10/13/2014

Contemporary Terrestrial Biosphere May Be More CO2 Limited than Previously Thought

Summary

In plants with C3 photosynthetic pathways, CO₂ concentrations drop considerably along leaf mesophyll diffusion pathways from sub-stomatal cavities to chloroplasts where CO₂ assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown, overestimating CO₂ available for carboxylation and underestimating photosynthetic responsiveness to atmospheric CO₂. Researchers at Oak Ridge National Laboratory sought to determine how mesophyll diffusion affects the global land CO₂ fertilization effect estimated by global carbon models. The team found that current carbon cycle models underestimate by 16% the long-term responsiveness of global terrestrial productivity to CO₂ fertilization. This underestimation of CO₂ fertilization is caused by an inherent model structural deficiency related to a lack of explicit representation of CO₂ diffusion inside leaves, which results in an overestimation of CO₂ available at the carboxylation site. The magnitude of CO₂ fertilization underestimation matches the long-term positive growth bias in the historical atmospheric CO₂ predicted by Earth system models. This finding implies that the contemporary terrestrial biosphere is more CO₂ limited than previously thought and will lead to improved understanding and modeling of carbon-climate feedbacks.

References

Sun, Y., L. Gu, R. E. Dickinson, R. J. Norby, S. G. Pallardy, and F. M. Hoffman. 2014. “Impact of Mesophyll Diffusion on Estimated Global Land CO₂ Fertilization,” Proceedings of the National Academy of Sciences (USA) 111(44), 15,774-779. DOI: 10.1073/pnas.1418075111.