Is CO2 a Pollutant?

Originally posted on August 9, 2005.

A recent news article illustrates a popular understanding of carbon dioxide as a pollutant. Referring to carbon permit trading it reports:

“These brokers don’t trade stocks or bonds or gold or oil. What they trade is pollution. To be exact, they buy and sell the right to foul the air with carbon dioxide, a greenhouse gas that the U.S. National Academy of Sciences says causes global warming.”

The term “foul” has a number of definitions according to the Webster New World Dictionary, but the most appropriate in the context of the above quote is that it means:

“so offensive to the senses as to cause disgust; stinking; loathsome” and “extremely dirty or impure”; disgustingly filthy.”

A “pollutant” is defined as:

“a harmful chemical or waste material discharged into the water or atmosphere.”

To “pollute” is to:

“make unclean, impure, or corrupt; defile; contaminate; dirty.”

The American Meteorological Society’s Glossary lists the definition as:

air pollutionThe presence of substances in the atmosphere, particularly those that do not occur naturally. These substances are generally contaminants that substantially alter or degrade the quality of the atmosphere. The term is often used to identify undesirable substances produced by human activity, that is, anthropogenic air pollution. Air pollution usually designates the collection of substances that adversely affects human health, animals, and plants; deteriorates structures; interferes with commerce; or interferes with the enjoyment of life. Compare airborne particulates, designated pollutant, particulates, criteria pollutants.

The question is: How does atmospheric carbon dioxide fit into this definition? Carbon dioxide does occur naturally, of course, and is essential to life on Earth, as it is an essential chemical component in the photosynthesis process of plants. This is in contrast with other trace gases in the lower atmosphere such carbon monoxide, ozone, and sulfur dioxide which are have direct health and environmental effects on humans and vegetation. Indeed, when combustion is optimized, less carbon monoxide and more carbon dioxide are produced. There are no positive effects that I am aware of at any level of these pollutants in the lower atmosphere.

Thus, it is more informative to define anthropogenic inputs of carbon dioxide as a climate forcing, as was done in the 2005 National Research Council Report. This provides the recognition that carbon dioxide does not have direct health effects as implied by the news article that carbon dioxide “fouls” the air, but it does significantly affect our climate. Of course, carbon monoxide, ozone, and sulfur dioxide are also climate forcings. When these other atmospheric constituents are referred to in news articles and elsewhere, we would benefit by a distinction between an “air pollutant” and a “climate forcing” depending on the context.

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