Why is East Africa so Dry?

Peter deMenocal, a marine geologist at Columbia University, has been investigating about the natural history of eastern Africa for more than a decade. There is evidence of desiccation and gradual change to open savannahs, grass-eating fauna and the appearance of modern humans. Now he and his team have an explanation for that climate change: the tropical oceans.

They analysed planktonic foraminifera (small single-celled organisms) in an old sediment in the Indian Ocean off the west coast of Australia. These calcifying organisms take more and more magnesium when the water temperature goes up. About two million years ago, the level of magnesium grew up, indicating hot climate on the eastern side of the Indian Ocean. But on the other side of the ocean, off the coast of Africa, just the opposite happened: the ocean got colder.

This change in sea temperatures in the Indian Ocean came at an important time, coinciding with a rapid drying of east Africa, and the beginning of modern circulation in the Pacific Ocean. They tested the theory with a series of computer models and there it is: rain over east Africa depends on the conditions in the Indian Ocean and the Pacific Ocean.

Two to four million years ago is a significant time: it is then that the ice ages began, and a lot about humans is related to that period of the Earth. Can this climate change be connected to the evolutionary changes that happened at the same time, including the appearance of Homo Erectus and of animals adapted to the grasslands? Well, there is a lot of changes happening at the same time but it is difficult to prove that they are all related.

By Jeff Tollefson at www.nature.com. Picture by Top-Pics TBK / Alamy.