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July 26, 2010

Dust from farming may affect rainfall

by Steve Baragona

New research suggests agriculture has greatly increased the amount of dust blowing off of West Africa, the world's largest source of atmospheric dust, and may have been one factor driving the decrease in rainfall in the region over the past several centuries.

Dust is more than a housekeeping nuisance. To climate scientists, dust is a force of nature. It's the most abundant particle in the atmosphere, and it reflects sunlight and heat.

The world's largest source of dust is the Sahara and Sahel region of West Africa. And its influence on the environment is surprisingly wide-ranging, according to Stefan Mulitza, a marine geologist at the University of Bremen in Germany.

"It probably interacts with cloud formation; people think that it has an influence on the quantity of precipitation; hurricane activity through the cooling of the sea surface is probably affected; and last but not least, the quality of the air we breathe" is also affected by dust, Mulitza says. Dust from severe West African dust storms can blow all the way across the Atlantic Ocean to the U.S. state of Florida.

But it's been hard for scientists to unravel what impact human activity has had on dust production in West Africa, in part because good data from satellites and ground stations has only been available for the last few decades.

In a new study in the journal Nature, Mulitza and his colleagues studied ocean sediments off the coast of Senegal and Mauritania. They constructed a record of West African dust production stretching back more than three thousand years.

For nearly that entire period, dust generation followed a predictable pattern: more dust in drier years, less in wetter ones. But then, beginning in the 19th century, something surprising happened: dust production increased dramatically.

Mulitza says that increase in dust coincides with a major economic change in the Sahel region.

"In the 19th century groundnuts were introduced into Senegal, just as one example," he says. "And there was a very widespread commercial agriculture to fuel the groundnut oil industry."

Farmers in the Sahel cleared forests to produce groundnuts and other cash crops. He says that disrupted the sandy soil and led to a sharp rise in the amount of dust blowing off the Sahel.

And that dust may have been one factor behind the drier climate in the region over the past few centuries. Although the effects of dust on climate are complex and not fully understood, it may cool surface temperatures, which can shift precipitation patterns away from the region.

Atmospheric scientist Charlie Zender at the University of California at Irvine says this is the first study to link farming with increased dust generation in the world's largest source of dust. And he says there may be lessons for farmers elsewhere in the world.

"In those regions where rainfall isn't plentiful and abundant, this study suggests that using those surfaces for agriculture, using that land, will lead to these types of unintended consequences, whether that's in Africa or not," he says.

Cornell University climate researcher Natalie Mahowald says if future studies confirm Mulitza's findings, it could have larger implications for climate change research.

"It would mean that there has been a cooling from dust over the 20th century that we haven't really been thinking about previously," she says. "And this cooling could be hiding some of the increase or the warming that should be happening from carbon dioxide."

Mahowald says that could mean the Earth is more sensitive to the warming effects of carbon dioxide than previously thought.

Mulitza says his findings may not be all bad news. He notes that increased dust over the tropical Atlantic may have lowered ocean surface temperatures, which may have reduced hurricane activity. "The climate system is coupled," he says. "And if you change something in the tropics, and you change something in the African dust source, it has global consequences" – although, he says, more research will be needed to quantify exactly what those consequences are.

VOA

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