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Messages - AbruptSLR

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1
For those who do not care to click on the link in Reply #1740, I provide the following four abstracts form the 2019 WAIS Workshop:

Title: "Could increased melting from East Antarctic ice shelves trigger runaway melting beneath Filcher-Ronne Ice Shelf?" by Matthew Hoffman et al. (2019)

Abstract: "The Filchner-Ronne Ice Shelf (FRIS) presently experiences modest basal melt rates, but model studies have highlighted the potential for an order of magnitude increase in melt rates should relatively warm modified Circum-polar Deep Water (mCDW) reach beneath the ice shelves. Models and observations have indicated that redirection of a coastal current by changes in sea-ice cover and wind stress has the potential to transport mCDW beneath FRIS and initiate melt instability there. We investigate this FRIS melt instability using the Energy Exascale Earth System Model (E3SM) run in a global low-resolution configuration that includes ocean circulation beneath Antarctic ice shelves with fixed geometry and prognostic calculation of freshwater and heat fluxes from ice-shelf melting. We demonstrate the potential of an additional mechanism for FRIS melt instability by freshening and increasing stratification due to high meltwater fluxes from nearby ice shelves in East Antarctica. We see this behavior in partially-coupled simulations with only active ocean and sea-ice, as well as in fully-coupled simulations that also include active atmosphere and land components. Freshening from ice-shelf meltwater reduces ocean density on the continental shelf, shoaling isopycnals near the shelf break and allowing sustained flow of mCDW from offshore onto the continental shelf and into the cavity beneath Filcher Ice Shelf. However, when ice-shelf melting is disabled from neighboring ice shelves in East Antarctica, the melt instability at FRIS is avoided, identifying meltwater from these ice shelves as another potential contributing trigger for this instability. While E3SM indicates the possibility of such a domino effect in ice shelf-melting, we identify biases in the E3SM simulations that precondition the ocean for FRIS melt instability. Reducing these biases through increased regional resolution and improved ocean model tuning is the focus of ongoing work."

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Title: "Vulnerability of Antarctica's ice shelves to meltwater-driven fracture", by Ching-Yao Lai et al. (2019)

Abstract: "Atmospheric warming threatens to accelerate the retreat of the Antarctic Ice Sheet by increasing surface melting and facilitating `hydrofracturing', where meltwater flows into and enlarges fractures on ice shelves. Hydrofracturing in turn could trigger ice-shelf collapse. The collapse of ice shelves that `buttress' the upstream ice sheet increases discharge from the ice sheet and accelerates sea-level rise. Despite progress in modelling hydrofracture, there is limited understanding of the extent to which ice-shelves are vulnerable to hydrofracturing, which is currently described simplistically in continent-scale ice-sheet models, hindering predictions ice-shelf collapse. Here we provide a new theoretical framework, based on Linear Elastic Fracture Mechanics, to predict fracture locations and for the first time quantify vulnerability to hydrofracture across Antarctica's ice shelves. To test theoretical predictions, we train a deep convolutional neural network (DCNN) to identify fractures in continent-wide satellite imagery, and demonstrate close agreement between the distribution of fracture and our model, without the need to tune any model parameters; 89% of 28000 locations identified by the DCNN as fractures across Antarctic ice-shelves lie in regions where our theory predicts fracture formation. We find that many regions regularly inundated with meltwater in the present climate are resilient to hydrofracturing. On the other hand, large regions that resist ice discharge (i.e., which provide significant buttressing to upstream grounded ice) are susceptible to hydrofracture if covered with meltwater. Our findings suggest that as the Antarctic atmosphere warms, increased meltwater production will generally only trigger hydrofracturing if water is formed in or flows into the vulnerable regions we identified. Our new theoretical framework could be included in ice-sheet models to improve predictions of Antarctica's response to atmospheric warming and impact on sea-level rise."

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Title: "Sensitivity of submarine melting of 79N glacier to ocean forcing" by Philipp Anhaus et al. (2019)

The Nioghalvfjerdsbrae (79NG) is a floating ice tongue on Northeast Greenland draining a large part of the Greenland Ice Sheet. A CTD profile from a rift on the ice tongue close to the northern front shows that Atlantic Water (AW) is present in the cavity below, with maximum temperature of approximately 1C at 610 m depth. The AW present in the cavity thus has the potential to drive submarine melting along the ice base. Here, we simulate melt rates from the 79NG with a 1D numerical Ice Shelf Water (ISW) plume model. A melt water plume is initiated at the grounding line depth (600 m) and rises along the ice base as a result of buoyancy contrast to the underlying AW. Ice melts as the plume entrains the warm AW. Maximum simulated melt rates are 50 - 76 m/yr within 10 km of the grounding line. Melt rates drop rapidly to 6 m/yr within the first 10-20 km from the grounding line. Further downstream, melt rates are between 15 m/yr and 6 m/yr. The melt-rate sensitivity to variations in AW temperatures is assessed by forcing the model with AW temperatures between 0.1- 1.4 C, as identified from the 10 ECCOv4 ocean state estimate. The melt rates increase quadratically with rising AW temperature, and overall mean melting changes from 10 m/yr to 21 m/yr with the changes in ocean forcing. The corresponding freshwater flux ranges between 11-30 km3/yr (0.4 - 1.0 mSv). If the 79NG has gone from steady state to the warm AW forcing between 2012 and 2016 it accounts for approximately 9% of the recent freshwater discharge from the ice sheet. Our results show that submarine melting of the marine-terminating 79NG is sensitive to changes in ocean temperature."

&

Title: "Glacial Earthquakes and Precursory Seismicity Reveal Thwaites-Glacier Calving Behavior" by Paul Winberry et al. (2019)

Abstract: "We observe two large (~Ms 5) long-period (10-50 s) seismic-events that originate from the terminus of Thwaites Glacier, Antarctica. Serendipitous acquisition of radar satellite images display a calving event of Thwaites Glacier at this time and confirm that the seismic events were glacial earthquakes generated during the capsizing of icebergs. The glacial earthquakes were preceded by 6 days of discrete high frequency events with an increasing rate of occurrence, culminating in several hours of sustained tremor co-eval with the long period events. A series of optical satellite images collected during this precursory time-period show that the high frequency events and tremor are the result of accelerating growth of ancillary fractures prior to the culminating calving event. Although Thwaites Glacier is one of the largest sources of Antarctic ice-mass loss, the physics of the processes that control discharge into the ocean remains poorly resolved. This study indicates that seismic data have the potential to elucidate the processes by which Thwaites glacier discharges into the ocean, and improve our ability to constrain future sea-level rise."

2
The linked reference indicates that the northern permafrost region already emits more carbon into the atmosphere than it absorbs:

Susan M. Natali et al. (2019), "Large loss of CO2 in winter observed across the northern permafrost region, "Nature Climate Change, DOI: https://doi.org/10.1038/s41558-019-0592-8

https://www.nature.com/articles/s41558-019-0592-8

Abstract: "Recent warming in the Arctic, which has been amplified during the winter, greatly enhances microbial decomposition of soil organic matter and subsequent release of carbon dioxide (CO2). However, the amount of CO2 released in winter is not known and has not been well represented by ecosystem models or empirically based estimates. Here we synthesize regional in situ observations of CO2 flux from Arctic and boreal soils to assess current and future winter carbon losses from the northern permafrost domain. We estimate a contemporary loss of 1,662 TgC per year from the permafrost region during the winter season (October–April). This loss is greater than the average growing season carbon uptake for this region estimated from process models (−1,032 TgC per year). Extending model predictions to warmer conditions up to 2100 indicates that winter CO2 emissions will increase 17% under a moderate mitigation scenario—Representative Concentration Pathway 4.5—and 41% under business-as-usual emissions scenario—Representative Concentration Pathway 8.5. Our results provide a baseline for winter CO2 emissions from northern terrestrial regions and indicate that enhanced soil CO2 loss due to winter warming may offset growing season carbon uptake under future climatic conditions."

