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Messages - AbruptSLR

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1
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 14, 2020, 10:07:18 PM »
The linked reference indicates that most of the accelerating ice mass loss from the GIS in recent years has come from discharge and calving from the ice face of marine terminating glaciers.

King, M.D., Howat, I.M., Candela, S.G. et al. Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat. Commun Earth Environ 1, 1 (2020). https://doi.org/10.1038/s43247-020-0001-2

https://www.nature.com/articles/s43247-020-0001-2

Abstract: "The Greenland Ice Sheet is losing mass at accelerated rates in the 21st century, making it the largest single contributor to rising sea levels. Faster flow of outlet glaciers has substantially contributed to this loss, with the cause of speedup, and potential for future change, uncertain. Here we combine more than three decades of remotely sensed observational products of outlet glacier velocity, elevation, and front position changes over the full ice sheet. We compare decadal variability in discharge and calving front position and find that increased glacier discharge was due almost entirely to the retreat of glacier fronts, rather than inland ice sheet processes, with a remarkably consistent speedup of 4–5% per km of retreat across the ice sheet. We show that widespread retreat between 2000 and 2005 resulted in a step-increase in discharge and a switch to a new dynamic state of sustained mass loss that would persist even under a decline in surface melt."

2
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 14, 2020, 03:51:53 PM »
The linked reference indicates that CMIP6 has a lot of uncertainty related to its Antarctic Sea Ice Area, SIA, change with global warming both between ensemble models and between forcing scenarios; which is not a good thing.  That said the attached Figure 4 from the reference does indicate a marked decrease (from current conditions) in February Antarctic SIA by 2035, while the Arctic SIA may be seasonally ice-free in September of 2035; and this pattern could trigger a marked increase in bipolar seesaw mechanisms circa 2035:

Lettie A. Roach et al. (17 April 2020), "Antarctic Sea Ice Area in CMIP6", Geophysical Research Letters, Volume 47, Issue 9, https://doi.org/10.1029/2019GL086729

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL086729

Abstract
Fully coupled climate models have long shown a wide range of Antarctic sea ice states and evolution over the satellite era. Here, we present a high‐level evaluation of Antarctic sea ice in 40 models from the most recent phase of the Coupled Model Intercomparison Project (CMIP6). Many models capture key characteristics of the mean seasonal cycle of sea ice area (SIA), but some simulate implausible historical mean states compared to satellite observations, leading to large intermodel spread. Summer SIA is consistently biased low across the ensemble. Compared to the previous model generation (CMIP5), the intermodel spread in winter and summer SIA has reduced, and the regional distribution of sea ice concentration has improved. Over 1979–2018, many models simulate strong negative trends in SIA concurrently with stronger‐than‐observed trends in global mean surface temperature (GMST). By the end of the 21st century, models project clear differences in sea ice between forcing scenarios.

Plain Language Summary
Coupled climate models are complex computer programs that simulate the interaction of the atmosphere, ocean, land surface, and cryosphere. An important feature of the Southern Ocean is its sea ice cover, which typically expands in winter to cover an area comparable to that of Russia. Climate models have shown very different amounts of Antarctic sea ice coverage and very different trajectories of sea ice change in response to expected greenhouse gas emissions. This year, new coupled climate models released under the Coupled Model Intercomparison Project (CMIP6) will form the basis of the next IPCC assessment report. Here, we compare output from those models to satellite observations of the areal coverage of sea ice. As a whole, the models successfully capture some elements of the observed seasonal cycle of sea ice but underestimate the summer minimum sea ice area. Compared to results from the previous model generation (CMIP5), the range across models has reduced, and the location of sea ice agrees better with observations. Models project sea ice loss over the 21st century in all scenarios, but confidence in the rate of loss is limited, as most models show stronger global warming trends than observed over the recent historical period.


Caption: "Figure 4. Antarctic SIA time series from 1950 to 2100 in February (b,d) and September (a,c). Upper plots (a,b) show the r1-ensemble-means for historical simulations and mid-range forcing scenarios: SRESA1B for CMIP3 (blue), RCP45 for CMIP5 (red) and SSP245 for CMIP6 (green). The mean plus or minus one standard deviation across the multi-model ensemble is shown as faint lines or shading corresponding to those colours. Lower plots (c,d) show historical simulations and three scenarios for the CMIP6 r1-ensemble. Three observational products are shown in black. Thick coloured lines denote multi-model means and faint lines show individual model trajectories. The number of models included in each mean is noted in brackets in the legend."

We should all remember that the fact that Antarctic sea ice extents have recently been, and currently are, higher than projected by CMIP6, this actual relatively high sea ice extent has been, and still is, promoting the upwelling of warm CDW; however, as indicated by the attached image, if the Antarctic sea ice extent actually decreases by 2035, the relative freshening of the Southern Ocean surface water will still continue to promote accelerated upwelling of warm CDW even with markedly reduced sea ice extent.

3
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 14, 2020, 09:31:44 AM »
As it is possible that we could reach conditions similar to those during the Mid-Pliocene, MP, before the end of this century, I provide the linked reference that discusses both ESS and ECS for the MP as simulated by CCSM4, CESM1.2 and CESM2:

Ran Feng et al. (2020), "Increasing Earth System Sensitivity in mid-Pliocene simulations from CCSM4 to CESM2", JAMES, https://doi.org/10.1029/2019MS002033

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019MS002033

Abstract: "Three new equilibrium Mid-Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4), Community Earth System Model version 1.2 (CESM1.2), and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2. These simulations reveal amplified MP climate change relative to preindustrial going from CCSM4 to CESM2, seen in global mean and polar amplification of surface warming, sea ice reduction in both Arctic and Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from both enhanced Earth System Sensitivity (ESS) and Equilibrium Climate Sensitivity (ECS) to CO forcing. ESS is amplified by up to 70% in CCSM4, and up to 100% in CESM1.2 and CESM2 relative to ECSs of respective models. Simulations also agree on the strengthened Atlantic Meridional Overturning Circulation, but disagree on several other climate metrics. Compared to preindustrial, CCSM4 features small increase in both low and high cloud cover and no change in the mean climate state of the equatorial Pacific. Whereas, both CESM1.2 and 2 show reduction of cloud cover at all heights, and an anomalous El Nino-like state of the equatorial Pacific. The performances of MP simulations are assessed with a new compilation of paleo-observations of sea surface temperature (SST). CESM1.2 and 2 show better skills than CCSM4 in simulating MP global mean warming and amplified SST warming in the northern middle and high latitudes, supporting the amplified ESS compared to the CCSM4."

4
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 14, 2020, 01:10:23 AM »
The linked reference indicates that the future release of ice nucleating particles (INPs) into the atmosphere associated with future permafrost thawing could promote Arctic cloud formation; which would accelerate Arctic Amplification.

Jessie M Creamean et al. (30 July 2020), "Thawing permafrost: an overlooked source of seeds for Arctic cloud formation", Environmental Research Letters, Volume 15, Number 8, https://doi.org/10.1088/1748-9326/ab87d3

https://iopscience.iop.org/article/10.1088/1748-9326/ab87d3

Abstract
As the Arctic warms at twice the global rate, radiative feedbacks from clouds will lead to compounding impacts on the surface energy budget that affect both regional and global weather, and climate. In a future warmer world, the Arctic is projected to become cloudier. However, the formation and evolution of Arctic clouds remain highly uncertain in part due to a limited understanding of current and future sources of ice nucleating particles (INPs). In particular, the sources and abundance of biologically-derived INPs are poorly characterized, yet they may be pivotal for cloud ice formation, especially at temperatures in which Arctic mixed-phase clouds (AMPCs) persist (i.e. >−15 °C). Here, we show for the first time that permafrost is a remarkably rich source of biologically-derived INPs, both heat labile (probably proteinaceous) and other organic INPs of biomolecular origin (41%–100% and 99%–100% of the total INPs, respectively). INP concentrations in 1,000 to 30,000 year old permafrost were comparable to the most active of other Arctic and midlatitude soil sources (up to 1010 INPs per gram of soil). Thawing of permafrost—which promotes metabolic activity in microbes—and subsequent mobilization of those soils directly into the atmosphere or into lakes, rivers, and the ocean, suggests the intriguing possibility that increasing emissions of INPs from this hitherto overlooked reservoir could be widespread, and, in time, greatly impact Arctic cloud cloud glaciation and radiative properties. This discovery is timely given the rapidly-thawing permafrost in Alaska and across Earth's high latitudes. Since permafrost covers 15% of Northern Hemisphere land, this novel and prevalent INP source may become central to predictions of aerosol-cloud-precipitation interactions in AMPCs.

5
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 14, 2020, 12:55:05 AM »
The linked reference discusses a newly identified positive feedback mechanism for sea ice reduction in the Eastern Arctic Ocean:

Igor V. Polyakov et al. (04 August 2020), "Intensification of Near‐Surface Currents and Shear in the Eastern Arctic Ocean", Geophysical Research Letters, https://doi.org/10.1029/2020GL089469

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL089469?af=R
https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2020GL089469

Abstract
A 15‐year (2004‐2018) record of mooring observations from the upper 50m ocean in the eastern Eurasian Basin reveals increased current speeds and shear, associated with an increasing coupling between wind, ice, and oceanic currents and their vertical shear over 2004‐2018, particularly in summer. Substantial increases in both current speeds and shears in the upper 50 m are dominated by a 2 times amplification of currents in the semidiurnal band, which includes tides and wind‐forced near‐inertial oscillations. For the first time the strengthened upper ocean currents and shear are observed to coincide with weakening stratification. This coupling links the Atlantic Water heat to the sea ice, a consequence of which would be reducing regional sea ice volume. These results point to a new positive feedback mechanism in which reduced sea ice extent facilitates more energetic inertial oscillations and associated upper‐ocean shear leading in enhanced ventilation of the Atlantic water.

Plain Language Summary
Previous studies demonstrated that in recent years strong density gradients above the warm and salty intermediate (~150‐900m) water of Atlantic origin in the eastern Arctic Ocean have weakened, allowing stronger upward transport heat to the bottom of the sea ice. Using mooring observations, we show that this weakening of stratification has been accompanied by stronger upper‐ocean currents and their vertical shear and by increasing coupling between the wind and sea ice with upper ocean currents and shear. Most of this enhanced energy and shear is in the semidiurnal band, which includes baroclinic tides and wind‐driven inertial oscillations. The increased shear together with the weakening stratification indicate a greater potential for shear‐driven turbulent mixing. We propose a new process, the ice/ocean‐heat positive feedback, that can accelerate current sea ice loss and impede the rate of recovery of eastern Arctic sea ice even if large‐scale climate warming conditions relax.

6
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 13, 2020, 08:24:50 PM »
The linked reference, and associated article, indicate that tropical forests are likely to transitions from carbon sinks to carbon sources much sooner than previously assumed by consensus climate science; which implies that ECS by 2100 may be greater than previously assumed:

Nottingham, A.T., Meir, P., Velasquez, E. et al. Soil carbon loss by experimental warming in a tropical forest. Nature 584, 234–237 (2020). https://doi.org/10.1038/s41586-020-2566-4

https://www.nature.com/articles/s41586-020-2566-4

Abstract: "Tropical soils contain one-third of the carbon stored in soils globally, so destabilization of soil organic matter caused by the warming predicted for tropical regions this century could accelerate climate change by releasing additional carbon dioxide (CO2) to the atmosphere. Theory predicts that warming should cause only modest carbon loss from tropical soils relative to those at higher latitudes, but there have been no warming experiments in tropical forests to test this. Here we show that in situ experimental warming of a lowland tropical forest soil on Barro Colorado Island, Panama, caused an unexpectedly large increase in soil CO2 emissions. Two years of warming of the whole soil profile by four degrees Celsius increased CO2 emissions by 55 per cent compared to soils at ambient temperature. The additional CO2 originated from heterotrophic rather than autotrophic sources, and equated to a loss of 8.2 ± 4.2 (one standard error) tonnes of carbon per hectare per year from the breakdown of soil organic matter. During this time, we detected no acclimation of respiration rates, no thermal compensation or change in the temperature sensitivity of enzyme activities, and no change in microbial carbon-use efficiency. These results demonstrate that soil carbon in tropical forests is highly sensitive to warming, creating a potentially substantial positive feedback to climate change."

