Environment: Interview

How climate change turned the Sahara green

Scientists say the world’s largest desert was once a lush paradise for fishermen, transformed by a dramatic - and temporary - rise in rainfall
Thousands of years ago the African monsoon shifted north and the Sahara supported flourishing .../ Credits: Reuters

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For much of the past 70,000 years, the Sahara has closely resembled the parched desert it is today. But for several thousand years a dramatic warming turned it lush with vegetation.

This upheaval, which ended as abruptly as it began, shows how our environment and whole societies can be utterly transformed by climate change.

A migrating monsoon


About 12,000 years ago, slight changes in the Earth’s orbit around the Sun made northern hemisphere summers warmer as more solar 'insolation' hit north of the Equator.

This drove monsoon climates like a huge heat engine.

Today, the West African monsoon avoids the Sahara, passing further south. But as the Earth’s orbit changed the rains intensified and shifted north. Slowly, the desert bloomed. By 10,000 years ago, the Sahara had turned into a savanna-like ecosystem.

The so-called ‘African Humid Period’ was also reinforced by newly grown vegetation that absorbed the Sun’s energy, which strengthened the monsoon system.

Increased surface ocean temperatures of about 0.4 degrees Celsius also supported a stronger monsoon, as well as numerous permanent lakes such as the 330,000 square kilometer Lake 'MegaChad'.

Humans follow rains

With the water came humans. For two millennia, fisher people known as the Kiffian lived in what is today one of the Sahara's most desolate regions, the Ténéré.

Kiffian garbage dumps reveal fish bones, including catfish and Nile perch, which can be bigger than a man. Cave engravings of ostriches, giraffes and elephants illustrate how fertile the land must have been. Kiffian skeletons showed them as tall, muscular people who likely ate a lot of animal protein.

About a thousand years after the Kiffian disappeared, the Tenerian people turned up. They were very different from their predecessors. Smaller and less muscular, they appear to have been herders and hunters. Tenerian trash contained smaller fish, suggesting the lakes had shrunk.

Cleary the Ténéré had changed.

What had happened? The seasonal monsoons must have started shifting south again. About 3,500 years ago, the lakes finally dried up, the vegetation disappeared, the people vanished.

The desert had returned. It has been expanding ever since.

Tipping point or transition?

We know what caused the greening of the Sahara: a complex interaction between solar insolation, vegetation cover, and ocean temperatures.

What is even more interesting is the speed at which these changes occurred.

Peter deMenocal, an expert at Columbia University’s Lamont-Doherty Earth Observatory, argues that it started and ended very abruptly—within a few decades to centuries. He speculates that there could be an insolation tipping point “whereby subtropical African climate flips abruptly between humid and arid”.

However, other evidence suggests a more gradual transition. Layers of sediment drilled from the bottom of Lake Yoa in northeastern Chad hint that the environment changed in phases from trees and bushes to shrubs and grasses and, finally, to sand.

It is hard to imagine the Sahara as anything but sand. But could global warming prompt another greening of the desert? Not likely. Insolation is still too low, the monsoon is shifting southwards, and vegetation cover is decreasing.

Anthropogenic warming will probably be warm enough to do serious damage to many parts of Africa. But it won’t be strong enough to turn its largest desert back into a fisherman's paradise.



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Comments (1)

Francisco Zaragoza: 17.08.2010, 10:44

Very, very informative



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