June 7, 2013
Severe African Drought In The 1980s Caused By Northern Pollution
April Flowers for redOrbit.com - Your Universe Online
In the 1980s, decades of drought in central Africa reached the worst point. This caused Lake Chad, a shallow lake used to water crops in neighboring countries, to dry out almost completely.
Initially blamed on overgrazing and bad agricultural practices, the shrinking lake and extended drought have become examples of global warming. A new study from the University of Washington reveals that the drought was cause in part by Northern Hemisphere air pollution. The findings of this study were published in the journal Geophysical Research Letters.
Coal burning factories in the US and Europe emitted aerosols from the 1960s to the 1980s that cooled the entire Northern Hemisphere by shifting tropical rain bands south. As a result of this shift, rains no longer reached the Sahel region, a band that spans the African continent just below the Sahara desert.
Clean-air legislation in the US and Europe resulted in a lessening of air pollution, which in turn led to the rain bands shifting back and the drought easing up.
Prior research by UW scientists and colleagues revealed that global warming is now causing the land-covered Northern Hemisphere to warm faster than the Southern Hemisphere, reversing the pre-1980s trend.
A connection was also thought to exist between coal-burning and the Sahel drought, but the current study was the first to use decades of historical observations to find that the drought was part of a global shift in tropical rainfall. The researchers then used multiple climate models to determine why.
“One of our research strategies is to zoom out,” said Yen-Ting Hwang, a UW doctoral student in atmospheric sciences. “Instead of studying rainfall at a particular place, we try to look for the larger-scale patterns.”
The researchers examined precipitation from all rain gauges that had continuous readings between 1930 and 1990 to determine that the Sahel drought was part of a broader shift. During the 1970s and 80s, other locales on the northern edge of the tropical rain band, including northern India and South America, also experienced dryer climates. Areas on the southern edge of the rain band, such as northeast Brazil and the African Great Lakes, were wetter than normal. The Intergovernmental Panel on Climate Change (IPCC) uses 26 climate models, which the research team looked at to understand the reason for this global shift. They discovered that nearly every model showed some southward shift, and that cooling from sulfate aerosols in the Northern Hemisphere was the primary cause.
“We think people should know that these particles not only pollute air locally, but they also have these remote climate effects,” Hwang said.
Dirty burning of coal emits the majority of light-colored sulfate aerosols, creating a hazy effect that reflects sunlight. The emissions also lead to more reflective, longer-lasting clouds.
The team says that people living in the Northern Hemisphere did not notice the cooling because it balanced the heating associated with the greenhouse effect from increased carbon dioxide, causing temperatures to remain steady.
“To some extent, science messed this one up the first time around,” said co-author Dargan Frierson, a UW associate professor of atmospheric sciences. “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.”
The observations revealed a stronger shift than the models, suggesting that ocean circulation also played a role in the drought.
Beyond improved air quality and related health benefits, the unintended positive effect of the US Clean Air Act and its European counterpart was the recovery of the Sahel region from the long-term drought that culminated in the 1980s.
“We were able to do something that was good for us, and it also benefited people elsewhere,” Frierson said.