Some 6,000 years ago, the Sahara Desert was regularly drowned by tropical rains. It wasn't a desert at all, but vast grasslands. Today, the Sahara features some of driest acreage on Earth.

Recently, a pair of researchers from Texas A&M University and Yale University set out to explain how such a vast climatic transformation can happen in such a short amount of time. In order to do so, the researchers built a model to contrast rain patterns of the Holocene era with those of today.

Their analysis offers new insights into the nature of the Hadley circulation, the cycle of warm air rising near the equator and descending in the subtropics. The Hadley circulation influences everything from the trade winds and tropical rain belts to jet streams and hurricanes.

"The framework we developed helps us understand why the heaviest tropical rain belts set up where they do," Robert Korty, associate professor of atmospheric sciences at Texas A&M, said in a news release. "Tropical rain belts are tied to what happens elsewhere in the world through the Hadley circulation, but it won't predict changes elsewhere directly, as the chain of events is very complex. But it is a step toward that goal."

Over time, the rain belt that once provided the Sahara with moderate rainfall has slowly moved northward toward the Mediterranean.

"It has been something of a mystery to understand how the tropical rain belt moved so far north of the equator," Korty said. "Our findings show that that large migrations in rainfall can occur in one part of the globe even while the belt doesn't move much elsewhere."

Korty and colleague William Boos of Yale argue the shifting rain belt alone fails to explain the transformation of the Sahara. Instead, falling precipitation totals likely created a sort of climatological feedback loop that triggered more drastic change in the soil and atmosphere.

"We were able to conclude that the variations in Earth's orbit that shifted rainfall north in Africa 6,000 years ago were by themselves insufficient to sustain the amount of rain that geologic evidence shows fell over what is now the Sahara Desert," Korty explained. "Feedbacks between the shifts in rain and the vegetation that could exist with it are needed to get heavy rains into the Sahara."

The pair of scientists hope their findings -- published in the journal Nature Geoscience -- will improve models designed to predict the impacts of climate change on regional weather patterns.