Once, the Sahara was a green and lush area covered in vegetation, with rainfall up to ten times more than it is today, scientists have said. They have come up with climate reconstruction spanning the last 25,000 years, which shows how wet the region was between 5,000 and 11,000 years ago.
Today, the Sahara desert is perhaps one of the most arid regions on Earth. Average rainfall is estimated at 35 to 100 millimetres per year. But during the 'Green Sahara' period, the area supported diverse vegetation, permanent lakes, and populations of hunter-gatherers.
Evidence for this has been found in paleolake deposits, pollen, and archaeological remains – which document ancient human activity deep into what is now a very dry desert.
However, scientists had so far been unable to determine the rates of rainfall at the time, or how long wet climatic conditions remained, because they did not have access to continuous sedimentary records.
Driving humans in and out the Sahara
In this new research published in the journal Science Advances, a team led by Jessica Tierney of the University of Arizona, have tackled this problem by using leaf wax isotopes preserved in marine sediment cores collected at four sites along the African coast.
This allowed them to come up with a quantitative reconstruction of rainfall over the last 25,000 years. The model indicates that during the 6,000 years of the 'Green Sahara' period, annual average rainfall was 640 millimetres per year across all sites.
However, the researchers were also able to infer that wet conditions stopped for a prolonged period of time, some 8000 years ago. This pause, right in the middle of the Green Sahara period lasted around a thousand years and was consistent with archaeological data that suggest humans left the area at the time.
"It looks like this thousand-year dry period caused people to leave. 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," Tierney explained.
The authors believe that a cooling in the Northern Hemisphere initiated this interruption in the 'Green Sahara' conditions.