Shift in SE Asian monsoon resulted in dry Sahara 4,000 years ago

Today, the Southeast Asian Monsoon provides critical water resources to more that 600 million people each year. Even slight variations in the strength and/or timing of the monsoon can have profound impacts on the region. But there's a problem, because while we do know more, very little is known about the range and mechanisms of Southeast Asian monsoon variability, particularly on timescales such as the Holocene. The mid- to late Holocene, roughly six to four thousand years ago, was characterized by one of the largest climate shifts since the last glacial termination.

New evidence from stalagmites from a cave in Laos, indicate a major decrease of monsoon rainfall in mainland Southeast Asia during the mid-to late Holocene (6,000 to 4,000 years ago). At the same time the African monsoon failed to deliver enough moisture and that signalled the end of the 'Green Sahara'[1].

It signalled the end of the 'Green Sahara' (also known as as the African Humid Period), when a vegetated northern Africa into the current desert landscape. It also resulted in the collapse of the Akkadian Empire of Mesopotamia, the de-urbanization of the Indus Civilization, and the spread of pastoralism along the Nile, as people were driven from the arid Sahara.

Computer modelling experiments show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation, the air flow in the tropics in the lower atmosphere, eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in mainland Southeast Asia. The results of the research indicate that reduced vegetation and increased dust from the Sahara may have been the catalyst for societal shifts in mainland Southeast Asia.

The megadrought it caused would have led to mass population movements and the adoption of new, more resilient subsistence strategies. It may even have led to the inception of Neolithic farming in mainland Southeast Asia, the researchers have concluded.

So, maybe as a result of an extreme El Niño, the Walker circulation was influenced, which resulted in an decrease in monsoon activity in Southeast Asia and Northern Africa. This in turn resulted in an arid and dusty Sahara which now feeds the Amazon rainforest and forced people in Southeast Asia to migrate.

[1] Griffiths et al: End of Green Sahara amplified mid- to late Holocene megadroughts in mainland Southeast Asia in Nature - 2020