Sensitivity of terrestrial water and energy budgets to CO₂-physiological forcing: an investigation using an offline land model

Increasing concentrations of atmospheric carbon dioxide (CO₂) influence climate by suppressing canopy transpiration in addition to its well-known greenhouse gas effect. The decrease in plant transpiration is due to changes in plant physiology (reduced opening of plant stomata). Here, we quantify such changes in water flux for various levels of CO₂ concentrations using the National Center for Atmospheric Research's (NCAR) Community Land Model. We find that photosynthesis saturates after 800 ppmv (parts per million, by volume) in this model. However, unlike photosynthesis, canopy transpiration continues to decline at about 5.1% per 100 ppmv increase in CO₂ levels. We also find that the associated reduction in latent heat flux is primarily compensated by increased sensible heat flux. The continued decline in canopy transpiration and subsequent increase in sensible heat flux at elevated CO₂ levels implies that incremental warming associated with the physiological effect of CO₂ will not abate at higher CO₂ concentrations, indicating important consequences for the global water and carbon cycles from anthropogenic CO₂ emissions.