Climate modelers: go talk to the trees.

Though the last ice age ended 10,000 years ago, it continues to chill the spirits of climate scientists around the world. In trying to understand how climate works, researchers use advanced computer models to simulate conditions during different periods of Earth's history. Yet even the best of these models have failed to reproduce the chilly temperatures that permitted vast glacial sheets to spread over parts of North America, Europe, and Asia during the last ice age.

The solution to this cold conundrum lies in the boreal forests of northern lands, according to new computer simulations described in the June 6 Nature. Though ignored in previous modeling studies, vegetation actually plays a significant role in climate-one large enough to send the world spiraling into an ice age, report Robert G. Gallimore and John E. Kutzbach of the University of Wisconsin-Madison.

"This has been a significant problem that has puzzled people, and I think they have offered a reasonable explanation," comments Thomas J. Crowley of Texas A&M University in College Station. In the new computer simulations, plants perform a reinforcing role, exaggerating an initial cooling caused by cyclical changes in Earth's orbit. At the start of the last ice age, 115,000 years ago, several factors cooled the Northern Hemisphere during summer. Earth traveled in a more elliptical orbit then and reached its farthest point from the sun in July. The planet's spin axis also tilted less than it does today, reducing the difference between seasons.

These changes caused an 8 percent reduction in the amount of summer radiation reaching far northern lands. Moreover, Earth's atmosphere held 20 percent less carbon dioxide, which cooled climate even further. These two effects represent only part of the answer, however, because in model simulations, they do not depress temperatures enough to create an ice age.

When Gallimore and Kutzbach included changes in plant communities, northern summers grew so cold that snow persisted year round north of 60#161#N latitude. Enough snow accumulated each year that kilometer-thick ice sheets could grow in 6,000 years, according to the simulation.

To mimic vegetation's influence on climate, the two researchers drew on the results of a previous study that examined ecological effects of orbital changes 115,000 years ago. This work suggested that as Earth's temperature dropped, about 25 percent of the land covered by boreal forests gave way to tundra.

This simple plant shift produced a profound cooling effect in the climate model. Snow-covered tundra reflects 50 to 70 percent of incoming sunlight back to space. In contrast, the dark branches of evergreen trees absorb solar radiation, so a boreal forest in winter reflects only 15 to 20 percent of sunlight. With the addition of tundra, summertime temperatures dropped by 8#161#C to 15#161#C at latitudes between 60#161#N and the pole, the researchers found.

These simulations, as well as other recent studies, demonstrate that vegetation changes can influence climate strongly, says Mark Chandler of NASA's Goddard Institute for Space Studies in New York. For that reason, modelers are now striving to marry ecological models to sophisticated global climate models-a union designed to let plants interact more realistically with climate and to improve forecasts of global change.