Life and climate in an ice age - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 9

temperature. Reliable separation of these two influences has proven very dicult.
The problem is compounded by the large seasonality in temperature and rainfall.
Because of this, two locales with the same mean annual temperature and rainfall
may have quite different plant cover. Thus, while in a historical sense pollen
abundances have provided very valuable qualitative evidence with regard to
changing climates, no means yet exists to convert these results into reliable
absolute temperature changes.''
Another problem is that pollen-bearing sediments are found only in lakes and
bogs. While providing an excellent record of the post-glacial succession of plants,
these lakes and bogs tell us these water bodies had not yet come into existence
until after the Ice Age had ended.
Broecker describes efforts to reconstruct precipitation during ice ages as ''an
extremely dicult task''. The use of pollen records for this purpose is dicult
because of the problem of separating influences of temperature and moisture. An
additional problem arises because the CO 2 content of the atmosphere was lower
during the Ice Age and plants needed more water to take in the CO 2 needed for
growth. Because of this, a drop in moisture availability suggested by the Ice Age
vegetation may not be a valid indicator of Ice Age rainfall because it may reflect
in part the atmosphere's low CO 2 content. Broecker described the use of the past
levels of lakes with closed drainage basins such as the Great Salt Lake, the Dead
Sea, and the Caspian Sea as indicators of past rainfall. The water that currently
enters these lakes via rain and via rivers must leave by evaporation. Hence, during
times of higher rainfall, these lakes expand in area until evaporation matches the
enhanced input of water to the lake. Accompanying the expansion in area is a rise
in lake level. Thus shorelines marking times when a closed basin lake stood higher
record times of greater precipitation. These lakes exist only in desert areas.
However, the lake level is likely to be more dependent on precipitation in the
neighboring mountains than in the region immediately surrounding the lake.
Broecker discusses several complications but concludes: ''despite these complica-
tions, the size of closed basin lakes is by far our best paleoprecipitation proxy.''
Finally, Broecker concludes: ''during late glacial time the tropics were less wet and
the subtropics less dry than now.''
Nevada's late Ice Age climate was much cooler and wetter than today.
The Great Basin's Ice Age was marked by increased precipitation and reduced
evaporation, known as a ''pluvial'' climate. This increased stream flow and
encouraged lake formation. The Great Basin received its name because rivers and
streams which originate in the mountains drain into the basin and end in lakes or
sinks within valley bottoms throughout the region. During the Ice Age, the Great
Basin region supported two major late Pleistocene pluvial lakes: Lake Lahontan
and Lake Bonneville. Lake Bonneville lay almost exclusively in western Utah, and
only a small area in eastern Nevada, while Lake Lahontan was mainly restricted
to western Nevada. Lake Lahontan reached a maximum depth of over 500 feet
and covered over 8,610 square miles.