Modelling the glacial–interglacial changes in the continental biosphere

A new estimate of the glacial–interglacial variations of the terrestrial carbon storage was obtained with the CARAIB biosphere model. The climatic data for the Last Glacial Maximum (LGM) necessary to drive the biosphere model are derived from results of the ECHAM2 General Circulation Model (GCM). Six model simulations (four under typical interglacial and two under typical glacial climatic conditions) were performed to analyse the roles of different environmental changes influencing the biospheric net primary productivity (NPP) and carbon stocks. The main differences between these simulations come from the adopted CO2 levels in the atmosphere, the presence or absence of crops and from changing continental boundaries. The variation of the terrestrial carbon stocks since the LGM are estimated by comparing the pre-agricultural (280 ppm of CO2, no crops, modern climate) and the full glacial simulations (200 ppm of CO2, LGM climate reconstruction). Our model predicts a global NPP increase from 38 Gt C year−1 to 53 Gt C year−1 during the deglaciation, a substantial part of that change being due to CO2 fertilization. At the same time, the terrestrial biosphere would have fixed between 134 (neglecting CO2 fertilization effects) and 606 Gt C. The treatment of both the C3 and C4 photosynthetic pathways in the CARAIB model enabled us further to reconstruct the partitioning between C4 and C3 plants. Following our experiments, 29.7% of the total biospheric carbon stock at the LGM was C4 material, compared to an interglacial fraction of only 19.8%. The average biospheric fractionation factor was 1.5‰ less negative at LGM than it is today. Considering an atmospheric δ13C 0.5±0.2‰ lower at LGM than at pre-industrial times, the 606 Gt C transfer would lead to a global ocean δ13C shift of roughly −0.41‰, fully consistent with currently available data. For the smaller change of 134 Gt C obtained without the CO2 fertilization effect, this shift would only be on the order of −0.10‰.