Climate-related uncertainties in projections of the twenty-first century terrestrial carbon budget: off-line model experiments using IPCC greenhouse-gas scenarios and AOGCM climate projections

A terrestrial ecosystem model (Sim-CYCLE) was driven by multiple climate projections to investigate uncertainties in predicting the interactions between global environmental change and the terrestrial carbon cycle. Sim-CYCLE has a spatial resolution of 0.5 degrees, and mechanistically evaluates photosynthetic and respiratory CO2 exchange. Six scenarios for atmospheric-CO2 concentrations in the twenty-first century, proposed by the Intergovernmental Panel on Climate Change, were considered. For each scenario, climate projections by a coupled atmosphere - ocean general circulation model (AOGCM) were used to assess the uncertainty due to socio-economic predictions. Under a single CO2 scenario, climate projections with seven AOGCMs were used to investigate the uncertainty stemming from uncertainty in the climate simulations. Increases in global photosynthesis and carbon storage differed considerably among scenarios, ranging from 23 to 37% and from 24 to 81 Pg C, respectively. Among the AOGCM projections, increases ranged from 26 to 33% and from 48 to 289 Pg C, respectively. There were regional heterogeneities in both climatic change and carbon budget response, and different carbon-cycle components often responded differently to a given environmental change. Photosynthetic CO2 fixation was more sensitive to atmospheric CO2, whereas soil carbon storage was more sensitive to temperature. Consequently, uncertainties in the CO2 scenarios and climatic projections may create additional uncertainties in projecting atmospheric-CO2 concentrations and climates through the interactive feedbacks between the atmosphere and the terrestrial ecosystem.