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. The administration has resisted until now delivering a firm pledge on emissions reductions because Congress has not yet acted on global warming legislation and because several large developing nations, including China and India, have not detailed their own plans.