See also:

Title: "Carbon dioxide loss from tropical soils increases on warming" by Eric Davidson

https://www.nature.com/articles/d41586-020-02266-9

Extract: "Besides the clear take-home message that the responses of soil respiration processes under climate change should not be represented in Earth-system models only by simple Q10 or Arrhenius functions, Nottingham and co-workers’ study adds to recently accumulating evidence that tropical forests are unlikely to continue indefinitely to be carbon sinks as the world warms. Tropical soil carbon does not receive as much attention as do the large and vulnerable soil-carbon stocks at high latitudes, which pose major concerns as a potential source of positive feedback to climate change. But tropical-forest soils also contain substantial carbon stores that might be more vulnerable to loss in a warming world than was previously recognized."

7
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 13, 2020, 11:44:43 AM »
The linked reference indicates that CMIP6 has a lot of uncertainty related to its Antarctic Sea Ice Area, SIA, change with global warming both between ensemble models and between forcing scenarios; which is not a good thing.  That said the attached Figure 4 from the reference does indicate a marked decrease (from current conditions) in February Antarctic SIA by 2035, while the Arctic SIA may be seasonally ice-free in September of 2035; and this pattern could trigger a marked increase in bipolar seesaw mechanisms circa 2035:

Lettie A. Roach et al. (17 April 2020), "Antarctic Sea Ice Area in CMIP6", Geophysical Research Letters, Volume 47, Issue 9, https://doi.org/10.1029/2019GL086729

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL086729

Abstract
Fully coupled climate models have long shown a wide range of Antarctic sea ice states and evolution over the satellite era. Here, we present a high‐level evaluation of Antarctic sea ice in 40 models from the most recent phase of the Coupled Model Intercomparison Project (CMIP6). Many models capture key characteristics of the mean seasonal cycle of sea ice area (SIA), but some simulate implausible historical mean states compared to satellite observations, leading to large intermodel spread. Summer SIA is consistently biased low across the ensemble. Compared to the previous model generation (CMIP5), the intermodel spread in winter and summer SIA has reduced, and the regional distribution of sea ice concentration has improved. Over 1979–2018, many models simulate strong negative trends in SIA concurrently with stronger‐than‐observed trends in global mean surface temperature (GMST). By the end of the 21st century, models project clear differences in sea ice between forcing scenarios.

Plain Language Summary
Coupled climate models are complex computer programs that simulate the interaction of the atmosphere, ocean, land surface, and cryosphere. An important feature of the Southern Ocean is its sea ice cover, which typically expands in winter to cover an area comparable to that of Russia. Climate models have shown very different amounts of Antarctic sea ice coverage and very different trajectories of sea ice change in response to expected greenhouse gas emissions. This year, new coupled climate models released under the Coupled Model Intercomparison Project (CMIP6) will form the basis of the next IPCC assessment report. Here, we compare output from those models to satellite observations of the areal coverage of sea ice. As a whole, the models successfully capture some elements of the observed seasonal cycle of sea ice but underestimate the summer minimum sea ice area. Compared to results from the previous model generation (CMIP5), the range across models has reduced, and the location of sea ice agrees better with observations. Models project sea ice loss over the 21st century in all scenarios, but confidence in the rate of loss is limited, as most models show stronger global warming trends than observed over the recent historical period.


Caption: "Figure 4. Antarctic SIA time series from 1950 to 2100 in February (b,d) and September (a,c). Upper plots (a,b) show the r1-ensemble-means for historical simulations and mid-range forcing scenarios: SRESA1B for CMIP3 (blue), RCP45 for CMIP5 (red) and SSP245 for CMIP6 (green). The mean plus or minus one standard deviation across the multi-model ensemble is shown as faint lines or shading corresponding to those colours. Lower plots (c,d) show historical simulations and three scenarios for the CMIP6 r1-ensemble. Three observational products are shown in black. Thick coloured lines denote multi-model means and faint lines show individual model trajectories. The number of models included in each mean is noted in brackets in the legend."

8
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 13, 2020, 11:17:46 AM »
...
One study published in 2000 showed mangrove plant remains found on the seabed recorded a catastrophic 16-metre sea level rise over several hundred years (about half a metre each decade). This event, known as Meltwater Pulse-1A, flooded the Sunda Shelf.

Our latest work has modified this story considerably. The new calibration curves reveal this extreme phase of sea level rise actually began 14,640 years ago and lasted just 160 years.

This equates to a staggering one-metre rise each decade – a sobering lesson for the future, considering the current much lower projected changes for the end of this century.

To be clearer, during Meltwater Pulse-1A SLR did not occur at a uniform rate but rather it occurred in a series of multi-meter/decade bursts with relatively slow rates of SLR in between.

9
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 12, 2020, 05:28:23 PM »
Maybe information entropy modeling methodology should be used in ESMs as it seems to improve ENSO forecasts as discussed in the linked reference:

Jun Meng et al. (January 7, 2020), "Complexity-based approach for El Niño magnitude forecasting before the spring predictability barrier", PNAS, 117 (1) 177- 183,  https://doi.org/10.1073/pnas.1917007117

https://www.pnas.org/content/117/1/177

Abstract: "Although El Niño events characterized by anomalous episodic warmings of the eastern equatorial Pacific can trigger disasters in various parts of the globe, reliable forecasts of their magnitude are still limited to about 6 mo ahead. A significant extension of this prewarning time would be instrumental for mitigating some of the worst damages. Here we introduce an approach relying on information entropy, which achieves some doubling of the prewarning time. The approach is based on our finding that the entropy in one calendar year exhibits a strong correlation with the magnitude of an El Niño that starts in the following year and thus allows us to forecast the onset and the magnitude of an El Niño event 1 y in advance."

See also:

Wang, X., Slawinska, J. & Giannakis, D. Extended-range statistical ENSO prediction through operator-theoretic techniques for nonlinear dynamics. Sci Rep 10, 2636 (2020). https://doi.org/10.1038/s41598-020-59128-7

https://www.nature.com/articles/s41598-020-59128-7


10
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 12, 2020, 04:17:15 PM »
With a hat-tip to kassy, the linked reference concludes that:

"The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable."

Gustaf Hugelius et al. (2020), "Large stocks of peatland carbon and nitrogen are vulnerable to permafrost thaw", PNAS, doi: 10.1073/pnas.1916387117

https://www.pnas.org/content/pnas/early/2020/08/04/1916387117.full.pdf

Abstract: "Northern peatlands have accumulated large stocks of organic carbon (C) and nitrogen (N), but their spatial distribution and vulnerability to climate warming remain uncertain. Here, we used machine-learning techniques with extensive peat core data (n > 7,000) to create observation-based maps of northern peatland C and N stocks, and to assess their response to warming and permafrost thaw. We estimate that northern peatlands cover 3.7 ± 0.5 million km2 and store 415 ± 150 Pg C and 10 ± 7 Pg N. Nearly half of the peatland area and peat C stocks are permafrost affected. Using modeled global warming stabilization scenarios (from 1.5 to 6 °C warming), we project that the current sink of atmospheric C (0.10 ± 0.02 Pg C·y−1 ) in northern peatlands will shift to a C source as 0.8 to 1.9 million km2 of permafrost-affected peatlands thaw. The projected thaw would cause peatland greenhouse gas emissions equal to ∼1% of anthropogenic radiative forcing in this century. The main forcing is from methane emissions (0.7 to 3 Pg cumulative CH4-C) with smaller carbon dioxide forcing (1 to 2 Pg CO2-C) and minor nitrous oxide losses. We project that initial CO2-C losses reverse after ∼200 y, as warming strengthens peatland C-sinks. We project substantial, but highly uncertain, additional losses of peat into fluvial systems of 10 to 30 Pg C and 0.4 to 0.9 Pg N. The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable."

11
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 12, 2020, 08:05:08 AM »
The linked website/article indicates that HadGEM3-GC3.1 has a TCR of about 2.7C and an ECS of about 5.5C; while E3SMv1 has a TCR of 2.93C and an ECS of 5.3C.  As HadGEM3-GC3.1 does a better job of modeling Arctic melt ponds on sea ice and E3SMv1 does a better job of assessing the impacts of freshwater on the AMOC; I am concerned that E3SMv1's TCR value of 2.93C may be more correct while HadGEM3-GC3.1's ECS value of 5.5C maybe more correct.  If so them the CMIP6 'Wolf Pack' may be erring on the side of least drama with regard to climate sensitivity this century.

Title: "New flagship climate models - UKESM1 and HadGEM3-GC3.1"

https://www.metoffice.gov.uk/research/approach/modelling-systems/new-flagship-climate-models

Extract: "Analysis shows the climate sensitivity of the models is high. For both models the Transient Climate Sensitivity (TCR) is about 2.7 °C, while the Equilibrium Climate Sensitivity (ECS)  is about 5.4°C for UKESM1 and about 5.5°C for GC3.1. Future projections using the new models are in progress. When these have been analysed, we will have a better understanding of how the climate sensitivity affects future warming and associated impacts."

12
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 12, 2020, 07:25:08 AM »
The linked article describes nine new studies that the DOE has funded in order to improve the E3SMv4 model.

Title: "DOE announces $7m for Energy Exascale Earth System Model"

https://www.datacenterdynamics.com/en/news/doe-announces-7m-energy-exascale-earth-system-model/

Extract: "The Department of Energy will fund nine studies designed to help improve the Energy Exascale Earth System Model program.
...
The studies funded include:

1. Improving the Capabilities and Computational Efficiency of the RTE+RRTMGP Radiation Code at The Trustees of Columbia University in the City of New York

2. Improving the initial state of biogeochemical components in Earth System Models at Regents of the University of California, Irvine

3. Improving the Parameterization of Cloud and Rain Microphysics in E3SM using a Novel ObservationallyConstrained Bayesian Approach at University Corporation for Atmospheric Research, Boulder, CO (Collaborative)

4. Interactions between land use, fires and dust as drivers of global climate change at Regents of the University of California, Irvine

5. Efficiently resolving the terrestrial-aquatic interface in E3SM with sub-grid methods to improve coastal simulations at University of Notre Dame du Lac

6. Investigating Cloud Feedbacks in Earth System Models at University of Miami (School of Marine and Atmospheric Science)

7. Ocean physical-biogeochemical interactions in the CMIP6 and E3SM Earth System Models at Georgia Tech Research Corporation

8. Evaluating the influence of plants on hydrologic cycling: Quantifying and validating the role of plant processes and stomatal conductance at University of Washington

9. The Role of Deep Convection and Large-scale Circulation in Driving Model Spread in Low Cloud Feedback and Equilibrium Climate Sensitivity at Regents of the University of California, Los Angeles"

13
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 07:39:50 PM »
Quote
A simulation of the future using the same model indicates that the Arctic may become sea ice-free by 2035."
Does this mean for a few days in September, or all year around? I would guess the former, but would not be surprised by the later.

While it means the former, still this period could well be sufficient to induce a reversal of the Beaufort Gyre of sufficient duration to create a significant freshwater hosing event with regard to both the Arctic Ocean halocline and the AMOC; which is likely not evaluated by HadGEM3 in CMIP6.

While consensus climate science seems to take the position that extraordinary (i.e. long tail risk) climate claims require extraordinary proof, it seems to me that the consensus climate science position that their models can safely ignore potential freshwater hosing events is an extraordinary claim that requires extraordinary proof from consensus climate scientists.

14
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 07:25:05 PM »
The linked video discusses how E3SM uses MPAS-Analysis and discusses how this helps to analysis the pattern effect of both SSTA, and sea ice, patterns.  For those only interested in findings you can skip ahead to minutes 9:25 to 14:50.

Title: "An Introduction to MPAS-Analysis"

https://www.youtube.com/watch?v=TCEzv7Vr3vc&feature=youtu.be

See also the pdf at:

https://e3sm.org/wp-content/uploads/2020/08/200806_Z_Davis_opt.pdf

15
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 07:00:25 PM »
The linked reference (& associated article) indicates that the Arctic is both warming faster and losing sea ice faster than projected by consensus climate models, and that Arctic Amplification is indeed currently proceeding at a rate indicating a present era of abrupt Arctic climate change.

