Long-term Decline of Global Atmospheric Ethane Concentrations and Implications for Methane
I.J. Simpson1, M.P.S. Andersen2, S. Meinardi1, L. Bruhwiler3, N.J. Blake1, D. Helmig4 and D.R. Blake1
1University of California at Irvine, Department of Chemistry, Irvine, CA 92697; 510-642-5882, E-mail: firstname.lastname@example.org
2University of California at Irvine and Jet Propulsion Laboratory, Department of Chemistry, Irvine, CA 92697
3NOAA Earth System Research Laboratory, Boulder, CO 80305
4Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309
Methane (CH4) and ethane (C2H6) are the most abundant hydrocarbons in the remote atmosphere. Both are precursors to tropospheric ozone and CH4 is a potent greenhouse gas. This work presents the longest continual record of global atmospheric CH4 and C2H6 levels. Their global trends have shown a remarkably strong correlation over the past 25 years, both in terms of long-term trends and short-term anomalies (Figure 1). The global CH4 growth rate has slowed considerably, from strong growth of ~1% yr-1 in the late 1970s and early 1980s to near-zero growth by the 2000s, with renewed growth of up to 0.4% yr-1 beginning in 2006. The global C2H6 concentration declined by 21% over a similar period, from 791 ± 19 pptv in 1986 to 625 ± 12 pptv in 2010. Based on simulations using the TM5 atmospheric tracer transport model, ethane's long-term global decline is attributed to decreasing fugitive emissions from ethane's fossil fuel source — most likely reduced venting and flaring of natural gas in oil fields. Because ethane's fossil fuel source is shared with CH4, and because CH4 and C2H6 are emitted from fossil fuel sources in characteristic emission ratios, we used our long-term C2H6 record to quantitatively investigate methane's slowing growth rate. Reduced fugitive fossil fuel emissions of CH4 were found to account for at least 10 ‒ 21 Tg yr-1 (30 ‒ 70%) of the decrease in methane's global emissions, significantly contributing to methane's slowing growth rate since the mid-1980s. This research helps to clarify conflicting findings from recent studies (Aydin et al., 2011; Kai et al., 2011), which have disagreed on whether reduced fossil fuel or microbial emissions caused methane's global growth rate to slow.