View source for Marine ice sheet instability

{{#ev:youtube||320|right|A collage of footage and animation to explain the changes that are occurring on the West Antarctic Ice Sheet, narrated by glaciologist Eric Rignot.|id=C1iumhQ4AE4}}
'''Marine ice sheet instability''' (MISI) describes the potential for ice sheets grounded below sea level to destabilize in a runaway fashion. The mechanism was first proposed in the 1970s<ref>Weertman, J. (1974) [https://www.cambridge.org/core/journals/journal-of-glaciology/article/stability-of-the-junction-of-an-ice-sheet-and-an-ice-shelf/489C95706873563AC12394FB713CD91C Stability of the Junction of an Ice Sheet and an Ice Shelf] Journal of Glaciology</ref><ref>Thomas, Robert H.; Bentley, Charles R. (1978) [https://www.cambridge.org/core/journals/quaternary-research/article/abs/model-for-holocene-retreat-of-the-west-antarctic-ice-sheet/3EE3894B569CE4C0AE72434EEA7E8A7F A Model for Holocene Retreat of the West Antarctic Ice Sheet A Model for Holocene Retreat of the West Antarctic Ice Sheet] Cambridge University Press</ref> by Johannes Weertman and was quickly identified as a means by which even gradual anthropogenic warming could lead to relatively rapid sea level rise.<ref>Mercer, J. H. (1978) [https://www.nature.com/articles/271321a0 West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster] nature</ref><ref>Vaughan, David G. (2008) [http://nora.nerc.ac.uk/id/eprint/769/1/The_return_of_a_paradigm_16_-_nora.pdf West Antarctic Ice Sheet collapse – the fall and rise of a paradigm] (PDF) [https://link.springer.com/article/10.1007/s10584-008-9448-3 DOI]</ref> In Antarctica, the West Antarctic Ice Sheet, the Aurora Subglacial Basin, and the Wilkes Basin are each grounded below sea level and are inherently subject to MISI.

==General==
The term marine ice sheet describes an ice sheet whose base rests on ground below sea level, and marine ice sheet instability describes the inherent precarious nature of marine ice sheets due to Archimedes' principle. Because seawater is denser than ice, marine ice sheets can only remain stable where the ice is thick enough for its mass to exceed the mass of the seawater displaced by the ice. In other words, wherever ice exists below sea level, it is held in place only by the weight of overlying ice. As a marine ice sheet melts, the weight of the overlying ice decreases. If melt causes thinning beyond a critical threshold, the overlying ice may no longer be heavy enough to prevent the submarine ice below it from lifting off the ground, allowing water to penetrate underneath.

The location of the grounding line, the boundary between the ice sheet and the floating ice shelves, is unstable in this case. The amount of ice flowing over the grounding line initially matches the production of ice from snow upstream. When the grounding line is pushed backwards, due to for instance melt by warm water, the ice sheet is thicker at the new location of the grounding line and the total amount of ice flowing through may increase. (This depends on the slope of the subaerial surface.) As this causes the ice sheet to lose mass, the grounding line is pushed back even further and this self-reinforcing mechanism is the cause of the instability. Ice sheets of this type have accelerated ice sheet retreat.<ref name="Pollard_2015">Pollard et al. (2015) [https://doi.org/10.1016%2Fj.epsl.2014.12.035 Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure] Earth and Planetary Science Letters</ref><ref>David Docquier (2016) [https://blogs.egu.eu/divisions/cr/2016/06/22/marine-ice-sheet-instability-for-dummies-2/ Marine Ice Sheet Instability "For Dummies"] EGU</ref>

Strictly speaking the MISI theory is only valid if the ice shelves are free floating and not constrained in an embayment.<ref name="Pattyn_2018">Pattyn, Frank (2018) [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6048022 The paradigm shift in Antarctic ice sheet modelling] Nature Communications</ref>

The initial perturbation or push-back of the grounding line might be caused by high water temperatures at the base of ice shelves so that melt increases (basal melt). The thinned ice shelves, which earlier stabilized the ice sheet, exert less of an buttressing effect (back stress).<ref name="Pollard_2015" />

==Marine Ice Cliff Instability==
A related process known as Marine Ice Cliff Instability (MICI) posits that due to the physical characteristics of ice, subaerial ice cliffs exceeding ~90 meters in height are likely to collapse under their own weight, and could lead to runaway ice sheet retreat in a fashion similar to MISI.<ref name="Pollard_2015" /> For an ice sheet grounded below sea level with an inland-sloping bed, ice cliff failure removes peripheral ice, which then exposes taller, more unstable ice cliffs, further perpetuating the cycle of ice front failure and retreat. Surface melt can further enhance MICI through ponding and hydrofracture.<ref name="Pattyn_2018" /><ref>Zappa et al. (2018) [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6007161/ Basal channels drive active surface hydrology and transverse ice shelf fracture] ScienceAdvances</ref>