See also:

Title: "Global impacts of thawing Arctic permafrost may be imminent"

https://www.sciencemag.org/news/2019/10/global-impacts-thawing-arctic-permafrost-may-be-imminent

Extract: "The Arctic permafrost, frozen soil that is chock full of carbon, is a ticking time bomb. When it thaws because of global warming, sometimes slumping into pits like on Herschel Island in Canada (above), scientists believe it is likely to release more carbon than it absorbs from new plant growth—adding to the atmosphere’s burden and accelerating climate change. But studies in the Arctic have been so limited that no one could say when that time would come.

It’s here now, according to research published today by a large team of scientists in Nature Climate Change. By pooling observations from more than 100 Arctic field sites, scientists from the Permafrost Carbon Network estimate that permafrost released an average of 1662 teragrams of carbon each winter from 2003 to 2017—double that of past estimates. Meanwhile, during the summer growing season, other surveys have found that the landscape absorbs only 1032 teragrams—leaving an average of more than 600 teragrams of carbon to escape to the atmosphere each year."

3
The abstract from the 2019 WAIS Workshop can be found at the following link, and I post one of these abstract below, to remind us all that hydrofracturing could cause the TEIS and/or the PIIS to collapse abruptly in the future:

https://drive.google.com/file/d/1o2-BqgOgPCEn7jY7IBpg_jugus0nwFCh/view


Title: "A poro-damage approach to simulating hydrofracture of glaciers and ice shelves" by Ravindra Duddu et al. (2019)

Abstract: "The dynamic ice mass loss from the Antarctic ice sheet into ocean is one of the greatest sources of uncertainty in predicting future sea level rise. The fracture and detachment of icebergs, that is, calving is an important control on the mass loss from the ice sheet, and is intricately linked to climate dynamics through processes such as hydrofracturing.  It has been hypothesized that hydrofracturing of ice shelves followed by ice cliff failure in Antarctica could contribute to rapid sea level rise over the coming centuries. Simulating hydrofracture propagation using process-based models can provide a better understanding of the conditions enabling full depth crevasse penetration and calving. To this end, we develop a new, nonlocal continuum poro-damage mechanics (CPDM) approach to simulate the propagation of water-filled surface crevasses in idealized rectangular glaciers based on creep-damage-mechanics and poro-mechanics. Using idealized simulations studies on rectangular glaciers in two-dimensions (plane strain conditions), we compare the penetration depths of isolated and closely-spaced water-filled surface crevasses predicted by the CPDM model with those from existing crevasse depth models. We find that the CPDM model is in good agreement with the linear elastic fracture mechanics (LEFM) models for an isolated surface crevasse and with the zero stress model (to a lesser extent) for closely-spaced surface crevasses, except when the glacier is near-floatation. We also examine crevasse propagation in relation to ice rheology, fracture process zone size, and basal boundary conditions using sharp crack and damage mechanics models. Based on these simulation studies, we argue that floating ice shelves are more vulnerable than grounded glaciers due to the combination of meltwater-induced hydrofracture and plate bending. To conclude, we discuss the limitations of the creep damage mechanics model and directions for future work, including modeling shear-dominated failure under compression using strain-energy-based damage models."

4
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."

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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.”"

5
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."

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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.

6
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.

7

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

8
...
We can agree to disagree, but I think the widening needs to happen before the "slumping" or MICI begins.  There is just an enormous amount of ice sitting behind Thwaites that will fill up the "narrow" (20km at most) trough and prevent the sea from encroaching too far inland.  Basically ice is viscous and will clog up any opening that is too small to let it through easily.

On a related note, I am preparing some maps overlaying current Sentinel-1 images and published bathymetry and grounding line data for the Thwaites Glacier thread that I hope might clear up some of the confusion for us casual observers.

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; which I believes helps to clarify/support my concerns that the sustained growth and episodic abrupt partial collapse of the subglacial cavities near the Thwaites gateway, may eventually lead to both slumping and/or ice-cliff failures (possibly in less than 10-years) within the approximately 50-km wide threshold roughly between Stations 1021.7 and 1021.12 in the second attached image, for reasons including:
1. The compressive ice force arch from south of the Big Ear kicking northeast into the mid-section of the TEIS (which I showed previously in heavy yellow arrows, see Reply #1733) will likely reduce the compressive ice loading on the icebergs at the upstream base of the Thwaites Ice Tongue, thus allowing the local calving front at the southwest corner of the Thwaites Ice Tongue to retreat southward towards the growing subglacial cavity near the Big Ear.
2. An abrupt partial collapse of the subglacial cavity near Big Ear (sometime within the next 10-years) causes the remaining icebergs at the base of the Thwaites Ice Tongue to float away leaving an exposed ice cliff face extending over 145m above sea level, leading to local slumping and a rapid deterioration of the downstream end compressive ice force arch (south of the Big Ear) kicking into the mid-section of the TEIS.
3. The mélange from the slumping would have icebergs with drafts too shallow to become pinned to the subsea ridges as they float seaward; which would make room for still more slumping within the trough leading to the BSB; which would allow the Thwaites gateway to widen to the approximately 50-km width shown in the second image between Stations 1021.7 and 1021.12.

Best, ASLR

9
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.

10
...

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.

11
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."

12
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."

13
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.

14
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.

15

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.

16
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.

17
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).


18
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).

19
The linked reference finds that simulating an armada of icebergs (from the WAIS) around the Southern Ocean substantially increases the positive feedback for most ice mass loss from the AIS:

Fabian Schloesser et al. (2019), "Antarctic iceberg impacts on future Southern Hemisphere climate", Nature Climate Change,  9, 672–677

DOI: https://doi.org/10.1038/s41558-019-0546-1

https://www.nature.com/articles/s41558-019-0546-1

Abstract: "Future iceberg and meltwater discharge from the Antarctic ice sheet (AIS) could substantially exceed present levels, with strong implications for future climate and sea levels. Recent climate model simulations on the impact of a rapid disintegration of the AIS on climate have applied idealized freshwater forcing scenarios rather than the more realistic iceberg forcing. Here we use a coupled climate–iceberg model to determine the climatic effects of combined iceberg latent heat of fusion and freshwater forcing. The iceberg forcing is derived from an ensemble of future simulations conducted using the Penn State ice-sheet model. In agreement with previous studies, the simulated AIS meltwater forcing causes a substantial delay in greenhouse warming in the Southern Hemisphere and activates a transient positive feedback between surface freshening, subsurface warming and ice-sheet/shelf melting, which can last for about 100 years and may contribute to an accelerated ice loss around Antarctica. However, accounting further for the oceanic heat loss due to iceberg melting considerably increases the surface cooling effect and reduces the subsurface temperature feedback amplitude. Our findings document the importance of considering realistic climate–ice sheet–iceberg coupling for future climate and sea-level projections."

May last series of posts provide some level of evidence that the Thwaites Glacier may initiate a MICI-type of collapse starting as early as 2035 to 2040; which might roughly match the information associated with the 5-year doubling time on Hansen et al (2016)'s attached plots.  However Schloesser et al. (2019), points out that analyses like Hansen et al.'s used meltwater hosing events (in both the Southern Ocean and/or the North Atlantic) instead of armadas of icebergs which have substantial latent heat of fusion; which both extends the timeline of cooling of the local SSTs and slows the rate of the positive ocean feedback to melt more ice from marine glaciers. 

