Supporters of natural gas promote it as a clean "bridge fuel" between the coal-and-oil dominated energy system of the present and the zero-emissions system of the future based on renewables and nuclear power. Burning gas rather than coal in a power plant produces roughly half the carbon dioxide emissions and sharply reduces airborne pollution from mercury and other heavy metals.
Switching to gas from coal could therefore slow the rise in global temperatures as well as produce significant health benefits. Shifting to gas is at the heart of the U.S. Environmental Protection Agency's (EPA) proposed new rules for power plant emissions.
But gas is only more environmentally friendly if it is produced, transported and burned carefully, without too much leaking into the atmosphere.