As recently as 5,000 years ago, the Sahara was not a desert, but green. Dramatic changes can arise from even gradual planetary shifts. First the change comes slowly, then fast, in just a few hundred years.
This is some high quality twitter content in this thread: https://t.co/TMy3pxXApj — Nicholas A. Christakis (@NAChristakis) September 2, 2019