Evaluating the CO2 Component of Climate Models Using ARM Southern Great Plains Site Data
Aircraft data collected over the Southern Great Plains (SGP) site were used to analyze changes in the three-dimensional (3D) distribution of atmospheric CO2 for several greenhouse gas concentration trajectories adopted by the Intergovernmental Panel on Climate Change for its fifth Assessment Report. Using the Community Earth System Model–Biogeochemistry (CESM1-BGC), scientists first compared CO2 concentrations simulated for 1850 to 2005 to surface, aircraft, and column observations. Second, the evolution of spatial and temporal gradients within the SGP’s 3D observational footprint during the twenty-first century was examined. By upscaling the results, the study showed that the mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net CO2 flux in the Community Land Model (the land component of CESM) was too weak. Over the last half century, the growth rate of atmospheric CO2 was higher in the model than in observations, suggesting that the overall sensitivity of land and ocean CO2 uptake to rising atmospheric CO2 (and other human global change perturbations) was too weak (i.e., model parameterization of the land and ocean to rising CO2 needs to be adjusted) . The diagnostics that were developed in this paper provide a means to test future generations of coupled carbon–climate models.
Keppel-Aleks, G., J. T. Randerson, K. Lindsay, B. B. Stephens, J. K. Moore, S. C. Doney, P. E. Thornton, N. M. Mahowald, F. M. Hoffman, C. Sweeney, P. P. Tans, P. O. Wennberg, and S. C. Wofsy. 2013. “Atmospheric Carbon Dioxide Variability in the Community Earth System Model: Evaluation and Transient Dynamics during the Twentieth and Twenty-First Centuries,” Journal of Climate 26, 4447–75. DOI: 10.1175/JCLI-D-12-00589.1.