Bloomberg: China and the US, the biggest sources of the greenhouse gas emissions heating the planet, have stood in the way of an international climate treaty for almost as long as there have been efforts to craft one. The US never ratified the 1997 Kyoto Protocol partly because the pact didn’t compel China and other developing economies to lower emissions. Now, the two countries may be moving toward agreement on how to rein in the 40 per cent of the world’s carbon dioxide pollution that comes …