Antarctic Sound-2016-Joinville Island-Ice Shelf

An ice shelf is a large floating platform of ice that forms where a

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...

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 La ...

flows down to a

coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

line and onto the ocean surface. Ice shelves are only found in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland ...

Northern Canada Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories an ...

, and the Russian Arctic. The boundary between the floating ice shelf and the anchor ice (resting on

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...

) that feeds it is the grounding line. The thickness of ice shelves can range from about to . In contrast,

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...

is formed on water, is much thinner (typically less than ), and forms throughout the

Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...

. It is also found in the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-smal ...

around the continent of

. The movement of ice shelves is principally driven by

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

-induced pressure from the grounded ice. That flow continually moves ice from the grounding line to the seaward front of the shelf. In steady state, about half of Antarctica's ice shelf mass is lost to basal melt and half is lost to calving, but the relative importance of each process varies significantly between ice shelves. In recent decades, Antarctica's ice shelves have been out of balance, as they have lost more mass to basal melt and calving than has been replenished by the influx of new ice and snow. Typically, a shelf front will extend forward for years or decades between major calving events. Snow accumulation on the upper surface and melting from the lower surface are also important to the

mass balance In physics, a mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have b ...

of an ice shelf. Ice may also accrete onto the underside of the shelf. The density contrast between glacial ice and liquid water means that at least of the floating ice is above the ocean surface, depending on how much pressurized air is contained in the bubbles within the glacial ice, stemming from compressed snow. The formula for the denominators above is

1/ \rho_\text - \rho_\text)/\rho_\text /math>, density of cold seawater is about 1028 kg/m

³ and that of glacial ice from about 850 kg/m³ to well below 920 kg/m³, the limit for very cold ice without bubbles. The height of the shelf above the sea can be even larger, if there is much less dense firn and snow above the glacier ice. The world's largest ice shelves are the

Ross Ice Shelf The Ross Ice Shelf is the largest ice shelf of Antarctica (, an area of roughly and about across: about the size of France). It is several hundred metres thick. The nearly vertical ice front to the open sea is more than long, and between h ...

and the Filchner-Ronne Ice Shelf in Antarctica. The term captured ice shelf has been used for the ice over a subglacial lake, such as Lake Vostok.

Canadian ice shelves

All Canadian ice shelves are attached to

Ellesmere Island Ellesmere Island ( iu, script=Latn, Umingmak Nuna, lit=land of muskoxen; french: île d'Ellesmere) is Canada's northernmost and third largest island, and the tenth largest in the world. It comprises an area of , slightly smaller than Great Br ...

and lie north of 82°N. Ice shelves that are still in existence are the Alfred Ernest Ice Shelf, Ward Hunt Ice Shelf, Milne Ice Shelf and Smith Ice Shelf. The M'Clintock Ice Shelf broke up from 1963 to 1966; the Ayles Ice Shelf broke up in 2005; and the Markham Ice Shelf broke up in 2008. The remaining ice shelves have also lost a significant amount of their area over time, with the Milne Ice Shelf being the last to be affected, with it breaking off in August 2020.

Antarctic ice shelves

A large portion of the Antarctic coastline has ice shelves attached. Their aggregate area is over .

Russian ice shelves

The Matusevich Ice Shelf was a ice shelf located in Severnaya Zemlya being fed by some of the largest ice caps on October Revolution Island, the Karpinsky Ice Cap to the south and the Rusanov Ice Cap to the north.Mark Nuttall, ''Encyclopedia of the Arctic'', p. 1887 In 2012 it ceased to exist.

Ice shelf disruption

In the last several decades, glaciologists have observed consistent decreases in ice shelf extent through melt, calving, and complete disintegration of some shelves. The Ellesmere ice shelf was reduced by 90% in the twentieth century, leaving the separate Alfred Ernest, Ayles, Milne, Ward Hunt, and Markham ice shelves. A 1986 survey of Canadian ice shelves found that 48 km² (3.3 cubic kilometres) of ice calved from the Milne and Ayles ice shelves between 1959 and 1974. The Ayles Ice Shelf calved entirely on August 13, 2005. The Ward Hunt Ice Shelf, the largest remaining section of thick (>) landfast sea ice along the northern coastline of Ellesmere Island, lost of ice in a massive calving in 1961–1962. It further decreased by 27% in thickness () between 1967 and 1999. In summer 2002, the Ward Ice Shelf experienced another major breakup, and other instances of note happened in 2008 and 2010 as well. The last remnant to remain mostly intact, the Milne Ice Shelf, also ultimately experienced a major breakup at the end of July 2020, losing over 40% of its area. Two sections of Antarctica's Larsen Ice Shelf broke apart into hundreds of unusually small fragments (hundreds of meters wide or less) in 1995 and 2002, Larsen C calved a huge ice island in 2017. Recent observations indicate that a portion of the West Antarctic Ice Sheet that holds back Thwaites Glacier is now showing

instability In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...

, as warming waters undermine the grounding zone. According to Ted Scambos, a glaciologist at the

University of Colorado Boulder The University of Colorado Boulder (CU Boulder, CU, or Colorado) is a public research university in Boulder, Colorado. Founded in 1876, five months before Colorado became a state, it is the flagship university of the University of Colorado sy ...

and a leader of the International Thwaites Glacier Collaboration, in a 2021 interview from

McMurdo Station McMurdo Station is a United States Antarctic research station on the south tip of Ross Island, which is in the New Zealand-claimed Ross Dependency on the shore of McMurdo Sound in Antarctica. It is operated by the United States through the ...

, “Things are evolving really rapidly here. It’s daunting.” The breakup events may be linked to the dramatic polar warming trends that are part of

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

. The leading ideas involve enhanced ice fracturing due to surface

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 ca ...

and enhanced bottom melting due to warmer ocean water circulating under the floating ice. The cold, fresh water produced by melting underneath the Ross and Filchner-Ronne ice shelves is a component of Antarctic bottom water. Although it is believed that the melting of floating ice shelves will not raise sea levels, technically, there is a small effect because seawater is ~2.6% more dense than fresh water combined with the fact that ice shelves are overwhelmingly "fresh" (having virtually no salinity); this causes the volume of the seawater needed to displace a floating ice shelf to be slightly less than the volume of the fresh water contained in the floating ice. Therefore, when a mass of floating ice melts, sea levels will increase; however, this effect is small enough that if all extant sea ice and floating ice shelves were to melt, the corresponding sea level rise is estimated to be ~.

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