Abstract:

An atmospheric transport model and observations of atmospheric CO2 were used to evaluate the performance of four Terrestrial Biosphere Models (TBMs) in simulating the seasonal dynamics of atmospheric CO2 between 1980 and 1991. The TBMs were forced with time varying atmospheric CO2 concentrations, climate, and land use to simulate the net exchange of carbon between the terrestrial biosphere and the atmosphere. An examination of the net carbon exchange simulated by the TBMs aggregated over 30° latitude bands shows some differences between the models in the phase and amplitude of the seasonal cycle. The monthly surface CO2 fluxes from the TBMs were used as surface constraints in the Model of Atmospheric Transport and Chemistry (MATCH) and the simulated seasonal cycles were compared with observations from several stations in the Climate Monitoring and Diagnostics laboratory network. In comparison to observations at stations north of the tropics, the TBMs underestimate the amplitude of the seasonal cycle and tnd to simulate earlier uptake of carbon during the spring by approximately one to two months. In general, the trends in amplitude agreed between the models and the observations at monitoring stations in the Northern hemisphere, but the magnitude of the trends varied among the models ass did the contributions to the trends of increasing CO2 climate change and variability, and land use to the trends. The comparison of the modeled anomalies with the observed anomalies indicates that the TBMs underestimated inter-annual variability in the exchange of CO2 between the terrestrial biosphere and the atmosphere.