So a concerted effort has been made over the last five years to understand what drives dry spells there and what will occur in a future warmed by accumulating heat-trapping emissions in the atmosphere. Most climate experts now say that significant human-caused warming is inevitable.
One new study bodes particularly poorly for southern Africa, indicating that a 50-year-long drying trend there is likely to continue and appears tightly linked to substantial warming of the Indian Ocean.
The authors of the study say that the heating of that ocean, which lacks the natural variability of the Pacific and Atlantic, is one of the clearest fingerprints pointing to human-caused climate change.
"In our models, the Indian Ocean shows very clear and dramatic warming into the future, which means more and more drought for southern Africa," said Dr. James W. Hurrell, a scientist at the National Center for Atmospheric Research in Boulder, Colo., and an author of the study. "It is consistent with what we would expect from an increase in greenhouse gases."
Dr. Hurrell is to present the findings Tuesday at a geophysics conference in New Orleans.
The study compared the observed 20th-century changes in the oceans and weather patterns with 60 simulations of climate run on five computer models developed by different research centers. The other researchers were Dr. Martin P. Hoerling and Dr. Jon K. Eischeid of the Commerce Department's Climate Diagnostics Center, also in Boulder.
Dr. Hurrell said the warming ocean changes the circulation of the atmosphere, causing more air to rise over the water, generating marine storms and rains, while at the same time causing the opposite motion in air masses over the adjacent continent, leading to less rain. By midcentury, he said, there could be a 10 to 20 percent drying in the February-to-April wet season compared with the average for the last half of the 20th century.
For the Sahel, beset by drought and famine for generations, the picture is less clear. The new study projects rainier times there as the boundary between warm and cooler portions of the Atlantic Ocean shifts. But other experts on African climate still see evidence that the Sahelian drought pattern could worsen. Dr. Kerry H. Cook, an atmospheric scientist at Cornell, said the frequency of extremely dry years matters more to vulnerable populations than longer-term shifts in average rainfall. She said such extreme dry seasons were directly related to warm temperatures in the Gulf of Guinea, the Atlantic waters beneath the westward bulge of central Africa.
Most climate experts agree that this gulf will warm as global temperatures rise, Dr. Cook said. If emissions of greenhouse gases are not curbed, she added, new computer simulations show that there could be twice as many "really harmful dry years" in the latter half of this century as there were in the late 20th century. Other experts on African climate said that uncertainties in projections were likely to remain high as long as big gaps persist in collecting basic meteorological data.
For the moment, those gaps are not likely to close, largely because of political turmoil in many African countries and a lack of attention by wealthier nations, said Dr. Washington at Oxford.
He was the lead author of a British report last year on African climate variability and trends that concluded, "The African climate-observing system is in a worse state than that on any continent and is deteriorating."