The most realistic experiments to date — free air experiments in an irrigated environment — indicate that C3 agricultural crops particularly respond favorably to the gradual rise of atmospheric CO2 concentrations over current levels (e.g., wheat yield increases by an average of 28%), although extrapolation of experimental results to real-world production where several factors (e.g., nutrients, temperature, precipitation, and others) are likely to be limiting at one time or another remains problematic. Moreover, little is known of crop response to elevated CO2 in the tropics, as most of the research has been conducted in the mid-latitudes.
Research suggests that for some crops, for example rice, CO2 benefits from a doubling of atmospheric CO2 concentrations over current concentrations may decline quickly as temperatures warm beyond optimum photosynthetic levels. However, crop plant growth may benefit relatively more from CO2 enrichment in drought conditions than in wet conditions.
Modeling studies suggest that any warming above current temperatures will diminish crop yields in the tropics while up to 2°C to 3°C of warming in the mid-latitudes may be tolerated by crops, especially if accompanied by increasing precipitation. Many developing countries are located in or near the tropics; this finding does not bode well for food production in those countries.
Recent advances in modeling of vegetation response suggest that transient effects associated with dynamically responding ecosystems to climate change will increasingly dominate over the next century, and that during these changes the global forest resource is likely to be adversely affected.
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