Why So Dry? Ocean Temperatures Alone Don't Explain Droughts

Article excerpt

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. …