Accounting for the missing carbon-sink with the CO2-fertilization effect

A terrestrial-biosphere carbon-sink has been included in global carbon-cycle models in order to reproduce past atmospheric CO₂, ¹³C and ¹⁴C concentrations. The sink is of large enough magnitude that its effect on projections of future CO₂ levels should not be ignored. However, the cause and mechanism of this sink are not well understood, contributing to uncertainty of projections. The estimated magnitude of the biospheric sink is examined with the aid of a global carbon-cycle model. For CO₂ emissions scenarios, model estimates are made of the resulting atmospheric CO₂ concentration. Next, the response of this model to CO₂-emission impulses is broken down to give the fractions of the impulse which reside in the atmosphere, oceans, and terrestrial biosphere - all as a perturbation to background atmospheric CO₂ concentration time-profiles that correspond to different emission scenarios. For a biospheric sink driven by the CO₂-fertilization effect, we find that the biospheric fraction reaches a maximum of roughly 30% about 50 years after the impulse, which is of the same size as the oceanic fraction at that time. The dependence of these results on emission scenario and the year of the impulse are reported.