On the contribution of CO2fertilization to the missing biospheric sink

A gridded biospheric carbon model is used to investigate the impact of the atmospheric CO 2 increase on terrestrial carbon storage. The analysis shows that the calculated CO 2 fertilization sink is dependent not just on the mathematical formulation of the “β factor” but also on the relative controls of net primary productivity (NPP), carbon residence times, and resource availability. The modeled evolution of the biosphere for the period 1850–1990 shows an increasing lag between NPP and the heterotrophic respiration. The time evolution of the modeled biospheric sink (i.e., difference between enhanced NPP and enhanced respiration) does not match that obtained by deconvolution of the ice core CO 2 time series. Agreement between the two is reasonable for the first half of the period, but during the recent decades the deconvoluted CO 2 increase is much too fast to be explained by the CO 2 fertilization effect only. Therefore other mechanisms than CO 2 fertilization should also contribute to the missing sink. Our results suggest that about two thirds to three fourths of the 1850–1990 integrated missing sink is due to the CO 2 greening of the biosphere. The remainder may be due to the increased level of nitrogen deposition starting around 1950.