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. The hydroxyl radical (OH), which is the dominant sink of methane (CH4),plays a key role in closing the global methane budget. Current top-downestimates of the global and regional CH4 budget using 3D models usuallyapply prescribed OH fields and attribute model–observation mismatches almostexclusively to CH4 emissions, leaving the uncertainties due toprescribed OH fields less quantified. Here, using a variational Bayesianinversion framework and the 3D chemical transport model LMDz, combined with10 different OH fields derived from chemistry–climate models (Chemistry–Climate Model Initiative, or CCMI, experiment), we evaluate the influence of OH burden, spatial distribution,and temporal variations on the global and regional CH4 budget. Theglobal tropospheric mean CH4-reaction-weighted [OH] ([OH]GM-CH4)ranges 10.3–16.3×105 molec cm−3 across 10 OH fieldsduring the early 2000s, resulting in inversion-based global CH4emissions between 518 and 757  Tg yr−1. The uncertainties in CH4inversions induced by the different OH fields are similar to the CH4emission range estimated by previous bottom-up syntheses and larger than therange reported by the top-down studies. The uncertainties in emissionsinduced by OH are largest over South America, corresponding to largeinter-model differences of [OH] in this region. From the early to the late2000s, the optimized CH4 emissions increased by 22±6  Tg yr−1(17–30  Tg yr−1), of which ∼25  % (on average) offsetsthe 0.7  % (on average) increase in OH burden. If the CCMI models representthe OH trend properly over the 2000s, our results show that a higherincreasing trend of CH4 emissions is needed to match the CH4observations compared to the CH4 emission trend derived using constantOH. This study strengthens the importance of reaching a better representationof OH burden and of OH spatial and temporal distributions to reduce theuncertainties in the global and regional CH4 budgets.

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