Not only is coal dirty, but anybody who took chemistry in high school can calculate that the CO2 emission per unit of energy is more than twice as high (2.26 to be more precise) for coal than for natural gas. Rik C + O2 -> CO2 = 393.5 kJ/mole of CO2 CH4 + 2 O2 -> CO2 + 2H2O = 890 kJ/mole of CO2 890/393.5 = 2.26, so per mole of produced CO2, natural gas yields 2.26 times more energy, or per given amount of energy, coal produces 2.26 times more CO2.