry, and sea level
• Changing snow cover, freshwater ice and thawing permafrost (terrestrial and subsea); carbon flux and climate feedbacks; impacts on infrastructure and ecosystems; community- based adaptation
• Changing sea ice; effects on ocean and atmospheric circulation and climate, including teleconnections; implications for ecosystems, coastal communities, transportation and industry
• Changing polar ocean (physical, dynamical and biogeochemical properties), implications for acidification, carbon uptake and release; impacts on ecosystems and their services (e.g., fisheries); adaptation options (e.g., ecosystem-based management and habitat protection) and limits to adaptation
• Access to resources and ecological, institutional, social, economic, livelihood and cultural consequences of polar change, including issues of international cooperation
• Responses to enhance resilience
…
Chapter 6: Extremes, Abrupt Changes and Managing Risks (~20 pages)
• Risks of abrupt change in ocean circulation and cryosphere and potential consequences
• Extreme ENSO events and other modes of variability and their implications
• Marine heat waves and implications
• Changes in tracks, intensity, and frequency of tropical and extra-tropical storms and associated wave height
• Cascading risks (e.g., storm surge and sea level rise), irreversibility, and tipping points
• Monitoring systems for extremes, early warning and forecasting systems in the context of climate change
• Governance and policy options, risk management, including disaster risk reduction and enhancing resilience"

[AbruptSLR]

The first linked Barron's article, explains that the information/guidance that consensus climate scientists have managed to convey to business, makes those businesses conclude that 'it still makes financial sense to burn the globe".  As the article indicates this leads some people to conclude that we are headed to a 'climate Minsky moment', comparable to the 2008 financial collapses where a few 'outlier' warnings (Minsky in the case of the market and Hansen in the case of climate) were discounted by the consensus which lead to a collapse (in 2008 for the economy and still unfolding for the climate).  It seems clear to me that consensus climate science has conveyed a message to the markets that climate change impacts will unfold so slowly that it makes good financial sense for business people to continue to game the market.  Clearly, climate scientists need to stop erring on the side of least drama, and instead they need to convey clearer information about the magnitude and the time-scale of our current climate risks (which are unfolding in real time in front of our eyes):


Title: "It Still Makes Financial Sense to Burn the Globe"

https://www.barrons.com/articles/climate-change-is-still-too-cheap-esg-investing-51553104880

Extract: "It still makes good financial sense to burn the globe, although we know that we will have to pay a steep price for it."

See also:

Title: "Minsky moment"

https://en.wikipedia.org/wiki/Minsky_moment

Extract: "The term was coined by Paul McCulley of PIMCO in 1998, to describe the 1998 Russian financial crisis, and was named after economist Dr. Hyman Minsky, who noted that bankers, traders, and other financiers periodically played the role of arsonists, setting the entire economy ablaze. Minsky opposed the deregulation that characterized the 1980s."
&

Title: "Mark Carney warns of climate change threat to financial system"

https://www.theguardian.com/business/2018/apr/06/mark-carney-warns-climate-change-threat-financial-system

Extract: "The governor of the Bank of England has warned of the “catastrophic impact” climate change could have for the financial system unless firms do more to disclose their vulnerabilities."

[AbruptSLR]

Aside from the RollingStone reporter's somewhat loose use of terms, the linked article indicates both how fragile the ice is near the base of the Thwaites Ice Tongue (as this fragility contributes to the surge of icebergs described in the article), and how the Thwaites ice velocity has almost doubled in the past five years, see the attached image; which probably is due to a combination of: a) reduced buttressing from the Thwaites Ice Tongue, b) reduced buttressing on the Southwest Tributary Glacier, and c) increased basal meltwater due to increased ice friction as the ice velocity accelerates.  The article is correct that we may very likely have crossed a tipping point that will lead to a collapse of the Thwaites Glacier, but in my opinion it will still take into the 2035 to 2040 timeframe before an MICI type of collapse is triggered:

Title: "Journey to Antarctica: Is This What a Climate Catastrophe Looks Like in Real Time?"

