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Nature Geoscience 1, 650–651 (1 October 2008) | doi:10.1038/ngeo323

Palaeoclimate: Africa on the edge

Peter B. deMenocal

Like the ice ages, the deserts of the Sahara also come and go. The slow wobble of the Earth's orbit periodically increases and decreases the amount of summer seasonal radiation over North Africa with a roughly 20,000-year beat.