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When southwestern deserts were wet.

When southwestern deserts were wet

For more than a decade scientists have puzzled over the climatic behavior of the United States' southwestern desert region during the last glacial period. Researchers have understood that the invasion of massive ice sheets into North America, down to latitudes of Seattle and New York, Made the Mojave and nearby deserts wet enough to sustain woodlands by changing the atmospheric circulation and storm patterns over the continent. But why did the deserts remain moist for several thousands of years after the ice sheets began their last retreat 12,000 years ago?

W. Geoffrey Spaulding at the University of Washington in Seattle and Lisa Graumlich at the University of Minnesota in Minneapolis present an answer in the April 3 NATURE. The two paleoclimatologists used the types of plants found in fossil pack-rat middens -- massive piles of plant fragments and debris collected by the rats and preserved for thousands of years by mummification -- to verify a National Center for Atmospheric Research computer model. This model, which has been used primarily to study global climate, is now being applied to continental and smaller scales by a few research groups.

From the model, the researchers conclude that the deserts stayed wet because of Milankovitch forcing, periodic changes in the earth's orbit, which increased solar radiation reaching the surface and intensified the monsoons. Spaudling says scientists have long recognized a Milankovitch wet spell starting about 11,000 years ago in African and Indian deserts; this wet spell stood out because it was preceded by a period that, due to existing weather patterns, was very dry. No one had recognized Milankovitch forcing at work in the southwestern deserts, he says, because this wet spell came right on the tail of the moist period brought on by the North American ice sheet.
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Publication:Science News
Date:Apr 26, 1986
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