MODELING THE GLACIAL-INTERGLACIAL CHANGES IN THE CONTINENTAL BIOSPHERE

A new estimate of the glacial-interglacial variations of the terrestri al carbon storage was obtained with the CARAIB biosphere model. The cl imatic data for the Last Glacial Maximum (LGM) necessary to drive the biosphere model are derived from results of the ECHAM2 General Circula tion Model (GCM). Six model simulations (four under typical interglaci al and two under typical glacial climatic conditions) were performed t o 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 CO, level s in the atmosphere, the presence or absence of crops and from changin g continental boundaries. The variation of the terrestrial carbon stoc ks 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 gl obal 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 CO, fert ilization. At the same time, the terrestrial biosphere would have fixe d between 134 (neglecting CO2 fertilization effects) and 606 Gt C. The treatment of both the C-3 and C-4 photosynthetic pathways in the CARA IB model enabled us further to reconstruct the partitioning between C, and C, plants. Following our experiments, 29.7% of the total biospher ic carbon stock at the LGM was C-4 material, compared to an interglaci al fraction of only 19.8%. The average biospheric fractionation factor was similar to 1.5 parts per thousand less negative at LGM than it is today. Considering an atmospheric delta(13)C 0.5 +/- 0.2 parts per th ousand lower at LGM than at pre-industrial times, the 606 Gt C transfe r would lead to a global ocean delta(13)C shift of roughly -0.41 parts per thousand, fully consistent with currently available data. For the smaller change of 134 Gt C obtained without the CO2 fertilization eff ect, this shift would only be on the order of -0.10 parts per thousand . (C) 1998 Elsevier Science B,V. All rights reserved.