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{{Short description|Large mass of glacial ice}}
{{Redirect|Continental glacier|the glacier located in Wyoming|Continental Glacier}}

[[File:Antarctica 6400px from Blue Marble.jpg|thumb|upright=1.3|One of Earth's two ice sheets: The [[Antarctic ice sheet]] covers about 98% of the [[Antarctica|Antarctic]] [[continent]] and is the largest single mass of [[ice]] on Earth. It has an average thickness of over 2&nbsp;kilometers.<ref name="NSFfactsheet">{{Cite web |title=Ice Sheets |url=https://www.nsf.gov/geo/opp/antarct/science/icesheet.jsp |publisher=National Science Foundation}}</ref>]]

In [[glaciology]], an '''ice sheet''', also known as a '''continental glacier''',<ref>[http://amsglossary.allenpress.com/glossary/search?id=ice-sheet1 American Meteorological Society, Glossary of Meteorology] {{webarchive|url=https://web.archive.org/web/20120623093132/http://amsglossary.allenpress.com/glossary/search?id=ice-sheet1 |date=2012-06-23 }}</ref> is a mass of [[glacier|glacial]] [[ice]] that covers surrounding terrain and is greater than {{convert|50000|km2|sqmi|abbr=on|lk=off}}.<ref>{{cite web|url=http://gemini.oscs.montana.edu/~geol445/hyperglac/glossary.htm|access-date=2006-08-22|title=Glossary of Important Terms in Glacial Geology|url-status=dead|archive-url=https://web.archive.org/web/20060829001754/http://gemini.oscs.montana.edu/~geol445/hyperglac/glossary.htm|archive-date=2006-08-29}}</ref> The only current ice sheets are the [[Antarctic ice sheet]] and the [[Greenland ice sheet]]. Ice sheets are bigger than [[ice shelf|ice shelves]] or alpine [[glacier]]s. Masses of ice covering less than 50,000&nbsp;km<sup>2</sup> are termed an [[ice cap]]. An ice cap will typically feed a series of glaciers around its periphery.

Although the surface is cold, the base of an ice sheet is generally warmer due to [[Geothermal activity|geothermal]] heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet&nbsp;— these are [[ice stream]]s.

Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.<ref name="IPCC_AR6_AnnexVII" />

Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing more [[supraglacial lake]]s. These lakes may feed warm water to glacial bases and facilitate glacial motion.<ref name="IPCCc4" />

In previous geologic time spans ([[glacial period]]s) there were other ice sheets. During the [[Last Glacial Period]] at [[Last Glacial Maximum]], the [[Laurentide Ice Sheet]] covered much of [[North America]]. In the same period, the [[Weichselian]] ice sheet covered [[Northern Europe]] and the [[Patagonian Ice Sheet]] covered southern [[South America]].

== Overview ==
[[File:Greenland_ice_sheet_USGS.jpg|thumb|upright=1.2|[[Greenland ice sheet]] as seen from space]]
An ice sheet is a body of ice which covers a land area of continental size - meaning that it exceeds 50,000&nbsp;km<sup>2</sup>.<ref name="IPCC_AR6_AnnexVII" /> The currently existing two ice sheets in [[Greenland ice sheet|Greenland]] and [[Antarctic ice sheet|Antarctica]] have a much greater area than this minimum definition, measuring at 1.7 million km<sup>2</sup> and 14 million km<sup>2</sup>, respectively. Both ice sheets are also very thick, as they consist of a continuous ice layer with an average thickness of {{cvt|2|km|mi|frac=2}}.<ref name="NSFfactsheet" /><ref>{{Cite web |date=21 November 2012 |title=About the Greenland Ice Sheet |url=https://nsidc.org/ice-sheets-today/analyses/about-greenland-ice-sheet |publisher=National Snow and Ice Data Center }}</ref> This ice layer forms because most of the snow which falls onto the ice sheet never melts, and is instead compressed by the mass of newer snow layers.<ref name="IPCC_AR6_AnnexVII" />

This process of ice sheet growth is still occurring nowadays, as can be clearly seen in an example that occurred in [[World War II]]. A [[Glacier Girl|Lockheed P-38 Lightning fighter plane]] crashed in Greenland in 1942. It was only recovered 50 years later. By then, it had been buried under 81 m (268 feet) of ice which had formed over that time period.<ref>{{cite web |title=Glacier Girl: The Back Story |url=https://www.airspacemag.com/history-of-flight/glacier-girl-the-back-story-19218360/?all |website=Air & Space Magazine |publisher=Smithsonian Institution |access-date=21 June 2020 |archive-date=21 June 2020 |archive-url=https://web.archive.org/web/20200621142843/https://www.airspacemag.com/history-of-flight/glacier-girl-the-back-story-19218360/?all |url-status=live }}</ref>

== Dynamics ==

===Glacial flows===
[[Image:Antarctica glacier flow rate.jpg|300 px|thumb|Glacial flow rate in the Antarctic ice sheet.]]
[[File:Flow of Ice Across Antarctica.ogv|thumb|upright=1.35|The motion of ice in Antarctica]]

Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.<ref name="IPCC_AR6_AnnexVII" /> This difference in slope occurs due to an imbalance between high ice accumulation in the central plateau and lower accumulation, as well as higher [[ablation]], at the margins. This imbalance increases the [[shear stress]] on a glacier until it begins to flow. The flow velocity and deformation will increase as the equilibrium line between these two processes is approached.<ref name="Easterbrook">Easterbrook, Don J., Surface Processes and Landforms, 2nd Edition, Prentice-Hall Inc., 1999{{page needed|date=February 2014}}</ref><ref name="GreveBlatter2009">{{cite book|author1=Greve, R. |author2=Blatter, H. |year=2009|title=Dynamics of Ice Sheets and Glaciers|publisher=Springer|doi=10.1007/978-3-642-03415-2|isbn=978-3-642-03414-5}}{{page needed|date=October 2021}}</ref> This motion is driven by [[gravity]] but is controlled by temperature and the strength of individual glacier bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on time scales ranging from hourly (i.e. tidal flows) to the [[wikt:centennial|centennial]] (Milankovich cycles).<ref name="GreveBlatter2009" />

On an unrelated hour-to-hour basis, surges of ice motion can be modulated by tidal activity. The influence of a 1&nbsp;m tidal oscillation can be felt as much as 100&nbsp;km from the sea.<ref name=Clarke2005>{{cite journal | author = Clarke, G. K. C. |title=Subglacial processes |journal=Annual Review of Earth and Planetary Sciences |volume=33 |issue=1 |pages=247–276 |year=2005 |doi=10.1146/annurev.earth.33.092203.122621 |bibcode = 2005AREPS..33..247C }}</ref> During larger [[spring tide]]s, an ice stream will remain almost stationary for hours at a time, before a surge of around a foot in under an hour, just after the peak high tide; a stationary period then takes hold until another surge towards the middle or end of the falling tide.<ref name=Bindschalder2003>{{cite journal |last1=Bindschadler |first1=Robert A. |last2=King |first2=Matt A. |last3=Alley |first3=Richard B. |last4=Anandakrishnan |first4=Sridhar |last5=Padman |first5=Laurence |title=Tidally Controlled Stick-Slip Discharge of a West Antarctic Ice |journal=Science |date=22 August 2003 |volume=301 |issue=5636 |pages=1087–1089 |doi=10.1126/science.1087231 |pmid=12934005 |s2cid=37375591 |url=https://zenodo.org/record/1230832 }}</ref><ref name=Anandakrishnan2003>{{cite journal |last1=Anandakrishnan |first1=S. |last2=Voigt |first2=D. E. |last3=Alley |first3=R. B. |last4=King |first4=M. A. |title=Ice stream D flow speed is strongly modulated by the tide beneath the Ross Ice Shelf |journal=Geophysical Research Letters |date=April 2003 |volume=30 |issue=7 |page=1361 |doi=10.1029/2002GL016329 |bibcode=2003GeoRL..30.1361A |s2cid=53347069 |doi-access=free }}</ref> At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours.<ref name=Bindschalder2003/>

Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing more [[supraglacial lake]]s. These lakes may feed warm water to glacial bases and facilitate glacial motion.<ref name=IPCCc4>Sections 4.5 and 4.6 of {{IPCC4/wg1/4}}</ref> Lakes of a diameter greater than ~300&nbsp;m are capable of creating a fluid-filled crevasse to the glacier/bed interface. When these crevasses form, the entirety of the lake's (relatively warm) contents can reach the base of the glacier in as little as 2–18 hours – lubricating the bed and causing the glacier to [[surge (glacier)|surge]].<ref name=Krawczynski2007>{{cite conference |last1=Krawczynski |first1=M. J. |last2=Behn |first2=M. D. |last3=Das |first3=S. B. |last4=Joughin |first4=I. |title=Constraints on melt-water flux through the West Greenland ice-sheet: modeling of hydro- fracture drainage of supraglacial lakes |date=1 December 2007 |pages=C41B–0474 |bibcode=2007AGUFM.C41B0474K |url=http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |archive-url = https://archive.today/20121228013531/http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |url-status=dead |archive-date=2012-12-28 |access-date=2008-03-04 |book-title=Eos Trans. AGU |volume=88 |issue=52 }}</ref> Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.<ref name="Bell2011">{{Cite journal | last1 = Bell | first1 = R. E. | last2 = Ferraccioli | first2 = F. | last3 = Creyts | first3 = T. T. | last4 = Braaten | first4 = D. | last5 = Corr | first5 = H. | last6 = Das | first6 = I. | last7 = Damaske | first7 = D. | last8 = Frearson | first8 = N. | last9 = Jordan | first9 = T. | last10 = Rose | doi = 10.1126/science.1200109 | first10 = K. | last11 = Studinger | first11 = M. | last12 = Wolovick | first12 = M. | title = Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base | journal = Science | volume = 331 | issue = 6024 | pages = 1592–1595 | year = 2011 | pmid =  21385719| bibcode = 2011Sci...331.1592B | s2cid = 45110037 }}</ref>

===Boundary conditions===

[[Image:Larsen B collapse.jpg|thumb|The collapse of the [[Larsen B]] ice shelf had profound effects on the velocities of its feeder glaciers.]]

