Why does the Sahara Desert alternate between dry and humid periods?

The Sahara Desert’s transition between arid and humid phases is driven by Earth’s precession, a 26,000-year cycle involving a wobble on the planet's axis.12

Currently, the Northern Hemisphere experiences perihelion—the closest approach to the Sun—during winter, resulting in more indirect sunlight for the Sahara. During humid periods, however, the precession aligns perihelion with the Northern Hemisphere's summer. This alignment increases solar radiation in the region by approximately 7 percent.12

This increased heating warms North Africa, creating rising air currents that draw in humid air from the Atlantic. This process effectively "supercharges" the West African monsoon, allowing it to penetrate deep into the desert and transform the arid landscape into a lush environment of rivers, grasslands, and forests. Evidence for these cycles, which have occurred at least 20 times in the last 800,000 years, is found in sedimentary rock layers and hydrogen isotope analysis of fossilized plant waxes.2