Infrastructure protectionMethane emissions in the U.S. exceed government estimates
Along with carbon dioxide, methane is one of the most important greenhouse gases in terms of its potential to raise global temperatures. It also encourages the formation of surface ozone in cities and affects other aspects of atmospheric chemistry. A new study finds that emissions of methane from fossil fuel extraction and refining activities in the South Central United States are nearly five times higher than previous estimates. The collaborative study indicates that in addition to fossil fuel extraction, animal husbandry is also a major contributor to the higher-than-expected methane emissions.
Emissions of methane from fossil fuel extraction and refining activities in the South Central United States are nearly five times higher than previous estimates, according to researchers at Harvard University and seven other institutions. Their study, published this week in Proceedings of the National Academy of Sciences (PNAS), also suggests that the contribution from livestock operations may be twice as high as previously thought.
A Harvard University release reports that methane, a potent greenhouse gas, is produced through natural gas production and distribution, cattle farming, landfills, coal mining, manure management, and other anthropogenic and natural sources, though human activities are thought to contribute approximately 60 percent of the total.
Overall, according to the new study, total methane emissions in the United States appear to be 1.5 times and 1.7 times higher than the amounts previously estimated by the U.S. Environmental Protection Agency (EPA) and the international Emissions Database for Global Atmospheric Research (EDGAR), respectively.
The difference lies in the methodology. The EPA and EDGAR use a bottom-up approach, calculating total emissions based on “emissions factors” — the amount of methane typically released per cow or per unit of coal or natural gas sold, for example. The new study takes a top-down approach, measuring what is actually present in the atmosphere and then using meteorological data and statistical analysis to trace it back to regional sources.
Generated by a large, multi-institutional team of researchers, the latest findings offer a comprehensive baseline for assessing policies designed to reduce greenhouse gases. They also point to a few areas where the assumptions built into recent emissions factors and estimated totals may be flawed.
“The bottom-up and top-down approaches give us very different answers about the level of methane gas emissions,” notes lead author Scot M. Miller, a doctoral student in Earth and Planetary Sciences through the Harvard Graduate School of Arts and Sciences. “Most strikingly, our results are higher by a factor of 2.7 over the South Central United States, which we know is a key region for fossil-fuel extraction and refining. It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions.”
