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.