When the biggest hot desert had lakes, hippos and crocodiles, and why it dried up.
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The Sahara is the biggest hot desert on Earth. But in the middle of it, people have dug up fish hooks and the bones of hippos and crocodiles. How did they get there?
The Sahara wasn’t always a desert. Drag the slider to go back in time, and watch the rain, the plants and Lake Chad. (The green land and the lake are drawn simpler than they really were.)
About 7,000 years ago, Lake Chad was a giant lake. Scientists call it Lake Mega-Chad. It was even a little bigger than the Caspian Sea, the biggest lake on Earth today! (Its shore here is simplified.)
People lived in the green Sahara, and they painted and carved pictures on the rocks. In Algeria there are giraffes, hippos and herds of cattle. In Egypt’s Cave of Swimmers, painted between 9,000 and 6,000 years ago, little people seem to swim!
The Earth slowly wobbles as it travels round the Sun. About 10,000 years ago, the wobble gave the northern half of the Earth stronger summer sunshine than today. The hot land pulled the wet monsoon wind in from the sea, far into Africa, and the rain came too.
The wobble kept going. Northern summers slowly lost their extra sunshine, and the monsoon rain moved back south. By about 5,000 years ago the grass had died, the giant lake had shrunk, and the sand came back.
The northern end of the old lake is now the Bodélé Depression, the lowest land in Chad. Its ground is full of the tiny shells of diatoms that lived in the lake. Winds whip them up into dust storms about 100 days a year, and the dust can blow right across the Atlantic.
The giant lake is gone, but its old beaches are still there.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).