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Photosynthesis and the incorporation of photosynthate into plant biomass (net primary productivity, NPP) varies spatially and through time at individual locations as a response to climate. In particular, the amount and seasonal distribution of precipitation and the length and warmth of the growing season are the primary determinants of biomass production in natural ecosystems and also for agroecosystems. Over the past century regional meteorological measurements have shown that global climate is changing (Easterling et al., 1997). Some of this change has been the result of rising atmospheric concentration of CO2 from fossil fuel burning and land-use change (Folland et al., 2001). As CO2 concentrations in the air continue to rise, additional solar radiation is trapped in the atmosphere and warms the planet. Over the next century, CO2 concentrations will continue to rise and additional changes in climate are expected.

Global climate models project significant changes in temperature regimes and precipitation patterns over the next 100 years when they are forced with expected scenarios of atmospheric CO2 concentration changes from increasing fossil fuel burning. The response of the terrestrial ecosystems to this CO2 increase and climate change may be modeled based on information from field studies that examine the response of ecosystems to interannual climate variation and from experiments where temperature, moisture, or CO2 concentration has been manipulated.

We have developed and employed a terrestrial bio-geochemistry model that uses fundamental processes of plant and soil carbon dynamics to estimate NPP of terrestrial ecosystems over the past century and then into the future for the next 100 years. We completed two simulations with this model to examine the response of terrestrial ecosystems to climate change and rising CO2 concentration. In both simulations, a changing climate was used to generate historical and projected estimates of NPP. In the first simulation terrestrial ecosystems were stimulated by rising atmospheric CO2 concentrations in accordance with experimental findings. In a second simulation, we eliminated this CO2 stimulation by running the model with a CO2 concentration fixed at the 1930 amount. From the difference in response we are able to infer the relative effects of rising CO2 compared to those of climate change. Terrestrial ecosystem NPP increases with rising atmospheric CO2 concentration, and decreases with recent historical and future projected climate. The relative strength of these two opposing trends indicates whether global change will result in an increase or decrease in terrestrial ecosystem NPP over the next century.

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