Mr Stern was bullish on America's negotiating position
America's chief climate negotiator has pledged billions of dollars a year to help developing countries acquire clean energy and adapt to climate change.
Todd Stern said it was morally right for rich nations to help the poor on climate change.
He also told BBC News that China deserved credit for its energy-saving programme.
And he defended America's negotiating position on emissions cuts, claiming they were of the same order as the cuts proposed by the EU.
Critics accept that the US has shifted hugely - but they say it needs to do much more.
On China, Mr Stern said the US was in serious dialogue on climate change. Some American politicians and commentators say China must agree to reduce greenhouse gases before the US agrees emissions cuts.
Does the world have to excuse America? I don't think it's a question of excusing
Todd Stern, US chief climate negotiator
It was this political blockage that prevented the Clinton Administration from pursuing the climate goals it agreed in the Kyoto Protocol.
But Mr Stern - who was also President Clinton's climate negotiator - said that, given China's current levels of development, it was impossible for the Chinese to reduce their emissions yet.
"Their emissions will continue to grow for some time," he said.
"But it's important that they grow a lot less then they would have. At some point the emissions will have to peak and then come down - but I don't yet know when that is.
He added: "We really do recognise and appreciate the significant steps China has already taken. Anyone who says China is not doing anything on climate change is wrong. They've done a lot - but they have to do a lot more.
It's not clear when China's emissions will peak
"They have to commit to it and it's got to be broadly in line with what the science is telling us the world needs to do in order to deal with this problem. Those are very challenging principles."
On developing countries, Mr Stern said clauses on emissions trading in the Waxman-Markey Bill being debated by the US Congress would provide $15-$20bn a year in allowances annually to poor nations, based on a carbon price of $15-$20 a tonne.
"We don't know the final shape of the bill - but that's just one provision. I can't say it's going to be the same amount developing countries have called for - and some of those (sums) aren't realistic - but we are talking about significant amounts for sustainable development.
"I think it's incumbent on donor major developed countries to provide financing for adaptation and help with mitigation - reducing emissions. It's a really critical part of the Copenhagen discussion."
On emissions cuts, Mr Stern insisted that America's effort was comparable to the EU's between now and 2020.
He explained: "There are games that can be played with respect to the baseline years. What we are doing as compared to where we are now is quite significant and virtually the same as what's proposed by the EU. It's different as compared to 1990 - that is true. But we're starting later - that's the reality."
What they are proposing now won't solve the problem
John Lanchbery, RSPB
The emissions caps in the Waxman Markey Bill will bring cuts of a few percent from the US economy by 2020, based on the UN's agreed 1990 baseline.
Developing countries say the US should be cutting 40-60% in light of recent science. They say that America should not be excused by the inaction of the Bush administration.
"Does the world have to excuse America? I don't think it's a question of excusing," said Mr Stern.
"You have an administration that's committed to moving on this issue. We are doing what is feasible and really quite bold and ambitious."
John Lanchbery, climate campaigner for the green group RSPB said: "I acknowledge that the US is making efforts, but it is a fact that emissions have risen massively since the Kyoto Protocol was signed - and all those emissions are now in the atmosphere.
Verlean Pack fears possible power price rises
"It may not be the fault of Obama - but it is the fault of America. What they are proposing now won't solve the problem."
Even worse from the perspective of campaigners is that the Waxman-Markey Bill is being whittled away as it passes through Congress.
It is hard to track gains and losses exactly but environmentalists claim that plans have been watered down on energy efficiency, renewables and on auctioning emissions permits.
Accommodations have had to be made not just with Republicans but with Democrats in states heavily dependent on fossil fuels. Kentucky, for instance, relies on coal for more than 90% of its energy - and for providing jobs.
The Big Sandy plant near Louisa, Kentucky, burns 2-2.5 million tonnes of coal a year. The manager Mitch Thomas told BBC News: "Apart from the folks who work here, you have folks who mine the coal, you have folks who transport the coal, so if you think about replacing that it's difficult to grasp exactly where you would go."
