WASHINGTON -- Emissions of the heat-trapping gas methane are considerably greater than government estimates, a problem significantly fueled by leaks from the U.S. natural gas system, according to a study released Thursday.
The leak rate probably is large enough to negate the value of switching buses and trucks from diesel to natural gas, as governments and private companies have done to help slow the warming of the planet, the scientists concluded.
But even with the current leaks, burning natural gas instead of coal is producing less heat-trapping gas and will slow the rate of climate change over 100 years, the researchers said in their study, published in the current issue of the journal Science.
They also determined that the controversial practice of hydraulic fracturing, or fracking, for gas trapped in rock formations is "unlikely to be a dominant contributor" to total methane emissions.
"If natural gas is to be a 'bridge' to a more sustainable energy future, it is a bridge that must be traversed carefully: Diligence will be required to ensure that leakage rates are low enough to achieve sustainability goals," the team wrote.
Fortunately, they added, that task is achievable, because a large share of the leaked gas comes from a tiny number of "super-emitters," devices or other parts of the gas and oil system that are allowing disproportionate emissions.
The scientists from universities, national laboratories and government agencies reviewed more than 200 studies with conflicting methodologies in what a news release called the first comprehensive look at North American methane emissions.
They considered studies that totaled leaks directly from equipment, as well as research that measured the gas in the atmosphere, using aircraft and towers. The research was led by Adam R. Brandt, an assistant professor in Stanford University's Department of Energy Resources Engineering.
While methane is much less common in the atmosphere than carbon dioxide, the primary contributor to global warming, it is also much more effective at trapping heat -- perhaps 30 times more potent, the researchers said.
Natural gas is mainly composed of methane. As it is extracted from the earth, processed and transported through pipes to consumers, about 1.5 percent of it escapes, the researchers concluded. Some gas is released intentionally by drillers. Oil exploration also releases methane.
The study concluded that estimates of methane in the atmosphere by the Environmental Protection Agency, begun in the 1990s, are probably 50 percent too low, for a variety of reasons. In a phone news conference, the researchers said they are working with the EPA to reconcile the differences.
"We are in discussion with EPA as scientists who have tried to synthesize the available evidence, and they are very interested in hearing" the researchers' views, said National Renewable Energy Laboratory senior scientist Garvin Heath, one of the study authors.
The researchers could not pinpoint the natural gas system's leakage rate, though they estimated it at 7 billion to 21 billion tons per year. They did not offer an estimate of how much the transition from coal to natural gas in sectors such as electricity generation is slowing global warming.
Livestock also emit substantial -- and probably undercounted -- amounts of methane, and more comes from natural openings in the earth above gas and coal deposits, the researchers noted.
The conclusion about fracking is based on methane totals before the boom in that method of extracting gas, and on counts of methane at fracking sites. "This is a lot methane, it's not trivial, but it's not considered a main contributor," Mr. Brandt said at the news conference. "The math just doesn't work out."
The diesel-to-natural gas switch in heavy vehicles probably has not been worthwhile so far in slowing climate change, even if it has helped with air quality, the researchers said. "Fueling trucks and buses with natural gas may help local air quality and reduce oil imports, but it is not likely to reduce greenhouse gas emissions" Mr. Brandt said in a news release.