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Prior to closure of isthmus After closure of isthmus
Figure 2.41. Arctic freezing induced by decreased flow of tropical waters to the north (adapted
from Uriarte, 2009).
At the other end point, a gigantic ice sheet forms over much of Canada and
Scandinavia which, in combination with Antarctica, sequesters so much of the
Earth's water as ice that the ocean level drops by more than 100 meters, 7 global
average temperatures drop significantly, and temperatures at high latitudes drop
even more. Figure 2.42 shows an estimate of global average temperature over
the past 3 million years. The data in this figure indicate that there have been
oscillations about a continuing downward trend, and over the past few hundred
thousand years the duration and depth of the cold periods has increased. We refer
to these cold periods as ice ages. Clearly, these temperature variations were not
nearly as extreme as the climate extremes described in previous sections (hothouse
or snowball Earth). Nevertheless, the occurrence of an ice age has a major world-
wide impact on the geography, weather, flora, and fauna of the planet. In the
ensuing chapters we will describe what is known from experimental data about
past ice ages and we will review the theories that have been offered to explain
their occurrence.
7 The removal of water from the oceans was actually about 50m greater than this because the
crust below the ocean rebounded about 50m when water was removed at the LGM.
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