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You wouldn't know it from the news, but there's a major fossil-fuelspill ongoing in the North Sea off the coast of Scotland. A leakfrom a gas platform operated by the French energy company Total SAwas first detected on March 25 and has been spilling around 7million cu. ft. (200,000 cu m) of natural gas every day since. Ofcourse gas, unlike oil, doesn't have a devastating — or visual— effect on the marine environment, which is one reason theElgin gas field, where the spill is taking place, hasn't become asinfamous as the Deepwater Horizon site in the Gulf of Mexico. Butthe leak is a disaster for the climate all the same; natural gas ismostly made up of methane, a greenhouse gas that has 25 times thewarming power of carbon dioxide. Engineers working for Totalestimate that it may take half a year to shut the leak, and if allof the methane released in that time reaches the atmosphere, thespill would approximate the annual global warming impact of putting300,000 new cars on the road. The Total leak is a reminder that natural gas — in the wrongplace — can do very real damage to the environment, even if itdoes so invisibly. That might sound surprising because natural gashas been hyped as the clean fossil fuel, a replacement for coalpower that's better for the atmosphere. That and the vast newreserves of shale gas found in states like Montana and Pennsylvaniahave kept the price of natural gas low — the lowest it's beenin over a decade in the U.S. — and led to something of a boomtime for the industry, enabling utilities to replace aging,polluting coal plants. "We, it turns out, are the Saudi Arabia ofnatural gas," President Barack Obama said in a speech in January."And developing it could power our cars and our homes and ourfactories in a cleaner and cheaper way." (MORE: Natural Gas Can Save the Climate? Not Exactly) The clean, green claim is more than just hype. Natural gas producesabout half as much carbon dioxide as coal does when burned. Butit's not quite that simple. Some amount of methane will always leakaway during the production of natural gas — particularlyduring the process of drilling or transporting via pipelines. Andthat means more warming. If enough methane leaks, natural gas losesits clean reputation — and the billions of dollars of newnatural-gas infrastructure now being built could end up making theclimate worse, not better. "We have to make sure that the climatebenefits are real and continuous," says Steven Hamburg, theEnvironmental Defense Fund's (EDF) chief scientist, who has beenstudying methane-leakage rates. That's what makes a new study published in the April 9 Proceedings of the National Academy of Sciences ( PNAS ) so important — and so controversial. Researchers fromPrinceton University, Duke University, the Rochester Institute ofTechnology and EDF have distilled some of the existing data onmethane leakage and calculated just how climate-friendlynatural-gas electricity and natural-gas-powered vehicles are. Andthe answer is: it depends. On the whole, for both power plants andvehicles, natural gas beats conventional energy sources only ifleakage rates are very low. The redline for power plants seems tobe a leakage rate of 3.2%. As long as the share of methane thatescapes remains below that level, natural gas beats coal. And sincea 2009 estimate from the Environmental Protection Agency (EPA) pegsthe average leakage rate at 2.4%, natural gas does seem to come outcomfortably ahead — especially when you factor in the muchlower levels of mercury and other traditional air pollutantsemitted when power plants burn gas as compared with coal. (PHOTOS: Terribly Beautiful: Industrial Pollution Seen from Above) But if we replace gasoline cars with ones powered by natural gas inan effort to reduce oil consumption — as some like natural-gastycoon T. Boone Pickens have urged — methane leakage wouldneed to be kept below 1.6% to provide a meaningful climate benefit.For heavy-duty trucks run on diesel — a fuel that has asmaller carbon footprint than gasoline — methane leakage wouldhave to be reduced by two-thirds below the EPA estimate of 2.4% toprovide a climate benefit. "It's a much steeper hill to climb,"says Hamburg. There are, of course, many caveats in all this data. Even the EPAhas admitted that its 2009 estimate of methane leakages is likelyoutdated and thus inaccurate, as the trade group America's NaturalGas Alliance pointed out in a response to the PNAS paper. But there's no guarantee that the actual methane-leakagerate is lower than the EPA estimate; it could be higher. In acontroversial 2011 study, Cornell scientist Robert Howarth peggedthe leakage rate at between 2.2% and 3.8%, while another studypublished this year in the Journal of Geophysical Research estimated that methane-leakage rates at a field in Colorado werecloser to 4%, which would make natural gas worse than coal, atleast over the short term. (While methane has a stronger warmingeffect than CO2, carbon dioxide lingers in the atmosphere farlonger than methane.) "We can't say, and I don't think anyone cansay, 'Here is the absolute truth about natural gas compared toother fuels,' because the emissions data is highly uncertain," saysRamon Alvarez, a senior scientist at EDF and a lead author on the PNAS study. "At this point, we just have to go with the bestinformation we have." (MORE: Frack: Is Shale Natural Gas Worse for the Climate ThanCoal?) That's why we need better information — and indeed, EDF isworking with a number of industry partners to get a much moreprecise figure on just how much methane is escaping during theproduction of natural gas, especially unconventional shale gas.(That sets EDF apart from a number of other environmental groups— including the Sierra Club — that are waging politicalwar on the shale-gas industry.) America may indeed be the SaudiArabia of natural gas, and air-pollution regulations and othereconomic factors will almost certainly speed the shift fromcoal-fired power to natural-gas plants. But it would be a terriblemistake to make that transition without accounting for the risks tothe climate — especially since reducing methane leaks is a loteasier than, say, taking the carbon out of coal. Just becausemethane is invisible doesn't mean it's harmless. MORE: The Future Of Oil MORE: Could Shale Gas Power the World?. 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