Editing Ice sheet (section)

==== Marine ice cliff instability ====

[[File:West Antarctic Collapse.ogv|thumb|A collage of footage and animation to explain the changes that are occurring on the West Antarctic Ice Sheet, narrated by glaciologist [[Eric Rignot]]]]

A related process known as ''Marine Ice Cliff Instability'' (MICI) posits that ice cliffs which exceed ~{{cvt|90|m|ft|frac=2}} in above-ground height and are ~{{cvt|800|m|ft|frac=2}} in basal (underground) height are likely to collapse under their own weight once the peripheral ice stabilizing them is gone.<ref name="Zhang2022" /> Their collapse then exposes the ice masses following them to the same instability, potentially resulting in a self-sustaining cycle of cliff collapse and rapid ice sheet retreat - i.e. sea level rise of a meter or more by 2100 from Antarctica alone.<ref name="Pollard2015" /><ref name="DeConto2016" /><ref name="Pattyn 2018">{{Cite journal |last=Pattyn |first=Frank |author-link=Frank Pattyn |date=2018 |title=The paradigm shift in Antarctic ice sheet modelling |journal=Nature Communications |language=En |volume=9 |issue=1 |page=2728 |bibcode=2018NatCo...9.2728P |doi=10.1038/s41467-018-05003-z |issn=2041-1723 |pmc=6048022 |pmid=30013142}}</ref><ref>{{Cite journal|last1=Dow|first1=Christine F.|last2=Lee|first2=Won Sang|last3=Greenbaum|first3=Jamin S.|last4=Greene|first4=Chad A.|last5=Blankenship|first5=Donald D.|last6=Poinar|first6=Kristin|last7=Forrest|first7=Alexander L.|last8=Young|first8=Duncan A.|last9=Zappa|first9=Christopher J.|date=2018-06-01|title=Basal channels drive active surface hydrology and transverse ice shelf fracture|journal=Science Advances|language=en|volume=4|issue=6|pages=eaao7212|doi=10.1126/sciadv.aao7212|issn=2375-2548|pmc=6007161|pmid=29928691|bibcode=2018SciA....4.7212D}}</ref> This theory had been highly influential - in a 2020 survey of 106 experts, the paper which had advanced this theory was considered more important than even the year 2014 [[IPCC Fifth Assessment Report]].<ref name="Horton2020">{{Cite journal |last1=Horton |first1=Benjamin P. |last2=Khan |first2=Nicole S. |last3=Cahill |first3=Niamh |last4=Lee |first4=Janice S. H. |last5=Shaw |first5=Timothy A. |last6=Garner |first6=Andra J. |last7=Kemp |first7=Andrew C. |last8=Engelhart |first8=Simon E. |last9=Rahmstorf |first9=Stefan |date=2020-05-08 |title=Estimating global mean sea-level rise and its uncertainties by 2100 and 2300 from an expert survey |journal=npj Climate and Atmospheric Science |volume=3 |issue=1 |page=18 |doi=10.1038/s41612-020-0121-5 |bibcode=2020npCAS...3...18H |s2cid=218541055 |hdl=10356/143900 |hdl-access=free }}</ref> Sea level rise projections which involve MICI are much larger than the others, particularly under high warming rate.<ref name="Slangen2022">{{cite journal |last1=Slangen |first1=A. B. A. |last2=Haasnoot |first2=M. |last3=Winter |first3=G. |date=30 March 2022 |title=Rethinking Sea-Level Projections Using Families and Timing Differences |journal=Earth's Future |volume=10 |issue=4 |page=e2021EF002576 |doi=10.1029/2021EF002576 |bibcode=2022EaFut..1002576S |url=https://www.vliz.be/imisdocs/publications/00/378000.pdf }}</ref>

