The Government's chief scientist today gave his starkest warning yet about the world's increasing carbon emissions saying that even the best-case scenario put millions of lives at risk by the end of the century.

Professor Sir David King said that a 3C rise in global temperatures is likely within 100 years, a process that will lead to a rise in sea levels and increase in desertification that will place 400 million people at the risk of hunger. Parts of Britain will be flooded as the UK comes under coastal attack.

Developing countries will be the hardest hit, with ecosystems failing to adapt and between 20 million to 400 million tonnes of cereal production being lost, according to Sir David.

He said the temperature rise would be the consequence of carbon dioxide levels of 500 parts per million, roughly double those of the Industrial Revolution. The current carbon dioxide concentration stands at 380 parts per million, already the highest levels likely to have been experienced on Earth for 740,000 years.

Sir David said that his grim prediction was based on an optimistic reading of the world's ability to control its carbon emissions. Many scientists believe emissions could rise to 550 million parts per million and even larger temperature rises.

"The temperature rise could well be in excess of 3C and yet we are saying 500 parts per million in the atmosphere is probably the best we can achieve through global agreement," Sir David said.

He said that further increases in CO2 concentrations could put the situation beyond the planet's control.

"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 told BBC Radio 4's Today programme.

Until recently, the Government has been committed to restraining global carbon emissions to around 475 parts per million, believing that such a concentration could keep temperatures rising more than 2C. The Kyoto Protocols, signed in 1997 and set to expire in 2012, aimed at a concentration of 450 parts per million.

Last month Margaret Beckett, the Environment Secretary, admitted that the Government was set to miss its target of reducing Britain's emissions by 20 per cent by 2010, saying it was more likely to achieve cuts of between 15 and 18 per cent.

Sir David said it was crucial to confront the gloomy outlook with information and investment.

"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. It is very important to understand that we can manage the risks to our population," he said.

"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."

But Sir David, who has been criticised in the past for restraining his warnings on the advice of Government ministers, had stern words for politicians who say that carbon emissions can be controlled by the use of new, environmentally-friendly technologies.

"There is a difference between optimism and 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."

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