Eystein Jansen et al. Past perspectives on the present era of abrupt Arctic climate change, Nature Climate Change (2020). DOI: 10.1038/s41558-020-0860-7

https://www.nature.com/articles/s41558-020-0860-7

Abstract: "Abrupt climate change is a striking feature of many climate records, particularly the warming events in Greenland ice cores. These abrupt and high-amplitude events were tightly coupled to rapid sea-ice retreat in the North Atlantic and Nordic Seas, and observational evidence shows they had global repercussions. In the present-day Arctic, sea-ice loss is also key to ongoing warming. This Perspective uses observations and climate models to place contemporary Arctic change into the context of past abrupt Greenland warmings. We find that warming rates similar to or higher than modern trends have only occurred during past abrupt glacial episodes. We argue that the Arctic is currently experiencing an abrupt climate change event, and that climate models underestimate this ongoing warming."

See also:

Title: "New study warns: We have underestimated the pace at which the Arctic is melting"

https://phys.org/news/2020-08-underestimated-pace-arctic.html

Extract: "Arctic sea ice is melting more quickly than once assumed. Today's climate models have yet to incorporate the steep rise in temperatures that have occurred over the past 40 years. This, according to a new study by researchers at the University of Copenhagen and other institutions.

Temperatures in the Arctic Ocean between Canada, Russia and Europe are warming faster than researchers' climate models have been able to predict.

Over the past 40 years, temperatures have risen by one degree every decade, and even more so over the Barents Sea and around Norway's Svalbard archipelago, where they have increased by 1.5 degrees per decade throughout the period."

16
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 04:34:18 PM »
The linked article indicates that a CMIP6 version of HadGEM3 (the UK's most advanced model) indicates that the Arctic may become seasonally sea ice free by 2035 (see also the associated attached image w.r.t. ECS).  If so, this would greatly increase the probability of a freshwater hosing event from the Beaufort Gyre, which, could likely trigger a bipolar seesaw cascade of subsequent freshwater hosing event such as: hosing from key ASE marine glaciers triggering hosing from key GIS marine terminating glaciers, triggering key AIS marine glaciers etc.  If so this might lead to 4 to 5m of SLR circa 2100.

Title: "Past evidence supports complete loss of Arctic sea ice by 2035"

https://phys.org/news/2020-08-evidence-loss-arctic-sea-ice.html

Extract: "During spring and early summer, shallow pools of water form on the surface of Arctic sea-ice. These 'melt ponds' are important for how much sunlight is absorbed by the ice and how much is reflected back into space. The new Hadley Centre model is the UK's most advanced physical representation of the Earth's climate and a critical tool for climate research and incorporates sea-ice and melt ponds.

Using the model to look at Arctic sea ice during the last interglacial, the team concludes that the impact of intense springtime sunshine created many melt ponds, which played a crucial role in sea-ice melt. A simulation of the future using the same model indicates that the Arctic may become sea ice-free by 2035."

&

Guarino, M., Sime, L.C., Schröeder, D. et al. Sea-ice-free Arctic during the Last Interglacial supports fast future loss. Nat. Clim. Chang. (2020). doi.org/10.1038/s41558-020-0865-2

https://www.nature.com/articles/s41558-020-0865-2

Abstract: "The Last Interglacial (LIG), a warmer period 130,000–116,000 years before present, is a potential analogue for future climate change. Stronger LIG summertime insolation at high northern latitudes drove Arctic land summer temperatures 4–5 °C higher than in the pre-industrial era. Climate model simulations have previously failed to capture these elevated temperatures, possibly because they were unable to correctly capture LIG sea-ice changes. Here, we show that the latest version of the fully coupled UK Hadley Center climate model (HadGEM3) simulates a more accurate Arctic LIG climate, including elevated temperatures. Improved model physics, including a sophisticated sea-ice melt-pond scheme, result in a complete simulated loss of Arctic sea ice in summer during the LIG, which has yet to be simulated in past generations of models. This ice-free Arctic yields a compelling solution to the long-standing puzzle of what drove LIG Arctic warmth and supports a fast retreat of future Arctic summer sea ice."

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 04:28:23 PM »
The linked reference indicates that from 1994 to 2018, Antarctic ice shelves have lost enough ice mass to fill the Grand Canyon.

Adusumilli, S., Fricker, H.A., Medley, B. et al. Interannual variations in meltwater input to the Southern Ocean from Antarctic ice shelves. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0616-z

https://www.nature.com/articles/s41561-020-0616-z

Abstract: "Ocean-driven basal melting of Antarctica’s floating ice shelves accounts for about half of their mass loss in steady state, where gains in ice-shelf mass are balanced by losses. Ice-shelf thickness changes driven by varying basal melt rates modulate mass loss from the grounded ice sheet and its contribution to sea level, and the changing meltwater fluxes influence climate processes in the Southern Ocean. Existing continent-wide melt-rate datasets have no temporal variability, introducing uncertainties in sea level and climate projections. Here, we combine surface height data from satellite radar altimeters with satellite-derived ice velocities and a new model of firn-layer evolution to generate a high-resolution map of time-averaged (2010–2018) basal melt rates and time series (1994–2018) of meltwater fluxes for most ice shelves. Total basal meltwater flux in 1994 (1,090 ± 150 Gt yr–1) was similar to the steady-state value (1,100 ± 60 Gt yr–1), but increased to 1,570 ± 140 Gt yr–1 in 2009, followed by a decline to 1,160 ± 150 Gt yr–1 in 2018. For the four largest ‘cold-water’ ice shelves, we partition meltwater fluxes into deep and shallow sources to reveal distinct signatures of temporal variability, providing insights into climate forcing of basal melting and the impact of this melting on the Southern Ocean."

18
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 11, 2020, 01:57:44 AM »
As most current ESMs (including most CMIP6 models) do not adequately account for potential upcoming significant freshwater hosing events, I believe that the first attached image (from the first linked website) is only relevant to current conditions.

Title: "Disequilibrium and the AMOC"

https://www.gfdl.noaa.gov/blog_held/64-disequilibrium-and-the-amoc/

Caption for the first image: "The strength of the Atlantic Meridional Overturning Circulation (AMOC) at 26N , in units of Sverdrups (106 m3/s), plotted against the ratio of the Transient Climate Response to the Equilibrium Climate Sensitivity (TCR/ECS) in a set of coupled atmosphere-ocean climate models developed at GFDL over the past 15 years.  Redrawn from Winton et al 2014. Also shown is the strength of  the AMOC as observed by the RAPID array at 26N,  from McCarthy et al 2015."

Next, I remind readers that the second linked reference finds that part of the E3SM version 1 projected high value of TCR of 2.93C (see the second attached image) is due to its projected slowing of the AMOC; and that E3SMv1 projected an ECS value of 5.3C; which gives a TCR/ECS ratio of 0.553; which is close to the TCR/ECS ratio given in the first image for the AMOC mass flux measured by RAPID.  In my opinion this provides some substantiation of the E3SMv1 projections.

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

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

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

Edit: As Hu et al. (2020) indicates that TCR (but not ECS) increases when the AMOC slows due to freshwater hosing.  Thus, the first image indicates that if the AMOC were to slow due to future freshwater hosing events that TCR would likely increase relatively rapidly.

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 09, 2020, 06:26:02 PM »
The linked reference (& associated article) reminds us that with continued global warming, Super El Nino events will become more frequent; which will more frequently telecommunicate large amounts of atmospheric heat energy from particularly the Central Tropical Pacific to the West Antarctica coastal regions, where it may likely accelerate the risk of hydro-fracturing inducing calving events, say for Thwaites:

Bin Wang, Xiao Luo, Young-Min Yang, Weiyi Sun, Mark A. Cane, Wenju Cai, Sang-Wook Yeh, and  Jian Liu (November 5, 2019), "Historical change of El Niño properties sheds light on future changes of extreme El Niño", PNAS, 116 ,(45), 22512-22517; https://doi.org/10.1073/pnas.1911130116

https://www.pnas.org/content/116/45/22512

Significance
How the magnitude of El Niño will change is of great societal concern, yet it remains largely unknown. Here we show analysis of how changing El Niño properties, due to 20th century climate change, can shed light on changes to the intensity of El Niño in the future. Since the 1970s, El Niño has changed its origination from the eastern Pacific to the western Pacific, along with increased strong El Niño events due to a background warming in the western Pacific warm pool. This suggests the controlling factors that may lead to increased extreme El Niño events in the future. If the observed background changes continue under future anthropogenic forcing, more frequent extreme El Niño events will induce profound socioeconomic consequences.

Abstract
El Niño’s intensity change under anthropogenic warming is of great importance to society, yet current climate models’ projections remain largely uncertain. The current classification of El Niño does not distinguish the strong from the moderate El Niño events, making it difficult to project future change of El Niño’s intensity. Here we classify 33 El Niño events from 1901 to 2017 by cluster analysis of the onset and amplification processes, and the resultant 4 types of El Niño distinguish the strong from the moderate events and the onset from successive events. The 3 categories of El Niño onset exhibit distinct development mechanisms. We find El Niño onset regime has changed from eastern Pacific origin to western Pacific origin with more frequent occurrence of extreme events since the 1970s. This regime change is hypothesized to arise from a background warming in the western Pacific and the associated increased zonal and vertical sea-surface temperature (SST) gradients in the equatorial central Pacific, which reveals a controlling factor that could lead to increased extreme El Niño events in the future. The Coupled Model Intercomparison Project phase 5 (CMIP5) models’ projections demonstrate that both the frequency and intensity of the strong El Niño events will increase significantly if the projected central Pacific zonal SST gradients become enhanced. If the currently observed background changes continue under future anthropogenic forcing, more frequent strong El Niño events are anticipated. The models’ uncertainty in the projected equatorial zonal SST gradients, however, remains a major roadblock for faithful prediction of El Niño’s future changes.

See also:

Title: "Super El Niño events may become more frequent as the climate warms"

https://www.washingtonpost.com/weather/2019/10/22/super-el-nio-events-may-become-more-frequent-climate-warms/

Extract: "Study co-author Mark Cane of Columbia University, who is a pioneer in El Niño forecasting, says computer models have failed to accurately simulate changes in the tropical Pacific Ocean during the past few decades.

He says if the West Pacific heats up faster than the East Pacific, it’ll cause more El Niño events to be centered toward the international date line, rather than farther east."

For what it is worth, I remind readers that currently Extreme El Nino events occur approximately once every 20 years (and that that return period is projected to decrease with continued global warming) and that the last Extreme El Nino event occurred in the 2015-2016 season, so it is reasonable to assume that the next Extreme El Nino event may likely begin roughly in the 2035-2036 season.  Next, I note that taking the current GMSTA (above pre-industrial) of about 1.3C and the 0.38C/decade trend line from the first attached image (from Hansen) and extending this trend line to 2035 gives a GMSTA (above pre-industrial) of about 1.9C.  Next, the second attached image (from a French CMIP6 model in the middle of the 'Wolf Pack') confirms that by 2035, GMSTA (above pre-industrial) will likely be about 1.9C.  Next, the third image (from Brown & Caldeira 2017) shows that the observationally-informed trend line supports that projection that GMSTA (above pre-industrial) will be roughly 1.9C by 2035.  Lastly, the fourth image (of a SIMIP/CMIP6 projection of September Arctic Sea Ice extent) indicates that the Arctic Ocean will be come seasonally ice-free when GMSTA (above pre-industrial) reaches roughly 1.9C (for all SSP scenarios); which may occur by about 2035 (as I just indicated).  Given that a seasonally ice-free Arctic could readily trigger a temporary reversal of the Beaufort Gyre, this is not good news.

Edit:  Furthermore, I note that an Extreme El Nino event in the 2035-2036 ENSO season would push more warm CDW into the ASE (Amundsen Sea Embayment) and would increase the mean regional sea level in the ASE sufficiently to break the ice seal in the Thwaites Gateway that is currently resisting the release of the BSB subglacial meltwater into the zone at the base of the Thwaites Ice Tongue; which might be sufficient to displace the pinned icebergs in this zone; which might trigger a MICI-type of collapse of the Thwaites Glacier as early as January-February 2036.