[[Image:Ice flow controls.jpg|thumb|Accelerated ice flows after the break-up of an ice shelf]]

As the margins end at the marine boundary, excess ice is discharged through ice streams or [[outlet glacier]]s. Then, it either falls directly into the sea or is accumulated atop the floating [[Ice shelf|ice shelves]].<ref name="IPCC_AR6_AnnexVII">IPCC, 2021: Annex VII: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref>{{Rp|page=2234}} Those ice shelves then [[Ice calving|calve]] icebergs at their periphery if they experience excess of ice. Ice shelves would also experience accelerated calving due to basal melting. In Antarctica, this is driven by heat fed to the shelf by the [[circumpolar deep water]] current, which is 3&nbsp;°C above the ice's melting point.<ref name=Walker2007>{{cite journal |last1=Walker |first1=Dziga P. |last2=Brandon |first2=Mark A. |last3=Jenkins |first3=Adrian |last4=Allen |first4=John T. |last5=Dowdeswell |first5=Julian A. |last6=Evans |first6=Jeff |title=Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough |journal=Geophysical Research Letters |date=16 January 2007 |volume=34 |issue=2 |pages=L02602 |doi=10.1029/2006GL028154 |bibcode=2007GeoRL..34.2602W |s2cid=30646727 |url=http://nora.nerc.ac.uk/id/eprint/1199/1/grl22452.pdf }}</ref>

The presence of ice shelves has a stabilizing influence on the glacier behind them, while an absence of an ice shelf becomes destabilizing. For instance, when [[Larsen B]] ice shelf in the [[Antarctic Peninsula]] had collapsed over three weeks in February 2002, the four glaciers behind it - [[Crane Glacier]], [[Green Glacier]], [[Hektoria Glacier]] and [[Jorum Glacier]] - all started to flow at a much faster rate, while the two glaciers (Flask and Leppard) stabilized by the remnants of the ice shelf did not accelerate.<ref name=Scambos2004>{{cite journal |last1=Scambos |first1=T. A. |title=Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica |journal=Geophysical Research Letters |date=2004 |volume=31 |issue=18 |pages=L18402 |doi=10.1029/2004GL020670 |bibcode=2004GeoRL..3118402S |s2cid=36917564 |doi-access=free |hdl=11603/24296 |hdl-access=free }}</ref>

The collapse of the Larsen B shelf was preceded by thinning of just 1&nbsp;metre per year, while some other Antarctic ice shelves have displayed thinning of tens of metres per year.<ref name="IPCCc4" /> Further, increased ocean temperatures of 1&nbsp;°C may lead to up to 10&nbsp;metres per year of basal melting.<ref name="IPCCc4" /> Ice shelves are always stable under mean annual temperatures of −9&nbsp;°C, but never stable above −5&nbsp;°C; this places regional warming of 1.5&nbsp;°C, as preceded the collapse of Larsen B, in context.<ref name="IPCCc4" />

===Marine ice sheet instability===

In the 1970s, [[Johannes Weertman]] proposed that because [[seawater]] is denser than ice, then any ice sheets grounded below [[sea level]] inherently become less stable as they melt due to [[Archimedes' principle]].<ref name="Weertman1974" /> Effectively, these marine ice sheets must have enough mass to exceed the mass of the seawater displaced by the ice, which requires excess thickness. As the ice sheet melts and becomes thinner, the weight of the overlying ice decreases. At a certain point, sea water could force itself into the gaps which form at the base of the ice sheet, and ''marine ice sheet instability'' (MISI) would occur.<ref name="Weertman1974">{{Cite journal|last=Weertman|first=J.|date=1974|title=Stability of the Junction of an Ice Sheet and an Ice Shelf|journal=Journal of Glaciology|language=en|volume=13|issue=67|pages=3–11|doi=10.3189/S0022143000023327|issn=0022-1430|doi-access=free}}</ref><ref name="Pollard2015" />

Even if the ice sheet is grounded below the sea level, MISI cannot occur as long as there is a stable ice shelf in front of it.<ref name="Pattyn 2018" /> The boundary between the ice sheet and the ice shelf, known as the ''grounding line'', is particularly stable if it is constrained in an [[Bay|embayment]].<ref name="Pattyn 2018" /> In that case, the ice sheet may not be thinning at all, as the amount of ice flowing over the grounding line would be likely to match the annual accumulation of ice from snow upstream.<ref name="Pollard2015" /> Otherwise, ocean warming at the base of an ice shelf tends to thin it through basal melting. As the ice shelf becomes thinner, it exerts less of a buttressing effect on the ice sheet, the so-called back stress increases and the grounding line is pushed backwards.<ref name="Pollard2015" /> The ice sheet is likely to start losing more ice from the new location of the grounding line and so become lighter and less capable of displacing seawater. This eventually pushes the grounding line back even further, creating a [[Positive feedback|self-reinforcing mechanism]].<ref name="Pollard2015">{{cite journal|journal=Nature|volume=412|pages=112–121|year=2015|title=Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure |author=David Pollard |author2=Robert M. DeConto |author3=Richard B. Alley |doi=10.1016/j.epsl.2014.12.035|doi-access=free|bibcode=2015E&PSL.412..112P}}</ref><ref>{{cite web|url=https://blogs.egu.eu/divisions/cr/2016/06/22/marine-ice-sheet-instability-for-dummies-2/|title=Marine Ice Sheet Instability "For Dummies"|work=EGU|year=2016|author=David Docquier}}</ref>