That said, I note that one of Hansen's more significant warnings on this topic is that an episodic release of an armada of icebergs, into the Southern Ocean and/or the North Atlantic, would generate tremendously large storms (e.g. Cat 6 hurricanes and larger); which would likely act as multidecadal positive feedbacks for higher transient values of ECS due both to: a) heat energy advected by such mega-storms directly to the Arctic, thus providing a multidecadal increase in Arctic Amplification; and b) a likely multidecadal increase in Arctic rainfall, which would result in a transient increase in ECS due to such positive feedbacks as: melting of Arctic Sea Ice, degradation of permafrost, enhance ice mass loss from the GIS; and increased methane emissions from high latitude peatlands and degraded permafrost.

20
This post is the last of my recent series of posts on mechanisms that may likely lead to local ice-cliff failures occurring in the trough of the bed of the Thwaites gateway after the TEIS and the Thwaites Ice Tongue may have collapsed circa 2030/35:

The first image reminds us of the geometry of the bed topology and the subglacial cavity in the Thwaites gateway; which makes the Thwaites Glacier uniquely susceptible to an MICI-type of collapse in the coming decades.

The second image shows how snow fall primarily in the coastal areas of the West Antarctic; where it progressively increases the gravitational driving force on key marine glaciers in the ASE like the Thwaites Glacier.

The third image shows that the eastern shear margin of the Thwaites Glacier is linked to the SW Tributary Glacier.  Thus now that the calving front of the Pine Island Ice Shelf, PIIS, has retreated upstream of the ice shelf for the SW Tributary Glacier, we can expect that the associated acceleration of the ice velocities of the SW Tributary Glacier will reduce the buttressing of the eastern shear margin; which should accelerate the ice flow velocities of the Thwaites Glacier in coming decades.

The fourth image of a 'Domino Wave' reminds us that a cascade of tipping points associated with the stability of the Thwaites Glacier can accelerate rapidly once triggered.

21
I provide the following four images to illustrate additional reasons that the Thwaites Eastern Ice Shelf, TEIS, and Thwaites Ice Tongue may become unstable circa 2030/35 (whether they have thinned sufficiently to become unpinned from the submarine ridge pinning points, or not):

The first image shows how the melt-pond mechanism (also called hydrofracturing) caused the Larsen B ice shelf to collapse in less than a week, and I note that if ECS is much above 3C then similar meltwater ponds will likely form during the austral summer on the TEIS circa 2030/35; which could cause the TEIS to collapse abruptly in that timeframe.

The second image (from sidd) shows that the air space in the firn of Antarctic ice shelves are coming saturated with ice (including the TEIS); which means that hydrofracturing in the TEIS is becoming increasingly likely.

The third image shows the firn system in an ice shelf and how meltwater can displace the associate firn air; which increases the likelihood of hydrofracturing.

The fourth image shows the current susceptibility of the firn system on Antarctic ice shelves to become saturated with salty seawater as the ice shelves become thinner; thus increasing the risk of hydrofracturing of the ice shelf.

22
To continue my series of posts on the instability of the Thwaites gateway and associated ice shelf & ice tongue:

The first image shows how warm CDW passes through troughs in the continental shelf to the grounding line of a representative marine glacier (like in the ASE); where it both melts the ice at the grounding line and along the underside of the ice shelf causing a relatively freshwater current of water to exist from the calving front of the ice shelf/tongue.  This freshwater flow not only thins the thickness of the ice shelf/tongue but also causes grooves in the underside of the shelf/tongue which have accelerated ice melting.

The second image (from Bassis) shows how warm CDW beneath an ice shelf can 'burn' upwards through crevasses/channels in the underside of the ice shelf, thus reducing the stability of the ice shelf.

The third & fourth images show that so much ice meltwater has already been discharged from Antarctic ice shelves and marine glaciers that the surface waters of much of the Southern Ocean have become colder and fresher, both of which cause positive ice-climate feedback mechanisms.

23
To continue my series of posts on the instability of the Thwaites gateway:

The first image shows the location of four Thwaites subglacial lakes that drain between June 2013 and July 2014, just after the local collapse of the subglacial cavities in the Thwaites gateway.

The second image shows that the ice surface elevation above these four Thwaites subglacial lakes dropped as the lakes drained, & the associated outburst of meltwater contributed to the surge of the Thwaites Ice Tongue during this period.

The third image shows a photograph of base of the Thwaites Ice Tongue from January 2013, showing both the location of the calving front and the grounding line and how icebergs are floating away from the calving front during the 2012 to 2014 surge of the Thwaites Ice Tongue.

The fourth image shows the geothermal heat flux beneath the Thwaites Glacier which contributes to the volume of basal meltwater in the drainage system beneath the Thwaites:

24
To continue my posts on why local ice-cliff failures may be initiated in the trough in the bed of the Thwaites gateway circa 2035:

The first image shows the ice flow velocities of the Thwaites Glacier and how those velocities change with distance from the calving front and with time & I note that both the internal ice friction and associated internal glacial ice melting has varied with distance from the calving front and with time.

The second image shows the typical phreatic water surface elevation within a marine glacier (here the Byrd Glacier) and how it varies in a similar fashion with the ice flow velocities with distance from the calving front.

The third image shows an idealized image of the subglacial meltwater conveyance system beneath the Thwaites Glacier and how this basal meltwater exists from beneath the glacier near the base of the Thwaites Ice Tongue & through the trough located near there.

The fourth image shows how when basal meltwater exists from beneath the calving front of a marine glacier it can cause mixing turbulence that can accelerate local ice mass loss.

25
As a follow-on to my last post:

The first image shows the relationship between the ENSO cycle and the surface elevation of the ice shelves in the Amundsen Sea Sector; clearly increasing that these ice shelves float up on El Nino events and down on La Nina events; which causes flexure and cracking of the ice shelves (which weakens them and makes them more susceptible to the influence of warm CDW).

The second image shows how during the combination of an El Nino event and a positive SAM event tropical heat energy is advected from the Tropical Pacific directly to the coastal West Antarctica; where it can episodically accelerate local ice mass loss (along the coastal areas).

The third image shows how the Amundsen Bellingshausen Sea Low, ABSL (or ASL), can direct winds directly into the ASE, which also drags along ocean currents that advect more warm CDW into the ASE which accelerates local ice mass loss.

The fourth image shows the average potential temperature of the warm CDW (above freezing) typically being advected into the ASE, and the associate marine glacier ice flow velocities (because of the reduced buttressing from the degrading ice shelves and the retreating grounding lines).  Also, I note that relatively rapid ice flow velocities cause internal friction within the ice of the marine glaciers, which induces more basal meltwater beneath the marine glaciers (which further destabilize the marine glaciers).

26
To continue with my series of posts reminding readers why I believe that ice-cliff failure mechanisms will be activated in the trough passing through the Thwaites gateway near the base of the Thwaites Ice Tongue:

The first image illustrates how the increased circumpolar westerly winds (due to the Antarctic ozone hole) causes the coastal surface water to move offshore (due to the Coriolis effect), which causes increased upwelling of warm CDW (circumpolar deep water), so of which moves toward the continental shelf.

The second image (from a computer simulation) illustrates how this upwelled CDW is advected towards the grounding line of key Antarctic marine glaciers (including those in the Amundsen Sea Embayment, ASE), due to the presences of ice meltwater near the coastal seawater surface; which accelerates both basal ice melting of the associated ice shelves (including the Thwaites Eastern Ice Shelf, TEIS, and the Thwaites Ice Tongue) and grounding line retreat.  This figure also shows that around 2030 this mechanism will begin delivering warmer CDW to still shallower continental shelf water depths.  Which will stress the ASE ice shelves and grounding lines more than what has been occurring since about 1990 to about 2030.

