A lockstep rhythm between the 22-year sunspot cycle and drought in the Western United States has seemed a handy guide for long-range climatic forecasts. But as many meteorologists warned might happen, it breaks down when looked at closely.
The roughly 22-year drought cycle only relates to conditions averaged over the Western two-thirds of the US. It doesn't show up in other parts of the world. Now Charles W. Stockton and David M. Meko have found that it doesn't show up when specific geographic areas within the Western US are studied, either.
Stockton and Meko, who work at the Laboratory for Tree Ring Research at the University of Arizona, have looked at patterns of growth rings in white oak, post oak, and ponderosa pine, mainly from southeastern Montana, eastern Wyoming, Iowa, and Oklahoma. These rings provide a year-by-year reflection of moisture conditions.
Using a similar technique, Stockton, Meko, and Murray Mitchell of the National Oceanic and Atmospheric Administration had previously uncovered the correlation between drought over the Western US and the sunspot cycle. This correlation seemed statistically well established throughout the past 300 years. However, as the scientists pointed out at the time, the drought history of smaller sections would be masked by averaging conditions over so large a region.
This has turned out to be the case. Stockton and Meko told a session of the recent annual meeting of the American Association for the Advancement of Science that, for specific regions the size of a state, or smaller areas, well-defined drought cycles were hard to find. They turned up only ill-defined intervals of from 15 to 25 years.
Thus the much-touted Great Plains drought cycle won't yield practical forecasts for farmers. One wonders whether or not there is really any link between drought and sunspots at all.
Jerome Namias of the Scripps Institution of Oceanography told the meeting that the ''tantalizing suggestion'' of such a linkage has always been suspect because no one has satisfactory physical reasons for it. Stockton and Meko said their findings do not rule out a possible sun-drought relationship. However, they added, ''whatever effect solar variability may have on drought, it is overwhelmed by other factors at particular locations.''
Namias, who has been making extensive studies of global weather relationships , does hold out some hope for drought forecasting. He said that meteorologists are beginning to understand the atmospheric mechanisms that cause drought. Calling the prospect for predicting the onset and duration of drought ''by no means pessimistic,'' he noted that some droughts have been foreseen a month, or even a year, in advance. However, he added, ''the ability to predict duration and sometimes abrupt termination is not at hand.''
Obviously, this is a field where scientific progress is being made slowly and where skepticism is wise. Seemingly sound statistical relationships such as that between sunspots and drought cycles can let you down.