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 hydr