At the same time, this theory has also been highly controversial.<ref name="Zhang2022">{{Cite conference |last=Zhang |first=Zhe |date=7 November 2021 |title=Reviewing the elements of marine ice cliff instability |conference=The International Conference on Materials Chemistry and Environmental Engineering (CONF-MCEE 2021) |location=California, United States |journal=Journal of Physics: Conference Series |volume=2152 |doi=10.1088/1742-6596/2152/1/012057 |doi-access=free }}</ref> It was originally proposed in order to describe how the large sea level rise during the [[Pliocene]] and the [[Last Interglacial]] could have occurred<ref name="Zhang2022" /><ref name="DeConto2016">{{Cite journal |last1=DeConto |first1=Robert M. |last2=Pollard |first2=David |date=30 March 2016 |title=Contribution of Antarctica to past and future sea-level rise |journal=Nature |language=en |volume=531 |issue=7596 |pages=591–597 |doi=10.1038/nature17145 |pmid=27029274 |bibcode=2016Natur.531..591D |s2cid=205247890 }}</ref> - yet more recent research found that these sea level rise episodes can be explained without any ice cliff instability taking place.<ref name="Gilford2020" /><ref name="Zhang2022" /><ref>{{Cite journal |last1=Edwards |first1=Tamsin L. |last2=Brandon |first2=Mark A. |last3=Durand |first3=Gael |last4=Edwards |first4=Neil R. |last5=Golledge |first5=Nicholas R. |last6=Holden |first6=Philip B. |last7=Nias |first7=Isabel J.|last8=Payne |first8=Antony J. |last9=Ritz |first9=Catherine |last10=Wernecke |first10=Andreas |date=6 February 2019 |title=Revisiting Antarctic ice loss due to marine ice-cliff instability |journal=Nature |language=en |volume=566 |issue=7742 |pages=58–64 |doi=10.1038/s41586-019-0901-4 |pmid=30728522 |bibcode=2019Natur.566...58E |s2cid=59606547 |issn=1476-4687 |hdl=1983/de5e9847-612f-42fb-97b0-5d7ff43d37b8 |url=https://oro.open.ac.uk/58538/1/Edwards_et_al_2019_Nature.pdf |hdl-access=free}}</ref> Research in [[Pine Island Bay]] in [[West Antarctica]] (the location of [[Thwaites Glacier|Thwaites]] and [[Pine Island Glacier]]) had found [[seabed gouging by ice]] from the [[Younger Dryas]] period which appears consistent with MICI.<ref name="Wise2017" /><ref name="Gilford2020" /> However, it indicates "relatively rapid" yet still prolonged ice sheet retreat, with a movement of >{{cvt|200|km|mi}} inland taking place over an estimated 1100 years <!-- Do the math ... or, "see" (e.g.), https://www.google.com/search?q=(-12%2C300)+-+(-11%2C200)+%3D -->(from ~12,300 years [[Before Present]] to ~11,200 B.P.)<ref name="Wise2017">{{Cite journal |last1=Wise |first1=Matthew G. |last2=Dowdeswell |first2=Julian A. |last3=Jakobsson |first3=Martin |last4=Larter |first4=Robert D. |date=October 2017 |title=Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks |url=https://nora.nerc.ac.uk/id/eprint/514800/1/Nature_final_accepted_ms.pdf |journal=Nature |language=en |volume=550 |issue=7677 |pages=506–510 |doi=10.1038/nature24458 |pmid=29072274 |bibcode=2017Natur.550..506W |issn=0028-0836 |archive-url=https://web.archive.org/web/20200506155034/https://nora.nerc.ac.uk/id/eprint/514800/1/Nature_final_accepted_ms.pdf |archive-date=May 6, 2020}}</ref>

[[File:Schlemm 2022 MICI embayment.png|thumb|left|If MICI can occur, the structure of the glacier [[embayment]] (viewed from the top) would do a lot to determine how quickly it may proceed. Bays which are deep or narrow towards the exit would experience much less rapid retreat than the opposite<ref name="Schlemm2022" />]]

