Oceans caused 1930s Dust Bowl

Influence: Tiny temperature changes in tropical waters created years of drought on the Plains, NASA computers find.

Medicine & Science

March 22, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

The Dust Bowl droughts that ruined millions of Great Plains farm families in the 1930s were triggered and sustained by barely perceptible temperature changes in oceans thousands of miles away, according to a new NASA study.

Computerized climate simulations suggest that abnormally warm water in the tropical Atlantic set up wind circulation changes that cut off the flow of moisture from the Gulf of Mexico during the summer and fall.

At the same time, unusually cool sea-surface temperatures in the tropical Pacific set up global patterns that suppressed storm development on the Plains.

The combination, scientists said, led to persistent drought and what many regard as the worst U.S. natural disaster of the 20th century.

The pattern has not recurred, and no Great Plains drought since has been as severe. But that might be only a matter of time, said Siegfried D. Schubert, of the Earth Sciences Directorate at NASA's Goddard Space Flight Center in Greenbelt, a co-author of the study published in the journal Science.

"There's no reason to believe we're not going to have another drought like that," he said. "There's evidence going back at least 400 and even 1,000 years where they've been able to show these mega-droughts tend to occur once or twice a century."

Nebraska tree rings suggest that one Great Plains drought began in the late 13th century and persisted for 38 years.

The long, dry decade that began in 1930 ended a period of benign weather that helped draw many new settlers to the Plains. Encouraged by mechanization, new crop varieties and 1920s optimism, they plowed up one-third of the region's 100 million acres of grasslands and exposed the fine soil to the wind.

When the rain stopped, the dust storms began, some of them terrifyingly dark and vast. In May 1934, a roiling cloud of topsoil 1,500 miles long and two miles high swept 900 miles across the Plains.

Crops failed, animals suffocated and people suffered from dust-related asthma and pneumonia. By 1937, one-fifth of all rural families on the Great Plains were on federal assistance. Millions lost their farms and fled.

Previous studies had found correlations between changing sea-surface temperatures and droughts on the Great Plains, Siegfried said, "but it wasn't clear specifically what was happening in 1930."

To find out, he and his colleagues from Goddard and the University of Maryland Baltimore County enlisted a NASA computer program that simulates, or "models," the complex interactions of the atmosphere, the ocean and the land.

Normally, climate modelers set some initial conditions, then allow the programs to run. Repeated runs will vary, yielding probabilities for future conditions.

This time, they plugged in Atlantic and Pacific sea-surface temperatures as actually measured or extrapolated throughout the 20th century. Then they ran the program 14 times to project conditions from 1902 to 2001, watching closely to see how the simulation evolved with respect to rainfall on the Great Plains in the 1930s.

"Each time evolved slightly differently," Siegfried said. But "almost all of them went dry [in the 1930s], which tells us that sea-surface temperatures really did have important control."

Subsequent manipulations of the program found the strongest influences were in the tropical regions of the Atlantic and Pacific, not the more temperate latitudes.

The abnormal warming or cooling was small, "actually only a few tenths of a degree Centigrade" averaged over nine years, Siegfried said. Drought victims would not have suspected it as the villain in their drama.

"Only through modeling studies could you really get at that," he said.

The study also revealed that parched soils reinforced the drought by reducing evaporation and further limiting rainfall. The modelers found this interaction of land and sky doubled the rainfall shortage, especially in summer.

"The sea-surface temperatures set the stage," Siegfried said, "but you needed that interaction" to create the Dust Bowl.

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