The leakage of methane - a potent heat-trapping gas - likely outweighs much, if not all of the climate benefit of natural gas versus coal. When burned in a power plant, natural gas gives off about 50 percent of the carbon dioxide emissions as coal. But this ignores the widespread problem of leaking methane. Methane's warming potential far exceeds that of carbon dioxide: On a twenty year time scale, methane traps heat up to 105 times more effectively than an equal mass of CO2. Unburned methane is a byproduct at every stage of the gas development lifecycle, including production, processing, transmission, and distribution.

Multiple studies (1, 2) suggest that "fugitive emissions" of methane from wells and pipelines are significant, thereby offsetting the climate benefits of gas versus coal.

Indeed, gas produced from fracked wells may actually turn out to be worse than coal: Once fugitive methane emissions exceed 2-3 percent of total gas production, natural gas's climate advantage over coal disappears over a 20-year time horizon. Recent studies (1, 2) suggest leakage rates well above this threshold.

In addition, shale gas represents one of the largest reserves of carbon on earth. If we burn more than a tiny fraction of it, putting its carbon into the atmosphere, it will be impossible to keep global temperatures from rising beyond a livable threshold.