Which sea terminating Glacier will be the first to Catastrophically Collapse?

[NotaDenier]

I’m just wondering what others think. Catastrophic is defined as ice cliff failures allowing the glacier to recede at an accelerating pace.
The climate record says this has happened in the past.
Please also post your reason for naming the glacier. Has it sped up? Bed slopes down inland? What is your timeframe?
Greenland glaciers are also allowable.


Post your response here.

[Neven]

Why not add a poll?

[prokaryotes]

Given the rapid progression of grounding-line retreat in the model simulations, thinning associated with the retreat of Smith Glacier may reach the ice divide and undermine a portion of the Thwaites catchment as quickly as changes initiated at the Thwaites terminus.

[..]

The ice presently within the Smith, Pope, and Kohler drainage could raise global mean sea level by a relatively modest 6 cm (Fretwell et al., 2013), but thinning can lead to drainage capture and therefore increased loss of ice volume. Thus, due to a shared divide, rapid thinning could potentially hasten the collapse of the larger reservoir of ice in the neighboring Thwaites catchment.

https://tc.copernicus.org/articles/13/2817/2019/

[blumenkraft]

Given Johns post here:

There's a cill of sorts thereabouts, so maybe 100m+?

I would add Jakobshavn. The grounding line seems to be on a reverse slope now.

[Stephan]

I wouldn't bet on a catastrophic collapse. I rather think that the ongoing destruction processes e.g. in the Amundsen Sector (PIG/TG/Haynes/Smith etc) will continue. They probably speed up, leading to more often calvings, smaller ice shelves and a faster retreat of the grounding line. But this seems to me a catastrophee in very slow motion.

[NotaDenier]

Why not add a poll?

I’m not sure which glaciers to include. I have opinions but I know there are more knowledgeable people on this forum.  Depending on the responses I might start a poll.

Stephan the slow drip drip of calving will eventually lead down a retrograde slope somewhere and we’ll actually see the real consequence of AGW in action. I am less pessimistic than someone like AbruptSLR, I think the timeline is more like 100 years before we really see massive ice sheet instabilities.

But again I trying to see which glacier will go first.
One vote for Jakobshavn it is.

[sidd]

What is 'catastrophic'  collapse ?

sidd

[oren]

I bet it'll be the PIG. Its shelf is receding, its bay clears bergs fast and is often ice free, its bed is retrograde and its height is tall enough for all kinds of problems.
JH is too fast and too narrow for a catastrophic collapse, but it can speed up, which also brings about more SLR.

[AbruptSLR]

I am less pessimistic than someone like AbruptSLR, I think the timeline is more like 100 years before we really see massive ice sheet instabilities.

But again I trying to see which glacier will go first.
One vote for Jakobshavn it is.

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.  Thus, if you are asking which glacier will be the first to lead to a collapse of a ice sheet, then the only marine glacier that is reasonable to cite is the Thwaites Glacier.

To me, it is more productive to discuss the validity of the reasons that I have previously cited (& for which I have previously provided references) as to why Thwaites Glacier has a good probability of triggering a MICI-type of collapse of the WAIS before 2045.  While it is not possible to cite one specific scenario I am slowly preparing a summary, but here I cite a few key points.

A freshwater hosing event (like the temporary collapse of Jakobshavn's calving front and/or as short-term reversal of the Beaufort Gyre) say between 2025 and 2035, could slow the MOC sufficiently to trigger a Super El Nino  say by 2035 to 2040 that would cause both a sufficient perturbation of both CDW pulse into the ASE and surface ice melting to trigger an ice cliff front at the base of the current Thwaites Ice Tongue in the bed trough leading to the BSB.

See also:

Title: "NASA Space Laser Missions Map 16 Years of Ice Sheet Loss"

https://www.nasa.gov/feature/goddard/2020/nasa-space-laser-missions-map-16-years-of-ice-sheet-loss

[gerontocrat]

I am less pessimistic than someone like AbruptSLR, I think the timeline is more like 100 years before we really see massive ice sheet instabilities.

But again I trying to see which glacier will go first.
One vote for Jakobshavn it is.

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.  Thus, if you are asking which glacier will be the first to lead to a collapse of a ice sheet, then the only marine glacier that is reasonable to cite is the Thwaites Glacier.