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 09, 2020, 05:08:58 PM »
For readers who would like access to an overview interactive tool for Thwaites Glacier I provide the following link.  That said this tool does not address any MICI mechanisms or risks:

Title: "Thwaites-Explorer online interactive tool for web browsers"

https://thwaitesglacier.org/education/thwaites-explorer-online-interactive-tool-web-browsers


21
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 09, 2020, 04:27:55 PM »
The linked reference concludes that:

"Calculations show that, maintaining the actual rate of population growth and resource consumption, in particular forest consumption, we have a few decades left before an irreversible collapse of our civilisation (see Fig. 5)."

Bologna, M., Aquino, G. Deforestation and world population sustainability: a quantitative analysis. Sci Rep 10, 7631 (2020). https://doi.org/10.1038/s41598-020-63657-6

https://www.nature.com/articles/s41598-020-63657-6

Abstract: "In this paper we afford a quantitative analysis of the sustainability of current world population growth in relation to the parallel deforestation process adopting a statistical point of view. We consider a simplified model based on a stochastic growth process driven by a continuous time random walk, which depicts the technological evolution of human kind, in conjunction with a deterministic generalised logistic model for humans-forest interaction and we evaluate the probability of avoiding the self-destruction of our civilisation. Based on the current resource consumption rates and best estimate of technological rate growth our study shows that we have very low probability, less than 10% in most optimistic estimate, to survive without facing a catastrophic collapse."

Caption for image: "Average time τ (in years) to reach Dyson value before hitting “no-return” point (success, left) and without meeting Dyson value (failure, right) as function of α and a for β = 170. Plateau region (left panel) where τ ≥ 50 corresponds to diverging τ, i.e. Dyson value not being reached before hitting “no-return” point and therefore failure. Plateau region at τ = 0 (right panel), corresponds to failure not occurring, i.e. success. Parameter a is expressed in Km2 ys−1."

22
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 09, 2020, 04:10:19 PM »
...
Ah I see why you used only ten years of the data .
Using the other six years data gives a decline of about 15 % .
Quote
The strength of the AMOC at 26.5°N has been observed by the RAPID array since 2004 and has an average of about 17.0 Sv (1 Sv ≡106 m3/s). The time series shows variability on all timescales, including a weakening of 15% over the length of the record (Smeed et al., 2018). Many climate models display variability of the AMOC on decadal and multidecadal timescales (e.g., Muir & Fedorov, 2015, 2017). Most models also predict a gradual weakening of the AMOC over the 21st century in response to anthropogenic forcing (e.g., Cheng et al., 2013; Weaver et al., 2012). In fact, Dima and Lohmann (2010), Rahmstorf et al. (2015), and Caesar et al. (2018) argue that such a gradual slowdown has already started and is noticeable in proxy records of the AMOC from the midtwentieth century.
Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis
W. Weijer  W. Cheng  S. S. Drijfhout  A. V. Fedorov  A. Hu  L. C. Jackson  W. Liu  E. L. McDonagh  J. V. Mecking  J. Zhang
First published: 24 July 2019
https://doi.org/10.1029/2019JC015083

I concur that a ten-year period is not sufficient to establish a trend.

23
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 08, 2020, 11:34:40 PM »
Here, I note that MIS-11c (394.7 to 424.2 kya) is a relatively good analogy for estimating our current climate sensitivity, and that per the linked reference, during that period the south Greenland Ice Sheet, GIS, crossed a stability threshold when GMSTA was only slightly warmer than present (∼0.5 ± 1.6 °C).  Finally, I note that a freshwater hosing event from the south GIS could help to trigger the collapse of Antarctic marine glaciers via the bipolar seesaw mechanisms.

Nil Irvalı, Eirik V. Galaasen, Ulysses S. Ninnemann, Yair Rosenthal, Andreas Born, and Helga (Kikki) F. Kleiven (January 7, 2020), "A low climate threshold for south Greenland Ice Sheet demise during the Late Pleistocene", PNAS  117 (1) 190-195; https://doi.org/10.1073/pnas.1911902116

https://www.pnas.org/content/117/1/190

Significance
Understanding how warmer climates affected Greenland in the past helps in determining how future warming will impact it. The Greenland Ice Sheet (GIS) has retreated during recent interglacials, suggesting a critical survival threshold within a few degrees of modern temperatures. Defining this temperature threshold requires records of the past climates responsible for GIS demise. Using microfossil temperature reconstructions, we show that the current interglacial is unusually moderate and that all 4 previous interglacials were warmer than present near Greenland. Both magnitude and duration of past warmth were important influences on the ice sheet. Notably, the critical temperature threshold for past GIS decay will likely be surpassed this century. The duration for which this threshold is exceeded will determine Greenland’s fate.

Abstract
The Greenland Ice Sheet (GIS) has been losing mass at an accelerating rate over the recent decades. Models suggest a possible temperature threshold between 0.8 and 3.2 °C, beyond which GIS decline becomes irreversible. The duration of warmth above a given threshold is also a critical determinant for GIS survival, underlining the role of ocean warming, as its inertia prolongs warmth and triggers longer-term feedbacks. The exact point at which these feedbacks are triggered remains equivocal. Late Pleistocene interglacials provide potential case examples for constraining the past response of the GIS to a range of climate states, including conditions warmer than present. However, little is known about the magnitude and duration of warming near Greenland during these periods. Using high-resolution multiproxy surface ocean climate records off southern Greenland, we show that the previous 4 interglacials over the last ∼450 ka all reached warmer than present climate conditions and exceeded the modeled temperature threshold for GIS collapse but by different magnitudes and durations. Complete deglaciation of the southern GIS in Marine Isotope Stage 11c (MIS 11c; 394.7 to 424.2 ka) occurred under climates only slightly warmer than present (∼0.5 ± 1.6 °C), placing the temperature threshold for major GIS retreat in the lower end of model estimates and within projections for this century.

Extract: "A low GIS threshold increases the plausibility that one of the most economically impactful long-term consequences of warming could be passed well below the 2 °C threshold and may already be unavoidable if even modestly warmer conditions were to persist long enough."

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 08, 2020, 10:39:07 PM »
New Study Shows Retreat of East Antarctic Ice Sheet During Previous Warm Periods
https://phys.org/news/2020-07-retreat-east-antarctic-ice-sheet.html

Questions about the stability of the East Antarctic Ice Sheet are a major source of uncertainty in estimates of how much sea level will rise as the Earth continues to warm. For decades, scientists thought the East Antarctic Ice Sheet had remained stable for millions of years, but recent studies have begun to cast doubt on this idea. Now, researchers at UC Santa Cruz have reported new evidence of substantial ice loss from East Antarctica during an interglacial warm period about 400,000 years ago.

The study, published July 22 in Nature, focused on the Wilkes Basin, one of several bowl-like basins at the edges of the ice sheet that are considered vulnerable to melting because the ice rests on land that is below sea level. The Wilkes Basin currently holds enough ice to raise sea level by 3 to 4 meters (10 to 13 feet).

"Our data shows that the grounding line in the Wilkes Basin retreated 700 kilometers [435 miles] inland during one of the last really warm interglacials, when global temperatures were 1 to 2 degrees Celsius warmer than now," Blackburn said. "That probably contributed 3 to 4 meters to global sea level rise, with Greenland and West Antarctica together contributing another 10 meters."


In other words, a period of global warming comparable to what is expected under current scenarios for manmade greenhouse gas emissions resulted in an increase in sea level of around 13 meters (43 feet). Of course, this wouldn't happen all at once—it takes time for that much ice to melt.

"We've opened the freezer door, but that block of ice is still cold and it's not going anywhere in the short term," Blackburn said. "To understand what will happen over longer time scales, we need to see what happened under comparable conditions in the past."

... What that history suggests is that the U-234 (proxy marker) in subglacial water in the Wilkes Basin was flushed out during the interglacial period 400,000 years ago as the ice melted and the grounding line retreated. That reset the U-234 concentration to low background levels, and accumulation then restarted when the ice advanced again.

Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial, Nature (2020)
https://www.nature.com/articles/s41586-020-2484-5
Clearly, MISI models are not dynamic to hind cast such behavior; which supports the idea that MICI-mechanism were involved during the ice retreat of the Wilkes Basin 400kya.

Blackburn, T., Edwards, G.H., Tulaczyk, S. et al. Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial. Nature 583, 554–559 (2020). https://doi.org/10.1038/s41586-020-2484-5

https://www.nature.com/articles/s41586-020-2484-5

Abstract: "Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth’s past interglacial warm periods. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater and sea level was 6 to 13 metres higher. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet3, which has been argued to have remained stable for millions of years before and includes MIS11. Here we show how the evolution of 234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet’s response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of 234U (the product of rock–water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of 234U enrichment place the inception of this reservoir at MIS11. Informed by the 234U cycling observed in the Laurentide Ice Sheet, where 234U accumulated during periods of ice stability6 and was flushed to global oceans in response to deglaciation7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The 234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines, indicating that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts10 and bacteria implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which—assuming current ice volumes—would have contributed about 3 to 4 metres to global sea levels."

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 08, 2020, 08:04:58 PM »
More food for thought about the implications of the bipolar seesaw:

Dome Fuji Ice Core Project Members (08 Feb 2017), "State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling", Science Advances, Vol. 3, no. 2, e1600446, DOI: 10.1126/sciadv.1600446

http://advances.sciencemag.org/content/3/2/e1600446.full

Extract: "Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation."

&

Turney, et al. (2017), "Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial", Nature Communications 8, Article No. 520, https://doi.org/10.1038/s41467-017-00577-6

https://www.nature.com/articles/s41467-017-00577-6

Extract: "An ensemble of transient meltwater simulations show that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train."

26
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 08, 2020, 08:00:00 PM »
In the way of lessons learned from the GIS, the following website (and extracted quote) indicates that data from a significant number of new boreholes in Greenland indicates that current ice sheet models (both for the GIS and the AIS) currently do not capture critical processes including the probability that as "... ice sheets accelerate, the acceleration itself opens up space between the ice and bedrock and expands the drainage network."  This means that model projections of ice mass loss from both the GIS and the AIS (particularly for the WAIS) need to be revised upward, once the models are corrected to account for such previously unrecognized ice sheet behavior.


http://www.sciencedaily.com/releases/2013/08/130815161541.htm

"Once the data was analyzed, the research team discovered that it didn't match up with the working hypotheses for water flow beneath the ice sheet. This led the scientists to surmise that there are other critical processes at work that had been missing -- one possibility being that as the ice sheet accelerates, the acceleration itself opens up space between the ice and bedrock and expands the drainage network.

"This process is largely neglected in current interpretations," Meierbachtol said. "We need to pull ourselves away from the narrow vision and start to explore some of the other options for transient growth."

Future warming likely will be enhanced over the Arctic. This body of research will provide a more accurate assessment of the impacts of future warming on Greenland."

27
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 08, 2020, 04:50:55 AM »
From both morganism and vox-mundi, the linked research implies that the MOC is currently slow:

Florida Current is Weaker Now Than at Any Point in the Past Century

https://www.whoi.edu/press-room/news-release/florida-current-weakening/

"Piecuch, who specializes in coastal and regional sea level change, used a connection between coastal sea level and the strength of near-shore currents to trace the evolution of the Florida Current, which forms the beginning of the Gulf Stream. "

Nature Communications published on August 7

&

Florida Current Weaker Now Than At Any Point In the Past Century
https://phys.org/news/2020-08-florida-current-weaker-century.html

A key component of the Gulf Stream has markedly slowed over the past century—that's the conclusion of a new research paper in Nature Communications published on August 7.

Weakening of the Florida Current transport is apparent on longer timescales. The centennial trend during 1909–2018 is  −1.7 ± 3.7 Sv century−1, which overlaps zero, but implies that the trend is likely negative (P = 0.82; Supplementary Fig. 3b).

