Natural gas from shale deposits such as the Marcellus has a bigger greenhouse gas footprint than coal, according to a study by researchers at Cornell University.

The peer-reviewed study concludes, “The large green house gas footprint of shale gas undercuts the logic of its use as a bridging fuel over coming decades, if the goal is to reduce global warming.”.

That could have far-ranging consequences, especially in Pennsylvania, where most of the Marcellus Shale is located and where state legislators are actively promoting the conversion of transportation vehicles from diesel to natural gas.

Industry groups immediately mocked the study as flawed and slanted, saying it was “an annual rite of spring” in Ithaca, N.Y., along with finals and farmer’s markets.

Former Secretary of Environmental Protection John Hanger was skeptical. An earlier version, he said, contained “extreme assumptions, controversial data and methods.”

While natural gas burns much more cleanly than oil and coal — a point the study does not dispute — the process of extracting it from shale contributes more harmful greenhouse gases, the researchers say, than its clean-burning benefits.

That’s because much of the shale gas — perhaps as much as 7.9 percent of the well’s total lifetime production, they say — is vented directly into the atmosphere during production.

That’s important because methane, the study says, has a global warming potential far greater than carbon dioxide.

But the study does have limitations, acknowledged by its authors.

One of the study’s major limitations is as clear as the night sky, which in the northern tier is often lit with a bright flickering glow as if a local cow barn were ablaze.

That’s a Marcellus well being “flared.”

In the very early stages of the well’s production, the first rush of gas is often set afire, or “flared,” until the fluids that flow up with it diminish.

That early flowback period is when the study says a majority of the methane is vented to the atmosphere.

But the study is based on wells that were not flared.

Lead author Robert Howarth agreed if the wells are flared, the results of the study change.

“It’s very difficult to figure out how often they do that rather than simply letting it vent,” he said.

“The industry is resisting reporting it,” he said, and an attempt by the EPA to determine the frequency of flaring guessed between 15 and 50 percent of wells are probably flared.

He said sources from northern Pennsylvania have indicated to him that wells usually are not flared in the Marcellus because it draws attention and complaints.

But Howarth freely admits the evidence is sketchy.

“We don’t pretend our study is the final study,” Howarth told The Patriot-News, but it is the first peer-reviewed study to look at greenhouse gas footprint of shale gas.

Howarth also explained — as the study does — “The data available is far from perfect,” in some instances culled from obscure Power Point presentations.

“It’s highly likely the conclusions will be refined,” he said.

The study also focuses only on the comparative global warming potential of the fuels, not their impacts on human health.

That’s what concerns John Hanger.

The burning of coal spews mercury to the extent that “one in six women have elevated levels of mercury to the point IQ in babies would be reduced,” said Hanger.

Hanger said coal also spews other noxious gasses and soot to the point the EPA estimates “36,000 people die each year from breathing power plant emissions from burning coal.”

The human health benefits of transitioning vehicles — especially diesel trucks — to natural gas are also undisputed, said Jan Jarrett, president of the environmental group PennFuture.

Jarrett is a proponent of the House Republicans’ plan to offer incentives to convert large vehicles to natural gas in Pennsylvania.

“It’s far cleaner,” she said. Particulates from diesel engines “are the main air pollution problem in the Harrisburg area because it is a transportation cross-roads.”

She noted a campaign to improve air quality in Carlisle area was started by physicians concerned about human health.

Jarrett said she cannot evaluate the science of the Cornell study, but its focus on global warming doesn’t change her position on natural gas for human health one bit.

Howarth does not dispute any of that.

“In other ways, I’d say coal is worse without question,” he said. “We don’t want our study used as an excuse to burn more coal.”

But if there’s going to be national policy based on global climate change calculations and the assumption that natural gas is always better, Howarth said, “we need to think through what the ramifications might be.”

“Our goal is to focus more attention on this and suggest one proceed with great caution.”

The study will be published Thursday in the journal "Climatic Change Letters."