Coupling between the climate system and the terrestrial carbon-cycle: study of the biogeochemical and bio-geographical response of biosphere to future climate change

This study describes the impact of both the CO₂ atmospheric concentration increase and of the induced climate change on terrestrial biosphere activity. It is shown that over the 21. century, the climate change reduces significantly the terrestrial absorption of CO₂. This results into a positive feedback of the carbon cycle onto climate change. The terrestrial global reduction is mainly explained by a reduction of the net uptake of tropical ecosystems which suffer from increasing drought. On the contrary, in high latitudes of Northern Hemisphere, increasing temperature stimulates the land biosphere. The robustness of our results is tested against uncertainties linked to both climate change and carbon cycle simulation. The use of biogeochemical, biophysical and dynamic vegetation in a complex terrestrial carbon model highlights the antagonism effects of vegetation on climate. We show that change in vegetation distribution tends to decrease the reduction of terrestrial carbon absorption under climate change in comparison with a fixed distribution. This induced a weaker positive feedback of carbon cycle on climate. Nevertheless, the modification of energy fluxes because of dynamic vegetation induced an increase of global surface temperature and also a more important positive carbon cycle-climate feedback. (author)