March 23, 2009 -- It was one of the worst natural disasters in American History. The fertile Great Plains were blighted by a decade-long drought in the 1930's that brought massive crop failures, dust storms and misery to the country's heartland.
But the Dust Bowl was more than just a cruel twist of nature; a new study has found that humans were partly to blame, too.
Largely unregulated farming practices left croplands vulnerable to degradation, and the combination of widespread plant death and soil erosion exacerbated already severe drought conditions.
Drought in North America is controlled by temperatures in the tropical Pacific Ocean, for the most part. When surface waters are cool, a so-called La Nina event, precipitation usually slackens over the central part of the continent. It's a regular cycle in Earth's climate that repeats every few decades, including the 1950's, and again around the beginning of the 21st century.
So what made the Dust Bowl different?
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"The droughts in the 1950's and 1998-2004 were in many ways just as bad," Benjamin Cook of Columbia University, lead author on the study, said. "But there was less dust available, because strong regulations were put in place after the 1930's to prevent land degradation."
Farmers in the 1930's stripped the plains of tough, native grasses, and replaced them with far more fragile crops, like corn and wheat. Moisture from the plants is critical to moderating air temperatures in the region, and when they failed, Cook and his team show that air temperatures spiked by as much as 0.5 degrees Centigrade (0.9 degrees Fahrenheit).
Dead crops also meant huge tracts of loose, dry soil that were easily scoured by wind. As dust kicked up into the atmosphere it blocked sunlight and disrupted air circulation in the region, which led to a further decrease in precipitation of between 15 and 2 percent.
The team's work appeared last week in the journal Proceedings of the National Academy of Sciences.
"Dust turns out to be very important to changes in precipitation," Ning Zeng of the University of Maryland said. "This is a pretty new discovery, and something that's always under-emphasized in climate models."
For example, Zeng said that dust emissions from Sahel region of Africa can account for fluctuations of up to 50 percent in regional precipitation.
Projections for climate change through the end of the 21st century call for significant warming, and increasing dryness in several parts of the world. Developed countries like the United States have subsidies and regulations in place to bolster farmers' resilience to drought. But developing regions often lack such infrastructure, and are particularly vulnerable to future droughts that could snowball into Dust Bowl-like disasters.
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