|MadSci Network: Earth Sciences|
The fate of every glacier and ice sheet is ultimately decided by its "budget" of accumulating precipitation (snow) versus the subtraction of ice by processes of ablation, especially melting. An ice sheet grows if the addition of new snowfall in its source regions is greater than losses through ablation. It shrinks if losses through ablation are greater than the addition of new snowfall. The geological evidence suggests that the glaciation of Antarctica began during the Oligocene epoch, about 25 million years ago, at which time glaciers formed in some parts of Antarctica while other areas remained relatively warm and ice free. By the early Miocene epoch, about 23 million years ago, a continental scale ice sheet is thought to have formed. Nevertheless, the history of the glaciation of Antarctica is not a simple case of a stable ice cap existing unchanged for 23 million years. According to the Australian Antarctic Division, "As recently as 2.5 to 3 million years ago, when the South Pole was where it is now, vegetation similar to the Tasmanian cold temperate rainforests grew near the Pole." Although today the centre of Antarctica is classed as a desert averaging about 50mm of precipitation per year, such dry conditions almost certainly did not exist during times when the ice cap was expanding. The existence of the ice cap is itself evidence that dry conditions could not have prevailed during the Oligocene and early Miocene, nor even in the mid- Pliocene about 3 million years ago before the onset of the Quaternary ice age. The present dry climate of the Antarctic interior is, in fact, largely a creation of the ice cap itself. Continental-scale ice sheets cool the atmosphere above them, and thereby create large, stable cells of high atmospheric pressure. High pressure sets up a cold anticyclonic atmospheric circulation, in which winds blow outward from the interior and block low pressure cyclones, which otherwise would penetrate the interior of the continent and bring in moisture from the surrounding seas. The coastal areas of Antarctica today receive much higher precipitation than the interior, up to 200 mm per year, which falls mostly as snow. This precipitation is brought in by cyclones which pick up moisture from the seas around Antarctica. Their influence in coastal areas is perhaps an indication of what the prevailing climate in the interior was like in the geological past and would be today if the ice cap did not exist. The fact that the ice cap is a stable feature at the present time (human induced global warming aside!) suggests that precipitation and ablation are in balance, the dry climate notwithstanding. I have been unable to locate data about the rate of ablation of the Antarctic ice cap, but I would predict that the rate of ablation is very low. References: · http://www.antdiv.gov.au/search/index.html (information provided by the Australian Antarctic Division). · Brian John (ed.), The Winters of the World, Earth Under The Ice Ages (Newton Abbott, David & Charles, 1979). · The Dynamic Earth (The Open University, 1997. A course unit for the course S269, Earth and Life). · Microsoft Encarta 97 Encyclopedia. David Scarboro
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