Abstract:
The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid-Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstructed with the CARbon Assimilation In the Biosphere (CARAIB) model forced with two different sets of climates simulated by the European Centre-HAMburg (ECHAM) and LMD general circulation models. It is found that the trends predicted on the basis of both sets of GCM climatic fields are generally consistent with each other, although substantial discrepancies in the magnitude of the changes may be observed. Actually, these discrepancies in the biospheric results associated with the use of different GCM climatic fields are usually smaller than the differences between biospheric runs performed while considering or neglecting the CO2 fertilization effect (which might, however, be overestimated by the model due to uncertainties concerning changes in nutrient availability). The calculated changes with respect to the present of the biosphere carbon stock range from −132 to +92 Gt C for the mid-Holocene and from −710 to +70 Gt C for the last glacial maximum. It is also shown that the relative contribution of the material synthesized by C4 plants to the total biomass of vegetation, litter and soils was substantially larger at mid-Holocene and last glacial maximum times than today. This change in the relative importance of the C3 and C4 photosynthetic pathways induced changes in the fractionation of the land biosphere. These changes in the average biospheric fractionation resulting from the redistribution of C3 and C4 plants were partly compensated for by changes of opposite sign in the fractionation of C3 plants due to the modification of the intercellular CO2 pressure within their leaves. With respect to present times, the combination of both processes reduced the discrimination (i.e., less negative fractionation) of the land biosphere by 0.03 to 0.32‰ during the mid-Holocene and by 0.30 to 1.86‰ at the last glacial maximum.