Dust Bowl affected midwestern climate.

During the 1930s, immense clouds of dust wafting over the Great Plains blocked so much sunlight that temperatures there were significantly lower than normal during summer months, a new analysis suggests.

From 1930 to 1938, an extended drought transformed the central United States into the Dust Bowl of popular legend. That dry spell, like many others that have struck the United States, occurred during a lengthy La Nina, when sea-surface temperatures in the central Pacific were cooler than normal (SN: 8/10/02, p. 85).

Travis A. O'Brien, a climatologist at the University of California, Santa Cruz, and his colleagues used a computer model to estimate the effect of airborne dust on the midwestern climate. To create the appropriate weather patterns in their simulations, the researchers forced their model to follow the sea-surface temperatures recorded throughout 1988, when a strong La Nina occurred and a major drought struck 40 percent of the United States. One simulation allowed dust to lift off the parched ground if winds were strong enough. In another simulation, dust stayed on the ground, regardless of wind speed, says O'Brien.

When dust went airborne, it kept at least 15 percent of the sunlight during June, July, and August from reaching the ground throughout a swath of land stretching from northern Texas to North Dakota.

Those dust clouds substantially cooled the underlying landscape, the researchers found. Across large areas of Oklahoma and Kansas, the average temperature at Earth's surface during the summer months was more than 1[degrees]C lower during the dusty simulation than in the dustfree scenario.--S.P.