A new study challenges our understanding of natural gas as a clean fuel, and raises new questions about the U.S. energy boom.
Sure, natural gas (or methane, its main component) burns with less pollution than coal, but release it directly to the atmosphere and it is a highly potent greenhouse gas–at least 25 times worse than carbon dioxide. (See related, “Methane: Good Gas, Bad Gas.”) The study indicates that far more methane is escaping than previously thought from both oil and gas operations and from livestock facilities.
The findings have especially great significance because the U.S. Environmental Protection Agency (EPA) has been grappling with the uncertainty over how much methane is escaping from the nation’s growing shale gas production. (See related “Quiz: What You Don’t Know About Natural Gas.”)
Faced with an onslaught of criticism from the industry that it had overestimated the fugitive emissions, the EPA this year incorporated the industry’s own studies to downgrade its estimate of U.S. methane emissions by 25 to 30 percent. But the study published Monday in the Proceedings of the National Academy of Sciences (PNAS), based on the relatively new data gathered from monitoring stations on tall towers and on aircraft, indicate that U.S. methane emissions instead are actually 50 percent higher than EPA has calculated. (See related blog post: “A Move to Capture ‘Fugitive’ Natural Gas Emissions.”)
The study only captures emissions for two years that were early in the shale boom: 2007 and 2008, so it does not provide enough data to see whether methane emissions are increasing over time as gas production ramps up. It also provides no answer on whether the latest estimated trend in overall U.S. greenhouse gas–that the emissions have been falling as natural gas has replaced coal as an electricity fuel–is incorrect. (See related, “U.S. Energy-Related Carbon Emissions Fall to an 18-Year Low” and “Natural Gas Nation: EIA Sees Future Shaped By Fracking.”)
But in an informative Dot Earth blog post by The New York Times’ Andy Revkin, who has been following the fugitive methane issue for years, the authors say that an analysis of more recent years’ data is in the works. The years 2007 and 2008 were the first time that data for such detailed analysis was available from the U.S. National Oceanic and Atmospheric (NOAA) and U.S. Department of Energy (DOE) cooperative air sampling network. (Both NOAA and DOE’s Lawrence Berkeley Laboratory, as well as the European Commission Joint Research Center and four other academic institutions, collaborated in the paper.)
“The beauty of the approach we’re using is that, because we’re taking measurements in the atmosphere, which carry with them a signature of everything that happened upwind, we get a very strong number on what that total should be,” said co-author Anna M. Michalak, of the Carnegie Institution for Science. “This paper provides the most solid and the most detailed estimate to date of total U.S. methane emissions.”
Lead author Scot M. Miller, a doctoral student in Earth and Planetary Sciences at Harvard, called it a “top-down” approach that provides an important check on the “bottom-up” approach of EPA and other regulatory agencies around the world, which perform a kind of accounting to estimate emissions, based on assumptions on the amount of gases that escape from various operations.
He said it was especially telling was the data showing that the highest emissions, and greatest discrepancy with EPA estimates, were over Texas and Oklahoma, two of the biggest states for natural gas production. “It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions,” said Miller. The researchers did geostatistical analysis, using data on population density, economic activity, as well as weather patterns, and concluded that natural gas and oil operations are a far more likely source of the excess emissions than, say, landfills, which also emit methane. Also, concentrations of propane, a tracer of fossil hydrocarbons, were much higher over those states.
Methane being released from livestock operations (both from the burping of ruminants like cows and from manure) are as much as double what is currently estimated by the EPA, and they may also be contributing to the emissions over Texas and Oklahoma, the paper said. But Marc Fischer, head of Berkeley Lab’s California Greenhouse Gas Emissions Measurement Project (CALGEM), said even if livestock emissions were ramped up several times higher than inventory estimates for the southwest, it wouldn’t be enough to cover the discrepancy the researchers saw. “That’s why it looks like oil and gas are likely responsible for a large part of the remainder,” he said.
The results concur with those in a separate paper Fischer co-authored earlier this year, in which researchers found that tall tower monitoring data showed California’s total methane emissions are 1.3 to 1.8 times higher than the current official inventory by the California Air Resources Board (CARB).
Miller and his co-authors said they didn’t have detailed enough information on the various sectors within the oil and gas industry to target the methane sources more specifically: Is the methane escaping from drilling sites? Or gas processing facilities? Or pipelines?
David Allen, director of the Center for Energy and Environmental Resources at the University of Texas at Austin, who was not involved in the new study, says that further regional measurements and analyses, and source-specific studies, are needed to identify how methane emissions might be reduced. (See related, “Natural Gas Stirs Hope and Fear in Pennsylvania,” and interactive, “Breaking Fuel From Rock.”) Allen is leading a research team, funded by the Environmental Defense Fund and nine natural gas producers, that is attempting to get a better handle on methane emissions in the industry. (See related, “For Natural Gas-Fueled Cars, A Long Road Looms Ahead,” The New Truck Stop: Filling Up With Natural Gas For the Long Haul,” “Trading Oil for Natural Gas in the Truck Lane.”)
Allen’s team published a paper in PNAS based on data from 190 production sites throughout the United States, including the first-ever direct measurements of some sources. It appeared to reach a hopeful conclusion, that methane control equipment being used at the well completion sites reduced emissions 99 percent. (See related blog, “Natural Gas Study: Allays Fears for Some, Inspires Hot Air From Others.”
But Allen noted in testimony at a Senate hearing earlier this month that the team also found emissions from certain types of pneumatic devices, which control devices such as valves on well sites, had emissions from 30 percent to several times higher than EPA estimates.
In an email, Allen said that the new study by Miller and colleagues makes “an important contribution by using a large number of measurements of ambient methane concentrations to estimate methane emissions to the atmosphere.” But he said more work is needed.
“Fossil fuel production and processing and animal husbandry are large and complex activities, with a large number of potential emission sources,” he said. “So, a logical follow-up question is which sources within these sectors are responsible for the emissions. Some emission sources may be more important than others.”
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