The strongest evidence yet that global warming has been triggered by human activity has emerged from a major study of rising temperatures in the world’s oceans.

The present trend of warmer sea temperatures, which have risen by an average of half a degree Celsius (0.9F) over the past 40 years, can be explained only if greenhouse gas emissions are responsible, new research has revealed.

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."

In the study, Dr Barnett’s team examined more than 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 that are predicted by computer models of various potential causes of climate change.

It found that 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, however, 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. They did it so well it was almost unbelieveable."

Climate change has affected the seas in different ways in different parts of the world: in the Atlantic, for example, rising temperatures can be observed up to 700 metres below the surface, while in the Pacific the warming is seen only up to 100m down.

Only the greenhouse models replicated the changes that have been observed in practice. "The fact that this has gone on in different ways gives us the chance to figure out who did it," Dr Barnett said.

"All the potential culprits have been ruled out except one.

"This is perhaps the most compelling evidence yet that global warming is happening right now, and it shows that we can successfully simulate its past and its likely future evolution. The statistical significance of these results is far too strong to be merely dismissed and should wipe out much of the uncertainty about the reality of global warming."

Dr Barnett said the results, which are about to be submitted for publication in a major peer-reviewed journal, should put further pressure on the Bush Administration to sign up to the Kyoto Protocol, which came into force on Wednesday. "It is now time for nations that are not part of Kyoto to reevaluate and see if it would be to their advantage to join," he said.

"We have got a serious problem ahead of us. 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, also presented to the conference, a team led by Ruth Curry of Woods Hole Oceanographic Institution in Connecticut has established that 20,000 square kilometres 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. "It is taking the first steps, the system is moving in that direction," Dr Curry said.

"The new ocean study, taken together with the numberous validations of the same models in the atmosphere, portends far broader changes. Other parts of the world will face similar problems to those expected, and being observed now, in the western US.

"The skill demonstrated by the climate models in handling the changing planetary heat budget suggests that these scenarios have a high enough probability of actually happening that they need to be taken seriously by decision-makers."

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