Wobbles of the Earth on its axis have created periods when the Sahara was savannah rather than desert. These periods recur roughly every 21,000 years, and the last one was around 10,000 years ago. Counterintuitively, the greeting happened when the tilt moved the Northern Hemisphere so it was warmer, which increased humidity and rainfall. It’s not clear whether climate change is going to have a similar effect, or if that would matter with all the other problems it causes.

https://theconversation.com/the-sahara-desert-used-to-be-a-green-savannah-new-research-explains-why-216555