Is CO2 a Pollutant Part II

The weblog RealClimate on May 18 2006 has a posting entitled “Thank you for emitting”. . In the post, which is written in response to an advertisement, there is text referring to CO2 as a pollutant; ;

” It contains the immortal lines ‘CO2: they call it pollution, we call it Life!’ – it is beyond parody and without content – and so you should definitely see it.”

Is CO2 a pollutant? This issue was discussed on the Climate Science weblog of August 9 2005 which included a constructive comment discussion on this question. The subject of whether CO2 is a pollutant is revisited here.

A search on google quickly shows that CO2 is not a pollutant in the context of traditional air quality regulation. As summarized by the US Environmental Protection Agency (EPA),

“The Clean Air Act established two types of National Ambient Air Quality Standards.

“Primary’ standards are designed to establish limits to protect public health, including the health of “sensitive” populations such as asthmatics, children, and the elderly.

‘Secondary’ standards set limits to protect public welfare, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings”

The six criteria pollutants, as defined by the EPA, are ozone, particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide and lead.

Having served two terms on the Colorado Air Quality Control Commission, published a number of papers on air quality modeling, and working with the EPA and other regulatory agencies, my understanding of when the term “pollution” should be used is based on the existing EPA context. The term “pollution” connotes an undesirable constituent. For example, ozone is undesirable at any concentration in the lower atmosphere where we can inhale it; O3 is a pollutant in the troposphere. In the stratosphere, however, its absence is undesirable! O3 is not a pollutant in the stratosphere.

Thus the referral to CO2 as a pollutant, in my view, is inaccurate. Indeed, one of the environmental successes was the development of vehicles which have very efficient combustion such that CO2 rather than CO is produced (see the National Research Council report where it is stated,

“The regulation of carbon monoxide has been one of the great success stories in air pollution control. While more than 90 percent of the locations with carbon monoxide monitors were in violation in 1971, today the number of monitors showing violations has fallen to only a few, on a small number of days and mainly in areas with unique meteorological and topographical conditions. ”

The reason that this is a success story is that vehicles have more efficient combustion today, so that more CO2 and less CO is produced!

Information on whether CO2 is a pollutant include a “Is CO2 a Pollutant?” Department of Energy Technical Report edited by B. R. Strain and J.D. Cure entitled “Direct effects of increasing carbon dioxide on vegetation”.

“CO2 is an essential environmental resource. It is required as a raw material of the orderly development of all green plants. As the availability of CO2 increases, perhaps reaching two or three times the concentration prevailing in preindustrial times, plants and all other organisms dependent on them for food will be affected. Humans are releasing a gaseous fertilizer into the global atmosphere in quantities sufficient to affect all life. This volume considers the direct effects of global CO2 fertilization on plants and thus on all other life. Separate abstracts have been prepared for individual papers.”

Recent research work has examined the issue of whether there is a limit to whether the effect on vegetation becomes saturated or affects different types of plants differently; e.g. see the August 1993 Nature paper by S. Díaz, J. P. Grime, J. Harris and E. McPherson entitled “Evidence of a feedback mechanism limiting plant response to elevated carbon dioxide” where the abstract reads,

“In short-term experiments under productive laboratory conditions, native herbaceous plants differ widely in their potential to achieve higher yields at elevated concentrations of atmospheric carbon dioxide. The most responsive species appear to be large fast-growing perennials of recently disturbed fertile soils. These types of plants are currently increasing in abundance9 but it is not known whether this is an effect of rising carbon dioxide or is due to other factors. Doubts concerning the potential of natural vegetation for sustained response to rising carbon dioxide have arisen from experiments on infertile soils, where the stimulus to growth was curtailed by mineral nutrient limitations. Here we present evidence that mineral nutrient constraints on the fertilizer effect of elevated carbon dioxide can also occur on fertile soil and in the earliest stages of secondary succession. Our data indicate that there may be a feedback mechanism in which elevated carbon dioxide causes an increase in substrate release into the rhizosphere by non-mycorrhizal plants, leading to mineral nutrient sequestration by the expanded microflora and a consequent nutritional limitation on plant growth.”

We also know that atmospheric concentrations of CO2 routinely double everyday within tropical forests, as soils and plants respire at night releasing CO2, with vegetation assimilating CO2 during the day. For example, a study by Jenny Rissler, Erik Swietlicki, Anders Vestin and Jingchuan Zhou entitled ” Diurnal Aerosol patterns and nucleation in the Amazon Region” (see their Figures 2 and 3) show atmospheric CO2 concentrations reaching as high as 500 ppm in the Amazon. Clearly, this local level of 500 ppm is not a pollutant for the rain forest.

There are, therefore, several conclusions from such studies with respect to the question of whether CO2 is a pollutant.

1. Atmospheric CO2 is essential for vegetation growth, and is not an air quality pollutant in the same context as the EPA Criteria pollutants.

2. Elevated atmospheric concentrations of CO2 affect vegetation differentially, with the result that species compositions should be expected to change.

3. High (doubled from background free atmospheric concentrations) of CO2 are routine in forests.

4. We do not know what (or even if) there is an optimal level of CO2 in the atmosphere with respect to vegetation. It appears there are a wide diversity of optimal conditions for each vegetation (and perhaps even genotype), weather, latitude and soil conditions.

Focusing on CO2 as a pollutant is, therefore, counterproductive if the goal is to limit its increase by human activities. The added CO2 is a radiative and biogeochemical climate forcing, and it should be communicated to policymakers in this context. We do not use the term “pollutant” when referring to land use/land cover change yet this is just as much a climate forcing as CO2.

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