https://www.rollingstone.com/politics/politics-features/journey-to-antarctica-is-this-what-a-climate-catastrophe-looks-like-in-real-time-810392/

Extract: "On this trip, satellite images have been indispensable in helping scientists track the ever-changing ice in the regions we’ve been exploring. But the map Queste received that morning was different. He noticed dark cracks in parts of the ice shelf, which floats out over the sea like a huge fingernail from the glacier itself. They had not been there before. The ice shelf was clearly starting to break up. Queste’s first thought: “Oh, shit.”
…
And here’s a graph that shows how quickly the ice flow on Thwaites has accelerated — it’s almost doubled in the last five years."

[AbruptSLR]

While consensus climate science of the likely range and the likely mean value of climate sensitivity has not changed much since 2002 (see the attached image), new results from CMIP6 may likely cause this value to move upward; which may well mean that our carbon budget is negative (as indicated by James Hansen):

https://www.carbonbrief.org/guest-post-why-results-from-the-next-generation-of-climate-models-matter

Extract: "Early results suggest ECS values from some of the new CMIP6 climate models are higher than previous estimates, with early numbers being reported between 2.8C (pdf) and 5.8C. This compares with the previous coupled model intercomparison project (CMIP5), which reported values between 2.1C to 4.7C. The IPCC’s fifth assessment report (AR5) assessed ECS to be “likely” in the range 1.5C to 4.5C and “very unlikely” greater than 6C. (These terms are defined using the IPCC methodology.)
…
For AR5, simple models constrained by observed changes in the instrumental record tended to give values of ECS generally in the lower part of the likely range of 1.5 to 4.5C, whereas global climate models tended to give ECS in the upper part of the likely range.

Climate scientists will need to assess how new understanding of ECS from the various lines of evidence compares. They will all be considered by the IPCC for AR6 due in 2021.
..
The next step is for climate scientists to understand in detail why some of the new models are showing this shift in ECS – and how this fits with other lines of evidence. This includes looking at other measures of sensitivity, including “transient climate response” (TCR), which measures the rate of warming.

TCR is defined as the temperature increase at the instant that atmospheric CO2 has doubled, following an increase of 1% each year. This measure is arguably more useful for looking at changes we might expect over the current century, as it deals with shorter timescales than ECS.
…
If it turns out that there is enough evidence to corroborate the higher ECS values from new-generation climate models then there would be important implications for carbon budgets. A higher ECS means a greater likelihood of reaching higher levels of global warming – even with deeper emissions cuts. Higher warming would allow less time to adapt and mean a greater likelihood of passing climate “tipping points” – such as thawing of permafrost, which would further accelerate warming."

Edit:  The linked following Climate Central article reminds us that NASA estimates that GMSTA in the period from 1986 to 2005 was about 0.66C above the late 19th century, while Hawkins estimates that GMSTA in the late 19th century was about 0.05C above the mid-18th century.  Thus as CMIP6 uses 1986-2005 as a temperature baseline, this implies that as the IPCC's goal of limiting GMSTA to 'well below 2C) as referenced to the mid-18th century, that we need to add about 0.71C to CMIP6 temperature outputs.  Furthermore, as the IPCC uses TCR to estimate the remaining carbon budget, if the attached image from the E3SMv1 projection of TCR is correct (see Reply #755), then by 2040 GMSTA (reference to the mid-18th century) should be at least about 1.7C + 0.71C = 2.41C (ignoring nonlinear responses); which is not 'well below 2C).  If so this would confirm James Hansen's point that we are likely already in a negative carbon budget situation today.

[AbruptSLR]

Just to be clear, thirty years of Hansen's 1988 projections were verified in 2018 (see the linked article); and if governments had following Hansen's recommendations (e.g. carbon fees with dividends and planting forests) in the 1980's we would be in much better shape today:

Title: "James Hansen wishes he wasn't so right about global warming"

https://phys.org/news/2018-06-james-hansen-wasnt-global.html

Extract: "NASA's top climate scientist in 1988, Hansen warned the world on a record hot June day 30 years ago that global warming was here and worsening. In a scientific study that came out a couple months later, he even forecast how warm it would get, depending on emissions of heat-trapping gases.

