WARMER oceans may be reponsible for triggering the droughts that have engulfed the Sahel region of Africa for much of the past 30 years. A new comparison of rainfall data with sea surface temperatures in the tropics undermines two alternative theories, which blame the drought on deforestation or the influence of sulphurous clouds wafting to the continent from Europe.
The Sahel droughts have been among the worst the world has ever seen, and caused the infamous famines that crippled countries such as Ethiopia in the 1980s. Now Alessandra Giannini of the International Institute for Climate Prediction in Palisades, New York, has compared 70 years of rainfall data for the drought-prone Sahel with sea surface temperatures in the tropics. She found a strong correlation between the two, especially between rainfall and temperatures in the Indian Ocean. As the ocean got warmer, rainfall decreased all along the heavily populated Sahel, south of the Sahara. The research was published online by Science ().
When Giannini ran a climate model in which the only variable was sea-surface temperatures, it accurately reproduced the arid 1940s, the wet 1950s and 1960s, the dry 1970s and 1980s, and the partial recovery in the 1990s. Giannini says the oceans have an effect because warm waters upset the atmospheric circulation, weakening the monsoon and triggering drought from Senegal to Ethiopia.
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The earliest theory about what caused the droughts, proposed in the 1970s, blamed deforestation in wetter regions close to the Sahel. Vegetation, the argument went, perpetuates rainfall, as plants take up water then transpire it through their leaves back into the atmosphere. But the theory has stalled, partly because vegetation loss in Africa is now thought to have been much less than commonly supposed.
Giannini suggests that the loss of vegetation as rainfall dries up may amplify the initial effect of warmer oceans. Jonathan Foley of the University of Wisconsin agrees. Last month he published a new study of ecological influences on Sahel rainfall in the journal Ecosystems (DOI: 10.1007/s10021-002-0227-0). “We think changes in vegetation, caused by declining rainfall, reinforce the drought,” he told 91av. It could explain why droughts in the Sahel last much longer than those elsewhere.
In the mid-1980s, Chris Folland from the Met Office in Bracknell, UK, established a statistical link between sea-surface temperatures in the Atlantic and Indian Oceans and African droughts, and used it to produce annual forecasts of droughts in the Sahel.
Last year Leon Rotstayn of the Australian government research agency, the CSIRO, suggested that clouds of industrial sulphur from Europe and North America could be driving the tropical rain belt south and drying up the Sahel (91av, 15 June 2002, p 4). Giannini did not specifically test this theory, but Rotstayn says the two ideas are not necessarily inconsistent. He thinks his mechanism might work by changing the heat balance of the atmosphere, and through it warm the oceans.
None of this gets humans off the hook, of course. The most likely cause of long-term warming of the oceans is global warming.