Once it was green and lush – but then the Sahara became a desert. This dehydration, associated with the development of dust, apparently had astonishingly far-reaching consequences, say researchers: They present evidence of a connection between desertification in North Africa and droughts that hit the interior of Southeast Asia before 5000 to 4000 and shaped the fate of the people there.
Where the sun burns on the bare ground today, green landscapes extended until about 6000 years ago, where the typical savannah animals of Africa grazed. This once friendly Sahara was also home to humans, as can be seen in rock carvings and archaeological finds. But then a drastic change set in: North Africa, once rich in vegetation, was transformed into the arid desert landscape that we know today. There is diverse evidence of how the climatic changes associated with the end of the green Sahara have affected people in North Africa and also in West Asia. The change was associated with the social developments along the Nile, with the collapse of the Akkadian Empire in Mesopotamia and with effects on the Indus civilization in today’s Pakistan and India.
A questioning look at Southeast Asia
As the researchers led by Michael Griffiths from William Paterson University in New Jersey report, changes in human society are also becoming apparent in far-off Southeast Asia during this time. This is a period marked by a clear lack of archaeological evidence compared to earlier and later epochs. One cause of the shift in human settlement patterns in the region could have been changes in the extent or timing of the monsoons, which are known to have profound social and economic effects. “Archaeologists and anthropologists have been studying this period for decades in terms of social adjustments and upheavals, but the exact background is not yet known,” says Griffiths.
In order to gain solid information on the development of the climate in Southeast Asia during the period in question, Griffiths and his colleagues examined samples of stalagmites from caves in northern Laos. As they explain, the analyzes of oxygen and carbon isotopes in the datable layers of these stalactites allow conclusions to be drawn about the past climatic developments. On the basis of the geochemical properties, they were now able to prove that there was indeed a sharp decrease in monsoon rains in the region during the period in question, which thus coincided with the drying up of the Sahara.
Dry link
The scientists then combined the data obtained with climate simulations in order to elucidate which processes could have led to the changes in the monsoons in Southeast Asia. As they report, the modeling experiments show that feedback and far-reaching connections between the ocean and the atmosphere may have led to the abrupt shift in precipitation. Specifically, it is becoming apparent how the reduced plant growth in the Sahara has led to increased dust in the air, which could have far-reaching effects.
According to the simulations, the dust could have cooled the Indian Ocean, which in turn was associated with a shift in the so-called Walker circulation to the east. This effect on the atmospheric flow cycle then led to the sharp decrease in monsoon humidity in Southeast Asia, which lasted for more than 1000 years. “Our high-resolution and well-dated record suggests a strong connection between North Africa and mainland Southeast Asia during this time,” says co-author Kathleen Johnson of the University of California at Irvine. “We are providing evidence of a close connection between the end of the green Sahara and the changes in the Southeast Asian monsoons during the mid to late Holocene,” said the scientist.
The researchers suspect that the droughts may have spurred changes in population patterns and the adoption of new, more resilient food supply strategies: they may have been a motor for the emergence of Neolithic agriculture on mainland Southeast Asia, the scientists say. “It was a climatic change that must have had an impact on society at the time, because it determined what plants and animals were available,” says co-author Joyce White of the University of Pennsylvania. Griffiths concludes: “The results can now help us to better understand the observed social shifts in many parts of the tropics to varying degrees”.
Source: University of California at Irvine, article: Nature Communications, doi: 10.1038 / s41467-020-17927-6