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    July 13, 2011 | 1:47 PM

    Past African droughts linked to air pollution

    Source: IRRI Images

    Disastrous droughts are making headlines again this week. In Texas, peanut crops are stunted and 4th of July firework celebrations were cancelled. In East Africa, famine looms due to the extremely dry conditions. In the shadow of these concerning reports comes new research that sheds light on some of the most severe droughts of last century: those that plagued the Sahel — a belt of grassland and savanna that cuts across the African continent just south of the Sahara desert.

    The scientific community has long debated the causes of the deadly string of Sahelian droughts that occurred between 1940 and 1980.
    Using global climate models, the authors of this recent study find that sulfate aerosols from the burning of fossil fuels are likely to explain most of the dry weather that caused the drought. “It is very unlikely that the observed drought was entirely due to natural climate variability,” the study concludes.

    Sulfate aerosols, unlike greenhouse gases, have a cooling effect on the climate. They scatter (rather than absorb) incoming solar radiation and make clouds more reflective. These pollutants, emitted at high levels in the industrialized north during the second half of the 20th century, caused a cooling in the northern hemisphere relative to the southern hemisphere. Ultimately, this imbalance shifted the band of clouds that influences the wet season in the tropics southward, dramatically decreasing the amount of rainfall that reached the Sahel.

    Although focused on the Sahel, this study has broader implications. Geoengineering proponents have suggested injecting sulfate aerosols into the stratosphere to manipulate climate and counteract the effects of global warming. Because we now know that sulfate aerosols can have dramatic effects on regional precipitation patterns, strategies like these may be unnecessarily risky — especially when they won’t address carbon pollution, the root cause of climate change.

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