Why so dry? Ocean temperatures alone don't explain droughts.

The western United States continues to struggle with the worst dry spell since the 1930s, and an international report on climate change predicts more and worse droughts to come (seepage 83). As scientists work to understand what triggers droughts, a new finding suggests that the causes may be more complex than many have supposed.

Researchers recently pieced together the most comprehensive history yet of drought in the Great Plains region. The record covers the 10,000 years since the end of the last ice age. This new time line shows three distinct megadroughts--periods of severe dryness lasting for centuries. Scientists often attribute drought to changes in ocean-surface temperature patterns, such as those associated with El Ninos. But when the research team compared its record with estimates of historical sea-surface temperatures, only the most recent of the three dry spells matched up.

"Linking Pacific sea-surface temperatures to drought doesn't explain the drought patterns that we see," says Joseph Mason, a geographer with the research team at the University of Wisconsin-Madison. The finding suggests that other factors, such as solar intensity or global wind patterns, sometimes play a role.

To detect drought in the distant past, the scientists studied buried dunes in the Sand Hills region of Nebraska. Land covered by vegetation is protected against wind erosion, but as drought lingers, the soil becomes exposed and dry. Wind can then push sand around more easily, forming migrating dunes. So, ancient dunes are a good indicator that drought has occurred.

The scientists dated the remnants of the dunes by measuring fluorescence in the long-buried sand grains. Exposing the grains to light releases a faint flash of fluorescence that's more intense the longer the period since the sand grain last saw sunlight. The technique yields age estimates good to within 10 percent, which is better than radiocarbon dating can achieve.

From these data, the scientists found a 300-year dry period that began about 1,000 years ago, coinciding with a well-known warm episode called the Medieval Climate Anomaly. They found two other epochs of desertlike conditions that ran from 4,500 to 2,300 years ago and from 9,600 to 6,500 years ago, the team reports in the February Geology. The Medieval Climate Anomaly appears linked to changes in ocean temperatures, but the earlier two droughts don't.

"It's a very nice piece of work," comments Daniel Muhs, a research geologist at the U.S. Geological Survey in Denver. The causes of droughts "may be a lot more complicated than we thought," he says. "The next step is for the atmospheric-science community to look at something like this and say, 'OK, maybe there are other mechanisms involved.'"