Seasonal variability of tropical wetland CH₄ emissions: the role of the methanogen-available carbon pool

Abstract. We develop a dynamic methanogen-available carbon model (DMCM) to quantify the role of the methanogen-available carbon pool in determining the spatial and temporal variability of tropical wetland CH₄ emissions over seasonal timescales. We fit DMCM parameters to satellite observations of CH₄ columns from SCIAMACHY CH₄ and equivalent water height (EWH) from GRACE. Over the Amazon River basin we found substantial seasonal variability of this carbon pool (coefficient of variation = 28 ± 22%) and a rapid decay constant (φ = 0.017 day⁻¹), in agreement with available laboratory measurements, suggesting that plant litter is likely the prominent methanogen carbon source over this region. Using the DMCM we derived global CH₄ emissions for 2003–2009, and determined the resulting seasonal variability of atmospheric CH₄ on a global scale using the GEOS-Chem atmospheric chemistry and transport model. First, we estimated that tropical emissions amounted to 111.1 Tg CH₄ yr⁻¹, of which 24% was emitted from Amazon wetlands. We estimated that annual tropical wetland emissions increased by 3.4 Tg CH₄ yr⁻¹ between 2003 and 2009. Second, we found that the model was able to reproduce the observed seasonal lag of CH₄ concentrations peaking 1–3 months before peak EWH values. We also found that our estimates of CH₄ emissions substantially improved the comparison between the model and observed CH₄ surface concentrations (r = 0.9). We anticipate that these new insights from the DMCM represent a fundamental step in parameterising tropical wetland CH₄ emissions and quantifying the seasonal variability and future trends of tropical CH₄ emissions.