BEIJING, June 14 — In the debate on global climate change it has long been a given that China, with its huge population and endless coal reserves, would overtake the United States early this century as the biggest source of the atmospheric pollution that scientists believe is warming the planet.
That specter of runaway Chinese emissions has been cited by President Bush as a major reason for describing as "fatally flawed" the 1997 Kyoto agreement to protect the climate. The treaty exempts developing countries, including China, from its initial, binding limits on the output of carbon dioxide and other so-called greenhouse gases that scientists believe are causing traumatic changes in the climate.
But treaty obligation or not, China has already achieved a dramatic slowing in its emissions of carbon dioxide in the last decade, Chinese and Western energy experts say. That record of progress has pushed further into the horizon the day that China will surpass the United States as the lead culprit, and it is something that Mr. Bush seems to have overlooked in his harsh appraisal.
Chinese officials insist that their country will do its fair share to combat a serious global threat.
"We already have one of the world's best records in improving energy efficiency," Zhou Dadi, director of the Energy Research Institute of the central government's State Development Planning Commission, said in an interview.
"Our challenge is this: Can we give people an acceptable lifestyle and also address the problem of climate change?" Mr. Zhou said.
"As an energy expert, I think we need a demonstration from a developed country to prove that a high living standard can be associated with lower carbon emissions," he said. "Then China will follow that example or even do better."
In the most surprising development, China's annual output of carbon dioxide in the last four years of rapid economic growth has actually declined, according to data compiled by the United States Department of Energy. While the numbers could be overstated because of flaws in both economic and energy statistics, some experts think, China does seem to have achieved a stunning if temporary reversal of the usual trend during economic expansion.
"China's emissions of carbon dioxide have shrunk by 17 percent since the mid-1990's," according to an April report from researchers at the Lawrence Berkeley National Laboratory in California. "Remarkably, over the same period, G.D.P. grew by 36 percent."
"Even without undertaking binding commitments under an international agreement," the researchers concluded, China "has nevertheless contributed substantially to reducing growth in global emissions."
This achievement has been a welcome side effect of China's shift to market prices for fuels, including an end to coal subsidies, and its programs to encourage energy conservation and fight urban air pollution, mainly by curbing the burning of coal.
Only a few years ago, many studies projected that China would emerge as the world's leading source of carbon dioxide by 2020, but these recent developments appear to have put off that day by years or even decades.
Although the United States has improved its energy efficiency since the oil crises of the 1970's, recent trends like the fad for large, gas-guzzling vehicles have undermined the former goal of returning carbon dioxide output to 1990 levels.
"There is a good basis to argue that China has done more to combat climate change over the past decade than has the United States," according to a new report by the Natural Resources Defense Council, an American environmental group that aids energy conservation projects in China.
Mr. Bush, most recently on Monday, has said he cannot support the 1997 Kyoto Protocol in large part because it exempts China and other developing countries from the initial limits on emissions of greenhouse gases that richer countries are supposed to accept.
With his condemnation of the hard- won treaty, Mr. Bush has set off a tempest in Europe and many developing countries, which are more convinced of the looming threat of climate change and had thought they had agreement to act.
The signatory countries will meet next month in Bonn to search for ways to save an agreement with some teeth.
In his speech on Monday, Mr. Bush complained that China, as the second-largest emitter of greenhouse gases, after the United States, "was entirely exempted from the requirements in the Kyoto Protocol."
Chinese officials point to what they feel is their unacknowledged progress, but they also say the rich countries, which account for most of the carbon dioxide that has already accumulated in the atmosphere, must show that they are serious.
"We've done what we can to reduce emissions, and we'll continue to do so," Gao Feng, a senior Foreign Ministry official here who has taken part in the climate negotiations, said in a recent interview. "But it's not fair to ask the developing countries to take the lead."
"Before the developed countries show that they will do something real and good to address this issue, why should the developing countries make a commitment?" Mr. Gao asked, repeating the arguments that have led to an impasse between developing nations and the Bush administration.
Because it is so large and makes such enormous, inefficient use of coal — the worst fuel in terms of climate effects — China is second only to the United States in emissions of carbon dioxide. At the same time, its people consume on average only one-tenth as much energy as Americans, and they hunger for economic advances.
In the last decade, according to data compiled by the United States Energy Department, China's carbon dioxide emissions from the burning of fossil fuels have climbed at annual rate of 0.9 percent — lower than the 1.3 percent a year registered in the United States, even as China's economy expanded much more rapidly.
Despite the recent slowdown, experts say, substantial future growth in carbon dioxide emissions is inevitable in China as the country develops. Yet officials here also say that China accepts the need to work against global warming and that at some point, they know, China will need to accept international targets.
Strategically, we have adopted climate change as an important concern in our energy planning," said Mr. Zhou of the Energy Research Institute.
Before 1980, Mr. Zhou said, China's energy use increased 1.6 times as fast as the economy. But in the last 20 years, he said, energy use has grown at less than half the rate of the economy — an exceptional advance in the efficient use of fuels.
India and other large developing countries have also improved efficiency but not as dramatically.
With a combination of increasingly stringent regulations, like energy codes for new buildings, as well as other conservation programs and rising prices, Chinese planners hope to preserve a similar low ratio of energy use to growth in the decade to come, Mr. Zhou said.
"It's not easy because there is no precedent anywhere in the world," he said. Fuel use in much of China remains extremely wasteful, however, leaving opportunities for large gains.
"Our per capita energy use is just one-tenth of that in the United States and one-seventh of that in Europe," Mr. Zhou said. "With development, it must be increased.
"I don't think China can achieve a unique style of development," he said. "Americans drive cars while we ride bicycles; you live in houses while we live in dormitories."
Frank Loy, who as under secretary of state under President Clinton helped negotiate climate issues and has since left government, said he believed that creative new approaches might allow the United States and other countries to proceed against greenhouse gases, but that this would require some give on all sides.
Mr. Loy said it was reasonable for the United States to insist on assurances that its efforts will be part of an effective, shared global plan to curb emissions.
At the same time, he said, it would not be fair to stifle the development of poor countries.
As one possible compromise, Mr. Loy said, developing countries like China could take on an obligation to keep emissions at a certain fraction of economic growth, rather than setting absolute limits. Or they could adopt targets for energy efficiency as their economies grow.
Mr. Loy said he believed that some in the Bush Administration started out with a clear goal: "to drive a stake through the heart of the Kyoto agreement." But the outcry at home and abroad, he said, has led them to second thoughts.
At the same time, Mr. Loy noted that poor countries, with their extreme vulnerability to climate-related natural disasters, have the most to lose if the agreement collapses, and he called for more flexibility on their part.
The original 1992 treaty laying out the framework for climate talks, he noted, called for "common but differentiated obligations" on the part of rich and poor countries.
"It's too bad that this has been transformed into a group of countries that have real obligations and a group of countries that don't have any," he said.