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Messages - AbruptSLR

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1
More troubling information about the potential net increase in carbon emissions from peatlands in a warming world, which current consensus climate change models underestimate:

Swindles, G.T. et al. (2019) Widespread drying of European peatlands in recent centuries, Nature Geoscience, doi.org/10.1038/s41561-019-0462-z

https://www.nature.com/articles/s41561-019-0462-z

Abstract: "Climate warming and human impacts are thought to be causing peatlands to dry, potentially converting them from sinks to sources of carbon. However, it is unclear whether the hydrological status of peatlands has moved beyond their natural envelope. Here we show that European peatlands have undergone substantial, widespread drying during the last ~300 years. We analyse testate amoeba-derived hydrological reconstructions from 31 peatlands across Britain, Ireland, Scandinavia and Continental Europe to examine changes in peatland surface wetness during the last 2,000 years. We find that 60% of our study sites were drier during the period 1800–2000 CE than they have been for the last 600 years, 40% of sites were drier than they have been for 1,000 years and 24% of sites were drier than they have been for 2,000 years. This marked recent transition in the hydrology of European peatlands is concurrent with compound pressures including climatic drying, warming and direct human impacts on peatlands, although these factors vary among regions and individual sites. Our results suggest that the wetness of many European peatlands may now be moving away from natural baselines. Our findings highlight the need for effective management and restoration of European peatlands."

&

Nichols, J.E. and Peteet, D.M. (2019) Rapid expansion of northern peatlands and doubled estimate of carbon storage, Nature Geoscience, doi.org/10.1038/s41561-019-0454-z

https://www.nature.com/articles/s41561-019-0454-z

Abstract: "Northern peatlands are an integral part of the global carbon cycle—a strong sink of atmospheric carbon dioxide and source of methane. Increasing anthropogenic carbon dioxide and methane in the atmosphere are thought to strongly impact these environments, and yet, peatlands are not routinely included in Earth system models. Here we present a quantification of the sink and stock of northern peat carbon from the last glacial period through the pre-industrial period. Additional data and new algorithms for reconstructing the history of peat carbon accumulation and the timing of peatland initiation increased the estimate of total northern peat carbon stocks from 545 Gt to 1,055 Gt of carbon. Further, the post-glacial increases in peatland initiation rate and carbon accumulation rate are more abrupt than previously reported. Peatlands have been a strong carbon sink throughout the Holocene, but the atmospheric partial pressure of carbon dioxide has been relatively stable over this period. While processes such as permafrost thaw and coral reef development probably contributed some additional carbon to the atmosphere, we suggest that deep ocean upwelling was the most important mechanism for balancing the peatland sink and maintaining the observed stability."

See also:

Title: "Europe’s carbon-rich peatlands show ‘widespread’ and ‘concerning’ drying trends"

https://www.carbonbrief.org/europes-carbon-rich-peatlands-show-widespread-and-concerning-drying-trends

Extract: "European peatlands could turn from carbon sinks to sources as a quarter have reached levels of dryness unsurpassed in a record stretching back 2,000 years, according to a new study.

This trend of “widespread” and “substantial” drying corresponds to recent climate change, both natural and human-caused, but may also be exacerbated by the peatlands being used for agriculture and fuel.

It comes as another study estimates that the amount of carbon stored in peatlands across northern regions could be as much as double previous, widely reported estimates.

The papers, both published in Nature Geoscience, indicate a need for efforts to conserve peatlands as sites of carbon storage at higher latitudes.

For his part, Nichols says that considering the threats facing peatlands, it is important for scientists to investigate the total volume of peat available across the world, in order to “put a number on how much there is to lose”:

“Peatlands are not usually part of global climate models. If we want to make realistic predictions of future climate, peatlands need to be a part of it.”"

2
While the first linked reference Clerc et al. (2019), certainly sounds reassuring that an abrupt MICI-type of collapse of glacial ice in the Byrd Subglacial Basin is not likely, I note that the authors use a highly simplified model that does not necessarily match reality.  For instance, Clerc et al. (2019) indicate that currently that marine ice cliff subaerial heights do not exist above ~90m (~100m); however, Parizek et al. (2019) states: 'New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs …' above ~100m exist today.  Also, in my past string of posts in this thread, I have indicated that it is conceivable/likely that a future partial collapse of the subglacial cavity near the 'Big Ear' in the 50km wide Thwaites gateway, could expose an subaerial ice cliff height on the order of 145m within hours if the downstream iceberg field/Thwaites Ice Tongue have substantially lost their ability to provide buttressing say over the next 10-years.

Fiona Clerc et al. (21 October 2019), "Marine Ice Cliff Instability Mitigated by Slow Removal of Ice Shelves", Geophysical Research Letters, https://doi.org/10.1029/2019GL084183

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL084183

Abstract

The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice‐cliff instability (MICI) hypothesis posits that cliffs taller than a critical height (~90‐m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from pre‐existing, static ice cliffs. Here we show how critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales > days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate MICI, implying less ice‐mass loss.

Plain Language Summary

The seaward flow of ice from grounded ice sheets to the ocean is often resisted by the buttressing effect of floating ice shelves. These ice shelves risk collapsing as the climate warms, potentially exposing tall cliff faces. Some suggest ice cliffs taller than ~90 m could collapse under their own weight, exposing taller cliffs further to the interior of a thickening ice sheet, leading to runaway ice‐sheet retreat. This model, however, is based on studies of pre‐existing cliffs found at calving fronts. In this study, we consider the transient case, examining the processes by which an ice cliff forms as a buttressing ice shelf is removed. We show that the height at which a cliff collapses increases with the timescale of ice‐shelf removal. If the ice shelf is removed rapidly, deformation may be concentrated, forming vertical cracks and potentially leading to the collapse of small (e.g., 90‐m) cliffs. However, if we consider ice‐shelf collapse timescales longer than a few days (consistent with observations), deformation is distributed throughout the cliff, which flows viscously rather than collapsing. We expect including the effects of such ice‐shelf collapse timescales in future ice‐sheet models would mitigate runaway cliff collapse and reduce predicted ice‐sheet mass loss.

See also:

Title: "Antarctic ice cliffs may not contribute to sea-level rise as much as predicted"

http://news.mit.edu/2019/antarctic-ice-cliffs-not-contribute-sea-level-rise-1021

Extract: "Scientists have assumed that ice cliffs taller than 90 meters (about the height of the Statue of Liberty) would rapidly collapse under their own weight, contributing to more than 6 feet of sea-level rise by the end of the century — enough to completely flood Boston and other coastal cities. But now MIT researchers have found that this particular prediction may be overestimated.

In a paper published today in Geophysical Research Letters, the team reports that in order for a 90-meter ice cliff to collapse entirely, the ice shelves supporting the cliff would have to break apart extremely quickly, within a matter of hours — a rate of ice loss that has not been observed in the modern record.

“Ice shelves are about a kilometer thick, and some are the size of Texas,” says MIT graduate student Fiona Clerc. “To get into catastrophic failures of really tall ice cliffs, you would have to remove these ice shelves within hours, which seems unlikely no matter what the climate-change scenario.”

If a supporting ice shelf were to melt away over a longer period of days or weeks, rather than hours, the researchers found that the remaining ice cliff wouldn’t suddenly crack and collapse under its own weight, but instead would slowly flow out, like a mountain of cold honey that’s been released from a dam."

&

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.

3
Attached is another status report on the progress being made to update the atmospheric component of the E3SM projections:

P. J. Rasch et al. (09 July 2019), "An Overview of the Atmospheric Component of the Energy Exascale Earth System Model", JAMES,
https://doi.org/10.1029/2019MS001629

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019MS001629

Abstract
The Energy Exascale Earth System Model Atmosphere Model version 1, the atmospheric component of the Department of Energy's Energy Exascale Earth System Model is described. The model began as a fork of the well‐known Community Atmosphere Model, but it has evolved in new ways, and coding, performance, resolution, physical processes (primarily cloud and aerosols formulations), testing and development procedures now differ significantly. Vertical resolution was increased (from 30 to 72 layers), and the model top extended to 60 km (~0.1 hPa). A simple ozone photochemistry predicts stratospheric ozone, and the model now supports increased and more realistic variability in the upper troposphere and stratosphere. An optional improved treatment of light‐absorbing particle deposition to snowpack and ice is available, and stronger connections with Earth system biogeochemistry can be used for some science problems. Satellite and ground‐based cloud and aerosol simulators were implemented to facilitate evaluation of clouds, aerosols, and aerosol‐cloud interactions. Higher horizontal and vertical resolution, increased complexity, and more predicted and transported variables have increased the model computational cost and changed the simulations considerably. These changes required development of alternate strategies for tuning and evaluation as it was not feasible to “brute force” tune the high‐resolution configurations, so short‐term hindcasts, perturbed parameter ensemble simulations, and regionally refined simulations provided guidance on tuning and parameterization sensitivity to higher resolution. A brief overview of the model and model climate is provided. Model fidelity has generally improved compared to its predecessors and the CMIP5 generation of climate models.

Plain Language Summary

This study provides an overview of a new computer model of the Earth's atmosphere that is used as one component of the Department of Energy's latest Earth system model. The model can be used to help understand past, present, and future changes in Earth's behavior as the system responds to changes in atmospheric composition (like pollution and greenhouse gases), land, and water use and to explore how the atmosphere interacts with other components of the Earth system (ocean, land, biology, etc.). Physical, chemical, and biogeochemical processes treated within the atmospheric model are described, and pointers to previous and recent work are listed to provide additional information. The model is compared to present‐day observations and evaluated for some important tests that provide information about what could happen to clouds and the environment as changes occur. Strengths and weaknesses of the model are listed, as well as opportunities for future work.

4

baking,

Thank you for creating the first attached image comparing grounding line information from Milillo et al. (2019) (see Reply #1704 for the reference) to the Sentinel-1 image for October 16, 2019; …

baking,

If you want to be of more service to the ASIF readers then in addition to overlaying panel B from the attached image from Milillo et al (2019) on top of the Sentinel-1 image from Oct 16, 2019, you could also overlay the information from panels C, D, E & F.  Herein, I note that panel D shows a very abrupt change in the 'Height of the ice surface above flotation, hf, in meters' between the ice surface above the subglacial cavity at the Big Ear and the ice surface immediately upstream; which is where I postulate that an ice cliff will form with hf more than 145m if/when the local part of the subglacial cavity at the Big Ear collapse in less than ten years.

Best,
ASLR

5
The accompanying four attached image provides one interpretation of how the influence of the November 2012 partial collapse of the subglacial cavity near the Little Ear and the subsequent growth of the size of the Big Ear subglacial cavity has change the gravitational load path between the buttressing action of the Thwaites Eastern Ice Shelf (TEIS), shown in yellow arrows, and the Thwaites Ice Tongue, shown in orange arrows, near the trough in the bed of the Thwaites gateway.  The first image shows the marked drop in the surface elevation of the ice near the Little Ear between Jan 2012 and Jan 2013 due to the Nov 2012 partial collapse of the subglacial cavity near the Little Ear.  The second image shows how this partial subglacial collapse provided a new load path from the gravitational load from the ice at the upstream end of the bed trough in the gateway to the buttressing action of the TEIS.  The third image reminds us of the subsequent growth of the subglacial cavity near the Big Ear, which likely resulted a reduction in the force on the load path to the ice tongue and an increase in the force on the load path to the middle of the TEIS as shown in the fourth image from May 23, 2019.  This conceptual sequence of events would also explain the significant calving of icebergs from the southwest corner of the TEIS (due to compression loading) and why the middle section of the TEIS appear to be shearing to the northeast past the pinning point at the downstream end of the TEIS (which is likely causing another iceberg calving event at the northeast region of the TEIS).

If correct, this sequence of events should: a) facilitate future calving of icebergs from the southwest corn of the ice tongue due to the reduced compressive force in this region; and b) accelerate the collapse of the TEIS by shearing the middle section of the TEIS into a northeastern direction past the TEIS pinning point.

6
...

One problem with AbrubtSLR's analysis is that a grounding line retreat in the "Trough" at the center of Thwaites Glacier is too narrow to lead to MICI all on it's own.  You would need at least a significant grounding line retreat on either the Eastern of Western sides of Thwaites, if not both, to open up the wide West Antarctic basin to a massive collapse.

...

I note that once potential ice-cliff failure mechanisms within the trough in the Thwaites gateway bed reaches the retrograde slope leading into the Byrd Subglacial Basin, that the most likely failure mechanism will be by slumping as described by Parizek et al. (2019); which would likely result in icebergs with much shallower drafts than those currently being formed in in Thwaites calving events; which would allow the Thwaites gateway to widen up to be 50-km wide; which would allow for the float-out of a large volume of ice mélange.

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.

