Influences of hydroxyl radicals (OH) on top-down estimates of the global and regional methane budgets

Authors: Zhao Y, M Saunois, P Bousquet, X Lin, A Berchet, MI Hegglin, JG Canadell, RB Jackson, EJ Dlugokencky, RL Langenfelds, M Ramonet, D Worthy, B Zheng

The hydroxyl radical (OH), which is the domi- nant sink of methane (CH4), plays a key role in closing the global methane budget. Current top-down estimates of the global and regional CH4 budget using 3D models usually apply prescribed OH fields and attribute model-observation mismatches almost exclusively to CH4 emissions, leaving the uncertainties due to prescribed OH fields less quanti- fied. Here, using a variational Bayesian inversion frame- work and the 3D chemical transport model LMDz, combined with 10 different OH fields derived from chemistry-climate models (Chemistry-Climate Model Initiative, or CCMI, ex- periment), we evaluate the influence of OH burden, spa- tial distribution, and temporal variations on the global and regional CH4 budget. The global tropospheric mean CH4- reaction-weighted [OH] ([OH]GM-CH4 ) ranges 10.3-16.3 × 105 molec cm-3 across 10 OH fields during the early 2000s, resulting in inversion-based global CH4 emissions between 518 and 757 Tg yr-1. The uncertainties in CH4 inversions in- duced by the different OH fields are similar to the CH4 emis- sion range estimated by previous bottom-up syntheses and larger than the range reported by the top-down studies. The uncertainties in emissions induced by OH are largest overnSouth America, corresponding to large inter-model differ- ences of [OH] in this region. From the early to the late 2000s, the optimized CH4 emissions increased by 22 ± 6 Tg yr-1 (17-30 Tg yr-1), of which ∼ 25 % (on average) offsets the 0.7 % (on average) increase in OH burden. If the CCMI mod- els represent the OH trend properly over the 2000s, our re- sults show that a higher increasing trend of CH4 emissions is needed to match the CH4 observations compared to the CH4 emission trend derived using constant OH. This study strengthens the importance of reaching a better representa- tion of OH burden and of OH spatial and temporal distribu- tions to reduce the uncertainties in the global and regional CH4 budgets.