The Sahara - largest hot desert in the world - was a green and lush area covered in vegetation 5,000 to 11,000 years ago, with rainfall up to ten times more than it is today, according to a new study.
The Sahara Desert was home to hunter-gatherers who made their living off the animals and plants in the region's savannahs and wooded grasslands, researchers said.
The study pinpointed the rainfall patterns in the Sahara during the 6,000-year "Green Sahara" period by analysing marine sediments. "It was 10 times as wet as today," said lead author Jessica Tierney of the University of Arizona (UA).
Annual rainfall in the Sahara now ranges from about four inches to less than one inch. Although other research had already identified the existence of the Green Sahara period, Tierney and her colleagues are the first to compile a continuous record of the region's rainfall going 25,000 years into the past.
Archaeological evidence shows humans occupied much of theSahara during the wet period, but left for about a thousand years around 8,000 years ago - the middle of the Green Sahara period.
Other researchers have suggested the Sahara became drier at the time people left, but the evidence was not conclusive, said Tierney, a UA associate professor of geosciences.
Her team's continuous rainfall record shows a thousand-year period about 8,000 years ago when the Sahara became drier. That drier period coincides with when people left, she said. "It looks like this thousand-year dry period caused people to leave," Tierney said.
"What's interesting is the people who came back after the dry period were different - most raised cattle. That dry period separates two different cultures. Our record provides a climate context for this change in occupation and lifestyle in the western Sahara," he said.
Researchers used their rainfall record to suggest ways current climate models can better replicate the Sahara'sancient climate and therefore improve climate projections.
They had long known the Sahara was much greener in the past, but how much of the Sahara was wetter and how much wetter was not well understood, Tierney said. Although scientists can learn about past climate by examining ancient lake sediments, in the Sahara the lakes dried up long ago and their sediments have blown away.
Instead of lake sediments, researchers used cores of marine sediments taken off the coast of West Africa at four different sites. As the cores were taken over a north-south distance of about 1,300 km - from offshore Cape Ghir, Morocco, to the northwest of Mauritania - the cores revealed both the ancient rainfall patterns and the areal extent of the Green Sahara.
The research was published in the journal Science Advances.