Also, see the following related abstract from the 2018 WAIS Workshop:

Title: "Across the Great Divide: The Flow-to-Fracture Transition and the Future of the West Antarctic Ice Sheet", by Richard B. Alley, Byron R. Parizek, Knut Christianson, Robert M. DeConto, David Pollard and Sridhar Anandakrishna

https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/Alley_R.pdf

Abstract: "Physical understanding, modeling, and available data indicate that sufficient warming and retreat of Thwaites Glacier, West Antarctica will remove its ice shelf and generate a calving cliff taller than any extant calving fronts, and that beyond some threshold this will generate faster retreat than any now observed. Persistent ice shelves are restricted to cold environments. Ice-shelf removal has been observed in response to atmospheric warming, with an important role for meltwater wedging open crevasses, and in response to oceanic warming, by mechanisms that are not fully characterized. Some marine-terminating glaciers lacking ice shelves “calve” from cliffs that are grounded at sea level or in relatively shallow water, but more-vigorous flows advance until the ice is close to flotation before calving. For these vigorous flows, a calving event shifts the ice front to a position that is slightly too thick to float, and generates a stress imbalance that causes the ice front to flow faster and thin to flotation, followed by another calving event; the rate of retreat thus is controlled by ice flow even though the retreat is achieved by fracture. Taller cliffs generate higher stresses, however, favoring fracture over flow. Deformational processes are often written as power-law functions of stress, with ice deformation increasing as approximately the third power of stress, but subcritical crack growth as roughly the thirtieth power, accelerating to elastic-wave speeds with full failure. Physical understanding, models based on this understanding, and the limited available data agree that, above some threshold height, brittle processes will become rate-limiting, generating faster calving at a rate that is not well known but could be very fast. Subaerial slumping followed by basal-crevasse growth of the unloaded ice is the most-likely path to this rapid calving. This threshold height is probably not too much greater than the tallest modern cliffs, which are roughly 100 m."

For those who do not understand the implications of Alley et al. (2018)'s comment that ice deformation is a power-function of stress, I provide the first image that translates this underlying ice-cliff behavior into terms of calving rate (deformation) per year for various values of marine glacier freeboard (ice face height minus water depth) and relative water depth (which combine determine the primary stresses near the ice cliff face).

Edit: For what it is worth, the 2019 WAIS Workshop is ending today, and thus the associated abstracts should be available online within a few months.

Edit2: The last three images show a sequence of slumping.

7
The linked 2014 article makes it clear that using the public's approach to dealing with uncertainty using delayed action is a bad idea.

Title: "Scientists unmask the climate uncertainty monster"

https://phys.org/news/2014-04-scientists-unmask-climate-uncertainty-monster.html

Extract: "Scientific uncertainty is a 'monster' that prevents understanding and delays mitigative action in response to climate change, according to The University of Western Australia's Winthrop Professor Stephan Lewandowsky and international colleagues, who suggest that uncertainty should make us more rather than less concerned about climate change.

In two companion papers published today in Climatic Change, the researchers investigated the mathematics of uncertainty in the climate system and showed that increased scientific uncertainty necessitates even greater action to mitigate climate change.

Professor Stephan Lewandowsky, who is also Chair in Cognitive Psychology and member of the Cabot Institute at the University of Bristol, said: "We can understand the implications of uncertainty, and in the case of the climate system, it is very clear that greater uncertainty will make things even worse. This means that we can never say that there is too much uncertainty for us to act. If you appeal to uncertainty to make a policy decision the legitimate conclusion is to increase the urgency of mitigation."

These new findings challenge the frequent public misinterpretation of uncertainty as a reason to delay action. Arguing against mitigation by appealing to uncertainty is therefore misplaced: any appeal to uncertainty should provoke a greater, rather than weaker, concern about climate change than in the absence of uncertainty."

8
The linked reference suggests that consensus scientists likely avoid talking about known unknowns and unknown unknowns with the public in order to maintain the publics trust in their projections.  This behavior does not improve our safety regarding a potential collapse of the WAIS, unless somehow very wise decision makers are diligently working behind the scenes to evaluate the potential impacts of such right-tail risks (which I doubt).

Lauren C. Howe et al. Acknowledging uncertainty impacts public acceptance of climate scientists' predictions, Nature Climate Change (2019). DOI: 10.1038/s41558-019-0587-5

https://www.nature.com/articles/s41558-019-0587-5

Abstract: "Predictions about the effects of climate change cannot be made with complete certainty, so acknowledging uncertainty may increase trust in scientists and public acceptance of their messages. Here we show that this is true regarding expressions of uncertainty, unless they are also accompanied by acknowledgements of irreducible uncertainty. A representative national sample of Americans read predictions about effects of global warming on sea level that included either a worst-case scenario (high partially bounded uncertainty) or the best and worst cases (fully bounded uncertainty). Compared to a control condition, expressing fully bounded but not high partially bounded uncertainty increased trust in scientists and message acceptance. However, these effects were eliminated when fully bounded uncertainty was accompanied by an acknowledgement that the full effects of sea-level rise cannot be quantified because of unpredictable storm surges. Thus, expressions of fully bounded uncertainty alone may enhance confidence in scientists and their assertions but not when the full extent of inevitable uncertainty is acknowledged."

See also:

Title: "How uncertainty in scientific predictions can help and harm credibility"

https://phys.org/news/2019-10-uncertainty-scientific-credibility.html

Extract: "The more specific climate scientists are about the uncertainties of global warming, the more the American public trusts their predictions, according to new research by Stanford scholars.

But scientists may want to tread carefully when talking about their predictions, the researchers say, because that trust falters when scientists acknowledge that other unknown factors could come into play."

9
Many left-tail climate-change pdf focused people argue that as all climate models are wrong (see the linked Wikipedia article), they are entitled to advise decision makers using the least dramatic linear approximation for climate change projections.  However, it is my opinion that some models match reality sufficiently to be useful, while overly simplified models can be counterproductive.  Furthermore, I note that the 'scientific method' is not a given model, but rather is a process that progressively iterates towards developing new models that better match the complexities of reality with time.

Title: "All models are wrong"

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

Extract: ""All models are wrong" is a common aphorism in statistics; it is often expanded as "All models are wrong, but some are useful". It is usually considered to be applicable to not only statistical models, but to scientific models generally. The aphorism is generally attributed to the statistician George Box, although the underlying concept predates Box's writings.…
Although the aphorism seems to have originated with George Box, the underlying concept goes back decades, perhaps centuries.

In 1947, the mathematician John von Neumann said that "truth … is much too complicated to allow anything but approximations".
 
In 1942, the French philosopher-poet Paul Valéry said the following.
Ce qui est simple est toujours faux. Ce qui ne l’est pas est inutilisable.
What is simple is always wrong. What is not is unusable."

Furthermore, the developer of the holographic method (Dennis Gabor), once stated:

"The future cannot be predicted, but futures can be invented."

In this regard, I note that:

1. Left-tail climate-change focused people can serve to 'invent'/promote a 'hothouse' future by making decision makers believe that they have more time to curtail GHG emission than what is advisable to avoid triggering a cascade of climate change tipping points.

2. Right-tail climate-change focused people can serve to 'invent'/promote a 'sustainable' future by citing model projections that are sufficiently complex to effectively attribute cause and effect in the various interconnected Earth Systems responses; but which are simple enough to be useful to decision makers.

Regarding my second point about the usefulness of a right-tail climate-change focus when it promotes the development of models that are sufficiently complex to effectively attribute cause and effect in Earth System responses, I close this post by noting that just because many ice-climate feedback mechanisms flush ice meltwater into both the North Atlantic and the Southern Ocean thus decreasing the SSTA and thus limiting the increase in GMSTA with increasing radiative forcing, does not mean that we are all safer, but rather the opposite.  However, in order to recognize the threat from fresh-meltwater/iceberg hosing one must have climate models that are sufficiently sophisticated to correctly account for such somewhat complex mechanisms as: Ekman Transport, Meridional Overturning Circulation, Vertical Eddy Flux and Ocean Heat Uptake, etc.  In this regard, one of the reasons that CMIP5 projected values of ECS that will likely be lower than the ECS values to likely be projected by CMIP6 is that the CMIP5 models underestimated the influence of many of these ocean dynamics.

10
First, in the linked article Hausfather notes that 2019 most likely will have the second highest GMSTA on record, and per the first attached image the 'very likely' range for the 2019 GMSTA is from 1.12 to 1.21C; which is getting close to the 1.5C aspirational goal set by the IPCC.

Title: "State of the climate: Low sea ice and near-record warmth define 2019 to date"

https://www.carbonbrief.org/state-of-the-climate-low-sea-ice-and-near-record-warmth-define-2019-to-date

Extract: "This year is shaping up to be the second warmest on record for most surface temperature datasets, behind only the super-El Niño year of 2016. This is particularly noteworthy because 2019 has been characterised by a weak El Niño that has played little role in boosting temperatures."

Second, in the second attached image (of a Hausfather tweet), a given CH4 emission will raise the GMSTA by more than 100 times what the same emission of CO2 will do over a 10-year period.  Thus, assuming that we do not meaningfully change either CH4 or CO2 emission rates for the next 20-years; methane emissions will contribute more to potentially pushing the world over any possible Earth System tipping points that exist in the next couple of decades, than are carbon dioxide emissions (over the same period); and remember that the higher GMSTA gets the more likely we collectively are to crossing such a tipping point.

11

It covers DeConto and Pollard's 2016 paper, the Edwards et. al 2019 response and discuss updates planned by DeConto and Pollard.



I do indeed consider Edwards et al. (2019) to be a representation of consensus climate science erring on the side of least drama; which is not to disrespect consensus climate scientists; but which is to say that in my opinion they are doing a poor job of communicating climate risk to both the public and to decision makers.

For example consensus climate science acknowledges that the probability density function (PDF) for climate sensitivity is right-skewed as shown in the first attached image; nevertheless, consensus climate scientists generally talk about the 'most likely' (or mode) value rather than the mean value which is considerably higher and which represents much more risk to our socio-economic system as discussed by the linked article and the second attached image; which assumes that the mode value for ECS is 3C instead of over 5C.


Title: "Climate Change Could End Human Civilisation as We Know It by 2050, Analysis Finds"

https://www.sciencealert.com/by-2050-climate-change-could-alter-human-civilisation-as-we-know-it

Extract: "The new report, co-written by a former executive in the fossil fuel industry, is a harrowing follow-up to the Breakthrough National Centre for Climate Restoration's 2018 paper, which found that climate models often underestimate the most extreme scenarios.

Endorsed by former Australian defence chief Admiral Chris Barrie, the message is simple: if we do not take climate action in the next 30 years, it is entirely plausible that our planet warms by 3°C and that human civilisation as we know it collapses.

Under this scenario, the authors explain, the world will be locked into a "hothouse Earth" scenario, where 35 percent of the global land area, and 55 percent of the global population, will be subject to more than 20 days a year of "lethal heat conditions, beyond the threshold of human survivability."

With a runaway event like this, climate change will not present as a normal distribution, but instead will be skewed by a fat tail – indicating a greater likelihood of warming that is well in excess of average climate models.
Under a business-as-usual scenario, the authors explain, warming is set to reach 2.4°C by 2050. If feedback cycles are taken into account, however, there may be another 0.6°C that current models do not assume.
"It should be noted," the paper adds, "that this is far from an extreme scenario: the low-probability, high-impact warming (five percent probability) can exceed 3.5–4°C by 2050.""

In this same vein of thought I note that Dr. Tasmin Edwards et al. (2019) demonstrated that when calibrating a MICI model to a left-tail set of assumptions from the paleorecord that it is possible to present less alarming MICI projections than if one were to calibrate an MICI model to either mean or right-tailed sets of assumptions from the paleorecord.

12
While many of my past posts in this thread have emphasized that ECS has a good chance to be considerably higher than the values cited in AR5; many of my other posts demonstrate that there is a good possibility that much of the WAIS may collapse in coming decades without any the need for either higher (than current consensus) values of ECS or even for more radiative forcing.  Still many of my other posts emphasize that the numerous transient positive ice-climate feedback mechanisms (e.g. slowing of the MOC, albedo flip, the bipolar seesaw mechanism, ocean-cloud feedbacks, etc.) can act to increase the effective equilibrium climate sensitivity (EffCS) for multidecadal periods; which raises the topic of this post.

If ECS is indeed currently above 5C as indicates by at least eight of the most sophisticated CMIP6 models, and if a collapse of the WAIS (beginning between 2030 & 2040) increases this value still higher for several (to multiple) decades, then it is possible that the North Hemisphere could tip into an equable climate, and stay in that pattern for at least centuries, for reason including:

1. The primary characteristic of an equable climate (as occurred during the Eocene) is that ocean heat energy is conveyed directly from the tropical oceans (particularly the Tropical Pacific) poleward (& particularly to the Arctic).  In this regard, Schneider et al. (2019) cites that the future risk of losing marine stratocumulus clouds (which currently produce a negative feedback) would would result in an abrupt increase in GMSTA.  While Schneider et al. (2019) showed that an increase of atmospheric carbon dioxide concentration to about 1,200ppm, would result in such a loss of marine stratocumulus cloud, I previously pointed out in Reply #652 (see also Replies: #633, #642 & #650), the risk of abruptly losing the marine stratocumulus clouds would also occur if the equatorial SST increases from about 27C to about 32C.  In this case the atmosphere for the North Hemisphere could be abruptly transitioned from modern to equable climate (or 'hot house') conditions.  Such a 5C SST increase in the equatorial oceans, could conceivable occur this century from a combination of: a) ice-climate feedbacks from the collapse of the WAIS & bipolar seesaw interaction with the Arctic & Greenland; b) a cascade of other tipping points (including methane feedbacks), c) a rapid decrease/redistribution of anthropogenic aerosol emissions and/or a high current value of ECS.