The hotter world that Hansen envisioned in 1988 has pretty much come true so far, more or less. Three decades later, most climate scientists interviewed rave about the accuracy of Hansen's predictions given the technology of the time.

Hansen won't say, "I told you so."

"I don't want to be right in that sense," Hansen told The Associated Press, in an interview is his New York penthouse apartment. That's because being right means the world is warming at an unprecedented pace and ice sheets in Antarctica and Greenland are melting.
…
"If scientists are not allowed to talk about the policy implications of the science, who is going to do that? People with financial interests?" Hansen asked."

[AbruptSLR]

The linked reference indicates that the Ross Ice Shelf, RIS, is currently being stabilized by 'nails' from adjoining EAIS outlet glaciers (like Byrd Glacier) that are pinning the ice shelf to the Transantarctic Mountains.  In the "Hazard Analysis for the FRIS/RIS in the 2012 to 20160 Timeframe" thread  (see Replies #3, 4, 7 & 18 in that thread) I make the case that global warming related changes in the Ross Gyre (and the associate collapse of the Getz Ice Shelf) will unpin the RIS by extracting the 'nail' from the Byrd Glacier by 2050.  In this scenario the RIS would substantially collapse circa 2060 (largely due to hydrofracturing); which would allow the collapse of the marine portions of the WAIS to proceed to completion circa 2100:

Terence Hughes, Zihong Zhao, Raymond Hintz & James Fastook (27 May 2017), "Instability of the Antarctic Ross Sea Embayment as climate warms", Reviews of Geophysics, DOI: 10.1002/2016RG000545

http://onlinelibrary.wiley.com/doi/10.1002/2016RG000545/abstract

Abstract: "Collapse of the Antarctic Ice Sheet since the Last Glacial Maximum 18,000 years ago is most pronounced in the Ross Sea Embayment, which is partly ice-free during Antarctic summers, thereby breaching the O-ring of ice shelves and sea ice surrounding Antarctica that stabilizes the ice sheet. The O-ring may have vanished during Early Holocene (5000 to 3000 B.C.), Roman (1 to 400 A.D.), and Medieval (900 to 1300 A.D.) warm periods and reappeared during the Little Ice Age (1300 to 1900 A.D.). We postulate further collapse in the embayment during the post-1900 warming may be forestalled because East Antarctic outlet glaciers “nail” the Ross Ice Shelf to the Transantarctic Mountains so it can resist the push from West Antarctic ice streams. Our hypothesis is examined for Byrd Glacier and a static ice shelf using three modeling experiments having plastic, viscous, and viscoplastic solutions as more data and improved modeling became available. Observed crevasse patterns were not reproduced. A new research study is needed to model a dynamic Ross Ice Shelf with all its feeder ice streams, outlet glaciers, and ice calving dynamics in three dimensions over time to fully test our hypothesis. The required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf. Calving must then successively pull the outlet glacier “nails” so collapse of the marine West Antarctic Ice Sheet proceeds to completion."

See also:

Title: "The Uncertain Future of the West Antarctic Ice Sheet"

https://eos.org/editors-vox/the-uncertain-future-of-the-west-antarctic-ice-sheet

[AbruptSLR]

As I have noticed in other threads that there is some confusion about where the Thwaites Eastern Ice Shelf and the Thwaites Ice Tongue are located, I provide the attached image from a couple of years ago:

[AbruptSLR]

I will let readers make-up their own minds about meaning of the linked research findings, but for me these findings only present an opportunity to better calibrate ESMs against paleo-data:

Title: "Changes in ocean 'conveyor belt' foretold abrupt climate changes by four centuries"

https://phys.org/news/2019-03-ocean-conveyor-belt-foretold-abrupt.html

Extract: "Comparing the data from the three cores revealed that the AMOC weakened in the time leading up to the planet's last major cold snap, called the Younger Dryas, around 13,000 years ago. The ocean circulation began slowing down about 400 years before the cold snap, but once the climate started changing, temperatures over Greenland plunged quickly by about 6 degrees.
A similar pattern emerged near the end of that cold snap; the current started strengthening roughly 400 years before the atmosphere began to heat up dramatically, transitioning out of the ice age. Once the deglaciation started, Greenland warmed up rapidly—its average temperature climbed by about 8 degrees over just a few decades, causing glaciers to melt and sea ice to drop off considerably in the North Atlantic.

