Geoscience Reference
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diffuse, and that is a much slower process. Seasonal changes rarely penetrate
more than about two meters. Ocean water mixes readily; cooling water contracts
and sinks to the bottom until the temperature of the entire depth of water drops
to 4 C. The uppermost 50 meters or so of the ocean, called the mixed layer,is
thoroughly mixed by wind and waves. The importance of land in the formation
of large glaciers is illustrated by the presence of glaciers in Greenland and
Antarctica. These are the only landmasses that extend close to the poles, and
they are the only ones with extensive glaciers remaining from the Ice Ages. In
Greenland, the glaciers extend southward almost 30 degrees from the North Pole;
in Antarctica, they extend about 20 degrees from the South Pole. Large areas in
Canada and Russia that are closer to the North Pole than southern Greenland,
but don't reach as close as northern Greenland, have no glaciation. This suggests
that polar roots on land are necessary and, if they exist, then the glaciers can
extend much further from the polar regions. Although sea ice covers the Arctic
Ocean, it does not appear able to provide the same kind of roots that are
provided by land.''
Once glaciers begin to form, the increased albedo from their surfaces may
provide a positive feedback to enhance their formation, although precipitation is
also needed. It is not obvious that increased cold alone will lead to increased ice.
The modeled history of the Earth's climate and its relationship to the
arrangement of landmasses is described in considerable detail by Christopher R.
Scotese at his website ( http://www.scotese.com ). He provides maps of the land-
masses roughly every 20 to 40 million years, starting around 540 million years
ago, with brief descriptions of the prevailing climates. However, it is not always
apparent why climates changed dramatically while the continents hardly changed
over the same interval. For example, Scotese's maps for 480 million years ago and
440 million years ago are quite similar; yet he says that mild climates covered most
of the globe 480 million years ago while a ''South Polar Ice Cap covered much of
Africa and South America'' 440 million years ago.
2.4.2 Evolution of glaciation near the South Pole 34 MYBP
Antarctica has been located over southern polar latitudes since about 150 mybp ,
yet it is believed to have remained mostly ice free, vegetated, and with mean
annual temperatures well above freezing until about 34 mybp . About 34 million
years ago, the Earth underwent a significant climate change. Evidence for cooling
and the sudden growth of an East Antarctic ice sheet (EAIS) comes from marine
records and other geological data (DeConto and Pollard, 2003).
''Fifty million years ago our planet was on average 6 C warmer, and
atmospheric greenhouse gases, such as carbon dioxide (CO 2 ), were four times
that of current levels. 6 Temperature changes at the poles were probably twice the
6 It should be noted that Zachos et al. estimated that 50 million years ago the Earth was about
12 C warmer than today.
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