Fortunat Joos, Robert Meyer, Gian-Kasper Plattner and Stefan Gerber,
Climate and Environmental Physics, Physics Insitute, University of
Bern, Bern, Switzerland
I. Colin Prentice, Max Planck Institute for Biogeochemistry, Jena,
Germany
Stephen Sitch, Potsdam Institute for Climate Impact Research, Potsdam,
Germany
Georg Hooss and Klaus Hasselmann, Max Planck Institute for
Meteorology, Hamburg, Germany
Global warming-terrestrial carbon cycle feedbacks are analyzed. A coupled physical-biogeochemical climate model that includes a dynamic global vegetation model and a representation of a coupled atmosphere-ocean general circulation model is driven by the non-intervention emission scenarios recently developed by the Intergovernmental Panel on Climate Change (IPCC). Atmospheric CO2, carbon sinks, radiative forcing by greenhouse gases (GHGs) and aerosols, changes in the fields of surface-air temperature, precipitation, cloud cover, ocean thermal expansion, and vegetation structure are projected. Until 2100, atmospheric CO2 increases to 540 ppm for the lowest and to 960 ppm for the highest scenario analyzed. Sensitivity analyses suggest an uncertainty in these projections of -10% to +30% . Radiative forcing is estimated to increase between 3 and 8 Wm-2 between now and 2100. Simulated warmer conditions in North America and Eurasia affect ecosystem structure: boreal trees invade high latitudes but are replaced by temperate trees and grasses in the mid latitudes. The consequences for terrestrial carbon storage depend on the assumed sensitivity of climate to radiative forcing, the sensitivity of soil respiration to temperature, and the rate of increase in radiative forcing by both CO2 and other GHGs. In the most extreme cases, the terrestrial biosphere becomes a source of carbon during the second half of the century. High GHG emissions, and high contributions of non-CO2 agents to radiative forcing favor a transient terrestrial carbon source.