BEIJING - China is preparing to unveil a target to curb carbon emissions ahead of a major climate summit in Copenhagen next month, but experts and negotiators worry Beijing's much-anticipated figure may disappoint.

Offers to tackle carbon pollution from China and the United States, the world's two top emitters, are key to the success of the U.N.-led talks, which originally aimed to seal a new framework to fight climate change.

The negotiations have run out of time and instead a political pact is expected to be agreed at the Dec 7-18 Denmark meeting with a legally binding agreement in 2010.

Beijing is considering a reduction of around 40 to 45 percent in carbon intensity -- the amount of greenhouse gases emitted for each yuan of national income -- from 2005 levels, sources with knowledge of the negotiations say.

It is expected to announce the number before the Copenhagen talks start, perhaps as early as this week.

But critics say Beijing is almost half-way to that goal after just four years of an energy efficiency drive, and warn that an unambitious target could slow a push for cleaner growth.

Maybe the Chinese government likes to claim by 2020 there will be 40 percent reduction ... but it is not aggressive enough, we have to argue for at least 45 percent, said Yang Fuqiang, director of Global Climate solutions at WWF.

China pledged in 2006 to cut its energy intensity 20 percent by 2010 and is more or less on track to meet that goal. This brings a matching improvement in emissions because every tonne of coal that is not burned keeps carbon dioxide out of the air.

The rapid roll-out of renewables and nuclear has also avoided extra greenhouse gas emissions that would have been created if that energy came from fossil fuels. So a 20 percent improvement in energy efficiency brings a greater carbon intensity cut.

If you convert that into carbon numbers, it's a little less than a 25 percent decrease, Jonathan Pershing, U.S. deputy special envoy for climate change, said of China's expected energy savings through 2010, in an interview earlier this month.

Even with a higher-end target of 45 percent, this leaves just over 20 percent to be achieved in the coming decade.

WHAT IS NOTABLE?

President Hu Jintao promised in September that China would unveil a goal for a notable cut in carbon intensity by 2020, compared with 2005, a landmark because it was the first time China had accepted it must put measurable controls on emissions.

Beijing's move was seen as a key step toward unblocking U.N. negotiations that had stalled as rich and poor nations argued over who should cut emissions, by how much and who should pay.

But support for China might wane if rich nations feel Beijing is dressing up its normal economic trajectory as an emissions goal -- and experts say that China does already seem on track to make significant cuts in the next decade, in part as it reaps the reward of its energy efficiency and renewables programs.

My view is that a Chinese target of a 40 percent reduction in carbon emissions intensity between 2005 and 2020 would be a continuation of historical trends, said Jim Watson, from the Tyndall Center for climate change research in Britain.

If 'business as usual' is no progress in carbon intensity reduction, which is very unlikely, then 40 percent is quite big. But compared to historic trends, it is not nearly as big as it sounds, he added in an email to Reuters.

With the U.S. commitment considered modest by many -- President Barack Obama has proposed a 17 percent emissions cut by 2020 from 2005 levels -- the two may be aiming for politically acceptable compromises that could potentially be ramped up later.

China's target has an advantage over any proffered by the United States, however, as its negotiating team do not have to take their deal back for government approval -- it will be already endorsed by the ruling Communist Party.

Whatever the government puts on the table will be a domestic commitment, said Wu Changhua, China head of The Climate Group.

PEAK YEAR

China might also be considering unveiling a target for the year it wants to see emissions peak, something it has so far resisted as it could imply an implicit cap on greenhouse gases.

The Chinese government's latest discussions internally have been very encouraging, said Wu, from the climate think-tank.

The second possibility (after a 2020 emissions intensity target) is setting out when China is going to peak in terms of emissions, she told Reuters.

The most optimistic analysts say an economic shift to greener growth that has already started could be sped up so that emissions peak in barely 10 years.

They point to China's dramatic transformation from poverty-stricken recluse to leading world power in just three decades as a sign of what the government can do.

In my road map for CO2 emissions, we reach the peak by 2020 and after that decline, said prominent economist Hu Angang.

Most estimates, however, are more conservative, suggesting a peak around 2030, the date laid out in a major government study on climate change policy options published earlier this year.

(Editing by David Fogarty)

United kingdom Energy Climate change China