Wetlands are one of the most important sources of atmospheric methane (CH4), but the strength of this source is still highly uncertain. To improve estimates of CH4 emission at the regional and global scales and predict future variation requires a process-based model integrating the controls of climatic and edaphic factors and complex biological processes over CH4 flux rates. This study used a methane emission model based on the hypothesis that plant primary production and soil organic matter decomposition act to control the supply of substrate needed by methanogens; the rate of substrate supply and environmental factors, in turn, control the rate of CH4 production, and the balance between CH4 production and methanotrophic oxidation determines the rate of CH4 emission into the atmosphere. Coupled to data sets for climate, vegetation, soil, and wetland distribution, the model was used to calculate spatial and seasonal distributions of CH4 emissions at a resolution of 1° latitude × 1° longitude. The calculated net primary production (NPP) of wetlands ranged from 45 g C m−2 yr−1 for northern bogs to 820 g C m−2 yr−1 for tropical swamps. CH4 emission rates from individual gridcells ranged from 0.0 to 661 mg CH4 m−2 d−1, with a mean of 40 mg CH4 m−2 d−1 for northern wetland, 150 mg CH4 m−2 d−1 for temperate wetland, and 199 mg CH4 m−2 d−1 for tropical wetland. Total CH4 emission was 92 Tg yr−1. Sensitivity analysis showed that the response of CH4 emission to climate change depends upon the combined effects of soil carbon storage, rate of decomposition, soil moisture and activity of methanogens.