* Stern says "not insurmountable" to get U.S. climate law

* China, other developing nations must do more (Adds quotes on major economies meeting next week)

By Jeff Mason and Deborah Zabarenko

WASHINGTON, April 23 (Reuters) - U.S. flexibility in negotiating a global warming pact depends on passing a law to cut greenhouse gas emissions, which is not an insurmountable task, Washington's top climate envoy said on Thursday.

"We are going to fundamentally be guided by what happens in our own legislative process," Todd Stern, the U.S. Special Envoy for Climate Change, told Reuters in an interview when asked whether there was room for maneuver in the U.S. stance at U.N. climate talks.

President Barack Obama wants to cut U.S. emissions by roughly 15 percent by 2020 -- back to 1990 levels -- mostly through a cap-and-trade system that limits how much carbon dioxide and other greenhouse gases big factories can emit.

That system is the crux of a bill the U.S. Congress is now studying. The measure faces its toughest hurdle in the 100-member Senate, where 60 votes are needed for passage, but Stern said he thought it would be possible to get enough support to pass the bill.

"I don't think the problem's too insurmountable," he said.

Opponents have called the cap-and-trade measure an energy tax, saying it would cripple the U.S. economy in the midst of a recession.

The 27-nation European Union -- which says it will cut emissions 20 percent below 1990 levels by 2020 or by 30 percent if other developed countries follow suit -- wants the United States to go further than Obama's stated goal.

Stern said the EU and U.S. emissions reduction targets were not an exact comparison and said the United States had other avenues to cut emissions aside from the cap-and-trade law.

"The EU puts out a number that is an overall target for everything that contributes to emissions reductions, so there may be other things that contribute to emissions reductions in the United States that aren't part of that ... bill," he said.

BIG ROLE FOR DEVELOPING COUNTRIES

Stern said the major developing countries, such as fast-growing China and India, would have to play a big part in fighting global warming for the world to succeed in tackling the problem.

"The major developing countries need to do significant things, need to make real commitments," he said. "There is no other way to solve this problem."

Leaders from around the globe will gather in Copenhagen in December to forge a successor treaty to the Kyoto Protocol, which caps greenhouse gas emissions.

Washington is kicking off a series of meetings among major economies next week to work toward facilitating global talks.

"What we want to do is to invigorate this forum and to infuse it with a mission and with substance," he said of the meetings, contrasting them with a similar effort by former President George W. Bush's administration that fell flat.

The role of developing countries will be a major issue at those international talks. Stern declined to spell out the U.S. position on how far developing countries needed to go, but he said specific commitments were crucial.

"We cannot any longer be in a position where there is ... a notion that the major developing countries don't need to make commitments in the context of an international treaty," he said. "They do."

Picking out one country in particular, Stern said China was doing a great deal to fight climate change but had to do more.

"I actually think that the Chinese are very serious about it. They are doing a lot. They're not doing enough, but they're doing a lot," he said.

Stern noted Chinese actions to boost renewable sources for electricity, set high auto efficiency standards and cut energy intensity, but he said the country's greenhouse gas emissions were still too high.

"Their emissions trajectory is huge and at an unsustainable level," he said. (Additional reporting by Arshad Mohammed, Sue Pleming and Richard Cowan; editing by John O'Callaghan and Todd Eastham)

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