The latter would be "almost impossible to get through the US Senate", he said, no matter whether incumbent Barack Obama or challenger Mitt Romney wins the US presidential election.

Mr de Boer, who is now special global advisor on climate change for KPMG, said the best prospect may be for nations to settle on targets that they write into their national laws, rather than a binding international deal.

Before then is the next end-of-year UN climate meeting in Doha, Qatar. Delegates will discuss a second commitment period for the Kyoto Protocol, the only legally binding accord to cut greenhouse gas emissions.

Three years on from Copenhagen, Mr de Boer said he has neither nightmares nor withdrawal symptoms from those failed talks, which included a final 12-hour marathon of discussions behind closed doors with Mr Obama, former Australian prime minister Kevin Rudd and 23 other global leaders.



"That was such a fantastic opportunity and it slipped between our fingers," said Mr de Boer. "That's my big frustration."



He said Copenhagen's legacy had changed political attitudes to climate talks.



"Most politicians will think twice about going to a climate event, and if they go again, they will be damned sure that they'll be celebrating success rather than be associated with failure,'' he said.



Expectations of global action on climate change have diminished, but Mr de Boer said it would still be "a very, very difficult conference" for his successor, Christiana Figueres.



He said the mandate for 2015 remained ambiguous and nations would be reluctant to sign up before they know what they were getting into.



The Kyoto Protocol has not been ratified by the US, and Canada, Japan and Russia have said they would not sign up to a second round when the current commitment lapses at the end of the year. Australia is yet to announce whether it will commit. Developing nations are holding back until they can see the rich acting.

They also want proof the UN's Green Climate Fund - aimed at supporting efforts to counter climate change - will get the $US100 billion in government and private funding promised.

He said superstorm Sandy may spur more Americans, and people elsewhere, to consider the risks of climate change, but warned: "It's a bit like being shocked into stopping smoking when you've been told you've got terminal cancer."