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 declared today.

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 (3.6F), 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 today 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.

The study, due to be presented this morning by Mr Annan, was reviewed by distinguished experts in the field, including Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change (IPCC), and Professor Jeffrey Sachs, director of the Earth Institute at Columbia University in New York. It projects that by 2030, the number of annual deaths directly resulting from the warming global climate will rise to 500,000.

The St James's Palace memorandum was agreed after three days of discussions attended by 60 leading scientists, policymakers and intellectuals. Participants included Steven Chu, the US Energy Secretary and Nobel physics laureate, Wole Solinka, the Nigerian literature laureate, and Wangari Maathai, the first environmentalist to win the Nobel Peace Prize.

The symposium, for which The Times was media partner, was organised by the Potsdam Institute and the University of Cambridge Programme for Sustainability Leadership, under the patronage of the Prince of Wales.

The memorandum called for an emergency package of financial support for tropical forest nations, as the loss of forests is responsible for about 18 per cent of global carbon emissions.

"The St James's Palace memorandum calls for a global deal on climate change that matches the scale and urgency of the human, ecological and economic crises facing the world today," the final document said.

"It urges governments at all levels, as well as the scientific community, to join with business and civil society to seize hold of this historic opportunity to transform our carbon-intensive economies into sustainable and equitable systems. We must recognise the fierce urgency of now."