In recent years, 2002-2004 fast retreat of [[Crane Glacier]] immediately after the collapse of the [[Larsen B]] ice shelf (before it reached a shallow [[fjord]] and stabilized) could have involved MICI, but there weren't enough observations to confirm or refute this theory.<ref name="Needell2023">{{cite journal |last1=Needell |first1=C. |last2=Holschuh |first2=N.  |date=20 January 2023 |title=Evaluating the Retreat, Arrest, and Regrowth of Crane Glacier Against Marine Ice Cliff Process Models |journal=Geophysical Research Letters |volume=50 |issue=4 |page=e2022GL102400 |doi=10.1029/2022GL102400 |doi-access=free |bibcode=2023GeoRL..5002400N }}</ref> The retreat of [[Greenland ice sheet]]'s three largest glaciers - [[Jakobshavn Glacier|Jakobshavn]], [[Helheim Glacier|Helheim]], and [[Kangerdlugssuaq Glacier]] - did not resemble predictions from ice cliff collapse at least up until the end of 2013,<ref name="Gilford2020" /><ref>{{cite journal |last1=Olsen |first1=Kira G. |last2=Nettles |first2=Meredith |date=8 June 2019 |title=Constraints on Terminus Dynamics at Greenland Glaciers From Small Glacial Earthquakes |journal=Journal of Geophysical Research: Earth Surface |volume=124 |issue=7 |pages=1899–1918 |doi=10.1029/2019JF005054 |bibcode=2019JGRF..124.1899O }}</ref> but an event observed at Helheim Glacier in August 2014 may fit the definition.<ref name="Gilford2020" /><ref>{{Cite journal |last1=Parizek |first1=Byron R. |last2=Christianson |first2=Knut |last3=Alley |first3=Richard B. |last4=Voytenko |first4=Denis |last5=Vaňková |first5=Irena |last6=Dixon |first6=Timothy H. |last7=Walker |first7=Ryan T. |last8=Holland |first8=David M. |date=22 March 2019 |title=Ice-cliff failure via retrogressive slumping |journal=Geology |volume=47 |issue=5 |pages=449–452 |doi=10.1130/G45880.1 |doi-access=free |bibcode=2019Geo....47..449P }}</ref> Further, modelling done after the initial hypothesis indicates that ice-cliff instability would require implausibly fast ice shelf collapse (i.e. within an hour for ~{{cvt|90|m|ft|frac=2}}-tall cliffs),<ref>{{cite journal |last1=Clerc |first1=Fiona |last2=Minchew |first2=Brent M. |last3=Behn |first3=Mark D. |date=21 October 2019 |title=Marine Ice Cliff Instability Mitigated by Slow Removal of Ice Shelves |journal=Geophysical Research Letters |volume=50 |issue=4 |pages=e2022GL102400 |doi=10.1029/2019GL084183 |bibcode=2019GeoRL..4612108C |hdl=1912/25343 |hdl-access=free }}</ref> unless the ice had already been substantially damaged beforehand.<ref name="Needell2023" /> Further, ice cliff breakdown would produce a large number of debris in the coastal waters - known as [[ice mélange]] - and multiple studies indicate their build-up would slow or even outright stop the instability soon after it started.<ref>{{cite news|url=https://www.sciencenews.org/article/climate-marine-ice-cliffs-sheets-collapse-not-inevitable-sea-level|title=Collapse may not always be inevitable for marine ice cliffs|last1=Perkins|first1=Sid|date=17 June 2021|access-date=9 January 2023|agency=ScienceNews}}</ref><ref>{{Cite journal |last1=Bassis |first1=J. N. |last2=Berg |first2=B. |last3=Crawford |first3=A. J. |last4=Benn |first4=D. I. |date=18 June 2021 |title=Transition to marine ice cliff instability controlled by ice thickness gradients and velocity |journal=Science |language=en |volume=372 |issue=6548 |pages=1342–1344 |doi=10.1126/science.abf6271 |pmid=34140387 |bibcode=2021Sci...372.1342B |hdl=10023/23422 |issn=0036-8075|hdl-access=free }}</ref><ref>{{Cite journal |last1=Crawford |first1=Anna J. |last2=Benn |first2=Douglas I. |last3=Todd |first3=Joe |last4=Åström |first4=Jan A. |last5=Bassis |first5=Jeremy N. |last6=Zwinger |first6=Thomas |date=11 May 2021 |title=Marine ice-cliff instability modeling shows mixed-mode ice-cliff failure and yields calving rate parameterization |journal=Nature Communications |volume=12 |issue=1 |page=2701 |doi=10.1038/s41467-021-23070-7 |pmid=33976208 |pmc=8113328 |bibcode=2021NatCo..12.2701C |hdl=10023/23200 |hdl-access=free }}</ref><ref name="Schlemm2022">{{Cite journal |last1=Schlemm |first1=Tanja |last2=Feldmann |first2=Johannes |last3=Winkelmann |first3=Ricarda |last4=Levermann |first4=Anders |date=24 May 2022 |title=Stabilizing effect of mélange buttressing on the marine ice-cliff instability of the West Antarctic Ice Sheet |journal=The Cryosphere |volume=16 |issue=5 |pages=1979–1996 |doi=10.5194/tc-16-1979-2022 |doi-access=free |bibcode=2022TCry...16.1979S }}</ref>

