Not that this issue has any importance whatsoever, since GW is a load of rubbish, but I suspect that one of the main reasons that emissions are down in the US is that (1) natural gas discoveries led to a lot of natural gas electric generation (which requires a lot lower capital investment than the coal plants it replaces), and (2) Obama crippled the coal generating industry with regulations. In the long run coal will make a come back unless there is another energy source (like Thorium nuclear) to provide the capacity needed for future growth.