Top U.S. officials on Wednesday left open the possibility that China might not have to accept a hard cap on its greenhouse gas emissions under a new global climate change treaty.

Right now, our purpose is really to convey the seriousness of this issue and the need for U.S.-China leadership, U.S. Commerce Secretary Gary Locke told Reuters on a trip to China to boost cooperation on energy conservation and cleaner fuels.

What the ultimate mechanism may be is to be decided at Copenhagen. But it is something that can not be shirked or delayed, Locke said.

Countries are scheduled to meet in Copenhagen in December to agree a new climate treaty to replace the Kyoto Protocol -- which expires in 2012 -- to reduce carbon dioxide (CO2) and other emissions blamed for raising global temperatures.

Scientists say major emissions cuts are needed to prevent dramatic climate changes that could cause more droughts, flooding and disease, and cause ocean levels to rise.

In a separate interview, U.S. Energy Secretary Steven Chu said he would leave it to the diplomats to figure out whether China will be required to cap emissions as part of the new pact.

He also challenged critics who say Beijing does not take the issue seriously to go to China and look at what they're doing.

CHINA SERIOUS ON CLIMATE

They are working very aggressively on this, anywhere from a more stringent mileage standard on cars to very aggressive subsidies and help in promoting green energy too, Chu said.

What they have been doing the past couple of years is proof positive that they know there is a problem with climate change due to CO2 and other greenhouse gas emissions, Chu said.

Instead of harping on differences, let's just help each other. Their leaders know the consequence of climate change. This is big time serious stuff to them, Chu said.

Under Kyoto, the treaty agreed in Japan in 1997, developing nations like China did not have to cap their emissions. Former President George W. Bush pulled the United States out of the pact which would have imposed U.S. emissions cuts.

President Barack Obama has made U.S. legislation to reduce the use of fossil fuels one of his top domestic priorities. The bill faces an uncertain fate in the Senate after barely passing the House of Representatives.

Many senators are reluctant to vote for what they see as an energy tax without major developing countries like China agreeing to specific caps on their greenhouse emissions as part of the new climate treaty.

But China, which recently passed the United States as the biggest source of CO2 pollution, has said it cannot agree to an emissions cap because that would hamper its efforts to raise standards of living.

Locke said it was important China, as the biggest developing country, and the United States, as the biggest developed country, set an example for others to follow in addressing climate change.

That's why we're having these discussions to make it very clear that these issues are of paramount importance to the world, Locke said.

(Via Beijing newsroom; editing by Robin Pomeroy)

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