Climate change will be big at the AAAS conference in Denver next week and the US is likely to come in for much criticism from scientists.

"It's very difficult to have a global effort when the number one polluter and economy isn't participating in the solution," laments Mohamed El-Ashry, one of the key speakers at the conference and head of the Global Environment Facility, the United Nations agency charged with galvanising international action to combat climate change, ozone depletion, biodiversity threats and desertification.

Noting continued US scepticism about global warming despite successive reports from the Intergovernmental Panel on Climate Change, he says: "It's very frustrating, particularly when you live in this country and see so many big gas-guzzling cars, and we are going to war to keep gas cheap so people can continue to waste it."

Washington DC-based El-Ashry compares the White House's recent commissioning of yet more research into global warming, which will further delay efforts to reduce it, to President Reagan's handling of acid rain in the 1980s. "Reagan wanted ten years' more research, even though the impact of acid rain on lakes was known."

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More research is needed, says El-Ashry, but "we now know enough to know that the time has come to take action".

Meantime, his AAAS conference speech will emphasise that for "science to drive the agenda forward it needs to be translated for decision-makers".

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Another of the main speakers in Denver is America's leading climate change expert, Warren Washington, who will plot a bleak prognosis for continued global warming in his address.

Latest projections point conservatively to a 2 to 3 degree rise in the temperature of the earth's lower atmosphere by the end of the next century, says Washington, head of the climate change division at US meteorological research facility, the National Center for Atmospheric Research.

"You're talking about a temperature change not too different from that seen from the last Ice Age to the present day," he adds. That increase played out over 20,000 years; this one could be telescoped into less than 200.

The ecological implications are manifold, Washington says. "It took thousands of years for trees to adjust to the climate change from the glacial period to the present." He also conjures the spectre of a warmer ocean stoking typhoons and hurricanes.

Sceptics with the ear of the White House say recent warming can be accounted for within normal climatic cycles and that cutting industrial emissions is unfounded and too costly.

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But Washington, who has advised US administrations since the Carter era, says that while natural variance is not ruled out, the view that warming is largely man-made is orthodoxy in informed scientific circles and prudence demands that "we start curbing the greenhouse gases we put into the atmosphere - even if all the scientific questions aren't answered".

Those questions - chiefly the impact of aerosols - remain a sticking point for the Bush administration, which has resisted calls to fetter emissions.

But Washington uses data showing how concentrations of greenhouse gases and sulphate aerosols remained constant throughout the last millennium until the late 20th century when they increased exponentially. Similarly, average temperatures oscillated within a narrow band before rising drastically in recent decades. Extrapolations for the 21st century leap off the chart.

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Long-range forecasting has grown more sophisticated since Washington, chair of the prestigious National Science Board, helped pioneer computer modelling in the 1960s. Atmospheric, oceanic and land variables are factored in, allowing various scenarios to be mapped out.

Researchers have particularly high hopes of the predictive possibilities opened up by Japan's $310 million (?189 million) Earth Simulator supercomputer, which has the data-crunching capacity of 20,000 desktop machines.

They already know something potentially catastrophic is afoot, though.

Washington is uneasy at stepping beyond his research role into policy prescription, "but on the other hand I'm a grandfather, it's an issue I care about for future generations".

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