Direct Physical Effects of CO2-Fertilization on Global Climate

CO2-fertilization affects plant growth, which modifies surface physical properties, altering the surface albedo and latent heat fluxes. Here we investigate how such changes to surface properties via CO2-fertilization, including changes in vegetation distribution, would directly affect the physical climate system. We know of no previous study that has investigated this question. Using a global three-dimensional climate-carbon model that simulates vegetation dynamics, we compare two multi-century simulations: a "Control" simulation with no emissions, and a "Fertilization-noGHG" simulation where the land biosphere is fertilized as a result of prescribed CO2 emissions, but where the climate model sees no additional greenhouse gas forcing. Our simulations indicate that the direct physical effect of CO2-fertilization could be warming over a timescale of a few centuries; we obtain an annual- and global-mean warming of about 0.65 K over 430 years in our model. The average land warming is 1.4 K. We find that this warming is mostly due to the albedo decrease in the Northern Hemisphere boreal forest regions. This albedo-based warming could partially offset the century-scale cooling effect of additional CO2 uptake due to CO2-fertilization. Further study is needed to confirm and better quantify our results.