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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.

What the science says...

While there are direct ways in which CO₂ is a pollutant (acidification of the ocean), its primary impact is its greenhouse warming effect. While the greenhouse effect is a natural occurrence, too much warming has severe negative impacts

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 occurring 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.

Intermediate rebuttal written by John Cook

Update July 2015:

Updated on 2015-07-07 by pattimer.

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