2.  The linked reference (Pistone et al 2019) calculates the radiative heating of a sea ice free Arctic Ocean during the sunlit part of the year and assuming constant cloudiness they '… calculate a global radiative heating of 0.71 W/m2 relative to the 1979 baseline state. This is equivalent to …' hastening global warming by an estimated 25 years.  If the Northern Hemisphere were to flip into an equable pattern this century, this would lead to a sea ice free Arctic Ocean during the sunlit part of the year (particularly due to rainfall on the Arctic Sea Ice); which (together with bipolar seesaw interaction between the GIS and the AIS) might well be sufficient to maintain an equable climate pattern even after the multidecadal pulse of planetary energy imbalance associated with glacial ice mass loss from the GIS & the AIS.

Kristina Pistone et al. (20 June 2019), "Radiative Heating of an Ice‐free Arctic Ocean", Geophysical Research Letters, https://doi.org/10.1029/2019GL082914

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL082914?af=R

&

3. The linked reference (Massoud et al 2019) indicates that using consensus science (CMIP5) analyses the frequency of Atmospheric Rivers (ARs) will increase in frequency by about 50% and in intensity by about 25% by 2100, without considering points 1. and 2. above.  As an AR rainfall event on the GIS would greatly accelerate the bipolar seesaw mechanism, these findings should be considered when evaluating future right-tail climate change risks this century:

E.C. Massoud et al. (12 October 2019), "Global Climate Model Ensemble Approaches for Future Projections of Atmospheric Rivers", Earth's Future, https://doi.org/10.1029/2019EF001249

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019EF001249?af=R

Abstract
Atmospheric rivers (ARs) are narrow jets of integrated water vapor transport that are important for the global water cycle, and also have large impacts on local weather and regional hydrology. Uniformly‐weighted multi‐model averages have been used to describe how ARs will change in the future, but this type of estimate does not consider skill or independence of the climate models of interest. Here, we utilize information from various model averaging approaches, such as Bayesian Model Averaging (BMA), to evaluate 21 global climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Model ensemble weighting strategies are based on model independence and atmospheric river performance skill relative to ERA‐Interim reanalysis data, and result in higher accuracy for the historic period, e.g. RMSE for AR frequency (in % of timesteps) of 0.69 for BMA vs 0.94 for the multi‐model ensemble mean. Model weighting strategies also result in lower uncertainties in the future estimates, e.g. only 20‐25% of the total uncertainties seen in the equal weighting strategy. These model averaging methods show, with high certainty, that globally the frequency of ARs are expected to have average relative increases of ~50% (and ~25% in AR intensity) by the end of the century.

Plain Language Summary

Atmospheric rivers (ARs) are storms of integrated water vapor transport that are important for the global water cycle, and also have large impacts on local weather and regional hydrology. An increase in the frequency of ARs is expected to occur by the end of the century throughout most of the globe. Usually, these types of assessments of future climate change rely on simple (i.e. equally‐weighted) multi‐model averages and do not consider the skill or independence of the climate models of interest. Here, we utilize information from various model averaging approaches to constrain a suite of 21 global climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The weighted model combinations are fit to reanalysis data (ERA‐Interim) and are useful because they provide higher skill as well as lower uncertainties compared to equal weighting. This work supports the claim that AR frequency will increase in the future by about ~50% (and intensity will increase by ~25%) globally by the end of the century.

13
As it may not be clear to some readers as to why the new upstream crevasses at the base of the Thwaites Ice Tongue might be at risk of creating icebergs that can float away, I note that:

1. The first attached image from Rignot et al. (2009) shows that the ice velocities exiting the Thwaites gateway at the base of the Thwaites Ice Tongue are particularly high.  So much so that when the ice tongue virtually collapsed in 2012, it was rapidly replaced by a new ice tongue that was about half as wide as the old ice tongue and was much more fractured than the old ice tongue (yet it maintains a fragile stability as its northerly end is pinned on the subsea ridge; which keeps the fragmented mass of icebergs in compression so they cannot readily float away).

2. I have previously posted the second image from Rignot et al. (2017) showing a second along the line AB that was near the centerline of the old ice tongue, but which now runs along the westerly edge of the new ice tongue, and passes upstream along the western edge of the 'Big Ear' subglacial cavity.  Thus the nearly 40 km-long ice tongue shown in this image use to be composed of less fractured glacial ice, but now consists of a relatively narrow field of confined floating icebergs; and even more importantly the figure shows a red line showing the hydrostatic bottom elevation calculated from the surface elevation which indicates whether the ice above the line can float if not restrained from floating as the over 2 km of ice upstream of the grounding line would do except that before 2017 it was restrained from floating by the weight of the adjoining upstream ice.  Thus, the new crevasses shown in the Oct 12, 2019 Sentinel-1 image define potential icebergs if their confinement is relieved as has occurred along their westerly edge as indicated by the area of new calving front indicated in the image in Reply #1715, and as this calving front (at the westerly side of the base of the Thwaites Ice Tongue) progresses in the southeasterly direction (both due to icebergs calving off this front and due to the 'Big Ear' subglacial cavity extending along the trough to the southwest) eventually the new Thwaites Ice Tongue will loss confinement and the icebergs currently confined in its mass will float away to the northwest (note the first 2009 image shows a subsea ridge to the northwest that might pin such icebergs, but the improved resolution of the third 2013 image shows that this subsea ridge to the northwest is not high enough to pin such future icebergs).


14
For readers who are uncertain of the timing, and direction, of when icebergs may float out from the bed trough near the 'Big Ear' (see Replies 1704 thru 1713) upstream of the base of the Thwaites Ice Tongue (TG), I provide the attached image that compares captures from the Sentinel 1 satellite in this area on May 23, 2019 and Oct 12, 2019 (less than 5 months apart).  This comparison shows new crevasses in the base of the TG just downstream of the 'Big Ear', and also the calving front at the western side of the TG base in moving towards the 'Big Ear'.  If this trend continues it seem probable that icebergs will be able to float in the northwest direction from the 'Big Ear' area of the bed trough prior to 2030.

Edit: If it is not clear from my prior posts, the new upstream crevasses in the Oct 12, 2019 occur as the glacial ice flows over the subglacial cavity (leading to the Big Ear) causing the surface elevation to drop abruptly, thus cracking the ice.  When the ice between these regularly spaced crevasses either thins enough, or reaches deep enough water, it will float; and when not confined it will float away as icebergs (see the third image in Reply #1710).

15
In about one more month we will say sayonara to Mt. Fuji :'(

16

The vast majority of the continent has experienced no net change in ice.  All the melt is concentration on a small section along the western coastline.  The ice rebound is likely to be  less than 10%, as so little area is affected.

First, the 10% increase in ice mass loss, calculated based on GRACE satellite data, only applies to the WAIS and not to the EAIS.


Note also that there has to be corresponding reduction in height in from surrounding areas. The location and amount depending on the viscosity and topography of the asthenosphere. I don't see where the rock is coming from mentioned in the paper (my guess is along the rift axis). If the grounding line is the point of the most ice loss, then i would think that it would act to increase the angle of slope to the interior as that is proportionally pushed down. The wavelength of any elastic response is short in rift zones as the lithosphere is elastically weak, on the orders of 5-20 kms. Uplift of the grounding line coupled with decreasing bedrock elevation in the interior does not sound like a recipe for stabilizing West Antarctica to me.

As this topic has been discussed several times in this thread, here I will note that the assessment of rapid uplift in the Amundsen Sea Embayment area is not based on theory but on physical observations as noted in the two linked references and as illustrated by the two accompanying images.

The first accompanying image shows an overview of the Amundsen Sea sector, West Antarctica. The red line defines the generalized drainage basins of Pine Island Glacier, Thwaites Glacier and Smith Glacier (PITS). Locations of three GPS campaign sites are marked by red triangles.

The second image shows how post-glacial rebound for current ice mass loss from a marine glacier consists of both quick elastic rebound and slower rebound due to the flow of magma in the mantle.  I note that the current GIA corrections to GRACE data are based on conservative assumptions about the viscosity of the magma beneath the Byrd Subglacial Basin, BSB

V.R. Barletta el al. (22 Jun 2018), "Observed rapid bedrock uplift in Amundsen Sea Embayment promotes ice-sheet stability," Science, Vol. 360, Issue 6395, pp. 1335-1339, DOI: 10.1126/science.aao1447.

http://science.sciencemag.org/content/360/6395/1335

&

An investigation of Glacial Isostatic Adjustment over the Amundsen Sea sector, West Antarctica
by: A. Groh; H. Ewert, M. Scheinert, M. Fritsche, A. Rülke, A. Richter, R. Rosenau, R. Dietrich
http://dx.doi.org/10.1016/j.gloplacha.2012.08.001

Edit: Also, I reiterate that such relatively slow rebound will not protect the WAIS from a possible MICI-type of collapse in the coming decades.

17
...
All these challenges are clearly solvable, but I'd guess we need a couple of decades to get models that are truly reliable as forecasting tools.

While your assessments of the key shortcomings of the CMIP5/6 models have some merit; I point-out that what one considers to be '... truly reliable as forecasting tools' depends very much on how one plans to use such tools.  Businesses (whether under capitalist or socialist systems) commonly face complex situations that cannot be deterministically solved using current business models/computers; but this does not stop such business from making successful use of risk-based evaluations from the output of those limited business models/computers.  In other words intelligent decision makers cannot be absolved from their responsibilities to make wise decisions in the face of uncertainties, just because climate change models are not perfect.

18

The vast majority of the continent has experienced no net change in ice.  All the melt is concentration on a small section along the western coastline.  The ice rebound is likely to be  less than 10%, as so little area is affected.

First, the 10% increase in ice mass loss, calculated based on GRACE satellite data, only applies to the WAIS and not to the EAIS.

Second, the first image (with a hat tip to sidd) shows that the firn for large portions of the Antarctic ice shelves are already saturated with ice and thus could soon be subject to rapid collapse due to hydrofracturing from surface meltwater during the austral summer months; which would leave bare ice-cliffs.

Third, the second image (for the Wilkes marine glacier) shows how relatively thin the 'ice plug' that stops MICI-types of marine glacier collapse once the ice shelves are lost.

Fourth, the third image [from P. Milillo et al. (30 Jan 2019)] shows a Manhattan-sized cavity in a trough in the seafloor/glacial-bed in the gateway for the Thwaites Glacier; where the trough spans the width of the 'ice plug' from the ocean to the negative slope of the Byrd Subglacial Basin (see the fourth image).  Thus if icebergs calved from ice cliffs in this trough area were to float-out to sea once the ice shelf/ice tongue is lost, then the MICI-type of failure for Thwaites could begin in this trough in the coming decades, thus bypassing the assumed 'ice plug'.

All of this indicates that the ice mass loss along the 'small section along the western coastline' of the key Antarctic marine glaciers is what is actually important for abrupt climate change in coming decades.

19
The linked reference indicates that without sufficient nitrogen, the grow potential of marsh plants is limited at elevated CO2 levels:

Meng Lu et al. (2019), "Nitrogen status regulates morphological adaptation of marsh plants to elevated CO2", Nature Climate Change  9, 764–768, DOI: https://doi.org/10.1038/s41558-019-0582-x

https://www.nature.com/articles/s41558-019-0582-x

Abstract: "Coastal wetlands provide valuable ecosystem services that are increasingly threatened by anthropogenic activities. The atmospheric carbon dioxide (CO2) concentration has increased from 280 ppm to 404 ppm since the Industrial Revolution and is projected to exceed 900 ppm by 2100 (ref. 2). In terrestrial ecosystems, elevated CO2 typically stimulates C3 plant photosynthesis and primary productivity leading to an increase in plant size. However, compared with woody plants or crops, the morphological responses of clonal non-woody plants to elevated CO2 have rarely been examined. We show that 30 years of experimental CO2 enrichment in a brackish marsh increased primary productivity and stem density but decreased stem diameter and height of the dominant clonal species Schoenoplectus americanus. Smaller, denser stems were associated with the expansion of roots and rhizomes to alleviate nitrogen (N) limitation as evidenced by high N immobilization in live tissue and litter, high tissue C:N ratio and low available porewater N. Changes in morphology and tissue chemistry induced by elevated CO2 were reversed by N addition. We demonstrate that morphological responses to CO2 and N supply in a clonal plant species influences the capacity of marshes to gain elevation at rates that keep pace with rising sea levels."

