Impact of Prior Terrestrial Carbon Fluxes on Simulations of Atmospheric CO2 Concentrations

Terrestrial ecosystems have a significant role in shaping the distribution of atmospheric CO2, but it is uncertain how much they affect CO2 concentrations. We assessed the impact of terrestrial fluxes derived from different biosphere models in the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) on atmospheric CO2 estimations based on a global chemical transport model (GEOS-Chem) when these fluxes were applied as prior information. We examined the spatiotemporal uncertainty in terrestrial flux estimations from 15 MsTMIP terrestrial fluxes. We found high uncertainties in the terrestrial fluxes for temperate North America, tropical and temperate South America, southern Africa, Europe and tropical Asia. Sensitivity simulations showed that the annual mean CO2 concentration changed by 6.0-8.0 ppmv with the spread of the terrestrial fluxes. The interannual trend in the terrestrial fluxes could significantly affect the simulated trend of atmospheric CO2 concentrations. As a result of the spread in the prior terrestrial fluxes, large differences in the daily mean CO2 anomalies changed with an interquartile range of -1.0 to +1.0 ppmv and the magnitude of change in the sub-daily CO2 concentrations was in the range 4.0-6.0 ppmv for China, the USA and Europe. Our results suggest an urgent need to increase the reliability of terrestrial flux estimates in CO2 simulations. Surface seasonal CO2 concentrations were simulated to change by +/- 2.0 ppmv in most regions of the world due to the differences in the ensemble mean fluxes, reflecting the impact of the uncertainties in the existing optimized terrestrial fluxes on CO2 simulations.