Christopher G. Piecuch, Likely weakening of the Florida Current during the past century revealed by sea-level observations, Nature Communications (2020)
https://www.nature.com/articles/s41467-020-17761-w

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 07, 2020, 11:51:26 PM »
The linked reference indicates that CMIP6 projects that the Arctic Ocean will be seasonally ice free (sea‐ice area <1 × 106 km2) in September well before 2050 (particularly for SSP5-8.5).  If so, this would increase the probability of a reversal of the Beaufort Gyre in the coming decades.

SIMIP Community (17 April 2020), "Arctic Sea Ice in CMIP6", Geophysical Research Letters, Volume 47, Issue 10, https://doi.org/10.1029/2019GL086749

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL086749

Abstract
We examine CMIP6 simulations of Arctic sea‐ice area and volume. We find that CMIP6 models produce a wide spread of mean Arctic sea‐ice area, capturing the observational estimate within the multimodel ensemble spread. The CMIP6 multimodel ensemble mean provides a more realistic estimate of the sensitivity of September Arctic sea‐ice area to a given amount of anthropogenic CO2 emissions and to a given amount of global warming, compared with earlier CMIP experiments. Still, most CMIP6 models fail to simulate at the same time a plausible evolution of sea‐ice area and of global mean surface temperature. In the vast majority of the available CMIP6 simulations, the Arctic Ocean becomes practically sea‐ice free (sea‐ice area <1 × 106 km2) in September for the first time before the Year 2050 in each of the four emission scenarios SSP1‐1.9, SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5 examined here.

Edit, see the attached image (particularly panel 'e') of CMIP6 Arctic Sea Ice projections:

Caption: "Evolution of Arctic sea‐ice area over the historical period and following three scenario projections in (a–c) March and (d–f) September as a function of (a,d) cumulative anthropogenic CO2 emissions, (b, e) global annual mean surface temperature anomaly, and (c, f) time for all available CMIP6 models. Thick lines denote the multimodel ensemble mean, where all models are represented by their first ensemble member, and the shading around the lines indicates one? standard deviation around the multimodel mean. Faint dots denote the first ensemble member of each model, and thick black lines and crosses denote observations. Note that discontinuities in the multimodel ensemble mean arise from a different number of available models for the historical period and the scenario simulations."

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 07, 2020, 11:01:56 PM »
The linked reference discusses the possibility that decreasing Arctic Sea Ice coverage may lead to a collapse of the Beaufort High; which would cause the Beaufort Gyre to reverse, which would release a pulse of relatively freshwater first into the Arctic Ocean and then into the North Atlantic:

Alek. A. Petty (2018), "A possible link between winter Arctic sea ice decline and a collapse of the Beaufort High?", Geophys Res Lett 45, 6, 2879-2882, doi:10.1002/2018GL077704

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6146396/

Abstract: "A new study by Moore et al., (2018, this issue) highlights a collapse of the anticyclonic ‘Beaufort High’ atmospheric circulation over the western Arctic Ocean in the winter of 2017 and an associated reversal of the sea ice drift through the southern Beaufort Sea (eastward instead of the predominantly westward circulation). The authors linked this to the loss of sea ice in the Barents Sea, anomalous warming over the region, and the intrusion of low-pressure cyclones along the eastern Arctic. In this commentary we discuss the significance of this observation, the challenges associated with understanding these possible linkages, and some of the alternative hypotheses surrounding the impacts of winter Arctic sea ice loss."


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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 07, 2020, 07:45:50 PM »
The linked article discusses the risks associated Chinese super-pollutants if these emissions are not effectively limited (to net-zero emissions) soon:

Title: "‘Super-Pollutant’ Emitted by 11 Chinese Chemical Plants Could Equal a Climate Catastrophe"

https://insideclimatenews.org/news/04082020/china-n2o-super-pollutant-nylon-emissions-climate-change

Extract: "Ranping Song, the author of the World Resources Institute report, said China should also set its sights on reducing non-CO2 greenhouse gases; pollutants like methane, hydrofluorocarbons, highly potent greenhouse gases used in refrigeration and air conditioning, and nitrous oxide.

"If China's non-CO2 emissions were a country, they would be the 7th largest emitter of total GHGs [greenhouse gases] in the world," Song wrote in a blog post last September.

To have a chance at limiting global warming to 1.5 degrees, the world's nations must reduce greenhouse gas emissions to net zero by 2050, Song said. Reducing non-CO2 emissions "offers a range of low cost, feasible technologies that should be deployed," Song told InsideClimate News. "We think non-CO2 emissions reductions are a good opportunity for China to demonstrate momentum" on climate change."

31
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 07, 2020, 04:50:47 PM »
The linked reference, and associated article, provides insights on differences of between one model projections that are part of CMIP5 and CMIP6.  I note that DeConto and Pollard (2016) indicated that the BSB would begin to collapse due to an MICI-mechanism before GMSTA reached 2.7C, which Wyser et al. (2020) indicates has a 50% chance of occurring before 2037 when following SSP5 (which we are currently doing), and Wyser et al. (2020) do not consider possible perturbations such as a freshwater hosing event associated with a reversal of the Beaufort Gyre.

Klaus Wyser, Erik Kjellström, Torben Koenigk, Helena Martins and Ralf Döscher (12 May 2020), "Warmer climate projections in EC-Earth3-Veg: the role of changes in the greenhouse gas concentrations from CMIP5 to CMIP6", Environmental Research Letters, Volume 15, Number 5, https://doi.org/10.1088/1748-9326/ab81c2

https://iopscience.iop.org/article/10.1088/1748-9326/ab81c2

Abstract: "Climate projections for the 21st century for CMIP6 are warmer than those for CMIP5 despite nominally identical instantaneous radiative forcing. Many climate modeling groups attribute the stronger warming in the CMIP6 projections to the higher climate sensitivity of the new generation of climate models, but here we demonstrate that also changes in the forcing datasets can play an important role, in particular the prescribed concentrations of greenhouse gases (GHG) that are used to force the models. In the EC-Earth3-Veg model the effective radiative forcing (ERF) is reduced by 1.4 W m−2 when the GHG concentrations from SSP5-8.5 (used in CMIP6) are replaced by the GHG concentrations from RCP8.5 (used in CMIP5), and similar yet smaller reductions are seen for the SSP2-4.5/RCP4.5 and SSP1-2.6/RCP2.6 scenario pairs. From the reduced ERF we can estimate the temperature at the end of the century in a full climate simulation with the CMIP6 version of the EC-Earth model but using CMIP5 GHG concentrations instead. For the new SSP5-8.5 and SSP2-4.5 scenarios we find that 50% or more of the temperature increase from CMIP5 to CMIP6 at the end of the century is due to changes in the prescribed GHG concentrations. The implication is that CMIP5 and CMIP6 projections for the 21st century are difficult to compare with each other not only as models differ but also as the forcing conditions are not equal. Therefore, the communication of CMIP6 results to the impact, mitigation and adaptation communities has to be carefully formulated, taking into account the role of the updated GHG concentrations when interpreting the warmer climate projections for the 21st century."

Extract: "A striking feature of the new CMIP6 projections are the differences in the warming at the end of the 21st century compared to CMIP5 despite nominally identical radiative forcing. Some of the additional warming in CMIP6 has previously been attributed to the higher climate sensitivity in many CMIP6 models. However, we demonstrate here that in the EC-Earth3-Veg model the change in the forcing datasets from CMIP5 to CMIP6 could also contribute substantially to the larger warming at the end of the century. In particular, we identify the changes in GHG concentrations from the RCPs in CMIP5 to the corresponding SSPs in CMIP6 as a major contributor to the additional warming in the climate projections with the EC-Earth3-Veg model. For the new SSP2-4.5 and SSP5-8.5 scenarios, more than 50% of the extra warming in CMIP6 can be explained by the changes in GHG concentrations. The extra warming is a consequence of the higher effective radiative forcing with SSP5-8.5 that would drop by 1.4 W m−2 if GHG concentrations from RCP8.5 would be used instead, despite the nominally equal instantaneous radiative forcing for SSP5-8.5 and RCP8.5. Similar, yet smaller, reductions of the ERF have been found for SSP2-4.5 and SSP1-2.6. We'd like to emphasize that this result is probably strongly model dependent because ERF depends on adjustments in the climate model in contrast to the instantaneous RF. Despite this caveat, we still think that changes in GHG concentrations could contribute to the warming from CMIP5 to CMIP6 even in other models, and we therefore encourage other groups to make similar RFMIP type experiments as those presented here to test the impact of the changes in GHG concentrations and possibly even other updates in the CMIP6 forcing datasets. Further studies along this line with other climate models will improve the possibility of assessing the robustness of the presented results."

Caption for first image: "Figure 1. Temperature anomalies relative pre-industrial levels for CMIP6 with the EC-Earth3-Veg model (solid), and the previous results for CMIP5 with EC-Earth v2.3 (dashed). The CMIP6 results are considerably warmer at the end of the 21st century for scenarios with same nameplate radiative forcing. The solid or dashed lines denote the ensemble mean and the shaded area marks the full width of the ensemble. The ensemble consists of 6 members for CMIP6 and 3 members for CMIP5, except for RCP26 for which we only have 1 member."

Caption for second image: "Figure 2. Annual global mean GHG concentration for CMIP5 (dashed) and CMIP6 (solid). Shown are only the GHGs that are used by EC-Earth. In the CFC-11 plot, the values shown for the RCPs are the plain CFC-11 concentrations while for the SSPs the CFC11eq concentrations are shown, see text for details."

Caption for third image: "Figure 3. Warming at the end of the 21st century vs ERF for the EC-Earth3-Veg model. The blue crosses are the warming of the ensemble means from the CMIP6 ScenarioMIP experiments while the ERF was obtained from the AMIP experiments with the SSP forcings for CMIP6 from the end of the century. The red line is the linear fit to the data given by (1)."


See also:

Title: "New insights into why new global climate projections show faster warming" April 2020

https://www.smhi.se/en/research/research-news/new-insights-into-why-new-global-climate-projections-show-faster-warming-1.159529

Extract: "Last year, the SMHI Rossby Centre presented its first future climate projections using the new version of the global climate model EC-Earth. The new results showed warming taking place at a faster rate than suggested by previous calculations, but the reasons behind it were not fully understood. A new study explains some of the causes.

32
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 07, 2020, 10:48:09 AM »
The linked website points out (among other points) that:

a) Some recent papers criticizing the high-end CMIP6 ECS values use analysis that over emphasize the faux hiatus years of 1999 to 2012, and thus may be biased.
b) Following SSP5 GMSTA could reach peak Pliocene conditions as early as 2050 (see the first attached image).
c) The long/fat tail of CMIP6 ECS values represent higher risks to society than that estimated using CMIP5 values (see the second image from Sutton 2018)

Title: "Climate Sensitivity in CMIP6: some initial findings"

https://www.crescendoproject.eu/climate-sensitivity-in-cmip6-causes-consequences-and-uses/

Extract:
"- A new paper, to appear soon, suggests models with ECS greater than ~5°C overestimate post-1990 warming rates compared to observations, suggesting their future warming may also be overestimated. A caveat to this finding is that the post-1990 observed warming includes the highly unusual “warming hiatus” from ~ 1999 to 2012, so the observed record may not fully constrain future warming.

- While the high ECS values (greater than ~5°C) seen in some CMIP6 models are very likely greater than the Earth’s actual climate sensitivity, for some of these models we are confident the underpinning feedback processes controlling the ECS response are now more realistic than in predecessor models. This gives us confidence our models are improving at the process level.

- Low climate sensitivities may be more likely but, due to the more moderate associated climate impacts, the risk to society may also be moderate. In contrast, high climate sensitivities may have a very low likelihood of occurrence, but they are associated with significantly higher potential impacts. Hence, while these high climate sensitivities are low probability, they are also high impact and thus carry significant risk to society.

- CMIP6 models with high climate sensitivities represent this category of low probability, high impact, high risk futures. It is therefore crucial future projections made with these models are used in combination with the more likely, lower impact projections, to develop sound adaptation strategies encompassing the full range of possible futures risks faced by society."