====Vulnerable locations====

[[File:Dotto_2022_PIB_meltwater.png|thumb|Distribution of meltwater hotspots caused by ice losses in [[Pine Island Bay]], the location of both Thwaites (TEIS refers to Thwaites Eastern Ice Shelf) and Pine Island Glaciers.<ref name="Dotto2022">{{Cite journal|last1=Dotto |first1=Tiago S. |last2=Heywood |first2=Karen J. |last3=Hall |first3=Rob A. |last4=Scambos |first4=Ted A. |last5=Zheng |first5=Yixi |last6=Nakayama |first6=Yoshihiro |last7=Hyogo |first7=Shuntaro |last8=Snow |first8=Tasha |last9=Wåhlin |first9=Anna K. |last10=Wild |first10=Christian |last11=Truffer |first11=Martin |last12=Muto |first12=Atsuhiro |last13=Alley |first13=Karen E. |last14=Boehme |first14=Lars |last15=Bortolotto |first15=Guilherme A. |last16=Tyler |first16=Scott W. |last17=Pettit |first17=Erin |date=21 December 2022 |title=Ocean variability beneath Thwaites Eastern Ice Shelf driven by the Pine Island Bay Gyre strength| display-authors= 3 |journal=Nature Communications|language=en |volume=13 |issue=1 |page=7840 |doi=10.1038/s41467-022-35499-5 |pmid=36543787 |pmc=9772408 |bibcode=2022NatCo..13.7840D }}</ref>]]

Because the entire West Antarctic Ice Sheet is grounded below the sea level, it would be vulnerable to geologically rapid ice loss in this scenario.<ref>{{Cite journal|last=Mercer|first=J. H.|date=1978|title=West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster|journal=Nature|language=En|volume=271|issue=5643|pages=321–325|doi=10.1038/271321a0|issn=0028-0836|bibcode=1978Natur.271..321M|s2cid=4149290}}</ref><ref>{{Cite journal|last=Vaughan|first=David G.|date=2008-08-20|title=West Antarctic Ice Sheet collapse – the fall and rise of a paradigm|journal=Climatic Change|language=en|volume=91|issue=1–2|pages=65–79|doi=10.1007/s10584-008-9448-3|bibcode=2008ClCh...91...65V|s2cid=154732005|issn=0165-0009|url=http://nora.nerc.ac.uk/id/eprint/769/1/The_return_of_a_paradigm_16_-_nora.pdf}}</ref> In particular, the [[Thwaites glacier|Thwaites]] and [[Pine Island glacier|Pine Island]] glaciers are most likely to be prone to MISI, and both glaciers have been rapidly thinning and accelerating in recent decades.<ref>{{cite web|url=https://www.theatlantic.com/science/archive/2018/06/after-decades-of-ice-loss-antarctica-is-now-hemorrhaging-mass/562748/|work=The Atlantic|year=2018|title=After Decades of Losing Ice, Antarctica Is Now Hemorrhaging It}}</ref><ref>{{cite web|url=http://www.antarcticglaciers.org/glaciers-and-climate/ice-ocean-interactions/marine-ice-sheets/|work=AntarcticGlaciers.org|year=2014|title=Marine ice sheet instability}}</ref><ref name="Gardner 2018">{{Cite journal|last1=Gardner|first1=A. S.|last2=Moholdt|first2=G.|last3=Scambos|first3=T.|last4=Fahnstock|first4=M.|last5=Ligtenberg|first5=S.|last6=van den Broeke|first6=M.|last7=Nilsson|first7=J.|date=2018-02-13|title=Increased West Antarctic and unchanged East Antarctic ice discharge over the last 7 years|journal=The Cryosphere|volume=12|issue=2|pages=521–547|doi=10.5194/tc-12-521-2018|bibcode=2018TCry...12..521G|issn=1994-0424|doi-access=free}}</ref><ref>{{Cite journal|author1=IMBIE team|date=2018|title=Mass balance of the Antarctic Ice Sheet from 1992 to 2017|journal=Nature|language=En|volume=558|issue=7709|pages=219–222|doi=10.1038/s41586-018-0179-y|issn=0028-0836|pmid=29899482|url=https://orbi.uliege.be/handle/2268/225208|bibcode=2018Natur.558..219I|hdl=2268/225208|s2cid=49188002}}</ref> As a result, sea level rise from the ice sheet could be accelerated by tens of centimeters within the 21st century alone.<ref name="IPCC AR6 WG1 Ch.9">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 9: Ocean, Cryosphere and Sea Level Change |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |pages=1270–1272 }}</ref>

