Role of atmospheric oxidation in recent methane growth

Significance

Methane, the second most important greenhouse gas, has varied markedly in its atmospheric growth rate. The cause of these fluctuations remains poorly understood. Recent efforts to determine the drivers of the pause in growth in 1999 and renewed growth from 2007 onward have focused primarily on changes in sources alone. Here, we show that changes in the major methane sink, the hydroxyl radical, have likely played a substantial role in the global methane growth rate. This work has significant implications for our understanding of the methane budget, which is important if we are to better predict future changes in this potent greenhouse gas and effectively mitigate enhanced radiative forcing caused by anthropogenic emissions.

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 ₃CCl ₃), which is lost primarily through reaction with OH, to estimate OH levels as well as CH ₃CC 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 ¹³CH ₄/ ¹²CH ₄ ratio and the recent growth in C ₂H 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.