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Just How Green Is Natural Gas?

• Tyler Hamilton via

Researchers at Cornell University are raising alarms over the expected increase in use of natural gas from shale deposits. They argue that replacing coal and gasoline with natural-gas alternatives could worsen, rather than improve, the impact of greenhouse gases. The greenhouse-gas footprint of shale gas over a 20-year period is at least 20 percent higher than that of coal and "perhaps more than twice as great," they say in a study published online in the journal Climatic Change. The culprit is the leakage of methane, the main component of natural gas.

To extract natural gas from shale, drillers hydraulically fracture the rock by injecting a cocktail of water and chemicals into a horizontally drilled well at high pressures. Robert Howarth, a biogeochemist and professor of earth sciences at Cornell, argues that a significant amount of gas also mixes with the water-chemical mix and escapes into the atmosphere when the fluid returns to the surface. The drilling out of well plugs that separate fracking stages also results in temporary emission releases, giving shale gas a "significantly larger" greenhouse-gas footprint than conventional natural gas.

The finding is a blow to environmentalists, many of whom have viewed natural gas as a "bridge fuel" on the way to cleaner energy sources. The U.S. Energy Information Administration projects that total U.S. natural-gas production will increase by 20 percent over the next 25 years and that shale gas will account for nearly half of the total, up from 23 percent in 2010. Major U.S. shale deposits include the Marcellus shale formations in the Northeast and the Barnett formations in Texas. China, France, Poland, Chile, and more than a dozen other countries also have significant recoverable shale gas resources, according to a recent Energy Information Administration study.

When burned to generate electricity, natural gas emits roughly half as much carbon dioxide per megawatt-hour as coal. But over its life cycle, natural gas could result in far more greenhouse-gas emissions, whether through intentional venting, equipment leaks, or fracking. And the leaks would consist of methane, which is a much more powerful greenhouse gas than carbon dioxide. The U.S. Environmental Protection Agency conceded in a December report that it had "significantly underestimated" the methane emissions that result from development of unconventional natural-gas resources, primarily shale gas. In some instances, methane emissions were nearly 9,000 times higher than previously thought.

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