Marine ice sheet instability (MISI) describes the potential for

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at Las ...

s grounded below

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

to destabilize in a runaway fashion. The mechanism was first proposed in the 1970s by

Johannes Weertman Johannes Weertman (May 11, 1925 – October 13, 2018) was an American materials scientist and geophysicist. Biography Born in 1925 in Fairfield, Alabama, Weertman served in the United States Marine Corps for three years. He then received from Carn ...

and was quickly identified as a means by which even gradual anthropogenic warming could lead to relatively rapid

sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...

. In Antarctica, the

West Antarctic Ice Sheet The Western Antarctic Ice Sheet (WAIS) is the segment of the Antarctic ice sheet, continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. The WAI ...

, the Aurora Subglacial Basin, and the

Wilkes Basin The Wilkes Basin is a large subglacial basin situated generally southward of George V Coast and westward of Prince Albert Mountains in East Antarctica. The feature is approximately 1400 km long and 400 km wide. The Wilkes Basin is conside ...

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 Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimede ...

. Because

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

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 In natural science, subaerial (literally "under the air"), has been used since 1833,Subaerial
in the Merriam ...

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. Strictly speaking the MISI theory is only valid if the ice shelves are free floating and not constrained in an

embayment A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a narr ...

. The initial perturbation or push-back of the grounding line might be caused by high water temperatures at the base of

ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...

so that melt increases (basal melt). The thinned ice shelves, which earlier stabilized the ice sheet, exert less of an buttressing effect (back stress).

Marine Ice Cliff Instability

A related process known as ''Marine Ice Cliff Instability'' (MICI) posits that due to the physical characteristics of ice,

subaerial In natural science, subaerial (literally "under the air"), has been used since 1833,Subaerial
in the Merriam ...

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. 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 Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...

Ocean warming

Schematic-of-stratification-and-precipitation-amplifying-feedbacks

According to a 2016 published study, cold

meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...

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. 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. 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.

Observations

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. In East Antarctica,

Totten Glacier Totten Glacier is a large glacier draining a major portion of the East Antarctic Ice Sheet, through the Budd Coast of Wilkes Land in the Australian Antarctic Territory. The catchment drained by the glacier is estimated at , extending approximate ...

is the largest glacier known to be subject to MISI 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, 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. The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.

References

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External links

Marine Ice Sheet Instability “For Dummies”
Ice sheets Glaciology Effects of climate change Articles containing video clips