Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase

Abstract

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....

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…5.12 PDX 5.12.1 δ 13 C-CH 4 Rice et al (2016) presented an offset of +0.024 ± 0.088 ‰ of the PDX measurements relative to UW by comparing coinciding measurements of archive air samples at PDX and δ 13 C-CH 4 records from Quay et al (1999) from stations Mauna Loa, Hawaii andTutuila, American Samoa (1995-1996). With the UW offset with respect to NIWA (Sect.…”

“…Portland State University (PDX) reported δ 13 C-CH 4 and δD-CH 4 measurements for archive air samples (Rice et al, 2016). The PDX measurement system has been described in Teama (2013) with some updates since Rice et al (2001).…”

“…The PDX measurement system has been described in Teama (2013) with some updates since Rice et al (2001). The δ 13 C-CH 4 and δD-CH 4 reproducibilities are 0.07 and 2.0 ‰, respectively, and PDX shares the standard scales with UCI for both δ 13 C-CH 4 and δD-CH 4 (Rice et al, 2016).…”

“…Sherwood et al, 2017), which could result in large uncertainties of isotope-based estimates of the global CH 4 budget (Schwietzke et al, 2016). Nonetheless, the value of isotope measurements was amply demonstrated by recent studies which suggested shifts in the global CH 4 source over the last decades (Schaefer et al, 2016;Rice et al, 2016;Nisbet et al, 2016;Schwietzke et al, 2016); without isotopic analyses such conclusions would have been difficult to achieve. The isotopic ratios are commonly reported using the delta notation:…”

“…Incorporating δ 13 C-CH 4 and δD-CH 4 data sets in chemistry transport models is useful for quantitatively separating different CH 4 source categories and attempts have been made to reduce uncertainties in the global CH 4 budget (e.g. Hein et al, 1997;Mikaloff Fletcher et al, 2004a, b;Monteil et al, 2011;Kirschke et al, 2013;Ghosh et al, 2015;Rice et al, 2016;Schaefer et al, 2016;Schwietzke et al, 2016;Röckmann et al, 2016;Turner et al, 2017;Rigby et al, 2017). However, although an increasing number of δ 13 C-CH 4 and δD-CH 4 data have been reported over the last decades, significant measurement offsets among laboratories have been found for both δ 13 C-CH 4 (e.g.…”