Seasonal variability of tropical wetland CH 4 emissions: the role of the methanogen-available carbon pool
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 CH 4 emissions over seasonal timescales. We fit DMCM parameters to satellite observations of CH 4 columns from SCIAMACHY CH 4 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 CH 4 emissions for 2003–2009, and determined the resulting seasonal variability of atmospheric CH 4 on a global scale using the GEOS-Chem atmospheric chemistry and transport model. First, we estimated that tropical emissions amounted to 111.1 Tg CH 4 yr −1, of which 24% was emitted from Amazon wetlands. We estimated that annual tropical wetland emissions increased by 3.4 Tg CH 4 yr −1 between 2003 and 2009. Second, we found that the model was able to reproduce the observed seasonal lag of CH 4 concentrations peaking 1–3 months before peak EWH values. We also found that our estimates of CH 4 emissions substantially improved the comparison between the model and observed CH 4 surface concentrations ( r = 0.9). We anticipate that these new insights from the DMCM represent a fundamental step in parameterising tropical wetland CH 4 emissions and quantifying the seasonal variability and future trends of tropical CH 4 emissions.