Huffington Post: Since international negotiations on global climate change began, it has been the case that the two countries most responsible for greenhouse gas emissions today -- the United States and China -- could lead the world on the issue if they could agree with one another. They haven't gotten there yet, but they took a meaningful step last Saturday when Presidents Obama and Xi Jinping agreed to cooperate on phasing out the use of HFCs -- a class of potent greenhouse gases used as refrigerants and in industrial...