A study at the University of Washington shows 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 so they no longer reached the Sahel region, a band that spans the African continent just below the Sahara desert.
Following the passage of clean-air legislation in the United States and Europe the rain band shifted back and the drought lessened, researchers said.
Light-colored sulfate aerosols, emitted mainly by burning of coal, create hazy air that reflects sunlight and also leads to more reflective, longer-lasting clouds, they said.
"We think people should know that these particles not only pollute air locally, but they also have these remote climate effects," UW atmospheric scientist and lead study author Yen-Ting Hwang said.
People living in the Northern Hemisphere did not notice the cooling, the researchers said, because it was balanced out the heating associated with the greenhouse effect from increased carbon dioxide, keeping temperatures steady.
"To some extent, science messed this one up the first time around," study co-author Dargan Frierson said. "People thought that a large part of that drought was due to bad farming practices and desertification. But over the last 20 years or so we've realized that that was quite wrong, and that large-scale ocean and atmosphere patterns are significantly more powerful in terms of shaping where the rains fall."