The controversial natural gas extraction technique known as fracking releases so much methane that it wipes out the supposed advantage of the fuel in slowing down climate change, a new study by Cornell University researchers concludes.

Natural gas, which is abundant in the U.S., has been widely touted and has gained political support as a cleaner-burning alternative to oil and coal.

But according to the Cornell study, to be published in the journal Climatic Change, methane emissions from shale gas production are at least 30 percent more than — and perhaps more than twice as high — as those from conventional gas production. Those higher emissions largely result from fracking, or hydraulic fracturing, the process of injecting water, sand and chemicals into a gas well to open up subterranean cracks that release natural gas from shale rock formations.

All told, the greenhouse gas emissions from the production and use of shale gas challenge “the logic of its use as a bridging fuel over coming decades, if the goal is to reduce global warming,” wrote the Cornell researchers, who were led by Robert Howarth, a professor of ecology and environmental biology. “We do not intend that our study be used to justify the continued use of either oil or coal, but rather to demonstrate that substituting shale gas for these other fossil fuels may not have the desired effect of mitigating climate warming.”

The study also stated that the greenhouse gas “footprint” for shale gas “is greater than that for conventional gas or oil when viewed on any time horizon, but particularly so over 20 years. Compared to coal, the footprint of shale gas is at least 20 percent greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.”

Fracking also raises concerns about groundwater contamination, an issue particularly controversial in states such as Pennsylvania, Texas and Arkansas that have shale rock formations.

Some experts said the Cornell study overstates the potency of methane as a greenhouse gas, and they questioned the researchers’ emphasis on the climatic impact over a 20-year period. “Methane only lasts in the atmosphere for about a decade, CO2 remains in the atmosphere for about a century,” Christopher Van Atten of the consulting firm M.J. Bradley & Associates told The Hill. “By focusing on the shorter timeframe, they show a greater impact from the shorter-lived chemical.”

The Energy Department estimates that shale gas will account for 45 percent of U.S. gas supplies in 2035, up from 14 percent in 2009.

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