From:    Karen Claxon 
Subject: Scientists show that vegetation conditions drive the North Africa
         drought

Scientists show that vegetation conditions drive the North Africa drought
 EMBARGOED FOR RELEASE: 18 NOVEMBER 1999 AT 14:00 ET US
Contact: Cynthia O'Carroll
Cynthia.M.OCarroll.1@gsfc.nasa.gov
301-614-5562

NASA/Goddard Space Flight Center--EOS Project Science Office

Scientists show that vegetation conditions drive the North Africa drought

New research by NASA Goddard and University of California, Los Angeles
scientists to be published in the Nov. 19th issue of the journal Science,
shows that the devastating drought that plagued North Africa for decades may
be a natural phenomenon-fueled by the land's naturally-changing vegetation
cover.
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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