Caption for the first image: "Figure 2. Global mean surface warming relative to 1850-1900 mean for 10 CMIP6 models (left side), following five SSP pathways shown in the legend and the full CMIP5 ensemble (right side, with time-axis reversed) following the earlier CMIP5 RCPs. The red plume (red line) plots the CMIP6/CMIP5 multi-model ensemble (ensemble mean) of SSP5-85/RCP85, while the blue plume plots the same for SSP1-26/RCP26. The grey plume and black line shows the historical temperature anomaly for CMIP6 and CMIP5. Figure produced by Erich Fischer ETH Zurich using the ESMValTool package."

Edit: The third image shows that CMIP6 has better skill than does CMIP5 at matching observed data.
The fourth image shows Zelinka's frequency histogram for ECS values from CMIP6 where the Wolf Pack histogram could substitute as a probability density function, pdf, for updating Sutton 2018's risk assessment shown in the second attached image.

33
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 11:47:11 PM »
The linked reference indicates that CMIP5 likely underestimated how negative aerosol forcing has been and indicates that the mean values of aerosol forcing and of TCR from CMIP6 are closer to matching the observed regional land temperatures.  If so, this indicates that the relatively high mean climate sensitivity values (both TCR and ECS) determined in CMIP6 are more likely than so far acknowledged by consensus climate documents like AR5.  I note that while the TCR value of 2.0K determined by this reference is relatively high, this value does not consider the impacts of freshwater hosing on TCR and that when E3SMv1 did so it projected a value of 2.95K for TCR.

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

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

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

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

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

34
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 11:04:51 PM »
The information at the linked Copernicus website indicates that July 2020 was the third warmest July on record and that for the 12-month period from August 2019 to July 2020, GMSTA was about 1.3C above pre-industrial:

Title: "Surface air temperature for July 2020"

https://climate.copernicus.eu/surface-air-temperature-july-2020

Extract: "Global temperatures were much above average in July 2020. The month was:
•   0.49°C warmer than the 1981-2020 average for July;
•   the third warmest July in this data record;
•   cooler by 0.07°C than July 2019, the warmest July;
•   cooler by 0.04°C than July 2016, the second warmest July."

Globally, the twelve-month period from August 2019 to July 2020 was 0.65°C warmer than the 1981-2010 average. The value for this period is marginally below that of the 12-month periods ending in May 2020 and September 2016, the two warmest periods in this record. 2016 is the warmest calendar year on record, with a global temperature 0.63°C above that for 1981-2010. 2019 is the second warmest calendar year in this data record, with a temperature 0.59°C above average.

0.63°C should be added to these values to relate recent global temperatures to the pre-industrial level defined in the IPCC Special Report on “Global Warming of 1.5°C”. The average temperature for the twelve months to July 2020 is a little under 1.3°C above the level."

35
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 10:49:18 PM »
While I have previous cited the linked open access reference, here I remind readers that Pollard et al (2018) confirms that ice mélange offers very little resistance to MICI-mechanisms in Antarctica and that if/when key Antarctic marine glaciers are subjected to mid-Pliocene type oceanic and atmospheric conditions that very rapid ice mass loss can occur in decades as indicated in the two attached images:

Pollard, D., R.M. DeConto and R.B. Alley (2018), "A continuum model (PSUMEL1) of ice mélange and its role during retreat of the Antarctic Ice Sheet", Geosci. Model Devel., 11, 5149-5172, https://doi.org/10.5194/gmd-11-5149-2018

https://gmd.copernicus.org/articles/11/5149/2018/

Abstract
Rapidly retreating thick ice fronts can generate large amounts of mélange (floating ice debris), which may affect episodes of rapid retreat of Antarctic marine ice. In modern Greenland fjords, mélange provides substantial back pressure on calving ice faces, which slows ice front calving rates. On the much larger scales of West Antarctica, it is unknown if mélange could clog seaways and provide enough back pressure to act as a negative feedback slowing retreat. Here we describe a new mélange model, using a continuum-mechanical formulation that is computationally feasible for long-term continental Antarctic applications. It is tested in an idealized rectangular channel and calibrated very basically using observed modern conditions in Jakobshavn fjord, West Greenland. The model is then applied to drastic retreat of Antarctic ice in response to warm mid-Pliocene climate. With mélange parameter values that yield reasonable modern Jakobshavn results, Antarctic marine ice still retreats drastically in the Pliocene simulations, with little slowdown despite the huge amounts of mélange generated. This holds both for the rapid early collapse of West Antarctica and for later retreat into major East Antarctic basins. If parameter values are changed to make the mélange much more resistive to flow, far outside the range for reasonable Jakobshavn results, West Antarctica still collapses and retreat is slowed or prevented only in a few East Antarctic basins.

Caption for first image: "Figure 6. Snapshots of mélange thickness (m, upper row) and speed (km yr−1 , middle row) in a simulation of Antarctic ice retreat, at selected times after a step-function transition from modern to warm mid-Pliocene climate. White regions are solid ice sheet or shelf, grey is ocean and the grounding line is shown by a black line. Mélange parameter values are as in the near-best-scoring run of the Jakobshavn ensemble shown in Fig. 3a (E = 106 , Hn(Hp) = 30(60), n = 5). Bottom row shows solid-ice extents from a corresponding simulation with no mélange. (a) 0 yr (modern). (b) 50 yr. (c) 200 yr. (d) 2000 yr. The first panel shows locations of features named in the text; EAIS: East Antarctic Ice Sheet; WAIS: West Antarctic Ice Sheet."

Caption for second image: "Figure 8. Time series in simulations of Antarctic retreat after a transition to warm mid-Pliocene climate with two different mélange parameter settings. Red curves: E = 101 , Hn(Hp) = 150(200), n = 5 (near-best scoring, as in Fig. 6). Green curves: E = 101 , Hn(Hp) = 150(200), n = 1 (much stiffer and thicker mélange, as in Fig. 7). (a) Equivalent global mean sea level rise (m), with black curve for a corresponding simulation with no mélange. (b) Total mélange volume (106 km3). (c) Total mélange area (106 km2), with thinner curves showing area where grounded. (d) Mélange thickness (m) averaged over all mélange grid cells adjacent to a solid ice face. (e) Total additional back force due to mélange summed over all Antarctic ice faces, compared to that due to ocean water pressure with no mélange (1015 N; see Appendix A)."

36
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 06:14:03 PM »
While the linked reference, and associated article, focuses on the environmental impacts of up-coming reservoirs due to dam construction, I note that the flooding of the reservoirs of the approximately 3,636 new dams currently under construction or planned for the next two decades, will result in a surge of associated GHG (largely methane) emissions as the flooded organics degrade underwater:

Michele L. Thieme et al. (22 May 2020), "Dams and protected areas: Quantifying the spatial and temporal extent of global dam construction within protected areas", Conservation Letters, https://doi.org/10.1111/conl.12719

https://conbio.onlinelibrary.wiley.com/doi/full/10.1111/conl.12719

Abstract: "Protected areas (PAs) are an essential tool for freshwater biodiversity conservation. Given past and expected future global increases in dams and impacts of dams on freshwater ecosystems, we document the number of dams existing or planned within PAs, their history, and the extent of PA downgrading, downsizing, and degazettement (PADDD) proximally caused by dams. Globally, at least 1,249 large dams are located within PAs; two‐thirds (907) were built before PA establishment. Additionally, 14% of planned geolocated hydropower dams (509 dams) are located within PAs. PADDD events have also legalized dam construction within existing PAs. Environmental safeguards should preclude development of dams within or adjacent to PAs and prioritize dams within PAs for possible removal and restoration."

See also:

Title: "More than 500 dams planned inside protected areas: Study"

https://news.mongabay.com/2020/08/more-than-500-dams-planned-inside-protected-areas-study/


37
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 05:57:45 PM »
The linked reference compares current changes in the Arctic to past Arctic changes can concludes that:

"… the Arctic is currently experiencing an abrupt climate change event, and that climate models underestimate this ongoing warming."

Jansen, E., Christensen, J.H., Dokken, T. et al. Past perspectives on the present era of abrupt Arctic climate change. Nat. Clim. Chang. 10, 714–721 (2020). https://doi.org/10.1038/s41558-020-0860-7

https://www.nature.com/articles/s41558-020-0860-7

Abstract: "Abrupt climate change is a striking feature of many climate records, particularly the warming events in Greenland ice cores. These abrupt and high-amplitude events were tightly coupled to rapid sea-ice retreat in the North Atlantic and Nordic Seas, and observational evidence shows they had global repercussions. In the present-day Arctic, sea-ice loss is also key to ongoing warming. This Perspective uses observations and climate models to place contemporary Arctic change into the context of past abrupt Greenland warmings. We find that warming rates similar to or higher than modern trends have only occurred during past abrupt glacial episodes. We argue that the Arctic is currently experiencing an abrupt climate change event, and that climate models underestimate this ongoing warming."

38
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 05:32:16 PM »
The linked open access reference describes yet another positive feedback mechanism for Arctic Amplification due to cloud-ice (CSI) feedback:

Daniel Philipp, Martin Stengel and Bodo Ahrens (28 JULY 2020), "Analyzing the Arctic Feedback Mechanism between Sea Ice and Low-Level Clouds Using 34 Years of Satellite Observations", J. Climate, 33, (17), 7479–7501, https://doi.org/10.1175/JCLI-D-19-0895.1

https://journals.ametsoc.org/jcli/article/33/17/7479/348638/Analyzing-the-Arctic-Feedback-Mechanism-between

Abstract: "Satellite-based cloud, radiation flux, and sea ice records covering 34 years are used 1) to investigate autumn cloud cover trends over the Arctic, 2) to assess its relation with declining sea ice using Granger causality (GC) analysis, and 3) to discuss the contribution of the cloud–sea ice (CSI) feedback to Arctic amplification. This paper provides strong evidence for a positive CSI feedback with the capability to contribute to autumnal Arctic amplification. Positive low-level cloud fractional cover (CFClow) trends over the Arctic ice pack are found in October and November (ON) with magnitudes of up to about +9.6% per decade locally. Statistically significant anticorrelations between sea ice concentration (SIC) and CFClow are observed in ON over melting zones, suggesting an association. The GC analysis indicated a causal two-way interaction between SIC and CFClow. Interpreting the resulting F statistic and its spatial distribution as a relation strength proxy, the influence of SIC on CFClow is likely stronger than the reverse. ERA-Interim reanalysis data suggest that ON CFClow is impacted by sea ice melt through surface–atmosphere coupling via turbulent heat and moisture fluxes. Due to weak solar insolation in ON, net cloud radiative forcing (CRF) exerts a warming effect on the Arctic surface. Increasing CFClow induces a large-scale surface warming trend reaching magnitudes of up to about +8.3 W m−2 per decade locally. Sensitivities of total CRF to CFClow ranges between +0.22 and +0.66 W m−2 per percent CFClow. Increasing surface warming can cause a melt season lengthening and hinders formation of perennial ice."

39
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 06, 2020, 09:24:19 AM »
"The dogmas of the quiet past are inadequate to the stormy present. The occasion is piled high with difficulty, and we must rise with the occasion. As our case is new, so we must think anew and act anew." Abraham Lincoln

Somehow, I do not think that today's world leaders will face our collective climate challenges as well as Lincoln faced his many challenges.

40
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 05, 2020, 07:13:12 PM »
Quote
« Reply #3599 on: August 04, 2020, 10:18:13 PM »
LikeQuote
The linked 2019 reference (that I have previously cited in this thread) indicates that in an MICI scenario that the calving front could retreat at a rate as high as 100 km/year and that Antarctica's contribution to sea level rise could be as high as 5.5 m by 2100 (see that attached image):

And 19.5 meters by 2200.

Also, I note that the attached image indicates that Arctic Amplification during the Pliocene Optimum was much higher that projected by current consensus climate models; indicating that climate sensitivity could increase abruptly if freshwater hosing events in coming decades lead to an abrupt loss of both Arctic and Antarctic sea ice.
For what little it is worth, I note that Lee et al (2020):

1. Refutes the conclusions of Edwards (2019) regarding MICI mechanisms during the Pliocene and,
2. Lee et al. (2020)'s calibration of there model to Pliocene conditions does not consider such modern considerations as: a) the Antarctic ozone hole; b) the freshwater accumulated in the Beaufort Gyre and in the entire Arctic Ocean surface layers; c) the high rate of current radiative forcing; and d) possible bipolar seesaw feedback mechanisms.

