They are called Milankovitch cycles, after the Serbian mathematician Milutin Milankovitch, who proposed in the 1930s that such regularly occurring variations could explain why Earth's climate goes in and out of ice ages. According to this theory, the most recent ice sheets began retreating from Canada and Eurasia around 17,000 years ago because that's when the northern hemisphere started getting enough sun during the summer to melt them. The axial tilt was higher than it is now, around 24 degrees, and perihelion was in July. Throughout the ice age, the Sahara was a desert. But as the summers got hotter, they also got wetter.
Hotter summers mean a stronger monsoon that brings more rainfall farther north, into the Sahara.
Until now, that has been the whole explanation for why the desert once bloomed - but it just doesn't work, says Claussen. You end up with a Sahara desert more or less where it is today. A bit more rain does fall on the desert, but not nearly enough to support abundant vegetation or to create lakes. Maybe the Sahara wasn't turning green in the models simply because the models, unlike reality, weren't letting it. Every gardener knows, he says, that climate zones dictate what plants can grow where.