SIX thousand years ago, the Sahara was covered in shrubs and grasses. Why it
suddenly turned into the driest region on Earth has long baffled researchers.
Now climate modellers in Germany claim they have the answer.
Today, bare sands and rock cover 9 million square kilometres of northern
Africa. But ancient pollen grains show that as recently 9000 to 6000 years ago,
the Sahara was carpeted by low brush and annual grasses. Archaeologists have
also shown that nomadic people grazed their animals by seasonal lakes, dug
wells, and built sophisticated ceremonial centres.
Climatologists have known for some time that slow changes in the Earth鈥檚
orbit and the tilt of its axis gradually cooled the northern hemisphere,
starting about 9000 years ago. But even the most sophisticated climate models
have been unable to explain how this gradual climate change caused the Sahara鈥檚
abrupt desertification.
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So Martin Claussen of the Potsdam Institute for Climate Impact Research in
Germany designed a model that can simulate the interplay between climate and the
planet鈥檚 vegetation鈥攁 difficult task that isn鈥檛 adequately tackled by most
climate models. To allow these computations to simulate the climate over
thousands of years, he sacrificed some of the fine detail present in other
models. 鈥淥ur model has everything in it,鈥� says Claussen, 鈥測et it runs 1000 times
faster than more complex models.鈥�
The model shows that the transition from grass to sand could have occurred in
just 300 years, Claussen and his colleagues report this week in Geophysical
Research Letters (vol 26, p 2037). Starting with the warmer and wetter
conditions of 9000 years ago, the researchers slowly reduced the solar heating
of the atmosphere to simulate the effect of the changes in the Earth鈥檚 tilt and
orbit. This gradually weakened the monsoons over India and North Africa,
thinning the vegetation cover.
After a few thousand years of gradual change, the decimated vegetation was no
longer able to preserve soil moisture and maintain the cycle of evaporation,
atmospheric circulation and precipitation that drove the African monsoon. This
triggered an abrupt switch to a desert climate about 5400 years ago.
Paul Valdes, a climate modeller at the University of Reading, is impressed.
鈥淚t鈥檚 one of the few models that gets the Sahara to be green 6000 years ago,鈥� he
says. 鈥淐laussen was able to get that and the transition from relatively green to
relatively dry.鈥� But Valdes says the findings now have to be evaluated by models
adapted to include Claussen鈥檚 sophisticated simulation of the interaction
between climate and vegetation.
Claussen is eager to have his findings checked. In the meantime, he鈥檚 using
his fast-running model to try and find out whether future global warming could
have the paradoxical effect of turning the Sahara green once again.