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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