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. Changes in atmospheric methane abundance haveimplications for both chemistry and climate as methane is both a stronggreenhouse gas and an important precursor for tropospheric ozone. A betterunderstanding of the drivers of trends and variability in methane abundanceover the recent past is therefore critical for building confidence inprojections of future methane levels. In this work, the representation ofmethane in the atmospheric chemistry model AM4.1 is improved by optimizingtotal methane emissions (to an annual mean of 580±34 Tg yr−1 ) tomatch surface observations over 1980–2017. The simulations with optimizedglobal emissions are in general able to capture the observed trend,variability, seasonal cycle, and latitudinal gradient of methane.Simulations with different emission adjustments suggest that increases inmethane emissions (mainly from agriculture, energy, and waste sectors)balanced by increases in methane sinks (mainly due to increases in OHlevels) lead to methane stabilization (with an imbalance of 5  Tg yr−1 )during 1999–2006 and that increases in methane emissions (mainly fromagriculture, energy, and waste sectors) combined with little change in sinks(despite small decreases in OH levels) during 2007–2012 lead to renewedgrowth in methane (with an imbalance of 14  Tg yr−1 for 2007–2017).Compared to 1999–2006, both methane emissions and sinks are greater (by 31 and 22  Tg yr−1 , respectively) during 2007–2017. Our taggedtracer analysis indicates that anthropogenic sources (such as agriculture,energy, and waste sectors) are more likely major contributors to the renewedgrowth in methane after 2006. A sharp increase in wetland emissions (alikely scenario) with a concomitant sharp decrease in anthropogenic emissions(a less likely scenario), would be required starting in 2006 to drive themethane growth by wetland tracer. Simulations with varying OH levelsindicate that a 1 % change in OH levels could lead to an annual meandifference of ∼4 Tg yr−1 in the optimized emissions and a0.08-year difference in the estimated tropospheric methane lifetime.Continued increases in methane emissions along with decreases intropospheric OH concentrations during 2008–2015 prolong methane's lifetimeand therefore amplify the response of methane concentrations to emissionchanges. Uncertainties still exist in the partitioning of emissions amongindividual sources and regions.

Investigation of the global methane budget over 1980–2017 using GFDL-AM4.1