"Those [400-year] lags are probably on the long side of what many would have expected," says Anders Svensson, who studies the paleoclimate at the University of Copenhagen, and who was not involved with the current study. "Many previous studies have suggested time lags of various lengths, but few have had the necessary tools to determine the phasing with sufficient accuracy."
Co-author William D'Andrea, a paleoclimatologist at Lamont-Doherty was surprised by what they found—he says the lag times are two or three times greater than he would have expected.
For now it's not fully clear why there was such a long delay between the AMOC changes and climatic changes over the North Atlantic.

It's also difficult to pinpoint what these patterns from the past could signify for Earth's future. Recent evidence suggests that the AMOC began weakening again 150 years ago. However, current conditions are quite different from the last time around, says Muschitiello; the global thermostat was much lower back then, winter sea ice stretched farther south than New York Harbor, and the ocean structure would have been much different. In addition, the past weakening of the AMOC was much more dramatic than today's trend so far.

Nevertheless, D'Andrea says that "if the AMOC were to weaken to the degree it did back then, it could take hundreds of years for major climate changes to actually manifest."

Muschitiello adds, "It is clear that there are some precursors in the ocean, so we should be watching the ocean. The mere fact that AMOC has been slowing down, that should be a concern based on what we have found."

The study should also help to improve the physics behind climate models, which generally assume the climate responds abruptly at the same time as AMOC intensity changes. The model refinements, in turn, could make climate predictions more accurate. As Svensson puts it: "As long as we do not understand the climate of the past, it is very difficult to constrain the climate models needed to make realistic future scenarios."

[AbruptSLR]

I agree with Trenberth's 2011 position that the null hypothesis should no longer be that human activity has no role in climate change, and that the null hypothesis should be changed to that human activity is assumed to be involved in a particular climate change observation unless proved otherwise.  Thus when consensus climate scientists make Type II errors they will be following the Precautionary Principle:

Kevin E. Trenberth (03 November 2011), "Attribution of climate variations and trends to human influences and natural variability", WIREs Climate Change, https://doi.org/10.1002/wcc.142

https://onlinelibrary.wiley.com/doi/abs/10.1002/wcc.142

Abstract: "Past attribution studies of climate change have assumed a null hypothesis of no role of human activities. The challenge, then, is to prove that there is an anthropogenic component. I argue that because global warming is “unequivocal” and ‘very likely’ caused by human activities, the reverse should now be the case. The task, then, could be to prove there is no anthropogenic component to a particular observed change in climate, although a more useful task is to determine what it is. In Bayesian statistics, this change might be thought of as adding a ‘prior’. The benefit of doubt and uncertainties about observations and models are then switched. Moreover, the science community is much too conservative on this issue and too many authors make what are called ‘Type II errors’ whereby they erroneously accept the null hypothesis. Global warming is contributing to a changing incidence of extreme weather because the environment in which all storms form has changed from human activities. WIREs Clim Change 2011, 2:925–930. doi: 10.1002/wcc.142"

[AbruptSLR]

The linked reference indicates that the Max Planck Institute ESM (MPI-ESM1.2) used for CMIP6 originally exhibited an ECS of 7C, which did not match observations so the modelers dialed-back the sensitivity and ended up with the lowest CMIP6 member sensitivity of 2.77C.  However, if aerosol feedbacks over the observed record have been more negative than MPI-ESM1.2 recognized then the modeler's fiddling with their model sensitivity may be a Type II error:

