Sahel drought: new look at causes

Ever since long-term droughts began parching the Sahel in the late 1960s, the West African region has served as a poster child for widespread hunger and the loss of farm and grazing land to encroaching deserts.

Now, scientists on three continents are planning an intensive set of field experiments to unlock the secrets that govern the region's annual rainy season, known as the West African monsoon.

With what they learn during the multiyear project, researchers expect to improve drought forecasts for the region and guide longer-term plans to improve weather and climate monitoring efforts there. The results also are expected to improve efforts to forecast Atlantic hurricanes and the impact of Saharan and Sahelian dust on air quality in North America and the Caribbean.

Already, recent research not associated with the monsoon project is pointing in tantalizing directions. It appears climate factors triggered the regional drought, not human activity, as many believed.

Individual researchers have scrutinized the Sahel for years, but this effort could well rank as the most ambitious yet. It comes at a time of growing interest in building a global network of environmental sensors to track and forecast long-term trends in weather, climate, vegetation, and ocean conditions. Next week, representatives from 34 countries are scheduled to meet in Baveno, Italy, to develop a blueprint for the United States-led Global Environmental Observation effort. The representatives are slated to present their results at two ministerial meetings next year.

When it comes to atmospheric and other environmental measurements, "West Africa is a data-sparse region," says Christopher Thorncroft, an atmospheric scientist at the State University of New York at Albany and one of two US coordinators for the international West Africa monsoon research effort.

Yet the region also is said to be one of the most sensitive on the planet to small changes in climate conditions. And weather and climate patterns there can influence conditions half a planet away.

Thus, he says, "field campaigns are needed to recommend future monitoring strategies for the region" that can feed and provide a reality check for forecasting models. Such measurements also help verify the accuracy of satellite instruments, which have become a critical tool for measuring conditions where ground-based instruments are lacking.

The monsoon-research effort, Dr. Thorncroft explains, grew out of a French proposal unveiled in 2001. Paris already has committed itself to the project, as have 14 African nations. Meanwhile, teams in Britain, Germany, and the US are developing specific research agendas to underpin their requests for the funds needed to participate in the project. The US team hopes to wrap up its science plan next month.

Two groups recently have published studies that shed light on the monsoon's often-tragic response to slight changes in ocean conditions as well as on the global reach of conditions that originate in the Sahel. These studies underscore the need for efforts to monitor interactions among the oceans, atmosphere, and land of West Africa. For decades, some scientists have held that human activity such as deforestation caused the drought. While these factors exacerbated the problem, recently studies have suggested another trigger.

In work that appeared in the Nov. 7 issue of the journal Science, a team led by Alessandra Giannini tried to tease out the causes of the Sahel drought, which began in the late 1960s and continued at least through the mid-80s. Dr. Giannini and her colleagues determined that warmer-than-average ocean temperatures on either side of the continent - but especially in the Indian Ocean - were the likely trigger. They used a new climate model developed at NASA's Goddard Space Flight Institute and compared the results with rainfall and sea-surface temperature data from the region.

The monsoons, Giannini says, rely on strong temperature differences between land and water to keep them flowing deep into the continent. When the difference shrinks, even by a fraction of a degree due to ocean warming, rainfall tends to remain over the oceans. The puzzle now, she says, is to determine why the Indian and tropical Atlantic got warmer.

Meanwhile, at the University of Miami in Coral Gables, Fla., Joseph Prospero has spent more than 30 years measuring dust blowing off Africa and landing in Barbados. On the basis of his measurements, published in the same issue of Science, he and Peter Lamb of the University of Oklahoma calculated that dust concentrations were substantially lower for much of the early 20th century. Then they rose after 1970, when rainfall became scarce. They conclude that future changes in climate could lead to large shifts in dust blowing across the Atlantic, with larger amounts crossing the Atlantic should conditions in the Sahel worsen.

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