Rising atmospheric methane: 2007–2014 growth and isotopic shift

Abstract

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr À1. Simultaneously, δ 13 C CH4 (a measure of the 13 C/ 12 C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13 C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

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

“…2a). In an intercomparison exercise reported by Nisbet (2005), the INSTAAR measurement was +0.14 ± 0.06 ‰ higher than NIWA (not shown in Fig. 2a), which is consistent with the above value.…”

“…5.11 RHUL 5.11.1 δ 13 C-CH 4 Nisbet (2005) reported that the RHUL DI-IRMS measurements agreed well with NIWA with an offset of 0.00 ± 0.02 ‰ (top in no. 11, Fig.…”

“…Earlier measurements of three air samples at both UHEI and NIWA indicated that the UHEI offset is −0.04 ± 0.04 ‰ relative to NIWA (Poß, 2003;. It is also noted that, in an intercomparison presented by Nisbet (2005), the UHEI measurement was −0.07 ± 0.04 ‰ lower than NIWA. As these earlier comparison results have been published before the rigorous corrections of the UHEI measurements, these values are not included in Fig.…”

“…Royal Holloway University of London (RHUL) measured atmospheric δ 13 C-CH 4 using an offline DI-IRMS technique (Lowry et al, 2001) and a GC-IRMS system (Fisher et al, 2006(Fisher et al, , 2011Nisbet et al, 2016). Reproducibility of the DI-IRMS measurement was evaluated to be 0.04 ‰ (Lowry et al, 2001) and that by the GC-IRMS is 0.05 ‰ (Fisher et al, 2006).…”

“…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:…”