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 America's energy future, two coming studies try to poke holes in the clean-and-green reputation of natural gas. 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 in far larger quantities than previously thought, with as much as 7.9 percent of it puffing out from shale gas wells, intentionally vented or flared, or seeping from loose pipe fittings along gas distribution lines. This offsets natural gas' most important advantage as an energy source, that it burns cleaner 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. 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.

"I think this is just the beginning of the story," said Howarth, who is an opponent of growing gas development in western New York, "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 are certain to stir debate. For much of the ast decade, the natural gas industry has carefully cultivated a green reputation, often with the help of environmental groups who 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.

Mark Whitley, a senior vice president of engineering and technology with Range Resources, a gas drilling company, said that the losses suggested by Howarth's study were simply too high.

Natural gas already is the principal source of heat in half of American households (including most in Utah). Advocates also have 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 Utah, Texas, Louisiana, Pennsylvania, New York, Wyoming and Colorado.

Such unconventional gas production accounts for roughly a quarter of total production in the U.S. That is expected to reach 45 percent by 2035.

But the cleanliness of natural gas is largely based on its lower carbon dioxide emissions when burned. It emits roughly half the amount of carbon dioxide as coal and about 30 percent that of oil.

Chris Tucker, a spokesman for Energy in Depth, a coalition of independent oil and natural gas producers, dismissed Howarth as an advocate who is opposed to hydraulic-fracturing or "fracking," a practice associated with unconventional gas development involving the high-pressure injection of water, sand and chemicals deep underground to break up shale formations and release gas deposits. Howarth said his credentials as a scientist spoke for themselves.

David Hawkins, the director of climate programs with the Natural Resources Defense Council, said that much could be done by regulators to nudge drillers to capture more of the fugitive methane, but that it often is more economical for industry to simply let it escape. Hawkins also said that too little was known about just how much methane was being lost and vented, and that studies like Howarth's relied on too slim a data set to be considered the final word.