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produced a strong circumpolar current in the Southern Ocean that is thought to
have thermally isolated the Antarctic continent, cooling it to a level where an
ice sheet could rapidly grow (Naish). This belief is supported by ocean general
circulation model simulations, which indicate that these changes would have
reduced southward oceanic heat transport, thus cooling Southern Ocean sea
surface temperatures (DeConto and Pollard, 2003).
However, an alternative explanation has been offered in which declining CO 2
(from 1,200 ppm at 50 mybp to 600 ppm at 34 mybp ) initiates ice sheet height/mass
balance feedbacks that cause the ice caps to expand rapidly with large orbital
variations, eventually coalescing into a continental-scale East Antarctic Ice Sheet.
According to this model, the opening of Southern Ocean gateways plays a second-
ary role in this transition, relative to CO 2 concentration. This model for the glacial
inception and early growth of the East Antarctic Ice Sheet used a general circula-
tion model with coupled components for atmosphere, ocean, ice sheet, and
sediment that incorporated palaeogeography, greenhouse gases, changing orbital
parameters, and varying ocean heat transport (DeConto and Pollard, 2003).
However, assumptions regarding past changes in CO 2 concentration seem some-
what contrived and, furthermore, it seems very unlikely that a CO 2 level of
1,200 ppm could drive global average temperature up by 12 C.
There is some uncertainty as to how stable the Antarctic ice sheets have been
over the past 34 million years. According to Naish, ''geological evidence shows
that global sea level has risen and fallen by 10 to 40m many times during the last
34 million years, with each cycle of sea level change lasting 40,000 or 100,000
years.'' The sea level changes imply that Antarctica had extensive periods when its
ice sheets were highly unstable, fluctuating in volume by up to 80%. This is
thought to be particularly true between 34 and 15 million years ago when atmo-
spheric CO 2 levels may have been twice as high as today and global average
temperatures may have been 2 C warmer than today. Naish claims: ''a second
major cooling step occurred about 15 million years ago when the Antarctic ice
sheets are thought to have expanded to their present size.''
2.4.3 Effect of the Isthmus of Panama on NH glaciation in the past 2,700,000 years
Haug (2004) raised some vital questions:
''Why did Antarctica become covered by massive ice sheets 34 million years
ago, while the Arctic Ocean acquired its ice cap only about 3 million years ago?
Since the end of the extremely warm, dinosaur-dominated Cretaceous Era 65
million years ago, heat-trapping greenhouse gases in the atmosphere have
...
...
declined
and the planet as a whole has steadily cooled. So why didn't both
poles freeze at the same time?''
According to Haug (2004), the explanation for glaciation of Antarctica is
straightforward:
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