The third image (from a computer simulation) shows how (under the right conditions, like an El Nino year) warm CDW can flow from the PIG westward along the coast to the Thwaites gateway, where it can accelerate the basal ice melting normally occurring due to tidally advected CDW into the Thwaites gateway.  As strong El Nino events happen every 15 years (with continued global warming), this mechanism may well accelerate basal ice mass loss from the TEIS & the Thwaites Ice Tongue circa 2015/16 +15 = 2030/31.

The fourth image shows how rapid the ice shelf/tongue mass loss has been in the ASE since the 1990's; and particularly shows how unstable the Thwaites Ice Tongue has been during this period of increase CDW advection to the Thwaites gateway.

27
As a follow on to my last post, I present four images in an attempt to get readers oriented in the area where I first expect ice cliff failures for Thwaites Glacier between to bed areas that I have labelled Big Ear and Little Ear in this series of images.

The first image shows the locations of the Big & Little Ears relative to a collapse of the cavity (see Kim et al. 2018) that lead to a surge of the Thwaites Ice Tongue after January 2012.

The second image shows the location of Big & Little Ears on a Sentinel 1 image from May 23, 2019.  This image shows ice bergs already floating near the Little Ear location.

The third image shows the sources of CDW leading from the continental rise to the bases of the TEIS and the Thwaites Ice Tongue.

The fourth image shows the bathymetry of the Thwaites Gateway prior to 2013, which confirms that icebergs can readily float-out of the trough leading to the BSB once the TEIS and the Thwaites Ice Tongue have collapsed and are no longer pinned by the ridge that is seaward of the gateway.

28
As a follow-on to my last post, I present four images in this post and four images in the next post, in an attempt to get readers oriented in the area where I first expect ice cliff failures for Thwaites Glacier between two bed areas that I have labelled Big Ear and Little Ear in this series of images.

The first image comes from Kim et al. (2018), and shows two local areas of subglacial cavities that collapsed circa 2012.

https://www.mdpi.com/2072-4292/10/8/1236

With a hat tip to Sleepy, the second image shows the ears relative to the growth of the subglacial cavity at the base of the Thwaites Ice Tongue (near the Big Ear), circa 2017.

The third and fourth images show the alignment (plan & section, respectively) of A-B (circa 2017) that Rignot believes has low stability, and which transects the bed trough (see Tinto & Bell 2012), and this figure clearly shows that in the trough the height of any future local ice cliff above sea level would be well above El 100m and the water depth would be at least 800 m; indicating that ice-cliff failure mechanisms would occur in the future if/when this ice is no longer buttressed by the floating glacial ice.  Here, I remind readers that any icebergs calved into the trough (once the ice shelf & ice tongue are gone) can simply float-out to sea.

29
I have referenced Milillo et al. (2019) previously; however, as it tells part of a complex/heterogeneous story of the portion of the Thwaites Glacier gateway between the east side of the base of the Thwaites Eastern Ice Shelf (TEIS) and the west side of the base of the Thwaites Ice Tongue, I use it to kick-off a series of Replies using previously posted information in order try to emphasize how this dynamic area of the Thwaites Glacier gateway could lead to the initiation of an MICI-type of failure for the Thwaites Glacier beginning around 2035 to 2045. 

In this regard, the first attached image (Fig 1) shows this critical portion of the Thwaites gateway where:
1. Panel A/B shows the bed topology (blue with white contour lines) and areas of high basal ice melting (red zones) associated with the influx of warm CDW (yellow arrows).
2. Panel C shows DinSAR data and points A, B & F (near what I later call the Big Ear) and points C, D & E (to the east of what I later call the Little Ear).
3. Panel D shows the height above floatation.
4. Panel E shows change in ice surface elevation, dh, between decimal years 2013.5 and 2016.66.
5. Panel F shows ice flow velocities (with the highest velocities at the west side of the base of the Thwaites Ice Tongue).

The second image (Fig 2) zooms in on the points A, B & F (with high basal ice mass loss near the grounding line and high changes in the ice surface elevation) and points C, D & E (with high basal ice mass loss near the grounding line but with lower changes in the ice surface elevation)

The third image (Fig 3) shows the subglacial cavity in this area with Panel C focused on the Big Ear area (points A, B & F).

The fourth image from Tinto & Bell (2011) shows what I call the Big Ear and Little Ear areas prior to 2011 in relation to both the TEIS, the Thwaites Ice Tongue and the bed trough that extends from the ocean to the Byrd Subglacial Basin, where I suspect that ice-cliff failures may begin as early as 2035.

P. Milillo, E. Rignot, P. Rizzoli, B. Scheuchl, J. Mouginot, J. Bueso-Bello, and P. Prats-Iraola (2019), "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica", Sci Adv. 5(1): eaau3433, doi: 10.1126/sciadv.aau3433
PMCID: PMC6353628
PMID: 30729155

https://advances.sciencemag.org/content/5/1/eaau3433
&
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6353628/

Abstract: "The glaciers flowing into the Amundsen Sea Embayment, West Antarctica, have undergone acceleration and grounding line retreat over the past few decades that may yield an irreversible mass loss. Using a constellation of satellites, we detect the evolution of ice velocity, ice thinning, and grounding line retreat of Thwaites Glacier from 1992 to 2017. The results reveal a complex pattern of retreat and ice melt, with sectors retreating at 0.8 km/year and floating ice melting at 200 m/year, while others retreat at 0.3 km/year with ice melting 10 times slower. We interpret the results in terms of buoyancy/slope-driven seawater intrusion along preferential channels at tidal frequencies leading to more efficient melt in newly formed cavities. Such complexities in ice-ocean interaction are not currently represented in coupled ice sheet/ocean models."

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

Caption for the second image (Fig 2): "Changes in ice surface elevation, h, of Thwaites Glacier.
(A to F) from TDX data (blue dots) for the time period 2011–2017 over grounded ice (red domain, dh/dt) at locations A to F, with height above floatation, hf (red lines), and 1σ uncertainty (dashed red lines), converted into change in ice thickness, H, over floating ice (blue domain, dH/dt) in meters per year. Black triangles are TDX dates in (G) to (J). (G and H) Main trunk. (I and J) TEIS. Grounding line position is thin black for 2016–2017 and white dashed blue for 2011."

Caption for the third image (Fig 3): "Ice thickness change of Thwaites Glacier.
(A) Ice surface elevation from Airborne Topographic Mapper and ice bottom from MCoRDS radar depth sounder in 2011, 2014, and 2016, color-coded green, blue, and brown, respectively, along profiles T1-T2 and (B) T3-T4 with bed elevation (brown) from (16). Grounding line positions deduced from the MCoRDS data are marked with arrows, with the same color coding. (C) Change in TDX ice surface elevation, h, from June 2011 to 2017, with 50-m contour line in bed elevation and tick marks every 1 km."

30
I have frequently recommended that climate scientists better discuss risk (probability times consequences) when presenting their findings.  The linked reference (about increasing damage from natural disasters due to climate change) is a very good example of how I believe more climate scientists should present their findings.  In particular, the associated attached image shows how, with time, increasing stress (i.e. radiative forcing) the pdf shifts to the right and the pdf becomes more right skewed with a fatter right tail; which dramatically increases the damage function:

Matteo Coronese, Francesco Lamperti, Klaus Keller, Francesca Chiaromonte, and Andrea Roventini (October 7, 2019), "Evidence for sharp increase in the economic damages of extreme natural disasters", PNAS https://doi.org/10.1073/pnas.1907826116

https://www.pnas.org/content/early/2019/10/01/1907826116

Significance
Observations indicate that climate change has driven an increase in the intensity of natural disasters. This, in turn, may drive an increase in economic damages. Whether these trends are real is an open and highly policy-relevant question. Based on decades of data, we provide robust evidence of mounting economic impacts, mostly driven by changes in the right tail of the damage distribution—that is, by major disasters. This points to a growing need for climate risk management.

