Uncertainty in Land Carbon Fluxes Simulated by CMIP6 Models from Treatments of Crop Distributions and Photosynthetic Pathways

Abstract. A reliable representation of the diversity of vegetation in terrestrial ecosystems is needed for the accurate simulation of present and future biogeochemical cycling and global climate, particularly as climate change affect different vegetation types differently. We compare the distributions of crops and of C₃ versus C₄ photosynthetic pathways in both natural vegetation and crops across Earth System Models in the 6th Coupled Model Intercomparison Project (CMIP6). We find a large range in vegetation type for area, gross primary production (GPP) and carbon stock change in both natural vegetation and croplands across the models. Even though 10 of 11 models used Land Use Harmonization (LUH2) crop areas as input data, modeled total crop area ranges from -28 to +10 % of the data-based estimate. The C₃ and C₄ crop areas were -56 to +15 % and -100 to +38 % of LUH2 for 2014, respectively. The C₄ fraction of total vegetation area in the models is 9–25 %, compared to 17 % in observation-based estimates. Total global GPP varies by a factor of two across the models, and the C₄ fraction of GPP ranges from 12 to 27 %. Simulated trends in the fraction of GPP by C₃ versus C₄ vegetation type (-20 to +29 %) would have changed global isotopic discrimination by -0.35 to +0.11 ‰ over 1975–2005, indicating that modeled changes in vegetation type do not account for the +0.7 ‰ increase indicated by atmospheric data. Disparity in vegetation types contributes to uncertainty in land carbon fluxes and further constraints and improvements in models are needed.