ESTIMATING THE CHANGE OF CARBON IN THE TERRESTRIAL BIOSPHERE FROM 18000-BP TO PRESENT USING A CARBON-CYCLE MODEL

The authors used a global High Resolution Biosphere Model (HRBM), consisting of a biome model and a carbon cycle model, to estimate the changes of carbon storage in the major pools of the terrestrial biosphere from 18 000 BP to present. The climate change data to drive the biosphere for 18000 BP were derived from an Atmospheric General Circulation Model. Using the AGCM anomalies interpolated to a 0-5-degrees grid, the HRBM data base of the present climate was recalculated for 18 000 BP. The most important processes which influenced the carbon storage include (1) climate-induced changes in biospheric processes and vegetation distribution, (2) the CO2 fertilization effect, (3) the inundation of lowland areas resulting from the sea level rise of 100 m. Two scenarios were investigated. The first scenario, which ignored the CO2 fertilization effect, led to total carbon losses from the terrestrial biosphere of -460 x 10(9) t. Scenario 2, which assumed that the model formulation of the CO2 fertilization effect as used for preindustrial to present could be extrapolated to the glacial 200 mul litre-1 (ppmv, parts per million per volume), gave a carbon fixation in the terrestrial biosphere of +213 x 10(9) t. The two scenarios were compared with CO2 concentration data and isotopic ratios from air in ice cores. The results of Scenario 1 are not in agreement with the data. Scenario 2 gives realistic deltaC-13 shifts in the atmosphere but the biospheric carbon storage at the end of the glacial period seems too large. The authors suggest that the low atmospheric CO2 concentration may have favoured the C-4 plants in ice age vegetation types. As a consequence the influence of the low CO2 concentration was eventually reduced and the glacial carbon storage in vegetation, litter, and soil was increased