See also:

Title: "High carbon dioxide can create 'shrinking stems' in marshes"

https://phys.org/news/2019-09-high-carbon-dioxide-stems-marshes.html

Extract: "For most plants, carbon dioxide acts like a steroid: The more they can take in, the bigger they get. But in a new study published Sept. 25, scientists with the Smithsonian discovered something strange happening in marshes. Under higher levels of carbon dioxide, instead of producing bigger stems, marsh plants produced more stems that were noticeably smaller."

20
In regards to ice mass loss from Antarctic ice shelves, I provide the following information:

Sutterley, T. C., Markus, T., Neumann, T. A., van den Broeke, M., van Wessem, J. M., and Ligtenberg, S. R. M.: Antarctic ice shelf thickness change from multimission lidar mapping, The Cryosphere, 13, 1801–1817, https://doi.org/10.5194/tc-13-1801-2019, 2019.

https://www.the-cryosphere.net/13/1801/2019/

Abstract

We calculate rates of ice thickness change and bottom melt for ice shelves in West Antarctica and the Antarctic Peninsula from a combination of elevation measurements from NASA–CECS Antarctic ice mapping campaigns and NASA Operation IceBridge corrected for oceanic processes from measurements and models, surface velocity measurements from synthetic aperture radar, and high-resolution outputs from regional climate models. The ice thickness change rates are calculated in a Lagrangian reference frame to reduce the effects from advection of sharp vertical features, such as cracks and crevasses, that can saturate Eulerian-derived estimates. We use our method over different ice shelves in Antarctica, which vary in terms of size, repeat coverage from airborne altimetry, and dominant processes governing their recent changes. We find that the Larsen-C Ice Shelf is close to steady state over our observation period with spatial variations in ice thickness largely due to the flux divergence of the shelf. Firn and surface processes are responsible for some short-term variability in ice thickness of the Larsen-C Ice Shelf over the time period. The Wilkins Ice Shelf is sensitive to short-timescale coastal and upper-ocean processes, and basal melt is the dominant contributor to the ice thickness change over the period. At the Pine Island Ice Shelf in the critical region near the grounding zone, we find that ice shelf thickness change rates exceed 40 m yr−1, with the change dominated by strong submarine melting. Regions near the grounding zones of the Dotson and Crosson ice shelves are decreasing in thickness at rates greater than 40 m yr−1, also due to intense basal melt. NASA–CECS Antarctic ice mapping and NASA Operation IceBridge campaigns provide validation datasets for floating ice shelves at moderately high resolution when coregistered using Lagrangian methods.

&

E. Rignot et al. (Jul 2013), "Ice-Shelf Melting Around Antarctica", Science, Vol. 341, Issue 6143, pp. 266-270, DOI: 10.1126/science.1235798

https://science.sciencemag.org/content/341/6143/266.abstract

Major Meltdown
The ice shelves and floating ice tongues that surround Antarctica cover more than 1.5 million square kilometers—approximately the size of the entire Greenland Ice Sheet. Conventional wisdom has held that ice shelves around Antarctica lose mass mostly by iceberg calving, but recently it has become increasingly clear that melting by a warming ocean may also be important. Rignot et al. (p. 266, published 13 June) present detailed glaciological estimates of ice-shelf melting around the entire continent of Antarctica, which show that basal melting accounts for as much mass loss as does calving.

Abstract
We compare the volume flux divergence of Antarctic ice shelves in 2007 and 2008 with 1979 to 2010 surface accumulation and 2003 to 2008 thinning to determine their rates of melting and mass balance. Basal melt of 1325 ± 235 gigatons per year (Gt/year) exceeds a calving flux of 1089 ± 139 Gt/year, making ice-shelf melting the largest ablation process in Antarctica. The giant cold-cavity Ross, Filchner, and Ronne ice shelves covering two-thirds of the total ice-shelf area account for only 15% of net melting. Half of the meltwater comes from 10 small, warm-cavity Southeast Pacific ice shelves occupying 8% of the area. A similar high melt/area ratio is found for six East Antarctic ice shelves, implying undocumented strong ocean thermal forcing on their deep grounding lines.

21

As is shown on the attached graphic from the GRACE-FO data
( http://gravis.gfz-potsdam.de/antarctica ) - interactive map

and as I extracted from the ASCII file they provide.
____________________________________________
Note: The German Partners in the GRACE-FO project ( Helmholtz Centre Potsdam
GFZ - German Research Centre for Geosciences) are being very helpful in getting data out to non-scientists like me - instant answers to my e-mails.. I must write & say thanks.

JPL/NASA seem all about the scientists - never an answer to queries. But maybe they are getting strife from Trump acolytes.

As noted in Reply #1673, almost certainly the GRACE-FO ice mass values need to be increase by about 10%, due to more rapid than previously assumed ice rebound (i.e. the increasing mass of the mantle associated with the rapid rebound, can fool gravity measurements into believing that less ice mass has been lost than is actually the case).

Edit:  Also, I note that to date most of the freshwater released from Antarctica into the Southern Ocean has come from ice shelves and this ice mass loss is not measured by GRACE-FO and needs to be added separately, in order to evaluate the impact of the surface water freshening/cooling; which not only reduces the local SSTA but also accelerates the accumulation of warm deep water in the Southern Ocean; which in turn accelerates ice mass loss from both ice shelves and from marine glaciers in Antarctia.

22
The linked article discusses how new 'Ghost Forests' are a visible sign of climate change:

Title: "New “Ghost Forests” Are a Sign of Climate Change"

https://www.nathab.com/blog/new-ghost-forests-are-a-sign-of-climate-change/

Extract: "Along vast stretches of North America’s East Coast, rising sea levels are killing trees by inundating them in saltwater. Researchers are calling these dead trees in what used to be thriving freshwater environments “ghost forests.” Although this is occurring around the world, new ghost forests are particularly apparent in the United States, with hundreds of thousands of acres of salt-killed trees spreading from Canada down to Florida and over to Texas.

One thing that scientists do agree upon, though, is that the startling sight of dead trees in once-healthy areas aren’t specters, but easy-to-grasp signs of the consequences of climate change."

Edit: If it is not clear to some readers, relative sea level rise can increase groundwater saliently for several miles inland; which can also stunt/kill vegetation along the world's coastlines.

23
The linked E3SM PPTX discusses the current high degree of uncertainty in current consensus climate change land models (see the attached image) and what the E3SMv1.1 program is doing to reduce that uncertainty (which is not our friend, despite what some left-tail PDF advocates seem to think):

Title: "Uncertainty quantification methods and application for the E3SM land model"

https://e3sm.org/wp-content/uploads/2019/10/190924_Ricciuto_Sargsyan_opt.pdf


24
Prior consensus climate science estimates of ice mass loss from the Amundsen Sea Embayment based on satellite measurements have underestimated the viscosity of the local mantle and consequently have previously underestimate ice mass loss from this area by approximately 10%.  If one relies on MISI-type of behavior the more rapid uplift of the local crust might help to stability the local marine glaciers, but if one considers MICI-types of behavior the increase rate of uplift of the local crust will not serve to stabilize the local marine glaciers:

Valentina R. Barletta et al. (22 Jun 2018), "Observed rapid bedrock uplift in Amundsen Sea Embayment promotes ice-sheet stability", Science, Vol. 360, Issue 6395, pp. 1335-1339, DOI: 10.1126/science.aao1447

https://science.sciencemag.org/content/360/6395/1335

Abstract
The marine portion of the West Antarctic Ice Sheet (WAIS) in the Amundsen Sea Embayment (ASE) accounts for one-fourth of the cryospheric contribution to global sea-level rise and is vulnerable to catastrophic collapse. The bedrock response to ice mass loss, glacial isostatic adjustment (GIA), was thought to occur on a time scale of 10,000 years. We used new GPS measurements, which show a rapid (41 millimeters per year) uplift of the ASE, to estimate the viscosity of the mantle underneath. We found a much lower viscosity (4 × 1018 pascal-second) than global average, and this shortens the GIA response time scale to decades up to a century. Our finding requires an upward revision of ice mass loss from gravity data of 10% and increases the potential stability of the WAIS against catastrophic collapse.

25
Gavin makes a valid point in his attached Tweet; however, I note that the impact of ice meltwater from both the GIS and the AIS distort the meaning of the SSTs indicated in many of the cool regions shown on his graphic:

26
Yes but .
The error goes both ways .
...

I concur.  For example the right-tail risk of the ECS PDF plot that you show for the Last Millennium is large.  Also, I note that various reported values for ECS do not report the same thing as they are determined using different assumptions and different procedures and AR5 do not both to normalize these different definitions of ECS.

Finally, I note that as a high current value for ECS means a high current variability (both high and low) for climate response; thus for example, the risk of an extreme storm impacting people's lives exists today.

27
...
I don't deny that GCM models can be useful to get a hunch about where climate is going, but at the end of the day they are nothing more than the educated guesses built on intuition that go into the tweaking and tuning efforts.

Any short-comings of such CMIP6 models only serve to increase mankind's climate change risk, rather than providing an excuse to minimize mankind's effort to combat climate change.  Furthermore, the E3SM program is scheduled to coming improving their projections until at least 2027 (see image), because of the difficulty and large number of the problems that they are trying to solve.

28
The linked article shows that meltwater ponding and drainage of an Antarctic ice shelf can cause not only immediate deformation, but also fracturing, of the ice-shelf.  If/when the meltwater flows into such fracture, this can trigger hydrofracturing of the ice-shelf.  Currently, consensus ice-self models do not consider the immediate deformation of ice-shelves from meltwater ponding and drainage:

Alison F. Banwell et al. (2019), "Direct measurements of ice-shelf flexure caused by surface meltwater ponding and drainage", Nature Communications  10, Article number: 730, DOI: https://doi.org/10.1038/s41467-019-08522-5

https://www.nature.com/articles/s41467-019-08522-5

Abstract: "Global sea-level rise is caused, in part, by more rapid ice discharge from Antarctica, following the removal of the restraining forces of floating ice-shelves after their break-up. A trigger of ice-shelf break-up is thought to be stress variations associated with surface meltwater ponding and drainage, causing flexure and fracture. But until now, there have been no direct measurements of these processes. Here, we present field data from the McMurdo Ice Shelf, Antarctica, showing that the filling, to ~2 m depth, and subsequent draining, by overflow and channel incision, of four surface lakes causes pronounced and immediate ice-shelf flexure over multiple-week timescales. The magnitude of the vertical ice-shelf deflection reaches maxima of ~1 m at the lake centres, declining to zero at distances of <500 m. Our results should be used to guide development of continent-wide ice-sheet models, which currently do not simulate ice-shelf break-up due to meltwater loading and unloading."

29
The linked article indicates that almost none of the countries that have promised to get to "net zero" by 2050 have presented measurable plans as to how to get to this goal; which makes such goals subject to 'greenwashing' such as the proposed use of 'offsets' that have been tried previously and has failed just as often.

Title: "The word nobody wanted to say at the UN Climate Action Summit
“Offsets.”"


https://www.salon.com/2019/10/05/the-word-nobody-wanted-to-say-at-the-un-climate-action-summit_partner/

Extract: "The leaders of more than 70 countries have made a promise that sounds nothing short of revolutionary. By 2050, they say they will reach “net zero,” putting no more carbon dioxide into the atmosphere than can be somehow canceled out.

Until countries start filling in the details of how they plan to get to net zero, Frédéric Hache, a former investment banker who scrutinizes environmental markets, worries the benefit could wind up existing only on paper. “Everybody talks about the level of ambition and nobody questions the how,” he said. “The how is at least as important, because that’s where all the greenwashing takes place.”"

30
The linked article indicates that the increasing number of observed meltwater ponds/lakes in Antarctica indicates that previously/currently consensus climate science has underestimated the vulnerability of Antarctic ice shelves and marine glaciers to hydrofracturing:

Title: "Antarctica now has more than 65,000 ‘meltwater lakes’ as summer ice melts"

https://theconversation.com/antarctica-now-has-more-than-65-000-meltwater-lakes-as-summer-ice-melts-124362

Extract: "During the Antarctic summer, thousands of mesmerising blue lakes form around the edges of the continent’s ice sheet, as warmer temperatures cause snow and ice to melt and collect into depressions on the surface. Colleagues of mine at Durham University have recently used satellites to record more than 65,000 of these lakes.

Though seasonal meltwater lakes have formed on the continent for decades, lakes had not been recorded before in such great numbers across coastal areas of East Antarctica. This means parts of the world’s largest ice sheet may be more vulnerable to a warming climate than previously thought.

Much of Antarctica is surrounded by floating platforms of ice, often as tall as a skyscraper. These are “ice shelves”. And when some of these ice shelves have collapsed in the past, satellites have recorded networks of lakes growing and then abruptly disappearing shortly beforehand. For instance, several hundred lakes disappeared in the weeks before the catastrophic disintegration of the Larsen B Ice Shelf – when 3,250 km² of ice broke up in just two months in 2002."

