Summary

Terrestrial NPP response to expected global changes has two opposing trends: (1) NPP increases with rising atmospheric CO2; and (2) NPP decreases with recent and projected climate change. The relative balance between these two trends determines the long-term response of the global carbon cycle to global changes. This balance also determines the physiological response of ecosystems to altered environmental factors that influence productivity. This applies also for agricultural ecosystems from which we derive food and biomass for fuels, and

Historical + Stabilization

Historical + Stabilization

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Figure 7.3 Global total estimate net primary production using the Oak Ridge National Laboratory global terrestrial ecosystem carbon (ORNL-GTEC) model. Two scenarios are presented. Both are forced with historical and projected climate change. One run used a constant CO2 concentration, and the other includes a response to historical and projected future CO2 concentration.

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Figure 7.3 Global total estimate net primary production using the Oak Ridge National Laboratory global terrestrial ecosystem carbon (ORNL-GTEC) model. Two scenarios are presented. Both are forced with historical and projected climate change. One run used a constant CO2 concentration, and the other includes a response to historical and projected future CO2 concentration.

for forest ecosystems from which we derive wood products, fiber, and some energy. Ecosystems with greatest NPP have the greatest expected change with respect to CO2 and climate, both for increases with rising CO2, and decreases with climate change.

Other factors to be considered when projecting future global NPP that are not considered in the analysis here include historical and current land-use patterns, ecosystem N cycling and impact of atmospheric N-deposition, changes in hydrology of wetland ecosystems, and potential changes in fire frequency, insect outbreaks, and other disturbances that could be associated with climate changes. We have presented only the main biochemical and biophysical responses of terrestrial ecosystems to recent historical and future anticipated global change. The analysis is thus not yet comprehensive.

In our simulations, the changes in NPP were driven by historical and prescribed global changes in atmospheric CO2 concentration and associated climate change. The simulated changes in NPP are large enough to influence the global carbon cycle, and therefore potentially to feed back and change atmospheric CO2 and alter climate further. The simulated CO2 fertilization response would result in a negative feedback with simulated NPP resulting in the potential to increase carbon sequestration in terrestrial ecosystems reducing atmospheric CO2. On the other hand, the simulated response to climate change alone suggests the possibility of a positive feedback where additional warming leads to greater decreases in NPP that result in higher CO2 concentrations. The relative strengths of positive and negative feedbacks will determine the relative contribution of terrestrial ecosystem ecophysiology to enhancement or mitigation of climate change.

Other climate models are likely to project stronger decreases in NPP, and other ecosystem biogeochemistry models (Pan et al., 1998) are known to have more subdued responses to increased CO2. In combination, responses to future global change could range from the negative or stabilizing response indicated with the detailed processes models used in this study, to positive or destabilizing responses. Unstable positive feedback where climate change reductions in NPP become larger and the increase in NPP by CO2 fertilization become saturated is, however, possible (Cox et al., 2000). Our simulations indicate that this outcome is less likely than stabilizing negative feedback, as suggested by the coupled carbon cycle-climate change simulations by Freidlingstein et al. (2001).

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