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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 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 co ...
and Greenland; during the Last Glacial Period at
Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
, the Laurentide Ice Sheet covered much of
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
, the Weichselian ice sheet covered
Northern Europe The northern region of Europe has several definitions. A restrictive definition may describe Northern Europe as being roughly north of the southern coast of the Baltic Sea, which is about 54th parallel north, 54°N, or may be based on other g ...
and the Patagonian Ice Sheet covered southern South America. Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an
ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...
. 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 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 — these are ice streams. The present-day polar ice sheets are relatively young in geological terms. The Antarctic Ice Sheet first formed as a small ice cap (maybe several) in the early
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
, 33.9-23.0 Ma, but retreated and advanced many times until the Pliocene, 5.33-2.58Ma, when it came to occupy almost all of Antarctica. The Greenland ice sheet did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation. This had the unusual effect of allowing
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s of
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s that once grew on present-day Greenland to be much better preserved than with the slowly forming Antarctic ice sheet.


Antarctic ice sheet

The
Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost a ...
is the largest single mass of ice on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
. It covers an area of almost 14 million km2 (5.4 million mi2) and contains 30 million km3 of ice. Around 90% of the Earth's ice mass is in Antarctica, which, if melted, would cause sea levels to rise by . The continent-wide average surface temperature trend of Antarctica is positive and significant at >0.05 °C (0.09 °F)/decade since 1957. The Antarctic ice sheet is divided by the Transantarctic Mountains into two unequal sections called the
East Antarctic Ice Sheet The East Antarctic Ice Sheet (EAIS) is one of two large ice sheets in Antarctica, and the largest on the entire planet. The EAIS lies between 45° west and 168° east longitudinally. The EAIS holds enough ice to raise global sea levels by and ...
(EAIS) and the smaller West Antarctic Ice Sheet (WAIS). The EAIS rests on a major land mass, but the bed of the WAIS is, in places, more than 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 standardise ...
. It would be
seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
if the ice sheet were not there. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies below sea level and its edges flow into floating ice shelves. The WAIS is bounded by 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 ...
, the Filchner-Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.


Greenland ice sheet

The
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equ ...
occupies about 82% of the surface of
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 ...
, and if melted would cause sea levels to rise by . Estimated changes in the mass of Greenland's ice sheet suggest it is melting at a rate of about per year. These measurements came from
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...
's Gravity Recovery and Climate Experiment satellite, launched in 2002, as reported by BBC News in August 2006.


Ice sheet dynamics

Ice movement is dominated by the motion of
glaciers 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 ...
, whose activity is determined by a number of processes. Their motion is the result of cyclic surges interspersed with longer periods of inactivity, on both hourly and centennial time scales.


Predicted effects of global warming

The Greenland, and possibly the Antarctic, ice sheets have been losing mass recently, because losses by ablation including outlet glaciers exceed accumulation of snowfall. According to the
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) a ...
(IPCC), loss of Antarctic and Greenland ice sheet mass contributed, respectively, about 0.21 ± 0.35 and 0.21 ± 0.07 mm/year to
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 ...
between 1993 and 2003. The IPCC projects that ice mass loss from melting of the Greenland ice sheet will continue to outpace accumulation of snowfall. Accumulation of snowfall on the Antarctic ice sheet is projected to outpace losses from melting. However, in the words of the IPCC, "Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude." More research work is therefore required to improve the reliability of predictions of ice-sheet response to
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 variability and change, Climate change in a broader sense also includes ...
. In 2018, scientists discovered channels between the East and West Antarctic ice sheets that may allow melted ice to flow more quickly to the sea. The effects on ice sheets due to increasing temperature may accelerate, but as documented by the IPCC the effects are not easily projected accurately and in the case of the Antarctic, may trigger an accumulation of additional ice mass. If an ice sheet were ablated down to bare ground, less light from the sun would be reflected back into space and more would be absorbed by the land. The Greenland Ice Sheet covers 84% of the island, and the Antarctic Ice Sheet covers approximately 98% of the continent. Due to the significant thickness of these ice sheets, global warming analysis typically focuses on the loss of ice mass from the ice sheets increasing sea level rise, and not on a reduction in the surface area of the ice sheets. Until recently, ice sheets were viewed as inert components of the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
and were largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon in excess of 100 billion tonnes, as well as nutrients (see diagram).


See also

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Quaternary glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describ ...
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Wisconsin glaciation The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the most recent glacial period of the North American ice sheet complex. This advance included the Cordilleran Ice Sheet, which nucleated in the northern North American Cor ...


References


Further reading

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External links


United Nations Environment Programme: Global Outlook for Ice and Snow
* http://www.nasa.gov/vision/earth/environment/ice_sheets.html {{Authority control
Sheet Sheet or Sheets may refer to: * Bed sheet, a rectangular piece of cloth used as bedding * Sheet of paper, a flat, very thin piece of paper * Sheet metal, a flat thin piece of metal * Sheet (sailing), a line, cable or chain used to control the cle ...
Snow or ice weather phenomena
Sheet Sheet or Sheets may refer to: * Bed sheet, a rectangular piece of cloth used as bedding * Sheet of paper, a flat, very thin piece of paper * Sheet metal, a flat thin piece of metal * Sheet (sailing), a line, cable or chain used to control the cle ...
Glaciology Effects of climate change Cryosphere