British Prime Minister Gordon Brown warns that the UK faces a "catastrophe" of floods, droughts and killer heatwaves if world leaders fail to agree on a new deal on climate change in December.
Mr Brown said negotiators had 50 days to save the world from global warming and break the "impasse".
The BBC reports he told a forum of 17 major economies in London that there was "no plan B".
World delegations are to meet in Copenhagen in December for talks on a new treaty.
The United Nations summit will aim to establish a deal to replace the 1997 Kyoto treaty as its targets for reducing emissions apply only to a small number of countries and expire in 2012.
Mr Brown warned that negotiators were not reaching agreement quickly enough and said it was a "profound moment" for the world involving "momentous choice".
He said an extraordinary summer heatwave of 2003 in Europe resulted in over 35,000 extra deaths.
The BBC reports there have been a number of warnings in recent days that progress is stalling.
