Imagine Northern Africa, now inhabited by the desiccated Sahara desert, as a green oasis, covered by plants and trees. That’s what it used to be 11,000-5,000 years ago until it became the hyper-arid Sahara we know now. But what triggered that shift? Now new research published in Nature Communications from scientists at the University of California, Irvine, the University of Pennsylvania, William Paterson University of New Jersey point toward the existence of a megadrought 4,000 to 5,000 years ago.
Using physical evidence in the form of stalagmite samples from caves in Northern Laos, the collaboration of researchers say their findings help reconstruct the mid-Holocene period of Earth’s history.
"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 co-author Kathleen Johnson, UCI associate professor of Earth system science. "Our high-resolution and well-dated record suggests a strong connection between Northern Africa and mainland Southeast Asia during this time."
The team measured the geochemical properties of the oxygen and carbon isotopes, carbon-14, and trace metals found in the stalagmites in order to confirm the existence of the megadrought and analyze how it affected the region.
"This is outstanding evidence for the type of climate change that must have affected society, what plants were available, what animals were available," said co-author Joyce White, adjunct professor of anthropology at the University of Pennsylvania. "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."
Their findings provide evidence for a shift in the Walker circulation pattern, explain the authors. They write: “Through a set of modeling experiments, we show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in mainland Southeast Asia. Our results indicate that vegetation-dust climate feedbacks from Sahara drying may have been the catalyst for societal shifts in mainland Southeast Asia via ocean-atmospheric teleconnections.”
These conclusions help fill in the gaps of archaeological evidence in interior Southeast Asia during the mid-Holocene. They also imply a reason for understanding the mass migrations that occurred during the time and offer a view on how the development of Neolithic farming in mainland Southeast Asia came to be.