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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?

What the science says...

Select a level... Intermediate Advanced

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 on agriculture, health and environment.

Climate Myth...

CO2 is not a pollutant

'To suddenly label CO2 as a "pollutant" is a disservice to a gas that has played an enormous role in the development and sustainability of all life on this wonderful Earth. Mother Earth has clearly ruled that CO2 is not a pollutant.' (Robert Balling, as quoted by Popular Technology)

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:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 7 July 2015 by pattimer. View Archives

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Further reading

For a good overview of CO2 acidification, read Ken Caldeira's What Corals are Dying to Tell Us About CO2 and Ocean Acidification.


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Comments 26 to 35 out of 35:

  1. I think the question hangs on whether you believe global warming is on net harmful at the levels predicted. If you don't believe that the net effects of a 2 degree temperature rise are harmful then it's hard to get to the point of calling CO2 a pollutant. While there have been many dire predictions based on the 2 degree theory I have yet to see a well reasoned quantification of the relative benefits versus harms. Where can I find that?
  2. I need a link to EPA's listing of CO2 as an air pollutant. This blog states "...the US Environmental Protection Agency (EPA) Administrator must publish a list of certain air pollutants....Greenhouse gases including CO2 unquestionably fit the Clean Air Act's broad definition of air pollutants and must be listed and regulated by the EPA if it can be determined that they endanger public heath and/or welfare." However, I cannot find an EPA list of air pollutants that includes CO2, and the links in this blog don't help. They lead to an EPA Endangerment Finding but not a statement that CO2 is an air pollutant. Or is that the same thing? I appreciate it.

  3. fake progress @27...  I believe they would be the same thing.

  4. Earthling,

    CO2 is poisoness to animals and (especially) marine life at low concentrations.  Past mass extinctions have been caused by high CO2 levels.  Calling anything that causes mass extinctions a fertilizer is ludicrous.

  5. Traditionally CO2 has not been considered a pollutant because it does not directly cause human harm (from inhaling it, like particulates), nor does it promote harm to human health indirectly by contributing to smog creation (as NOx leads to the creation of ozone, which in turn may trigger athsma), nor does it lead to property or wildlife damage (as SO2 causes acid rain).  CO2, like H2O, is an odorless, harmless emission that is beneficial to humans and wildlife.  However, since about 2010, the EPA has tried to intentionally conflate CO2 with other pollutants using the new "carbon pollution" moniker.  There is a deliberate intent to confuse CO2--the odorless gas--with black carbon soot (particulates).  CO2 itself does not cause any direct or indirect human harm or property damage.  There is a theoretical link of global warming to more inense adverse weather events, and a theoretical link of increased CO2 to sea level rise, which may necessitate waterfront modification, depending on the magnitude of the sea level rise.  However, in practice, CO2 could double or triple in the atmosphere without any temperature change because there is so much H2O in the system that has about the same greenhouse gas effect.  So if you want to classify CO2 as a pollutant, then H2O would also fit the definition.  Historically, CO2 has not caused warming.  Rather, global warming was caused by some other factor (volcanoes, meteors, sun, etc.), and the warming has caused CO2 to rise as tundra melts.


    [TD] Provide evidence for your claim of deliberate intent to confuse CO2 with carbon soot. Else do not make such baseless, conspiratorial claims here.

    Your claim of CO2's influence being trivial because of the H2O in the atmosphere is completely, factually wrong, because water vapor is a feedback rather than a forcing. Read the post "Explaining How the Water Vapor Greenhouse Effect Works." Read the Basic tabbed pane there, and then the Intermediate tabbed pane. If you want to comment on that topic, do so there rather than here; comments on that topic here will be deleted for being off topic.

    Your claim that historically CO2 has not caused warming also is completely, factually incorrect. Read the post "CO2 Is Not the Only Driver of Climate"--the Basic tabbed pane and then the Intermediate one. If you want to comment on that topic, do so there, not here. Then watch climatologist Richard Alley's talk "The Biggest Control Knob: Carbon Dioxide in Earth's Climate History."

    [RH] Extended warning snip. It would be worthy to note that the EPA initially rejected the idea of making a ruling on CO2. It was a court case "Massachusetts v. EPA" that went to the Supreme Court where the court ruled that the EPA was required to make a ruling on greenhouse gases under the Clean Air Act (Wiki). The commenter here clearly lacks sufficient understanding of the issue s/he's opining on and would be well advised to fully inform her/himself before continuing commentary.

  6. Sorry, actually, the "carbon pollution" moniker was introduced by the Obama administration in 2013.


    [RH] Searching Google Trends for the usage of the phrase "carbon pollution" shows that you are incorrect. Try it out.

  7. Actually aed939, that CO2 was defined as a pollutant dates back to Massachusetts vs EPA in the US Supreme Court in April2007 when SCOTUS ruled that the greenhouse gases are 'air pollutants' as defined under the Clean AIr Act.

    President Bush issued an executive order in May 2007, authorising the EPA to regulate GHGs for mobile sources.

  8. If we can define pollution as the contamination of air, water, or soil by substances that are harmful to living organisms, most substances can fill-the-bill, especially when we consider high/low concentrations and temperatures.  Plants that grow in areas that we don't want them will be considered as weeds.  Carbon Dioxide that becomes too concentrated for our environment certainly should be considered a pollutant, but the question, and I think it is still unanswered scientifically, is what is that level?  The dynamic complexity of the Earth's ecosystem suggests that when we begin tinkering with nature we may find ourselves creating additional problems for mankind that are currently unforseen.

  9. I think carbon dioxide is not a pollution. Let talk about definition ,the definition of pollution in Webster’s dictionary is "to make physically impure or unclean: Befoul, dirty." By that definition, carbon dioxide is not pollution. However, Webster's also has the definition: "to contaminate (an environment) esp. with man-made waste." Carbon dioxide is a waste gas produced by fossil fuel combustion, so can be classified as man-made waste.

  10. Aomsin@34,

    You provided two definitions of pollution then provided the connection between human created CO2 from burning ancient buried hydrocarbons to the second definition.

    Think seriously about why you opened your comment by declaring "I think carbon dioxide is not a pollution."

    The interpretation of the definition is a game played by legal-minded people trying to argue against government regulations to limit harmful impacts of human activity. The EPA written legal mandate is to limit 'pollution'. That is the cause of the attempts to legally argue that CO2 from burning fossil fuels is not 'pollution' (an argument has been lost in the courts). Continuing to argue about the definition distracts from the undeniable harmful consequences of the activity. And regulatory bodies like the EPA should act to restrict the creation of harmful consequences, no matter what term is used to describe them.

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