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Sahara
Geographica  
Climatology

Green Sahara
The great desert where hippos once wallowed

The Sahara sets a standard for dry land. It's the world's largest desert. Relative humidity can drop into the low single digits. There are places where it rains only about once a century. There are people who reach the end of their lives without ever seeing water come from the sky.
 
Yet beneath the Sahara are vast aquifers of fresh water, enough liquid to fill a small sea. It's fossil water, a treasure laid down in prehistoric times, some of it possibly a million years old. Just 6,000 years ago the Sahara was a much different place.
 
It was green. Prehistoric rock art in the Sahara shows something surprising: hippopotamuses, which need year-round water.
 
"We don't have much evidence of a tropical paradise out there, but we had something perfectly livable," says Jennifer Smith, a geologist at Washington University in St. Louis.
 
The green Sahara was the product of the migration of the paleomonsoon. In the same way that ice ages come and go, so too do monsoons migrate north and south. The dynamics of the Earth's motion are responsible. The tilt of the Earth's axis varies in a regular cycle—sometimes the planet is more tilted toward the sun, sometimes less so.
The axis also wobbles like a spinning top. The date of Earth's perihelion—its closest approach to the sun—varies in a cycle as well.
 
At times when the Northern Hemisphere tilts sharply toward the sun and the planet makes its closest approach, the increased blast of sunlight during the north's summer months can cause the African monsoon (which currently occurs between the Equator and roughly 17°N latitude) to shift to the north as it did 10,000 years ago, inundating North Africa.
 
Around 5,000 years ago the monsoon shifted dramatically southward again. The prehistoric inhabitants of the Sahara discovered that their relatively green surroundings were undergoing something worse than a drought (and perhaps they migrated toward the Nile Valley, where Egyptian culture began to flourish at around the same time).
 
"We're learning, and only in recent years, that some climate changes in the past have been as rapid as anything under way today," says Robert Giegengack, a University of Pennsylvania geologist.
 
As the land dried out and vegetation decreased, the soil lost its ability to hold water when it did rain. Fewer clouds formed from evaporation. When it rained, the water washed away and evaporated quickly. There was a kind of runaway drying effect. By 4,000 years ago the Sahara had become what it is today.
 
No one knows how human-driven climate change may alter the Sahara in the future. It's something scientists can ponder while sipping bottled fossil water pumped from underground.
 
"It's the best water in Egypt," Giegengack said—clean, refreshing mineral water. If you want to drink something good, try the ancient buried treasure of the Sahara.
 
—Joel Achenbach
Washington Post staff writer


Web Links

Sahara
www.pbs.org/sahara/index.htm
Explore the geography, climate, people, and wildlife of the largest desert on Earth with resources from a PBS movie.
 
What Is a Desert?
pubs.usgs.gov/gip/deserts/what
Read about the different types of deserts and how they form.
 
Desert USA
www.desertusa.com
Learn about desert wildlife, how sand dunes form, and where to go to hear booming and singing sand.

More Articles by Joel Achenbach
www.washingtonpost.com/wp-dyn/style/columns/achenbach
Read some of writer Joel Achenbach's columns for the Washington Post.


Free World Map
Bibliography

Allaby, Michael. Deserts. Facts on File, Inc., 2001.
 
Alley, R. B., and others. "Abrupt Climate Change." Science (March 28, 2003), 2005-9. 
 
Crowley, Thomas. "Cycles, Cycles Everywhere." Science (February 22, 2002), 1473-4.
 
DeVilliers, Marc, and Sheila Hirtle. Sahara: A Natural History. Walker and Co., 2002.
 
Lamb, H. H. Climate, History and the Modern World. Routledge, 1995.




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