Negotiators from many countries have said that a crucial moment in the path to the Paris accord came last year in the United States, when Mr. Obama enacted the nation’s first climate change policy — a set of stringent new Environmental Protection Agency regulations designed to slash greenhouse gas pollution from the nation’s coal-fired power plants. Meanwhile, in China, the growing internal criticism over air pollution from coal-fired power plants led President Xi Jinping to pursue domestic policies to cut coal use.