America’s methane emissions may be 1.5 times larger than the EPA’s current estimate, according to a new study. Researchers from eight different institutions, including Harvard University, arrived at that conclusion using a top-down methodology.Rather than measuring emissions from representative samples of known sources of methane—like livestock and oil and gas drilling—the authors analyzed air samples (quite literally a top-down approach). They found that America’s total methane emissions are significantly higher than the bottom-up approach had suggested, and that the south central United States is emitting more than double the methane previously believed.The top-down method is useful insofar as it shows that our understanding of our cumulative methane emissions may be incomplete. But unlike the bottom-up approach, researchers have to make inferences to identify the specific causes of this increase. Harvard has more:
NOAA and the U.S. Department of Energy collect observations of methane and other gases from the tops of telecommunications towers, typically about as tall as the Empire State Building, and during research flights. The team combined this data with meteorological models of the temperatures, winds, and movement of air masses from the same time period, and then used a statistical method known as geostatistical inverse modeling to essentially run the model backward and determine the methane’s origin.The team also compared these results with regional economic and demographic data, as well as other information that provided clues to the sources—for example, data on human populations, livestock populations, electricity production from power plants, oil and natural gas production, production from oil refineries, rice production, and coal production. In addition, they drew correlations between methane levels and other gases that were observed at the time. For example, a high correlation between levels of methane and propane in the South Central region suggests a significant role for fossil fuels there.
This is where it gets tricky. Some could look at this study and conclude that oil and gas drilling is emitting a lot more methane than we thought it was, simply because there’s a lot of drilling going on in a part of the country that’s emitting significantly more methane. But there’s plenty of agricultural sources of methane in the south central US as well, making it difficult to pinpoint which source is responsible for what increase.Ideally, a top-down approach like this one complements a bottom-up one; this study suggests that America’s big picture methane emissions are greater, but bottom-up approaches are our best bet for understanding why. The EPA has already announced that it’s reviewing this latest study, and we’ll be watching to see what the rest of the academic community has to say about it as well.But this points most of all to the degree of uncertainty that remains about some of the most basic facts about human impact on the climate. Sweeping policy changes based on such shaky scientific ground are going to be as politically toxic as they are unworkable.[Oil rig image courtesy of Shutterstock]