31
The linked commentary reminds us that the wildfires currently burning in the Gran Chaco rainforest of Bolivia are on track to post a record setting year:

Title: "Fires still being set in blazing Bolivia (commentary)"

https://news.mongabay.com/2019/10/fires-still-being-set-in-blazing-bolivia-commentary/

Extract: "Despite over six weeks of firefighting, the infernos destroying Bolivia’s forests continue to spread. 5.3 million hectares (about 13.1 million acres) — an area larger than the whole of Costa Rica — have been destroyed, and about 40 percent of that area was forest. A perfect storm of factors — from an unusually dry year, probably linked to climate change, to a new law allowing burning of forest lands — have combined to make this one of the worst years this century for forest fires in the megadiverse nation.
But are these fires out of the ordinary?

Fires are set every year in Bolivia, usually to clear land for agriculture. But Assistant Professor Carwil Bjork-James of Vanderbilt University says that the fires this year are especially severe: “We are seeing a dramatic year in terms of the numbers of fires blazing in Bolivia, and the acreage consumed by them. If the fires continue at their current pace, it will be the second worst year of the twenty-first century.”"

32
By using a high resolution global coupled climate model the authors of the linked reference identified an important mechanism for slowing the AMOC; which is associated with the impacts of weaker local winds on the North Atlantic Subpolar Gyre (SPG); which needs to be added to other mechanisms for slowing the MOC (with continued global warming) such as a collapse of the WAIS (sending an armada of icebergs into the Southern Ocean) and a release of excess freshwater from the Beaufort Gyre triggered by rainfall into the Artic Ocean (particularly on to sea ice):

D. A. Putrasahan  K. Lohmann  J.‐S. von Storch  J. H. Jungclaus  O. Gutjahr  H. Haak (16 April 2019), "Surface Flux Drivers for the Slowdown of the Atlantic Meridional Overturning Circulation in a High‐Resolution Global Coupled Climate Model", JAMES, https://doi.org/10.1029/2018MS001447

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018MS001447

Abstract: "This paper investigates the causation for the decline of the Atlantic Meridional Overturning Circulation (AMOC) from approximately 17 Sv to about 9 Sv, when the atmospheric resolution of the Max Planck Institute‐Earth System Model is enhanced from ∼1° to ∼0.5°. The results show that the slowdown of the AMOC is caused by the cessation of deep convection. In most modeling studies, this is thought to be controlled by buoyancy fluxes in the convective regions, for example, by surface freshwater flux that is introduced locally or via enormous input from glacier or iceberg melts. While we find that freshwater is still the key to the reduction of AMOC seen in the higher‐resolution run, the freshening of the North Atlantic does not need to be directly caused by local freshwater fluxes. Instead, it can be caused indirectly through winds via a reduced wind‐driven gyre circulation and salinity transport associated to this circulation, as seen in the higher‐resolution run."

Extract: "While the enhancement of model resolution is typically expected to provide beneficial improvements to a climate model, challenges still abound. We see that in MPI‐ESM, AMOC of 17 Sv starting from an equilibrium state achieved at a lower atmospheric resolution slows down to almost half its original value when atmospheric resolution is enhanced. This decline of the AMOC is strongly related to the freshening and cooling of the Labrador Sea through the salinity‐advection feedback (Figure 8 ). While resolution‐induced differences in local heat and freshwater fluxes can contribute to the decay in AMOC, weaker surface wind stress seen in the higher‐resolution simulation plays a much more prominent role, particularly the winds over the subpolar North Atlantic. These weaker winds dynamically spin down the SPG, resulting in reduced heat and salt transport from the subtropics, thereby freshening and cooling the subpolar region. This promotes sea ice formation in the Labrador Sea, and the increased presence of sea ice inhibits deep convection and deep water formation, thus slowing down the AMOC. This in turn further reduces the northward transport heat and salt and provides the positive feedback that would prevent the recovery of the AMOC. For future work, it would be of interest to evaluate the impact of enhanced ocean resolution on the AMOC."

Edit: If it is not clear to some readers why a slowdown of the AMOC/MOC would contribute to potential abrupt global warming in coming decades; such a slowdown would increase the SST of the Tropical Oceans; which would accelerate the evaporation of tropical seawater; which would have major impacts of the poleward telecommunication of heat energy from the tropics, and also would promote the formation of more high altitude clouds (resulting in an increase in this positive feedback) and would promote a decrease in low altitude cloud formation (resulting in a decrease in this negative feedback).

33
As in recent posts I have been mentioning that an increase in Arctic rainfall (with continued global warming) could cause an albedo flip in coming decades; I thought that I should post the linked reference that helps to quantify the contributions of both sea ice and land snow to climate feedback:

Lei Duan  Long Cao  Ken Caldeira (07 January 2019), "Estimating Contributions of Sea Ice and Land Snow to Climate Feedback", JGR Atmospheres, https://doi.org/10.1029/2018JD029093

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JD029093

Abstract
In this study, we use the National Center for Atmospheric Research Community Earth System Model to investigate the contribution of sea ice and land snow to the climate sensitivity in response to increased atmospheric carbon dioxide content. We focus on the overall effect arising from the presence or absence of sea ice and/or land snow. We analyze our results in terms of the radiative forcing and climate feedback parameter. We find that the presence of sea ice and land snow decreases the climate feedback parameter (and thus increases climate sensitivity). Adjusted radiative forcing from added carbon dioxide is comparatively less sensitive to the presence of sea ice or land snow. The effect of sea ice on the climate feedback parameter decreases as sea ice cover diminishes at higher CO2 concentration. However, the influence of both sea ice and land snow on the climate feedback parameter remains substantial under the CO2 concentration range considered here (to eight times preindustrial CO2 content). Approximately, one quarter of the effect of sea ice and land snow on the climate feedback parameter is a consequence of the effect of these components on longwave feedback that is mainly associated with cloud change. Polar warming in response to added CO2 is approximately doubled by the presence of sea ice and land snow. Relative to the case in which sea ice and land snow are absent in the model, in response to increased CO2 concentrations, the presence of sea ice and land snow results in an increase in global mean warming by over 40%.

Plain Language Summary
Sea ice and land snow are two crucial components that affect the climate response to external forcings. Feedbacks between ice/snow and climate change cause amplified surface warming in high latitudes. In this study, we use a climate model to estimate the contribution of sea ice and land snow to climate change in response to increased CO2 concentrations. We compare the climate response to increased CO2 between the simulations with sea ice and/or land snow and the simulations without them. We show that the existence of sea ice and land snow substantially amplifies the global temperature response to increased CO2 with sea ice having a stronger effect than land snow. Under higher CO2 levels, the effect of sea ice diminishes more rapidly than does the effect of land snow. About one quarter of the total climate feedback from sea ice and land snow is associated with the change in longwave radiation. Also we show that the effect of sea ice and land snow on the sensitivity of top‐of‐atmosphere net energy flux to the global mean temperature change is approximately additive.

34
ENSO conditions are neutral this year and yet we already having several months of record-breaking heat for such months; with the monthly GMSTA for September 2019 being more than 1.2C above pre-industrial.  We are steadily moving towards an annual GMSTA of 1.5C above pre-industrial:

Title: "Earth just experienced its hottest September, as 2019 heads for the record books"

https://www.washingtonpost.com/weather/2019/10/04/earth-just-experienced-its-hottest-september-heads-record-books/

Extract: "September 2019 was the warmest such month on record, tying the old record set in 2016, according to the Copernicus Climate Change Service, an organization funded by the European Union that tracks global temperatures. This makes September the fourth-straight month “to be close to or breaking a temperature record,” according to an agency statement.

Based on Copernicus’s data, which uses computer models fed with billions of observations from air, land and sea, June 2019 set a record high for that month, July 2019 was the warmest month ever recorded, and August 2019 was the second-warmest such month globally.

According to Copernicus, September was about 1.02 degrees above the 1981-2010 average for the month, and about 1.2 degrees above the preindustrial level. It was also slightly warmer, by about 0.04 degrees, than September 2016, which had been the warmest such month on record."

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The first linked reference indicates that currently major atmospheric river (AR) events can markedly increase ice melting in Greenland, and currently some of the precipitation from such AR events fall as snow, so imagine what will happen with continuing global warming when more of such precipitation will fall as rain.

The second linked reference shows that extratropical cyclones (ECs) and atmospheric rivers (ARs) frequently reinforce one another.  Here I note that both ECs and ARs telecommunicate heat energy poleward from the tropical oceans, and that some of this heat energy get advected into the Artic and Antarctic regions where it can help induce local rainfall that can melt for snow & ice, particularly with continuing global warming.

The third linked reference (& associated article) indicate that with continued global warming, the frequency of intense atmospheric river (AR) events will likely double in coming decades.  As large ARs telecommunicate more energy poleward than less intense ARs this trend will certainly contribute to Polar Amplification:

K. S. Mattingly et al. (25 July 2018) "Atmospheric River Impacts on Greenland Ice Sheet Surface Mass Balance", JGR Atmospheres, https://doi.org/10.1029/2018JD028714

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JD028714

Abstract: "Greenland Ice Sheet (GrIS) mass loss has accelerated since the turn of the twenty‐first century. Several recent episodes of rapid GrIS ablation coincided with intense moisture transport over Greenland by atmospheric rivers (ARs), suggesting that these events influence the evolution of GrIS surface mass balance (SMB). ARs likely provide melt energy through several physical mechanisms, and conversely, may increase SMB through enhanced snow accumulation. In this study, we compile a long‐term (1980–2016) record of moisture transport events using a conventional AR identification algorithm as well as a self‐organizing map classification applied to MERRA‐2 data. We then analyze AR effects on the GrIS using melt data from passive microwave satellite observations and regional climate model output. Results show that anomalously strong moisture transport by ARs clearly contributed to increased GrIS mass loss in recent years. AR activity over Greenland was above normal throughout the 2000s and early 2010s, and recent melting seasons with above‐average GrIS melt feature positive moisture transport anomalies over Greenland. Analysis of individual AR impacts shows a pronounced increase in GrIS surface melt after strong AR events. AR effects on SMB are more complex, as strong summer ARs cause sharp SMB losses in the ablation zone that exceed moderate SMB gains induced by ARs in the accumulation zone during summer and in all areas during other seasons. Our results demonstrate the influence of the strongest ARs in controlling GrIS SMB, and we conclude that projections of future GrIS SMB should accurately capture these rare ephemeral events."

&

Zhenhai Zhang et al. (10 September 2018), "The Relationship Between Extratropical Cyclone Strength and Atmospheric River Intensity and Position", Geophysical Research Letters, https://doi.org/10.1029/2018GL079071

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL079071

Abstract
Extratropical cyclones (ECs) and atmospheric rivers (ARs) impact precipitation over the U.S. West Coast and other analogous regions globally. This study investigates the relationship between ECs and ARs by exploring the connections between EC strength and AR intensity and position using a new AR intensity scale. While 82% of ARs are associated with an EC, only 45% of ECs have a paired AR and the distance between the AR and EC varies greatly. Roughly 20% of ARs (defined by vertically integrated water vapor transport) occur without a nearby EC. These are usually close to a subtropical/tropical moisture source and include an anticyclone. AR intensity is only moderately proportional to EC strength. Neither the location nor intensity of an AR can be simply determined by an EC. Greater EC intensification occurs with stronger ARs, suggesting that ARs enhance EC deepening by providing more water vapor for latent heat release.

Plain Language Summary
Both extratropical cyclones and atmospheric rivers have impact on precipitation over the U.S. West Coast, and they are often mentioned together. However, the relationship between the two is not completely understood. In this study, we have examined the connections between extratropical cyclone strength and atmospheric river intensity and position. While 82% of atmospheric rivers are related to a cyclone, only 45% of cyclones have an accompanied atmospheric river. The distance between the two varies from about 300 km to over 2,000 km. Roughly 20% of atmospheric rivers occur without a nearby cyclone. These cases are close to the subtropical/tropical moisture source and are related to a high pressure. While cyclones can enhance atmospheric rivers with stronger wind, neither the location nor the intensity of an atmospheric river can be simply determined by a cyclone. On the other hand, atmospheric rivers with strong water vapor transport provide favorable conditions for cyclone intensification. Our results provide a comprehensive analysis of the relationship between atmospheric rivers and extratropical cyclones. This work improves the understanding of the dynamical mechanism for heavy precipitation over the U.S. West Coast and thus provides more reliable information on long‐term flood control and water planning.

&


Vicky Espinoza et al. (19 April 2018), "Global Analysis of Climate Change Projection Effects on Atmospheric Rivers", Geophysical Research Letters, https://doi.org/10.1029/2017GL076968

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2017GL076968

Abstract
A uniform, global approach is used to quantify how atmospheric rivers (ARs) change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports (IVTs) under RCP8.5. These changes result in pronounced increases in the frequency (IVT strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes. The models exhibit systematic low biases across the midlatitudes in replicating historical AR frequency (~10%), zonal IVT (~15%), and meridional IVT (~25%), with sizable intermodel differences. A more detailed examination of six regions strongly impacted by ARs suggests that the western United States, northwestern Europe, and southwestern South America exhibit considerable intermodel differences in projected changes in ARs.