Edit: Also, I note that for those who do not know, that the IPCC's projections of SLR have progressively increased from FAR to SAR, to TAR to AR4 to AR5 and due to past scientific reticence I expect the IPCC SLR projections will continue to increase from AR6 thru AR10 by which time we may be observing MICI mechanisms in real time.

41
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 05, 2020, 12:30:26 PM »
As a follow-on my last post, the Lee et al. paper has now been published in a peer-reviewed journal as Lee et al (2020):

Lee, Ben Seiyon; Haran, Murali; Fuller, Robert W.; Pollard, David; Keller, Klaus. A fast particle-based approach for calibrating a 3-D model of the Antarctic ice sheet. Ann. Appl. Stat. 14 (2020), no. 2, 605--634. doi:10.1214/19-AOAS1305

https://projecteuclid.org/euclid.aoas/1593449318
or
https://pennstate.pure.elsevier.com/en/publications/a-fast-particle-based-approach-for-calibrating-a-3-d-model-of-the

Abstract: "We consider the scientifically challenging and policy-relevant task of understanding the past and projecting the future dynamics of the Antarctic ice sheet. The Antarctic ice sheet has shown a highly nonlinear threshold response to past climate forcings. Triggering such a threshold response through anthropogenic greenhouse gas emissions would drive drastic and potentially fast sea level rise with important implications for coastal flood risks. Previous studies have combined information from ice sheet models and observations to calibrate model parameters. These studies have broken important new ground but have either adopted simple ice sheet models or have limited the number of parameters to allow for the use of more complex models. These limitations are largely due to the computational challenges posed by calibration as models become more computationally intensive or when the number of parameters increases. Here, we propose a method to alleviate this problem: a fast sequential Monte Carlo method that takes advantage of the massive parallelization afforded by modern high-performance computing systems. We use simulated examples to demonstrate how our sample-based approach provides accurate approximations to the posterior distributions of the calibrated parameters. The drastic reduction in computational times enables us to provide new insights into important scientific questions, for example, the impact of Pliocene era data and prior parameter information on sea level projections. These studies would be computationally prohibitive with other computational approaches for calibration such as Markov chain Monte Carlo or emulation-based methods. We also find considerable differences in the distributions of sea level projections when we account for a larger number of uncertain parameters. For example, based on the same ice sheet model and data set, the 99th percentile of the Antarctic ice sheet contribution to sea level rise in 2300 increases from 6.5 m to 13.1 m when we increase the number of calibrated parameters from three to 11. With previous calibration methods, it would be challenging to go beyond five parameters. This work provides an important next step toward improving the uncertainty quantification of complex, computationally intensive and decision-relevant models."

42
The fasted deglaciation model in GIS that i have seen is about 500 yr.

...

Applegate et al. 2015 is a peer-reviewed reference that with a Greenland T of 9K indicates a 7m,  or a with a Greenland T of 12K indicates an 8m, contribution to SLR by 2500 from the GIS (see the first attached image). Here a Greenland T of 9 to 12K roughly corresponding to a 4.5 to 6K GMSTA increase (see the second image); which is for RCP 8.5 and which shows 12 K is reached over Greenland around 2200.

Applegate, P.J., Parizek, B.R., Nicholas, R.E. et al. (2015), "Increasing temperature forcing reduces the Greenland Ice Sheet’s response time scale", Clim Dyn 45: 2001. https://doi.org/10.1007/s00382-014-2451-7

https://link.springer.com/article/10.1007/s00382-014-2451-7#citeas
&
https://static-content.springer.com/esm/art%3A10.1007%2Fs00382-014-2451-7/MediaObjects/382_2014_2451_MOESM1_ESM.pdf

Abstract: "Damages from sea level rise, as well as strategies to manage the associated risk, hinge critically on the time scale and eventual magnitude of sea level rise. Satellite observations and paleo-data suggest that the Greenland Ice Sheet (GIS) loses mass in response to increased temperatures, and may thus contribute substantially to sea level rise as anthropogenic climate change progresses. The time scale of GIS mass loss and sea level rise are deeply uncertain, and are often assumed to be constant. However, previous ice sheet modeling studies have shown that the time scale of GIS response likely decreases strongly with increasing temperature anomaly. Here, we map the relationship between temperature anomaly and the time scale of GIS response, by perturbing a calibrated, three-dimensional model of GIS behavior. Additional simulations with a profile, higher-order, ice sheet model yield time scales that are broadly consistent with those obtained using the three-dimensional model, and shed light on the feedbacks in the ice sheet system that cause the time scale shortening. Semi-empirical modeling studies that assume a constant time scale of sea level adjustment, and are calibrated to small preanthropogenic temperature and sea level changes, may underestimate future sea level rise. Our analysis suggests that the benefits of reducing greenhouse gas emissions, in terms of avoided sea level rise from the GIS, may be greatest if emissions reductions begin before large temperature increases have been realized. Reducing anthropogenic climate change may also allow more time for design and deployment of risk management strategies by slowing sea level contributions from the GIS."

Also, the Purdue researchers who studied the Cordilleran Ice Sheet feel that the GIS could lose around half of its ice mass in as little as 500-years.

Title: "Research shows ice sheets as large as Greenland’s melted fast in a warming climate"

https://www.purdue.edu/newsroom/releases/2017/Q4/research-shows-ice-sheets-as-large-as-greenlands-melted-fast-in-a-warming-climate.html

Extract: "New research published in Science shows that climate warming reduced the mass of the Cordilleran Ice Sheet by half in as little as 500 years, indicating the Greenland Ice Sheet could have a similar fate.

The Cordilleran Ice Sheet covered large parts of North America during the Pleistocene - or last ice age - and was similar in mass to the Greenland Ice Sheet. Previous research estimated that it covered much of western Canada as late as 12,500 years ago, but new data shows that large areas in the region were ice-free as early as 1,500 years earlier. This confirms that once ice sheets start to melt, they can do so very quickly."

43
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 05, 2020, 03:18:13 AM »
The linked 2019 reference (that I have previously cited in this thread) indicates that in an MICI scenario that the calving front could retreat at a rate as high as 100 km/year and that Antarctica's contribution to sea level rise could be as high as 5.5 m by 2100 (see that attached image):

Ben Seiyon Lee, Murali Haran, Robert Fuller, David Pollard, Klaus Keller (24 March 2019), "A Fast Particle-Based Approach for Calibrating a 3-D Model of the Antarctic Ice Sheet", arXiv:1903.10032v2

https://arxiv.org/abs/1903.10032

Abstract: "We consider the scientifically challenging and policy-relevant task of understanding the past and projecting the future dynamics of the Antarctic ice sheet. The Antarctic ice sheet has shown a highly nonlinear threshold response to past climate forcings. Triggering such a threshold response through anthropogenic greenhouse gas emissions would drive drastic and potentially fast sea level rise with important implications for coastal flood risks. Previous studies have combined information from ice sheet models and observations to calibrate model parameters. These studies have broken important new ground but have either adopted simple ice sheet models or have limited the number of parameters to allow for the use of more complex models. These limitations are largely due to the computational challenges posed by calibration as models become more computationally intensive or when the number of parameters increases. Here we propose a method to alleviate this problem: a fast sequential Monte Carlo method that takes advantage of the massive parallelization afforded by modern high performance computing systems. We use simulated examples to demonstrate how our sample-based approach provides accurate approximations to the posterior distributions of the calibrated parameters. The drastic reduction in computational times enables us to provide new insights into important scientific questions, for example, the impact of Pliocene era data and prior parameter information on sea level projections. These studies would be computationally prohibitive with other computational approaches for calibration such as Markov chain Monte Carlo or emulation-based methods. We also find considerable differences in the distributions of sea level projections when we account for a larger number of uncertain parameters."

Extract: "The wider range for CLIFFVMAX explores a fundamental uncertainty in MICI – the rate at which very tall ice cliffs will disintegrate back into the ice sheet interior. If grounding lines retreat into the interior of deep Antarctic basins, the exposed ice cliffs will be taller than any observed today, and the wastage velocities (CLIFFVMAX) could conceivably be much greater than the approximately 12 km per year observed today at the ice fronts of major Greenland glaciers (which might not even be approximate analogs for MICI, being driven instead mainly by buoyant calving; Murray et al. (2015)). The bimodal character of the posterior densities in the top panels of Figure 9 for 2300 and 2500 are due to the very large CLIFFVMAX range. The upper peak centered on around 20 m is produced by CLIFFVMAX values of approximately 100 km per year and above, which produce collapse of almost all marine ice in both East and West Antarctica. The lower peak centered on around 5 m occurs for many lower CLIFFVMAX values, for which the more vulnerable West Antarctic ice sheet collapses, but marine basins in East Antarctica do not retreat.

We use this new method to assess the impacts of neglecting parametric uncertainties on sea level projections. Emulation-calibration methods using fewer parameters yield lower and more overconfident projections of sea level rise than using more parameters through the particle-based calibration approach. This method includes the recent study of Edwards et al. (2019), who found that the important mechanism of marine ice cliff instability (MICI) is not necessary to capture past variations. In this case, future sea level projections are considerably lower. In contrast, our new approach that accounts for more parametric uncertainties suggests that MICI may still be important and future sea level projections may be much higher, especially considering potential Pliocene windows. Using emulation-calibration in a high-dimensional parameter space induces considerable emulator-model discrepancy and can result in large projection uncertainties. Our method utilizes the actual ice sheet model; thereby preserving the highly non-linear ice dynamics as well as the complex interactions between model parameters. This has clear policy relevant implications because projections from ice sheet models inform economic and engineering assessments (cf. Sriver et al., 2018; Diaz and Keller, 2016; Johnson et al., 2013)."

44
The linked 2019 reference indicates that in an MICI scenario that the calving front could retreat at a rate as high as 100 km/year and that Antarctica's contribution to sea level rise could be as high as 5.5 m by 2100 (see that attached image):

Ben Seiyon Lee, Murali Haran, Robert Fuller, David Pollard, Klaus Keller (24 March 2019), "A Fast Particle-Based Approach for Calibrating a 3-D Model of the Antarctic Ice Sheet", arXiv:1903.10032v2

https://arxiv.org/abs/1903.10032

Abstract: "We consider the scientifically challenging and policy-relevant task of understanding the past and projecting the future dynamics of the Antarctic ice sheet. The Antarctic ice sheet has shown a highly nonlinear threshold response to past climate forcings. Triggering such a threshold response through anthropogenic greenhouse gas emissions would drive drastic and potentially fast sea level rise with important implications for coastal flood risks. Previous studies have combined information from ice sheet models and observations to calibrate model parameters. These studies have broken important new ground but have either adopted simple ice sheet models or have limited the number of parameters to allow for the use of more complex models. These limitations are largely due to the computational challenges posed by calibration as models become more computationally intensive or when the number of parameters increases. Here we propose a method to alleviate this problem: a fast sequential Monte Carlo method that takes advantage of the massive parallelization afforded by modern high performance computing systems. We use simulated examples to demonstrate how our sample-based approach provides accurate approximations to the posterior distributions of the calibrated parameters. The drastic reduction in computational times enables us to provide new insights into important scientific questions, for example, the impact of Pliocene era data and prior parameter information on sea level projections. These studies would be computationally prohibitive with other computational approaches for calibration such as Markov chain Monte Carlo or emulation-based methods. We also find considerable differences in the distributions of sea level projections when we account for a larger number of uncertain parameters."

Extract: "The wider range for CLIFFVMAX explores a fundamental uncertainty in MICI – the rate at which very tall ice cliffs will disintegrate back into the ice sheet interior. If grounding lines retreat into the interior of deep Antarctic basins, the exposed ice cliffs will be taller than any observed today, and the wastage velocities (CLIFFVMAX) could conceivably be much greater than the approximately 12 km per year observed today at the ice fronts of major Greenland glaciers (which might not even be approximate analogs for MICI, being driven instead mainly by buoyant calving; Murray et al. (2015)). The bimodal character of the posterior densities in the top panels of Figure 9 for 2300 and 2500 are due to the very large CLIFFVMAX range. The upper peak centered on around 20 m is produced by CLIFFVMAX values of approximately 100 km per year and above, which produce collapse of almost all marine ice in both East and West Antarctica. The lower peak centered on around 5 m occurs for many lower CLIFFVMAX values, for which the more vulnerable West Antarctic ice sheet collapses, but marine basins in East Antarctica do not retreat.

