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Antarctic ice sheet
The Antarctic ice sheet is one of the two polar ice caps of the Earth.
It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It covers an area of almost 14 million square km (5.4 million sq. miles) and contains 26.5 million cubic km of ice (6.36 million cubic miles). That is, approximately 61 percent of all fresh water on the Earth is held in the Antarctic ice sheet, an amount equivalent to about 58 m of sea-level rise. In East Antarctica, the ice sheet rests on a major land mass, but in West Antarctica the bed can extend to more than 2,500 m below sea level. Much of the land in this area would be seabed if the ice sheet were not there.
In contrast to the melting of the Arctic sea ice, sea ice around Antarctica was expanding . The reasons for this are not fully understood, but suggestions include the climatic effects on ocean and atmospheric circulation of the ozone hole,〔 and/or cooler ocean surface temperatures as the warming deep waters melt the ice shelves.
==History==
The icing of Antarctica began with ice-rafting from middle Eocene times about 45.5 million years ago〔(Sedimentological evidence for the formation of an East Antarctic ice sheet in Eocene/Oligocene time ) Palaeogeography, palaeoclimatology, & palaeoecology ISSN 0031-0182, 1992, vol. 93, no1-2, pp. 85–112 (3 p.)〕 and escalated inland widely during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm〔(New CO2 data helps unlock the secrets of Antarctic formation ) September 13th, 2009〕 and had been decreasing from earlier levels in the thousands of ppm. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation. The glaciation was favored by an interval when the Earth's orbit favored cool summers but oxygen isotope ratio cycle marker changes were too large to be explained by Antarctic ice-sheet growth alone indicating an ice age of some size.〔(Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean ) ''Nature'' 433, 53–57 (6 January 2005) | ; Received 1 September 2004; Accepted 25 October 2004〕 The opening of the Drake Passage may have played a role as well〔(Eocene-Oligocene transition in the Southern Ocean: History of water mass circulation and biological productivity ) Geology February 1996 v. 24 no. 2 p. 163-166 〕 though models of the changes suggest declining CO2 levels to have been more important.〔(Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2 ) ''Nature'' 421, 245–249 (16 January 2003) | ; Received 25 July 2002; Accepted 12 November 2002〕
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