China and the United States ratified the Paris agreement to cut climate-warming emissions on Saturday, marking a major step toward the enactment of the pact as early as the end of the year and setting the stage for other countries to follow suit. The world's two biggest emitters of greenhouse gases made the landmark announcement as heads of state from the Group of 20 biggest economies, or G20, arrived for a summit in the city of Hangzhou, parts of which resembled a ghost town as Chinese security locked down the area.