Twenty Nobel prizewinners, including US energy secretary Steven Chu, Nigerian writer Wole Soyinka, and Kenyan environmentalist Wangari Maathai, have compared the threat of climate change to that posed to civilisation by nuclear weapons.

Borrowing a phrase from US civil rights leader Martin Luther King, they said at the end of a three-day climate change symposium hosted by Prince Charles in London: "We must recognise the fierce urgency of now. The evidence is compelling for the range and scale of climate impacts that must be avoided, such as droughts, sea level rise and flooding leading to mass migration and conflict. The scientific process, by which this evidence has been gathered, should be used as a clear mandate to accelerate the actions that need to be taken. Political leaders cannot possibly ask for a more robust, evidence-based call for action."

The laureates, who included physics and chemistry Nobel winners, called for urgent reduction in emissions. "Without directing current economic recovery resources wisely, and embarking on a path towards a low carbon economy, the world will have lost the opportunity to meet the global sustainability challenge. Decarbonising our economy offers a multitude of benefits, from addressing energy security to stimulating unprecedented technological innovation. A zero carbon economy is an ultimate necessity and must be seriously explored now."

The St James's Palace Memorandum urged politicians to make far faster and deeper emission cuts than most countries were contemplating. "[There must be] a peak of global emissions of all greenhouse gases by 2015 and at least a 50% emission reduction by 2050 on a 1990 baseline. This in turn means that developed countries have to aim for a 25-40% reduction by 2020. A robust measure of assessing the necessary emission reductions is a total carbon budget, which should be accepted as the base for measuring the effectiveness of short-term (2020) and long-term (2050) targets. They also called for an emergency package to provide "substantial" funding to tropical forest nations to help them halt deforestation.

This was a theme that Prince Charles, who was hosting the event, picked up on Wednesday. He said, "Saving the rainforests is not an option, it is an absolute necessity." On Tuesday, Chu stressed the importance of energy efficiency in combating climate change.

They concluded: "The solutions to the extraordinary environmental, economic and human crises of this century will not be found in the political arena alone.

"Global climate change represents a threat of similar proportions [to that ] threat posed to civilization by the advent of thermonuclear weapons, and should be addressed in a similar manner. All scientists should be urged to contribute to raising levels of public knowledge on these threats to civilisation and engage in a massive education effort.

"We know what needs to be done. We cannot wait until it is too late. We cannot wait until what we value most is lost."

• Read the full St James's Palace Memorandum here.