Some U.S. Methane Sources May Be Larger than Previously Thought

Inventories underestimate emissions from fossil fuel industry and animal husbandry, study finds.

dan-mike-miller-anthropogenic-methane-sources-pnas-large.jpg — Image used with permission from Miller, S., *et al.* “Anthropogenic emissions of methane in the United States,” *Proc. Natl. Acad. Sci. USA* **110**(50), 20018–20022 (2013).

The Science

Methane (CH₄) is the second most prevalent greenhouse gas emitted in the United States from anthropogenic (human) activities. Current estimates for total CH₄ emissions in the U.S. vary significantly, and estimates of individual source sectors are even more uncertain. Effective national and state greenhouse gas reduction strategies may be difficult to develop without appropriate estimates of CH₄ emissions from these sectors. To help obtain such data, researchers have estimated the spatial distribution of anthropogenic CH₄ sources in the United States by combining comprehensive atmospheric CH₄ observations, extensive spatial datasets, and a high-resolution atmospheric transport model.

The Impact

Investigators concluded that CH₄ emissions associated with both the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories.

Summary

Atmospheric CH₄ observations from eddy covariance towers and aircraft, along with results from a high-resolution atmospheric transport model, were compared to inventories from the U.S. Environmental Protection Agency and the Emissions Database for Global Atmospheric Research (EDGAR). The study indicates that these inventories underestimate CH₄ emissions nationally by a factor of ∼1.5 to ∼1.7, respectively, and that emissions due to ruminant animals (livestock) and manure are up to twice the magnitude of existing inventories. The discrepancy in CH₄ estimates is particularly pronounced in the south-central United States, where total emissions are ∼2.7 times greater than in most inventories and account for ~24% of national emissions. The spatial patterns of emission fluxes and observed CH₄ -propane correlations indicate that fossil fuel extraction and refining are major contributors (~45%) in the south-central United States. This result suggests that regional CH₄ emissions due to fossil fuel extraction and processing could be nearly five times larger than in EDGAR, the most comprehensive global CH₄ inventory.

Contact

Scot M. Miller
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
scot.m.miller@gmail.com

Funding

Data were obtained from the Atmospheric Radiation Measurement (ARM) program sponsored by the Climate and Environmental Sciences Division of the Office of Biological and Environmental Research within the U.S. Department of Energy’s (DOE) Office of Science. Authors acknowledge the funding support of the National Basic Research Program (2013CB955804), DOE’s Atmospheric System Research (ER65319), and National Science Foundation (1118325).

Publications

Miller, S., et al. “Anthropogenic emissions of methane in the United States,” Proc. Natl. Acad. Sci. USA 110 (50), 20018–20022 (2013). [DOI: 10.1073/pnas.1314392110].