The United States and China announced new goals for reducing their global warming pollution in the coming decades, with the U.S. ramping up its rate of decarbonization in five to 10 years and China promising that its carbon emissions will peak in the next 15 years. The announcements, which came at a multinational summit in Beijing Tuesday, made clear for the first time the commitments that the two biggest sources of greenhouse gases will make as part of the urgent United Nations negotiations.