Sahara

In the July 15, 1999 paper published by the journal, Geophysical Research Letters, the Sahara desert's arid climate change occurred quickly and dramatically 4000 to 3600 years ago. A team of researchers headed by Martin Cluassen 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. 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" 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 dried up. The scientists do not say what caused the change in the tilt of Earth's axis.

Offered by John.

ABC News, July 15, 1999

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 today 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.

Desert Shrubland

Some 9,000 years ago, Earth’s tilt was 24.14 degrees, compared with the current 23.45 degrees, and the point in the Earth’s orbit that is closest to the sun occurred at the end of July, as compared with early January now. At that time, the Northern Hemisphere received more summer sunlight, which amplified the African and Indian summer monsoon, they reported. 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.

 

Mass Migration

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 in response to questions via e-mail. “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.”