Some tentative clues to the effects of increased C02 on plants come from short-term observations of individual leaves exposed to artificially high C02 concentrations. It is possible to estimate how fast a leaf is photosynthesizing by measuring the uptake of C02 labeled with radioactive 14C. The more radioactive the leaf is at the end of the experiment, the more carbon it has managed to fix by photosynthesis. Such small-scale experiments on raised C02 have tended to involve a doubling of C02 from about 350 ppm—the approximate "background" level of C02 during the past couple of decades—to 700 ppm, a level that C02 is likely to reach well before the end of this century. Short-term exposure to high C02 tends to result in a major increase in the amount of sugars fixed—a typical sort of change observed would be a doubling or tripling of the rate of photosynthesis. In these experiments, C02 tends to increase photosynthesis by proportionately greater amounts at higher temperatures. At 30°C, the relative "gain" from C02 fertilization is about 30% greater than at 20°C. This is because at high temperatures plants suffer quite badly from a process known as photorespiration where oxygen "gets in the way" of the photosynthetic reaction. Raising the C02 level helps push carbon instead of oxygen into the reaction, preventing photorespiration. Hence the greater benefit that comes at higher temperatures where the problem of photorespiration is especially acute.
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