"It's just an impossible number," he said. Cathles' estimates that it is more likely less than 2 percent of production.

Still, even if 10 percent of methane is being released, conversion to natural gas will still mean an almost 40 percent reduction in greenhouse gases beginning in a little more than a century and extending out for centuries to come.

Howarth and Ingraffea analyzed the impacts of natural gas compared with coal over the next two decades and one century.

"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," they wrote in their paper published in the journal Climatic Change in April 2011.

Cathles said that 20 years is too short of a horizon to analyze the impact of greenhouse gases, which have been building up for more than a century. Over time, power production will need to shift to wind, solar and nuclear production.

"If we can encourage steps like the conversion to gas, we'll buy ourselves time to convert," he said.

Cathles' paper was published in the journal Geochemistry, Geophysics, Geosystems on June 19. It was released to the public by Cornell's public affairs office today.

Howarth and Ingraffea did not immediately respond to e-mail messages seeking comment on Cathles' study.

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