Beijing has not released recent official data on greenhouse gas from the nation's fast-growing use of coal, oil and gas. Researchers abroad estimate China's carbon dioxide emissions now easily outstrip that of the United States, long the biggest emitter.

But in a break with official reticence, researchers from the Chinese Academy of Sciences and other major state-run institutes have concluded that, without dramatic counter-steps, their nation's emissions will tower over all others' much sooner and higher than an earlier government forecast indicated.

The projected leap in emissions underscores the pressures that China will face in looming climate change negotiations, and the immense challenges it will face in meeting any commitments.

By 2020, China's burning of fossil fuels could annually emit carbon dioxide equal in mass to 2.5bn metric tonnes of pure carbon and up to 2.9bn tonnes, depending on varying scenarios for development and technology, the new report states. By 2030, those annual emissions may reach 3.1bn tonnes a year and up to 4.0bn tonnes.

That compares with global carbon emissions of about 8.5bn tonnes in 2007. Emissions are also often estimated in tonnes of Co2, which weighs 3.67 times as much as carbon alone.

The think-tank report does not give its own estimate of China's current Co2 emissions, but cites data from a U.S. Department of Energy institute that put them at 1.4bn tonnes of carbon in 2004.

The U.S. Oak Ridge National Laboratory estimated that the United States emitted about 1.6bn tonnes of carbon in 2007, compared to China's 1.8bn tonnes.

The Chinese researchers' "China Energy Report" for 2008 warns of drastic risks from inaction in the face of this projected growth, and yet also s

ays economic development must not be hobbled.

"No matter how historical responsibility is defined, our country's development path cannot repeat the unconstrained emissions of developed countries' energy use," states the Chinese-language report, which recently went on public sale without fanfare.

"Therefore, we must soon prepare and plan ahead to implement emissions reduction concepts and measures in a long-term and stable energy development strategy."

The main author, Wei Yiming, has participated in a U.N. scientific panel to assess global warming. He was not immediately available for comment on the findings and why they appeared now.

A Chinese government report of last year which stressed the country's will to fight global warming estimated that by 2050 its total greenhouse gas emissions could reach 2.0 billion tonnes of carbon a year, thus surpassing current U.S. levels some time before then. It did not say when China would reach that point.

This was much slower projected growth than the new report's estimates, which only count emissions of carbon dioxide, the main greenhouse gas from human activities. The new report also leaves out emissions sources other than burning fossil fuels; for instance, from clearing forests.

The study may add to contention over China's response to global warming at a time of accelerating international negotiations. Beijing will be at the heart of efforts to forge a treaty next year to succeed the first phase of the Kyoto Protocol, which expires at the end of 2012.

The European Union this week said developing countries should accept a 15-30 percent cut in their greenhouse gas emissions from "business-as-usual" levels.

But under the Protocol, a UN-led pact, poor nations do not assume targets to cap emissions. And Washington has refused to ratify Kyoto partly because it says the treaty is ineffective without Beijing's acceptance of such mandatory caps.

Carbon dioxide and other greenhouse gases trap solar radiation, heating the atmosphere and threatening to stoke worsening drought, disrupted rainfall and wild weather.

But China points out that per capita emissions of its 1.3bn people are much lower than rich countries' and says the developed countries bear overwhelming responsibility for the dangerous accumulation of greenhouse gases.

Beijing officials have also often said they will not sacrifice hard-won economic development to greenhouse gas caps.

For China, "relative to reducing carbon dioxide emissions, economic development is even more important," the study says.