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Carbon dioxide


Besides solar energy, the photosynthesis  of farm crops requires raw material. The raw materials that solar energy makes into carbohydrates are tex2html_wrap_inline2319 and water. Carbon dioxide furnishes the carbon and oxygen, and water furnishes the hydrogen for carbohydrate, tex2html_wrap_inline2321. Like solar energy, the quantity of tex2html_wrap_inline2319 and water form another elementary limit on food production. So the silver lining in the growing cloud of atmospheric tex2html_wrap_inline2319 that may warm the planet is more raw material for photosynthesis. Optimists see a stimulation of photosynthesis from more tex2html_wrap_inline2319, 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 tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 to raise yield.

But will more tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 under varying temperature, water supply, nutrient supply, light, and so forth. Cure (1985,[Cur85] 99-116) surveyed many tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 around it was raised, but subsequently photosynthesis slowed to that in normal tex2html_wrap_inline2319. In contrast, in an experiment initiated in 1987, Idso et al. (1991[IKA91]) grew orange trees in Arizona, at about twice the present tex2html_wrap_inline2319 concentration. Tree growth nearly tripled, and increases in growth and in photosynthetic rates continue unabated. Therefore, the interaction between tex2html_wrap_inline2319 and temperature as well as species will affect how vegetation responds as tex2html_wrap_inline2319 increases across the wide range of present climates. If climate as well as tex2html_wrap_inline2319 changes, then the ultimate responses are even harder to predict.

figure6.2.1 Figure 6.2.1. The net photosynthesis of wheat and maize leaves at different concentrations of tex2html_wrap_inline2319 (Akita and Moss, 1973).[AM73] 

While research sorts out the effects of tex2html_wrap_inline2319 on a global scale, the best refuge continues to be the curves showing how photosynthesis speeds up with rising tex2html_wrap_inline2319 (Figure 6.2.1). From 100 ppm to near 900 ppm, raising tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 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 tex2html_wrap_inline2319 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.gif Although rising concentration of tex2html_wrap_inline2319 won't materially augment, neither will it limit food production for ten billion people nor lessen the land that they can spare for Nature.

next up previous contents index
Next: Water Up: Do GlobalPhysical Limits Previous: SunWarmth, and land

Yasuko Kitajima
Thu Jun 19 16:20:56 PDT 1997