We could conceivably be over-estimating the effect of human activity on the Earth's climate, but alternatively we could also be under-estimating it

Dr Robert Watson IPCC Chair

Dr Watson, who chairs the IPCC (Intergovernmental Panel on Climate Change), dismissed President George W Bush's doubts about the reality of global warming.

"We know enough to say climate change is a serious environmental issue," said Dr Watson.

A host of recent studies have predicted catastrophic consequences for the environment because of global warming. Other research papers, although not as numerous, have been far more circumspect in their analysis of climate change, pointing up the many uncertainties that still grip the research field.

But the UN's IPCC report has the weight of 3,000 scientists, including several of the world's most distinguished meteorologists, behind it.

Computer models

They have given their unqualified backing to the argument that global warming is happening, and at a much faster rate than was expected.

Their prediction - based on computer models - is that temperatures could rise by as much as 5.8C by the end of the century.

Industrial pollution is the main offender

And they stress that human activity is responsible for this crisis - that industrial pollution, and in particular its gas emissions, is the worst offender.

President Bush withdrew US support for the Kyoto Protocol, questioning the link between higher temperatures and pollution.

But Dr Watson told the BBC that, while there are some scientific uncertainties about global warming, "science should not be the reason for inaction".

"We could conceivably be over-estimating the effect human activities have on the Earth's climate," he said," but alternatively we could also be under-estimating it."

Dr Watson said it was necessary to start using cost-effective technologies to reduce greenhouse gas concentrations.

Key talks

President Bush has argued that it is unfair to expect the US and other industrialised countries to bear responsibility for the problem.

But Dr Watson said most of the greenhouse gas emissions to date had come from the industrialised world.

Farmers like these in Africa face a bleak future

"Even in the future, per capita emissions from India and China will still be well below those of the US and Europe."

The timing of the formal publication of the Third Assessment Report (Tar) - its contents have already been well reported - is important. Politicians from more than 150 countries meet in Germany next week to try to salvage the Kyoto agreement.

They are keen to coax the United States back into the fold.

Indeed, some countries have questioned the point in implementing a treaty that does not have the support of the world's most prolific carbon gas polluter.

But the chairman of the Bonn talks, Dutch Environment Minister Jan Pronk, says the agreement cannot be delayed any longer. If it is, he says, the Kyoto Protocol really will become nothing more than a dead letter.