The majority of the East Antarctic Ice Sheet would not be affected. [[Totten Glacier]] is the largest glacier there which is known to be subject to MISI - yet, its potential contribution to sea level rise is comparable to that of the entire West Antarctic Ice Sheet.<ref>{{Cite journal|last1=Young|first1=Duncan A.|last2=Wright|first2=Andrew P.|last3=Roberts|first3=Jason L.|last4=Warner|first4=Roland C.|last5=Young|first5=Neal W.|last6=Greenbaum|first6=Jamin S.|last7=Schroeder|first7=Dustin M.|last8=Holt|first8=John W.|last9=Sugden|first9=David E.|date=2011-06-02|title=A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes|journal=Nature|language=En|volume=474|issue=7349|pages=72–75|doi=10.1038/nature10114|pmid=21637255|issn=0028-0836|bibcode=2011Natur.474...72Y|s2cid=4425075}}</ref> Totten Glacier has been losing mass nearly monotonically in recent decades,<ref>{{Cite journal|last=Mohajerani|first=Yara|date=2018|title=Mass Loss of Totten and Moscow University Glaciers, East Antarctica, Using Regionally Optimized GRACE Mascons|journal=Geophysical Research Letters|volume=45|issue=14|pages=7010–7018|doi=10.1029/2018GL078173|bibcode=2018GeoRL..45.7010M|s2cid=134054176 |url=https://escholarship.org/uc/item/21c3r9dv|doi-access=free}}</ref> suggesting rapid retreat is possible in the near future, although the dynamic behavior of Totten Ice Shelf is known to vary on seasonal to interannual timescales.<ref>{{Cite journal|last1=Greene|first1=Chad A.|last2=Young|first2=Duncan A.|last3=Gwyther|first3=David E.|last4=Galton-Fenzi|first4=Benjamin K.|last5=Blankenship|first5=Donald D.|date=2018|title=Seasonal dynamics of Totten Ice Shelf controlled by sea ice buttressing|journal=The Cryosphere|language=en|volume=12|issue=9|pages=2869–2882|doi=10.5194/tc-12-2869-2018|issn=1994-0416|doi-access=free|bibcode=2018TCry...12.2869G}}</ref><ref>{{Cite journal|last1=Roberts|first1=Jason|last2=Galton-Fenzi|first2=Benjamin K.|last3=Paolo|first3=Fernando S.|last4=Donnelly|first4=Claire|last5=Gwyther|first5=David E.|last6=Padman|first6=Laurie|last7=Young|first7=Duncan|last8=Warner|first8=Roland|last9=Greenbaum|first9=Jamin|date=2017-08-23|title=Ocean forced variability of Totten Glacier mass loss|journal=Geological Society, London, Special Publications|volume=461|issue=1|pages=175–186|doi=10.1144/sp461.6|issn=0305-8719|url=https://eprints.utas.edu.au/25611/1/SP461.6.full.pdf|bibcode=2018GSLSP.461..175R|doi-access=free}}</ref><ref>{{Cite journal|last1=Greene|first1=Chad A.|last2=Blankenship|first2=Donald D.|last3=Gwyther|first3=David E.|last4=Silvano|first4=Alessandro|last5=Wijk|first5=Esmee van|date=2017-11-01|title=Wind causes Totten Ice Shelf melt and acceleration|journal=Science Advances|language=en|volume=3|issue=11|pages=e1701681|doi=10.1126/sciadv.1701681|issn=2375-2548|pmc=5665591|pmid=29109976|bibcode=2017SciA....3E1681G}}</ref> The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.<ref name="Gardner 2018" /> Ultimately, even geologically rapid sea level rise would still most likely require several millennia for the entirety of these ice masses (WAIS and the subglacial basins) to be lost.<ref name="ArmstrongMcKay2022">{{Cite journal |last1=Armstrong McKay |first1=David|last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda  |last4=Sakschewski |first4=Boris  |last5=Loriani |first5=Sina  |last6=Fetzer |first6=Ingo|last7=Cornell|first7=Sarah |last8=Rockström |first8=Johan |last9=Staal |first9=Arie |last10=Lenton |first10=Timothy |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=Science |language=en |volume=377 |issue=6611 |pages=eabn7950 |doi=10.1126/science.abn7950 |pmid=36074831 |hdl=10871/131584 |s2cid=252161375 |issn=0036-8075|hdl-access=free }}</ref><ref name="Explainer">{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en}}</ref>

==== 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>

== Earth's current two ice sheets ==

=== Antarctic ice sheet ===
{{excerpt|Antarctic ice sheet|paragraphs=1}}

====West Antarctic ice sheet====
{{Infobox glacier
|name=West Antarctic ice sheet
|photo=File:Antarctica_ice_shelves-en.svg
|photo_width=300
|area=<{{convert|1970000|km2|mi2|abbr=on}}<ref name="WAISDavies">{{cite web |last=Davies |first=Bethan |title=West Antarctic Ice Sheet |url=https://www.antarcticglaciers.org/antarctica-2/west-antarctic-ice-sheet-2/west-antarctic-ice-sheet/ |website=AntarcticGlaciers.org |date=21 October 2020 }}</ref> 
|thickness=~{{convert|1.05|km|mi|1|abbr=on}} (average),<ref name="BEDMAP2-2013">{{cite journal |last1=Fretwell |first1=P. |display-authors=et al |title=Bedmap2: improved ice bed, surface and thickness datasets for Antarctica |journal=The Cryosphere |volume=7 |issue=1 |page=390 |date=28 February 2013 |url=http://www.the-cryosphere.net/7/375/2013/tc-7-375-2013.pdf |access-date=6 January 2014 |doi=10.5194/tc-7-375-2013 |bibcode=2013TCry....7..375F |s2cid=13129041 |doi-access=free |archive-date=16 February 2020 |archive-url=https://web.archive.org/web/20200216072841/https://www.the-cryosphere.net/7/375/2013/tc-7-375-2013.pdf |url-status=live }}</ref>  ~{{convert|2|km|mi|1|abbr=on}} (maximum)<ref name="WAISDavies" /> 
|type=Ice sheet
|status=Receding
}}
{{excerpt|West Antarctic ice sheet}}

====East Antarctic ice sheet====
{{Infobox glacier
|name=East Antarctic ice sheet
|photo=File:Antarctica.svg
|photo_width=300
|thickness=~{{convert|2.2|km|mi|1|abbr=on}} (average),<ref name="Torsvik2008">{{cite book|last1=Torsvik|first1=T. H.|first2=C. |last2=Gaina |first3=T. F. |last3=Redfield|chapter=Antarctica and Global Paleogeography: From Rodinia, Through Gondwanaland and Pangea, to the Birth of the Southern Ocean and the Opening of Gateways|title=Antarctica: A Keystone in a Changing World|year=2008|pages=125–140|chapter-url=http://www.nap.edu/openbook.php?record_id=12168&page=125|doi=10.17226/12168|isbn=978-0-309-11854-5|url=http://pubs.usgs.gov/of/2007/1047/ }}</ref> ~{{convert|4.9|km|mi|1|abbr=on}} (maximum) <ref name="Fretwell2013">{{Cite journal|last1=Fretwell|first1=P.|last2=Pritchard|first2=H. D.|last3=Vaughan|first3=D. G.|last4=Bamber|first4=J. L.|last5=Barrand|first5=N. E.|last6=Bell|first6=R.|last7=Bianchi|first7=C.|last8=Bingham|first8=R. G.|last9=Blankenship|first9=D. D.|date=2013-02-28|title=Bedmap2: improved ice bed, surface and thickness datasets for Antarctica|journal=The Cryosphere|volume=7|issue=1|pages=375–393|doi=10.5194/tc-7-375-2013|issn=1994-0424|doi-access=free|bibcode=2013TCry....7..375F|hdl=1808/18763|hdl-access=free}}</ref>
|type=Ice sheet
}}
{{excerpt|East Antarctic ice sheet}}

