Natural gas, with its reputation as a crucial linchpin in the effort to wean the nation off dirtier fossil fuels and reduce global warming, may not be as clean overall as its proponents say.
Even as natural gas production in the United States increases and Washington gives it a warm embrace as a crucial component of the nation's energy future, two studies try to poke holes in natural gas' clean-and-green reputation.
They suggest that the rush to develop the nation's vast, unconventional sources of natural gas is logistically impractical and likely to do more to heat up the planet than mining and burning coal.
The problem, the studies suggest, is that planet-warming methane, the chief component of natural gas, is escaping into the atmosphere -- puffing out from shale gas wells, intentionally vented or flared, or seeping from loose pipe fittings along gas distribution lines -- in far larger quantities than previously thought.
This offsets natural gas' most important advantage as an energy source: it burns more cleanly than other fossil fuels and releases lower carbon dioxide emissions.
"The old dogma of natural gas being better than coal in terms of greenhouse gas emissions gets stated over and over without qualification," said Robert Howarth, a professor of ecology and environmental biology at Cornell University and the lead author of one of the studies. Mr. Howarth said his analysis, which looked specifically at methane leakage rates in unconventional shale gas development, was among the first of its kind and that much more research was needed.
An opponent of growing gas development in western New York, he said, "I think this is just the beginning of the story, and before governments and the industry push ahead on gas development, at the very least we ought to do a better job of making measurements."
The findings, published recently, are certain to stir debate. For much of the last decade, the natural gas industry has carefully cultivated a green reputation, often with the help of environmental groups which embrace the resource as a clean-burning "bridge fuel" to a renewable-energy future. The industry argues that it has vastly reduced the amount of fugitive methane with new technologies and upgraded pipe fittings and other equipment.
Natural gas is the principal source of heat in half of American households. Advocates including former oil tycoon T. Boone Pickens have also long sought to promote it as a substitute for coal in electricity generation or gasoline in a new generation of natural gas cars. And the development of new ways to tap reserves of natural gas means production is likely to increase sharply.
The ability to pull natural gas economically from previously inaccessible formations deep underground has made huge quantities of the resource available in wide areas of the country, from Texas, Louisiana, Pennsylvania, New York, Wyoming and Colorado.
Such unconventional gas production accounts for roughly nearly a quarter of total production in the United States, according to the latest figures from the Energy Information Administration. That is expected to reach 45 percent by 2035.
Methane leaks have long been a concern because although it dissipates in the atmosphere more quickly than carbon dioxide, it is far more efficient at trapping heat. Recent evidence has suggested that the amount of leakage has been underestimated. A report in January by the nonprofit journalism organization ProPublica, for example, noted that the Environmental Protection Agency had recently doubled its estimates for the amount of methane that is vented or lost from natural gas distribution lines.
Chris Tucker, a spokesman for Energy in Depth, a coalition of independent oil and natural gas producers, dismissed Mr. Howarth as an opponent of hydraulic fracturing or "fracking," a practice associated with unconventional gas development. It involves high-pressure injection of water, sand, and chemicals deep underground to break up shale formations and release gas deposits.
Mr. Howarth said his credentials as a scientist spoke for themselves.
He included methane losses associated with flow-back and drill-out processes in hydraulic fracturing and other unconventional gas drilling techniques. The study combined these emissions with studies of other methane losses along the processing and distribution cycle to arrive at an estimated total methane loss range from 3.6 to 7.9 percent for the shale gas industry.
The researchers also include a recent study from the Goddard Institute for Space Studies at NASA suggesting that an interaction of methane with certain aerosol particles significantly amplifies methane's greenhouse gas effects, particularly over a 20-year time horizon.
When all is factored together, Mr. Howarth and his colleagues conclude that the greenhouse-gas footprint of shale gas can be as much as 20 percent greater than, and perhaps twice as high as, coal per unit of energy.
David Hawkins, director of climate programs with the Natural Resources Defense Council, said regulators could nudge drillers to capture more of the fugitive methane, but it's often more economical for industry to simply let it escape.
He said too little is known about the quantity of methane being lost and vented and that studies like Mr. Howarth's, although needed, rely on too slim a data set to be considered the final word.