Ice sheet
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
South America.
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 its 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.
|
A satellite composite image of Antarctica |
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
[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. However, if the ice sheet were actually removed, isostatic rebound would occur and Antarctica would rise to an average height of 800m above sea level.
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
[http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113].
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
[http://www.grida.no/climate/ipcc_tar/wg1/418.htm]. See:
sea level rise.
