Modelling the Terrestrial Biosphere Under Long Term Climate Change Scenarios With a Complex Earth System Model

A complex coupled earth system model, consisting of an atmosphere and ocean general circulation model, an ocean biogeochemistry model, a 3-dimensional thermodynamical ice sheet model and a dynamic global vegetation model, was used to study the long term behaviour of climate and carbon cycle under increased atmospheric CO2 levels. A set of experiments was performed, forced with CO2 emissions from historical reconstructions (1750-2000), the SRES IPCC scenarios B1, A1B and A2 (2001-2100) and an exponential decay of the emissions for the period 2101-3000. The experiments give a reasonable reconstruction of the measured CO2 concentrations up to 2000. After stabilization of the CO2 concentration (between 2200 and 2500, depending on the scenario), the terrestrial biosphere equilibrates in a few hundred years. The ocean stays a sink for carbon all up to the end of the experiments. Carbon uptake by the terrestrial biosphere is fastest and largest in the tropical regions. The northern hemisphere high latitudes tend to store large amounts of carbon as well due to a northward shift of the boreal forests. This process is slower, but in the end of the same size as in the tropical regions. Offline experiments with only climate change or CO2 concentration as forcing are used to distinguish between climate change and CO2 change effects. CO2 increase alone causes an uptake of carbon by the terrestrial biosphere, the accompanying climate change reduces the uptake of carbon almost everywhere, mainly due to higher respiration rates caused by higher temperatures.