China and the US have begun an ambitious new phase of talks on curbing their carbon dioxide emissions that observers say is the most promising development in nearly 20 years of global climate change negotiations.
A lack of co-ordination between the two superpowers, whose combined emissions nearly match the rest of the world's put together, has helped torpedo previous rounds of international climate talks, including the Copenhagen summit of 2009.
But the pair have recently begun detailed discussions about the sensitive issue of their respective carbon-cutting goals as world leaders try to seal a global climate deal in Paris next year that will hinge on a US-Chinese agreement.
Xie Zhenhua, China's chief climate negotiator, told the Financial Times that Beijing's "down to earth" talks with Washington were paving the way for "China and the US to build a new big country relationship".
"We should be confident that the Paris meeting will not be another Copenhagen," he said, explaining the air pollution choking China's cities now made it "a must" for Beijing to build a greener economy, regardless of outside pressure.
Todd Stern, US climate envoy, confirmed there was a new level of intensity in the talks, launched last year shortly after John Kerry, a veteran climate champion, became US secretary of state.
"The pace of our co-operative efforts is increasing and I think they hold a lot of promise," Mr Stern said.
China was now clearly committed to taking strong action, he added. "Their air pollution problem is extremely serious, and I think they are seized at the highest levels by the need to take action that deals with both air pollution and climate change at the same time."
Harvard University's Professor Robert Stavins, an expert in global climate negotiations, said the extent to which the US and China were now co-operating marked a substantial shift. "It is the most promising development since the Kyoto protocol," he said, referring to the 1997 climate treaty that is the only legally binding UN climate pact ever agreed, but is moribund because the US never ratified it and China was not bound by it.
Lord Nicholas Stern, British climate economist, said the Obama administration's commitment to climate action and China's appetite to tackle its domestic pollution problems meant there was far more agreement between the two countries than many realised.
"I have been working in China for 25 years now and in the last five years, there has been a remarkable change in terms of what is being discussed," he said.
China and the US have said they planned to talk about their climate actions, but it was not known they had reached the point of discussing their respective emissions strategies in such detail.
Still, it remains to be seen if they can overcome their long-running disagreement over the extent to which China should be required to meet similar emissions targets to advanced industrial nations, a key hurdle in past climate negotiations.
US, China carbon talks 'most promising' in 20 years