Paleoclimate Ocean Current Shutdown Study - 100,000+ Yrs For Restart

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The circulation of the deep oceans reversed abruptly some 55 million years ago, according to a study of fossilized sea creatures. This rings alarm bells about today's climate change, because the reversal coincided with a period of global warming driven by greenhouse gases.

Deep ocean currents are intimately linked to our climate, and are driven by colder, saltier water at the poles plummeting to the bottom of the ocean. Recent studies have suggested that the circulation of the North Atlantic could be weakened by the influx of fresh water from melting polar ice. As this watery conveyor belt also brings warm water northwards from the tropics, some scientists fear that a major shift in the circulation could plunge western Europe into a deep freeze.

Although computer models can predict how ocean circulation reacts to global warming, many researchers prefer to look for similar situations in the past to see how Earth responded. One useful period to look at is the Palaeocene/Eocene Thermal Maximum (PETM) about 55 million years ago, when the average temperature of the oceans shot up by 7 or 8 °C. Flavia Nunes and Richard Norris, oceanographers at the University of California, San Diego, have now worked out how ocean currents changed during the PETM, they report in Nature1. They have read the chemical records held in the shells of tiny fossilized deep-sea creatures called foraminifera.

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Comparing the composition of foraminifera shells at different locations throughout the Pacific and Atlantic Oceans, the scientists found that a gradient in the ratio of carbon isotopes revealed which way the oceans' conveyor belts were turning during the PETM. They found that although the Atlantic's deep-water flow normally travels from south to north, it ran in the opposite direction during the period of global warming. Similar flips were seen in other oceans around the world. Perhaps most remarkable is how abrupt the changes were. "The switch in circulation took just a few thousand years," says Norris. "But then it took 100,000 years to revert."

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http://www.nature.com/news/2006/060102/full/060102-5.html