=== Greenland ice sheet ===
{{Infobox glacier
|name=Greenland ice sheet
|photo=File:Greenland ice sheet AMSL thickness map-en.png
|other_name=''Grønlands indlandsis''<br />''Sermersuaq''
|photo_width=300
|area={{convert|1710000|km2|mi2|abbr=on}}<ref name="BBC2017">{{cite web |title=How Greenland would look without its ice sheet |date=14 December 2017 |url=https://www.bbc.com/news/science-environment-42260580 |publisher=[[BBC News]] |access-date=7 December 2023 |archive-date=7 December 2023 |archive-url=https://web.archive.org/web/20231207201039/https://www.bbc.com/news/science-environment-42260580 |url-status=live }}</ref>
|length={{convert|2400|km|mi|abbr=on}}<ref name="BRGIS">{{Cite encyclopedia|title=Greenland Ice Sheet|date=24 October 2023|url=https://www.britannica.com/place/Greenland-Ice-Sheet|language=en|access-date=26 May 2022|archive-date=30 October 2017|archive-url=https://web.archive.org/web/20171030220950/https://www.britannica.com/place/Greenland-Ice-Sheet|url-status=live}}</ref>
|width={{convert|1100|km|mi|abbr=on}}<ref name="BRGIS" />
|thickness={{convert|1.67|km|mi|1|abbr=on}} (average), ~{{convert|3.5|km|mi|1|abbr=on}} (maximum)<ref name="BBC2017" />
|type=Ice sheet
|coordinates = {{coord|76.7|N|41.2|W|region:GL_dim:{{Infobox dim|length_km=2400}}|format=dms|display=inline}}<ref name="BRGIS" />
}}
{{excerpt|Greenland ice sheet}}

== Role in carbon cycle ==
[[File:Carbon stores and fluxes in present day ice sheets.webp|thumb|Carbon stores and fluxes in present-day ice sheets (2019), and the predicted impact on carbon dioxide (where data exists). <br /> Estimated carbon fluxes are measured in Tg C a<sup>−1</sup> (megatonnes of carbon per year) and estimated sizes of carbon stores are measured in Pg C (thousands of megatonnes of carbon). DOC = [[dissolved organic carbon]], POC = [[particulate organic carbon]].<ref name="Wadham2019" />]]

Historically, ice sheets were viewed as inert components of the [[Marine carbon cycle|carbon cycle]] and were largely disregarded in global models. In 2010s, research had demonstrated the existence of uniquely adapted [[Marine microorganisms|microbial communities]], high rates of [[Marine biogeochemical cycles|biogeochemical]] and physical weathering in ice sheets, and storage and cycling of organic carbon in excess of 100 billion tonnes.<ref name="Wadham2019" />

There is a massive contrast in carbon storage between the two ice sheets. While only about 0.5-27 billion tonnes of pure carbon are present underneath the Greenland ice sheet, 6000-21,000 billion tonnes of pure carbon are thought to be located underneath Antarctica.<ref name="Wadham2019">{{cite journal |last1=Wadham |first1=J. L. |last2=Hawkings |first2=J. R. |last3=Tarasov |first3=L. |last4=Gregoire |first4=L. J. |last5=Spencer |first5=R. G. M. |last6=Gutjahr |first6=M. |last7=Ridgwell |first7=A. |last8=Kohfeld |first8=K. E. |title=Ice sheets matter for the global carbon cycle |journal=Nature Communications |date=15 August 2019 |volume=10 |issue=1 |page=3567 |doi=10.1038/s41467-019-11394-4 |pmid=31417076 |pmc=6695407 |bibcode=2019NatCo..10.3567W |hdl=1983/19a3bd0c-eff6-48f5-a8b0-1908c2404a24 |hdl-access=free }}</ref> This carbon can act as a [[climate change feedback]] if it is gradually released through meltwater, thus increasing overall [[carbon dioxide emissions]].<ref>{{Cite journal |last1=Ryu |first1=Jong-Sik |last2=Jacobson |first2=Andrew D. |date=6 August 2012 |title=CO2 evasion from the Greenland Ice Sheet: A new carbon-climate feedback |journal=Chemical Geology |language=en |volume=320 |issue=13 |pages=80–95 |bibcode=2012ChGeo.320...80R |doi=10.1016/j.chemgeo.2012.05.024}}</ref>

