YEARS ago, the Sahara was not a desert. When CO2 levels dropped, the plants could not survie, and desert sand encroached, unchallenged by soil/sand holding roots.
How 'bout THAT for a theory!!
If only we had more CO2, N Africa would be a thriving green-land.
Well...when the Sahara was green, roughly 5,000 years ago, CO2 levels were scarcely higher than they were say, in the 1700s.
What was different was that summer sun was stronger, creating monsoonal conditions in north Africa. More sun = warm rising air over land = advection of moist marine air over land = precipitation in the Sahara.