Abstract
Climate change has increased the frequency and intensity of natural disasters. Does this translate into increased economic damages? To date, empirical assessments of damage trends have been inconclusive. Our study demonstrates a temporal increase in extreme damages, after controlling for a number of factors. We analyze event-level data using quantile regressions to capture patterns in the damage distribution (not just its mean) and find strong evidence of progressive rightward skewing and tail-fattening over time. While the effect of time on averages is hard to detect, effects on extreme damages are large, statistically significant, and growing with increasing percentiles. Our results are consistent with an upwardly curved, convex damage function, which is commonly assumed in climate-economics models. They are also robust to different specifications of control variables and time range considered and indicate that the risk of extreme damages has increased more in temperate areas than in tropical ones. We use simulations to show that underreporting bias in the data does not weaken our inferences; in fact, it may make them overly conservative.

Extract: "Inconclusive results in the literature might be due to the use of statistical techniques ill-suited to capture the evolution of the damage distribution. We hypothesize that relevant patterns may in fact correspond to changes in its right skew and tail. To investigate this, we use a different modeling and statistical strategy. First, we include control variables for socio-demographic factors as covariates in our models alongside time—this generalizes the Actual-to-Potential-Loss approach, allowing for multiple controls, and improves upon procedures that normalize damage values prior to modeling (Normalization). Second, and perhaps most importantly, we characterize the behavior of the damage distribution fitting quantile regressions over disaggregated, event-level data.

Our approach avoids 2 common pitfalls: 1) linear aggregation—i.e., summing damages associated to disasters occurring in a given year over a specified geographical area, which may lead to a substantial loss of information—and 2) the use of ordinary least squares (OLS)—i.e., mean regression, which captures only average trends in damages (changes in expected losses). With increasing evidence that natural disasters induce fat-tailed damage distributions and that fat tails can dramatically change policy implications in a variety of climate-economics models, analyzing quantiles can be an effective way to inspect extreme, low-probability events. In addition, OLS regression can be a rather blunt instrument to analyze skewed data. In contrast, quantile regressions do not rely on Gaussianity or even symmetry assumptions for the error distribution and have already been used to characterize the evolution of cyclone strength.

The Devil Is in the Tails: From Climate Stressors to Damages
We hypothesize that what changes over time is the right skew and tail behavior (as opposed to the average) of the damage distribution. This can be explained using the concept of damage function. Damage functions are widely used in the Integrated Assessment Modeling literature to link climate-related stressors (e.g., wind speed for tropical cyclones or storm surges) to damages.

Rightward Skewing and Tail Fattening: Economic Impacts Are Mounting
Turning from simulated to actual data, we find strong evidence of an accelerating rightward skewing and tail fattening of the damage distribution over time."

31
As a public service, I note that the Twenty-Sixth Annual WAIS Workshop is being held next week in California

Twenty-Sixth Annual WAIS Workshop
October 16-18, 2019
Camp Cedar Glen
Julian, California, U.S.A.

https://www.waisworkshop.org/workshop-2019


Extract: "For 2019, sessions topics will include:
•   Will the snowflakes save us? Near-surface science in West Antarctica using shallow geophysics, ice cores, firn studies, atmospheric reanalysis, meteorological observations, and modeling
•   Processes Beneath: Deep geophysics, direct-access experiments, microbiology, sediment coring, and modeling for quantifying critical processes from the basal interface to the core
•   The Great Glacier Conveyer - A John Nye & Wally Broecker Appreciation Session: Connecting paleo understanding with modern processes
•   From The Sea: Polar oceanography, tropical teleconnections, marine life, and ice front dynamics and their impact on a changing WAIS
•   The Leading Edge: New boundary conditions, new modeling techniques, Operation IceBridge, ICESat-2, GRACE, GRACE-FO, NISAR, and data science applications for West Antarctic science
•   Dynamics across the grounding line: Past, present, and future understanding of the ice stream-grounding zone-ice shelf transition seen from the ground, air, satellites, and models
•   Science Communication: Taking WAIS science beyond our room
•   Community Health: Avoiding, acknowledging, and approaching issues with field harassment"


32
Consensus climate scientists have been slow to embrace the possibility that outbursts of subglacial meltwater may likely accelerate future grounding line retreat in the Thwaites Glacier gateway, largely because they do not yet fully understand the mechanism of such interactions.  For reasons to complex to go into in this post, there are multiple reasons to believe that lessons learned from current and paleo subglacial hydrology in the western Ross Sea reason can readily be applied to the Thwaites gateway region.  Thus, I provide the following linked reference about 'a meltwater drainage system beneath the ancestral East Antarctic ice sheet', which located in West Antarctica; which supports the position that '… ice stream dynamics in this region were sensitive to the underlying hydrological system.'  While this may not mean much to many readers, to me it supports the idea that the glacial ice in the trough through the Thwaites gateway is subject to being destabilized at a decadal-scale by outbursts of meltwater that pass through this very trough:

Simkins, L.M., Anderson, J.B., Greenwood, S.L., Gonnermann, H.M., Prothro, L.O., Halberstadt, A.R.W., Stearns, L.A., Pollard, D. and DeConto, R.M., 2017. Anatomy of a meltwater drainage system beneath the ancestral East Antarctic ice sheet. Nature Geoscience, 10(9), pp.691-697, doi: 10.1038/NGEO3012

https://www.nature.com/articles/ngeo3012

Abstract: "Subglacial hydrology is critical to understand the behaviour of ice sheets, yet active meltwater drainage beneath contemporary ice sheets is rarely accessible to direct observation. Using geophysical and sedimentological data from the deglaciated western Ross Sea, we identify a palaeo-subglacial hydrological system active beneath an area formerly covered by the East Antarctic ice sheet. A long channel network repeatedly delivered meltwater to an ice stream grounding line and was a persistent pathway for episodic meltwater drainage events. Embayments within grounding-line landforms coincide with the location of subglacial channels, marking reduced sedimentation and restricted landform growth. Consequently, channelized drainage at the grounding line influenced the degree to which these landforms could provide stability feedbacks to the ice stream. The channel network was connected to upstream subglacial lakes in an area of geologically recent rifting and volcanism, where elevated heat flux would have produced sufficient basal melting to fill the lakes over decades to several centuries; this timescale is consistent with our estimates of the frequency of drainage events at the retreating grounding line. Based on these data, we hypothesize that ice stream dynamics in this region were sensitive to the underlying hydrological system."

33
As a follow-on to my last post, it should be noted that WAIS is that remaining marine ice sheet on earth, and that it is unique from all of the other previous marine ice sheets in that the WAIS can retreat from four different regions (the Bellingshausen Sea region, the Amundsen Sea region; the Ross Sea region and the Weddell Sea region) all converging on a common central subglacial basin area (roughly below the WAIS divide).  The accompanying figure from Bingham et al 2012 shows that the West Antarctic with WAIS removed has numerous subglacial troughs that can lead warm ocean water (driven by: advection, currents and tidal action) from all four seas directly into the heart of the WAIS subglacial basins.  In this figure the area for the Ferrigno Glacier is indicated by the black rectangle; which rests in a rift valley that leads directly into the back side of the trough that the PIG rests in:

34
As a follow-on to my last post about seaways in West Antarctica, the accompanying figures are from Vaughan et al. 2011, the researcher postulate that the indicated seaways opened in the WAIS during the Eemian period some 124,000 years ago.  Vaughan et al. proved that for an upper bound that the longest of these seaways formed within less than a thousand years (which is well within the timeframe of the Eemian peak proving that WAIS could have contributed at least 3.4 to 3.8 m to eustatic SLR in that period).

