A rapid rise in global temperature 55 million years ago caused major disruption to ocean currents, new research shows. Scientists found that the disruption took 140,000 years to reverse.

Writing in the journal Nature, the scientists say the phenomenon may be important for understanding the impact of present day climate warming.

Recent research suggests north Atlantic currents which bring heat to northern Europe may be weakening.

The new study, by Flavia Nunes and Richard Norris from the Scripps Institution of Oceanography in La Jolla, California, looked at tiny fossil animals called foraminifera in marine sediments from 14 ocean-floor locations around the world.

Analysing the ratios of two isotopes of carbon in the shells of these foraminifera allowed them to determine ocean current patterns at the time the creatures died. Time of change The time in question was an extraordinary epoch in Earth history - the Palaeocene-Eocene Thermal Maximum (PETM), when the global average temperature rose by anything between four and seven Celsius in a few thousand years. It has been cited as the reason for the spread of mammals around the world, and for the evolution of bats.

Computer models of modern climate suggest that temperature changes could affect ocean currents, and recent research has found indications that it is happening now in the north Atlantic. But the disruption 55 million years ago took in more than a single ocean; the entire global system appears to have altered course. Before the PETM, Nunes and Norris found, surface waters sank principally in the southern hemisphere, with deep currents then flowing north. As temperatures rose, this pattern abruptly reversed. The new north-to-south system endured for 40,000 years, and currents took a further 100,000 to return to their previous polarity.

The reason why temperatures shot up during the PETM are unclear; but carbon dioxide concentrations in the atmosphere appear to have been extremely high, about a thousand times higher than currently. The suspicion is that some kind of feedback mechanism may have been involved. One theory is that an initial warming changed the distribution of heat in the oceans so that deposits of gas hydrates on the sea floor were released, with carbon dioxide and methane rising to the surface and entering the atmosphere, causing further greenhouse warming.

The new research provides some support for this theory, as well as demonstrating that abrupt temperature changes can have a long-term impact on ocean currents which are, as the Gulf Stream demonstrates, intimately tied to weather systems. Some researchers have raised concern that release of gas hydrates could contribute to present-day global warming.


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