Environment: Interview

Green Sahara: How climate change transformed the desert

Between 10,000 and 8,000 years ago fishermen thrived in the Sahara, demonstrating how climate change can prompt dramatic change on Earth.
Thousands of years ago the African monsoon shifted north and the Sahara supported flourishing .../ Credits: Reuters

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 according to some scientists, shows how our environment and whole societies can be utterly transformed by climate change.

About 12,000 years ago, slight changes in the Earth’s orbit around the sun brought the northern hemisphere into the limelight. Summers became warmer as more solar radiation hit the lands north of the Equator. Solar ‘insolation’ levels were up to 8 percent higher than today.

With insolation driving monsoonal climates like a huge heat engine, rainfall increased. One climate model estimated that the 8 percent increase in radiation in North Africa resulted in a 40 percent increase in precipitation.

Today, the West African monsoon avoids the Sahara, passing further south. But as the Earth’s orbit changed the rains intensified and shifted five degrees north. Slowly, the desert started to bloom. By 10,000 years ago, the Sahara had turned into a savanna-like ecosystem with trees and grass and grazing animals.

It wasn’t just down to the sun. The so-called ‘African Humid Period’ was also reinforced by newly grown vegetation that absorbed the sun’s energy, which strengthened the monsoon system, rather than reflecting the sun’s energy back into space.

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, four times the size of Lake Superior. Now, Lake Chad is a pitiful shadow of its former glory.

Humans Follow Rains

With the water came the people. The Ténéré, known to the nomadic Tuareg people as a "desert within a desert", was once home to fisher people. For two millennia, the Kiffian fishermen lived in what is today one of the most desolate parts of the Sahara.

Kiffian garbage dumps were full of 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 turned up. Clearly the Ténéré was still habitable, but it must have changed considerably.

The Tenerian people 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 started shrinking.

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

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

The Grass is Greener

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 for today’s civilizations 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—triggered largely by summer insolation crossing the threshold of 470 Watts per meter squared, 4.2 percent higher than today.

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 nothing but sand.

It’s hard to imagine the Sahara as anything but sand. But could global warming prompt another greening of the desert? Not likely. Solar 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 deserts back into a lush paradise for fishermen.

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Comments (1)

Francisco Zaragoza: 17.08.2010, 10:44

Very, very informative

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