Climate Change and Net Primary Production

Three globally significant issues are as follows:

30.1.1.1 Climate Change: CO2 Fertilization Impacts on Terrestrial Ecosystem Production

The relative contributions of CO2 fertilization and climate change effects on terrestrial CO2 sources and sinks have been estimated for different ecoregions. They suggest that the CO2 effect on total net primary production (NPP) content will be positive, and that NPP will decrease without it. On the other hand, the effect of projected climate changes on total NPP appear to be negative. Aggregate global impacts of CO2 fertilization and climate change on CO2 appear to be positive.

Modeling results also suggest that the geographic distribution of NPP will change along with changes in CO2 and climate. Ecosystems with the highest NPP show the greatest change. The effects of other factors, such as land use history and nitrogen cycling, have generally not been considered in these simulation models. They need to be carefully assessed.

30.1.1.2 Climate Change Impacts on Forest Ecosystems

Free-air CO2 enrichment (FACE) experiments have shown that trees grow faster under elevated levels of CO2. These findings suggest that temperate forests may be stimulated by higher atmospheric CO2 levels. However, data also suggest that tree growth may be negatively affected by increasing temperatures. In warmer climates, tropical forests could become a source rather than a sink for CO2. If all forests in the world were to experience stimulated growth from increased atmospheric CO2 levels, the effect would be minor relative to CO2 emissions from continued fossil fuel burning.

30.1.1.3 Climate Change Impacts on Water Supply

The potential impacts of climate change on global water supplies are of great importance. Results of a general circulation model (GCM) based on watershed data suggest that water flows will increase during the fall, winter, and spring seasons, but decrease during the summer. However, the increased demand for water associated with projected population increases will offset any additional water supplies over time. Future climate changes may reduce the carrying capacity of major reservoirs and water flows in rivers around the world because of large water demands for irrigated agriculture.

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