I don't trust the Sierra Club or any other activist environmental group. They usually have agendas that are far out of the mainstream.
The U.S. is on track to reduce its coal-consumption without activism or legislative mandates. Before 2020, solar becomes cheaper than fossil fuels, and this will only further fuel the trend toward more use of renewable energy, which is a good thing. In the meantime, cheap natural gas from shale-gas extraction is further pushing down coal's share of the energy market in the U.S.
Now, the U.S. coal industry will continue to thrive, as coal is becoming a hot export commodity for the U.S. China and India are buying U.S. coal at premium prices and in record quantities. This means the pollution will simply be exported to those countries.