The Weather Notebook: Sahara Switch

Sahara Switch
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Dave Thurlow, Host

Photo: Dan Heller

The Sahara Desert is the world's biggest zone of sand and scrubland, covering nearly half of Africa. But it wasn't always like that. About six thousand years ago, most of the current Sahara Desert was covered by grass. What caused the Sahara to go from being green to being dusty? The tilt of the earth. Hi, I'm Dave Thurlow and this is The Weather Notebook.

A few thousand years ago, the earth tilted a few more degrees than it does now. This made for an African monsoon, much like the Indian monsoon. The monsoon pulled moisture from the tropics deep into the Sahara, making for much more rain than there is now, and for a fertile pre-desert grassland.

The earth's orbit changes over thousands of years, but the Sahara's transition from grassland to desert happened very quickly, in only a few hundred years. That's a pretty quick change in climate terms. When the earth's axis got to a certain point, the monsoon shut down, boom, just like that, and the climate switched.

The grasslands began to diminish, lakes started to disappear and the cropland, home to most of the North African population, withered. Civilization began to concentrate in cities near rivers, such as the Nile, where there was at least some water. A shift in the earth's axis that lasted for tens of thousands of years shifted civilization in just a matter of centuries.

Thanks to today's contributing writer Bob Henson and for more information on the Sahara, be sure to visit our website at mountwashington.org.

The Weather Notebook is a production of the Mount Washington Observatory, a non profit education and research organization focusing on the atmospheric sciences. Our show is underwritten by Subaru, with major support provided by the National Science Foundation.