Thorsten Mauritsen et al. (13 January 2019), "Developments in the MPI‐M Earth System Model version 1.2 (MPI‐ESM 1.2) and its response to increasing CO₂", JAMES, https://doi.org/10.1029/2018MS001400

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018MS001400

Abstract: "A new release of the Max Planck Institute for Meteorology Earth System Model (MPI‐ESM 1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility and overall user friendliness. In addition to new radiation‐ and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multi‐layer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model, and improving the representation of wildfires. The ocean biogeochemistry now represents cyano‐bacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions, and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end a very high climate sensitivity of around 7 K caused by low‐level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over pre‐industrial conditions of 2.77 K, maintaining the previously identified highly non‐linear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two‐layer model."

[AbruptSLR]

The attached image shows various previously published CMIP6 higher-end values for ECS through mid-March 2019:

[vox_mundi]

Tall Ice-Cliffs Trigger Big Calving Events—and Fast Sea-Level Rise 
https://phys.org/news/2019-03-tall-ice-cliffs-trigger-big-calving.html

... Although much calving occurs when the ocean melts the front of the ice, and ice cliff above falls down, a new study presents another method of calving: slumping. And this process could break off much larger chunks of ice at a quicker rate.

... the research team noted that features on Helheim glacier are typical of what you might see in a slump-prone terrestrial landscape and they wondered if ice might suffer the same fate. "You've got a crevasse that serves as a head scarp and then you've got the stresses [within the ice] maximized down at the water level," he says.

To test if slumping occurs on ice cliffs, the team monitored Helheim glacier during a calving event, using real-aperture terrestrial radar interferometery. They measured speed, position, and motion of the calving ice. The researchers observed an ice-flow acceleration just before an initial slump, followed by a rotating, full ice-thickness calving of the glacier—including the entire remaining ice-cliff, reaching both above and below the water line.

Removing the weight of the upper ice by slumping encourages the underlying ice to pop upward. "Because it's still attached at the back, it's going to rotate a little bit," says Alley. The rotation causes a crack to form at the bottom of the glacier as the ice flexes. In turn, the crack can weaken the ice, creating a large calving event—all triggered by the initial slump on top of the ice cliff. ...

... With slumping, the calving occurs without waiting for the melt. "We'll go slump... basal crevasse... boom," he says, noting that when the calving happens it will take the 100 meters of ice above the water—and the 900 meters below the water—very quickly.

And 1000 meters of ice calving at once isn't the limit. Alley says that in some places in Antarctica, the glacial ice bed can be 1500 to 2000 meters below sea level, creating a much taller cliff above water. He says the worry is that taller cliffs are even more susceptible to slumping. "The scary thing is that if pieces of west Antarctica start doing what Helheim is doing, then over the next hundred years, models indicate that we get rapid sea level rise at rates that surpass those predicted," says Alley. 

Byron R. Parizek et al. Ice-cliff failure via retrogressive slumping, Geology (2019)

[AbruptSLR]

Tall Ice-Cliffs Trigger Big Calving Events—and Fast Sea-Level Rise 
https://phys.org/news/2019-03-tall-ice-cliffs-trigger-big-calving.html
...

Thanks for the links.  Richard Alley talked about this mechanism verbally in some of his videos dating back to before 2013, and I am glad that he has finally co-authored a paper describing it more rigorously.

Also, for those who did not follow the link to the source reference, I provide the following:

Byron R. Parizek et al. Ice-cliff failure via retrogressive slumping, Geology (2019). DOI: 10.1130/G45880.1

https://pubs.geoscienceworld.org/gsa/geology/article/569567/Icecliff-failure-via-retrogressive-slumping

Abstract: "Retrogressive slumping could accelerate sea-level rise if ice-sheet retreat generates ice cliffs much taller than observed today. The tallest ice cliffs, which extend roughly 100 m above sea level, calve only after ice-flow processes thin the ice to near flotation. Above some ice-cliff height limit, the stress state in ice will satisfy the material-failure criterion, resulting in faster brittle failure. New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs are prone to failure by slumping, unloading submarine ice to allow buoyancy-driven full-thickness calving. Full-Stokes diagnostic modeling shows that the threshold cliff height for slumping is likely slightly above 100 m in many cases, and roughly twice that (145–285 m) in mechanically competent ice under well-drained or low-melt conditions."

