My understanding has been that dry conditions result from cooling — water gets locked up in ice and the world becomes drier. In warm periods, the Sahara gets greener. More water, more evaporation, more rain. This article talks about human-caused warming and a megadrought. I think they just say what they want to say. There is no science here.
We have scarce written records in North America before the Europeans got here. All we have for written records here are the past 5 centuries, which includes the Little Ice Age and the Current Warm Period. And the western most part of the U.S. was sparsely populated during the Little Ice Age.
But we have plenty of recorded history in much the rest of the world for the past 3,000 years. And that backs up ClearCase_guy's statement. During the Little Ice Age, Sub-saharan Africa had not just years of droughts, but decades. It was during the Dark Age cooling period that the Huns' rain patterns were unlivable, forcing them to move south into the land of the Gothic tribes, which forced the Goths to move south and takeover what was left of the Roman Empire and keep moving south to force my ancestors, the Anglos out of modern day Germany. It was during the Little Ice Age that many indigenous American tribes fought each over in part to take over each other's land to eek out more food (though there were other motivations too, like revenge wars in the northeast).
Earth’s orbit changes on a reoccurring pattern as does its axis tilt. Both of those caused the Sahara to green then dry out.
Heres the science
https://www.space.com/10527-earth-orbit-shaped-sahara.html
“My understanding has been that dry conditions result from cooling — water gets locked up in ice and the world becomes drier.”
That is exactly right. Antarctica is the largest driest area on earth.