Livestock were the single largest source of methane gas emissions in the United States in 2004, releasing 70% more into the atmosphere than the oil and gas industry as estimated by other inventories, according to a new study by a team from Harvard University, JPL/CalTech and UC Irvine. The paper is published in the Journal of Geophysical Research: Atmospheres, a publication of the American Geophysical Union.
The team estimated methane emissions from North America with high spatial resolution by inversion of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations using the Goddard Earth Observing System Chemistry (GEOS-Chem) chemical transport model and its adjoint. They focused on summer 2004 when data from the Intercontinental Chemical Transport Experiment-North America (INTEX-A) aircraft campaign over the eastern US are available to validate the SCIAMACHY retrievals and evaluate the inversion.
From the INTEX-A data we identify and correct a water vapor-dependent bias in the SCIAMACHY data. We conduct an initial inversion of emissions on the horizontal grid of GEOS-Chem (1/2° × 2/3°) to identify correction tendencies relative to the Emission Database for Global Atmospheric Research (EDGAR) v4.2 emission inventory used as a priori. We then cluster these grid cells with a hierarchical algorithm to extract the maximum information from the SCIAMACHY observations. A 1000 cluster ensemble can be adequately constrained, providing ~100 km resolution across North America.—Wecht et al.
Among their findings:
The Hudson Bay Lowland wetlands source is 2.1 Tg a−1, lower than the a priori but consistent with other recent estimates.
Anthropogenic U.S. emissions are 30.1 ± 1.3 Tg a−1, compared to 25.8 Tg a−1 and 28.3 Tg a−1 in the EDGAR v4.2 and Environmental Protection Agency (EPA) inventories, respectively.
US livestock emissions are 40% greater than in these two inventories; no such discrepancy is apparent for overall US oil and gas emissions.
US livestock emissions are 70% greater than the oil and gas emissions, in contrast to the EDGAR v4.2 and EPA inventories where these two sources are of comparable magnitude.
There is little reason to suspect that the amount of methane released by livestock has changed since 2004 (before the SCIAMACHY instrument used to collect the data stopped working), said Wecht. Although oil and gas production has increased significantly since 2004, scientists don’t know how the increased production has affected methane emissions over the entire US or if methane emissions from the oil and gas industries have surpassed methane emissions from livestock.
Using a “bottom-up” approach, EPA estimated that, in 2004, 28.3 megatonnes (31 million tons) of methane were released into the atmosphere by human activities in the United States. The EPA estimates that 8.8 megatonnes (9.7 million tons) of methane came from livestock while 9.0 megatonnes (9.9 million tons) came from the oil and gas industries.
The satellite tool used in the new study directly measured the amount of gas released into the atmosphere, which is more accurate than estimating methane emissions using the “bottom-up” approach, according to the paper.
A new instrument, the TROPOspheric Monitoring Instrument (TROPOMI), is set to launch in 2015 and will map air pollution from space, including methane emissions.
Wecht, K. J., D. J. Jacob, C. Frankenberg, Z. Jiang, and D. R. Blake (2014) “Mapping of North American methane emissions with high spatial resolution by inversion of SCIAMACHY satellite data,” J. Geophys. Res. Atmos., 119, doi: 10.1002/2014JD021551