BOULDER--Boston buried under ice? Idaho sun-dried into desert? No, this isn't Hollywood's latest foray into climate change. Benjamin Felzer, a climatologist and geologist at the National Center for Atmospheric Research in Boulder, has used NCAR computer models of climate and vegetation to find which plant types our ancestors would have wandered among during the last glacial maximum (LGM) 21,000 years ago. Felzer presented his work on Wednesday, October 30, at the annual meeting of the Geological Society of America in Denver.
These models present a world blanketed by massive ice sheets over Canada and Eurasia, with tundra covering much of Europe and North America as the Sahara Desert crept southward toward central Africa. Rain forests existed in South America and Africa, as now, but there were fewer trees globally. The whole world was colder in both summer and winter, with a global average temperature four degrees Celsius lower than now and atmospheric carbon dioxide (a greenhouse gas) at just over half of today's levels.
Felzer's study checks the NCAR model's reliability by simulating past climates and comparing its results to geological data, such as pollen deposited in lake sediments at the time, fossilized, and recently retrieved from lake cores. Once verified, the model can be used to estimate what will happen to today's plants in the next century as increasing greenhouse gases warm the climate by several degrees.
"The plants we see around us today had 21,000 years to adapt to a several- degree warming. Now these same plant types may have a hundred years or less to make the same transition," explains Felzer. Will they have time to migrate and adapt or will they die? The model may eventually shed light on such questions as Jon Bergengren, an NCAR colleague, shifts the model's gaze out of the distant past and into the next century.
In the real world, the growth and melt of 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 larches. The Sahara expanded southward as the Asian monsoon weakened and drier conditions prevailed across Africa and Southest Asia.
Felzer checked his results against earlier LGM vegetation scenarios based on data from fossilized pollen retrieved from lake cores. His simulated vegetation matched up fairly well
with these geological reconstructions of plant life. Where differences exist, a colder simulation might help produce the correct vegetation, since there is evidence in the data for a slightly colder world than the model shows, especially in the tropics. According to Felzer, the model is accurate enough in showing past climates and vegetation that it can be a useful tool in simulating the future.
The two NCAR models used in the study were GENESIS (Global Environmental and Ecological Simulation of Interacitve Systems) created by Starley Thompson and Dave Pollard with funding from the Environmental Protection Agency, and EVE (Equilibrium Vegetation Ecology Model) developed by Bergengren and also funded by the EPA.