Rains in the plains less common in past.

After weathering a severe drought in 1988 and record floods in the 1990s, residents of the Great Plains believe they are keenly aware of climatic inconstancy. Yet these recent meteorological turns provide only a small taste of what nature can dole out.

Before the year 1200, the northern Great Plains region faced far more frequent and severe droughts than it has in recent history, report scientists who have traced the area's climate patterns for the last 2,300 years. "This shows that the last 100 years are not representative of the kind of climate variability that we potentially can have in this region," says Kathleen R. Laird of Queen's University in Kingston, Ontario.

Laird and her colleagues reconstructed the climatic history of the region by studying fossil diatoms--shells from single-celled algae--preserved in sediments from the bottom of a lake in eastern North Dakota. They identified droughts by looking for particular diatom species that thrive when the water level drops and the lake's dissolved salts and nutrients become concentrated. The scientists describe their work in the Dec. 12 NATURE.

During the last 750 years, droughts have occurred relatively infrequently, a finding that supports previous tree-ring analysis reaching back 500 years in near-by by Alberta. Before 1200, however, the diatoms record a much greater frequency of extreme droughts, many of which exceeded in intensity the Dust Bowl period of the 1930s. The longest dry spells lasted for centuries, from the years 200 to 370, 700 to 850, and 1000 to 1200.

Some of these droughts apparently affected large sections of North America, judging from studies of prehistoric climate in California. Rivers, lakes, and marshes in and adjacent to the Sierra Nevada dried up during the same three intervals recorded in North Dakota, according to Scott Stine of California State University in Hayward.

Before drawing further conclusions about the drought history of North America, climate scientists need to study other lakes in the Great Plains and elsewhere, says Laird. Her study is the only high-resolution climate record of the midcontinent that reaches back more than 500 years.

Because droughts were more common before 1200, when temperatures were warmer, future glow warming may increase the frequency of droughts in the Great Plains, says Laird. This scenario draws some support from computer models that forecast how climate will react to greenhouse gas pollution. According to the models, greenhouse warming is likely to make climate more variable and to promote drying in middle North America, says James E. Hansen of NASA's Goddard Institute for Space Studies in New York.