Observations of atmospheric methane (CH 4 ) since the late 1970s and measurements of CH 4 trapped in ice and snow reveal a meteoric rise in concentration during much of the twentieth century. Since 1750, levels of atmospheric CH 4 have more than doubled to current globally averaged concentration near 1,800 ppb. During the late 1980s and 1990s, the CH 4 growth rate slowed substantially and was near or at zero between 1999 and 2006. There is no scientific consensus on the drivers of this slowdown. Here, we report measurements of the stable isotopic composition of atmospheric CH 4 ( 13 C/ 12 C and D/H) from a rare air archive dating from 1977 to 1998. Together with more modern records of isotopic atmospheric CH 4 , we performed a time-dependent retrieval of methane fluxes spanning 25 y (1984-2009) using a 3D chemical transport model. This inversion results in a 24 [18,27] Tg y −1 CH 4 increase in fugitive fossil fuel emissions since 1984 with most of this growth occurring after year 2000. This result is consistent with some bottom-up emissions inventories but not with recent estimates based on atmospheric ethane. In fact, when forced with decreasing emissions from fossil fuel sources our inversion estimates unreasonably high emissions in other sources. Further, the inversion estimates a decrease in biomass-burning emissions that could explain falling ethane abundance. A range of sensitivity tests suggests that these results are robust.atmospheric methane | greenhouse gas emissions | methane isotopic composition | methane trends | Bayesian inversion C onsiderable research since the 1970s has established the role of methane (CH 4 ) in climate, as an infrared active gas, and as a chemically reactive species affecting hydroxyl radical, ozone, and carbon monoxide in the troposphere and chlorine, ozone, and water vapor in the stratosphere. At a globally averaged mixing ratio of 1,800 ppb, the abundance of CH 4 in the atmosphere has more than doubled since the industrial revolution as a result of population growth, agricultural practices, and fossil fuel use (1). The rise in CH 4 concentration is considered to contribute 0.48 Wm −2 of the 2.83 Wm −2 radiative forcing by wellmixed greenhouse gases since 1750 (2). Including indirect effects from CH 4 emissions roughly doubles its effective radiative forcing. Its global warming potential (not including feedbacks) is 28 based on a 100-y time horizon, but 84 based on a 20-y timescale [global warming potential (GWP) is relative to CO 2 ], illustrating the potential of large changes in the burden of CH 4 to influence climate on short timescales (2).Both the decrease in the CH 4 growth rate and its interannual variability since 1984 are well documented by at least four global networks of atmospheric measurements; agreement between time series is excellent with some exceptions early on. Recently, a review and synthesis of the CH 4 budget (3) pointed to some consensus of measurement and modeling studies and their comparisons toward understanding temporal changes in the CH 4 budget....

[1] Recent studies indicate that plants may be a previously overlooked but significant source of atmospheric CH 4 , though there is considerable disagreement on the mechanism of production. Our work sought to verify that woody deciduous trees grown under inundated conditions had the capacity for transporting CH 4 from an anaerobic subsurface to the atmosphere and to consider if such a source could be important globally. Here, we report results from a greenhouse mesocosm study that indicate significant emissions of anaerobically produced CH 4 transmitted to the atmosphere through broadleaf riparian tree species grown under flooded conditions. Using a leaf area normalized mean emission rate (0.7 ± 0.3 mg cm −2 hr −1 ), results were scaled globally for flooded forest regions and estimated to be 60 ± 20 Tg year −1 , ∼10% of the global CH 4 source. The carbon isotopic composition of CH 4 emitted was found to be significantly enriched compared with expectations (d 13 C ∼ −54‰) and provided an important isotopic constraint on the global source which coincides with the mean of the globally scaled greenhouse-based estimate. Citation: Rice, A.

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