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

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 affects different vegetation types differently. We compare the distributions of crops and of C 3 vs. C 4 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 C 3 and C 4 crops, natural and total vegetation for area and gross primary production (GPP) across the models. Even though 10 of the 11 models used Land Use Harmonisation (LUH2) crop areas as input data, modelled total crop area ranges from − 28 % to + 10 % of a satellite-based estimate. The C 3 and C 4 crop areas were − 56 % to + 15 % and − 100 % to + 38 % of LUH2 for 2014, respectively. The C 4 fraction of total vegetation area in the models is 9 %–25 %, compared to 20 % ± 3 % in observation-based estimates for the year 2014. Total global GPP varies by a factor of two across the models, and the C 4 fraction of GPP ranges from 12 % to 27 %. Simulated trends in the fraction of GPP by C 3 vs. C 4 vegetation type (− 20 % to + 29 %) would have changed global stable carbon isotopic discrimination by − 0.35 ‰ to + 0.11 ‰ over 1975–2005, not including changes in discrimination over time within C 3 and C 4 vegetation, indicating that modeled changes in the fraction of GPP by C 3 and C 4 vegetation do not account for the + 0.7 ‰ increase indicated by atmospheric data. Disparity in vegetation with these photosynthetic pathways in models contributes to uncertainty in land carbon flux simulations, and further constraints and improvements in models are needed.