The Big Sandy plant burns 2-2.5 million tonnes of coal per year
Critics point out that the poorest states like Kentucky also tend to be the most heavily dependent on coal and wonder whether these factors are related. But the political sensitivity over power prices persists nevertheless.
American Electric Power (AEP), which runs the Big Sandy plant, maintains that if carbon capture equipment is fitted, coal will still be competitive in Kentucky along with other forms of power - but they say it will inevitably put up prices for households.
Verlean Pack, who lives in a small pre-fabricated home in the Kentucky woods near the coal plant, told BBC News that she feared power price rises: "I just don't know how we would make it," she said, "I don't see how some of them are making it now. And it's hard for me to make it."
Jobs at stake
Democrats in oil-rich areas have lowered the climate targets too. Gene Green, who represents Houston and belongs to the Blue Dog Democrats, said he had to represent his voters who worked in oil refineries.
"I am totally in favour of cleaning up," he told BBC News, "but it mustn't be too much at a time, or too fast so that jobs are affected."
But it is widely acknowledged that the complexity of the Waxman-Markey Bill coupled with cap-and-trade policies have created a truckload of political pork which is being fought over by fossil fuel industries who are running a multi-million dollar lobbying campaign.
President Obama hoped to raise billions of dollars by auctioning pollution permits, having witnessed European utilities make windfall profits by charging customers for pollution permits that they had been given free of charge.
Congress is making the running on climate
Now the legislation is likely to lead to 85% of permits being given away in the first stages after 2012 when the US laws are supposed to come into place.
"The US is playing politics with the science and that is unacceptable," says Damon Moglen of Greenpeace USA.
"We have a situation where the international scientific community has reached a consensus about how serious the problem is - perhaps even more serious than we thought - and that we need to reach significant reductions very quickly."
"Unfortunately, under pressure from the oil coal and gas industry in this country and after tens of millions of dollars in lobbying we see totally inadequate reductions being discussed by our Congress.
"And President Obama cannot sit in the back seat and let Congress drive this - he needs to lead on this and fulfil the expectation of the international community."
Good to give?
The give-away of pollution permits has been condemned not just by environmentalists but also by many economists and commentators in the US.
But, as the Energy Secretary Steven Chu told BBC News, the compromises are seen by the Obama team as the price for making an early start on reversing America's growth in emissions.
The poorer states tend to be more dependent on coal
Some Democratic Congressmen from rural states are said to be refusing to agree to the bill unless it continues with the current subsidies for corn ethanol which are accused of pushing up the cost of food and wasting energy rather than saving it.
With many Republican politicians either uninformed on the latest climate science after eight years in which the topic was politically off limits, or simply sceptical about the need to cut emissions, renegade Democrats hold very strong cards.
Meanwhile, the rest of the world can only watch and wait. Developing countries are asking how America will be sanctioned for the 12 years of inaction that scientists say are endangering the poor.
But US emissions cuts look much more impressive if you count cuts in agricultural pollution, the contributions to preserving forests in developing nations and buying emissions allowances from poor countries.
The World Resources Institute estimate that if all the credits were translated into genuine pollution cuts it would lead to the equivalent of perhaps 23% American reductions on 1990 levels, which is very much in line with Europe's 20% offer.
Poor nations are divided about this - they want the US to buy credits from them, but they also want to ensure that America makes deeper cuts at home.
Studies suggest that internationally traded carbon credits often fail to deliver genuine cuts.
From the perspective of many in the Obama administration, it is not possible to move fast with major cuts at home without hitting a political wall in a nation that takes cheap and plentiful energy as a right. In Texas, for instance, the latest gizmo is a trailer tent with air-conditioning.
Bob Pitney from the North Texas Camping Association explained to us that his air-conditioned tent would actually save emissions. The argument might prove a hard sell to politicians from Tuvalu or Bangladesh - but it does illustrate the scale of the challenge facing politicians in the US.
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