Gas likes to escape. That's bad news for the atmosphere when the gas in question is methane, the primary component in natural gas that is a much stronger greenhouse gas than carbon dioxide. But burning natural gas results in half the greenhouse gas pollution than coal, making it appealing as fuel in an era of combating climate change.



Thanks to a bonanza of natural gas liberated from deep shales by new techniques, the U.S. is burning more and more of the fuel—and considering using more natural gas in more places, such as fuel for trucking. But if the amount of methane escaping is too high, such widespread use might prove a disaster for climate change. And that's why a group of scientists set out to better estimate how much methane is escaping in the U.S. To do that, they surveyed more than 200 sets of field measurements and scientific papers from the past 20 years to learn whether increasing use of natural gas could prove a climate boon or bane.



"A relatively small leakage rate can have significant impacts," notes energy resource specialist Adam Brandt of Stanford University, who lead the team that reported its finding on February 14 in Science. That's because methane (or CH4) has more than 30 times the global warming impact of carbon dioxide (CO2) over 100 years (and its more than 80 times more powerful over 20 years, since methane disappears from the atmosphere far more quickly than CO2). It turns out that the Environmental Protection Agency (EPA) has been underestimating how much natural gas leaks based on its own surveys. According to the new research, all told, the U.S. leaks about 14 million metric tons of CH4, or 50 percent more than EPA had estimated.



That means the U.S. is likely leaking 2.25 percent, not 1.5 percent, of all the natural gas used, according to the new study, which helps explain why concentrations measured in the atmosphere keep rising above expected rates.



Not all of that leaking gas is coming from the controversial practice known as fracking—fracturing shale rock with water at high pressure and using horizontally drilled wells to access the freed natural gas. The bulk of the leaking natural gas is coming from either more conventional wells—like the natural gas allowed to leak by oil producers—or compressors, pipelines and other kit from the nation's infrastructure to move natural gas around the country. Roughly 1 million metric tons of all leakage seems to come from fracking-related activities. "This is a lot of methane; it's not trivial," Brandt adds. "But [fracking] doesn't appear to be the main contributor." What is? Other parts of the oil and natural gas industry, livestock, garbage dumps and even wetlands.



With methane released at that rate, the drawbacks of its use may outweigh its global warming benefits compared with other transportation fuels, such as gasoline or diesel. For example, burning compressed natural gas emits roughly 30 percent less CO2 than burning diesel but, if the new methane leakage estimates are true, the practice ends up being worse for climate change. Of course, buses that burn natural gas do cut down on soot and other forms of air pollution, reducing known deaths from inhaling such small particles. "There are all kinds of reasons we want to switch away from diesel buses," Brandt notes. "But from a climate change perspective, it's not a big win," because diesel engines burn the fuel efficiently, delivering more miles per liter of fuel than natural gas alternatives (though natural gas engines can be improved).



The key to sustainable use of natural gas is identifying and halting "super emitters"—valves or hatches that are stuck open, corroded holes in pipes or other major leaks—according to this study. The good news: natural gas producers want to shut down such leaks as well. "If they know where the leakage is, they want to go and fix it," Brandt notes. "It's costing them money."



The bad news: there's no easy way to find seepage, given that pipelines and distribution pipes cover millions of kilometers and at least 500,000 active wells and more than 100,000 abandoned and forgotten old wells exist across the U.S. "We need to develop ways to quickly and cheaply find these sources," Brandt says. "Just like a car breaking down, there are many ways a machine can fail."



Even worse, scientists still cannot tell why there is more methane than expected in the atmosphere—or where exactly it is coming from. "The exact contribution of natural gas to this overall excess is unknown at this point and there's not enough scientific evidence to figure out how much is contributed by the gas industry," Brandt notes.



In any case, the EPA has mandated that the U.S. natural-gas industry must employ leak-reducing technology—known as green completions—at all new wellheads starting in 2015. Such green completions would capture the natural gas that flows back with the water used to frack the well. "It will be reducing emissions on the order of 95 or even 99 percent," says methane researcher and co-author Robert Harriss of the Environmental Defense Fund, a pro-gas environmental group. "It looks very effective." Better measurements on site might also permit a better understanding of how to reduce the climate impacts of natural gas, but the gas industry has, for the most part, not cooperated with attempts by either the EPA or independent scientists.



In the end, natural gas will always be a fossil fuel that would need to be phased out (or its global warming pollution captured and stored) in the next few decades to avoid even worse climate change. And there are other challenges associated with fracking for natural gas besides climate change, from what to do with the wastewater produced to drinking water contamination and even improperly drilled wells that leak or explode and get out of control (a blowout). This study is part of an ongoing series of studies into the costs and benefits of natural gas funded by the George Mitchell Foundation; Mitchell (now deceased) was the petroleum engineer and gas magnate who first popularized hydraulically fracturing shale to release natural gas but also donated some of his fortune to sustainability research.



But, despite any challenges for natural gas, it remains a more sustainable option than at least one other fossil fuel when it comes to climate change. "We believe leakage rates are higher than official estimates," Brandt concludes, but that leakage rate, even if the worst is assumed, is "unlikely to be high enough to disadvantage the switch from coal to natural gas."