Was the Sahara really green once?

Yes. Between roughly 14,500 and 5,000 years ago, much of the Sahara was covered with grasses, trees and lakes. Scientists call this time the African humid period, or the Green Sahara. Rivers flowed, lakes filled, and people settled across what is now one of the driest places on Earth.

The trigger was a slow wobble in Earth's orbit. Between 11,000 and 10,000 years ago, our planet passed closest to the Sun during the northern summer, which raised summer sunshine by about 8%. Hotter land draws in moist air from the Atlantic, so the African monsoon grew stronger and pushed its rains much farther north. Then the landscape amplified the change: more rain meant more plants, and plants absorb more sunlight than bare sand, which fed the monsoon even more.

People left their mark. In 1933, the Hungarian explorer László Almásy found a cave in the Egyptian desert painted with a giraffe, a hippopotamus and human figures that look like swimmers. He argued that the climate must have changed since. The idea was so new that his own editor added footnotes to say he disagreed.

The green did not fade gently. Around 6,000 to 5,000 years ago, as summer sunshine kept slowly falling, the Sahara dried out, and people gradually moved toward reliable water, like the Nile Valley, where early complex societies, including Ancient Egypt, took root.