What climate phenomenon made the Sahara greener during the African Humid Period?

Increased summer insolation made the Sahara greener during the African Humid Period. Stronger summer sunlight in the Northern Hemisphere strengthened the West African monsoon and brought more rainfall into North Africa.

The African Humid Period occurred broadly between about 14,500 and 5,000 years ago, although its timing varied by location. Lakes, rivers, grasslands and wetlands expanded across regions that are now desert, and people and animals occupied areas that are currently extremely dry.

The underlying astronomical driver was changes in Earth’s orbital configuration, especially precession, which altered the seasonal distribution of sunlight. More intense Northern Hemisphere summers helped draw moisture inland through a stronger monsoon system.

The Sahara did not become uniformly tropical. Conditions varied across the region, and the transition back to aridity was uneven. As summer insolation declined, monsoon rainfall weakened, lakes shrank and desert environments expanded. This history shows that Sahara climate can change dramatically without the desert’s location moving.