Long-term decline of global atmospheric ethane concentrations and implications for methane

Abstract

After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere’s oxidative capacity through its reaction with the hydroxyl radical, ethane’s primary atmospheric sink1,2,3. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane’s fossil fuel source—most probably decreased venting and flaring of natural gas in oil fields—rather than a decline in its other major sources, biofuel use and biomass burning. Ethane’s major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane’s atmospheric growth rate to slow4,5. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10–21 teragrams per year (30–70 per cent) of the decrease in methane’s global emissions, significantly contributing to methane’s slowing atmospheric growth rate since the mid-1980s.

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Figure 1: Individual air sampling locations for the UCI global trace gas monitoring network.
Figure 2: Latitudinal distribution of ethane mixing ratios from 1984–2010.
Figure 3: Simulated and observed ethane mixing ratios from 2000 to 2010.
Figure 4: Running global averages of ethane mixing ratios and methane growth rate.

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Acknowledgements

This research was funded by NASA (grant NAG5-8935), with contributions from the Gary Comer Abrupt Climate Change Fellowship. We acknowledge discussions with many colleagues, especially M. Aydin and C. Wiedinmyer. We thank colleagues at the Norfolk Island Bureau of Meteorology and the NOAA research stations in Samoa and Barrow for sample collection; the UCI team for sample collection and analysis, especially B. Chisholm, R. Day, G. Liu, B. Love and M. McEachern; and K. Masarie for work with the NOAA/INSTAAR data. M.P.S.A. is supported at JPL by an appointment to the NASA Postdoctoral Program, administered by Oak Ridge Associated Universities through a contract with NASA.

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I.J.S. was responsible for data quality assurance, global averaging and emission calculations and manuscript preparation. M.P.S.A. prepared the figures and improved the manuscript. S.M. was responsible for sample analysis and calibration, and data quality assurance. L.B. did the ethane modelling. N.J.B. improved the manuscript. D.H. made the NOAA/INSTAAR measurements and improved the manuscript. F.S.R. was responsible for study design and data quality assurance. D.R.B. was responsible for study design and data quality assurance, and improved the manuscript.

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Correspondence to Isobel J. Simpson.

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The authors declare no competing financial interests.

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This file contains a Supplementary Discussion, Supplementary References, Supplementary Table 1 and Supplementary Figures 1-3. (PDF 1039 kb)

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Simpson, I., Sulbaek Andersen, M., Meinardi, S. et al. Long-term decline of global atmospheric ethane concentrations and implications for methane. Nature 488, 490–494 (2012). https://doi.org/10.1038/nature11342

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