Simulating effects of land use changes on carbon fluxes:: past contributions to atmospheric CO2 increases and future commitments due to losses of terrestrial sink capacity

The impact of land use on the global carbon cycle and climate is assessed. The Bern carbon cycle-climate model was used with land use maps from HYDE3.0 for 1700 to 2000 A.D. and from post-SRES scenarios for this century. Cropland and pasture expansion each cause about hall of the simulated net carbon emissions of 188 Gt C over the industrial period and 1.1 Gt C yr(-1) in the 1990s, implying a residual terrestrial sink of 113 Gt C and of 1.8 Gt C yr(-1), respectively. Direct CO2 emissions due to land conversion as simulated in book-keeping models dominate carbon fluxes due to land use in the past. They are, however, mitigated by 25% through the feedback of increased atmospheric CO2 stimulating uptake. CO2 stimulated sinks tire largely lost when natural lands are converted. Past land use change has eliminated potential future carbon sinks equivalent to emissions of 80-150 Gt C over this century. They represent a commitment of past land use change, which accounts for 70% of the future land use flux in the scenarios considered. Pre-industrial land use emissions are estimated to 45 Gt C at most, implying a maximum change in Holocene atmospheric CO2 of 3 ppm. This is not compatible with the hypothesis that early anthropogenic CO2 emissions prevented a new glacial period.