Strong monsoon influence on South Asian methane emissions in 2020 revealed by a Bayesian inversion constrained by satellite observations

Abstract. South Asia is a major contributor to global methane (CH₄) emissions, yet its emissions remain poorly constrained, limiting targeted mitigation. Current bottom-up inventories do not consistently capture the magnitude and seasonality of CH₄ emissions in this region, particularly during the monsoon. Here we quantify South Asian CH₄ emissions for 2020 using column observations from TROPOMI, a Lagrangian transport model (FLEXPART), and a Bayesian inversion system (FLEXINVERT+). We estimate a posteriori emission of 73.0 ± 0.5 Tg yr⁻¹ for South Asia, including 35.6 ± 0.5 Tg yr⁻¹ for India and 13.2 ± 0.2 Tg yr⁻¹ for Bangladesh. Agriculture and wetlands contribute substantially to the regional budget, with the flux increments coincident with rice‑growing areas and inundated lowlands. The inversion indicates pronounced monsoon‑modulated seasonality: posterior fluxes are higher than the prior during June–September by ~70 % and lower during January–May by ~46 %. Localized enhancements seen over the lower Indus Basin align with runoff patterns, while the seasonal peaks here are absent in inventories. By resolving monsoon seasonality with satellite constraints, our results point towards key uncertainties in the South Asian CH₄ budget and underscore the need for process-based, seasonally responsive inventories to inform mitigation strategies and reconcile bottom-up and top-down estimates.