"the green sahara" might be a long-ago thing in a geological timespan (i wouldn't know), but a significantly green-er sahara was pretty recent - enough that it comes up in some theories/reconstructions of where proto-afroasiatic speakers lived and the history of predynastic egypt, which is where i started learning about it - environmental changes in north africa is a really interesting topic and if you're interested in how one of the richest areas of the roman empire turned into a big dusty boy i would recommend it because it goes down some neat rabbitholes be careful though in that a fair bit of the resources online about it are things originating in french colonialism or eugenicists trying to make everything the dirty arabs' fault - if you see a story relying a little too heavily on the banu hilal just give the author a quick spotcheck