An extended drought from 1970 to 1985 killed 1.2 million people in one of the most devastating famines ever recorded in Africa. Several factors probably contributed to the famine, but now scientists are pointing the finger at a new culprit. They suspect sulfur dioxide, spewed from factories and power plants thousands of miles away, may have reduced rainfall by as much as 50% in the Sahel, a semiarid region of tropical North Africa.
Tiny airborne particles called sulfate aerosols, which are most common in the industrialized Northern Hemisphere, boost the number of small droplets in clouds; researchers have found that this extends the lifetime of clouds. Some suspect that the particles also make clouds reflect more sunlight, cooling Earth's surface below, reducing evaporation, and ultimately decreasing rainfall.
To test how sulfate aerosols affected tropical rainfall patterns from 1900 to 1998, climatologists Leon Rotstayn of the Commonwealth Scientific and Industrial Research Organization in Aspendale, Victoria, Australia, and Ulrike Lohmann of Dalhousie University in Halifax, Nova Scotia, ran global climate models. They simulated tropical rainfall patterns with and without industrial sulfur emissions. Simulations that didn't account for emissions showed no drought in the Sahel. But when emissions from Europe and North America were factored in, tropical rains shifted southward, with less rain north of the equator. Most severely affected by the reduced rainfall was the Sahel, the researchers report in the August issue of the Journal of Climate. Checking their findings against rainfall gauge records for the 20th century worldwide, the researchers found that, on average, tropical rainfall had indeed moved south during the century. Rotstayn notes that during the 1990s, when industrialized countries tightened aerosol pollution control laws, rain returned to the Sahel.
Climatologist V. Ramanathan of the Scripps Institution of Oceanography in La Jolla, California, says it's plausible that sulfur emissions have affected tropical rainfall, but more studies are needed to make a compelling case. "It's a good hypothesis, but there are competing effects between different aerosols, and we don't yet understand many of them."