The main product of the discussions with Beijing so far has therefore been agreement to hold more discussions.

Yet the clock is ticking. Only six months remain before the opening of United Nations-sponsored talks in Copenhagen to produce a climate change treaty to replace the 1997 Kyoto Protocol.

Without the full participation of the United States and China, most negotiators believe that any agreement is doomed to fail. Congress and two American presidents refused to accept the Kyoto accord, which expires in 2012, because it imposed no pollution limits on China or other developing countries. The American refusal to ratify the treaty and the lack of participation by China and other developing nations have left the pact all but toothless.

“China may not be the alpha and omega of the international negotiations, but it is close,” said Todd D. Stern, the top American climate negotiator at the three-day talks in Beijing. “Certainly no deal will be possible if we don’t find a way forward with China.”

The Obama administration has pledged to be a leader in the talks that culminate in December in Copenhagen, although it is far from clear that Congress has the will to approve emissions targets and furnish enough aid to developing countries to satisfy the Europeans, Chinese, Indians, Brazilians and other major participants. Mr. Stern described the demands from China and other countries as “not serious,” and said the United States was “jumping as high as the political system will tolerate.”

As a measure of how far apart the two nations are, China says the United States should reduce its greenhouse gas emissions by 40 percent below 1990 levels by 2020. The bill before Congress, which could be further weakened, now calls for less than a 4 percent reduction over that period.

The Chinese have begun to consider a series of unilateral actions to reduce carbon dioxide emissions, stepping up production of renewable electricity and increasing the efficiency of their manufacturing, buildings and vehicles. But Beijing insists it will not sacrifice China’s economy to meet the demands of outsiders, particularly those in the developed world that are responsible for the vast majority of human-caused carbon dioxide already in the atmosphere.