Sir David King said that, even by the most optimistic forecasts, carbon dioxide levels are set to rise to double what they were at the time of the Industrial Revolution in the 19th century.

That will lead to a three-degree centigrade rise in temperature, King said, adding that if nothing is done to manage such change, few eco-systems on Earth will be able to adapt.

Even worse, said King in an interview on BBC radio, up to 400 million people around the world would find themselves at risk of hunger, because 20 million to 400 million tonnes of cereal production will be lost.

In Britain, the main threat will be flooding and "coastal attack" as a result of rising sea levels.

"If you ask me where do we feel the temperature is likely to end up if we move to a level of carbon dioxide of 500 parts per million -- which is roughly twice the pre-industrial (revolution) level and the level at which we would be optimistically hoping we could settle -- the temperature rise could well be in excess of three degrees centigrade," he said.

"Yet we are saying 500 parts per million in the atmosphere is probably the best we can achieve through global agreement."

King, who has the ear of Prime Minister Tony Blair and other key policy makers, said it was essential that the world should act now to be able to cope with such climate change.

"We don't have to succumb to a state of despondency where we say that there is nothing we can do, so let's just carry on living as per usual," he said. "It is very important to understand that we can manage the risks."

"What we are talking about here is something that will play through over decades. We are talking 100 years or so. We need to begin that process of investment. It is going to be a major challenge for the developing countries."

Blair wants to see a world-wide consensus on global warming that goes beyond the disputed Kyoto protocol and puts China and India -- both economically booming -- at the heart of the issue.

But the United States in particular is opposed to slashing carbon dioxide emissions, while the European Union -- including Britain -- proposed in 2002 limiting a rise in the average global temperature to two degrees centigrade.

King said the situation would be even worse if the average global temperature broke through the three degrees centigrade level.

"If we go beyond 500 parts per million, we reach levels of temperature increase and sea level rise in terms of the coming century which would be extremely difficult for world populations to manage," he said.

He lashed out at politicians who feel the answer lies in new technologies which produce cleaner fuels, saying they ought to start listening to scientists instead.

"There is a difference between optimism and (putting one's) head in the sand," he said.

"Quite clearly what we have to do as we move forward with these discussions is see that this consensus position of the scientific community is brought right into the table where the discussions are taking place."

Last month, Blair's government unveiled plans to fight global warming by cutting greenhouse gases in every sector of the nation's economy, including imposing stricter emission caps on industry.

It claimed that Britain was on course to well exceed the 12.5 percent reduction target by 2012 under the Kyoto protocol -- but would fall short of its own goal of 20 percent by 2010.

Tony Juniper, the director of environmental campaign group Friends of the Earth, said: "It is technologically possible to significantly reduce our emissions and deliver two degrees centigrade.

"Professor King should be pressing for government polices to deliver on this rather than accepting the current lack of political will and talking of three degrees as an inevitability."

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