Sensitivity of wetland methane emissions to model assumptions: Application and model testing against site observations

A rice paddy module is also incorporated into the model, where the fraction of land used for rice production is explicitly prescribed. The model is evaluated at the site level with vegetation cover and water table prescribed from measurements. Explicit site level evaluations of simulated methane emissions are quite different than evaluating the grid-cell averaged emissions against available measurements.

Using a baseline set of parameter values, our model-estimated average global wetland emissions for the period 1993-2004 were 256 Tg CH4 yr⁻¹ (including the soil sink) and rice paddy emissions in the year 2000 were 42 Tg CH4 yr⁻¹. Tropical wetlands contributed 201 Tg CH4 yr⁻¹, or 78% of the global wetland flux. Northern latitude (>50 N) systems contributed 12 Tg CH4 yr⁻¹.

However, sensitivity studies show a large range (150-346 Tg CH4 yr⁻¹) in predicted global methane emissions (excluding emissions from rice paddies). The large range is sensitive to (1) the amount of methane transported through aerenchyma, (2) soil pH (±100 Tg CH4 yr⁻¹), and (3) redox inhibition (±45 Tg CH4 yr⁻¹). Results are sensitive to biases in the CLMCN and to errors in the satellite inundation fraction.

In particular, the high latitude methane emission estimate may be biased low due to both underestimates in the high-latitude inundated area captured by satellites and unrealistically low high-latitude productivity and soil carbon predicted by CLM4.