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
National Center for Atmospheric Research/University Corporation for Atmospheric Research