By Jeff Mason

SEATTLE, Feb 8 (Reuters) - U.S. presidential hopeful Barack Obama promised on Friday to start working on an international pact to reduce global warming if he becomes the Democratic nominee, touting his plan to reduce U.S. emissions as stronger than that of Republican front-runner John McCain.

Global warming has become a key issue in the race for the White House, with the top candidates in both political parties seeking to put a cap on greenhouse gases blamed for rising global temperatures.

Obama, an Illinois senator who is battling New York Senator Hillary Clinton to become their party's presidential nominee, said he would start developing the U.S. position on a pact to replace the Kyoto Protocol before the general election in November.

"I've been in conversations with former Vice President (Al) Gore repeatedly, and his recommendation, which I think is sound, is that you can't wait until you are sworn into office to get started," Obama told a news conference in Seattle.

"I think we need to start reaching out to other countries ahead of time, not because I'm presumptuous, but because there's such a sense of urgency about this."

Nearly 200 nations, including the United States, agreed at U.N.-led talks in December to launch negotiations on a new pact to fight global warming. But many environmentalists say real progress will only be made once President George W. Bush, who was long a global warming skeptic, leaves office.

Obama said he would not wait until January 2009, when the new president takes office, to get started.

"The moment I secure the nomination, I want to bring together experts in this area to start putting together the U.S. position ... what we're going to be doing internally, what we can agree to with other countries," he said.

"I know that my climate change plan is stronger than John McCain's," Obama said, citing his intention to make industrial polluters pay for the right to emit greenhouse gases.

Obama, Clinton, and McCain all support building a so-called "cap and trade" system that would issue big polluters such as oil companies and power producers permits to emit carbon dioxide (CO2), the main gas blamed for global warming.

Under such a system, companies that exceed their CO2 limits must buy more permits to pollute, while those that come in beneath their limits may sell the permits on a market.

Obama said his plan was superior to McCain's because it required companies to buy all of those permits up front -- a process known as auctioning.

"I've been very specific about proposing 100 percent auctioning, which makes an enormous difference in terms of how effective it's going to be," Obama said.

The European Union, which now has the biggest CO2 emissions trading scheme in the world, is changing that system to increase the amount of auctioning required.

(Editing by Stuart Grudgings)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/

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