WORLD carbon emissions must start to decline in only six years if humanity is to stand a chance of preventing dangerous global warming, a group of 20 Nobel prize-winning scientists, economists and writers has declared.

The United Nations climate summit in Copenhagen in December must agree to halve greenhouse-gas emissions by 2050 to stop temperatures from increasing by more than 2C, the St James's Palace Nobel Laureate Symposium concluded.

While even a 2C temperature rise will have adverse consequences, a bigger increase would create "unmanageable climate risks", according to the St James's Palace memorandum, signed by 20 Nobel laureates in physics, chemistry, economics, peace and literature.

The temperature target "can only be achieved with a peak of global emissions of all greenhouse gases by 2015", the document said. If emissions continue to rise after that date, the required cuts would become unachievable.

Professor Hans Joachim Schellnhuber, director of the Postdam Institute for Climate Impact Research, a convenor of the symposium, likened the urgency for action on climate change to the threat of thermonuclear weapons during the Cold War.

"We are facing a crisis as deep as the arms race of the 1950s and 1960s and the Cold War notion of mutually assured destruction," he said. "Today we have mutually assured increases in greenhouse gases."

He said the memorandum echoed a manifesto signed in 1955 when Bertrand Russell, Albert Einstein and nine other intellectuals called for world leaders to seek peaceful resolutions to international conflict.

"Global climate change represents a threat of similar proportions and should be addressed in a similar manner," the memorandum said.

The extent of the climate threat is also highlighted today by a report that suggests global warming is already killing an estimated 300,000 people per year - equivalent to the loss of life that resulted from the 2004 Indian Ocean tsunami.

The report from the World Humanitarian Forum, an independent organisation led by Kofi Annan, the former UN Secretary-General, claims that 90 per cent of those deaths are related to gradual environmental degradation resulting from the warming climate - principally malnutrition, diarrhoea and malaria. The remaining 10 per cent are linked with weather-related disasters.