Life and climate in an ice age - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 7

with far less rainfall than today. As a result of dry conditions over tens of
thousands of years, much of the rainforests were transformed to savannas. Only
local pockets, here and there, in the tropics stayed wet enough to maintain rain-
forest conditions. These ''refuges'' were isolated from one another, and this theory
is referred to as the ''refuge theory'', In order to generate large numbers of new
species, it is necessary for species to be isolated to prevent crossbreeding. The
refuge theory provided a mechanism for such speciation to take place during ice
ages, with consequent spreading of domains during interglacial periods. This
theory was widely accepted; however, there was a dearth of data supporting the
theory, both in regard to the existence of the putative refuges as well as the belief
that the tropics were arid during ice ages.
Colinvaux accepted the refuge theory at first, but he relentlessly went into the
Amazon jungles over several decades to seek lake sediments more than 20,000
years old that could provide evidence for the theory. What he found instead was
that during the Last Glacial Maximum (about 20,000 years ago) the tropical rain-
forests persisted; however, they were infused with coniferous trees that do not
presently grow in the tropical rainforests. Thus he concluded that the refuge
theory was wrong, that tropical rainforests persisted, and the climate change was
not aridification, but rather simple cooling. But the cooling was not draconian.
The Amazon lowlands were still suitable for most of the species that are common
today. But cooling allowed other plants, normally restricted to higher altitudes, to
invade the lowlands. Colinvaux estimated the lowering of the mean temperature in
the Amazon lowlands to be 4.5 C during the Last Glacial Maximum. However,
Colinvaux's viewpoint remains somewhat controversial and his conclusion does
not explain the great species diversity in the tropics. Some believe that cooling in
the tropics was more like 2 C.
According to Dawson (1992):

''The presence of extensive sea ice as far south as 40 Nto45 N during winter
months drastically reduced moisture evaporation and by cooling the overlying
air, resulted in southward extension of high pressure. The formation of a land
bridge across the Bering Straits due to lowering of the oceans reduced transfer of
warmer water from the Pacific to the Arctic. Summer melt water in the Arctic
produced a layer of fresh water that increased salinity stratification and

promoted formation of sea ice.''

In his classic topic, Mithen (2003) describes the world of 20,000 years ago as:

inhospitable, a cold, dry and windy planet with frequent storms and a
dust-laden atmosphere. The lower sea level has joined some landmasses together
and created extensive coastal plains. Tasmania, Australia and New Guinea are
one; so are Borneo, Java and Thailand that form mountain chains within the
largest extent of rainforest on planet Earth. The Sahara, Gobi and other sandy
deserts are greatly expanded in extent. Britain is no more than a peninsula of

'' ...