Rich nations should cut their greenhouse gas emissions by at least 40 percent by 2020 from 1990 levels as part of a new global climate change pact, China said on Thursday, spelling out its stance ahead of negotiations.

The pact must ensure wealthy nations take on quantified targets to drastically reduce emissions, said the statement, issued by the National Development and Reform Commission (www.ndrc.gov.cn), which steers Chinese climate change policy.

Developed countries should also give 0.5 to 1.0 percent of their annual economic worth to help other nations cope with global warming and curtail greenhouse gas emissions, China said in the document, laying down demands for a conference in Copenhagen in December meant to seal a new climate change pact.

The Copenhagen conference is looking to agree on a treaty that will build on the current Kyoto Protocol.

The document echoed one that Beijing made submitted to the United Nations climate change body (unfccc.int) last month.

The new document pointedly says a new treaty ensure developed countries that have not ratified the Kyoto Protocol assume corresponding and comparable emissions reduction commitments.

That demand appears aimed at the Obama administration.

The United States under President George W. Bush set aside the Kyoto Protocol, citing among its reasons that China and other big developing countries did not assume emissions caps.

Beijing's latest statement also says that it and other developing countries must be allowed to balance efforts to combat climate change with the need to develop.

China's own emissions of the main greenhouse gas, carbon dioxide, are likely to keep growing until 2035, experts told an official newspaper, urging massive spending to create a low-carbon economy and contain global warming.

Experts from the state-run Energy Research Institute told the China Daily the nation's emissions of carbon dioxide could reach 5.5 billion tonnes in 2010 and 8.8 billion tonnes in 2035.

But from 2035 to 2050, emissions will remain stable or decline marginally if the proper technological route is followed, the paper said on Thursday, quoting the experts.

China is widely believed to be the world's biggest emitter of CO2, the gas from fossil fuels, industry, farming and land clearance that is accumulating in the air, trapping more solar radiation and threatening to dangerously overheat the globe.

The latest reported estimates of China's emissions are much lower than other recent estimates, including those from Chinese experts. The report did not say what economic and technological assumptions lay behind the projections.

The projected 2035 peak and then gradual falloff in emissions may be an unsettling prospect for governments and experts who have urged Beijing to take swifter action to contain and eventually cut its fast-rising emissions.

(Additional reporting by Tom Miles; Editing by Nick Macfie)

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