35
This post is a reminder (see the second linked reference) that in 2012 Strugnell et al. provided evidence about octopi (octopods) that indicated that during the Eemian (last interglacial) period seaways existed through the WAIS; which supports the idea of at least a partial collapse of the WAIS at that time.  While in the first linked reference, Strugnell et al. (2018) lays-out a detailed plan to use a molecular genetic approach to confirm that such seaways existed through the WAIS during the Eemian.  Also, I note that if such seaways were to occur in the coming decades they would change the local ocean current circulation patterns in a way that would likely accelerate ice mass loss from the WAIS:

Jan M. Strugnell et al. (1 January 2018), "Dating Antarctic ice sheet collapse: Proposing a molecular genetic approach", Quaternary Science Reviews, Volume 179, Pages 153-157, https://doi.org/10.1016/j.quascirev.2017.11.014

https://www.sciencedirect.com/science/article/abs/pii/S0277379117306340?via%3Dihub

Abstract
Sea levels at the end of this century are projected to be 0.26–0.98 m higher than today. The upper end of this range, and even higher estimates, cannot be ruled out because of major uncertainties in the dynamic response of polar ice sheets to a warming climate. Here, we propose an ecological genetics approach that can provide insight into the past stability and configuration of the West Antarctic Ice Sheet (WAIS). We propose independent testing of the hypothesis that a trans-Antarctic seaway occurred at the last interglacial. Examination of the genomic signatures of bottom-dwelling marine species using the latest methods can provide an independent window into the integrity of the WAIS more than 100,000 years ago. Periods of connectivity facilitated by trans-Antarctic seaways could be revealed by dating coalescent events recorded in DNA. These methods allow alternative scenarios to be tested against a fit to genomic data. Ideal candidate taxa for this work would need to possess a circumpolar distribution, a benthic habitat, and some level of genetic structure indicated by phylogeographical investigation. The purpose of this perspective piece is to set out an ecological genetics method to help resolve when the West Antarctic Ice Shelf last collapsed.
&

J. M. Strugnell, P. C. Watts, P. J. Smith, A. L. Allcock. Persistent genetic signatures of historic climatic events in an Antarctic octopus. Molecular Ecology, 2012; DOI: 10.1111/j.1365-294X.2012.05572.x

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-294X.2012.05572.x

Abstract: "Repeated cycles of glaciation have had major impacts on the distribution of genetic diversity of the Antarctic marine fauna. During glacial periods, ice cover limited the amount of benthic habitat on the continental shelf. Conversely, more habitat and possibly altered seaways were available during interglacials when the ice receded and the sea level was higher. We used microsatellites and partial sequences of the mitochondrial cytochrome oxidase 1 gene to examine genetic structure in the direct‐developing, endemic Southern Ocean octopod Pareledone turqueti sampled from a broad range of areas that circumvent Antarctica. We find that, unusually for a species with poor dispersal potential, P. turqueti has a circumpolar distribution and is also found off the islands of South Georgia and Shag Rocks. The overriding pattern of spatial genetic structure can be explained by hydrographic (with ocean currents both facilitating and hindering gene flow) and bathymetric features. The Antarctic Peninsula region displays a complex population structure, consistent with its varied topographic and oceanographic influences. Genetic similarities between the Ross and Weddell Seas, however, are interpreted as a persistent historic genetic signature of connectivity during the hypothesized Pleistocene West Antarctic Ice Sheet collapses. A calibrated molecular clock indicates two major lineages within P. turqueti, a continental lineage and a sub‐Antarctic lineage, that diverged in the mid‐Pliocene with no subsequent gene flow. Both lineages survived subsequent major glacial cycles. Our data are indicative of potential refugia at Shag Rocks and South Georgia and also around the Antarctic continent within the Ross Sea, Weddell Sea and off Adélie Land. The mean age of mtDNA diversity within these main continental lineages coincides with Pleistocene glacial cycles."


36
The linked article discusses the use of state-of-the-art autonomous floating senors since 2014 to document a significant reduction in the net amount of carbon dioxiode that the Southern Ocean has been aborbing in the past few years (which is not included in consensus climate change models).  While a marked increase in CO2 outgassing in the southern sector of the Southern Ocean apparently due to increased upwelling (possibly due to increased westerly wind velocities associated with both increasing atmospheric GHG concentrations and the Antarctic ozone hole); I am personnelly concerned that the decreasing surface water salinity associated with ice meltwater may be reducing the ability of microorganisms to absorb and sequester CO2 (which could become markedly worse if the WAIS were to undergo a MICI-type of collapse in the coming decades):

Title: "Southern Ocean is absorbing less carbon"

https://www.earthmagazine.org/article/southern-ocean-absorbing-less-carbon

Extract: "In the Southern Ocean surrounding Antarctica, complex and dynamic interactions among the atmosphere, cryosphere, and surface and deep ocean waters play an important role in climate. Although it covers only a quarter of Earth’s oceanic surface area, the Southern Ocean — with its cold temperatures and carbon-sucking algal blooms — has been estimated to take up 40 percent of anthropogenic carbon dioxide emissions. However, new data collected by a fleet of autonomous floating sensors show that the Southern Ocean is taking up significantly less carbon than scientists thought.
...
In the Antarctic southern zone, the team found that the ocean was outgassing about 0.36 billion metric tons of carbon — a “pretty significant amount of carbon dioxide,” Gray says. “This was surprising because previous estimates, based on shipboard data, [suggested] that that region was not emitting anything,” she says.

The float observations show that with strong outgassing and upwelling, the entire Southern Ocean has a net annual carbon dioxide uptake of 0.08 billion metric tons a year. Not only are carbon emissions about 10 times higher than previously thought, carbon uptake is only one-tenth of what observations had suggested — which is “quite a big shift,” Gray says."

37
Many left-tail climate-commentators point to paleo climate variability as an example of why people should ignore climate change warnings, as climate has always fluctuated.  However, in reality high climate variability is a first-order indicator of high climate sensitivity.  Also, for the past two millennia the ice sheets have been much as they were during the pre-industrial era, and thus this period provides a good proxy for representing baseline Earth System conditions.  Finally, the linked reference studied the variability of the Northern North Atlantic and Artic Oceans for the last two millennia and found periods of strong variability such as the Medieval Warm Period followed by the Little Ice Age (see the attached image), and as the AMOC is strongly connected to climate sensitivity (note that a slowing MOC results in warmer tropical SSTs, which can increase the positive feedback from high altitude clouds), this paleodata supports the idea that ECS is currently relatively high:

P. Moffa‐Sánchez et al. (18 June 2019), "Variability in the Northern North Atlantic and Arctic Oceans Across the Last Two Millennia: A Review", Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003508

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018PA003508

Abstract
The climate of the last two millennia was characterized by decadal to multicentennial variations, which were recorded in terrestrial records and had important societal impacts. The cause of these climatic events is still under debate, but changes in the North Atlantic circulation have often been proposed to play an important role. In this review we compile available high‐resolution paleoceanographic data sets from the northern North Atlantic and Nordic Seas. The records are grouped into regions related to modern ocean conditions, and their variability is discussed. We additionally discuss our current knowledge from modeling studies, with a specific focus on the dynamical changes that are not well inferred from the proxy records. An illustration is provided through the analysis of two climate model ensembles and an individual simulation of the last millennium. This review thereby provides an up‐to‐date paleoperspective on the North Atlantic multidecadal to multicentennial ocean variability across the last two millennia.

