A shift in tropical ocean circulations could explain a historical shift in global climate.
Peter deMenocal has been pondering the ancient natural history of eastern Africa for more than a decade, documenting evidence of desiccation as well as the gradual shift toward open savannahs, grass-eating fauna and the rise of the ancestors of modern humans. Now, the marine geologist at Columbia University in New York and his colleagues think they have pinned down a culprit for the shift in climate: the tropical oceans. deMenocal previewed his team's findings at the American Geophysical Union conference in Santa Fe last week.
The breakthrough came after deMenocal analysed the shells of planktonic foraminifera (minute single-celled organisms) in an old sediment core drilled in the Indian Ocean off the west coast of Australia. These calcifying organisms live at the surface of the ocean and increase their uptake of magnesium at a predictable rate as the water temperature rises. He and his team found that the magnesium content of the foraminifera shells began to increase in sediments around 2 million years old, indicating the start of a warming trend on this side of the Indian Ocean. This trend diverged sharply from a significant cooling trend already documented on the other side of the ocean off the east coast of Africa.
This divergence is visible alongside a roughly 20,000-year wet–dry cycle, which is due to variations in solar energy caused by the earth's natural wobble and elliptical orbit. "It's a monster signal," says deMenocal.
This shift in sea surface temperatures in the Indian Ocean came at an important time, coinciding with a rapid uptick in the drying of east Africa, as well as the beginning of modern circulation trends in the Pacific Ocean1. deMenocal suggested that the Indian Ocean trends could explain the climate shift over east Africa and joined up with a pair of modellers at Yale University in New Haven to test the theory out. Alexey Fedorov and Chris Brierley ran a series of experiments analyzing the effects of tropical sea surface temperature changes on a global climate model and found that the combination of the estimated Indian and Pacific ocean temperature trends spurred a 30–80% reduction in rainfall over east Africa, the size of decrease depending on the exact location.
"What we found is that to get the changes in east Africa, you need to know the right conditions both in the Indian Ocean and in the Pacific Ocean," Fedorov says.
Christina Ravelo, a palaeoclimatologist at the University of California, Santa Cruz, calls the results "pretty robust" but says they need to be kept in context. Details are still being worked out, she says, but the climate and oceans experienced a long cooling trend that occurred between 2 and 4 million years ago as the world exited the Pliocene warm period — the most recent analogue to a world warmed by greenhouse gases — and entered the ice ages. "The 2-million-year event is when things started looking like today, but the transition was a long time in the making," she says. "But it could be that it was the establishment of the modern ocean gradients that helped dry out Africa."
Given that the tropical climate drives what happens in the rest of the globe, Fedorov says that this transition is as central to Earth's history as the ice ages themselves. Theories about the cause of the transition range from tectonic shifts that altered ocean circulation to climatic shifts driven by carbon dioxide, but so far none of them has taken hold. "We still don't understand why it happened," Fedorov says.
For deMenocal, it's clear that the east African story is coming together. "At least in terms of the drying climate, I think we've nailed that now," he says. The next question is whether this climatic shift can be tied to some of the major evolutionary changes that seem to arise around the same time, including the appearance of Homo erectus and of animals adapted to living on the grasslands.
"There's a whole bunch of stuff happening around this time, and it's coincident with this huge change in climate," deMenocal says. "But we can't prove it's related."
Wara, M. W., Ravelo, A. C. & Delaney, M. L. Science 309, 758-761 (2005).
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Tollefson, J. The drying of East Africa. Nature (2011). https://doi.org/10.1038/news.2011.204