CO2: Villain or Friend? An Exclusive Interview with Keith E. Idso

Published January 1, 1999

Dr. Keith E. Idso earned his Ph.D. in botany from Arizona State University in 1997. Prior to that, he received his M.S. degree in agronomy and plant genetics and his B.S. degree in agriculture, both from the University of Arizona.

In 1992, Idso compiled one of the most comprehensive reviews ever made of the peer-reviewed scientific literature pertaining to the effects of increasing atmospheric CO2 levels on plant growth. In 1994, he was the senior author of a paper that reviewed plant responses to elevated atmospheric CO2 levels when the plants were simultaneously exposed to various environmental stresses (high temperature, high soil salinity, and aerial pollutants) or resource limitations (inadequate soil moisture, soil nutrients, and sunlight).

Dr. Idso currently teaches biological science classes within the Maricopa County Community College District and is vice president of the Center for the Study of Carbon Dioxide and Global Change. He spoke recently about his work with EN contributing editor Bonner Cohen.

Cohen: Proponents of the theory of global warming frequently refer to carbon dioxide (CO2) as a pollutant. Is it?

Idso: Carbon dioxide is not a pollutant. It is a colorless, odorless trace gas that actually sustains life on this planet.

Consider the simple dynamics of human energy acquisition, which occurs daily across the globe. We eat plants directly, or we consume animals that have fed upon plants, to obtain the energy we need. But where do plants get their energy? Plants produce their own energy during a process called photosynthesis, which uses sunlight to combine water and carbon dioxide into sugars for supporting overall growth and development. Hence, CO2 is the primary raw material that plants depend upon for their existence.

Because plants reside beneath animals (including humans) on the food chain, their healthy existence ultimately determines our own. Carbon dioxide can hardly be labeled a pollutant, for it is the basic substrate that allows life to persist on Earth.

Cohen: Throughout the Earth’s history, atmospheric levels of CO2 have fluctuated considerably. How does the present time measure up?

Idso: Three-and-a-half billion years ago, there may have been as much as 70,000 ppm CO2 in the air. Its concentration has fluctuated greatly throughout Earth’s geologic history, dropping to as low as 180 ppm during the peak of the last great ice age.

When most plant life was evolving some 300 to 200 million years ago, the CO2 concentration of the air was around 2,000 to 3,000 ppm. Hence, most plants developed their photosynthetic machinery when the atmospheric CO2 concentration was much higher than the meager 365 ppm of today. It is thus only natural that they should grow so much better when the air’s CO2 content rises.

Cohen: So there are real benefits to be derived from higher levels of CO2?

Idso: There are many direct biological benefits that result from higher atmospheric CO2 concentrations. Two of the most important are increased plant photosynthesis and water-use efficiency.

Because most plants developed their photosynthetic machinery at a time when the CO2 content of the air was much higher than it is today, their “biological engines” are tuned to operate better at higher CO2 concentrations, which makes them grow better. And this is precisely what has been shown for most plants in literally thousands of atmospheric CO2 enrichment experiments.

In addition, during photosynthesis, tiny pores on the surfaces of leaves open to allow the inward diffusion of CO2 for sugar production. When these pores open, water vapor exits the leaves via a process called transpiration. With higher amounts of CO2 in the atmosphere, plants do not need to open these pores as wide or as often as they do at lower CO2 concentrations to acquire this vital gas, so transpirational water losses decrease. Hence, the amount of atmospheric carbon incorporated into sugars per unit of water loss (that is, water-use efficiency) increases with rising CO2 levels.

Cohen: Then increased CO2 levels benefit agriculture, right?

Idso: As the CO2 content of the air continues to climb, most agricultural crops should respond by increasing their yields. Agricultural crop responses to elevated CO2 have been studied extensively, and the peer-reviewed scientific literature shows that for a doubling of the atmospheric CO2 content, yield is boosted by 30 to 40 percent for most species.

Moreover, because transpirational water loss is generally reduced at elevated CO2 levels, these greater yields are achieved with about one-third less water loss per leaf. Hence, the water-use efficiencies of the leaves of most agricultural plants actually double with a doubling of the atmospheric CO2 content. And this phenomenon will become increasingly more important as agricultural water supplies are reduced to offset increased human needs in growing urban areas.

Cohen: Is there a point beyond which higher CO2 levels are no longer beneficial?

Idso: In controlled experiments, most plants continue to show significant increases in photosynthesis to CO2 concentrations of 1000 ppm. Beyond that point, growth responses may still be achieved in many cases, but the returns are not as great.

Cohen: Why is there so much ignorance about CO2?

Idso: There appears to be a lot of confusion about the biological benefits of elevated CO2 because the popular media typically fails to report them. Doomsday scenarios of global warming are much more dramatic than the good news of global greening.