We use this new method to assess the impacts of neglecting parametric uncertainties on sea level projections. Emulation-calibration methods using fewer parameters yield lower and more overconfident projections of sea level rise than using more parameters through the particle-based calibration approach. This method includes the recent study of Edwards et al. (2019), who found that the important mechanism of marine ice cliff instability (MICI) is not necessary to capture past variations. In this case, future sea level projections are considerably lower. In contrast, our new approach that accounts for more parametric uncertainties suggests that MICI may still be important and future sea level projections may be much higher, especially considering potential Pliocene windows. Using emulation-calibration in a high-dimensional parameter space induces considerable emulator-model discrepancy and can result in large projection uncertainties. Our method utilizes the actual ice sheet model; thereby preserving the highly non-linear ice dynamics as well as the complex interactions between model parameters. This has clear policy relevant implications because projections from ice sheet models inform economic and engineering assessments (cf. Sriver et al., 2018; Diaz and Keller, 2016; Johnson et al., 2013)."

45
I think that there is a DeConto and Pollard paper that comes up with a few hundred years for MICI for Thwaites/PIG/W Antarctica.

The fasted deglaciation model in GIS that i have seen is about 500 yr.

sidd

The first attached image from DeConto & Pollard (2016) clarifies sidd's approximation for the timing of a WAIS collapse for RCP 8.5 forcing.  That said, DeConto & Pollard (2016):

a) arbitrarily limited the rate of ice-cliff failure propagation upstream to about 1/2 of that observed for Jakobshavn (while subsequent research indicates that this rate of upstream propagation can increase nonlinearly with freeboard and water depth as illustrated by the second image);
b) they assumed an ECS value that is lower than the high-end CMIP6 estimates and
c) they initiated their model with the Thwaites Ice Tongue (not to mention the PIIS) with much better structural integrity than what we see today.

As DeConto and Pollard have admitted that their efforts to better calibrate their MICI model is on-going, I think that we likely will not have a clear idea of the risk, and rate, of an MICI type of collapse of the combined PIG and Thwaites drainage basin until the 2035 to 2045; by which time such an MICI-type of mechanism might already have been triggered.

46
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 04, 2020, 10:48:28 PM »
For those who do not know, or remember, what the Thwaites Ice Tongue looked like before iceberg B-22 calved in 2020, I provide the linked article & associated image.  Also, the NASA article states:

“What the satellites are showing us is a glacier coming apart at the seams,” said Ted Scambos, a senior scientist at the University of Colorado. “Every few years a new area seems to be letting go and accelerating. Like taffy being stretched out, this glacier is being drawn into the ocean.”

Title: "Thwaites Glacier Transformed"

https://earthobservatory.nasa.gov/images/146247/thwaites-glacier-transformed

Extract: "This image pair demonstrates the changes that have occurred since the start of this century. The first image, acquired with the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7, shows the glacier’s floating ice tongue on December 2, 2001 shortly before it calved Iceberg B-22. The second image, acquired with the Operational Land Imager (OLI) on Landsat 8, shows the glacier on December 28, 2019.

Both images show the glacier where it exits the land in West Antarctica and stretches over the Amundsen Sea as thick floating ice. Ice that originates on land can raise sea level if it is delivered to the ocean at a faster rate than it is being replaced inland by snowfall. Indeed, Thwaites Glacier is one of the largest contributors to global sea level rise from the West Antarctic Ice Sheet. The flow speed of Thwaites has been increasing, while inland snowfall has not changed significantly.

Notice the size of the glacier’s main ice tongue in 2001, when the glacier was advancing by about 4 kilometers per year. The large rift across the glacier eventually spawned Iceberg B-22 in 2002.

In the past ten years, the tongue has continued to fracture and separate from the Thwaites Eastern Ice Shelf. By the time the 2019 image was acquired, the main tongue had retreated substantially, and the ocean in front of Thwaites had become filled with mélange, a mixture of icebergs and sea ice."

The melting of floating ice as it makes contact with the ocean is a key reason why the glacier is coming unglued. Seawater that is a few degrees above freezing is melting the ice shelf from below. Warm water has recently been recorded near the Thwaites Glacier grounding line—the location where the glacial ice rests on the seafloor.

“What the satellites are showing us is a glacier coming apart at the seams,” said Ted Scambos, a senior scientist at the University of Colorado. “Every few years a new area seems to be letting go and accelerating. Like taffy being stretched out, this glacier is being drawn into the ocean.”"

47
The linked reference indicates that the projected increase (with continued global warming) of more frequent strong El Nino events combined with the projected increase in positive SAM, will significantly increase ice mass loss from the ASE, which will increase the risk of a collapse of the WAIS:

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

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

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

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

48
The ENSO cycle has repeatedly been demonstrated to generate decadal oceanic pulses of alternately relatively warm CDW (circumpolar deep water) and relative cooler surface water into the Amundsen Sea Embayment, ASE.  The linked reference provides both field and model results that help to better delineate the influence of these ENSO driven oceanic pulses on ice mass loss from key marine glaciers in the ASE; which the attached image indicates has accelerated markedly since about 1995.  Furthermore, research indicates that as climate change increases the frequency of strong El Nino events, the frequency of warm CDW pulses into the ASE should increase, resulting in increased ice mass loss from this key region:

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

http://www.nature.com/articles/s41561-018-0207-4.epdf?referrer_access_token=Bc93rPzj5mAIgFxIj7ONaNRgN0jAjWel9jnR3ZoTv0PHtUgk_ZOT39EqrANp0b8eqygnJyFYtkZtZrrvzpzzWFxFRxOjGyBuySjpDsnaRQh7XJnWxZ3ao5NgE_FXw2TbspGSBS1Ou39d7UURpwlPi_Pto2nRLEma6yWSJG3jZtjtHknyJEJlg9BIxSQMv28PGhskTGPjzqBOEvvM0U4N9vO_qHWkDtkY-E5jhH1DvWdJkNePrE5W2mXS98uEvX9LRJGTRyR_k2N9kxRVb0DlMnr7Jn6NgoQ-PnofJG67wP8%3D&tracking_referrer=www.carbonbrief.org

Abstract: "Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The non-linear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high."

See also:

Title: "Scientists find ‘natural pulses’ in recent melting of West Antarctic ice sheet"

https://www.carbonbrief.org/scientists-find-natural-pulses-in-recent-melting-of-west-antarctic-ice-sheet

Extract: "Natural ocean variability is heightening the rate of retreat of the West Antarctic ice sheet, a new study finds.

A 16-year study of ocean conditions in Antarctica suggests that the periodic arrival of warm currents as a result of natural variability is worsening the rate of ice mass loss from key glaciers in the region.

The natural pulses of warm water could be key to driving short-term changes in glacier ice mass loss, the lead author tells Carbon Brief. In the long term, this periodic ocean warming is likely to be exacerbated by climate change, he adds.

The new findings serve as a “smoking gun” by helping scientists to understand the mechanisms behind the ice sheet’s retreat, another scientist tells Carbon Brief.

The researchers believe that El Niño is altering the strength of these ocean currents, periodically pulling or pushing the CDW towards or away from the glaciers on Antarctica’s coast, driving the “warm” and “cool” ocean phases, respectively."

49
While Jakobshavn is already undergoing ice cliff failures and may very well undergo and temporary acceleration of ice cliff failures once the grounding line / calving front reaches the retrograde bed slope, but once that bed slope changes to a prograde slope then the temporary acceleration will stop and the ice cliff failures will slowdown to something like their current rate of calving. 
Is there evidence of such cliff failures at Jakobshavn? Do you have a link or a reference? I would imagine the calving front is getting quite a bit of lateral support from the channel walls which is holing the glacier back and preventing extra thinning and grounding-line retreat.

The linked reference by Hughes et al (2015) presents an analysis about the current type of ice-cliff failures occurring at Jakobshavn (see that attached image), which focused on progressive ice-bed uncoupling due to such factors as: basal meltwater, buoyancy friction (particularly with changing surface elevation), boundary constraints of the fjord.  This work has relevance to multiple marine-terminating, and marine, glaciers in both Greenland and Antarctica (see the extract for concerns about the PIG an the Thwaites Glacier, among other Antarctic marine glaciers).  However, I note that with regard to the Thwaites Glacier: a) it has a 50-km wide gateway for floating-out calved icebergs; b) it has little lateral boundary constraints; c) as its ice velocity has already increased this induces internal friction that induces ice melt that migrates to the glacial bed that facilitates more ice mass loss from calving; and d) one the calving retreats upstream of the gateway, the ice-cliff calving face can/will migrate in a 2-D horizontal plane.

Hughes, T., Sargent, A., Fastook, J., Purdon, K., Li, J., Yan, J.-B., and Gogineni, S.: Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica, and Jakobshavn Isbrae, Greenland, The Cryosphere Discuss., 9, 4271-4354, doi:10.5194/tcd-9-4271-2015, 2015.

http://www.the-cryosphere-discuss.net/9/4271/2015/tcd-9-4271-2015.pdf

Abstract. The first-order control of ice thickness and height above sea level is linked to the decreasing strength of ice-bed coupling alone flowlines from an interior ice divide to the calving front of an ice shelf. Uncoupling progresses as a frozen bed progressively thaws for sheet flow, as a thawed bed is progressively drowned for stream flow, and as lateral and/or local grounding vanish for shelf flow. This can reduce ice thicknesses by 90 % and ice elevations by 99 % along flowlines. Original work presented here includes (1) replacing flow and sliding laws for sheet flow with upper and lower yield stresses for creep in cold overlying ice and basal ice sliding over deforming till, respectively, (2) replacing integrating the Navier–Stokes equations for stream flow with geometrical solutions to the force balance, and (3) including resistance to shelf flow caused by lateral confinement in a fjord and local grounding at ice rumples and ice rises. A comparison is made between our approach and two approaches based on continuum mechanics. Applications are made to Byrd Glacier in Antarctica and Jakobshavn Isbrae in Greenland.


Extract: "Equation (24), based only on the force balance, is especially useful here because of its robust simplicity that applies to all flowlines and flowbands (ice streams) that end at a specified ice thickness h0. It gives phi variations along x that are usually somewhat higher than when the mass balance is also included, but with the same general trend. Using Eq. (24), Pingree et al. (2011) showed how Eq. (24) produced ice elevations before and after a former surge lifecycle of Lambert Glacier in East Antarctica, and for impending surge lifecycles of Thwaites Glacier and Pine Island Glacier entering the Pine Island Bay polynya in West Antarctica that continue into East Antarctica. Using Eq. (24), Hughes (2011) has tentatively assigned inception, growth, mature, declining, and terminal lifecycle stages shown in Table 2 to all major Antarctic ice streams at the present time."

50
Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: August 04, 2020, 05:27:43 PM »
The linked article cites an IPCC report that finds that the number of cooling units worldwide are projected to quadruple by 2050 and that with associated direct and indirect emissions increasing by 90%.

Title: "World to need 10 bln-plus more cooling machines: Report"

https://www.downtoearth.org.in/news/climate-change/world-to-need-10-bln-plus-more-cooling-machines-report-72353

Extract: "The world will need more than 10 billion new cooling appliances by 2050, according to a recent estimate. That would take the total up to around 14 billion such machines in a warming world.

An estimated 3.6 billion appliances are currently in use. This number, however, is increasing by up to 10 cooling appliances every second, said the Cooling Emissions and Policy Synthesis Report: Benefits of cooling efficiency and the Kigali Amendment report released July 17, 2020.

At this rate, the world will have four times more or at least 14 billion cooling appliances by 2050, contributing significantly to rising temperatures, the report by the UN Environment Programme (UNEP) and the International Energy Agency (IEA) pointed out.

Most ACs use hydrofluorocarbons (HFCs), extremely potent greenhouse gases and also need significant energy to run. Without policy intervention, direct and indirect emissions from air conditioning and refrigeration are projected to rise 90 per cent above 2017 levels by the year 2050."

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