See also:

Title: "NASA Space Laser Missions Map 16 Years of Ice Sheet Loss"

https://www.nasa.gov/feature/goddard/2020/nasa-space-laser-missions-map-16-years-of-ice-sheet-loss

Or you can go to https://forum.arctic-sea-ice.net/index.php/topic,2903.msg273889.html#msg273889

to see the latest data from GFZ who provide GRACE + GRACE-FO Antarctic Ice Sheet (AIS) Mass loss data by basin, which I have also summarised by region.

It shows how the basin that includes the Thwaites & PIG is where the AIS is losing most mass by far.  As far as the West Antarctic Ice Sheet is concerned, the sheer amount of annual mass loss suggests to me that "massive ice sheet instabilities" already exist.

ps: The Greenland Ice Sheet and its glaciers are huge, but compared with the glaciers of the AIS and the AIS itself, they are pigmies.

[interstitial]

The PIG is certainly moving faster than most other glaciers but it is fed by so many tributaries that most of western Antarctica would have to melt before I would consider it collapsed. It still amazes me that ice can flow at 13 m a day.

[nukefix]

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.

[oren]

I think the best reference is the film "Chasing Ice" which caught such an event on video (2012). I know I once watched a much longer version, but the last half minute of this gives a nice visual overview of how each calving step causes the next.

[AbruptSLR]

While the 'Chasing Ice' video is a great example of ice cliff failures at Jakobshavn, I am primarily concerned about slumping calving failures (see the first linked article & associated first linked reference & the associated image) in the BSB/Thwaites that would produce somewhat shallow draft icebergs (at least shallower than tabular icebergs) that can readily float out of the Thwaites gateway that would allow more calving leading to an MICI-type of collapse.  Also, see the second linked reference on ice cliff failures of Jakobshavn and see also the linked video:

Title: "Tall ice-cliffs may trigger big calving events -- and fast sea-level rise"

https://www.sciencedaily.com/releases/2019/03/190322163342.htm

Extract: "Glaciers that drain ice sheets such as Antarctica or Greenland often flow into the ocean, ending in near-vertical cliffs. As the glacier flows into the sea, chunks of the ice break off in calving events. Although much calving occurs when the ocean melts the front of the ice, and ice cliff above falls down, a new study presents another method of calving: slumping. And this process could break off much larger chunks of ice at a quicker rate.

The ice-cliff research was spurred by a helicopter ride over Jakobshavn and Helheim glaciers on Greenland's eastern coast. Helheim ends abruptly in the ocean, in near-vertical ice-cliffs reaching 30-stories high (100 meters). On the flight, scientists viewed large cracks (called crevasses) on top of the ice that marched towards the end of the glacier."

See also:

Byron R. Parizek, Knut Christianson, Richard B. Alley, Denis Voytenko, Irena Vaňková, Timothy H. Dixon, Ryan T. Walker, David M. Holland. Ice-cliff failure via retrogressive slumping. Geology, 2019; DOI: 10.1130/G45880.1

https://pubs.geoscienceworld.org/gsa/geology/article/47/5/449/569567/Ice-cliff-failure-via-retrogressive-slumping

&

Xie, S., Dixon, T. H., Voytenko, D., Deng, F., and Holland, D. M.: Grounding line migration through the calving season of Jakobshavn Isbræ, Greenland, observed with terrestrial radar interferometry, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-231, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2017-231/

Abstract. "Ice velocity variations near the terminus of Jakobshavn Isbræ, Greenland were observed with a terrestrial radar interferometer (TRI) during three summer campaigns in 2012, 2015, and 2016. Ice velocity variations appear to be largely modulated by ocean tides. We estimate a ∼ 1 km wide floating zone near the calving front in early summer of 2015 and 2016, where ice moves in phase with ocean tides. Digital Elevation Models (DEMs) generated by the TRI show that the glacier front here is thin (ice surface is < 125 m above local water level). However, in late summer 2012, there is no evidence of a floating ice tongue in the TRI observations. Ice surface elevation near the glacier front was also higher, > 140 m above local sea level within a very short distance (< 1 km) from the ice cliff. We hypothesize that during Jakobshavn Isbræ's recent calving seasons, the ice front advances ∼ 3 km from winter to spring, forming a > 1 km floating ice tongue. During the subsequent calving season in mid- and late-summer, the glacier retreats by losing its floating portion through a sequence of iceberg calving events. By late summer, the entire glacier is likely grounded. In addition to ice velocity variations driven by tide rise and fall, we also observed a transverse velocity variation in the mélange and floating ice front. This across flow-line signal is in phase with the first time derivative of tidal height, and is likely associated with tidal currents or bed topography."