However, when I discuss the biological benefits of elevated CO2 with groups of people from all walks of life, most of them can recall some high school biology class where they learned that CO2 was something plants needed for photosynthesis. And they rapidly make the connection between that fact and the rising atmospheric CO2 concentration and realize that it is completely logical to expect a great “greening of the Earth” in the not so distant future.

Cohen: Nowhere is the debate over greenhouse gas emissions and their possible effect on the Earth’s climate more controversial than in the U.S. How have your findings been received here and abroad?

Idso: My general findings–which are representative of those of hundreds of other scientists–have been received rather well throughout the world, especially when people are given the opportunity to learn of them. My 1994 review paper, for example, has received a large number of citations in the peer-reviewed scientific literature because it establishes an important biological fact: namely, that the CO2-induced growth enhancement is typically greater for stressed plants than it is for non-stressed plants on a percentage basis. Let me give a very simplified example to illustrate this point.

Let’s say you have two bean seedlings, one growing under optimal conditions and one that is stressed for water. The healthier, well-watered plant should logically be bigger, and after a week of growth, you might actually measure its height to be 10 cm, while that of the water-stressed plant is only 2 cm. If both plants then received atmospheric CO2 enrichment for an entire week, one might find them to be 11 and 2.5 cm tall, respectively. In this case, the healthy plant displayed an absolute CO2-induced growth increase of 1 cm, while the stressed plant exhibited only half as much (0.5 cm). However, on a percentage basis, the 25 percent increase displayed by the stressed plant is clearly much larger than the 10 percent increase exhibited by the healthy plant.

In short, rising levels of atmospheric CO2 may be more important than we initially thought, as most of the world’s natural vegetation is subject to all kinds of environmental stresses and resource limitations, the effects of which would be significantly ameliorated with increasing atmospheric CO2.

Cohen: The Kyoto Protocol establishes an elaborate international regulatory system designed primarily to reduce carbon emissions as a way of combating global warming and promoting alternative sources of energy. Is this a realistic goal? Is it a worthy goal?

Idso: The Center for the Study of Carbon Dioxide and Global Change takes no official stand on any political issue, but instead supplies the science that can be used by others to construct scientifically sound environmental policies.

As an informed individual, however, I know that in order to sustain the increasing human population, we need to produce greater agricultural and timber yields to meet humanity’s increasing requirements for food, fuel, and shelter. As a scientist, I have analyzed over one thousand observations from the peer-reviewed scientific literature dealing with plant responses to atmospheric CO2 enrichment, of which 93 percent were favorable, 4 percent were indifferent, and only 3 percent were negative.

With this information, in light of the increasing human demands on vegetation, it is my personal opinion that capping CO2 emissions or reducing them to some prior level would be akin to “biting the hand that feeds us.”

Cohen: Could you tell us something about the Center for the Study of Carbon Dioxide and Climate Change? What kind of work are you doing?

Idso: The Center for the Study of Carbon Dioxide and Global Change is a nonprofit organization specializing in research and educational developments related to the rising CO2 content of Earth’s atmosphere.

We maintain a Web site at that is dedicated to the timely interpretation and dissemination of the peer-reviewed scientific literature addressing the effects of rising atmospheric CO2 levels on the biosphere. We discuss both the biological effects of elevated CO2 and the more elusive role it plays in climate regulation. Our site also contains an on-line “Global Change Laboratory,” where we actually conduct real-time CO2 enrichment and depletion experiments on plants and show others how to do so in their own homes or schools. This aspect of our site is an excellent educational tool that can be used by science teachers everywhere to document for their classes the very real beneficial effects of elevated CO2 on plants.

Cohen: As this debate unfolds, how much success are you having countering the disinformation that is common currency in the media and elsewhere?

Idso: At our newly established Web site, we hope to counter gross misinformation that may appear in the popular media. In fact, we have a section called “Fact or Fiction” that we designed for that very purpose.

As far as success is concerned, we have only recently begun this endeavor, and we will have to wait a bit to see how our efforts will be received. Our Web site is designed to be a clearinghouse of scientific information pertaining to the effects of rising CO2 levels on various aspects of global change. Its primary products are timely and objective reviews of scientific research reports on the biological and climatological effects of atmospheric CO2 enrichment, which are posted on the first and fifteenth day of each month in our “Biological Reviews” and “Climatological Reviews” sections.

Each issue also contains a short editorial written by a member of the center’s staff or one of the members of its Scientific Advisory Board. Other Web site materials designed to inform the public about the scientifically demonstrable effects (and non-effects) of atmospheric CO2 enrichment are our “Topical Reviews,” “Fact Sheets,” “Fact or Fiction,” and “Questions & Answers” sections.

The last two sections of our Web site are the “Book Reviews” and “Educational Materials” sections, the latter of which I have already briefly described. We try to make the information contained in our site both interesting and useful. We welcome the comments of your readers and look forward to providing a valuable public service for many years to come!