Washington - For months the United States and China, by far the world’s two biggest emitters of greenhouse gases, have been warily circling each other in hopes of breaking a long impasse on global warming policy.

They are, as President Obama’s chief climate negotiator puts it, “the two gorillas in the room,” and if they do not reach some sort of truce, there is no chance of forging a meaningful international treaty in Copenhagen later this year to restrict emissions.

As a senior American team arrived in Beijing on Sunday for climate talks, the standoff was taking on the trappings of cold-war arms control negotiations, with gigatons of greenhouse gas emissions replacing megatons of nuclear might as a looming risk for people across the globe.

Both sides are demanding mutually assured reductions of emissions that are, in the current jargon, “measurable, verifiable and reportable.” In the background hover threats of massive retaliation in the form of tariffs or other trade barriers if one nation does not agree to ceilings on emissions.

“This is going to be one of the most complex diplomatic negotiations in the history of the world,” said Representative Edward J. Markey, Democrat of Massachusetts, the co-sponsor of an energy bill being debated in the House, who just returned from a week in China.

Many take the simple fact that the two nations, jointly responsible for more than 40 percent of the world’s greenhouse gas emissions, are even talking seriously to each other about the issue as a propitious sign after years of mutual distrust.

But there is cause for profound skepticism as well. The Chinese continue to resist mandatory ceilings on their emissions and are making financial and environmental demands on the United States that are political nonstarters.

The United States, despite optimistic words from the White House and Congress, has yet to enact any binding targets on greenhouse-gas emissions. The energy bill now before Congress proposes emissions targets that are far short of what China and other nations say they expect of the United States.

Compounding the difficulty is the fact that both countries are struggling economically and the Chinese and American publics appear far more interested in jobs than in tackling environmental problems, a task that would necessarily be costly.

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 remains 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 players. 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.

“What they are saying right now is, ’We can do a lot of things, but we don’t want to commit to any targets,’ ” said Jin Jiaman, executive director of the Global Environmental Institute, based in Beijing, which has helped pave the way for the current talks. “They want to preserve their right to develop.”

One of China’s senior climate negotiators, Su Wei, has said that although China will not accept absolute limits on its emissions, officials have begun to consider putting in place their own domestic targets to significantly reduce the carbon intensity of its heaviest-emitting industries. Under the current official five-year plan, China is trying to reduce the amount of energy emitted per unit of gross domestic product by approximately 20 percent by 2010, a goal it may or may not meet. Some experts question the accuracy of China’s official reports, and say it will be impossible to monitor the nation’s progress without a better system for tracking greenhouse gas pollution.

In a tough speech in Washington this week, Mr. Stern said that such modest reductions would do little to affect atmospheric concentrations of climate-altering gases. He also noted that China emitted four times as much carbon dioxide as the United States and six times as much as the European Union or Japan for every unit of gross domestic product.

“China and other developing countries do not need to take the same actions that developed countries are taking,” Mr. Stern said, “but they do need to take significant national actions that they commit to - internationally - that they quantify, and that are ambitious enough to be broadly consistent with the levels of science.”

The House speaker, Nancy Pelosi of California, who led a delegation of lawmakers to China at the end of May, said in an interview that she was hopeful about the dialogue between the two countries but fearful that they would fall into the old trap of hiding behind each other.

“They told us if we’re not going to do something, they’re not going to do anything,” she said. “Some of the people we talked to there said we should do more. I think we should do more, too. But we all have to go down this path together.”

John M. Broder reported from Washington, and Jonathan Ansfield from Beijing.

For More Information: New York Times