Plain Language Summary
Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. These “rivers in the sky” have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds. This study takes a uniform, global approach that is used to quantify how ARs change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios globally. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports under RCP8.5. These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes.

See also:

Title: "Climate change may lead to bigger atmospheric rivers"

https://climate.nasa.gov/news/2740/climate-change-may-lead-to-bigger-atmospheric-rivers/

Extract: ""The results project that in a scenario where greenhouse gas emissions continue at the current rate, there will be about 10 percent fewer atmospheric rivers globally by the end of the 21st century," said the study's lead author, Duane Waliser, of NASA's Jet Propulsion Laboratory in Pasadena, California. "However, because the findings project that the atmospheric rivers will be, on average, about 25 percent wider and longer, the global frequency of atmospheric river conditions -- like heavy rain and strong winds -- will actually increase by about 50 percent."

The results also show that the frequency of the most intense atmospheric river storms is projected to nearly double."

36
The linked article discusses how in large portions of Siberia not only is the permafrost degrading rapidly, but also the local rise of mean surface temperature is already over 3C above pre-industrial conditions.  While this is bad enough, I remind readers that as the Tropical Pacific Ocean SST increases, this heat energy is telecommunicated via an Atmospheric Bridge directly to the Bering Sea, parts of Alaska and parts of Canada; and all of this heat (in Siberia, the Bering Sea, Alaska and Canada) eventually gets transferred by local winds over the Arctic Ocean, where the heat can cause rain to fall instead of snow; and in the coming decades such rainfall over the Arctic Ocean could trigger an abrupt loss of Arctic Sea Ice area and an associated albedo flip:

Title: "Radical warming in Siberia leaves millions on unstable ground"

https://www.washingtonpost.com/graphics/2019/national/climate-environment/climate-change-siberia/

Extract: "… Siberia has warmed up faster than almost anywhere else on Earth. Scientists say the planet's warming must not exceed 1.5 degrees Celsius — but Siberia's temperatures have already spiked far beyond that.

A Washington Post analysis found that the region near the town of Zyryanka, in an enormous wedge of eastern Siberia called Yakutia, has warmed by more than 3 degrees Celsius since preindustrial times — roughly triple the global average.

For the 5.4 million people who live in Russia’s permafrost zone, the new climate has disrupted their homes and their livelihoods. Rivers are rising and running faster, and entire neighborhoods are falling into them. Arable land for farming has plummeted by more than half, to just 120,000 acres in 2017.

In Yakutia, an area one-third the size of the United States, cattle and reindeer herding have plunged 20 percent as the animals increasingly battle to survive the warming climate’s destruction of pastureland."

37

Edit: I note that consensus models project that with continuing global warming, rainfall will increase in the Arctic in coming decades; which will not only impact: Arctic sea ice extent, potential release of excess freshwater from the Arctic Ocean, glacial ice, but also permafrost (which will not only increase CO2 emissions, but also methane emissions from thermal karst lakes).

Imagine what Bintanja (2018) would project if it had used a model with ECS greater than 5C:

R. Bintanja (2018), "The impact of Arctic warming on increased rainfall", Scientific Reports,  8, Article number: 16001, DOI: https://doi.org/10.1038/s41598-018-34450-3

https://www.nature.com/articles/s41598-018-34450-3

Abstract: "The Arctic region is warming two to three times faster than the global mean, intensifying the hydrological cycle in the high north. Both enhanced regional evaporation and poleward moisture transport contribute to a 50–60% increase in Arctic precipitation over the 21st century. The additional precipitation is diagnosed to fall primarily as rain, but the physical and dynamical constraints governing the transition to a rain-dominated Arctic are unknown. Here we use actual precipitation, snowfall, rainfall output of 37 global climate models in standardised 21st-century simulations to demonstrate that, on average, the main contributor to additional Arctic (70–90°N) rainfall is local warming (~70%), whereas non-local (thermo)dynamical processes associated with precipitation changes contribute only 30%. Surprisingly, the effect of local warming peaks in the frigid high Arctic, where modest summer temperature changes exert a much larger effect on rainfall changes than strong wintertime warming. This counterintuitive seasonality exhibits steep geographical gradients, however, governed by non-linear changes in the temperature-dependent snowfall fraction, thereby obscuring regional-scale attribution of enhanced Arctic rainfall to climate warming. Detailed knowledge of the underlying causes behind Arctic snow/rainfall changes will contribute to more accurate assessments of the (possibly irreversible) impacts on hydrology/run-off, permafrost thawing, ecosystems, sea ice retreat, and glacier melt."

38
The linked reference warns that potential future increases in precipitation directly into the Arctic Ocean has potential to trigger the release of excess freshwater currently being stored in the Arctic Ocean (see prior discussion of how the Beaufort Gyre stores freshwater, e.g. Reply #1547) into the North Atlantic Ocean in sufficient quantities to ' … affect the global ocean circulation'.  This is not good news as such an abrupt release of freshwater would slow the MOC; which would at least temporarily increase climate sensitivity:

Nicola Jane Brown et al. (21 June 2019), "Arctic Ocean Freshwater Dynamics: Transient Response to Increasing River Runoff and Precipitation", JGR Oceans, https://doi.org/10.1029/2018JC014923

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JC014923

Abstract
Simulations from a coupled ice‐ocean general circulation model are used to assess the effects on Arctic Ocean freshwater storage of changes in freshwater input through river runoff and precipitation. We employ the climate response function framework to examine responses of freshwater content to abrupt changes in freshwater input. To the lowest order, the response of ocean freshwater content is linear, with an adjustment time scale of approximately 10 years, indicating that anomalies in Arctic Ocean freshwater export are proportional to anomalies in freshwater content. However, the details of the transient response of the ocean depend on the source of freshwater input. An increase in river runoff results in a fairly smooth response in freshwater storage consistent with an essentially linear relation between total freshwater content and discharge of excess freshwater through the main export straits. However, the response to a change in precipitation is subject to greater complexity, which can be explained by the localized formation and subsequent export of salinity anomalies which introduce additional response time scales. The results presented here suggest that future increases in Arctic Ocean freshwater input in the form of precipitation are more likely to be associated with variability in the storage and release of excess freshwater than are increases in freshwater input from river runoff.

Plain Language Summary
This paper shows that the Arctic Ocean adjusts to changes in freshwater input over time scales of about one decade. How much of the added freshwater is stored in the Arctic depends, however, on how the freshwater enters the ocean. If it arrives as additional river runoff, the response in Arctic freshwater storage is relatively smooth and predictable. If it falls, instead, as increased precipitation, the response is less easy to predict because it is complicated by interactions between the ocean and sea ice. This is important because the part of the freshwater that is not stored in the Arctic Ocean is exported to the North Atlantic, where it can affect the global ocean circulation.

Edit: I note that consensus models project that with continuing global warming, rainfall will increase in the Arctic in coming decades; which will not only impact: Arctic sea ice extent, potential release of excess freshwater from the Arctic Ocean, glacial ice, but also permafrost (which will not only increase CO2 emissions, but also methane emissions from thermal karst lakes).

39
While it is not correct to directly compare our current climate situation with paleo cases, the fact that the linked reference (& associate article) finds GMSL was about 20 meters higher during the mid-Pliocene warm period (when GMSTA was about 2C about pre-industrial) than today is not comforting news:

G. R. Grant et al. (2019), "The amplitude and origin of sea-level variability during the Pliocene epoch", Nature, DOI: https://doi.org/10.1038/s41586-019-1619-z

https://www.nature.com/articles/s41586-019-1619-z

Abstract: "Earth is heading towards a climate that last existed more than three million years ago (Ma) during the ‘mid-Pliocene warm period’, when atmospheric carbon dioxide concentrations were about 400 parts per million, global sea level oscillated in response to orbital forcing and peak global-mean sea level (GMSL) may have reached about 20 metres above the present-day value. For sea-level rise of this magnitude, extensive retreat or collapse of the Greenland, West Antarctic and marine-based sectors of the East Antarctic ice sheets is required. Yet the relative amplitude of sea-level variations within glacial–interglacial cycles remains poorly constrained. To address this, we calibrate a theoretical relationship between modern sediment transport by waves and water depth, and then apply the technique to grain size in a continuous 800-metre-thick Pliocene sequence of shallow-marine sediments from Whanganui Basin, New Zealand. Water-depth variations obtained in this way, after corrections for tectonic subsidence, yield cyclic relative sea-level (RSL) variations. Here we show that sea level varied on average by 13 ± 5 metres over glacial–interglacial cycles during the middle-to-late Pliocene (about 3.3–2.5 Ma). The resulting record is independent of the global ice volume proxy (as derived from the deep-ocean oxygen isotope record) and sea-level cycles are in phase with 20-thousand-year (kyr) periodic changes in insolation over Antarctica, paced by eccentricity-modulated orbital precession between 3.3 and 2.7 Ma. Thereafter, sea-level fluctuations are paced by the 41-kyr period of cycles in Earth’s axial tilt as ice sheets stabilize on Antarctica and intensify in the Northern Hemisphere. Strictly, we provide the amplitude of RSL change, rather than absolute GMSL change. However, simulations of RSL change based on glacio-isostatic adjustment show that our record approximates eustatic sea level, defined here as GMSL unregistered to the centre of the Earth. Nonetheless, under conservative assumptions, our estimates limit maximum Pliocene sea-level rise to less than 25 metres and provide new constraints on polar ice-volume variability under the climate conditions predicted for this century."

See also:

Title: "If warming exceeds 2°C, Antarctica’s melting ice sheets could raise seas 20 metres in coming centuries"

https://theconversation.com/if-warming-exceeds-2-c-antarcticas-melting-ice-sheets-could-raise-seas-20-metres-in-coming-centuries-124484

Extract: "The Pliocene was the last time atmospheric carbon dioxide concentrations were above 400 parts per million and Earth’s temperature was 2°C warmer than pre-industrial times. We show that warming of more than 2°C could set off widespread melting in Antarctica once again and our planet could be hurtling back to the future, towards a climate that existed three million years ago.

Our study has important implications for the stability and sensitivity of the Antarctic ice sheet and its potential to contribute to future sea levels. It supports the concept that a tipping point in the Antarctic ice sheet may be crossed if global temperatures are allowed to rise by more than 2℃. This could result in large parts of the ice sheet being committed to melt-down over the coming centuries, reshaping shorelines around the world."

40
...

Recent studies indicate that the aerosol effect has been overestimated, not underestimated.

...

The linked reference, and associated article, indicate that previous estimates of the cooling effect of aerosols in recent decades have been underestimated.  Thus, by the time consensus climate scientists finally converge on a more certain determination of the impact of aerosols (including cloud feedbacks); we may already have crossed various Earth System tipping points:

Daniel Rosenfeld, Yannian Zhu, Minghuai Wang, Youtong Zheng, Tom Goren, Shaocai Yu. Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds. Science, 2019; eaav0566 DOI: 10.1126/science.aav0566

https://science.sciencemag.org/content/363/6427/eaav0566

Structured Abstract
INTRODUCTION

Human-made emissions of particulate air pollution can offset part of the warming induced by emissions of greenhouse gases, by enhancing low-level clouds that reflect more solar radiation back to space. The aerosol particles have this effect because cloud droplets must condense on preexisting tiny particles in the same way as dew forms on cold objects; more aerosol particles from human-made emissions lead to larger numbers of smaller cloud droplets. One major pathway for low-level cloud enhancement is through the suppression of rain by reducing cloud droplet sizes. This leaves more water in the cloud for a longer time, thus increasing the cloud cover and water content and thereby reflecting more solar heat to space. This effect is strongest over the oceans, where moisture for sustaining low-level clouds over vast areas is abundant. Predicting global warming requires a quantitative understanding of how cloud cover and water content are affected by human-made aerosols.

RATIONALE
Quantifying the aerosol cloud–mediated radiative effects has been a major challenge and has driven the uncertainty in climate predictions. It has been difficult to measure cloud-active aerosols from satellites and to isolate their effects on clouds from meteorological data. The development of novel methodologies to retrieve cloud droplet concentrations and vertical winds from satellites represents a breakthrough that made this quantification possible. The methodologies were applied to the world’s oceans between the equator and 40°S. Aerosol and meteorological variables explained 95% of the variability in the cloud radiative effects.

RESULTS
The measured aerosol cloud–mediated cooling effect was much larger than the present estimates, especially via the effect of aerosols on the suppression of precipitation, which makes the clouds retain more water, persist longer, and have a larger fractional coverage. This goes against most previous observations and simulations, which reported that vertically integrated cloud water may even decrease with additional aerosols, especially in precipitating clouds. The major reason for this apparent discrepancy is because deeper clouds have more water and produce rainfall more easily, thus scavenging the aerosols more efficiently. The outcome is that clouds with fewer aerosols have more water, but it has nothing to do with aerosol effects on clouds. This fallacy is overcome when assessing the effects for clouds with a given fixed geometrical thickness.

