Results

Figure 7.1 gives the estimated annual NPP of the most geographically extensive terrestrial ecosystems (Table 7.1) as calculated by the GTEC model using historical climate data for 1930 to 1995 and projected climate data for 1995 to 2100. The CO2 concentration of the atmosphere used in the simulation follows observed concentrations for the historical period that are rising exponentially to 385 ppm by 1995. The rate of atmospheric CO2 increase slows thereafter according to the stabilization scenario and reaches equilibrium of 550 ppm by 2150.

Historical + Stabilization

Historical + Stabilization

Year

Figure 7.1 Model estimate of terrestrial net primary productivity for ecosystem types that cover large areas. Simulations are forced with historical climate and CO2 concentration and projected climate change and CO2. Projected climate change is output from parallel climate model using a stabilization CO2 emission scenario.

Year

Figure 7.1 Model estimate of terrestrial net primary productivity for ecosystem types that cover large areas. Simulations are forced with historical climate and CO2 concentration and projected climate change and CO2. Projected climate change is output from parallel climate model using a stabilization CO2 emission scenario.

During the historical period and into the future, even after CO2 concentration stabilizes, the model simulations indicate an increase in terrestrial ecosystem NPP. This increase in NPP is greatest in the tropical ecosystems (broadleaf evergreen forest and wooded C4 grassland), but holds for all ecosystems including cultivation. Cultivated ecosystems are expected to show even greater increases in NPP than indicated here due to expected improvements in management (irrigation, fertilization, genetic selection and engineering, etc.) that are not included in the simulations. Tropical ecosystems show a greater interannual variation in NPP. This is the result of interannual climate fluctuations from the El

Nino-Southern Oscillation affecting the balance of ecosystem production and vegetation respiration.

Most of the increase in ecosystem NPP may be attributed to CO2 fertilization effects. Both the direct influence on photosynthesis and indirect effect on water balance contribute to the simulated response (DeLucia et al., 2005, this volume). We can demonstrate the influence of CO2 on the simulation results by observing what happens in simulations in which we effectively turn off the CO2 effects. Figure 7.2 shows simulation results with historical and projected climate change, but with CO2 held constant at the 1930 concentration. In this simulation, which shows the influence of climate alone, terrestrial ecosystem NPP remains largely constant for the period 1930 to 1990 and then begins to decline. The decline in terrestrial ecosystem NPP, due to climate alone, is most noticeable for the tropical ecosystems with a stronger decrease in NPP after 2000.

The global total NPP over all ecosystems, including those with small land areas not presented in Figures 7.1 and 7.2, shows the opposing effects of CO2 fertilization and climate change more dramatically. With CO2 fertilization included, global NPP increases more or less linearly, although the simulation starts to level off toward the end of the next century after the CO2 concentration stabilizes. The influence of rising CO2 concentration is stronger than indicated since climate change impact on NPP has counteracted some of the potential increase in NPP. This is indicated by the climate change only simulation results that show a decrease in NPP from 47 to 40 Pg C year-1 (Pg = petagram = 1 gigaton). With a CO2 effect included, NPP increased to 63 Pg C year-1. If we add the estimated decrease of 7 Pg C year-1 due to climate change, we estimate that the effect of CO2 alone would have increased NPP to nearly 70 Pg C year-1 by 2100. The effect of CO2 therefore was to counteract a 15% decrease in global NPP due to climate change and result in a net 34% increase in NPP by the end of the 21st century.

Historical + Stabilization

Historical + Stabilization

Year

Figure 7.2 Model estimate of terrestrial net primary productivity forced with historical climate and projected climate estimated using output from the parallel climate model as in Figure 7.1 but without a CO2 fertilization effect. The effect of rising atmospheric CO2 concentration on photosynthesis, in this simulation, is effectively turned off by holding CO2 constant at the 1930 concentration.

Year

Figure 7.2 Model estimate of terrestrial net primary productivity forced with historical climate and projected climate estimated using output from the parallel climate model as in Figure 7.1 but without a CO2 fertilization effect. The effect of rising atmospheric CO2 concentration on photosynthesis, in this simulation, is effectively turned off by holding CO2 constant at the 1930 concentration.

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