Tree ring cycles in the Corn Belt.

Ten years ago, David M. Meko and Charles W. Stockton, looking at tree ring data, found that the total area of drought-stricken land in the western United States has fluctuated over the last 300 years almost in synic with the 22-year sunspot cycle. Other studies have suggested a link between rainfall, as measured by the width of tree rings, and an 18.6-year rhythm in the lunar tide. Finding such periodicities is a vital step toward understanding what triggers droughts like those that laid waste to sections of the United States in the 1930s and 1950s. Recent research, however, indicates that some of these connections are tenuous at best.

Links between climate and celestial forces are only as strong as the data used. And when Meko and Stockton, of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson, did their 1975 study, they had essentially no direct information on the grain-producing regions of the Great Plains, where past droughts have taken an especially great toll. Recently, with a vastly improved data set for the Corn Belt, the researchers looked again for cyclic influences of the sun and moon on that region's climate. Together with Terence Blasing at Oak Ridge (Tenn.) National Laboratory, they examined tree rings grown from 1680 to 1980 at 15 sites in lowa and Illinois.

They found no evidence for periodicity in the Illinois sites, the researchers report in the July 26 SCIENCE. In the Iowa data, they found an 18.33-year rhythm, which they cannot ascribe directly to either the lunar or solar cycles. Further statistical analysis, however, weakly supports their previous finding that the most severe droughts follow two years after alternate lows in the number of sunspots observed.

"These results offer more support for a solar rather than lunar cycle influence," says Meko. "But they are not conclusive." It's still possible that the moon has some subtle influence on climate and that the sun combines with other forces to trigger droughts. The new results are tantalizing, says Meko, but there are no clear-cut answers and "we are still a long way from being able to forecast drought in the western United States."