SENSITIVITY OF THE TERRESTRIAL BIOSPHERE TO CLIMATIC CHANGES - IMPACTON THE CARBON-CYCLE

The biosphere is a major pool in the global carbon cycle; its response to climatic change is therefore of great importance. We developed a 5 -degrees x 5-degrees longitude-latitude resolution model of the biosph ere in which the global distributions of the major biospheric variable s, i.e. the vegetation types and the main carbon pools and fluxes, are determined from climatic variables. We defined nine major broad veget ation types: perennial ice, desert and semi-desert, tundra, coniferous forest, temperate deciduous forest, grassland and shrubland, savannah , seasonal tropical forest and evergreen tropical forest. Their geogra phical repartition is parameterized using correlations between observe d vegetation type, precipitation and biotemperature distributions. The model computes as a function of climate and vegetation type, the vari ables related to the continental biospheric carbon cycle, i.e. the car bon pools such as the phytomass, the litter and the soil organic carbo n; and carbon fluxes such as net primary production, litter production and heterotrophic respiration. The modeled present-day biosphere is i n good agreement with observation. The model is used to investigate th e response of the terrestrial biosphere to climatic changes as predict ed by different General Circulation Models (GCM). In particular, the i mpact on the biosphere of climatic conditions corresponding to the las t glacial climate (LGM), 18 000 years ago, is investigated Comparison with results from present-day climate simulations shows the high sensi tivity of the geographical distribution of vegetation types and carbon content as well as biospheric trace gases emissions to climatic chang es. The general trend for LGM compared to the present is an increase i n low density vegetation types (tundra, desert, grassland) to the detr iment of forested areas, in tropical as well as in other regions. Cons equently, the biospheric activity (carbon fluxes and trace gases emiss ions) was reduced