THE POTENTIAL RESPONSE OF TERRESTRIAL CARBON STORAGE TO CHANGES IN CLIMATE AND ATMOSPHERIC CO2

We use a georeferenced model of ecosystem carbon dynamics to explore t he sensitivity of global terrestrial carbon storage to changes in atmo spheric CO2 and climate. We model changes in ecosystem carbon density, but we do not model shifts in vegetation type. A model of annual NPP is coupled with a model of carbon allocation in vegetation and a model of decomposition and soil carbon dynamics. NPP is a function of clima te and atmospheric CO2 concentration. The CO2 response is derived from a biochemical model of photosynthesis. With no change in climate, a d oubling of atmospheric CO2 from 280 ppm to 560 ppm enhances equilibriu m global NPP by 16.9%; equilibrium global terrestrial ecosystem carbon (TEC) increases by 14.9%. Simulations with no change in atmospheric C O2 concentration but changes in climate from five atmospheric general circulation models yield increases in global NPP of 10.0-14.8%. The ch anges in NPP are very nearly balanced by changes in decomposition, and the resulting changes in TEC range from an increase of 1.1% to a decr ease of 1.1%. These results are similar to those from analyses using b ioclimatic biome models that simulate shifts in ecosystem distribution but do not model changes in carbon density within vegetation types. W ith changes in both climate and a doubling of atmospheric CO2, our mod el generates increases in NPP of 30.2-36.5%. The increases in NPP and litter inputs to the soil more than compensate for any climate stimula tion of decomposition and lead to increases in global TEC of 15.4-18.2 %.