==Ice Mélange==
A main control on [[hydrofracturing]] is the thickness of the ice mélange encased in and around preexisting rifts that penetrate the entire ice shelf thickness. If the ice mélange thins beyond a threshold value, the rifts reactivate and trigger iceberg calving. This process linking climate forcing and ice shelf retreat is missing from models and does not require hydrofracture. Model simulations indicate that thinning of the ice mélange by 10 to 20 m is sufficient to reactivate the rifts and trigger a major calving event, thereby establishing a link between climate forcing and ice shelf retreat that has not been included in ice sheet models. Rift activation could initiate ice shelf retreat decades prior to hydrofracture caused by water ponding at the ice shelf surface.<ref>E. Larour, E. Rignot, M. Poinelli, and B. Scheuchl (2021) [https://www.pnas.org/doi/10.1073/pnas.2105080118 Physical processes controlling the rifting of Larsen C Ice Shelf, Antarctica, prior to the calving of iceberg A68] PNAS</ref>
<blockquote>For the ice melange itself, I would call it the “Achilles Heel” of ice shelves. If the ice melange melts, it will hurt the ice shelves very badly and they will fall apart. We believe that this melange is very sensitive to climate change and could disappear decades before melt water ponds at the surface of these ice shelves form and as a result they breakup, disintegrate. ~Eric Rignot (2021)<ref>Eric Rignot (2021) [https://climatestate.com/2021/10/06/eric-rignot-sea-level-rise-there-is-a-distinct-possibility-it-could-go-faster/ Sea level rise there is a distinct possibility it could go faster] Climate State</ref></blockquote>

==Ocean warming==
{{See also|Ocean heat content}}
[[File:Schematic-of-stratification-and-precipitation-amplifying-feedbacks.jpg|thumb|Schematic of stratification and precipitation amplifying feedbacks. Stratification: increased freshwater flux reduces surface water density, thus reducing AABW formation, trapping NADW heat, and increasing ice shelf melt. Precipitation: increased freshwater flux cools ocean mixed layer, increases sea ice area, causing precipitation to fall before it reaches Antarctica, reducing ice sheet growth and increasing ocean surface freshening. Ice in West Antarctica and the Wilkes Basin, East Antarctica, is most vulnerable because of the instability of retrograde beds.]]
According to a 2016 published study, cold meltwater provides cooling of the ocean's surface layer, acting like a lid, and also affecting deeper waters by increasing subsurface ocean warming and thus facilitating ice melt.
<blockquote>Our “pure freshwater” experiments show that the low-density lid causes deep-ocean warming, especially at depths of ice shelf grounding lines that provide most of the restraining force limiting ice sheet discharge.<ref name="Hansen2016">Hansen et al. (2016) [https://acp.copernicus.org/articles/16/3761/2016/ Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous] Atmospheric Chemistry and Physics</ref></blockquote>

Another theory discussed in 2007 for increasing warm bottom water is that changes in air circulation patterns have led to increased upwelling of warm, deep ocean water along the coast of Antarctica and that this warm water has increased melting of floating ice shelves.<ref>Various Authors (2017) [http://www.jsg.utexas.edu/news/2007/05/statement-thinning-of-west-antarctic-ice-sheet-demands-improved-monitoring-to-reduce-uncertainty-over-potential-sea-level-rise/ Statement: Thinning of West Antarctic Ice Sheet Demands Improved Monitoring to Reduce Uncertainty over Potential Sea-Level Rise]</ref> An ocean model has shown how changes in winds can help channel the water along deep troughs on the sea floor, toward the ice shelves of outlet glaciers.<ref name="ThomaJenkins2008">Jenkins et al. (2008) [http://epic.awi.de/25479/1/2008_Modelling_Circumpolar_Deep_Water_intrusions_on_the_Amundsen_Sea_continental_shelf_Antarctica.pdf Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf, Antarctica] Geophysical Research Letters</ref>

==Observations==
{{See also|West Antarctic Ice Sheet}}
In West Antarctica, the Thwaites and Pine Island glaciers have been identified to be potentially prone to MISI, and both glaciers have been rapidly thinning and accelerating in recent decades.<ref>Robinson Meyer (2018) [https://www.theatlantic.com/science/archive/2018/06/after-decades-of-ice-loss-antarctica-is-now-hemorrhaging-mass/562748/ After Decades of Losing Ice, Antarctica Is Now Hemorrhaging It] The Atlantic</ref><ref name="Copernicus_2018">Gardner, A. S., Moholdt, G., Scambos, T., Fahnstock, M., Ligtenberg, S., van den Broeke, M., and Nilsson, J. (2018) [https://tc.copernicus.org/articles/12/521/2018/ Increased West Antarctic and unchanged East Antarctic ice discharge over the last 7 years] The Cryosphere</ref><ref>IMBIE team (2018) [https://orbi.uliege.be/handle/2268/225208 Mass balance of the Antarctic Ice Sheet from 1992 to 2017] Nature</ref> In East Antarctica, Totten Glacier is the largest glacier known to be subject to MISI<ref>Young, D., Wright, A., Roberts, J. et al. (2011) [https://www.nature.com/articles/nature10114 A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes] Nature</ref> and its sea level potential is comparable to that of the entire West Antarctic Ice Sheet. Totten Glacier has been losing mass nearly monotonically in recent decades,<ref>Mohajerani, Y;Velicogna, I;Rignot, E (2018) [https://escholarship.org/uc/item/21c3r9dv Mass Loss of Totten and Moscow University Glaciers, East Antarctica, Using Regionally Optimized GRACE Mascons] UC Irvine</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>Chad A. Greene, Duncan A. Young, David E. Gwyther, Benjamin K. Galton-Fenzi, and Donald D. Blankenship (2018) [https://tc.copernicus.org/articles/12/2869/2018/ Seasonal dynamics of Totten Ice Shelf controlled by sea ice buttressing] EGU</ref><ref>(2017) [https://eprints.utas.edu.au/25611/1/SP461.6.full.pdf Ocean forced variability of Totten Glacier mass loss]</ref><ref>Greene CA, Blankenship DD, Gwyther DE, Silvano A, van Wijk E. (2017) [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5665591/ Wind causes Totten Ice Shelf melt and acceleration] ScienceAdvances</ref> The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.<ref name="Copernicus_2018" />

==References==
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[[Category:Climate change]]
[[Category:Glaciology]]
[[Category:Ice sheets]]