Some scientists  - including the originators of the hypothesis, Robert DeConto and David Pollard - have suggested that the best way to resolve the question would be to precisely determine sea level rise during the [[Last Interglacial]].<ref name="Gilford2020" /> MICI can be effectively ruled out if SLR at the time was lower than {{cvt|4|m|ft|frac=2}}, while it is very likely if the SLR was greater than {{cvt|6|m|ft|frac=2}}.<ref name="Gilford2020">{{cite journal |last1=Gilford |first1=Daniel M. |last2=Ashe |first2=Erica L. |last3=DeConto |first3=Robert M. |last4=Kopp |first4=Robert E. |last5=Pollard |first5=David |last6=Rovere |first6=Alessio |date=5 October 2020 |title=Could the Last Interglacial Constrain Projections of Future Antarctic Ice Mass Loss and Sea-Level Rise? |journal=Journal of Geophysical Research: Earth Surface |volume=124 |issue=7 |pages=1899–1918 |doi=10.1029/2019JF005418 |bibcode=2020JGRF..12505418G |hdl=10278/3749063 |hdl-access=free }}</ref> As of 2023, the most recent analysis indicates that the Last Interglacial SLR is unlikely to have been higher than {{cvt|2.7|m|ft|frac=2}},<ref name="Dumitru2023" /> as higher values in other research, such as {{cvt|5.7|m|ft|frac=2}},<ref>{{cite journal |last1=Barnett |first1=Robert L. |last2=Austermann |first2=Jacqueline |last3=Dyer |first3=Blake |last4=Telfer |first4=Matt W. |last5=Barlow |first5=Natasha L. M. |last6=Boulton |first6=Sarah J. |last7=Carr |first7=Andrew S. |last8=Creel |first8=Roger |date=15 September 2023 |title=Constraining the contribution of the Antarctic Ice Sheet to Last Interglacial sea level |journal=Science Advances |volume=9 |issue=27 |pages=eadf0198 |doi=10.1126/sciadv.adf0198 |pmid=37406130 |pmc=10321746 |bibcode=2023SciA....9F.198B }}</ref> appear inconsistent with the new [[paleoclimate]] data from [[The Bahamas]] and the known history of the Greenland Ice Sheet.<ref name="Dumitru2023">{{cite journal |last1=Dumitru |first1=Oana A. |last2=Dyer |first2=Blake |last3=Austermann |first3=Jacqueline |last4=Sandstrom |first4=Michael R. |last5=Goldstein |first5=Steven L. |last6=D'Andrea |first6=William J. |last7=Cashman |first7=Miranda |last8=Creel |first8=Roger |last9=Bolge |first9=Louise |last10=Raymo |first10=Maureen E. |date=15 September 2023 |title=Last interglacial global mean sea level from high-precision U-series ages of Bahamian fossil coral reefs |journal=Quaternary Science Reviews |volume=318 |page=108287 |doi=10.1016/j.quascirev.2023.108287 |doi-access=free |bibcode=2023QSRv..31808287D }}</ref>