Biogeophysical effects of CO₂ fertilization on global climate

CO₂ fertilization affects plant growth, which modifies surface physical properties, altering the surface albedo, and fluxes of sensible and latent heat. We investigate how such CO₂-fertilization effects on vegetation and surface properties would affect the climate system. Using a global three-dimensional climate-carbon model that simulates vegetation dynamics, we compare two multicentury simulations: a ‘Control’ simulation with no emissions and a ‘Physiol-noGHG’ simulation where physiological changes occur as a result of prescribed CO₂ emissions, but where CO₂-induced greenhouse warming is not included. In our simulations, CO₂ fertilization produces warming; we obtain an annual- and global-mean warming of about 0.65 K (and land-only warming of 1.4 K) after 430 yr. This century-scale warming is mostly due to a decreased surface albedo associated with the expansion of the Northern Hemisphere boreal forests. On decadal timescales, the CO₂ uptake by afforestation should produce a cooling effect that exceeds this albedo-based warming; but if the forests remain in place, the CO₂-enhanced-greenhouse effect would diminish as the ocean equilibrates with the atmosphere, whereas the albedo effect would persist. Thus, on century timescales, there is the prospect for net warming from CO₂ fertilization of the land biosphere. Further study is needed to confirm and better quantify our results.