Its mission: to fly over the busy natural-gas drilling operations of northeastern Pennsylvania so a pair of scientists could measure how much of the stuff was leaking into the atmosphere.

In particular, the researchers were interested in the prime component of natural gas, an odorless substance called methane that gets much of the blame for global warming.

"This is what we're going to fly today," said atmospheric scientist Anna Karion, indicating a zigzag pattern on her iPad map, covering an area that measured 50 miles by 80 miles.

The red line crisscrossed the heart of the Marcellus Shale region, hotbed of the practice called hydraulic fracturing. Methane leakage is the rare element of the "fracking" debate on which industry and environmental groups can find some agreement.

The less methane that escapes during all phases of gas production, the more there is to sell.

"This is just waste," said Karion, who works for the National Oceanic and Atmospheric Administration and also is affiliated with the University of Colorado.

The three-year study, led by Pennsylvania State University with the help of NOAA and the University of Colorado, is one of the most comprehensive looks at methane yet. Although it is agreed that methane emissions are worth curtailing, the exact scope of the problem remains unclear, said Penn State meteorology professor Ken Davis.

"You can't manage what you can't measure," said Davis, whose colleague Thomas Lauvaux is the project leader. "If we don't know quantitatively what the source of the problem is, it's hard to do something about it."

Over several weeks this spring, Karion and other scientists took more than 40 hours of measurements aboard the Twin Otter, a blue-striped, turboprop aircraft with slender, gas-sniffing rods protruding on one side.

The plane took off each day from Williamsport Regional Airport for four hours each time.

The team also has measured methane emissions from the ground, with equipment mounted in a sport-utility vehicle, and it plans to measure the gas for two years with sensors mounted on four communications towers in the region. The $1.8 million effort is funded by the U.S. Department of Energy.

Regulation is moving forward in the meantime. The U.S. Environmental Protection Agency is expected to announce a plan this summer to reduce methane emissions from the oil and gas industry generally, not just from fracking.

Carbon dioxide accounts for the vast majority of the greenhouse emissions implicated in climate change. When burned as fuel, natural gas produces less carbon dioxide than coal, so it is touted as a way to slow the rate of global warming.

But methane leaks change the equation. When it escapes to the air before getting burned in home stoves and furnaces, the gas has about 30 times the heat-trapping ability of carbon dioxide, as measured over a 100-year time span.

Estimates differ, but if more than 2 percent to 3 percent of the methane in natural gas leaks into the air, that negates its advantage over coal, said Drexel University engineer Peter DeCarlo. Previous short-term studies have suggested leak rates both above and below that threshold from natural-gas production in various parts of the country.

The single biggest source of methane is agriculture, according to the EPA. That includes "enteric fermentation" from cows - belching and flatulence - and manure from all livestock, for 36 percent of the total.

Natural gas and petroleum systems are next, at 29 percent, then microbial activity from landfills, at 18 percent, followed by a variety of other sources.

The Pennsylvania gas industry already has taken steps to reduce emissions. Chief among them is a "green completion," which involves filtering and capturing the methane from the water and other fluids that flow from a newly tapped well.

This technique is now required, though some operators were using it even before the rules took effect.

Dave Spigelmyer, president of the industry's Marcellus Shale Coalition, described his member companies as having a "laser focus on leveraging new advanced technologies aimed at protecting our environment."

Not all recovery technologies will pay for themselves, as measured by the added amount of gas that companies can sell.

Data collected by the EPA suggest that the measures taken to date are making a difference. Methane emissions from activities associated with hydraulic fracturing have dropped 79 percent since 2005, the agency says.

But those data, reported by the industry, are based partly on estimates, said Howard J. Feldman, senior director of regulatory and scientific affairs at the American Petroleum Institute. Gauging the exact amount of gas is tricky, as it can escape at multiple steps in the production and transmission process, he said.

"It's very different from measuring emissions from a smokestack," Feldman said. "It's not something that is routinely or easily quantified."

Hence the Penn State study. In the plane, the team used onboard analyzers to measure methane and other gases in real time, and also collected air samples in the suitcase-sized containers for later study in a lab.

The Washington-based Environmental Defense Fund also has coordinated research on the issue. It has taken some heat from fellow environmental groups for working with industry partners, along with others from academia, on a series of studies.

Steven Hamburg, the group's chief scientist, said cooperation with all parties was important in collecting the best data.

Once the scope of the problem becomes clearer, discussion about solutions will follow, whether it is replacing and upgrading leaky equipment or refining techniques, he said.

"The key here is that in fact these are tractable problems," Hamburg said. "We know how to do it."

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