A team of US geoscientists and climate scientists found that severe doughts lasting several decades and sometimes even centuries have been the norm in west Africa for the past three centuries.

The most recent such drought lasted from 1400 to 1750, according to the researchers, whose study was published in the journal Science.

It was the first study to examine climate conditions in west Africa over thousands of years by analyzing annual layers of mud and tree sediment in Ghana's Lake Bosumtwi, a crater lake.

"Clearly, much of west Africa is already on the edge of sustainability," said University of Arizona geoscience professor Jonathan Overpeck, the study's lead author, "and the situation could become much more dire in the future with increased global warming."

The latest Sahel drought killed over 100,000 people and displaced scores more, according to a 2002 report by the United Nations Environment Program.

"What's disconcerting about this record is that it suggests that the most recent drought was relatively minor in the context of the west African drought history," said Timothy Shanahan of the University of Texas at Austin, who co-authored the study.

As global warming progresses due to emissions of mostly human-generated greenhouse gases, the temperature rise could make droughts more severe and prolonged, a potentially "devastating" development for Africa, Overpeck warned.

"They must plan for possible droughts that last much longer than a couple of decades," he told reporters by telephone. "We have strong confidence that continued warming will take place in the absence of reduction in greenhouse gases."

The periods of drought, especially those that lasted for 30-40 years, pointed to a pattern of sea surface temperature changes called the Atlantic Multidecadal Oscillation (AMO), the researchers found.

According to this hypothesis, temperatures in the Atlantic naturally fluctuate over 60-year cycles.

Although the oscillation has not yet been confirmed over long time periods, computer simulations and data sets, such as tree-ring variations from sites around the West Atlantic, have hinted at the possibility.

"More and more, it's starting to look like the AMO is a big player affecting climate change around the Northern Hemisphere, including drought variability over western Africa and western North America," said Overpeck.

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