Sahara made slow transition from green to desert: study

A picture taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASAs Terra satelliteon shows dust blowing northward out of the Sahara Desert and over the Mediterranean Sea. The Sahara became the world's biggest hot desert some 2,700 years ago after a very slow fade from green, according to a new study which clashes with the theory that desertification came abruptly.

The Sahara became the world's biggest hot desert some 2,700 years ago after a very slow fade from green, according to a new study which clashes with the theory that desertification came abruptly.

Six thousand years ago, the massive arid region dominating northern Africa was quite green, a patchwork of trees and savannas as well as many sparkling lakes.

The region, larger than Australia, also was inhabited, according to the European-US-Canadian team of scientists behind a study in Science dated May 9.

Most of the physical elements that could tell the tale of the Sahara's geographic evolution have been lost. The scientists studied layers of sediment in one of the largest remaining Sahara lakes, Yoa, in a remote spot in northern Chad, which took them back through six millennia of climate history.

They looked at sediments, did soil tests and reviewed biological indicators such as plant and tree pollen and spores that were present before the desert encroached. They also studied the remains of aquatic microorganisms.

Their findings contradicted previous modeling that indicated a rapid collapse of vegetation in the region in a sudden end to the African Humid Period, about 5,500 years ago, said Stefan Kropelin, a geologist at the Prehistoric Archaeology Institute of the University of Cologne who took part in the new study.

In 2000, a study by Peter de Menocal of Columbia University of sediments in the west of Mauritania found a sudden increase in wind-carried dust blown off the Sahara region, suggesting swift climate change.

But data from Lake Yoa shows the opposite, and the transition to desert took its time, said Kropelin. He said he believed de Menocal's data were not wrong but misinterpreted.

Š 2008 AFP

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Posted by Seto 05/10/08 14:00

Rank: 5/5 after 1 vote

That's just like in Dune! These sci-fi sagas just have a lot of ideas to offer...

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Posted by DeeSmith 05/10/08 21:50

Rank: 4/5 after 3 votes

You know, it just MIGHT NOT be climate change that helped reform the Sahara in the Holocene. It was a desert before the humid period - its been dated as being approximately 3 millions years old.

What may have hastened it's return to desert (beyond the shift of a certain East_West weather pattern) was domestication of animals in the savannah-lands of North Africa. That human activity began, in Africa, about 10-11,000 BC; by 8000BC, goats, pigs, and sheep were domesticated. Overgrazing in the Middle East has been implicated in significant ecotope changes that resulted in irreversible soil erosion and desertification, at about this same time period. The concept of the humid period doesn't necessarily mean that there weren't intermittant dry periods (extended El Nino like conditions); heavy grazing by herd and wild animals may have damaged grasslands sufficiently to alter rainfall deposition. This is exactly the case for the savanna in the area of Lake Chad, and has, along with irrigation diversions, resulted in a loss of nearly 30 percent of this historic lake. It would not be a surprise that somewhat ephemeral lakes and grasslands might be lost due to human use.

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Posted by CWFlink 05/12/08 10:11

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What is fascinating to me is the realization that most of what we consider "human" is the product of such a short interval in time. We should NOT be shocked that the climate may change of the next 100 years, but rather than so little climate change has happened in the last 5000 years. In the earth's history, 5000 years is but a blink. How many times have creatures on this planet been granted a period of such favorable conditions with such stability? How many times did that result in the development of a civilization among those creatures? And if not, why not?

In order to deal with "Global Warming", we need an understanding of how fragile our world is, which requires an understanding of how stable conditions actually have been over much greater spans of time... and how creatures responded to those stable periods.

Any references anyone?