Impact of mesophyll diffusion on estimated global land CO₂ fertilization

In C₃ plants, CO₂ concentrations drop considerably along mesophyll diffusion pathways from substomatal cavities to chloroplasts where CO₂ assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdownand therefore overestimate CO₂ available for carboxylation and underestimate photosynthetic responsiveness to atmospheric CO₂. An explicit consideration of mesophyll diffusion increases the modeled cumulative CO₂ fertilization effect (CFE) for global gross primary production (GPP) from 915 to 1,057 PgC for the period of 1901-2010. This increase represents a 16% correction, which is large enough to explain the persistent overestimation of growth rates of historical atmospheric CO₂ by Earth system models. Without this correction, the CFE for global GPP is underestimated by 0.05 PgC/y/ppm. This finding implies that the contemporary terrestrial biosphere is more CO₂ limited than previously thought.