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

A reliable representation of the diversity and variability of terrestrial ecosystems, both natural and managed, is crucial to the accurate simulation of their present and future roles in biogeochemical cycles and global climate. In this study we compare the treatment of vegetation distributions and of photosynthetic pathways (C₃ versus C₄) of both natural vegetation and crops across Earth System Models (ESMs) in the 6th Coupled Model Intercomparison Project (CMIP6). Of the 11 CMIP6 models reporting variables on crop and C₃ versus C₄ distribution use, 10 models use the crop distributions of the Land Use Harmonization v2 (LUH2) dataset, which has an increase of ~188 and ~254% in C₃ and C₄cropland, respectively, from 1850 to 2014. The models simulate a 10% decrease in the area coverage of natural vegetation with the C₃ photosynthetic pathway but disagree on the trend of C₄. The impact on carbon isotopic discrimination from simulated C₃ and C₄ GPP trends only, not accounting for physiological effects, is generally to drive a decreasing trend in discrimination, especially in models with increasing C₄ vegetation cover, opposite to the trend derived from atmospheric data. Our findings suggest that implementation of C₃ and C₄ vegetation area abundance and GP of C₃ and C₄ vegetation contribute to uncertainty in land carbon fluxes and need further constraints and improvement in ESMs.