&

[AbruptSLR]

For those who do not know how much at risk the grounded icebergs at the base of the Thwaites Ice Tongue (see the first image from 2013) already have a negative height above floatation at Point B shown in the second attached image (from Milillo et al. 2019) and thus could easily float away (likely exposing an ice cliff face) once the melange downstream of Point B is disrupted, say due to a coming Super El Nino.

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

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

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

Edit: For those who cannot see the heights above floatation for the indicated Points A, B, C, D, E & F, I provide the last two attached images.

[AbruptSLR]

For those who doubt that the pinned icebergs at the base of the Thwaites Ice Tongue cannot float-away, then look at the two images from Jordan et al (2020) that show that once unpinned these icebergs could float away in a northwest direction around the current subsea mount/ridge that is currently pinning the Thwaites Ice Tongue:

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

https://www.the-cryosphere-discuss.net/tc-2019-294/

Extract: "Airborne gravity provides a good first order estimate of sub-ice-shelf bathymetry. Despite the relatively high uncertainty (~100 m standard deviation) comparisons with different gravity inversion techniques, and new observational bathymetric data, indicate that the pattern of sub-ice-shelf bathymetry is well resolved.

Thwaites Glacier is connected to the deep ocean by a major trough >800 m deep and 20 km wide. In contrast the grounding lines of the of Dotson and Crosson ice shelves are accessible through relatively narrow channels and thin sub shelf cavities. In the Thwaites, Dotson and Crosson region, areas of ice shelf which developed before and after 1993 form distinct populations. The most recently un-grounded areas are underlain by thin cavities (average 112 m) where the ice shelf base closely tracks the underlying bed topography."

Caption of image 2: "Figure 2: New bathymetry and cavity maps. a) Final topography from terrain shift method. White lines A-D mark profiles in Fig. 3. Yellow outline encloses region constrained by gravity data. Pink line shows -800m depth contour. Light grey lines mark grounding lines and ice shelf edge."

Caption of image 3: "Figure 3: Profiles across ice shelves. Upper panel shows ice surface from REMA DEM (Howat et al., 2019) and base of ice shelf calculated assuming hydrostatic equilibrium, together with gravity-derived bathymetric estimates. Second panel shows input freeair gravity anomaly. Third panel shows magnetic anomalies derived from ITGC survey data (REF data doi) and ADMAP2 (Golynsky et al., 2018). a) Thwaites Eastern Ice Shelf. b) Thwites Glacier Tongue. c) Crosson Ice Shelf. d) Dotson Ice Shelf. Note 520 thin cavity in regions of ice sheet grounding line retreat since 1993 (grey boxes)."

[AbruptSLR]

For those who still do not believe that once unpinned that the icebergs within the Thwaites Ice Tongue can float away to the northwest I attach an image of the ice tongue from May 31, 2020 annotated by baking where the feature that he names the 'Shear Zone' are going just that and the feature that he names 'Calving Zone' indicates the iceberg that I believe could expose ice cliffs once they float away due to a perturbation (such as a Super El Nino event).

[AbruptSLR]

For those who think that a Super El Nino is the only perturbation that might trigger an MICI-type of collapse of the Thwaites/BSB ice, I note that in September 2012 the Thwaites Ice Tongue flow rate surged and continued flowing at a high rate through the end of 2012 (and this high flow rate can be associated with the surface elevation depression shown in the first image)

In this regards, the linked reference studies a subglacial draining event beneath Thwaites Glacier from June 2013 to January 2014 (see the last three attached images), and that these subglacial lakes can refill within 20-years which indicates that another associated surge of ice in the Thwaites Ice Tongue may occur in the 2032 to 2033 timeframe:

Smith et. al. (2017), "Connected subglacial lake drainage beneath Thwaites Glacier, West Antarctica", The Cryosphere, 11, 451–467, doi:10.5194/tc-11-451-2017

http://www.the-cryosphere.net/11/451/2017/tc-11-451-2017.pdf

Abstract. We present conventional and swath altimetry data from CryoSat-2, revealing a system of subglacial lakes that drained between June 2013 and January 2014 under the central part of Thwaites Glacier, West Antarctica (TWG). Much of the drainage happened in less than 6 months, with an apparent connection between three lakes spanning more than 130 km. Hydro-potential analysis of the glacier bed shows a large number of small closed basins that should trap water produced by subglacial melt, although the observed largescale motion of water suggests that water can sometimes locally move against the apparent potential gradient, at least during lake-drainage events. This shows that there are important limitations in the ability of hydro-potential maps to predict subglacial water flow. An interpretation based on a map of the melt rate suggests that lake drainages of this type should take place every 20–80 years, depending on the connectivity of the water flow at the bed. Although we observed an acceleration in the downstream part of TWG immediately before the start of the lake drainage, there is no clear connection between the drainage and any speed change of the glacier."