The large aerosol sensitivity of the water content and coverage of shallow marine clouds dispels another belief that the effects of added aerosols are mostly buffered by adjustment of the cloud properties, which counteracts the initial aerosol effect. For example, adding aerosols suppresses rain, so the clouds respond by deepening just enough to restore the rain amount that was suppressed. But the time scale required for the completion of this adjustment process is substantially longer than the life cycle of the cloud systems, which is mostly under 12 hours. Therefore, most of the marine shallow clouds are not buffered for the aerosol effects, which are inducing cooling to a much greater extent than previously believed.

CONCLUSION
Aerosols explain three-fourths of the variability in the cooling effects of low-level marine clouds for a given geometrical thickness. Doubling the cloud droplet concentration nearly doubles the cooling. This reveals a much greater sensitivity to aerosols than previously reported, meaning too much cooling if incorporated into present climate models. This argument has been used to dismiss such large sensitivities. To avoid that, the aerosol effects in some of the models were tuned down. Accepting the large sensitivity revealed in this study implies that aerosols have another large positive forcing, possibly through the deep clouds, which is not accounted for in current models. This reveals additional uncertainty that must be accounted for and requires a major revision in calculating Earth’s energy budget and climate predictions. Paradoxically, this advancement in our knowledge increases the uncertainty in aerosol cloud–mediated radiative forcing. But it paves the way to eventual substantial reduction of this uncertainty.

Also see:

Title: "We need to rethink everything we know about global warming"

https://www.sciencedaily.com/releases/2019/01/190122104611.htm
Extract: "New calculations show scientists have grossly underestimated the effects of air pollution

New research shows that the degree to which aerosols cool the earth has been grossly underestimated, necessitating a recalculation of climate change models to more accurately predict the pace of global warming."

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There are large areas of peatlands in both the Northern and Tropical areas of the Earth; which in the observed record have been a net carbon sink but increasingly appear to be headed to become significant carbon sources.  Therefore, without further comment, I provide the four linked recent references the discuss different aspects of the various ways that peatland are currently trending towards eventually becoming carbon sources, with continued climate change for the next few decades:

Susan Waldron et al. C mobilisation in disturbed tropical peat swamps: old DOC can fuel the fluvial efflux of old carbon dioxide, but site recovery can occur, Scientific Reports (2019). DOI: 10.1038/s41598-019-46534-9

https://www.nature.com/articles/s41598-019-46534-9

Abstract: "Southeast-Asian peat swamp forests have been significantly logged and converted to plantation. Recently, to mitigate land degradation and C losses, some areas have been left to regenerate. Understanding how such complex land use change affects greenhouse gas emissions is essential for modelling climate feedbacks and supporting land management decisions. We carried out field research in a Malaysian swamp forest and an oil palm plantation to understand how clear-felling, drainage, and illegal and authorized conversion to oil palm impacted the C cycle, and how the C cycle may change if such logging and conversion stopped. We found that both the swamp forest and the plantation emit centuries-old CO2 from their drainage systems in the managed areas, releasing sequestered C to the atmosphere. Oil palm plantations are an iconic symbol of tropical peatland degradation, but CO2 efflux from the recently-burnt, cleared swamp forest was as old as from the oil palm plantation. However, in the swamp forest site, where logging had ceased approximately 30 years ago, the age of the CO2 efflux was modern, indicating recovery of the system can occur. 14C dating of the C pool acted as a tracer of recovery as well as degradation and offers a new tool to assess efficacy of restoration management. Methane was present in many sites, and in higher concentrations in slow-flowing anoxic systems as degassing mechanisms are not strong. Methane loading in freshwaters is rarely considered, but this may be an important C pool in restored drainage channels and should be considered in C budgets and losses."

&

Anna Ferretto et al. (2019), "Potential carbon loss from Scottish peatlands under climate change", Regional Environmental Change, pp 1–11, DOI: https://doi.org/10.1007/s10113-019-01550-3

https://link.springer.com/article/10.1007/s10113-019-01550-3

Abstract: "The Scottish Government is committed to reduce carbon emissions by 80% by 2050 (compared to a 1990–1995 baseline). Peatlands have been recognised as a key environment for the carbon balance as they sequester and store great quantities of carbon, but they also have the potential to release it. In Scotland, peatlands cover more than 20% of the surface (more than 90% of which is blanket bog) and store more than 2500 Mt of carbon. Blanket bogs are very climate reliant, and as a consequence of climate change, many areas in Scotland may not be able to support peatlands in the near future. In this study, two bioclimatic envelope models (Linsday Modified model and Blanket Bog Tree model) have been used to obtain a first estimate of how the distribution of blanket bogs in Scotland could vary according to climate change in the 2050s and in the 2080s. The potential losses of carbon arising from climate change have then been calculated. Results showed that in 2050, more than half of the carbon currently stored in Scottish blanket bogs will be at risk of loss. This is 4.4–6.6 times the amount of carbon emitted in 2016 from all the sectors in Scotland and, if emissions from peatland occur and are taken into account, it will greatly hamper efforts to meet emission reduction targets set out in the Climate Change (Scotland) Act of 2009."

&

Anna M. Laine et al. Warming impacts on boreal fen CO 2 exchange under wet and dry conditions, Global Change Biology (2019). DOI: 10.1111/gcb.14617

https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14617

Abstract: "Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub‐arctic sedge fen carbon dioxide (CO2) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO2 exchange is more strongly affected by drying than warming. Experimental water level draw‐down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO2 exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO2 uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO2 exchange."

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Korkiakoski, M., Tuovinen, J.-P., Penttilä, T., Sarkkola, S., Ojanen, P., Minkkinen, K., Rainne, J., Laurila, T., and Lohila, A.: Greenhouse gas and energy fluxes in a boreal peatland forest after clear-cutting, Biogeosciences, 16, 3703–3723, https://doi.org/10.5194/bg-16-3703-2019, 2019.

https://www.biogeosciences.net/16/3703/2019/

Abstract
The most common forest management method in Fennoscandia is rotation forestry, including clear-cutting and forest regeneration. In clear-cutting, stem wood is removed and the logging residues are either removed or left on site. Clear-cutting changes the microclimate and vegetation structure at the site, both of which affect the site's carbon balance. Peat soils with poor aeration and high carbon densities are especially prone to such changes, and significant changes in greenhouse gas exchange can be expected. We measured carbon dioxide (CO2) and energy fluxes with the eddy covariance method for 2 years (April 2016–March 2018) after clear-cutting a drained peatland forest. We observed a significant rise (23 cm) in the water table level and a large CO2 source (first year: 3086±148 g CO2 m−2 yr−1; second year: 2072±124 g CO2 m−2 yr−1). These large CO2 emissions resulted from the very low gross primary production (GPP) following the removal of photosynthesizing trees and the decline of ground vegetation, unable to compensate for the decomposition of logging residues and peat. During the second summer (June–August) after the clear-cutting, GPP had already increased by 96 % and total ecosystem respiration decreased by 14 % from the previous summer. The mean daytime ratio of sensible to latent heat flux decreased after harvesting from 2.6 in May 2016 to 1.0 in August 2016, and in 2017 it varied mostly within 0.6–1.0. In April–September, the mean daytime sensible heat flux was 33 % lower and latent heat flux 40 % higher in 2017, probably due to the recovery of ground vegetation that increased evapotranspiration and albedo of the site. In addition to CO2 and energy fluxes, we measured methane (CH4) and nitrous oxide (N2O) fluxes with manual chambers. After the clear-cutting, the site turned from a small CH4 sink into a small source and from N2O neutral to a significant N2O source. Compared to the large CO2 emissions, the 100-year global warming potential (GWP100) of the CH4 emissions was negligible. Also, the GWP100 due to increased N2O emissions was less than 10 % of that of the CO2 emission change.

42
The linked article provides an update on the progress being made with the CMIP6 program.  In the extracts below, I focus on the fact that all preliminary assessments of ECS from CMIP6 are substantially higher than that reported in either AR5 or CMIP5:

Title: "The CMIP6 landscape", (September 2019)
Nature Climate Change,  9, 727, DOI: https://doi.org/10.1038/s41558-019-0599-1

https://www.nature.com/articles/s41558-019-0599-1

Extract: "But the CMIP6 archive does appear to be reaching critical mass, and results are trickling into scientific discourse. One major discussion point centres on the models’ equilibrium climate sensitivity (ECS) — the global temperature change estimated from a doubling of CO2. As of March 2019, more than half of CMIP6 models exhibited an ECS of 5 °C or higher (https://go.nature.com/2kJv8YV; ref. 3), notably larger than the upper value of the CMIP5 range of 4.5 °C. By late August, with additional models available, a similar proportion still registered at 4.7 °C or higher (https://go.nature.com/2lOJHdZ).

If the higher CMIP6 ECS estimates hold true as the archive fills out, this will represent a departure from over four decades of research. Higher-sensitivity climates experience a greater probability of long-term temperature pauses and short-term trends, which can translate to more warming hiatuses or periods of fast temperature increase."

43
One significant reason (others include: more than expected heat has gone into the oceans and into melting ice and that the negative impact of aerosols were greater than previously assumed) that estimates of ECS based on observed changes in mean global temperature are lower than what society is likely going to face for the rest of this century is that a surge of plant growth has temporarily absorbed meaningful amounts of CO2 from the atmosphere.  However, the linked article (base on per reviewed research) indicates that 'vegetated coastal ecosystems' are susceptible to release carbon back into the atmosphere when subjected stressed by such factors as: storms, heatwaves, dredging, etc., and 'vegetated coastal ecosystems' are currently being degraded globally twice as fast as rain forests.  It does not take much imagination to see the significant risk that global biosystems may turn from carbon sinks into carbon sources this century:

Title: "Australia’s vast carbon sink releasing millions of tonnes of CO2 back into atmosphere "

https://www.theguardian.com/environment/2019/oct/01/australias-vast-carbon-sink-releasing-millions-of-tonnes-of-co2-back-into-atmosphere

Extract: "Australia’s mangroves, tidal marshes and seagrass meadows are absorbing about 20m tonnes of carbon dioxide every year, according to a major new study that is the first to measure in detail the climate benefits of the coastal ecosystems.

But the study, published in the journal Nature Communications, warns that degradation of these “vegetated coastal ecosystems” was already seeing 3 million tonnes of CO2 per year being released back into the atmosphere.

The study reveals Australia’s vast coastlines represent between 5% and 11% of all the so called “blue carbon” locked up in mangroves, seagrasses and tidal marshes globally.
...
Serrano said: “When these ecosystems are damaged by storms, heatwaves, dredging or other human development, the carbon dioxide stored in their biomass and soils beneath them can make its way back into the environment, contributing to climate change.

“Globally, vegetated coastal ecosystems are being lost twice as fast as tropical rainforests despite covering a fraction of the area.”"

44
The linked reference confirms the important role that SST of the Western Tropical Pacific will likely play in increasing ECS with continued anthropogenic radiative forcing (& I note that increasing glacial ice mass loss, in the future, will work to slow the MOC, which will serve to warm the Western Tropical Pacific:

Dong, Y., C. Proistosescu, K.C. Armour, D.S. Battisti (2019): Attributing historical and future evolution of radiative feedbacks to regional warming patterns using a Green’s function approach: The preeminence of the western Pacific. Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0843.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0843.1

Abstract
Global radiative feedbacks have been found to vary in global climate model (GCM) simulations. Atmospheric GCMs (AGCMs) driven with historical patterns of sea surface temperatures (SSTs) and sea ice concentrations produce radiative feedbacks that trend toward more negative values, implying low climate sensitivity, over recent decades. Freely evolving coupled GCMs driven by increasing CO2 produce radiative feedbacks that trend toward more positive values, implying increasing climate sensitivity, in the future. While this time variation in feedbacks has been linked to evolving SST patterns, the role of particular regions has not been quantified. Here, a Green’s function is derived from a suite of simulations within an AGCM (NCAR’s CAM4), allowing an attribution of global feedback changes to surface warming in each region. The results highlight the radiative response to surface warming in ascent regions of the western tropical Pacific as the dominant control on global radiative feedback changes. Historical warming from the 1950s to 2000s preferentially occurred in the western Pacific, yielding a strong global outgoing radiative response at the top of the atmosphere (TOA) and thus a strongly negative global feedback. Long-term warming in coupled GCMs occurs preferentially in tropical descent regions and in high latitudes, where surface warming yields small global TOA radiation change but large global surface air temperature change, and thus a less-negative global feedback. These results illuminate the importance of determining mechanisms of warm pool warming for understanding how feedbacks have varied historically and will evolve in the future.

