Implications of CO2 fertilization for future climate change in a coupled climate–carbon model

The terrestrial carbon cycle plays a critical role in determining levels of atmospheric CO₂ that result from anthropogenic carbon emissions. Elevated atmospheric CO₂ is thought to stimulate terrestrial carbon uptake, through the process of CO₂ fertilization of vegetation productivity. This negative carbon cycle feedback results in reduced atmospheric CO₂ growth, and has likely accounted for a substantial portion of the historical terrestrial carbon sink. However, the future strength of CO₂ fertilization in response to continued carbon emissions and atmospheric CO₂ rise is highly uncertain. In this paper, the ramifications of CO₂ fertilization in simulations of future climate change are explored, using an intermediate complexity coupled climate–carbon model. It is shown that the absence of future CO₂ fertilization results in substantially higher future CO₂ levels in the atmosphere, as this removes the dominant contributor to future terrestrial carbon uptake in the model. As a result, climate changes are larger, though the radiative effect of higher CO₂ on surface temperatures in the model is offset by about 30% due to reduced positive dynamic vegetation feedbacks; that is, the removal of CO₂ fertilization results in less vegetation expansion in the model, which would otherwise constitute an important positive surface albedo‐temperature feedback. However, the effect of larger climate changes has other important implications for the carbon cycle – notably to further weaken remaining carbon sinks in the model. As a result, positive climate–carbon cycle feedbacks are larger when CO₂ fertilization is absent. This creates an interesting synergism of terrestrial carbon cycle feedbacks, whereby positive (climate–carbon cycle) feedbacks are amplified when a negative (CO₂ fertilization) feedback is removed.