Edit, the following caption is for Figure 1 from the reference with the figure subdivided into three panels (A, B & C) shown in the attached three images:

Caption: "Figure 1. Helheim Glacier (East Greenland) ice-cliff geometry schematic during calving event. Glacier-front geometries along the last 1 km of the A-A′ profile shown in Figure 3A are from terrestrial radar interferometer data and assumed hydrostatic equilibrium for ice mélange before slumping begins (A), during slump (B), and after calving event concludes (C). Black arrows indicate relative glaciostatic and hydrostatic stress imbalance along ice front assuming negligible backpressure from mélange (A,C) and after mass loss due to slumping (B). Bed elevation is based on mass-conservation gridding (Morlighem et al., 2014) where available, and is unknown elsewhere, as denoted by question marks. Time (UTC, 12 August 2014) for each panel is noted in upper right corner."

[AbruptSLR]

The linked reference finds that the stratosphere has a large role in climate change response than previously assumed and that stratospheric ozone plays an important role in this response:

Williams, R. S., Hegglin, M. I., Kerridge, B. J., Jöckel, P., Latter, B. G., and Plummer, D. A.: Characterising the seasonal and geographical variability in tropospheric ozone, stratospheric influence and recent changes, Atmos. Chem. Phys., 19, 3589-3620, https://doi.org/10.5194/acp-19-3589-2019, 2019.

https://www.atmos-chem-phys.net/19/3589/2019/

Abstract: "The stratospheric contribution to tropospheric ozone (O3) has been a subject of much debate in recent decades but is known to have an important influence. Recent improvements in diagnostic and modelling tools provide new evidence that the stratosphere has a much larger influence than previously thought. This study aims to characterise the seasonal and geographical distribution of tropospheric ozone, its variability, and its changes and provide quantification of the stratospheric influence on these measures. To this end, we evaluate hindcast specified-dynamics chemistry–climate model (CCM) simulations from the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model and the Canadian Middle Atmosphere Model (CMAM), as contributed to the International Global Atmospheric Chemistry – Stratosphere-troposphere Processes And their Role in Climate (IGAC-SPARC) (IGAC–SPARC) Chemistry Climate Model Initiative (CCMI) activity, together with satellite observations from the Ozone Monitoring Instrument (OMI) and ozone-sonde profile measurements from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) over a period of concurrent data availability (2005–2010). An overall positive, seasonally dependent bias in 1000–450 hPa (∼0–5.5 km) sub-column ozone is found for EMAC, ranging from 2 to 8 Dobson units (DU), whereas CMAM is found to be in closer agreement with the observations, although with substantial seasonal and regional variation in the sign and magnitude of the bias (∼±4∼±4  DU). Although the application of OMI averaging kernels (AKs) improves agreement with model estimates from both EMAC and CMAM as expected, comparisons with ozone-sondes indicate a positive ozone bias in the lower stratosphere in CMAM, together with a negative bias in the troposphere resulting from a likely underestimation of photochemical ozone production. This has ramifications for diagnosing the level of model–measurement agreement. Model variability is found to be more similar in magnitude to that implied from ozone-sondes in comparison with OMI, which has significantly larger variability. Noting the overall consistency of the CCMs, the influence of the model chemistry schemes and internal dynamics is discussed in relation to the inter-model differences found. In particular, it is inferred that CMAM simulates a faster and shallower Brewer–Dobson circulation (BDC) compared to both EMAC and observational estimates, which has implications for the distribution and magnitude of the downward flux of stratospheric ozone over the most recent climatological period (1980–2010). Nonetheless, it is shown that the stratospheric influence on tropospheric ozone is significant and is estimated to exceed 50 % in the wintertime extratropics, even in the lower troposphere. Finally, long-term changes in the CCM ozone tracers are calculated for different seasons. An overall statistically significant increase in tropospheric ozone is found across much of the world but particularly in the Northern Hemisphere and in the middle to upper troposphere, where the increase is on the order of 4–6 ppbv (5 %–10 %) between 1980–1989 and 2001–2010. Our model study implies that attribution from stratosphere–troposphere exchange (STE) to such ozone changes ranges from 25 % to 30 % at the surface to as much as 50 %–80 % in the upper troposphere–lower stratosphere (UTLS) across some regions of the world, including western Eurasia, eastern North America, the South Pacific and the southern Indian Ocean. These findings highlight the importance of a well-resolved stratosphere in simulations of tropospheric ozone and its implications for the radiative forcing, air quality and oxidation capacity of the troposphere."