For comparison, 1400–1650&nbsp;billion tonnes are contained within the Arctic [[permafrost]].<ref>{{cite journal |last1=Tarnocai, C. |author2=Canadell, J.G. |author3=Schuur, E.A.G. |author4=Kuhry, P. |author5=Mazhitova, G. |author6=Zimov, S. |date=June 2009 |title=Soil organic carbon pools in the northern circumpolar permafrost region |journal=Global Biogeochemical Cycles |volume=23 |issue=2 |page=GB2023 |bibcode=2009GBioC..23.2023T |doi=10.1029/2008gb003327 |doi-access=free}}</ref> Also for comparison, the annual human caused carbon dioxide emissions amount to around 40&nbsp;billion tonnes of {{CO2}}.<ref name="IPCC AR6 WG1 Ch.9" />{{rp|1237}}

In Greenland, there is one known area, at [[Russell Glacier (Greenland)|Russell Glacier]], where meltwater carbon is released into the atmosphere as [[methane]], which has a much larger [[global warming potential]] than carbon dioxide.<ref name="Christiansen2018">{{cite journal |last1=Christiansen |first1=Jesper Riis |last2=Jørgensen |first2=Christian Juncher |date=9 November 2018 |title=First observation of direct methane emission to the atmosphere from the subglacial domain of the Greenland Ice Sheet |journal=Scientific Reports |volume=8 |issue=1 |page=16623 |doi=10.1038/s41598-018-35054-7 |pmid=30413774 |pmc=6226494 |bibcode=2018NatSR...816623C }}</ref> However, it also harbours large numbers of [[methanotroph]]ic bacteria, which limit those emissions.<ref>{{cite journal |last1=Dieser |first1=Markus |last2=Broemsen |first2=Erik L J E |last3=Cameron |first3=Karen A |last4=King |first4=Gary M |last5=Achberger |first5=Amanda |last6=Choquette |first6=Kyla |last7=Hagedorn |first7=Birgit |last8=Sletten |first8=Ron |last9=Junge |first9=Karen |last10=Christner |first10=Brent C |date=17 April 2014 |title=Molecular and biogeochemical evidence for methane cycling beneath the western margin of the Greenland Ice Sheet |journal=The ISME Journal |volume=8 |issue=11 |pages=2305–2316 |doi=10.1038/ismej.2014.59 |pmid=24739624 |pmc=4992074 |bibcode=2014ISMEJ...8.2305D }}</ref><ref>{{cite journal |last1=Znamínko |first1=Matěj |last2=Falteisek |first2=Lukáš |last3=Vrbická |first3=Kristýna |last4=Klímová |first4=Petra |last5=Christiansen |first5=Jesper R. |last6=Jørgensen |first6=Christian J. |last7=Stibal |first7=Marek |title=Methylotrophic Communities Associated with a Greenland Ice Sheet Methane Release Hotspot |journal=Microbial Ecology |date=16 October 2023 |volume=86 |issue=4 |pages=3057–3067 |doi=10.1007/s00248-023-02302-x |pmid=37843656 |pmc=10640400 |bibcode=2023MicEc..86.3057Z }}</ref>

==In geologic timescales==
[[File:Schannwell 2024 Heinrich events.png|thumb|A reconstruction of how Heinrich events would have likely proceeded, with the Laurentide ice sheet first growing to an unsustainable position, where the base of its periphery becomes too warm, and then rapidly losing ice until it is reduced to sustainable size<ref name="Schannwell2024">{{Cite journal|last1=Schannwell |first1=Clemens |last2=Mikolajewicz |first2=Uwe |last3=Kapsch |first3=Marie-Luise |last4=Ziemen |first4=Florian |date=5 April 2024 |title=A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles |journal=Nature Communications|language=en |volume=15 |issue=1 |page=2961 |doi=10.1038/s41467-024-47141-7 |pmid=38580634 |pmc=10997585 |bibcode=2024NatCo..15.2961S }}</ref>]]
Normally, the transitions between glacial and interglacial states are governed by [[Milankovitch cycles]], which are patterns in [[insolation]] (the amount of sunlight reaching the Earth). These patterns are caused by the variations in shape of the Earth's orbit and its angle relative to the Sun, caused by the gravitational pull of other planets as they go through their own orbits.<ref>{{cite journal |last1=Kerr |first1=Richard A. |date=14 July 1978 |title=Climate Control: How Large a Role for Orbital Variations? |url=https://www.jstor.org/stable/1746691 |journal=Science |volume=201 |issue=4351 |pages=144–146 |doi=10.1126/science.201.4351.144 |jstor=1746691 |pmid=17801827 |bibcode=1978Sci...201..144K |access-date=29 July 2022|url-access=subscription }}</ref><ref>{{cite web |title=Why Milankovitch (Orbital) Cycles Can't Explain Earth's Current Warming |last=Buis |first=Alan |date=27 February 2020 |url=https://climate.nasa.gov/ask-nasa-climate/2949/why-milankovitch-orbital-cycles-cant-explain-earths-current-warming/ |publisher=NASA |access-date=29 July 2022}}</ref>

For instance, during at least the last 100,000 years, portions of the ice sheet covering much of North America, the [[Laurentide Ice Sheet]] broke apart sending large flotillas of icebergs into the North Atlantic. When these icebergs melted they dropped the boulders and other continental rocks they carried, leaving layers known as [[ice rafted debris]]. These so-called [[Heinrich events]], named after their discoverer [[Hartmut Heinrich]], appear to have a 7,000–10,000-year [[Periodic function|periodicity]], and occur during cold periods within the last interglacial.<ref>{{cite journal |last1=Heinrich |first1=Hartmut |title=Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years |journal=Quaternary Research |date=March 1988 |volume=29 |issue=2 |pages=142–152 |doi=10.1016/0033-5894(88)90057-9 |bibcode=1988QuRes..29..142H |s2cid=129842509 }}</ref>

