Estimating permissible carbon emission for CO2 concentration stabilization using an Integrated Earth System Model

For mitigation of the global warming, the CO2 concentration must be stabilized at some level, and the estimate of the acceptable carbon emission for the stabilization is valuable information for society. The CO2 stabilization means that anthropogenic CO2 emission is balanced by nature uptake. Thus, using the Integrated Earth System Model (so called MIROC-"eKISSME' , Kawamiya et al, 2005), we calculated the carbon fluxes in and out of the natural terrestrial and oceanic carbon cycle under prescribed CO2 stabilization concentration scenarios from year 1850 to 2300. We used three CO2 stabilization concentration scenarios: SP450, SP550 and SP1000. To quantify climate-carbon cycle feedback effect, we set up the runs with and without the effect of the changing climate with the global warming on the carbon cycle model. As the land process relatively quickly adjusts to equilibrium in our model under stabilizing CO2 level, the carbon uptake by the land approaches to zero. Furthermore, the global warming significantly reduces carbon uptake by enhanced respiration. High surface air temperature in SP1000 even turns the land into the CO2 source. On the other hand, the ocean needs long time to equilibrium. CO2 uptake decreases with time, but remains as sink for long time. The changed climate with the global warming again reduces carbon uptake by the ocean. However, this feedback is relatively small compared with the land. The "epermissible emissions"f by human can be estimated by the atmospheric CO2 increase plus the modeled natural carbon uptake (Jones et al 2006, Hibbard et al. 2007). As the nature carbon uptake decreases, human must cut carbon emission in order to stabilize CO2 concentration. The reduced nature CO2 uptake by the climate feedback narrows the room for the anthropogenic carbon release. Fortunately, sustained by the long-lasting ocean carbon uptake, some emission is tolerated within the calculation time range.