Ask any expert on U.S.-China relations where opportunities for the two rivals to work together still lie, and they will usually give the same answer: combating the threat of climate change. Yet one recent major development shows how competition between the world’s two largest economies is set to be as much of a factor as cooperation in the fight to temper global warming. Last month, China formally ratified the Kigali Amendment, an international treaty that aims to phase down the use of hydrofluorocarbons (HFCs) — a super-pollutant used in air conditioners, refrigerators, and foam insulation that traps thousands of times more heat in the atmosphere than carbon dioxide. If the accord is fully implemented globally, it may prevent up to 0.5 degrees Celsius of global warming by 2100, experts estimate.  As the world’s largest producer of HFCs, China’s ratification alone should lead to a significant reduction in emissions. Beijing has come late to signing up: though it indi