In the popular media, CO2 is often called a pollutant;
and on television, global warming is frequently discussed against a background
of industrial smokestacks spewing out ominous columns of smoke and ash.
Is there any justification for this verbal and visual classification of CO2?
Answer:
As defined in Webster's New Universal Unabridged Dictionary, a pollutant
is something that makes things foul or unclean; i.e., pollutants taint,
contaminate and defile things. In the case of carbon dioxide, however,
we have a colorless odorless gas that cannot be seen nor smelled.
It currently comprises only 0.036% of the air we breathe (Houghton et
al., 1996); and although there is very little of it in the atmosphere,
it is vital for nearly all forms of life. 
Without it, in fact, we would not be here, as carbon dioxide is the principle
"food" that plants use to construct their tissues and which we either consume
directly or indirectly when we eat animals that have fed upon them (Wittwer,
1995). Consequently, it is clear that carbon dioxide does not contaminate
or defile things; it enhances and actually makes possible the very existence
of life on earth. By all counts, then, CO2 would
seem to be just the opposite of a pollutant. Indeed, it is a vital atmospheric
ingredient that makes our planet a place where all forms of life may thrive
(Idso, 1995).
References
Houghton, J.T., Miera Filho, L.G., Callander, B.A., Harris, N., Kattenberg, A. and Maskell, K (Eds.). 1996. Climate Change 1995: The Science of Climate Change. Cambridge University Press, Cambridge, UK.
Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN.
Wittwer, S.H. 1995. Food, Climate, and Carbon Dioxide: The Global Environment and World Food Production. CRC Press, Boca Raton, FL.