Role of atmospheric oxidation in recent methane growth.

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)CC(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.