The direct effects of increased carbon dioxide (CO₂) on plant growth refers to the change in plant grow with the levels of temperature, precipitation, evaporation and growing season at their present values. The indirect effects include the results of any changes in the other variables which affect plant growth that come as a result of the effect of increased CO₂ on global climate.

Photosynthesis and the C3/C4 Plant Classification

Life is based upon chemical reactions; many, many chemical reactions; but the chains of chemical reactions known as photosynthesis are the basis in one way or another of all life. Photosynthesis involves the input of carbon dioxide and water with radiant energy and the presence of a catalyst called chlorophyll. The outputs are carbohydrates and oxygen. The formal statement of the process is:

6CO₂ + 6H₂O + ν → C₆H₁₂O₆ + 6O₂

where ν represents photons of radiation.

The catalyst for the reaction, chlorophyll, is an organo-metallic compound containing magnesium. It is one of the three organo-metallic compounds which are the basis for life. The other two are the vital elements of the blood of mammals, hemoglobin, and of crustaceans, hemocyanin. Just as chlorophyll contains magnesium, hemoglobin contains iron and hemocyanin contains copper.

The process of photosynthesis is very complex and chemists could find out little about the processes until radioactive isotopes became available. First, the heavier isotope of oxygen, ¹⁸O, was used to create water, H₂O. When plants were exposed to this abnormal water the ¹⁸O isotope showed up in the mass spectrographic analysis of the oxygen exhaled from the plants. This showed that the oxygen created by plants comes from the water it uses rather than from the CO₂. The oxygen in the CO₂ gets incorporated in the carbohydrates created by the plants.

Second, a radioactive isotope of carbon, ¹⁴C, was used to create carbon dioxide. Plants were exposed to this radioactive CO₂ for a few seconds and then the leaf material was chemically analyzed. In most plants the radioactive carbon showed up in a compound called phosphoglyceric acid (PGA). The molecule of this compound contains three carbon atoms and one atom of phosphorus:

  
                           H   H   H
                           |   |   |
                        O- C - C - C -H
                        ||  |   |
                         O  O O-P-O
                            |   |                         
                            H   O-H

Most plants, including trees and flowering plants, produce PGA as the first step in photosynthesis. A few plant species, including tropical grasses such as sugar cane and corn (maize), produce malic acid or aspartic acid as the first step. The molecules of these compounds contain four carbon atoms and one nitrogen atom. The aspartic acid molecule is:

                              H H
                              | |
                        H-O-O-C-C-O-O-H
                              | |
                              H N-H
                                |
                                H

Because the initial products of photosynthesis for plants in this category involve compounds containing four carbon atoms this class is called C4. The other category of plants produces PGA which contains three carbon atoms so it is called C3. This classification is important because the responses of the two categories of plants to increased CO₂ are different.

The C3 vegetation developed about 3.5 billion years ago whereas the C4 vegetation developed only about 12 million years ago. C4 vegetation is better able to survive in hot climates than is C3 vegetation so now C4 vegetation dominates the tropics and C3 the temperate zone.

The Direct Effect of an Increase in CO₂

Over the years there have been numerous laboratory experiments which conclude that increases levels of CO₂ result in increased plant growth no matter how that plant growth is quantified. Sylvan Wittwer in Food, Climate and Carbon Dioxide tabulates the results. He observes

The effects of an enriched CO₂ atmosphere on crop productivity, in large measure, are positive, leaving little doubt as the benefits for global food security …. Now, after more than a century, and with the confirmation of thousands of scientific reports, CO₂ gives the most remarkable response of all nutrients in plant bulk, is usually in short supply, and is nearly always limiting for photosynthesis … The rising level of atmospheric CO₂ is a universally free premium, gaining in magnitude with time, on which we can all reckon for the foreseeable future.

The quantification of the enhanced growth due to higher levels of CO₂ has been given by H. Poorter in an article in the journal Vegetation:

About 95 percent of all plants on Earth are of type C3. C4 plants constitute only 1 percent but the C4 crops of sugar cane, corn, sorghum and millet are economically significant. The other 4 percent of plants are not economically significant. They include desert plants such as cactus. These plants are labeled CAM for Crassulacean Acid Metabolism. They keep their stomata closed during the day to prevent excessive loss of water and open during the night to absorb CO₂.

The Effect of Temperature on Plant
Response to Higher Levels of CO₂

Photosynthesis consists of chemical reactions. Chemical reactions proceed at a higher rate at higher temperatures. The rule of thumb is that there is a doubling of the reaction rate for every 10°F rise in temperature. Plants grow faster at a higher temperature providing they have adequate levels of CO₂, water, sunlight and plant nutrients. The C4 plants have a great response rate for a higher temperature than does the C3 plants.

A higher temperature without adequate level of the necessary ingredients for growth might produce no response or even damage. Sylvan Wittwer, quoted above, states that under most circumstances the availability of CO₂ is the factor which limits growth. Thus with a higher level of CO₂ in the air plants can grow faster with a higher temperature.

Plants transpire water vapor to keep an even temperature. There are tiny holes on the underside of plant leaves, called stomata, which are the openings through which the plant absorbs CO₂. With higher level of CO₂ concentration in the air the stomata do not have to be open as wide. The narrower opening means that less water is transpired and thus less water is required by the plants. In other words, higher levels of CO₂ increase the efficiency of water use by plants. This was confirmed in experiments reported by K.E. Idso and S.B. Idso. They found that enhanced CO₂ increased growth by 31 percent in plants with adequate moisture but it increase growth by 62 percent for plants in moisture-stressed condition. In effect, enhanced CO₂ by reducing water loss created the same effect as providing more water. Thus the effect in moisture-stressed plants was the effects of enhanced CO₂ plus the effect of increased water.

The effect of increased CO₂ in narrowing the stomata of plants has the additional benefit that a lesser amount of pollutants in the air will make it through the narrower openings. Thus enhanced CO₂ has the effect of protecting plants against damage from air pollutants such as ozone or sulfur dioxide.

The effect of enhanced CO₂ is even greater for plants grown under low light conditions. The enhance growth is greater than 100 percent for a 100 percent increase in CO₂. This compares to less than 50 percent for plants grown in normal light conditions.

The evidence that clinches the argument is that some greenhouse owners artificially elevate the CO₂ level to triple what the level in the atmosphere is.

(To be continued.)

Sources:
 
Sylvan H. Wittwer, "Flower power: rising carbon dioxide is great for plants", Policy Review (Fall 1992), pp. 4-10.
 
H. Poorter, "Interspecific variation in the growth response to an elevated and ambient CO₂ concentration," Vegetation (1993), pp. 77-97.
 
Sylvan H. Wittwer, Food, Climate and Carbon Dioxide, CRC Press, Boca Raton, Fla., 1995.
 
Patrick J. Michaels and Robert C. Balling, Jr., The Satanic Gases: Clearing the Air about Global Warming, Cato Institute, Washington, D.C., 2000.
 
Fred Pearce, "Global green belt," New Scientist, (September 15, 2001), p.15.