By Roger Harrabin
BBC environment analyst, Bonn
Lights go out in New York during last weekend's call to action on climate change
The US must balance science with what is politically and technologically achievable on climate change, America's lead negotiator has said.
Speaking at UN talks in Bonn, Jonathan Pershing said the US must not offer more than it could deliver by 2020.
Poor countries said the latest science showed rich states should cut emissions by 40% on 1990 levels by 2020.
President Barack Obama's plan merely to stabilise greenhouse gases at 1990 levels by 2020 is much less ambitious.
Mr Pershing, the US delegation head, previously spent many years promoting clean energy for the International Energy Agency and at the Washington think-tank WRI - World Resources Institute.
He told the BBC he was very worried the Earth might already be committed to dangerous climate change.
But he said the US should not make promises for 2020 that it could not keep: "It is not the point in time in 2020 that matters - it is a long-term trajectory against which the science measures cumulative emissions.
There is still much negotiation to be done between now and December
"The president has also announced his intent to pursue an 80% reduction by 2050.
"It is clear that the less we do in the near-term, the more we have to do in the long-term. But if we set a target that is un-meetable technically, or we can't pass it politically, then we're in the same position we are in now
where the world looks to us and we are out of the regime.
"We want to be in (the regime), we want to be pragmatic, we want to look at the science. There is a small window of where they overlap. We hope to find it."
This is a radical change of tone and content from the Bush administration which envisaged that emissions would continue to grow to 2025.
When asked if the Obama team would be able to improve its offer further before the crucial Copenhagen talks in December that will attempt to produce a new climate treaty, Mr Pershing said: "I don't know".
It would depend on the level of domestic political resistance in Congress and the amount of progress on technology policies.
The US has a key role to play in developing clean technologies
Mr Pershing did promise that the US would help poor countries to fund clean technology. He would not mention figures but he hinted the sums would be much less than many developing countries demanded.
"The notion that the USA would transfer funds to pay for the entirety of the world's development is implausible. The characterisation often made in these meetings [from developing countries] is that we will only do actions that are paid for. That's a limited vision and we'd like to turn that round."
He said the best role for governments would be to incentivise the private sector to develop energy efficiency, clean technologies and reduce deforestation. He said China did not want money for technology from the USA but co-operation on technology development.
Negotiators from China, India and Papua (representing vulnerable states) all told BBC News that the US and other rich nations needed to cut emissions much harder and offer concrete funding.
Surya Sethi from India said: "Progress is extremely slow. Rich nations seem to think that developing countries can help the world out of the climate problem. But the poorest 50% have just 11% of emissions.
"It is crystal clear that the answer is for the United States and other rich nations to change their lifestyles and their methods of production and consumption. We do not see any real evidence that they have grasped that issue properly yet."
One European delegate said: "We are in a dilemma. We want to support Obama - we know he has political difficulties. But we want him to know he has to do more."
The current talks in the German city are not expected to produce a substantial step forward, except perhaps on limited measures over land use and forestry. Delegates are frustrated there will be no negotiating text until the next meeting in Bonn in June.
So much work remains that the UN is planning two extra preparatory meetings before Copenhagen.
Mr Pershing expressed cautious optimism that the framework for a legally binding deal might emerge by December, although he said some details might take longer to be settled. There was an energy behind the issue in the United States that he had never previously witnessed, he said.