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Grass, lakes and cattle, 6,000 years ago

The Sahara Turns Green, and Will Again

The largest hot desert on Earth is not permanent. It has a cycle, and we know its length.

The Sahara today The largest hotdesert on EarthThe Sahara 6,000 yearsago Lakes, grassland,hippos and cattle
Same place. Not a long time ago.

  1. Step 1 of 5

    There are paintings on the rocks, and they are wrong for a desert.

    Rock walls in themiddle of theSahara Covered in cattle,herders and swimmers

    Deep in the Sahara, on cliff faces hundreds of kilometres from any water, there are thousands of paintings and engravings of cattle, giraffe, hippos and people swimming. Somebody painted them from life.

    One site in south-west Egypt is known as the Cave of Swimmers. It is roughly as far from open water as it is possible to get in Africa.

  2. Step 2 of 5

    The ground underneath agrees with the paintings.

    Today's sandDry dunes on thesurfaceLake mudLayered sediment:standing waterGrass pollenPreserved in themudFish and hippo bonesIn places now 500km from waterBuried river channelsSeen by radarunder the dunes
    Four separate lines of evidence, all saying wet.

    Cores drilled into the sand find lake sediments — the fine, layered mud that only settles at the bottom of standing water. Satellite radar sees the channels of enormous vanished rivers under the dunes. Fossil pollen in those layers is grassland pollen.

  3. Step 3 of 5

    The cause is a wobble in how Earth leans and turns.

    Earth's axis slowly circles, over ~21,000 years Which season isnearest the Sunshifts Northern summers heat up more, then less The rain beltmoves north,then back21,000years

    Earth's tilt drifts, and the direction the axis points slowly circles, taking about 21,000 years to come round. That changes which season happens when Earth is closest to the Sun, which changes how strongly the northern summer is heated.

  4. Step 4 of 5

    A hotter northern summer drags the rain north.

    Stronger northernsummer heating The rain beltshifts north overthe SaharaThe same rain belt that wets West Africa, moved north.

    The monsoon is driven by land heating faster than sea and pulling wet air inland. Heat the Sahara's latitude harder in summer and that pull strengthens, so the rain belt that today stops in the Sahel pushes hundreds of kilometres further north.

  5. Step 5 of 5

    It ended surprisingly fast, and it is due back.

    Thousands ofyears the greenSahara lastedThousands ofyears since itendedThousands ofyears, roughly,until the next

    The wet phase ran from roughly 11,000 to 5,000 years ago, and the drying took centuries rather than millennia — vegetation dying back reduced the rainfall further, which killed more vegetation. On the orbital schedule the next green phase is due in some thousands of years.

    What human greenhouse warming does to that schedule is an open research question, and not one this page can settle.

The short version

A 21,000-year wobble in Earth's orbit periodically strengthens the African monsoon and pushes it over the Sahara, which last turned the desert to lakes and grassland until about 5,000 years ago.

Try it yourself

Spin a top and watch the slow circle its tilted axis traces while it spins fast. That slow circle is precession, and it is the clock behind the green Sahara.

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