Life-cycle analysis of shale gas is complicated by the debate of how methane emissions should be measured. Methane emissions are measured in carbon dioxide equivalents, or the warming potential over time that a greenhouse gas has compared to the warming potential of carbon dioxide. Carbon dioxide can reside in the atmosphere for almost a century after it is released, but methane has a residence time of about 10 years.
While methane is in the atmosphere for less time, researchers say a ton of methane traps up to 25 times more heat than a ton of carbon dioxide (Revkin & Krauss, 2009). As a result, controversy arises. When the Environmental Protection Agency (EPA) calculated the average concentration of methane in the Earth’s atmosphere over a period of 100 years, it determined that one ton of methane in the atmosphere is the same as 21 tons of carbon dioxide. However, other scientists argue that because of methane’s shorter residence time in the atmosphere, its impact should be calculated over the span of 20 years rather than 100. Robert Howarth, a professor of ecology and environmental biology at Cornell University, calculated that when a 20-year period is used in calculations instead of 100 years, one ton of methane is the same as 72 tons of carbon dioxide (Lustgarten, 2011b).
In 2009, the EPA concluded that the amount of methane released by routine operations at gas wells was 12 times its original estimates of nine billion cubic feet (Revkin & Krauss, 2009). According to an article in ProPublica, a non-profit independent investigative journalism organization, these greenhouse gas emissions are comparable to annual emissions from “35 million automobiles” (Lustgarten, 2011b; 2011a). Using EPA’s revised estimates, methane levels from fracking in the Marcellus Shale are 9,000 times higher than previously reported levels (Lustgarten, 2011b). Current total methane emissions from the gas industry are 261 million metric tons (Lustgarten, 2011a). The World Bank estimates that gas-drilling emissions are at least one-fifth of human-caused methane residing in the atmosphere (Lustgarten, 2011b).
Howarth (2011) estimated the life-cycle emissions of an average shale gas well are 3.6 to 7.9 percent of the total lifetime production of that well (p. 685). The total emissions of shale gas are 20 to 50 percent higher than that of coal, based on the quantity of energy available during combustion over 20 years. Meanwhile, when analyzed over a period of 100 years, the total emissions of shale gas and coal are comparable, Howarth stated (p. 686). J.D. Hughes of the Post-Carbon Institute, a non-profit think tank in California, tried to reconcile Howarth’s findings with other studies in an Institute 2011 report because Howarth’s life-cycle analysis did not include the efficiency of final use of the fuel. When the efficiency of fuel use is taken into account, natural gas gains some advantage over coal when used to generate electricity because of power plant efficiencies (p. 686). Hughes found that shale gas does have higher total emissions than coal if the well’s lifetime production is less than 1.5 billion cubic feet, using the 20-year global warming factor value for methane. However, if the well produces more than 1.5 billion cubic feet or if the 100- year factor is used, then shale gas has lower total emissions than coal (Klemow & Fetcher, 2011).