Methane is spewing from natural gas wells in southwestern Pennsylvania. Only seven wells, out of dozens observed by airborne sensors and hundreds in the region, spit out high levels of the potent greenhouse gas. However, those seven wells produced four to 20 percent of methane emissions in the areas around the wells.

Those seven wells released the methane during drilling, a stage in the natural gas extraction process previously not thought to result in serious greenhouse gas pollution. Horizontal drilling precedes hydraulic fracturing, or fracking, and allows fracking operation to reach larger areas of gas-rich shale rock.

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"These findings present a possible weakness in the current methods to inventory methane emissions..." said study co-author Paul Shepson, professor of analytical and atmospheric chemistry at Purdue University, in a press release.

"This small fraction of the total number of wells was contributing a much larger portion of the total emissions in the area, and the emissions for this stage were not represented in the current inventories."

Methane pollution has serious climate consequences. Methane, a major ingredient in natural gas, traps 28 to 34 times more heat than carbon dioxide over a century-long timeline.

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The actual methane emissions during drilling from the seven wells were 100 to 1,000 times higher than industry estimates. Airplanes measured the plumes of methane above the wells, which gave Sherson's team a top-down view of pollution sources.

The Proceedings of the National Academy of Sciences published the results.

Where these seven wells malfunctioning or leaking? Or do all wells spew methane during drilling? Sherson couldn't comment on that, he told Discovery News.

“During our study, we quantified emissions from several high-emitting wells,” Sherson said. “After the fact, we determined from our analysis that these were all in the drilling phase.”

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Keeping an eye in the sky with airborne sensors could bolster weaknesses in current methods of monitoring methane release.

“The top-down approach clearly represents an important complementary method that could be added to better define the impacts of shale gas development,” Sherson said in the press release.