Terrestrial ecosystems in high latitudes are predicted to experience more dramatic environmental changes from global warming compared with lower latitude ecosystems. The expected changes include lengthening of the growing season and melting of permafrost, both of which have implications for the methane (CH4) exchanges between terrestrial ecosystems and the atmosphere. To date, most models of large-scale methane dynamics have ignored key aspects of the water and soil thermal regimes in high latitudes that control the timing and magnitude of CH4 exchanges between the land and the atmosphere. Extant methane models have not been coupled with well-validated terrestrial ecosystem models, and so the methane models do not simulate the important links among plant productivity, the availability of labile carbon compounds to microorganisms, and CH4 emissions. In this study we have modified our biogeochemistry model, the Terrestrial Ecosystem Model (TEM 5.0), to include the processes of methanogenesis and methanotrophy in areas with and without permafrost. We enhanced TEM's hydrological module to better simulate soil water dynamics including water table fluctuations. We applied our model to the terrestrial ecosystems at northern high latitudes (45oN above) to evaluate the responses of CH4 emissions to the climate change during the 20th century. Our simulations indicated current CH4 emissions to the atmosphere are about 96 Tg yr-1 from natural ecosystems at northern high latitudes, while methane consumption by soil microbes is about 36 Tg yr-1. Therefore, we estimate that this region is a net source of about 60 Tg CH4 yr-1. The simulations showed there is strong interannual variability in CH4 fluxes and a trend of increasing net CH4 emissions with a rate of 0.06 Tg CH4 yr-1 during the 20th Century. If this increase continues, it will create a strong positive feedback to the climate system.