[AbruptSLR]

The linked reference finds that: "The drying effect of the BDC is counteracted by the moistening effects of the tropical tropopause warming and methane oxidation. This leads to the moistening in both the lower and upper stratosphere."  Thus with continued global warming more water vapor (which has a positive feedback mechanism on warming) is accumulating in the stratosphere:

Yan Xia et al (2019), "Impacts of tropical tropopause warming on the stratospheric water vapor", Climate Dynamics, pp 1–10, https://doi.org/10.1007/s00382-019-04714-3

https://link.springer.com/article/10.1007%2Fs00382-019-04714-3

Abstract: "We investigate the impact of tropical tropopause warming on the stratospheric water vapor using the Specified-Dynamics version of the NCAR Whole Atmosphere Community Climate Model. We find that the tropical tropopause warming results in a strengthening of the Brewer–Dobson circulation (BDC). The strengthening of BDC induced by a narrow warming of tropical tropopause within 12° latitude, which is much stronger in boreal winter than that in boreal summer, propagates more dry air from the tropical tropopause into the stratosphere and thus causes a reduction of water vapor in the middle stratosphere. On the contrary, the seasonal difference of the BDC strengthening is weaker in the experiment with a broader tropical tropopause warming within 25° latitude. The drying effect of the BDC is counteracted by the moistening effects of the tropical tropopause warming and methane oxidation. This leads to the moistening in both the lower and upper stratosphere. The results suggest the control of the stratospheric humidity by the tropical tropopause temperature could be significantly offset by the associated BDC changes."

[AbruptSLR]

The linked reference indicates that future Arctic Sea Ice loss under BAU forcing will be significantly influenced by Ocean Heat Transport (OHT) from the North Atlantic through the Barents Sea and into the Arctic Ocean Basin and that:  Internal OHT variability is associated with both upstream ocean circulation changes, including AMOC, and large-scale atmospheric circulation anomalies reminiscent of the Arctic Oscillation."  Furthermore, the reference finds that: "The future long-term increase in Atlantic heat transport is carried by warmer water as the current itself is found to weaken."  This last sentence indicates that as the AMOC slows down the ocean water in the associated North Atlantic current gets warmer and it is the increased warmth of the ocean water which is critical to Arctic Sea Ice loss.  Finally, I note that ice-climate feedback associated with both future GIS and future AIS ice mass loss will work to accelerated the current slow down of the AMOC which will result in additional warming of the ocean water in the associated North Atlantic current; which will then accelerate future Arctic Sea Ice loss:

Marius Årthun et al. (13 March 2019), "The role of Atlantic heat transport in future Arctic winter sea ice loss", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0750.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0750.1

