The Biogeophysical Effects of Carbon Fertilization of the Terrestrial Biosphere

Abstract. The response of the terrestrial biosphere to increasing atmospheric carbon dioxide (CO₂), i.e., the carbon fertilization effect represents a significant source of uncertainty in future climate projections. The climate impacts of carbon fertilization include cooling associated with the biogeochemical effects of enhanced land carbon storage, whereas the non-carbon cycle biogeophysical effects associated with changes in surface energy and turbulent heat fluxes may warm or cool the climate system. Here, I analyze 15 state-of-the-art Earth system models that conducted simulations driven by 1 % per year increases in atmospheric CO₂ concentration that isolate the CO₂ fertilization effect (i.e., CO₂ radiative effects are not active). At the time of CO₂ quadrupling, the biogeophysical effects yield multimodel global mean near-surface warming of 0.16 ± 0.09 K with 13 of the 15 models yielding warming. Most of this warming is associated with decreases in surface latent heat flux associated with reduced canopy transpiration. Decreases in surface albedo and increases in downwelling shortwave and longwave radiation—both of which are modulated by cloud reductions—are also associated with the warming. Overall, however, the biogeophysical warming is about an order of magnitude smaller than the corresponding cooling associated with enhanced land carbon storage at -1.38 K (-1.92 to -0.84 K).