Role of atmospheric oxidation in recent methane growth

Abstract

The growth in global methane (CH 4 ) concentration, which had been ongoing since the industrial revolution, stalled around the year 2000 before resuming globally in 2007. We evaluate the role of the hydroxyl radical (OH), the major CH 4 sink, in the recent CH 4 growth. We also examine the influence of systematic uncertainties in OH concentrations on CH 4 emissions inferred from atmospheric observations. We use observations of 1,1,1-trichloroethane (CH 3 CCl 3 ), which is lost primarily through reaction with OH, to estimate OH levels as well as CH 3 CCl 3 emissions, which have uncertainty that previously limited the accuracy of OH estimates. We find a 64-70% probability that a decline in OH has contributed to the post-2007 methane rise. Our median solution suggests that CH 4 emissions increased relatively steadily during the late 1990s and early 2000s, after which growth was more modest. This solution obviates the need for a sudden statistically significant change in total CH 4 emissions around the year 2007 to explain the atmospheric observations and can explain some of the decline in the atmospheric 13 CH 4 / 12 CH 4 ratio and the recent growth in C 2 H 6 . Our approach indicates that significant OH-related uncertainties in the CH 4 budget remain, and we find that it is not possible to implicate, with a high degree of confidence, rapid global CH 4 emissions changes as the primary driver of recent trends when our inferred OH trends and these uncertainties are considered., the second most important partially anthropogenic greenhouse gas, is observed to vary markedly in its year to year growth rate (Fig. 1). The causes of these variations have been the subject of much controversy and uncertainty, primarily because there is a wide range of poorly quantified sources and because its sinks are ill-constrained (1). Of particular recent interest are the cause of the "pause" in CH4 growth between 1999 and 2007 and the renewed growth from 2007 onward (2-7). It is important that we understand these changes if we are to better project future CH4 changes and effectively mitigate enhanced radiative forcing caused by anthropogenic methane emissions.The major sources of CH4 include wetlands (natural and agricultural), fossil fuel extraction and distribution, enteric fermentation in ruminant animals, and solid and liquid waste. Our understanding of the sources of CH4 comes from two approaches: "bottom up," in which inventories or process models are used to predict fluxes, or "top down," in which fluxes are inferred from observations assimilated into atmospheric chemical transport models. Bottom-up methods suffer from uncertainties and potential biases in the available activity data or emissions factors or the extrapolation to large scales of a relatively small number of observations. Furthermore, there is no constraint on the global total emissions from bottom-up techniques. The topdown approach is limited by incomplete or imperfect observations and our understanding of atmospheric transport and chemical sinks. For CH4, these di...

Recent increases in the atmospheric growth rate and emissions of HFC-23 (CHF<sub>3</sub>) and the link to HCFC-22 (CHClF<sub>2</sub>) production

“…For the gases in this paper, potential variations in OH concentration (e.g. Rigby et al, 2017) were not found to lead to a large change in the derived emissions (see Sect. S4).…”

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

“…Dictated by global mass balance, the average isotopic composition of CH 4 in the atmosphere (δ 13 C-CH 4 or δD-CH 4 ) equals the flux-weighted isotopic composition of the sources, corrected for the total kinetic isotope effects of removal processes (e.g. Stevens and Rust, 1982;Cicerone and Oremland, 1988;Quay et al, 1991Quay et al, , 1999Miller et al, 2002;Turner et al, 2017;Rigby et al, 2017). It has been pointed out that assignment of representative isotopic signatures of various CH 4 sources remains uncertain due to their large spatial and temporal variability across the globe (e.g.…”

“…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.…”

Overexplaining or underexplaining methane’s role in climate change

“…Past work has separately used measurements of methane, its isotopes, and related gases to interpret the methane history. Two new publications (24,25) combine these complementary data into a consistent Bayesian modeling framework and use advanced statistical methods to match all observations simultaneously subject to the prior constraints. Notably, they advance our understanding of what could have caused the variability.…”

“…So we might conclude that an important process, some chemistry-climate feedback, is missing from the 3D models. The Bayesian models used in these new papers reduce the available observations into hemispheric averages (24,25), which makes their complex statistical methods computationally tractable. However, the spatial distributions of methane, its isotopes, and MCF may contain additional information that can distinguish between the alternative scenarios and provide causal explanations for the inferred OH trends.…”