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Tropical Oceans Dried Out East Africa

VANCOUVER, CANADA—What happens in the sea doesn’t always stay in the sea. Large differences in the temperature of the Indian and Pacific oceans 2 million years ago shifted rainfall patterns and dried out East Africa, replacing woodland with grassland and leading to an explosion in the number of species that grazed the region, according to results presented here today at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW).

Scientists have proposed several hypotheses for the climate shift that struck East Africa, said Peter deMenocal, a marine geologist and geochemist at Columbia University. They include cooling of the North Atlantic Ocean, decreasing atmospheric CO2 concentrations, and increasing volcanic activity—all of which could have shifted rainfall patterns away from East Africa. But none of these explanations can fully account for the dramatic changes that took place, he said.

Instead, deMenocal focused on the east-to-west ocean-temperature gradients in the Indian and Pacific oceans that run along the equator. These gradients shape modern-day rainfall patterns: The western equatorial Pacific and eastern Indian oceans hold a warm pool of water that acts like a bull’s-eye for rain, drawing it away from its ancient, more evenly distributed position from East Africa across to Australia, he said.

So deMenocal led a team to see if changes in ancient ocean temperatures triggered expansion of arid grasslands in East Africa. The researchers examined sea-surface temperature records from the western and eastern Indian Ocean and compared them with previously published temperature records from sea-floor sediment cores for the Pacific Ocean.

The team found that before 2 million years ago, ocean temperatures across locations in the Indian Ocean were fairly uniform, hovering around 27° to 28°C. But around 2 million years ago, the western Indian Ocean near the Arabian Sea cooled to 25°C, while the eastern Indian Ocean near northwestern Australia warmed to 28° to 29°C. The researchers saw a similar pattern in the Pacific Ocean, except it was the western Pacific Ocean that warmed and the eastern Pacific that cooled. These changes coincided with shifting rainfall patterns over East Africa.

To test the effects of these temperature gradients on rainfall patterns over the Indian Ocean, deMenocal and his colleagues ran climate models that erased temperature differences within the Indian and Pacific Oceans. The researchers found that once those gradients disappeared, rainfall increased in eastern Africa. Because rainfall follows warm ocean water, formation of the modern western Pacific Ocean warm pool shifted precipitation over areas including Borneo and northwestern Australia, deMenocal said. The changing seas led to the rise of new grazing species, including ancestors to modern-day antelopes and equines.

“It was a lovely study,” says Andrew Weaver, a climate scientist at the University of Victoria in Canada, who was not involved with the work. He adds that deMenocal’s research raises many intriguing questions. How did these changing ocean conditions interact with changing water currents between the Pacific and Indian oceans, and how did the presence of the Himalayas influence air currents in the region? “Something’s going on 2 million years ago," he says, "and the question is, 'What’s the big driver?' "

Full coverage of AAAS 2012