Ice sheet
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An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² (19,305 mile²). The only current ice sheets are Antarctic and Greenland; during the last ice age at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada and North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern Chile.
Ice sheets are bigger than ice shelves or glaciers. Masses of ice covering less than 50,000 km² are termed an ice cap. An ice cap will typically feed a series of glaciers around their periphery.
Although the surface is cold, the base of an ice sheet is generally warmer, in places it melts 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, but retreating and advancing many times until the Pliocene, 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 fossils of plants 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 is the largest single mass of ice on Earth. It covers an area of almost 14 million km² and contains 30 million km³ of ice. Around 90% of the fresh water on the Earth's surface is held in the ice sheet, and, if melted, would cause sea levels to rise by 61.1 metres. [1] (http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113) In East Antarctica the ice sheet rests on a major land mass, but in West Antarctica the bed is in places more than 2500 m below sea level. It would be seabed if the ice sheet were not there.
Greenland Ice Sheet
The Greenland ice sheet occupies about 82% of the surface of Greenland, and if melted would cause sea levels to rise by 7.2 metres. [2] (http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113)
Possible Effects of Global Warming
If global warming occurs, over the next century the Antarctic ice sheet is predicted to gain mass primarily because it is so cold that the extra warmth will not melt it significantly but will supply extra moisture; conversely the Greenland ice sheet is expected to lose mass through melting. These effects are expected to approximately cancel [3] (http://www.grida.no/climate/ipcc_tar/wg1/418.htm). See: sea level rise.fa:یخسار ja:氷床