Saturday, June 8, 2013

Pollution in Northern Hemisphere helped cause 1980s African drought

Hannah Hickey in the University of Washington News: Decades of drought in central Africa reached their worst point in the 1980s, causing Lake Chad, a shallow lake used to water crops in neighboring countries, to almost dry out completely. The shrinking lake and prolonged drought were initially blamed on overgrazing and bad agricultural practices. More recently, Lake Chad became an example of global warming.

New University of Washington research, to be published in Geophysical Research Letters, shows that the drought was caused at least in part by Northern Hemisphere air pollution.

Aerosols emanating from coal-burning factories in the United States and Europe during the 1960s, ’70s and ’80s cooled the entire Northern Hemisphere, shifting tropical rain bands south. Rains no longer reached the Sahel region, a band that spans the African continent just below the Sahara desert. When clean-air legislation passed in the U.S. and Europe, the rain band shifted back, and the drought lessened.

Related research by the UW researchers and their collaborators shows that global warming is now causing the land-covered Northern Hemisphere to warm faster than the Southern Hemisphere, further reversing the pre-1980s trend.

Previous research has suggested a connection between coal-burning and the Sahel drought, but this was the first study that used decades of historical observations to find that this drought was part of a global shift in tropical rainfall, and then used multiple climate models to determine why.

...“We think people should know that these particles not only pollute air locally, but they also have these remote climate effects,” Hwang said. Light-colored sulfate aerosols are emitted mainly by dirty burning of coal. They create hazy air that reflects sunlight, and also lead to more reflective, longer-lasting clouds. People living in the Northern Hemisphere did not notice the cooling, the authors said, because it balanced the heating associated with the greenhouse effect from increased carbon dioxide, so temperatures were steady....

Smokestacks in Champaign, Illinois, shot by Dori, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 United States license


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