Conventional wisdom says the United States has to bring concrete commitment to international climate talks in Copenhagen in December or no other country will act.

But power shifts at the White House, the State Department and in Congress mean Washington may not have to pass a law limiting carbon dioxide emissions to convince the world it intends to help lead the fight against global warming.

The ebullient atmosphere at last week's Washington conference of the planet's biggest greenhouse gas polluters showed that, even without U.S. legislation, other countries are ready to negotiate.

It's more positive and more constructive now ... because the position of the United States has changed so dramatically, Yvo de Boer, the U.N. climate chief, told reporters after the two-day Major Economies Forum at the State Department.

The change came with President Barack Obama, who took office determined to limit climate-warming pollution at home and participate actively in international negotiations in Copenhagen to craft a follow-up agreement to the carbon-capping Kyoto Protocol, which expires in 2012.

Under President George W. Bush, the United States was the only industrialized country to oppose the Kyoto pact, arguing that participation in the treaty would damage the U.S. economy and give fast-growing developing countries like China and India an unfair exemption from capping emissions.

Obama has made it clear he wants to cut U.S. emissions, either through regulation or legislation, by about 15 percent by 2020, bringing them down to 1990 levels. That's less than the European Union and environmentalists would like, but far more than favored by the previous Bush administration.

ECSTASY IN EUROPE

De Boer said the United States need not pass a fully enacted law to show its good intentions in Copenhagen.

While a law would show a serious U.S. commitment, he said, it also risked prematurely locking Washington into position.

It then becomes much more difficult to come back to the Senate and say, 'Sorry, guys, but we agreed something else in Copenhagen, would you please change the law?' de Boer said.

Secretary of State Hillary Clinton's acknowledgment that the United States needs to make up for lost time to help combat the effects of climate change went a long way to raise international morale, de Boer said.

The international environmental community has been in a rare good mood since Obama was elected last November, said Robert Stavins, director of the Harvard Project on International Climate Agreements.

If you think there is a honeymoon here, there is ecstasy

in Europe, Stavins said by telephone. The sentiment is extremely strong ... and the honeymoon is going to continue in the international policy process, especially in the European Union.

At home, Congress, which is controlled by Obama's Democrats, is likely to make enough progress on climate change legislation to boost chances of success in Copenhagen, according to Senate Majority Leader Harry Reid.

Dan Weiss of the Center for American Progress, a liberal think tank, figures there will be enough movement in Congress to enable meaningful U.S. participation in international climate talks.

It is possible that the Copenhagen talks will lead to a series of multi- and bi-lateral agreements rather than a single treaty, Weiss said by e-mail.

The U.S. Environmental Protection Agency said last month that greenhouse gas emissions threatened human health and welfare and therefore can be regulated as pollutants under the Clean Air Act.

The U.S. Chamber of Commerce, which opposes a cap-and-trade system the Obama team wants to use to curb emissions, released a study last week that this plan would cause 1.9 million job losses and cost the average U.S. household $1,400 by 2020.

The cap-and-trade system allows firms emitting more carbon dioxide than the limit to buy credits from those that emit less.

(Editing by Paul Simao)

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