"I'm not prepared to believe that the whole world was warmer," said Dr. Malcolm K. Hughes, a paleoclimatologist who directs the Laboratory of Tree Ring Research at the University of Arizona in Tucson. Nor is it possible to say for sure that future global warming would bring back the large-scale droughts of the past. "We've not done the work on the actual mechanisms," he said.

But he said that Dr. Stine's findings, coupled with similar conclusions that can be drawn from other tree-ring studies by scientists in his laboratory, are a "serious cause for concern." There appears to be little doubt that the epic dry spells of the past did occur, he said, adding that "what has happened can happen."

The findings also emphasize the importance of precipitation changes, rather than simply changes in temperature, when weighing the potential impact of future global climate change, Dr. Stine wrote in Nature. Periods of 'Epic Drought'

The Sierra Nevada, where Dr. Stine conducted his study, is California's most important area for the collection of water. Runoff from the Sierras provides two-thirds of the state's surface-water supply for cities and farms. The study involved trees at four places: Mono Lake, Tenaya Lake, the West Walker River and Osgood Swamp. Dr. Stine's tree-ring analysis found that live trees had covered dry beds of lakes, streams and swamps for overlapping periods of 50, 100, 141 and 220 years and that these "lowstand" periods were clustered in two major dry spells separated by a century-long wet period. "Epic drought," he wrote in Nature, is "the only plausible explanation for the site-to-site contemporaneity of the stumps."

In the period separating the two long droughts, Dr. Stine said, the water in Mono Lake rose to a level higher than any in the last 150 years, suggesting that the California climate was even wetter then than it is today. The last century and a half, Dr. Stine found, has been the third wettest period in the last three millenniums. But, he said, "the vast majority of years during the past 3,500 years have been much drier than what we've come to expect to be normal in California."

The evidence for the medieval drought periods is especially strong, Dr. Hughes said, because the lake basins are closed, with no natural outlets; consequently, their water levels are influenced only by inflow and evaporation, making them ideal gauges of drought. And even though radiocarbon dating is somewhat imprecise, he said, it is good enough "to show two major phases of tree growth and lowstands; there's no doubt about that." This, he said, shows that "often, for sure, there have been periods of 100 years or more" that have been "markedly drier" than the 20th century.

The 10th to the 14th centuries, encompassing the prolonged droughts reported by Dr. Stine, saw "dramatic changes" in Western Hemisphere civilizations, Dr. Street-Perrott wrote in Nature, and some have been attributed by archeologists to changes in rainfall. One example is the sudden decline of the Anasazi cliff-dwellers in the American southwest at about the year 1300. Another, even more striking, is the collapse of Tiwanaku.