Trends in Seasonal Variability of Global Methane Budget constrained by GOSAT observations

Seasonal variability of atmospheric methane (CH4) is governed by the seasonal cycle of surface emissions and the abundance of the tropospheric hydroxyl radical (OH). As substantial uncertainties remain in the variability of both sources and sinks, we constrain monthly emissions and tropospheric OH using the GEOS-Chem chemistry transport model and 14 years of GOSAT XCH4 observations. The seasonal cycle amplitude (SCA) of posterior global methane emissions shows an increasing trend of 4.23 Tg a-1 a-1, substantially larger than the prior estimate of 1.52 Tg a-1 a-1. Boreal wetlands in North America and Siberia dominate this amplification, accounting for 30% and 27% of the global SCA trend, respectively, with additional contributions from tropical wetland regions in central Africa and the Bengal region. Interannual variability (IAV) in tropospheric OH also plays a compensatory role by modulating the methane sink. While OH IAV amplifies the sink SCA trend in the northern midlatitudes, it dampens the trend over the tropics (–0.47 Tg a-1 a-1) through declining posterior tropical OH. Sensitivity tests are performed to attribute the observed XCH4 SCA trends to emissions and tropospheric OH across latitude bands. In the northern hemisphere midlatitudes, the posterior XCH4 SCA trend is 0.41 Tg a-1 a-1 predominantly driven by increasing emission SCA. In contrast, the tropics exhibit a larger XCH4 SCA trend of 0.81 Tg a-1 a-1, where tropical emissions act to suppress the XCH4 SCA trend. This SCA trend analysis improves our understanding of recent methane dynamics and provides information for projecting future atmospheric methane concentrations.