TsuBiMo:: a biosphere model of the CO2-fertilization effect

Stimulation of photosynthesis at higher CO2 concentrations is repeatedly observed in short-term experiments at the single leaf level. We scaled up from leaf to globe by use of a model of half-degree spatial resolution (ca 50 x 50 km) and came to the conclusion that the CO2-induced increase in the total terrestrial net primary production (NPP) may be described by the same simple formula that has been frequently used over the last 2 decades: NPP(C-a) = NPP0 [1 +gammaln(C-a/C-0)], where C-a is the atmospheric CO2 concentration. We also concluded that NPP,) can be set at 61.6 Pg C yr(-1) and gamma at 0.35 (for C-0 = 340 ppmv) in this formula, Proceeding from aircraft measurements Of CO2 concentrations and the isotopic carbon ratio, we assumed that the turnover time of the biological cycle is 19.2 yr, and we estimated the sink induced by CO2 fertilization to be 1.3 Pg C yr(-1) (averaged for the period 1980-90), It is worth mentioning, however, that the estimate of the turnover time is so sensitive to the average isotopic shift that occurs on air-plant exchange that one can hardly define a feasible confidence interval for it.