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 as ...

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 L ...

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 ...

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 contine ...

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 is ...

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 and ...

, and the

Russian Arctic The Extreme North or Far North (russian: Крайний Север, Дальний Север) is a large part of Russia located mainly north of the Arctic Circle and boasting enormous mineral and natural resources. Its total area is about , ...

. The boundary between the floating ice shelf and the

anchor ice Anchor ice is defined by the World Meteorological Organization as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation". It may also be called bottom-fast ice. Anchor ice is most commonly observed in fast ...

(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 bedro ...

) 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 oce ...

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-small ...

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 strong ...

-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 Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: ...

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 be ...

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 __NOTOC__ Firn (; from Swiss German "last year's", cognate with ''before'') is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that ...

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 hi ...

and the Filchner-Ronne Ice Shelf in Antarctica. The term captured ice shelf has been used for the ice over a

subglacial lake A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where gravitational pressure decreases the pressure melting point ...

, such as

Lake Vostok Lake Vostok (russian: озеро Восток, ''ozero Vostok'') is the largest of Antarctica's almost 400 known subglacial lakes. Lake Vostok is located at the southern Pole of Cold, beneath Russia's Vostok Station under the surface of the cen ...

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 Bri ...

and lie north of 82°N. Ice shelves that are still in existence are the

Alfred Ernest Ice Shelf The Alfred Ernest Ice Shelf is an ice shelf on the north-west part of Ellesmere Island, Canada. This ice mass is one of four remaining ice shelves on the island. This ice shelf lies between Alert Point and Cape Woods on the Wootton Peninsula. The ...

Ward Hunt Ice Shelf The Ward Hunt Ice Shelf is the largest ice shelf in the Arctic, located near Ward Hunt Island, on the north coast of Ellesmere Island, Nunavut, Canada. During the 20th century the Ellesmere Ice Shelf broke up into six separate shelves, the large ...

Milne Ice Shelf The Milne Ice Shelf, a fragment of the former Ellesmere Ice Shelf, is located in the Qikiqtaaluk Region, Nunavut, Canada. It is the second largest ice shelf in the Arctic Ocean. Situated on the north-west coast of Ellesmere Island, it is about w ...

and

Smith Ice Shelf Smith may refer to: People * Metalsmith, or simply smith, a craftsman fashioning tools or works of art out of various metals * Smith (given name) * Smith (surname), a family name originating in England, Scotland and Ireland ** List of people wit ...

. The

M'Clintock Ice Shelf The M'Clintock Ice Shelf was a Canadian ice shelf attached to northern 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 ...

broke up from 1963 to 1966; the

Ayles Ice Shelf The Ayles Ice Shelf was one of six major ice shelves in Canada, all on the north coast of Ellesmere Island, Nunavut. The ice shelf broke off from the coast on August 13, 2005, forming a giant ice island thick and measuring around in size (appr ...

broke up in 2005; and the

Markham Ice Shelf The Markham Ice Shelf was one of five major ice shelves in Canada, all on the north coast of Ellesmere Island, Nunavut. The ice shelf broke off from the coast in early August 2008, becoming adrift in the Arctic Ocean. The 4,500-year-old ice shelf w ...

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 Severnaya Zemlya (russian: link=no, Сéверная Земля́ (Northern Land), ) is a archipelago in the Russian high Arctic. It lies off Siberia's Taymyr Peninsula, separated from the mainland by the Vilkitsky Strait. This archipelago ...

being fed by some of the largest ice caps on

October Revolution Island October Revolution Island (Russian: Остров Октябрьской Революции, ''Ostrov Oktyabrskoy Revolyutsii'') is the largest island of the Severnaya Zemlya group in the Russian Arctic. It is named after the October Revoluti ...

, 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 The Ellesmere Ice Shelf was the largest ice shelf in the Arctic, encompassing about of the north coast of Ellesmere Island, Nunavut, Canada. The ice shelf was first documented by the British Arctic Expedition of 1875–76, in which Lieutenant Pel ...

was reduced by 90% in the twentieth century, leaving the separate Alfred Ernest, Ayles, Milne,

Ward Hunt Ward Hunt (June 14, 1810 – March 24, 1886) was an American jurist and politician. He was Chief Judge of the New York Court of Appeals from 1868 to 1869, and an associate justice of the U.S. Supreme Court from 1872 to 1882. Early life Hunt wa ...

, 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 The Larsen Ice Shelf is a long ice shelf in the northwest part of the Weddell Sea, extending along the east coast of the Antarctic Peninsula from Cape Longing to Smith Peninsula. It is named after Captain Carl Anton Larsen, the master of the ...

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 The Western Antarctic Ice Sheet (WAIS) is the segment of the 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 WAIS is classified as ...

that holds back

Thwaites Glacier Thwaites Glacier, nicknamed the Doomsday Glacier, is an unusually broad and vast Antarctic glacier flowing into Pine Island Bay, part of the Amundsen Sea, east of Mount Murphy, on the Walgreen Coast of Marie Byrd Land. Its surface speeds exc ...

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 sys ...

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 Unit ...

, “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 E ...

. 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 can be ...

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 ~.

References

External links