At the international climate meetings in Copenhagen next month, Mr. Obama will tell the delegates that the United States intends to reduce its greenhouse gas emissions in the range of 17 percent below 2005 levels by 2020 and 83 percent by 2050, officials said.
The figures reflect targets specified by legislation that passed the House in June but is stalled in the Senate. Congress has never enacted legislation that includes firm emissions limits or ratified an international global warming agreement with binding targets.
Mr. Obama will travel to the United Nations talks to deliver the promise in hopes of spurring significant progress there. He will appear December 9, near the beginning of the 12-day session, on his way to accept the Nobel Peace Prize in Oslo on December 10, officials said.
By making the pledge in an international forum, Mr. Obama is laying a bet that Congress will complete action on a climate bill next year and will be prepared to ratify an international agreement based on the commitment.
But White House officials acknowledged that those outcomes were uncertain. They will depend in large measure on whether the Democratic sponsors of the legislation can win 60 votes for a measure that is at the moment unpopular and whether major developing nations, notably China and India, deliver credible emissions reduction pledges of their own.
Mr. Obama has met over the past two weeks with the leaders of China and India, the fastest-growing sources of greenhouse gases, to discuss climate change and the Copenhagen conference. American officials said that both countries told the president they would be prepared to announce steps to reduce the rate of growth of emissions if the United States put a pledge on the table.
Neither has done so yet, although Chinese officials have hinted that they will announce a near-term target for reducing energy use relative to economic growth, or carbon intensity, before the Copenhagen conference opens.
The Chinese Foreign Ministry announced that Prime Minister Wen Jiabao would attend the conference.
Obviously, we hope other major economies will put forth ambitious action plans of their own, Carol M. Browner, the presidents senior adviser for energy and climate change, said at a White House briefing.
Mr. Obama, who had not previously committed either to emissions targets or to going to Copenhagen, has been under considerable pressure from other world leaders and environmental advocates to reassert American leadership on climate change.
Andreas Carlgren, the Swedish environment minister, said that Mr. Obama had now raised expectations for the Copenhagen talks, but he expressed a note of disappointment about the timing of his visit. He said he hoped Mr. Obama would come in the final days of negotiations, when dozens of other heads of government were planning to arrive.
A White House official said a return trip was highly unlikely.
It was unclear what effect Mr. Obamas promise of domestic emissions reductions would have on the slow progress of climate legislation through Congress. Until now, the administrations negotiators have said they will not get ahead of Congress in making promises in an international forum, but Mr. Obama has now essentially adopted the targets of a climate and energy bill that passed the House in June.
The House bill aims at greenhouse gas reductions of 17 percent below 2005 levels by 2020 and sharper cuts in the following decades, through a cap-and-trade system that includes most of the nations major sources of carbon dioxide emissions. Last month, a Senate committee passed a measure calling for a 20 percent cut by 2020, but that is expected to be weakened as the legislation moves through other Senate committees and onto the floor, perhaps next spring.
By putting a serious number for U.S. emission reductions on the table, the president has just called the worlds bet and then raised it for our negotiating partners, said Representative Edward J. Markey, co-sponsor of the House legislation.
Senator John Kerry, Democrat of Massachusetts, co-sponsor of the Senate legislation, said he believed that the presidents actions would give a boost to the Copenhagen talks and help move the Senate bill. He called the decision to declare an American target a game changer, domestically and internationally.
By announcing a provisional target, contingent on the support of Congress, the president has defined a path to an international agreement that challenges the developed and developing nations to fulfill their obligations, he said. It lays the groundwork for a broad political consensus at Copenhagen that will strip climate obstructionists here at home of their most persistent charge, that the United States shouldnt act if other countries wont join with us.
But Senator James M. Inhofe, the Senates most outspoken skeptic on climate change, said that Mr. Obamas public pledge would do little to speed an international agreement and foolishly prejudged the outcome of a Senate debate that had barely started. Mr. Inhofe, Republican of Oklahoma, said that Senate climate legislation was dying on the vine and that the Senate would never ratify a treaty that did not require strong emissions reductions from major developing countries.
The U.S. Senate has made clear on numerous occasions that unilateral action by the United States is unacceptable, because it will harm our economy and have virtually no effect on climate change, Mr. Inhofe said.
Mr. Obama takes little risk in appearing briefly at the Copenhagen conference because he and other world leaders punctured expectations for the session 10 days ago in a side meeting of leaders of Pacific nations. The leaders agreed that they would work at Copenhagen toward an interim political declaration on climate change that stopped short of a binding international treaty. Delegates are expected to pledge to complete work on a treaty next year.
Mr. Obama came to office promising to end eight years of relative inaction on climate change under the Bush administration, but the inaction of Congress has limited the administrations ability to negotiate with other nations. At the Kyoto climate conference in 1997, the Clinton administration joined other industrialized nations in pledging to reduce greenhouse gas emissions by 5.2 percent by 2012, but Congress refused to ratify the agreement because it made no demands of developing nations.
Many foreign leaders, particularly those in European nations that have been more aggressive in dealing with climate change, have become critical of Mr. Obamas seeming passivity on the issue. The White House appears to hope that the announcement of the targets and the trip to Copenhagen will quiet some of the dissension and help Mr. Obama re-establish American leadership on what he calls one of the signature issues of the time.
Mr. Obama said recently that he would attend the session if his presence could help lead to a successful outcome. It is significant that he will appear at the beginning rather than at the end of the 12-day meeting. Most major decisions at such environmental talks come in the closing days.
Yvo de Boer, head of the United Nations climate directorate, said in an e-mail message that he would like to see the American target in writing and a pledge of money to help poorer nations adapt to a changing climate.
If the president comes in the first week to announce that, Mr. de Boer said, it would be a major boost to the conference.
The White House also announced that several cabinet secretaries would speak at the Copenhagen conference: Lisa P. Jackson, the Environmental Protection Agency administrator; Steven Chu, the secretary of energy; and Ken Salazar, the secretary of interior.