In the popular media, CO2 is often referred to as a pollutant; and on television, the theory of CO2-induced global warming is frequently discussed against a backdrop of industrial smokestacks spewing out ominous columns of smoke and ash. In reality, however, CO2 is just the opposite of a pollutant [see our answer to the question Is Carbon Dioxide a Pollutant?], as this colorless and odorless trace constituent of the atmosphere actually stimulates the growth and development of nearly all plants as its aerial concentration gradually rises, while having no adverse effects on human health. In fact, elevated concentrations of atmospheric CO2 typically tend to counteract the deleterious effects of real air pollutants on plant life [see, for example, Ozone (Effects on Plants) in our Subject Index, as well as the study of Deepak and Agrawal (2001), which describes the ability of CO2-enriched air to completely alleviate the adverse effects of SO2 pollution on two different cultivars of soybean].
So what does CO2 have to do with what we normally consider to be air pollutants? For one thing, it's starting to look like a scapegoat for them, as certain scientists [Cifuentes et al. (2001), for example] are suggesting we fight air pollution by reducing greenhouse gas emissions [see our Editorial The Distempered Brain Strikes Again!]. What makes this proposal doubling disconcerting is that many of the air pollutants singled out for attack within this context may not be as deadly as has long been believed [see our Editorial What Kills People During Air Pollution Episodes?]. With respect to tiny particulates, for example, Smith et al. (1999) conclude there are "too many uncertain issues to allow us to make definitive statements about a causal relationship between PM10 and mortality." And in a study of the human-mortality effects of sulfur dioxide (SO2), carbon monoxide (CO), smoke, and a number of meteorological parameters during high-pollution days in London over the period 1976-1995, Keatinge and Donaldson (2001) conclude that "the large, delayed increase in mortality after low temperature is specifically associated with cold and is not due to associated patterns of wind, rain, humidity, sunshine, SO2, CO, or smoke." This fact makes the suggestion of Cifuentes et al. even more disturbing, as their proposal to fight air pollution by curtailing greenhouse gas emissions would work at cross purposes with the potential for atmospheric CO2 enrichment to elevate daily minimum air temperature and thus reduce the true death-dealing influence of many air pollution episodes, i.e., the highly-negative impact of cold air temperatures.
More straightforward attacks upon supposed air pollutants can also go awry. In the state of Arizona, USA, where we reside, a taxpayer-funded program designed to encourage the private purchase of cars designed to run on alternative fuels quickly ballooned from an initial estimated cost of approximately three million dollars to something close to three-quarters of a billion dollars, threatening to bust the state's budget until the legislature intervened and halted the program (see our Editorial The Best Laid Schemes of Mice and Men ...). Though costly, the fiasco may yet have served a useful purpose, if it encourages the citizens of the world to think twice before encouraging their national leaders to ratify the infinitely more complex Kyoto Protocol program for enhancing the environment. Instead of fixing the planet, it could well "fix" them.
Last of all, we mention something else that may be severely complicating our ability to correctly identify what is responsible for various plant, animal and human maladies around the world, i.e., the global nature of air pollution. In a study of the long-range aerial transport of dust, pollutants and biological materials from Eurasia to North America, Wilkening et al. (2000) report that "the once-pristine air above the North Pacific Ocean is polluted." Hence, there is a real possibility - if not a likelihood - that certain biological phenomena, such as the apparent growing inability of corals to withstand extended periods of regional warming and intense solar radiation reception, may be linked to the enhanced delivery of various substances that impair their natural ability to successfully cope with these environmental stresses (see, for example, our Journal Review Deadly Dustfalls: Scourge of Coral Reefs?).
In summary, the rising CO2 content of earth's atmosphere is (1) being blamed for certain things for which it may not be responsible, such as coral bleaching, (2) being targeted for curtailment in an attempt to alter things for which it is clearly not responsible, such as deaths due to cold weather, and (3) being ignored as a therapeutic agent when it could totally alleviate certain plant maladies, such as ozone- and sulfur dioxide-induced yield reductions in crops.
References
Cifuentes, L., Borja-Aburto, V.H., Gouveia, N., Thurston, G. and Davis, D.L. 2001. Hidden health benefits of greenhouse gas mitigation. Science 293: 1257-1259
Deepak, S.S. and Agrawal, M. 2001. Influence of elevated CO2 on the sensitivity of two soybean cultivars to sulphur dioxide. Environmental and Experimental Botany 46: 81-91.
Keatinge, W.R. and Donaldson, G.C. 2001. Mortality related to cold and air pollution in London after allowance for effects of associated weather patterns. Environmental Research 86: 209-216.
Smith, R.L., Davis, J.M. and Speckman, P. 1999. Assessing the human health risk of atmospheric particles. Environmental Statistics: Analyzing Data for Environmental Policy, Novartis Foundation Symposium 220: 59-79.
Wilkening, K.E., Barrie, L.A. and Engle, M. 2000. Trans-Pacific air pollution. Science 290: 65-67.