f the ice sheets over the past two million years has resulted from long-term cyclical changes in the earth's axial tilt, its precession (or motion around the tilted axis), and its elliptical orbit around the sun. The cold temperatures of the last glacial maximum resulted from the existence of the continental ice sheets and were intensified by reduced amounts of atmospheric carbon dioxide, which insulates the earth by trapping heat reflected from the surface toward space.

While a 4- degree global average cooling might not sound like much, it translates into regional average temperatures that were colder than today's temperatures by 2 to 20 degrees in various parts of North America. Above the ice sheets there and elsewhere, average temperatures may have plummeted to 40 degrees colder for those regions. Besides temperature, the model calculates all other climate indicators, including moisture levels (precipitation and evaporation).

To the climate model Felzer added a vegetation model that included 110 different plant types divided into 12 categories, including needle-leaf evergreen and broadleaf deciduous forests, savanna, shrub land, and desert. The model ranked the vegetation by which plant type was best adapted to which regional climate. It computed how much surface area each type occupied based on competition for light (related to canopy cover) and disturbances such as tree fall and lightning-sparked fire. The result was a global picture of vegetation 21,000 years ago.

The model shows fragile, treeless tundra covering most of Europe. Desert spread into the northern Rocky Mountains. A wetter Southwest still bore mostly desert plants, while the Pacific Northwest dried slightly. Forests gave way to tundra and polar desert in Alaska. Worldwide, the biggest vegetation changes occurred in central Asia, where needle-leaf evergreen spruces were replaced by needle-leaf deciduous larch
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Contact: Anatta
anatta@ucar.edu
303-497-8604
National Center for Atmospheric Research/University Corporation for Atmospheric Research
30-Oct-1996

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