Internal ice sheet "binge-purge" cycles may be responsible for the observed effects, where the ice builds to unstable levels, then a portion of the ice sheet collapses. External factors might also play a role in forcing ice sheets. [[Dansgaard–Oeschger event]]s are abrupt warmings of the northern hemisphere occurring over the space of perhaps 40 years. While these D–O events occur directly after each Heinrich event, they also occur more frequently – around every 1500 years; from this evidence, paleoclimatologists surmise that the same forcings may drive both Heinrich and D–O events.<ref>{{cite book |doi=10.1029/GM112p0035 |chapter=The North Atlantic's 1–2 kyr climate rhythm: Relation to Heinrich events, Dansgaard/Oeschger cycles and the Little Ice Age |title=Mechanisms of Global Climate Change at Millennial Time Scales |series=Geophysical Monograph Series |year=1999 |last1=Bond |first1=Gerard C. |last2=Showers |first2=William |last3=Elliot |first3=Mary |last4=Evans |first4=Michael |last5=Lotti |first5=Rusty |last6=Hajdas |first6=Irka |last7=Bonani |first7=Georges |last8=Johnson |first8=Sigfus |volume=112 |pages=35–58 |doi-broken-date=2024-11-26 |isbn=978-0-87590-095-7 }}</ref>

''Hemispheric asynchrony in ice sheet behavior'' has been observed by linking short-term spikes of methane in Greenland ice cores and Antarctic ice cores. During [[Dansgaard–Oeschger event]]s, the northern hemisphere warmed considerably, dramatically increasing the release of methane from wetlands, that were otherwise tundra during glacial times. This methane quickly distributes evenly across the globe, becoming incorporated in Antarctic and Greenland ice. With this tie, paleoclimatologists have been able to say that the ice sheets on Greenland only began to warm after the Antarctic ice sheet had been warming for several thousand years. Why this pattern occurs is still open for debate.<ref>{{cite journal |last1=Turney |first1=Chris S. M. |last2=Fogwill |first2=Christopher J. |last3=Golledge |first3=Nicholas R. |last4=McKay |first4=Nicholas P. |last5=Sebille |first5=Erik van |last6=Jones |first6=Richard T. |last7=Etheridge |first7=David |last8=Rubino |first8=Mauro |last9=Thornton |first9=David P. |last10=Davies |first10=Siwan M. |last11=Ramsey |first11=Christopher Bronk |last12=Thomas |first12=Zoë A. |last13=Bird |first13=Michael I. |last14=Munksgaard |first14=Niels C. |last15=Kohno |first15=Mika |last16=Woodward |first16=John |last17=Winter |first17=Kate |last18=Weyrich |first18=Laura S. |last19=Rootes |first19=Camilla M. |last20=Millman |first20=Helen |last21=Albert |first21=Paul G. |last22=Rivera |first22=Andres |last23=Ommen |first23=Tas van |last24=Curran |first24=Mark |last25=Moy |first25=Andrew |last26=Rahmstorf |first26=Stefan |last27=Kawamura |first27=Kenji |last28=Hillenbrand |first28=Claus-Dieter |last29=Weber |first29=Michael E. |last30=Manning |first30=Christina J. |last31=Young |first31=Jennifer |last32=Cooper |first32=Alan |title=Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica |journal=Proceedings of the National Academy of Sciences |date=25 February 2020 |volume=117 |issue=8 |pages=3996–4006 |doi=10.1073/pnas.1902469117 |pmid=32047039 |pmc=7049167 |bibcode=2020PNAS..117.3996T |doi-access=free }}</ref><ref>{{cite journal |last1=Crémière |first1=Antoine |last2=Lepland |first2=Aivo |last3=Chand |first3=Shyam |last4=Sahy |first4=Diana |last5=Condon |first5=Daniel J. |last6=Noble |first6=Stephen R. |last7=Martma |first7=Tõnu |last8=Thorsnes |first8=Terje |last9=Sauer |first9=Simone |last10=Brunstad |first10=Harald |title=Timescales of methane seepage on the Norwegian margin following collapse of the Scandinavian Ice Sheet |journal=Nature Communications |date=11 May 2016 |volume=7 |issue=1 |pages=11509 |doi=10.1038/ncomms11509 |pmid=27167635 |pmc=4865861 |bibcode=2016NatCo...711509C }}</ref>

===Antarctic ice sheet during geologic timescales===
{{excerpt|Antarctic ice sheet#Situation during geologic time scales}}

===Greenland ice sheet during geologic timescales===
{{excerpt|Greenland ice sheet#Geological history}}

==See also==
*{{annotated link|Cryosphere}}
*{{annotated link|Ice planet}}
*{{annotated link|Quaternary glaciation}}
*{{annotated link|Snowball Earth}}
*{{annotated link|Wisconsin glaciation}}
* {{annotated link|Ice-sheet model}}

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20070608011925/http://www.unep.org/geo/geo_ice/ United Nations Environment Programme: Global Outlook for Ice and Snow]
*[https://blogs.egu.eu/divisions/cr/2016/06/22/marine-ice-sheet-instability-for-dummies-2/ Marine Ice Sheet Instability "For Dummies"]

{{Global warming}}
{{Earth's landforms}}
{{Authority control}}

[[Category:Bodies of ice|Sheet]]
[[Category:Ice sheets| ]]
[[Category:Snow or ice weather phenomena]]
[[Category:Water ice|Sheet]]
[[Category:Glaciology]]
[[Category:Effects of climate change]]
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