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*Sahara turned to desert in abrupt climate
change*
Copyright 1999 The Associated Press
July 15, 1999
WASHINGTON (AP) -- The rains stopped coming, the temperature rose and
the great grasslands of North Africa turned to desert a few thousand
years ago -- changes that may have helped spur development of
civilization in the Nile Valley.
The change to today's arid climate was not gradual, but occurred in two
episodes -- the first 6,700 to 5,500 years ago and the second 4,000 to
3,600 years ago, according to a paper published Thursday by the journal
Geophysical Research Letters.
"The latter was very severe, ruining ancient civilizations and
socio-economic systems," the researchers wrote.
A team of researchers headed by Martin Claussen of Germany's Potsdam
Institute for Climate Impact Research analyzed computer models of
climate over the past several thousand years.
They concluded that the change to today's desert climate in the Sahara
was triggered by changes in the Earth's orbit and the tilt of Earth's axis.
While the changes in Earth's orbit occurred gradually, the switch in
North Africa's climate and vegetation was abrupt. In the Sahara, "we
find an abrupt decrease in vegetation from a green Sahara to a desert
shrubland within a few hundred years," the scientists reported.
No longer were grasses and other plants collecting water and releasing
it back into the atmosphere; now sand baked in the stronger sun and
rivers and streams dried up.
This event devastated ancient civilizations in the moist desert, now
remembered only by rock paintings. The change may have spurred them to
move to the Nile Valley and other river valleys where great
civilizations developed.
"The migration of people from the Sahara to the Nile is a hypothesis,"
Claussen said.
"Whether or not this migration was the stimulus for the high
civilization there is not yet known. ... For me it seems plausible," he
said.
Claussen and his team used computer models of climate to calculate the
impact of weather, oceans and vegetation separately and in various
combinations. They concluded that oceans played only a minor role in the
Sahara's desertification.
The research also suggested that land use practices of humans who lived
in and cultivated the Sahara were not significant causes of the
desertification. Claussen noted that changes in the Earth's orbit and
tilt will continue to occur in the future.
As to their effects, he said: "What will happen in the future, frankly,
we can only speculate."
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