THE strongest evidence yet that global warming has been triggered by human activity has emerged from a study of rising temperatures in the oceans.

The rise in marine temperatures — by an average of 0.5C (0.9F) in 40 years — can be explained only if greenhouse gas emissions are responsible, research has shown. The results are so compelling that they should end controversy about the causes of climate change, one of the scientists who led the study said yesterday.

“The debate about whether there is a global warming signal now is over, at least for rational people,” said Tim Barnett, of the Scripps Institution of Oceanography in La Jolla, California. “The models got it right. If a politician stands up and says the uncertainty is too great to believe these models, that is no longer tenable.”

Dr Barnett’s team examined seven million observations of temperature, salinity and other variables in the world’s oceans collected by the US National Oceanic and Atmospheric Administration and compared the patterns with those predicted by computer models of potential causes of climate change.

Natural variation in the Earth’s climate, or changes in solar activity or volcanic eruptions, which have been suggested as alternative explanations for rising temperatures, could not explain the data collected in the real world. Models based on man-made emissions of greenhouse gases matched the observations almost precisely.

“What absolutely nailed it was the greenhouse model,” Dr Barnett told the American Association for the Advancement of Science conference in Washington. Two models, one designed in Britain and one here in the US, got it almost exactly. We were stunned.”

Climate change has affected the seas in different ways in different parts of the world: in the Atlantic, rising temperatures can be observed up to 2,300ft below the surface, while in the Pacific the warming is seen only up to 330ft down.

Only the greenhouse models replicated the changes that have been observed in practice. “All the potential culprits have been ruled out except one,” Dr Barnett said.

The results, which are about to be submitted for publication in a peer-reviewed journal, should increase pressure on the US Administration to sign the Kyoto Protocol, which came into force this week, he said. “It is time for nations that are not part of Kyoto to re-evaluate and see if it would be to their advantage to join,” he said. “The debate is not — have we got a clear global warming signal; the debate is — what we are going to do about it.”

In a separate study a team led by Ruth Curry, of Woods Hole Oceanographic Institution in Connecticut, has established that 20,000 sq km of freshwater ice melted in the Arctic between 1965 and 1995. Further melting on this scale could be sufficient to turn off the ocean currents that drive the Gulf Stream, which keeps Britain up to 6C warmer than it would otherwise be.

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