There is more information on the June 2013 to Jan 2014 drainage of four subglacial lakes beneath the Thwaites Glacier.  The article is entitled: "Hidden lakes drain below West Antarctica’s Thwaites Glacier".

http://www.washington.edu/news/2017/02/08/hidden-lakes-drained-under-west-antarcticas-thwaites-glacier/

Extract: "Researchers at the University of Washington and the University of Edinburgh used data from the European Space Agency’s CryoSat-2 to identify a sudden drainage of large pools below Thwaites Glacier, one of two fast-moving glaciers at the edge of the ice sheet. The study published Feb. 8 in The Cryosphere finds four interconnected lakes drained in the eight months from June 2013 and January 2014. The glacier sped up by about 10 percent during that time, showing that the glacier’s long-term movement is fairly oblivious to trickles at its underside.
…
Melting at the ice sheet base would refill the lakes in 20 to 80 years, Smith said. Over time meltwater gradually collects in depressions in the bedrock. When the water reaches a certain level it breaches a weak point, then flows through channels in the ice. As Thwaites Glacier thins near the coast, its surface will become steeper, Smith said, and the difference in ice pressure between inland regions and the coast may push water coastward and cause more lakes to drain."

[AbruptSLR]

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."

[AbruptSLR]

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."

[AbruptSLR]

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)."

[Tom_Mazanec]

The calving on that Chasing Ice video took 75 minutes. How long would a collapse of the Thwaites/BSB take? 75 hours? 75 days? How much would sea level rise?

[sidd]

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

[AbruptSLR]

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.

[AbruptSLR]

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)."

[prokaryotes]

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

In the next 50 years, the model shows that melting at the present rate could contribute one to four inches to global sea level rise. This number jumps to five to 13 inches by 2100 and 19 to 63 inches by 2200. These numbers are considerably higher than previous estimates, which forecasted up to 35 inches of sea level rise by 2200. The updated model is the first to include outlet glaciers — river-like bodies of ice that connect to the ocean.

Outlet glaciers play a key role in how ice sheets melt, but previous models lacked the data to adequately represent their complex flow patterns. The study found that melting outlet glaciers could account for up to 40% of the ice mass lost from Greenland in the next 200 years.

13 inch = 33 cm estimated without mechanism like MICI(?), and potentially still 'conservative'.

[AbruptSLR]

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."

[Tom_Mazanec]

According to « Reply #24 on: August 04, 2020, 10:14:55 PM » the Antarctic contribution to sea level rise could be as high as 19.5 meters by 2200.

[anaphylaxia]

My bets are on 79N. There is a considerable area for which the only catchment is Blaso lake, located at the grounding point of the 79N glacier, that is reciving an awful lot of warm meltwater, and the water level is rising day by day. It is also above sea level, with the glacier acting as a dam. Also the bedrock topography behind the grounding point is below sea level, facilitating fast slippage if the grounding zone gives way. I believe a Vavilov style ice stream could start after that.

[blumenkraft]

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

Nioghalvfjerdsbrae... No wonder people say 79N glacier. 

[johnm33]

79N is my choice too, i no longer pay enough attention to the Antarctic to have an opinion there.
If 79n is grounded on rocky permafrost as i suspect then the vast basin in front of it needs only to establish tidal contact with the ocean and warm to above 0C to have melt commence, and it may [is imho] already be happening. The shelf will be reinforced by the constant resupply of fresh water and the signs of breakdown will be massive releases of freshwater via, i guess, the northern fjord containing Spaltegletsche since there is a sill inhibitng exchange in front of 79Ns shelf. A secondary sign of breakdown may be the slumping of Zacheriae where it lies over deep valleys where water can accumulate.

[anaphylaxia]

Also, the whole Spaltegletcher part calved on in July, so the sunwarmed water got like 30 km closer to the ice tongue of 79N.

[oren]

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

Nioghalvfjerdsbrae... No wonder people say 79N glacier. 

Some difficulty pronouncing, but mainly because nioghalvfjerds is Danish for 79.

[blumenkraft]

Well, that makes sense. Thanks, Oren, for this crucial bit of information.