Saharan desiccation
▶ Explore on the mapThe African humid period, often called the greening of the Sahara, ended a long era in which the desert was grassland dotted with lakes and rivers; its desiccation from about 5000 to 3000 BCE turned the region into the arid desert known today. Sediment cores from lakes in the Sahara, rock art of swimmers and giraffes in the Tassili n'Ajjer, and the fossils of hippos and crocodiles all show a wetter world. The change was driven by the slow wobble of Earth's orbit, which shifted the African monsoon northwards and then back; the drying was not smooth, and a final collapse around 3000 BCE was probably accelerated by overgrazing and vegetation loss once farming and herding reached the interior. As the lakes shrank, people moved to the Nile valley, where the concentration of population and water may have helped set the conditions for the Egyptian state. Pastoral communities crossed the desert in its last wet centuries, and their cattle burials and herding culture are found far from any water today. The episode is a standard case study of how climate rewrites where people can live. Later, when the desert dried, its rock art was left where no rain fell, which is why it survives so well today.