The Sahara was grassland dotted with lakes until about 5,000 years ago
Around 14,500 years ago the Sahara began turning green. Grass, trees and lakes spread across North Africa, people herded cattle and paddled canoes where there is now sand, and painted swimmers on cave walls. When the rains failed again, they moved toward the Nile and Mesopotamia and built early complex societies.
Scientists call this the African humid period. It was driven by a shift in Earth's axial tilt that strengthened the African monsoon, amplified by feedbacks from spreading vegetation, reduced dust and rising greenhouse gases. Before it, during the Last Glacial Maximum, the Sahara was bigger than today and largely empty, and even Lake Victoria and the White Nile were dry or low. The wet phase began at the close of a cold spell known as Heinrich event 1, alongside a northern warming, and Lake Chad swelled while glaciers grew on Kilimanjaro.
It was not steady. Two big dry setbacks interrupted it, one in the Younger Dryas and another in the brief 8.2-kiloyear event, and the period ended between 6,000 and 5,000 years ago, apparently in stages. People settled widely across the Sahara and Arabia as hunter-gatherers and herders of cattle, goats and sheep, leaving one of the oldest canoes known and rock art such as the Cave of Swimmers. Its influence reached the birth of ancient Egyptian civilization.
Rock art was the first clue. Heinrich Barth wondered in 1850 about a wetter past after finding petroglyphs in the Murzuq Desert, Ahmed Hassanein saw savanna animals depicted at Gabal El Uweinat in 1923, and László Almásy coined the idea of a Green Sahara in the 1930s. An orbital explanation for monsoon strength was floated in 1921, and precession became the favoured cause from the 1970s. The name itself was coined in 2000 by Peter de Menocal and colleagues.
Much remains uncertain, including exactly when it began and ended, how intense it was and how strongly the land fed back, and climate models struggle to reproduce a Sahara that green. It was not a one-off: evidence hints at as many as 230 earlier wet phases, possibly back to the desert's origin 7 to 8 million years ago, some of which opened routes for early humans across Arabia and North Africa.
Source: African humid period
