Long-term CO2 fertilization effect on the terrestrial above-ground biomass

Live vegetation, the largest reservoir of biomass carbon, absorbed more than one third of carbon emissions, mitigating the growth of CO2 in the atmosphere. Earlier studies suggested that CO2 fertilization is the primary cause for the substantial increase in global photosynthesis, however, few studies investigate whether and to what extent such stimulation of photosynthesis by CO2 can increase the stocks of vegetation biomass pool at the global scale. Here, we assess the global effects of CO2 fertilization on above-ground biomass (AGB) during the past three decades using 13 dynamical global vegetation models (DGVMs). We find that the magnitude of this CO2 fertilization differs strongly between models, ranging from +10 PgC to +72 PgC. We then constrain the global CO2-induced biomass changes using the long-term gross primary productivity (GPP) records (i.e., covering more than 10- or 15-yrs) from FLUXNET and a newly-developed satellite-based AGB product (called BIOMASCAT). This emergent-constraint approaches can be successfully used because the effects of CO2 fertilization at global levels is linearly related to the effects at site-level or regional-level, and different DGVMs do agree on a relationship between AGB variation and GPP variation induced by CO2. Three emerging data-constrained CO2 fertilization-induced AGB change ranges from 30 PgC to 82 PgC. This value suggests that CO2 fertilization alone explains most of the simulated increase in global biomass changes since the 1980s, which were underestimated by most DGVMs. This implies that that clarifying the mechanisms underlying elevated CO2 effects on terrestrial vegetation biomass, continued collaboration between experimentalists and modelers, is very necessary.