Climate change in the Sahara linked to a megadrought in Asia

Around 11,000 years ago, the Sahara desert was a heavily vegetated landscape covered with grasslands, trees, rivers, and lakes. Extreme environmental changes transitioned the Sahara back into a dry desert about 5,000 years ago. 

Now, a study from the University of Pennsylvania has found that the end of the Green Sahara triggered a previously unknown megadrought that devastated Southeast Asia.

An international team of scientists is describing how this major climate transformation led to a shift in human settlement patterns in Southeast Asia.

“In this study, we provide the first proof for a strong link between the end of the Green Sahara and Southeast Asian monsoon failure during the mid- to late Holocene period,” said study co-author Professor Kathleen Johnson. 

“Our high-resolution and well-dated record suggests a strong connection between Northern Africa and mainland Southeast Asia during this time.”

The research was focused on stalagmite samples from caves in Northern Laos. In her lab at UC Irvine, Professor Johnson and colleagues obtained the paleoclimate record contained in the samples through various analyses.

This data was combined with a series of climate model simulations that were conducted by co-author Francesco Pausata of the University of Quebec in Montreal.

For the computer simulations, Saharan vegetation and dust concentrations were altered. This enabled the experts to examine the potential ocean-atmosphere feedbacks that were associated with the abrupt shift in precipitation across the Sahara.

The simulations indicated that reduced vegetation in the Sahara led to increased airborne dust that cooled the Indian Ocean and shifted its circulation pattern eastward – causing an event similar to a modern-day El Niño. 

Ultimately, this system led to a large reduction in monsoon rainfall across Southeast Asia that lasted more than 1,000 years, explained Professor Johnson.

Study lead author Michael Griffiths is a professor of Environmental Science at William Paterson University. He said the link between the end of the Green Sahara and the origin of the Southeast Asia megadrought had not been previously investigated.

“Archaeologists and anthropologists have been studying this event for decades now, in terms of societal adaptations and upheavals, but its exact cause has eluded the scientific community,” said Professor Griffiths, who collaborated with Professor Johnson on the for more than 10 years.

“Results from this work provide a novel and convincing explanation for the origin of the Southeast Asia megadrought and could help us better understand, to varying degrees, the observed societal shifts across many parts of the tropics and extra-tropics.”

The researchers propose that the centuries-long megadrought corresponds to the “missing millennia” in Southeast Asia between 4,000 and 6,000 years ago. This time frame is marked by a noticeable lack of evidence in the interior region of Southeast Asia compared to before and after.

The research indicates that the mid-Holocene megadrought led to major changes in human settlement. The event likely forced the creation of new resilient subsistence strategies and possibly contributed to the start of Neolithic farming in mainland Southeast Asia.

“This is outstanding evidence for the type of climate change that must have affected society, what plants were available, what animals were available,” said study co-author Joyce White. 

“All of life had to adjust to this very different climate. From an archaeological point of view, this really is a game changer in how we try to understand or reconstruct the middle Holocene period.”

The study is published in the journal Nature Communications.

—

By Chrissy Sexton, Earth.com Staff Writer