Extract: "Surface ocean reconstructions of the relatively warm Atlantic waters show variable patterns in the three regions studied. The most southern region (30–45°N) shows large variability in the surface temperature conditions, with the most common signal being the anomalous conditions (either cooling or warming) starting in ~1850 CE (Figure 2). This diverging pattern perhaps arises from the complex regional interactions between the Gulf Stream and the Labrador Current (including changes in the Gulf Stream detachment) and/or larger basin scale circulation changes. Farther north in the subpolar North Atlantic, reconstructions from the NAC waters reveal diverging millennial trends likely resulting from seasonal or preferred habitat depth proxy biases (Figure 3). The records from the slope waters across the Scottish shelf are shorter and mostly reveal a common warming trend from 1800 CE to present not clearly recorded in the sediment cores of the subpolar North Atlantic (Figure 3). This geographical difference could be explained by differential warming of coastal/shelf environments versus central subpolar gyre. Surface reconstructions of the Atlantic inflow in the Nordic Seas largely reveal a millennial‐scale cooling and increase drift ice with higher‐resolution records showing a shift around 1300–1450 CE to colder conditions with more drift ice (Figure 4)."

Caption for the first image: "Surface ocean records from the southwest and northwest Atlantic. Percentages of the polar foraminiferal species N. pachyderma from (a) Laurentian Fan (Keigwin & Pickart, 1999) (b) Nova Scotia (Emerald Basin; blue; note that superimposed are two records from the same core: Mg/Ca‐based temperatures based on benthic foraminifera C. lobatus, in pink, and sea surface temperature (SST) record from alkenones, in gray (Keigwin et al., 2003); (c, d) % N. pachydermafrom Laurentian Fan (25MC‐A and 10MC, respectively; Thornalley et al., 2018); (e) δ18O from the bivalve Arctica islandica from the Gulf of Maine (Wanamaker et al., 2008); (f) Ostracod Mg/Ca‐based temperature reconstructions based on three spliced sediment cores from Chesapeake Bay (Cronin et al., 2010); (g) temperature reconstructions based on Sr/Ca in brain coral from Bermuda (Goodkin et al., 2008). Bold lines are weighted three‐point smoothing."

See also:

Title: "New Perspectives on 2,000 Years of North Atlantic Climate Change"

https://eos.org/research-spotlights/new-perspectives-on-2000-years-of-north-atlantic-climate-change

Extract: "Historical and natural clues suggest that Earth’s climate underwent small changes over the past 2,000 years, and variations in North Atlantic ocean circulation may have been a key driver. In a new paper, Moffa-Sánchez et al. compile recent advancements in analytical tools used to probe this period of ocean circulation, presenting a comprehensive overview of current knowledge.

The past 2 millennia have seen several centuries-long climate shifts, such as the Medieval Warm Period, followed by the Little Ice Age, which were particularly recorded around the North Atlantic. The North Atlantic is an important climatological region  because of the strong interactions between ocean, atmosphere, and sea ice, as well as the overturning circulation between surface and deep-ocean currents. Scientists have traditionally proposed that changes in this Atlantic Meridional Overturning Circulation played a key role in the observed climate shifts, but this view remains under debate because of the lack of clear evidence."

38
Most people follow the 'KISS' (Keep It Simple Stupid) rule when making decisions, including those made about climate change.  As climate change is complex, this means that most people prefer to think about their own economic wellbeing, like the Russians are doing according to the linked article.  Of course this increases the likelihood that the world will remain on a BAU pathway for decades to come:

Title: "New icebreakers, ports and satellites for Northern Sea Route"

https://thebarentsobserver.com/en/2019/10/new-icebreakers-ports-and-satellites-northern-sea-route

Extract: "Russia has big plans for its Arctic and is in the process of building capacities that will enable it to ship out major volumes of goods from the region.

Nuclear icebreakers, sea ports, support vessels and satellites are our main priority, says Maksim Kulinko, a top representative of Rosatom.

According to Kulinko, Russia will not only build eight nuclear powered icebreakers by year 2035, but also 16 rescue and support ships.

In addition, several major seaports are under development, and a total of 12 new satellites are to be put in orbit, he said in a recent conference on Russian offshore developments."

39
The linked reference discusses yet another positive feedback mechanism (to contribute to a cascade of other positive feedbacks) associated with the projected increase in Arctic rainfall with continued warming:

Rebecca B. Neumann, Colby J. Moorberg,  Jessica D. Lundquist,  Jesse C. Turner,  Mark P. Waldrop,  Jack W. McFarland,  Eugenie S. Euskirchen,  Colin W. Edgar & Merritt R. Turetsky (03 January 2019), "Warming Effects of Spring Rainfall Increase Methane Emissions From Thawing Permafrost" Geophysical Research Letters, https://doi.org/10.1029/2018GL081274

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GL081274

Abstract
Methane emissions regulate the near‐term global warming potential of permafrost thaw, particularly where loss of ice‐rich permafrost converts forest and tundra into wetlands. Northern latitudes are expected to get warmer and wetter, and while there is consensus that warming will increase thaw and methane emissions, effects of increased precipitation are uncertain. At a thawing wetland complex in Interior Alaska, we found that interactions between rain and deep soil temperatures controlled methane emissions. In rainy years, recharge from the watershed rapidly altered wetland soil temperatures, warming the top ~80 cm of soil in spring and summer and cooling it in autumn. When soils were warmed by spring rainfall, methane emissions increased by ~30%. The warm, deep soils early in the growing season likely supported both microbial and plant processes that enhanced emissions. Our study identifies an important and unconsidered role of rain in governing the radiative forcing of thawing permafrost landscapes.

Plain Language Summary
Because the world is getting warmer, permanently frozen ground around the arctic, known as permafrost, is thawing. When permafrost thaws, the ground collapses and sinks. Often a wetland forms within the collapsed area. Conversion of permanently frozen landscapes to wetlands changes the exchange of greenhouse gases between the land and atmosphere, which impacts global temperatures. Wetlands release methane into the atmosphere. Methane is a potent greenhouse gas. The ability of methane to warm the Earth is 32 times stronger than that of carbon dioxide over a period of 100 years. In our study, we found that methane release from a thaw wetland in Interior Alaska was greater in rainy years when rain fell in spring. When spring rainwater entered the wetland, it rapidly warmed wetland soils. Rain has roughly the same temperature as the air, and during springtime in northern regions, the air is warmer than the ground. The microbial and plant processes that generate methane increase with temperature. Therefore, wetland soils, warmed by spring rainfall, supported more methane production and release. Northern regions are expected to receive more rainfall in the future. By warming soils and increasing methane release, this rainfall could increase near‐term global warming associated with permafrost thaw.

40
In about one more month we will say sayonara to Mt. Fuji :'(

41
The linked reference identifies yet another observed mechanism for destabilizing Antarctic ice shelves that is not including in consensus climate models, including in those cited by the SROCC report and/or by DeConto & Pollard:

Karen E. Alley, Ted A. Scambos, Richard B. Alley and Nicholas Holschuh (09 Oct 2019), "Troughs developed in ice-stream shear margins precondition ice shelves for ocean-driven breakup", Science Advances, Vol. 5, no. 10, eaax2215, DOI: 10.1126/sciadv.aax2215

https://advances.sciencemag.org/content/5/10/eaax2215

Abstract
Floating ice shelves of fast-flowing ice streams are prone to rift initiation and calving originating along zones of rapid shearing at their margins. Predicting future ice-shelf destabilization under a warming ocean scenario, with the resultant reduced buttressing, faster ice flow, and sea-level rise, therefore requires an understanding of the processes that thin and weaken these shear margins. Here, we use satellite data to show that high velocity gradients result in surface troughs along the margins of fast-flowing ice streams. These troughs are advected into ice-shelf margins, where the locally thinned ice floats upward to form basal troughs. Buoyant plumes of warm ocean water beneath ice shelves can be focused into these basal troughs, localizing melting and weakening the ice-shelf margins. This implies that major ice sheet drainages are preconditioned for rapid retreat in response to ocean warming.

