The Sahara wasn’t always a desert. Trees and grasslands dominated the landscape from roughly 10,000 years ago to 5,000 years ago. Then, abruptly, the climate changed, and north Africa began to dry out.
Previous research has suggested that the end of the African Humid Period came gradually, over thousands of years, but a study published last month in Science says it took just a few hundred. The shift was initially triggered by more sunlight falling on Earth’s northern hemisphere, as Earth’s cyclic orientation toward the sun changed. But how that orbital change caused North Africa to dry out so fast–in 100 to 200 years, says the study–is a matter of debate.