An Ancient Megadrought

Between the Last Glacial Maximum (the peak of the most recent full fledged ice age), and the Neolithic Revolution (the domestication of plants and animals), there were the worst droughts of the last 50,000 years in the regions that were home to most of the modern human population. The timing suggests that these droughts may have set in place key events from pre-history that followed including proto-farming of non-domesticated plants and the settlement of the Americas.

[O]ne of the most widespread and intense droughts of the last 50,000 years or more struck Africa and Southern Asia 17,000 to 16,000 years ago.

Between 18,000 and 15,000 years ago, large amounts of ice and meltwater entered the North Atlantic Ocean, causing regional cooling but also major drought in the tropics,
"The height of this time period coincided with one of the most extreme megadroughts of the last 50,000 years in the Afro-Asian monsoon region with potentially serious consequences for the Paleolithic humans that lived there at the time[.]" The "H1 megadrought," as it's known, was one of the most severe climate trials ever faced by anatomically modern humans.

Africa's Lake Victoria, now the world's largest tropical lake, dried out, as did Lake Tana in Ethiopia, and Lake Van in Turkey. The Nile, Congo and other major rivers shriveled, and Asian summer monsoons weakened or failed from China to the Mediterranean, meaning the monsoon season carried little or no rainwater.

What caused the megadrought remains a mystery, but its timing suggests a link to Heinrich Event 1 (or "H1"), a massive surge of icebergs and meltwater into the North Atlantic at the close of the last ice age.

Previous studies had implicated southward drift of the tropical rain belt as a localized cause, but the broad geographic coverage in this study paints a more nuanced picture. "If southward drift were the only cause," says Stager, lead author of the Science paper, "we'd have found evidence of wetting farther south. But the megadrought hit equatorial and southeastern Africa as well, so the rain belt didn't just move--it also weakened."

From here.