No room left now for doubt. The world is warming

WorldView: 'The debate about whether there is or is not global warming is over. There is no doubt, at least for rational people". So Prof Tim Barnett told the American Association for the Advancement of Science last week as he presented exhaustive research showing that human production of greenhouse gases has warmed the oceans.

As he puts it, "the implications are huge . . . and in the short term we're sort of screwed." The findings are so significant that he wants the Bush administration to convene a research programme on the scale of the Manhattan Project that developed the nuclear bomb in the second World War.

"The debate is no longer whether there is a global warming signal. The debate is what we are going to do about it," he adds. "If somebody from the White House, or any place else, says everything's still far too uncertain . . . that argument just no longer holds".

Barnett is a marine physicist at the Scripps Institution of Oceanography at the University of California, San Diego. Working with a team from the Lawrence Livermore Laboratory, he and his colleague, David Pierce, concentrated on oceanographic rather than atmospheric evidence for global warming.

Since oceans cover 71 per cent of the world's surface this is the place to search, not only on temperature but on the 90 per cent of the energy released which has been absorbed by the oceans. This is enough to heat California at current rates for the next 200,000 years.

Global warming occurs when "greenhouse gases", largely produced by burning fossil fuels, accumulate in the upper atmosphere and prevent heat escaping from the Earth.

The two teams combined computer models with nine million ocean temperature and salinity readings over the last 40 years. Barnett says the warming signals found in the upper 5-700 metres of the oceans predicted by the models correspond to the measurements obtained at sea, with confidence levels exceeding 95 per cent.

Such a high degree of statistical significance is most unusual. Efforts to explain the ocean changes through naturally occurring variations in the climate or external forces - such as solar or volcanic factors - do not come close to reproducing the observed warming.

The climate mechanisms involved will produce broadscale changes on land and in the atmosphere which will be felt by millions of people in the decades immediately ahead. They will affect regional water supplies in the west of the United States, the Andes and western China, putting millions at risk without adequate summertime water.

Barnett says these scenarios "have a high enough probability of actually happening that they need to be taken seriously by decision-makers".

I was surprised by the resignation or complacency of several groups against whom I bounced this information during the week. Do we not know all this already? was one refrain. Is it not too alarmist and doom-laden? What can we do about it anyway?

What strikes me as crucial is the scientific conviction coming through Barnett's presentation of the evidence and the plausibility of researching the subject in the oceans rather than the atmosphere.

For non-specialists on climate change, like me, the modelling, statistical and observational methodology rings true. It was not contested at the AAAS meeting or in following days.

And yet it contradicts the policy line taken by the White House and sceptics on the subject of global warming, such as the Danish researcher Bjorn Limborg, who says the science is uncertain and unresolved. If we cannot take such warnings seriously we are certainly doomed if they are true. Such a fatalistic attitude runs counter to a precautionary approach.

It is all the more pressing in the week following ratification of the Kyoto Protocol and on George Bush's trip to Europe, during which the subject of climate change was dealt with in passing. Significantly, the British government aims to give it priority in chairing the Group of Eight and the European Union this year. Tony Blair heard similar research findings at a seminar on climate change and global warming in Exeter recently.

Other evidence to the AAAS reinforces the findings reported by Barnett and his colleagues. Ruth Curry, from the Woods Hole Oceanographic Institute in Massachusetts, told the meeting that between 1965 and 1995 about 4,800 cubic miles of fresh water - more than in the north American Great Lakes combined - melted from the Arctic region and into the normally salty northern Atlantic.

If this influx continues it could shut down the great ocean conveyor belt which regulates air and water temperatures. A system of currents, it moves water around the world and depends on heavier salt water sinking to pull warm water from the tropics to higher latitudes.

According to Curry, early calculations show it would take an additional 4,300 cubic miles of fresh water from the Arctic to do this, which could take two decades, at most. It is only half as thick as it was 30 years ago.

Worryingly, too, Greenland's ice is beginning to thaw and as it begins to float, it also raises water levels, which would rise by seven metres if it were to disappear, according to Prof Sharon Smith of the University of Miami.

All Ireland is warmed by the Gulf Stream, as Joyce reminded us. Without it we would revert to our latitudinal destiny of sub-Arctic tundra, comparable to northern Newfoundland. The Gulf Stream emerged over 10,000 years ago - probably in the space of a couple of decades and with immediate effect on the previous Ice Age. It was interrupted during a mini Ice Age 8,200 years ago, according to Curry. It could go just as quickly.

This should remind us that events in natural history can occur just as suddenly as in human history. These findings show the two histories are now inextricably intertwined. The consequences of human activity cannot be disguised as natural processes, for which we do not have direct responsibility.

When next you hear a White House spokesperson say this science is uncertain, you ought to respond, with Barnett and his courageous colleagues: e pur si scalda ("but it does warm").