Ice-dammed lakes had cooling effect.

About 90,000 years ago, an ice sheet blocked the flow of rivers in northern Russia, leading to the formation of massive lakes. New computer models suggest that those frigid bodies of water significantly cooled the region in summer months. That change in the local climate, in turn, allowed the massive ice sheet to expand more quickly than if the lakes had never formed.

At their largest, the lakes covered an area more than three times the size of North America's five Great Lakes, says Martin Jakobsson, a marine geologist at the University of New Hampshire in Durham. Computer simulations by him and his colleagues indicate that the lakes were frozen between 7 and 11 months of the year. This stifled evaporation and thereby squelched winter precipitation that would have nourished the ice sheet.

However, in the summer, the lakes' cooling effect cut down the ice sheet's rate of melting. Air temperatures at the sheet's southern edge were as much as 10[degrees]C lower than they would have been if the lakes weren't present, the researchers say. Overall, the team's computer models indicate that the ice sheet expanded at a faster rate after the lakes formed.

The lakes' cooling effect also delayed the demise of the ice sheet after the climate * began warming 85,000 years ago, says Jakobsson. He and his colleagues report their findings in the Jan. 29 Nature.

--S. P.