Abstract: "During recent decades Arctic sea ice variability and retreat during winter have largely been a result of variable ocean heat transport (OHT). Here we use the Community Earth System Model (CESM) large ensemble simulation to disentangle internally and externally forced winter Arctic sea ice variability, and to assess to what extent future winter sea ice variability and trends are driven by Atlantic heat transport. We find that OHT into the Barents Sea has been, and is at present, a major source of internal Arctic winter sea ice variability and predictability. In a warming world (RCP8.5), OHT remains a good predictor of winter sea ice variability, although the relation weakens as the sea ice retreats beyond the Barents Sea. Warm Atlantic water gradually spreads downstream from the Barents Sea and further into the Arctic Ocean, leading to a reduced sea ice cover and substantial changes in sea ice thickness. The future long-term increase in Atlantic heat transport is carried by warmer water as the current itself is found to weaken. The externally forced weakening of the Atlantic inflow to the Barents Sea is in contrast to a strengthening of the Nordic Seas circulation, and is thus not directly related to a slowdown of the Atlantic meridional overturning circulation (AMOC). The weakened Barents Sea inflow rather results from regional atmospheric circulation trends acting to change the relative strength of Atlantic water pathways into the Arctic. Internal OHT variability is associated with both upstream ocean circulation changes, including AMOC, and large-scale atmospheric circulation anomalies reminiscent of the Arctic Oscillation."

[AbruptSLR]

The linked open access reference finds a 'widespread decrease of firn air content in western Greenland'.  This is not good new, as the air content of the firn decreases (to zero) the surface meltwater runoff will increase substrantially:

Vandecrux, B., MacFerrin, M., Machguth, H., Colgan, W. T., van As, D., Heilig, A., Stevens, C. M., Charalampidis, C., Fausto, R. S., Morris, E. M., Mosley-Thompson, E., Koenig, L., Montgomery, L. N., Miège, C., Simonsen, S. B., Ingeman-Nielsen, T., and Box, J. E.: Firn data compilation reveals widespread decrease of firn air content in western Greenland, The Cryosphere, 13, 845-859, https://doi.org/10.5194/tc-13-845-2019, 2019.

https://www.the-cryosphere.net/13/845/2019/

[AbruptSLR]

The linked open access reference indicates that falling ice radiative effects (FIREs) "… results in the first occurrence of an “ice-free” Arctic (monthly mean extent <1×10⁶<1×106  km²) at 550 ppm CO₂, compared with 680 ppm otherwise."  Furthermore, this reference indicates that CMIP5 ensemble of projections underestimate the influence of FIREs, and thus underestimates likely future Arctic Sea Ice loss:

Li, J.-L. F., Richardson, M., Lee, W.-L., Fetzer, E., Stephens, G., Jiang, J., Hong, Y., Wang, Y.-H., Yu, J.-Y., and Liu, Y.: Potential faster Arctic sea ice retreat triggered by snowflakes' greenhouse effect, The Cryosphere, 13, 969-980, https://doi.org/10.5194/tc-13-969-2019, 2019.

https://www.the-cryosphere.net/13/969/2019/

Abstract

Recent Arctic sea ice retreat has been quicker than in most general circulation model (GCM) simulations. Internal variability may have amplified the observed retreat in recent years, but reliable attribution and projection requires accurate representation of relevant physics. Most current GCMs do not fully represent falling ice radiative effects (FIREs), and here we show that the small set of Coupled Model Intercomparison Project Phase 5 (CMIP5) models that include FIREs tend to show faster observed retreat. We investigate this using controlled simulations with the CESM1-CAM5 model. Under 1pctCO2 simulations, including FIREs results in the first occurrence of an “ice-free” Arctic (monthly mean extent <1×106<1×106  km2) at 550 ppm CO2, compared with 680 ppm otherwise. Over 60–90∘ N oceans, snowflakes reduce downward surface shortwave radiation and increase downward surface longwave radiation, improving agreement with the satellite-based CERES EBAF-Surface dataset. We propose that snowflakes' equivalent greenhouse effect reduces the mean sea ice thickness, resulting in a thinner pack whose retreat is more easily triggered by global warming. This is supported by the CESM1-CAM5 surface fluxes and a reduced initial thickness in perennial sea ice regions by approximately 0.3 m when FIREs are included. This explanation does not apply across the CMIP5 ensemble in which inter-model variation in the simulation of other processes likely dominates. Regardless, we show that FIRE can substantially change Arctic sea ice projections and propose that better including falling ice radiative effects in models is a high priority.

[rboyd]

They just woke up to the reduced ice albedo effect? On a CO_2e basis, we are already beyond 550ppm.