See also:

Title: "Warm ocean water attacking edges of Antarctica's ice shelves"

https://phys.org/news/2019-10-ocean-edges-antarctica-ice-shelves.html

Extract: "Upside-down "rivers" of warm ocean water are eroding the fractured edges of thick, floating Antarctic ice shelves from below, helping to create conditions that lead to ice-shelf breakup and sea-level rise, according to a new study.

The findings, published today in Science Advances, describe a new process important to the future of Antarctica's ice and the continent's contribution to rising seas. Models and forecasts do not yet account for the newly understood and troubling scenario, which is already underway."

42
...
In summary, Rob DeConto, one of the co-authors of the MICI papers, believes their is limited agreement or evidence for MICI, which may cause a partial collapse of the WAIS in a few centuries.

While the SROCC is a valuable document, and Rob DeConto is an honorable scientist; that does not mean that the SROCC (including portions written by committee's that DeConto was on) has not minimized much of the potential right-tailed risks associated with MICI-type mechanisms in Antarctica (particularly in West Antarctica) in the coming decades (as opposed to centuries), including off the top of my head:

a) DeConto & Pollard assumed that ECS is close to 3C rather than over 5C, and they ignored many mechanisms for cascades of tipping mechanisms associated with the ice-climate feedback mechanisms cited by James Hanson (another honorable scientist).
b) The Southern Ocean has more heat content than assumed by DeConto & Pollard and the upwelling of warm CDW (circumpolar deep water) appears to be occurring more regularly than their models assumed (largely being driven by continuing relatively high velocity westerly winds around the Southern Ocean).
c) The DeConto & Pollard models do not include the bed trough through the Thwaites gateway leading to the Byrd Subglacial Basin, where a large subglacial cavity is growing and through which icebergs from local ice cliff calving events can float-out without grounding.
d) The current rate of increase of radiative forcing is more than a hundred time faster than the during the paleo-periods that DeConto & Pollard conservative calibrated the MICI mechanism against.
e) When DeConto determined the current rate of ice cliff failures for the Jakobshavn Glacier, he divided this observed rate in half to use in his/Pollard's MICI models; while in reality the susceptibility of the Thwaites Glacier to future ice cliff failures may be many time higher than that for Jakobshavn.
f) DeConto & Pollard did not consider effective bipolar mechanisms associated with Arctic freshwater hosing events either from the Beaufort Gyre and/or from Greenland.

I could go on but I would only be repeating points that I have previously made in this thread.

43

The point is that Net Ice-Mass Loss is a very small figure when looking at SMB gain (i.e. snowfall) & Ice Loss (discharge) for the 39 years covered by the study.


Readers who have been following my posts for the past few (six) years know that Eric Rignot has made it clear that:

a) for many decades the SMB for East Antarctic has changed little except for the fact that in the past decade a few extreme precipitation (snowfall) events associated with atmospheric river events have episodically increased the SMB for East Antarctic as illustrated by the Dronning Maud Land events discussed in the two references cited below.

b) While snowfall (and thus SMB) has increased recently in West Antarctica, the added gravitational load from this mass is serving (& will increasing serve to) accelerate glacial ice flow velocities in key marine glaciers in West Antarctica; which in turn is serving to destabilize the WAIS.

The first and second attached images taken from Tsukernik et al:

May 2009 Atmospheric River Event in the Dronning Maud Land
By: Maria Tsukernik, Amanda Lynch, Maya Wei and Irina Gorodetskaya

The first attached image shows the accumulative and per day precipitation in Dronning Maud Land from 1979 to the end of 2011; which indicates exceptional (unusually high) snowfall/accumulation in this area particularly in May of 2009.  The second figure indicates that this (and possibly other subsequent) high precipitation event(s) was/were due to an Atmospheric River event(s) coming from the Indian Ocean tropical region.  While many scientists who have projected low ice mass loss from AIS have grabbed on to this (and possible subsequent) high precipitation event to say that the future accumulation of large amounts of snow in East Antarctica will largely offset the projected future dynamic ice mass loss in West Antarctica, I do not feel good about any such use of this precipitation data as: (a) The atmospheric rivers are not captured in the GCM models used by these reticent researchers; yet they are happy to grab on to field data without a long trend line and which could be a natural fluctuation; and (b) with increasing global warming future atmospheric river events may bring sufficient warm water from the tropics to Antarctica so that the precipitation falls as rain and not snow; which contribute to episodic rapid ice mass loss from any such impacted area.

&

The following reference and abstract provide valuable analysis of weather data from a station in Dronning Maud Land; and provide further insight into the various factors involved in such events. 

Meteorological regimes and accumulation patterns at Utsteinen, Dronning Maud Land, East Antarctica: Analysis of two contrasting years by: I. V. Gorodetskaya, N. P. M. Van Lipzig, M. R. Van den Broeke, A. Mangold,W. Boot, and C. H. Reijmer; JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 1–16, doi:10.1002/jgrd.50177, 2013

Abstract: "Since February 2009, an automatic weather station (AWS) has been operating near Utsteinen Nunatak, north of the Sør Rondane Mountains, in Dronning Maud Land at the ascent to the East Antarctic Plateau. This paper gives an assessment of the meteorological conditions, radiative fluxes, and snow accumulation for the first 2 years of operation, 2009 to 2010, analyzed in terms of meteorological regimes. Three major meteorological regimes— cold katabatic, warm synoptic, and transitional synoptic—are identified using cluster analysis based on five parameters derived from the AWS measurements (wind speed, specific humidity, near-surface temperature inversion, surface pressure, and incoming longwave flux indicative of cloud forcing). For its location, the relatively mild climate at Utsteinen can be explained by the high frequency of synoptic events (observed 41%–48% of the time), and a lack of drainage of cold air from the plateau due to mountain sheltering. During the cold katabatic regime, a strong surface cooling leads to a strong near-surface temperature inversion buildup. A large difference in accumulation is recorded by the AWS for the first 2 years: 235mm water equivalent in 2009 and 27mm water equivalent in 2010. Several large accumulation events during the warm synoptic regime occurring mainly in winter were responsible for the majority of the accumulation in 2009. Mostly, small accumulation events occurred during 2010, frequently followed by snow removal. This interannual variability in snow accumulation at the site is related to the intensity of the local synoptic events as recorded by meteorological regime characteristics."

44

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.

45
Is the powerful methane seep in the Arctic Ocean cited in the linked article a 'canary-in-the-coal-mine' for a possible rapid acceleration of methane emissions from such seeps with continued global warming for the next few decades?

Title: "Russian scientists find 'most powerful' ever methane seep in Arctic Ocean"

https://www.telegraph.co.uk/news/2019/10/08/russian-scientists-find-powerful-ever-methane-seep-arctic-ocean/

Extract: "A research expedition from the Tomsk polytechnic university found the seep, as methane leaks are known, east of Bennett Island in the East Siberian Sea, where its violent bubbles seemed to make the water “boil” over an area of 50 square feet.

The concentration of methane in the air there was up to 16 parts per million, more than nine times higher than the atmospheric average.

A recent Russian study found that the thawing of underwater permafrost has doubled in the past three decades, reaching 18 centimetres a year. One of the consequences has been massive releases of methane from the seafloor, including from hydrates, ice-like formations of solid methane that can explode into gas if they are destabilised."

46
...
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.

47

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.

48
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."

49
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.

50

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.

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