72 / Basics of Environmental Science
is roughly equivalent to the North American Wisconsinian. It began about 70000 years ago and
ended about 10000 years ago.
Glaciations and interglacials are associated with climatic changes more extensive than the advance
or retreat of ice sheets. Precipitation patterns also change. Today, central Antarctica is possibly
the most arid place on Earth and the Arctic is also dry, though less extremely so. Ice sheets
accumulate because fallen snow is removed only slowly by ablation, not because precipitation
is heavy. At the peak of the Devensian (Wisconsinian) glaciation, 18000 years ago, most deserts
were larger than they are today, but during the warm episode following the end of the Younger
Dryas the Sahara was smaller than it is now and lakes in that part of Africa were larger (GENTRY
AND SUTCLIFFE, 1981, p. 239). Generally, climatic cooling implies increasing aridity, climatic
warming increasing precipitation. Indeed, the correlation with rainfall makes it possible to equate
interglacials with pluvials (periods with wet climates) and glacials with interpluvials (HOLMES,
1965, pp. 715–716).
Sea levels also change with the advance and retreat of ice sheets. As more and more water reaches
the frozen regions and is trapped there, sea levels fall. At times they have been 100 m below their
present limit. At times during the Devensian (Wisconsinian), the North Sea was dry land, crossed by
rivers, Alaska was joined to Siberia by a wide strip of land, and New Guinea was linked to Australia.
The exposure of land bridges presents living organisms with opportunities to expand their ranges
and it was during the Pleistocene that humans reached most of the regions in which they live now.
About 60000 years ago, people arrived in Australia (MORELL, 1995), presumably having crossed
about 100 km of open sea to migrate from Asia to New Guinea-Australia, and some time later there
was the first of several migrations from Asia into North America. It is only the more isolated Pacific
islands, Madagascar, and New Zealand that have been colonized during the Holocene, about 3000,
2000, and 1000 years ago respectively (ROBERTS, 1989, p. 56).
People did not live on the ice sheets, of course, but the tundra landscapes bordering the ice
supported abundant game and the seas provided fish and seals as well as marine invertebrates.
Climate change, especially if it is rapid, might be supposed to challenge species and, indeed,
many animals became extinct during the Pleistocene. These extinctions are unlikely to have
been linked solely to changing climate, however. It is much more likely that they were caused
by overhunting as humans expanded their range. Vast quantities of animal bones have been
found in certain places, sometimes at the foot of cliffs over which entire herds appear to have
been driven, presumably by hunters who then took the meat and other materials they needed
from the pile of carcasses. One such site, at Solutré, France, has the bones of more than 100000
horses (ROBERTS, 1989, p. 59), and in North America many large mammals became extinct
within 1000 years of the emergence of the earliest human culture. The extinctions were confined
to the late Pleistocene and are not linked to earlier climate changes, some of which were at least
as rapid; they principally affected large mammals, and they occurred on all continents. Australia
lost some 81 per cent of its large mammals 26000–15000 years ago, South America 80 per cent
13000–8000 years ago, North America 73 per cent 14000–10000 years ago, Europe 39 per cent
14000–9000 years ago, and Africa 14 per cent 12000–9500 years ago.
Glaciers scour away soil and their retreat leaves a barren landscape, but one to which plants and
animals soon start to return. Holocene recolonization has been well documented, together with the
limits species reached before rising sea levels prevented their further migration. The mole (Talpa
europea), common shrew (Sorex araneus), beaver (Castor fiber), aurochs (Bos primigenius),
European elk or North American moose (Alces alces), and roe deer (Capreolus capreolus) never
reached Ireland, for example, because of the formation of the Irish Sea 9200 years ago (SIMMONS
ET AL., 1981, pp. 86–89).