The United States wants China to accept slow increases in its greenhouse gas emissions until it hits a "peak year," beyond which a real decrease must occur, US negotiator Todd Stern said Friday.

Stern, briefing reporters following a trip to China, said he believed the Chinese authorities have taken "on board" the concept of a peak year for carbon gases that cause climate change but have not specified what year that will be.

"It matters a whole lot when that peak year is," said Stern, President Barack Obama's envoy for climate change. "And we don't know yet where they're going to come out on that."

The United States wants "significant actions in the mid-term range between now and 2020 that will reduce emissions very significantly from where they otherwise would have been," Stern said.

He said he had also talked to Chinese officials about a greenhouse gas concentration target of 450 parts per million (ppm) in the atmosphere, but did not get a clear position from them.

"I wouldn't say that they disputed it," he added.

The planet's average temperatures have warmed by nearly one degree Celsius (1.8 degrees Fahrenheit) since the pre-industrial era.

Most of the warming is due to emissions of greenhouse gases, chief among them carbon dioxide, according to a recent study of the US National Center for Atmospheric Research (NCAR).

Those gases have increased from a pre-industrial level of about 284 ppm in the atmosphere to more than 380 ppm today. Recent studies have found that temperatures would reach the threshold for dangerous climate change if they rise by an additional one degree Celsius.

If carbon dioxide emissions were to plateau and maintained at 450 ppm -- cited as an attainable target by the US Climate Change Science Program if dramatic emissions cuts are enacted -- global temperatures would rise by 0.6 degrees Celsius (one degree Fahrenheit) by the end of the century, the study said.

But if emissions were allowed to continue their current trend, temperatures would rise by almost four times that amount, to 2.2 degrees Celsius (four degrees Fahrenheit).

Chinese foreign ministry spokesman Qin Gang said Thursday that, given the need for a developing country like China to boost its economy and ease poverty, "it is natural for China to have some increase in its emissions.

"So it is not possible for China in that context to accept a binding or compulsory target."

China and the United States are the world's two largest emitters of greenhouse gases that cause global warming.

The US government has already issued a statement describing the meetings between Stern and Chinese officials as "a step in the right direction on the road to Copenhagen and to charting a global path to a clean energy future."

Stern told reporters here he was "sympathetic" to the Chinese position.

He pointed out that although modern cities like Shanghai and Beijing would rank with cities in the developed worlds, whole swathes of rural China put the country in the developing category.