45
Given the importance of the various snow/ice albedo feedback mechanisms wr.t. the risk of abrupt climate change, I provide following three linked references, all of which indicate that snow/ice albedo will likely be significantly reduced in coming decades:

Jieru Ma (2019), "The Dominant Role of Snow/Ice Albedo Feedback Strengthened by Black Carbon in the Enhanced Warming over the Himalayas", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0720.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0720.1

Abstract: "An obvious warming trend in winter over the Tibetan Plateau (TP) in the recent decades has been widely discussed, with studies emphasizing the dominant effects of local radiative factors, including those due to black carbon (BC). The Himalayas are one of the largest snowpack- and ice-covered regions in the TP, and an ideal area to investigate local radiative effects on climate change. In this study, the coupled climate feedback response analysis method (CFRAM) is applied to quantify the magnitude of warming over the Himalayas induced by different external forcing factors and climate feedback processes. The results show that snow/ice albedo feedback (SAF) resulted in a warming of approximately 2.6°C and was the primary contributor to enhanced warming over the Himalayas in recent decades. This warming was much greater than the warming induced by dynamic and other radiative factors. In particular, the strong radiative effects of BC on the warming over the Himalayas are identified by comparing control and BC-perturbed experiments of the Community Earth System Model (CESM). As a result of strong BC effects on the Himalayas, evaporation and reduced precipitation were strengthened, accounting for local drying and land degradation, which intensified warming. These results suggest that more investigations on the local radiative effects on the climate and ecosystem are needed, especially in the high-altitude cryosphere."

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Adrienne M. Marshall et al.  (08 August 2019), "Projected Changes in Interannual Variability of Peak Snowpack Amount and Timing in the Western United States", Geophysical Research Letters, https://doi.org/10.1029/2019GL083770

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL083770

Abstract
Interannual variability of mountain snowpack has important consequences for ecological and socioeconomic systems, yet changes in variability have not been widely examined under future climates. Physically based snowpack simulations for historical (1970–1999) and high‐emission scenario (RCP 8.5) mid‐21st century (2050–2079) periods were used to assess changes in the variability of annual maximum snow water equivalent (SWEmax) and SWEmax timing across the western United States. Models show robust declines in the interannual variability of SWEmax in regions where precipitation is projected to increasingly fall as rain. The average frequency of consecutive snow drought years (SWEmax < historical 25th percentile) is projected to increase from 6.6% to 42.2% of years. Models also project increases in the variability of SWEmax timing, suggesting reduced reliability of when SWEmax occurs. Differences in physiography and regional climate create distinct spatial patterns of change in snowpack variability that will require adaptive strategies for environmental resource management.

Plain Language Summary
A wealth of research has established that warming temperatures associated with climate change in the western United States will generally reduce snowpack accumulation and result in earlier snowmelt timing, with important consequences for water resources and ecosystems. However, changes in the variability of snowpack conditions between years have not been well established. We analyze simulated snowpack data for historical and future climate scenarios and find that changes in variability differ across the western United States. Variability of annual maximum snowpack between years decreases while the timing of peak snow accumulation becomes more variable, particularly in areas transitioning from snow‐ to rain‐dominated precipitation. We also find that consecutive years with very low or early snowpack will become much more frequent. These findings highlight the need to consider changes in snowpack variability in climate change impact assessments and adaptation planning

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Chunxia Zhou et al. (2019), "The Characteristics of Surface Albedo Change Trends over the Antarctic Sea Ice Region during Recent Decades", Remote Sens., 11(7), 821, doi: 10.3390/rs11070821

https://www.mdpi.com/2072-4292/11/7/821

Abstract: "Based on a long-time series (1982–2015) of remote sensing data, we analyzed the change in surface albedo (SAL) during summer (from December to the following February) for the entire Antarctic Sea Ice Region (ASIR) and five longitudinal sectors around Antarctica: (1). the Weddell Sea (WS), (2). Indian Ocean, (3). Pacific Ocean (PO), (4). Ross Sea, and (5). Bellingshausen–Amundsen Sea (BS). Empirical mode decomposition was used to extract the trend of the original signal, and then a slope test method was utilized to identify a transition point. The SAL provided by the CM SAF cloud, Albedo, and Surface Radiation dataset from AVHRR data-Second Edition was validated at Neumayer station. Sea ice concentration (SIC) and sea surface temperature (SST) were also analyzed. The trend of the SAL/SIC was positive during summer over the ASIR and five longitudinal sectors, except for the BS (−2.926% and −4.596% per decade for SAL and SIC, correspondingly). Moreover, the largest increasing trend of SAL and SIC appeared in the PO at approximately 3.781% and 3.358% per decade, respectively. However, the decreasing trend of SAL/SIC in the BS slowed down, and the increasing trend of SAL/SIC in the PO accelerated. The trend curves of the SST exhibited a crest around 2000–2005; thus, the slope lines of the SST showed an increasing–decreasing type for the ASIR and the five longitudinal sectors. The evolution of summer albedo decreased rapidly in the early summer and then maintained a relatively stable level for the whole ASIR. The change of it mainly depended on the early melt of sea ice during the entire summer. The change of sea ice albedo had a narrow range when compared with composite albedo and SIC over the five longitudinal sectors and reached a stable level earlier. The transition point of SAL/SIC in several sectors appeared around the year 2000, whereas that of the SST for the entire ASIR occurred in 2003–2005. A high value of SAL/SIC and a low value of the SST existed in the WS which can be displayed by the spatial distribution of pixel average. In addition, the lower the latitude was, the lower the SAL/SIC and the higher the SST would be. A transition point of SAL appeared in 2001 in most areas of West Antarctica. This transition point could be illustrated by anomaly maps. The spatial distribution of the pixel-based trend of SAL demonstrated that the change in SAL in East Antarctica has exhibited a positive trend in recent decades. However, in West Antarctica, the change of SAL presented a decreasing trend before 2001 and transformed into an increasing trend afterward, especially in the east of the Antarctic Peninsula."

46
While I like the concept of developing calibrated language to better communicate the results of extreme event attribution studies, I do not like the language recommended in the linked study.  Nevertheless, I provide this information for those who might have use for the study's findings.

Also, I note that as an increase in climate variability is an indication of increasing climate sensitivity, attribution is a important topic:

Sophie C. Lewis et al. (27 August 2019), "Toward Calibrated Language for Effectively Communicating the Results of Extreme Event Attribution Studies", Earth's Future, https://doi.org/10.1029/2019EF001273

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019EF001273

Abstract: "Extreme event attribution studies attempt to quantify the role of human influences in observed weather and climate extremes. These studies are of broad scientific and public interest, although quantitative results (e.g., that a specific event was made a specific number of times more likely because of anthropogenic forcings) can be difficult to communicate accurately to a variety of audiences and difficult for audiences to interpret. Here, we focus on how results of these studies can be effectively communicated using standardized language and propose, for the first time, a set of calibrated terms to describe event attribution results. Using these terms and an accompanying visual guide, results are presented in terms of likelihood of event changes and the associated uncertainties. This standardized language will allow clearer communication and interpretation of probabilities by the public and stakeholders."

47
Information from the linked articles and references can help scientists to between compare abrupt climate change events in the past with the changes that are happening in modern times:

Title: "Humanity's emissions '100-times greater' than volcanoes"

https://phys.org/news/2019-10-humanity-emissions-times-greater-volcanoes.html

Extract: "The Deep Carbon Observatory (DCO), a 500-strong international team of scientists, released a series of papers outlining how carbon is stored, emitted and reabsorbed by natural and manmade processes.

Celina Suarez, Associate Professor of Geology at the University of Arkansas, said modern manmade emissions were the "same magnitude" as past carbon shocks that precipitated mass extinction.

See also:

Giancarlo Tamburello et al. Global-scale control of extensional tectonics on CO2 earth degassing, Nature Communications (2018). DOI: 10.1038/s41467-018-07087-z

J.M. de Moor et al. Short-period volcanic gas precursors to phreatic eruptions: Insights from Poás Volcano, Costa Rica, Earth and Planetary Science Letters (2016). DOI: 10.1016/j.epsl.2016.02.056

Brendan McCormick Kilbride et al. Observing eruptions of gas-rich compressible magmas from space, Nature Communications (2016). DOI: 10.1038/ncomms13744

Louis Johansson et al. The Interplay Between the Eruption and Weathering of Large Igneous Provinces and the Deep-Time Carbon Cycle, Geophysical Research Letters (2018). DOI: 10.1029/2017GL076691

Peter B. Kelemen et al. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up, Proceedings of the National Academy of Sciences (2015). DOI: 10.1073/pnas.1507889112

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Title: "Scientists quantify global volcanic CO2 venting; estimate total carbon on Earth"

https://phys.org/news/2019-10-scientists-quantify-global-volcanic-co2.html


48

Thus, I reject calling any of these models "the best".

https://isccp.giss.nasa.gov/role.html

I prefer to think of the scientific method as a process that is continually improving climate change models, and as an example of the next generation (of new & improved) climate change models, I provide the following link to special issues of the JGR Atmospheres publication, update September 13, 2019, focused on the Energy Exascale Earth System Model (E3SM), which is one of the CMIP6 preliminarily indicating that the mean value of ECS is currently over 5C (as discussed earlier in this thread):

Title: "The Energy Exascale Earth System Model"

https://agupubs.onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)2169-8996.ENERGY1

Edit: Also note that recent models (e.g. see Reply #1642) indicate that with ice shelf & MISI ice mass loss, ice-climate interaction (including slowing of the MOC & heating of the tropical oceans [which promotes the formation of high elevation clouds]) become significant without MICI mechanisms.

Two French model projections for CMIP6 have now been officially released, and they confirm their preliminary findings that ECS is likely over 5C, and per Olivier Boucher, head of the Institute Pierre Simon Laplace Climate Modelling Centre in Paris: "The most respected ones—from the United States, and Britain's Met Office—also show a higher ECS" than the previous generation of models:

Title: "Earth warming more quickly than thought, new climate models show"

https://phys.org/news/2019-09-earth-quickly-climate.html

Extract: "'Tipping points'

A core finding of the new models is that increased levels of CO2 in the atmosphere will warm Earth's surface more—and more easily—than earlier calculations had suggested.

If confirmed, this higher "equilibrium climate sensitivity", or ECS, means humanity's carbon budget—our total emissions allowance—is likely to shrink.

"CMIP6 clearly includes the latest modelling improvements," even as important uncertainties remain, Joeri Rogelj, an associate professor at Imperial College London and an IPCC lead author, told AFP.

These include increased supercomputing power and sharper representations of weather systems, natural and man-made particles, and how clouds evolve in a warming world.

"We have better models now," said Boucher. "They have better resolution, and they represent current climate trends more accurately."

The French models are the first to be released.

"The French modelling groups are to be congratulated for being the first to complete their simulations," said Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds.

But other models developed independently have come to the same unsettling conclusion, Boucher confirmed.

"The most respected ones—from the United States, and Britain's Met Office—also show a higher ECS" than the previous generation of models, he said."

49
The linked Youtube video describes a major calving event for the Amery Ice Shelf (East Antarctica).  This reminds us that ice mass loss from ice shelves in Eastern Antarctica are actively contributing to the freshening of the Southern Ocean, and thus is contributing in real time to ice-climate feedback mechanisms like the slowing of the MOC:

Title: "City-sized iceberg separates from Antarctic ice shelf"



Extract: "A gigantic iceberg has broken away from the Amery ice shelf in east Antarctica. The tabular iceberg, officially named D28, is 1,636 square kilometres in size, or about 50 x 30 kilometres - the size of greater London or greater Sydney. It separated from the ice shelf last week, on 26 September but scientists said it was not related to climate change."

50
The linked reference indicates that even relatively small ice mass losses (not included in CMIP5 models, and not including any MICI mechanism) from Antarctica and Greenland are sufficient to slow the MOC, thus increasing climate variability which is a sign of increasing ECS.  Imagine the ice-climate feedbacks associated with a MICI-type of collapse of the WAIS in coming decades.

Nicholas R. Golledge et al. (2019), "Global environmental consequences of twenty-first-century ice-sheet melt", Nature,  566, 65–72, DOI: https://doi.org/10.1038/s41586-019-0889-9

https://www.nature.com/articles/s41586-019-0889-9

Abstract: "Government policies currently commit us to surface warming of three to four degrees Celsius above pre-industrial levels by 2100, which will lead to enhanced ice-sheet melt. Ice-sheet discharge was not explicitly included in Coupled Model Intercomparison Project phase 5, so effects on climate from this melt are not currently captured in the simulations most commonly used to inform governmental policy. Here we show, using simulations of the Greenland and Antarctic ice sheets constrained by satellite-based measurements of recent changes in ice mass, that increasing meltwater from Greenland will lead to substantial slowing of the Atlantic overturning circulation, and that meltwater from Antarctica will trap warm water below the sea surface, creating a positive feedback that increases Antarctic ice loss. In our simulations, future ice-sheet melt enhances global temperature variability and contributes up to 25 centimetres to sea level by 2100. However, uncertainties in the way in which future changes in ice dynamics are modelled remain, underlining the need for continued observations and comprehensive multi-model assessments."

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