In a stark warning issued on Friday, Sir David King said a rise of this magnitude would cause famine and drought and threaten millions of lives.

It would also cause a worldwide drop in cereal crops of between 20 and 400 million tonnes, put 400 million more people at risk of hunger, and put up to 3 billion people at risk of flooding and without access to fresh water supplies.

Few ecosystems could adapt to such a temperature change, equivalent to a level of carbon dioxide of 550 parts per million in the atmosphere, which would result in the destruction of half the world's nature reserves and a fifth of coastal wetlands

Many of Professor King's predictions come from a report published by the U.K.'s Hadley Centre, a world leader in climate change modelling, called Avoiding Dangerous Climate Change.

Tony Blair wants governments around the world to set a target of a rise of no more than 2º C — equivalent to 450ppm of CO2 in the atmosphere — in global temperatures. This had already been agreed as an upper limit by the European Union, but Prof. King said this agreement would be difficult, given the refusal of the United States to cut emissions and those of China and India rising as they develop.

Tony Juniper, director of Friends of the Earth, said despondency was not the answer. "George Bush won't be in office forever. When he's gone there will be a change of policy inside the United States. To what degree that change takes place is still an open question. We should be concentrating on working with those elements of U.S. society that see the danger and seeking to change the opinion within the U.S. to a point where that country can embrace a really ambitious programme of de-carbonisation," he said.

Prof. King said in an interview that the Government would not go into any future negotiations with its mind already made up. "Our position back in 2003 was that we must aim to get global agreement to keep levels below 550ppm.

"The science has moved on since then and we're now aware that even 550ppm poses the impacts of dangerous climate change. The government's position now is that we must get negotiations on an agreed upper limit of carbon dioxide in the atmosphere," he said.

"It must be below 550ppm but if we can get international agreement and there's public agreement around the world behind action required then that can be ratcheted down to, say, 500."

Mr. Juniper said that even such limits on CO2 emissions went against what the Government had previously indicated it would do. "It is surprising in so far as they've said that they want a science-based approach to this and other environmental issues," he said.

"Science says that 550ppm is far too high. We should be aiming for 450ppm and we should have the political strategies to deliver that rather than saying its inevitable we're going to go way above the thresholds that are likely to deliver devastating global warming."

Prof. King's latest comments are partly designed to raise the profile of the climate change debate among members of the public. Mr. Juniper added that messages from campaigners and scientists to the public needed to stress the opportunities as well as the negatives when discussing climate change. While it was important to face up to danger, it should be done with a sense of optimism. "We can be generating many millions of jobs out of this, we can be helping to end fuel poverty, we can bring electric power to people who have never had it before with renewable energy technologies."

He added that perhaps the British could put their own house in order first. "The thing that Dave King and Tony Blair could focus on to underline that message is to start some reductions at home. It's all very well talking about America and China but the reality is that CO2 emissions are going up in this country," said Mr. Juniper. —