MadSci Network: Earth Sciences

Re: How did the ice cap originate at Antartica if it is so dry?

Date: Fri Nov 3 18:36:19 2000
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 969369773.Es

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: (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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