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. We analyse historical (1850–2014) atmospheric hydroxyl(OH) and methane lifetime data from Coupled Model Intercomparison ProjectPhase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project(AerChemMIP) simulations. Tropospheric OH changed little from 1850 up toaround 1980, then increased by around 9 % up to 2014, with an associatedreduction in methane lifetime. The model-derived OH trends from 1980 to 2005are broadly consistent with trends estimated by several studies that inferOH from inversions of methyl chloroform and associated measurements; mostinversion studies indicate decreases in OH since 2005. However, the modelresults fall within observational uncertainty ranges. The upward trend inmodelled OH since 1980 was mainly driven by changes in anthropogenicnear-term climate forcer emissions (increases in anthropogenic nitrogenoxides and decreases in CO). Increases in halocarbon emissions since 1950have made a small contribution to the increase in OH, whilst increases inaerosol-related emissions have slightly reduced OH. Halocarbon emissionshave dramatically reduced the stratospheric methane lifetime by about15 %–40 %; most previous studies assumed a fixed stratospheric lifetime.Whilst the main driver of atmospheric methane increases since 1850 isemissions of methane itself, increased ozone precursor emissions havesignificantly modulated (in general reduced) methane trends. Halocarbon andaerosol emissions are found to have relatively small contributions tomethane trends. These experiments do not isolate the effects of climatechange on OH and methane evolution; however, we calculate residual terms thatare due to the combined effects of climate change and non-linearinteractions between drivers. These residual terms indicate that non-linearinteractions are important and differ between the two methodologies we usefor quantifying OH and methane drivers. All these factors need to beconsidered in order to fully explain OH and methane trends since 1850; thesefactors will also be important for future trends.

Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP