Scientists have been trying to understand what caused the paralyzing drought that began in the 1970s, ravaging North Africa and causing the Sahara desert to advance to the south. Some studies suggested that by using the land for farming and livestock grazing, humans were responsible for bringing about the drought and keeping the land from recovering.

Researchers have found that previous drought conditions in Africa grasslands cause following years to remain drier, over-riding meteorological conditions which otherwise would have caused more rain to fall.

The approach scientists took was to put natural vegetation in a (computer) climate model and make it fully interactive with land surface processes and atmospheric processes," said William Lau, a Goddard Space Flight Center (Greenbelt, Md.) atmospheric scientist. Lau, along with UCLA atmospheric scientists Ning Zeng and David Neelin, found that the addition of vegetation to climate computer models proved to be the missing link in what was driving the drought.

"The Sahel has gone through a well-known drought in the 70s and 80s, and it is a huge economic concern for the countries in the Sahel region like Nigeria, Niger and Mali," said Zeng. "One school of thought has blamed the drought on the lack of rainfall caused by changing temperatures of the ocean surface." Other studies, said Zeng, claim the drought is more man-made.

In trying to decipher the true cause, the team used a computer climate model to see how much of the drought could be accounted for by the cooler sea surface temperatures that suppress summer monsoons and bring less rain to the Sahel region.

Lau said the model showed that changing sea surface temperatures couldn't account for much of the drought at all. "If you look at the sea surface temperature as a drought forcing phenomenon over 50 years, you see a very weak drought, not enough to explain the observed variance and the long term severity of the drought," he said.

To find what else may be causing the decades-long dry spell, the scientists added another factor -soil moisture-to the climate model. They found that as the soil dried out, the air lost humidity, making the chance of rain decrease even more. But still, said Lau, it was only enough to account for a drought half as severe as what actually happened in the Sahel.

Finally, the team added natural vegetation to the model and found that the natural vegetation interacts significantly with climate, and in the case of the Sahel drought, caused enhanced drying.

"The way it works is when it gets dryer due to changes in sea surface temperature, less vegetation grows. And less vegetation leads to higher surface albedo," said Zeng. A higher surface albedo-or a greater amount of reflected solar radiation-leads to a drier, cooler climate. The cooling effect weakens the monsoon circulation, and less moisture comes in from the south and the west, he said.

Also, since plants transpire by losing water through their leaves, less vegetation decreases humidity. The loss of a direct moisture supply means less rainfall, which causes weaker circulation, dampening the monsoon season. "This is the first time a realistic global model is able to show the interactive natural vegetation on a time scale of decades," said Zeng.

Lau said that the new model including natural vegetation changes is a much better reflection of what the Sahel region actually experienced. And it suggests that without the addition of man-made landscape changes, the climate system is fully capable of generating this devastating type of drought.

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