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New Research Suggests Ancient Humans Helped Make the World's Largest Desert

Causing climate change since 6,000 BCE.

MIKE MCRAE
15 MAR 2017
 

It seems impossible to imagine, but where the Sahara is dominated today by a landscape of sand dunes, just a few thousand years ago, it was a vast grassland dotted with lakes.

An archaeologist has now suggested that we humans played a pivotal role in bringing on the rapid changes in ecology that caused a drop in rainfall, which could provide us with insights into how we'll cope with large scale climate change in the future.

 

Deep beneath the Sahara's sands are signs of old rivers and traces of plants and animals, preserving reminders of the region's greener past.

The roughly 10,000-year stretch of what's called the African Humid Period is largely considered to have been caused by an episode of monsoons that washed over the continent - just one of a cycle of such wet periods dating back 9 million years that current hypotheses claim ended with changes in Earth's orbit.

While the current thinking blames a 20,000 year 'wobble' in our planet's orbital axis for ending the last big wet, David Wright from Seoul National University isn't convinced.

"In East Asia there are long established theories of how Neolithic populations changed the landscape so profoundly that monsoons stopped penetrating so far inland," he says.

The Sahara is currently the world's largest desert, taking in 10 African countries and stretching over 9 million square kilometres (3.5 million square miles).

Its average rainfall varies, with most of the desert receiving less than 20 millimetres (0.79 in) each year.

 

Wind the clock back about 11,000 years, however, and the weather was vastly different.

"It was 10 times as wet as today," says Jessica Tierney of the University of Arizona, a paleoclimatologist who measured the rainfall over the last of the 'Green Sahara' periods by analysing marine sediments taken off the coast of West Africa.

That time period happens to coincide with a human migration into the area, bringing agriculture with them.

But then 8,000 years ago, over the course of just 1,000 years, the monsoons started to weaken.

"It looks like this 1,000-year dry period caused people to leave," says Tierney.

"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."

It was the impact of the crop-growers who ultimately tipped the climate from damp to dry, according to Wright.

His research involved analysing archaeological evidence of the first signs of pastoralism in the Saharan region and matching it with the coverage of ancient scrub vegetation.

The story that emerged suggests that as communities of people spread, they changed the landscape to accommodate crops and livestock, causing an exchange in plant species that covered the ground for specimens that exposed the soil.

As sunlight bounced from the brighter soil, it warmed the air, building a feedback loop that shifted the atmospheric conditions enough to reduce the frequent monsoon rains and benefit scrub vegetation over grasslands until rainfall virtually vanished, leaving only a scattering of hardy desert plants.

It's an interesting idea - and one that isn't necessarily mutually exclusive with the 'wobble' hypothesis, which Wright suggests had a small impact about 8,200 years ago but wasn't permanent.

But Wright will need to build a stronger case to convince other researchers to rewrite the textbooks just yet - he's now planning to hunt down more evidence to support his ideas.

"There were lakes everywhere in the Sahara at this time, and they will have the records of the changing vegetation. We need to drill down into these former lake beds to get the vegetation records, look at the archaeology, and see what people were doing there," he says.

Since approximately one-sixth of the world's population lives in a desert, research that describes our relationship with our wet and dry ecosystems is important.

Who knows, maybe one day we'll even find a way to make the Sahara bloom again.

David Wright's research was published in Frontiers in Earth Science; Jessica Tierney's work was published in Science Advances.

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