Besides solar energy, the photosynthesis of farm crops requires raw material. The raw materials that solar energy makes into carbohydrates are and water. Carbon dioxide furnishes the carbon and oxygen, and water furnishes the hydrogen for carbohydrate, . Like solar energy, the quantity of and water form another elementary limit on food production. So the silver lining in the growing cloud of atmospheric that may warm the planet is more raw material for photosynthesis. Optimists see a stimulation of photosynthesis from more , and pessimists see little benefit or even harm.
Compare the following two headlines. On September 18, 1992. The New York Times headlined, ``Report Says Carbon Dioxide Rise May Hurt Plants" (The New York Times, 1992.[NYT92] Announced Korner and Arnone III, 1992[KJA92]). Within five days The Des Moines Register (1992,[DMR92] 1A. Announced Wittwer, 1992[Wit92]) headlined, ``Researcher Says Global Warming Would Help Crops" and wrote, ``Plants--including Iowa's two major crops, maize and soybeans--would benefit from global warming and its higher levels of carbon dioxide." The headlines demonstrate that the long-term effect of more is important enough to be headlined in the popular press and uncertain and controversial enough that the headlines disagree.
Scientists have known for nearly two centuries that elevated levels of in the air enhance the growth of plants (Wittwer, 1986,[Wit86] 3-15). Indeed, since the early 1960s, horticulturalists have enriched greenhouses about three times today's outdoor concentration whenever the greenhouses require no ventilation for cooling (Enoch and Kimball, 1986[EBAK86]). Under more or less ideal greenhouse conditions, practical people have long exploited the benefit of increased to raise yield.
But will more outdoors help ten billion spare more land for Nature? The outdoors typically has a wider range of temperature, more frequent limitations of water and nutrients, and brighter sunlight than do greenhouses. So people need to know whether plants respond the same to under varying temperature, water supply, nutrient supply, light, and so forth. Cure (1985,[Cur85] 99-116) surveyed many enrichment experiments with ten important crop species and with temperature and other conditions varied as they might be outdoors. Numerous variations were not explored, and although some unexplored variations since have been evaluated, many more have not. Experiments revealing behavior for an entire season and allowing some acclimation are, of course, needed. So much remains to be done.
To illustrate the range of possible outcomes outdoors and in natural circumstances, I cite Tissue and Oechel (1987[TO87]), who observed that photosynthesis in an Alaskan tundra grass increased for a few weeks after the around it was raised, but subsequently photosynthesis slowed to that in normal . In contrast, in an experiment initiated in 1987, Idso et al. (1991[IKA91]) grew orange trees in Arizona, at about twice the present concentration. Tree growth nearly tripled, and increases in growth and in photosynthetic rates continue unabated. Therefore, the interaction between and temperature as well as species will affect how vegetation responds as increases across the wide range of present climates. If climate as well as changes, then the ultimate responses are even harder to predict.
Figure 6.2.1. The net photosynthesis of wheat and maize leaves at different concentrations of (Akita and Moss, 1973).[AM73]
While research sorts out the effects of on a global scale, the best refuge continues to be the curves showing how photosynthesis speeds up with rising (Figure 6.2.1). From 100 ppm to near 900 ppm, raising 1% generally speeds the photosynthesis of wheat and maize 0.7%. Within that generality lie some important exceptions. At present concentrations, photosynthesis is faster in the C4, or maize, class than in the C3, or wheat, class. Above 300 ppm, however, raising 1% speeds the photosynthesis of the wheat class only 0.4% and of the maize class only 0.2% (Akita and Moss, 1973[AM73]).
Controversy clouds the direct effect on growth of increasing the supply of the raw material in the air. In the end, however, the effect of the gas on photosynthesis seems almost surely to be as beneficial outdoors as it is in greenhouses. Carbon dioxide in the air likely will continue rising 1 to 2 ppm/yr, raising the concentration approximately one-third while population grows to ten billion. Measurements of photosynthesis rates suggest that this will increase photosynthesis and perhaps crop growth some 10% in crops such as wheat and 5% in those such as maize. Although rising concentration of won't materially augment, neither will it limit food production for ten billion people nor lessen the land that they can spare for Nature.