BERLIN - World powers have a very good chance of reaching a political deal to target global curbs in carbon emissions at a U.N. conference in Copenhagen next month, British climate change expert Nicholas Stern said on Wednesday.

Stern, author of an influential 2006 report for the British government on the impact of global warming, said he did not expect a formal treaty to be signed in Copenhagen, partly due to resistance in the United States.

But we can and should put together a strong and clear political deal. I think that a complete failure in Copenhagen would be very damaging. But I don't think that will happen.

I think so many people want it to succeed that we've got a very good chance, Stern told a news conference before delivering a lecture at Berlin's Technical University.

A deal at the December 7-18 Copenhagen conference ought to contain a commitment to cutting global annual output of carbon dioxide-equivalent emissions to 35 gigatonnes from almost 50 gigatonnes now, and to around 20 gigatonnes by 2050, he said.

Those numbers are crucial. This is the first generation that through its negligence could destroy the relationship between human beings and the planet, said the 63-year-old Stern, a former chief economist of the World Bank.

We must agree in Copenhagen to those maxima for 2030 and 2050, he added, noting that the numbers implied a reduction in emissions in Europe of around 80 percent per capita by 2050.

Failure to take the necessary steps would lead to major population shifts and create serious conflict, Stern said.

The biggest difficulty I think will come in the U.S., but I think we've seen progress there as well, he added.

German Chancellor Angela Merkel and U.S. President Barack Obama discussed climate change in Washington on Tuesday, with Obama saying the U.S. and the European Union had agreed to redouble their efforts to achieve success in Copenhagen.

Progress toward a climate deal has been hampered by obstacles in the U.S. Senate and at U.N. negotiations this week in Barcelona, Spain -- the last session before Copenhagen.

Stern said he felt the United States should spend more on helping developing economies to lower carbon emissions.

If there is $50 billion per year to help the poorer parts of the world make their adjustments by 2015, I would hope that the U.S. would make a significant share of that -- maybe $15 or $20 billion, with a priority for adaptation in Africa and fighting deforestation, he said.

(Editing by Mark Trevelyan)