China must swiftly decouple its rapid economic growth from rising carbon dioxide emissions for global greenhouse gas levels to stay manageable, the authors of a new study said, urging sweeping support to help that transition.

The study from Britain's Tyndall Center for Climate Change Research by Tao Wang and Jim Watson finds China can transform into a low-carbon economy with the right mix of clean energy, carbon storage technology and development policies.

But at the release of the report to officials and experts in Beijing on Wednesday, Wang said the task of turning the world's biggest greenhouse gas emitter into a green economy will be difficult, even in the easier scenarios.

And it would require big commitments of technology and funding from wealthy countries.

It's very crucial to slow the growth as early as possible and to reach a peak as early as possible, Wang, a researcher at the University of Sussex, told the meeting.

It's vital for China to have the technical and financial assistance to make the fast transition which is necessary, he told Reuters in a separate interview.

Wang and Watson said their study suggested China's CO2 output should peak between 2020 and 2030, because keeping accumulated emissions within tolerable levels would be increasingly difficult if output keeps growing beyond then.

What we're not saying is that China should take on a target now, Watson, a researcher at the Tyndall Center, told Reuters. But, he added, slowing the trajectory from the steep rise it's been on is needed, whatever future you conceive of.

Their study can be found on the Centre's website (www.tyndall.ac.uk).

LESS AMBITIOUS

Chinese climate change policy officials and experts are developing the government's position for negotiations aiming to agree the outlines of a new pact on fighting global warming by the end of the year.

China is mankind's biggest source of CO2, the main greenhouse gas. On a per-capita basis, China's 1.3 billion people produce about 4 tons of greenhouse gases, compared with the U.S. average of about 20 tons per person.

The Tyndall study will add to debate here and abroad about how China can balance hopes for prosperity with efforts to contain greenhouse gases from industry, vehicles, farming and land clearance.

China produces about 80 percent of its electricity from coal-fired power stations and is also the world's largest producer of power from coal.

The Chinese government is exploring pathways to a low-carbon economy, but the emissions growth reductions envisaged by Chinese studies are less ambitious than those Wang and Watson examine.

How low is low?, Lu Xuedu, a Chinese environment policy official said at the release of the report, speaking of a low-carbon economy. To do this well, and not treat it as a mere slogan, will not be easy.

Wang and Watson take the total budget of CO2 emissions throughout this century that a U.N. scientific panel concluded was likely to keep average global temperature rises 1.9 to 4.4 degrees Celsius above pre-industrial levels.

They then tested how China might be able to grow while staying within the carbon budget it could receive in an international apportionment of emissions.

Of global CO2 emissions throughout this century equal to 490 gigatonnes of pure carbon, China may potentially get to emit 70 to 111 gigatonnes, they wrote. Emissions are also often estimated in tons of CO2, which weighs 3.67 times as much as pure carbon.

China can stay within carbon bounds and keep growing if it adopts sweeping measures to divert energy generation away from dirty coal to clean sources, and puts increasingly wealthy consumers on a path to less carbon-intensive homes and transport, said Wang and Watson.

Under various energy and development settings, China's economy could expand to between 8 and 13 times its current size by 2050 while sticking within the emissions budget, they found.

But while China's massive market might help speed the spread of wind and solar power, other bigger technological challenges such as mastering carbon capture and storage would be expensive, and wealthy nations should pitch to help, said Watson.

Such trade-offs will be at the heart of the global climate negotiations culminating in Copenhagen in December.

They would not be signing up to just a number, Watson said of China. They'd be signing up to a huge set of infrastructural changes, behavioral changes, institutional changes.

(Editing by David Fogarty)

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