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President Obama is pledging a provisional target for reductions in greenhouse gas emissions in the United States, the first time an American administration offered even a tentative promise to reduce production of climate-altering gases, White House officials said Wednesday. Mr. Obama will travel to the United Nations climate change conference in Copenhagen next month to deliver the pledge in hopes of spurring significant progress at the talks.

Mr. Obama will tell the delegates to the climate conference 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. No American administration has ever delivered even a tentative pledge on emissions reductions because Congress...