such deserts probably characterized almost 50 %
of the land area at those latitudes. In this period,
tropical rainforests and adjacent savannas were
reduced to a narrow corridor, and were much less
extensive than they are today.
Causes of such variability are evident as
interpreted from the changing configuration and
position of the continents and oceans (caused by
plate tectonics), together with the uplift of
mountain chains; these have strongly affected
world climate. For most of the Earth’s history,
the continents and oceans have been so arranged
that warm ocean currents from the tropics were
able to flow easily into the northern and southern
Polar Regions; there were no barriers to the flow
of currents from low to high latitudes. This is not
the situation today. At present, a continent
(Antarctica) covers a large area centered on the
South Pole; the Arctic Ocean, centered on the
North Pole, is almost cut off from surrounding
oceans because of the arrangement of northern
continents around it. Because land barriers prevent warm ocean currents from circulating the
sun’s energy away from tropical and temperate
latitudes toward polar ones, we are now
experiencing a glacial age.
Another geological change that can affect
world climates is volcanic activity. A prolonged
period of severe eruptions can inject large
amounts of gases and particulates into the atmosphere, both of which, especially together, can
decrease the quantity of solar radiation reaching
the Earth’s surface and so cause a phase of
cooling. A meteorite that struck the Earth at the
Fig. 10.1 Shorelines of
ancient Lake Bonneville,
Utah. These terraces
formed at various times
during the ice age as the
level of the lake came up
and fell as the climate
changed from humid to arid
and back again. From G.K.
Gilbert (1890)
Fig. 10.2 Changes in
glacial history derived from
the evidence of deep sea
cores obtained from the
Indian Ocean. From Imbrie
and Imbrie (1979), p. 169;
reproduced with
permission
96
10 Ecoregions and Climate Change
of the land area at those latitudes. In this period,
tropical rainforests and adjacent savannas were
reduced to a narrow corridor, and were much less
extensive than they are today.
Causes of such variability are evident as
interpreted from the changing configuration and
position of the continents and oceans (caused by
plate tectonics), together with the uplift of
mountain chains; these have strongly affected
world climate. For most of the Earth’s history,
the continents and oceans have been so arranged
that warm ocean currents from the tropics were
able to flow easily into the northern and southern
Polar Regions; there were no barriers to the flow
of currents from low to high latitudes. This is not
the situation today. At present, a continent
(Antarctica) covers a large area centered on the
South Pole; the Arctic Ocean, centered on the
North Pole, is almost cut off from surrounding
oceans because of the arrangement of northern
continents around it. Because land barriers prevent warm ocean currents from circulating the
sun’s energy away from tropical and temperate
latitudes toward polar ones, we are now
experiencing a glacial age.
Another geological change that can affect
world climates is volcanic activity. A prolonged
period of severe eruptions can inject large
amounts of gases and particulates into the atmosphere, both of which, especially together, can
decrease the quantity of solar radiation reaching
the Earth’s surface and so cause a phase of
cooling. A meteorite that struck the Earth at the
Fig. 10.1 Shorelines of
ancient Lake Bonneville,
Utah. These terraces
formed at various times
during the ice age as the
level of the lake came up
and fell as the climate
changed from humid to arid
and back again. From G.K.
Gilbert (1890)
Fig. 10.2 Changes in
glacial history derived from
the evidence of deep sea
cores obtained from the
Indian Ocean. From Imbrie
and Imbrie (1979), p. 169;
reproduced with
permission
96
10 Ecoregions and Climate Change