Shale gas, produced by "hydraulic fracturing" or "fracking," could create as much as twice the greenhouse gasses as coal, according to a study soon to be published by Cornell University professors.

Over the past few years, the Washington D.C. consensus has been that shale gas is better for the environment than coal. President Obama has praised natural gas and given it partial credit in his proposed "clean energy standard."

But Cornell Prof. Robert Howarth argues in the new study that making natural gas available through "fracking" contributes more to global warming than conventional gas and coal over 20 years.

"Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the life-time of a well," a pre-publication version of the study (.pdf) -- obtained by The Hill -- said.

"These methane emissions are at least 30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur at the time wells are hydraulically fractured -- as methane escapes from flow-back return fluids -- and during drill out following the fracturing," the study added.

"The 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% greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years."

The Department of Energy predicts that shale gas will provide 45 percent of total U.S. gas production by 2035, an increase over the 14 percent it provided in 2009.

Gas is often promoted as a fuel that could bridge energy needs while renewable energy solutions are brought online in coming years.

"Recent innovations have given us the opportunity to tap large reserves -- perhaps a century's worth of reserves, a hundred years worth of reserves -- in the shale under our feet," Obama said during a recent speech at Georgetown University.

While Howarth argues that the methane emissions caused by fracking are enough to negate the advantages over coal, experts funded by the energy industry disagree.

"It needs to be understood as a study that has several key assumptions that are highly uncertain or based on limited data points," Christopher Van Atten, a vice president with the M.J. Bradley & Associates energy consulting firm, told The Hill.

"They focus some of their results on a 20 year period which is not particularly relevant in terms of climate change. Methane only lasts in the atmosphere for about a decade, co2 remains in the atmosphere for about a century. By focusing on the shorter timeframe, they show a greater impact from the shorter lived chemical," he added.

The study explains, however, that "the 20-year horizon is critical, given the need to reduce global warming in coming decades."

This is the second study this year that seems to indicate fracking does not hold up to its promises.

ProPublica revealed in January that an EPA analysis showed that methane levels from hydraulic fracturing were 9,000 times higher than expected.