Deutsche Bank's head of carbon emissions research Mark Lewis can't see how the world can avoid dangerous global warming - judged as greater than two degrees Celsius - based on the targets agreed at climate change summits in Copenhagen and Cancun.

Mr Lewis said a target to restrict global warming to those two degrees is "probably unrealistic now, because of the politics; not because the technology isn't there, not because with the right policies it's just not possible - it's just that there isn't the political will".

Without the United States signing up, there's unlikely to be a global deal on emissions reduction negotiated at the next climate summit in Durban, South Africa. Without such a pact, there will be no successor to the Kyoto Protocol, which expires in 2012.

"Frankly the UN process has become, not totally irrelevant, but it's not the forum where a meaningful global deal is going to be brokered that will get you on a two degrees centigrade trajectory," Paris-based Mr Lewis said on a recent visit to Australia.

"That is not the political framework through which this (trajectory) is going to be delivered. I can't tell you there is a ready-made alternative framework in place that will allow a global deal to be delivered because I don't believe that either."