19 February 2005

Scientists have found the first unequivocal link between man-made greenhouse gases and a dramatic heating of the Earth's oceans. The researchers - many funded by the US government - have seen what they describe as a "stunning" correlation between a rise in ocean temperature over the past 40 years and pollution of the atmosphere.

The study destroys a central argument of global warming sceptics within the Bush administration - that climate change could be a natural phenomenon. It should convince George Bush to drop his objections to the Kyoto treaty on climate change, the scientists say.

Tim Barnett, a marine physicist at the Scripps Institution of Oceanography in San Diego and a leading member of the team, said: "We've got a serious problem. The debate is no longer: 'Is there a global warming signal?' The debate now is what are we going to do about it?"

The findings are crucial because much of the evidence of a warmer world has until now been from air temperatures, but it is the oceans that are the driving force behind the Earth's climate. Dr Barnett said: "Over the past 40 years there has been considerable warming of the planetary system and approximately 90 per cent of that warming has gone directly into the oceans."

He told the American Association for the Advancement of Science in Washington: "We defined a 'fingerprint' of ocean warming. Each of the oceans warmed differently at different depths and constitutes a fingerprint which you can look for. We had several computer simulations, for instance one for natural variability: could the climate system just do this on its own? The answer was no.

"We looked at the possibility that solar changes or volcanic effects could have caused the warming - not a chance. What just absolutely nailed it was greenhouse warming."

America produces a quarter of the world's greenhouse gases, yet under President Bush it is one of the few developed nations not to have signed the Kyoto treaty to limit emissions. The President's advisers have argued that the science of global warming is full of uncertainties and change might be a natural phenomenon.

Dr Barnett said that position was untenable because it was now clear from the latest study, which is yet to be published, that man-made greenhouse gases had caused vast amounts of heat to be soaked up by the oceans. "It's a good time for nations that are not part of Kyoto to re-evaluate their positions and see if it would be to their advantage to join," he said.

The study involved scientists from the US Department of Energy, the Lawrence Livermore National Laboratory in California and the US National Oceanic and Atmospheric Administration, as well as the Met Office's Hadley Centre.

They analysed more than 7 million recordings of ocean temperature from around the world, along with about 2 million readings of sea salinity, and compared the rise in temperatures at different depths to predictions made by two computer simulations of global warming.

"Two models, one from here and one from England, got the observed warming almost exactly. In fact we were stunned by the degree of similarity," Dr Barnett said. "The models are right. So when a politician stands up and says 'the uncertainty in all these simulations start to question whether we can believe in these models', that argument is no longer tenable." Typical ocean temperatures have increased since 1960 by between 0.5C and 1C, depending largely on depth. Dr Barnett said: "The real key is the amount of energy that has gone into the oceans. If we could mine the energy that has gone in over the past 40 years we could run the state of California for 200,000 years... It's come from greenhouse warming."

Because the global climate is largely driven by the heat locked up in the oceans, a rise in sea temperatures could have devastating effects for many parts of the world.

Ruth Curry, from the Woods Hole Oceanographic Institution, said that warming could alter important warm-water currents such as the Gulf Stream, as melting glaciers poured massive volumes of fresh water into the North Atlantic. "These changes are happening and they are expected to amplify. It's a certainty that these changes will put serious strains on the ecosystems of the planet," Dr Curry said.