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Direct Effects of CO2
Photosynthetic rates in C3 plants increase by 25-75% for a doubling of CO2. For C4 plants the data are less conclusive and range from
no response to an increase of 10-25%. Results likely are temperature
dependent. Increases in CO2, with accompanying increases in
photosynthetic rate and decreased water requirement, translate into increased growth and crop yield in C3 plants, increased growth in
C4 plants, and increased tree seedling growth. The response to elevated CO2 will be most pronounced in regions where water
availability is a limiting factor.
The net responses of ecosystems to increases in CO2, both directly and indirectly through changes in temperature and water availability, are quite complex and only poorly understood. The actual growth enhancements expected in response to gradually increasing CO2 concentrations are likely to have only a small and gradual impact on terrestrial ecosystems globally.
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