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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Is CO2 a pollutant?

Posted on 11 February 2010 by John Cook

We commonly think of pollutants as contaminants that make the environment dirty or impure. A vivid example is sulphur dioxide, a by-product of industrial activity. High levels of sulphur dioxide cause breathing problems. Too much causes acid rain. Sulphur dioxide has a direct effect on health and the environment. Carbon dioxide, on the other hand, is a naturally occuring gas that existed in the atmosphere long before humans. Plants need it to survive. The CO2 greenhouse effect keeps our climate from freezing over. How can CO2 be considered a pollutant?

A broader definition of pollutant is a substance that causes instability or discomfort to an ecosystem. Over the past 10,000 years, the level of atmospheric carbon dioxide in the atmosphere has remained at relatively stable levels. However, human CO2 emissions over the past few centuries have upset this balance. The increase in CO2 has some direct effects on the environment. For example, as the oceans absorb CO2 from the atmosphere, it leads to acidification that affects many marine ecosystems. However, the chief impact from rising CO2 is warmer temperatures.

Figure 1: CO2 levels (parts per million) over the past 10,000 years. Blue line from Taylor Dome ice cores (NOAA). Green line from Law Dome ice core (CDIAC). Red line from direct measurements at Mauna Loa, Hawaii (NOAA).

Rising CO2 levels causes an enhanced greenhouse effect. This leads to warmer temperatures which has many consequences. Some effects are beneficial such as improved agriculture at high latitudes and increased vegetation growth in some circumstances. However, the negatives far outweigh the positives. Coast-bound communities are threatened by rising sea levels. Melting glaciers threaten the water supplies of hundreds of millions. Species are becoming extinct at the fastest rate in history.

How we choose to define the word 'pollutant' is a play in semantics. To focus on a few positive effects of carbon dioxide is to ignore the broader picture of its full impacts. The net result from increasing CO2 are severe negative impacts on our environment and the living conditions of future humanity.

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Comments 251 to 252 out of 252:

  1. suibhne, I read the G&T paper. I've rarely seen a worse article, EVER. I would suggest you look at some of the responses to that which have been linked to from this blog. G&T pose a strawman argument, that the greenhouse effect (radiative heating by greenhouse gasses in the atmosphere 55 pages in their paper) is not the same mechanism as surface greenhouses (blocking convection/conduction heat exchanges, 20 pages). This gets covered in the first two minutes of review in any thermodynamics discussion on the topic - G&T claim this these aren't equivalent, therefore greenhouse gasses don't hold heat, which is bull****. The surface of the earth would be ~33C colder if they didn't. Nobody argues that these two mechanisms hold heat the same way, just that they are similar in effect (warming things up). Side note - they also seem to claim that the IPCC came up with radiative forcing and radiative equilibrium. They're a bit off with that statement; I believe that dates to 1791. Please read that, the references are excellent. G&T misderive path length intercepts for IR and CO2, derive wavelength dependent energy retention for glass but then claim that can't possibly work for CO2, contradicting themselves, etc. There are (by my count) about 2 misstatements per page. The core of their argument, described in regards to a car heating in the sun, is "Conduction, condensation and radiation, which slow down the rise in temperature, work practically the same inside and outside the car. Therefore, the only possible reason for a difference in final temperatures must be convection...". G&T essentially argue that radiation would rapidly equalize temperatures. This means that they haven't done their math! Total energy into the atmosphere (solar plasma emission spectra) warms the surface of the earth, which radiates a different spectra (thermal IR) back up. Increasing CO2 blocks part of this thermal spectra. For the energy output to balance the solar input in a steady state condition, the entire thermal spectra must increase in intensity to counter the CO2 blockage, which means that energy will accumulate (rising temperature) until the sum spectral energies in/out are equivalent. This is pretty basic; G&T either don't understand radiation equilibrium or choose to ignore it. The paper is worthless. Read this, this, and finally this for some useful descriptions of radiative equilibrium. There are some excellent thermodymanics links from those pages as well.
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  2. suibhne, also look at this, which is an excellent explanation of what G&T's attempted equivalence of convective greenhouse heating with radiative greenhouse gas heating really means.
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