A new publication in Nature shows that a shift in air circulation is responsible for the eastern African climate having changed to dry conditions about two million years ago. The authors studied a marine core to prove their hypothesis. Björn Backeberg is a co-author – he has an adjunct position with NERSC and is affiliated with Deltares in the Netherlands and the Nansen-Tutu Centre for Marine Environmental Research in South Africa.

Today, eastern Africa is quite dry, and it has been that way for about two million years. We know that the climate in that region used to be wetter before, but something shifted around two million years ago, leading to a change in the region’s climate.

It is established that today’s dry climate there is linked to an air circulation pattern called the “atmospheric Walker circulation”: Moist air rises from the warm eastern Indian Ocean and is transported westwards towards the cool western Indian Ocean off the shore of eastern Africa. On the way, the air loses its moisture – it becomes dry. The dry air then sinks down and causes the dry conditions over eastern Africa. This is the prevailing situation in the region. 

Evidence from several studies show that this circulation pattern has been active for a long time – affecting the eastern African climate for more than hundreds of thousands of years. It might even have had an influence on how humans evolved and left Africa for the first time to spread all over the globe. The new study “Indo-Pacific Walker circulation drove Pleistocene African aridification” published in Nature presents data covering the past seven million years that support this theory!

The authors looked in an unconventional spot to learn something about the atmosphere: in a marine drill core from below the seafloor. What they were interested in was to find out how fast the ocean water was flowing at the bottom of the ocean in between eastern Africa and Madagascar. Analyzing the sediments in multiple ways showed that the flow speed was quite weak and steady for about five million years, until about 2.1 million years ago. Then, something shifted, and the flow speed increased to approximately the same as today. At the same time, the Walker circulation increased. The authors show that the flow speed increase and Walker circulation increase are connected, their results indicate the establishment of the Walker circulation in the region at around 2.1 million years ago. They claim that this has led to the drier climate in eastern Africa from about 2.1 million years ago until today, compared to before.

This piece of evidence for the intensification of the Walker circulation at around 2.1 million years ago has implications for research investigating how humans evolved in Africa and how and why they ventured out to new continents about 2 million years ago!

Tarangire National Park in Tanzania, eastern Africa. Photo by Jorge Láscar. CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/)

Reference:

van der Lubbe, H.J.L., Hall, I.R., Barker, S. et al. Indo-Pacific Walker circulation drove Pleistocene African aridification. 2021. Nature 598, 618–623. https://doi.org/10.1038/s41586-021-03896-3