Interpreting contemporary trends in atmospheric methane

Abstract

Atmospheric methane plays a major role in controlling climate, yet contemporary methane trends (1982-2017) have defied explanation with numerous, often conflicting, hypotheses proposed in the literature. Specifically, atmospheric observations of methane from 1982 to 2017 have exhibited periods of both increasing concentrations (from 1982 to 2000 and from 2007 to 2017) and stabilization (from 2000 to 2007). Explanations for the increases and stabilization have invoked changes in tropical wetlands, livestock, fossil fuels, biomass burning, and the methane sink. Contradictions in these hypotheses arise because our current observational network cannot unambiguously link recent methane variations to specific sources. This raises some fundamental questions: (i) What do we know about sources, sinks, and underlying processes driving observed trends in atmospheric methane? (ii) How will global methane respond to changes in anthropogenic emissions? And (iii), What future observations could help resolve changes in the methane budget? To address these questions, we discuss potential drivers of atmospheric methane abundances over the last four decades in light of various observational constraints as well as process-based knowledge. While uncertainties in the methane budget exist, they should not detract from the potential of methane emissions mitigation strategies. We show that net-zero cost emission reductions can lead to a declining atmospheric burden, but can take three decades to stabilize. Moving forward, we make recommendations for observations to better constrain contemporary trends in atmospheric methane and to provide mitigation support.

Keywords: greenhouse gas mitigation; methane trends; tropospheric oxidative capacity.

Fig. 1.

Observations of atmospheric methane over the past 2,000 y. Shown are Law Dome ice core record (blue) (4) and direct atmospheric observations from the South Pole (black, deseasonalized in gray) (6). Red line illustrates if the 7-y stabilization period is removed.

Fig. 2.

Constraints on atmospheric methane over the past 40 y. Left column illustrates atmospheric constraints: methane (6), ethane (18), ${\delta }^{13}$C-${\mathrm{C}\mathrm{H}}_4$ (ftp://aftp.cmdl.noaa.gov/data/ and www.iup.uni-heidelberg.de/institut/forschung/groups/kk/en/) (36, 37), and OH sink inferred from methyl chloroform (27, 28, 38), assuming a global methane source of 550 Tg/y. Black lines in the ethane panel are taken directly from Hausmann et al. (18). Right column illustrates deseasonalized process and inventory representations for the same time period: total anthropogenic (35), anthropogenic disaggregated to three most important anthropogenic sectors, wetland models (30, 39, 40), and fire emission estimates (41). The stabilization period is indicated in both columns by the vertical gray area.

Fig. 3.

Projections of atmospheric methane over the next 30 y. (Left) The methane emissions from 1980 to 2050 under four different emission scenarios: continued growth in anthropogenic emissions (case A, red), stabilization of emissions in 2012 (case B, blue), and an emission decrease over 10 y (case C, orange) or instantaneously (case D, green). Anthropogenic emissions from 1980 to 2012 are from Emission Database for Global Atmospheric Research (EDGAR) v4.3 (black). (Center) The simulated methane concentrations under the four emission scenarios with interactive OH (solid line) and a constant OH concentration (dashed line; no OH or CO feedback). Colored vertical lines to the right of the panel show the range of the ${\mathrm{C}\mathrm{H}}_4$ concentrations in 2050 for interactive and constant OH. (Right) The OH anomaly due to changes in methane and CO. The stabilization period in all panels is indicated by the vertical gray shading.