Evaluation of two process-based models used to estimate global CH4 emissions from natural wetlands

Reliable models are required to estimate global wetland CH₄ emissions. This study aimed to test two process-based models, CH4MOD_wetland and TEM, against the CH₄ flux measurements of marsh, swamps, peatland and coastal wetland sites across the world; specifically, model accuracy and generality were evaluated for different wetland types and in different continents, and then the global CH₄ emissions from 2000 to 2010 were estimated. Both models showed similar high correlations with the observed seasonal CH₄ emissions, and the regression of the observed versus computed total seasonal CH₄ emissions resulted in R² values of 0.78 and 0.72 by CH4MOD_wetland and TEM, respectively. The CH4MOD_wetland predicted more accurately in marsh, peatland and coastal wetlands, with model efficiency (EF) values of 0.22, 0.55 and 0.72, respectively; however, the model showed poor performance in swamps (EF<0). The TEM model predicted better in peatland and swamp, with EF values of 0.77 and 0.71, respectively, but it could not accurately simulate the marsh and coastal wetland (EF<0). There was a good correlation between the simulated CH₄ fluxes and the observed values on most continents. However, CH4MOD_wetland showed no correlation with the observed values in South America and Africa. TEM showed no correlation with the observations in Europe. The global CH₄ emissions for the period 2000­­–2010 were estimated to be 105.31±2.72 Tg yr^-1 by CH4MOD_wetland and 134.31±0.84 Tg yr^-1 by TEM. Both models simulated a similar spatial distribution of CH₄ emissions across the world and among continents. Marsh contributes 36%–39% to global CH₄ emissions. Lakes and rivers and swamp are the second and third contributors, respectively. Other wetland types account for only approximately 20% of global emissions. Based on the models’ generality, if we use the more accurate model to estimate each continent/wetland type, we obtain a new assessment of 116.99–124.74 Tg yr^-1 for the global CH₄ emissions for the period 2000–2010.