Land‐atmosphere CO2 exchange simulated by a land surface process model coupled to an atmospheric general circulation model

CO 2 uptake during plant photosynthesis and CO 2 loss during plant and microbial respiration were added to a land surface process model to simulate the diurnal and annual cycles of biosphere‐atmosphere CO 2 exchange. The model was coupled to a modified version of the National Center for Atmospheric Research Community Climate Model version 2, and the coupled model was run for 5 years. The geographic patterns of annual net primary production are qualitatively similar to other models. When compared by vegetation type, annual production and annual microbial respiration are consistent with other models, except for needleleaf evergreen tree vegetation, where production is too high, and semidesert vegetation, where production and microbial respiration are too low. The seasonally of the net CO 2 flux agrees with other models in the southern hemisphere and the tropics. The diurnal range is large for photosynthesis and lower for plant and microbial respiration, which agrees with qualitative expectations. The simulation of the central United States is poor due to temperature and precipitation biases in the coupled model. Despite these deficiencies the current approach is a promising means to include terrestrial CO 2 fluxes in a climate system model that simulates atmospheric CO 2 concentrations, because it alleviates important parameterization discrepancies between standard biogeochemical models and the land surface models typically used in general circulation models, and because the model resolves the diurnal range of CO 2 exchange, which can be large (15–45 μmol CO 2 m −2 s −1 ).