1) Harm to plants or animals.
2) Harm to human health.
3) Hindrance of human activities - for example, making water unsafe to swim or fish in.
4) Reducing "ecosystem functionality" - making nature less able to do the important things it does, like processing waste and providing fresh water, food, oxygen.
A complication that confuses people is that many pollutants are also naturally-occurring substances, which only become harmful when humans put them into the environment at unnaturally high levels or in contexts where they are inappropriate. These are called "Quantitative Pollutants" and include things like nutrients, ozone, and carbon dioxide. My favorite example of a quantitative pollutant is pure fresh water - if you dump too much fresh water into an estuary all at once it can cause harm by killing the saltwater-dependent organisms.
The less-confusing type of pollutants are "Qualitative Pollutants" - substances that NEVER occur naturally, like plastics and synthetic chemicals. Their identity as pollutants is independent of context. For example, there's no normal, healthy level of plastic in the environment.
Carbon dioxide (CO2) is a quantitative pollutant. It's occurs naturally as a minor but important component of Earth's atmosphere. Minor because it makes up less than 0.05% of atmospheric gas composition, but important because it's essential in photosynthesis and other cycles of life, it strongly affects ocean chemistry and acidity, and it strongly affects the insulative properties of the atmosphere (and therefore weather and climate). Over the long history of earth there have been natural ups and downs in CO2, which have had huge consequences for climate and life. Even the relatively minor oscillations in CO2 from 0.018% - 0.030% over the last 800,000 years have affected our repeated cycling into and out of glacial periods. (You can see the CO2 record of both the recent and distant past at https://keelingcurve.ucsd.edu/).
Given how sensitive vital climate and ecosystem processes are to atmospheric CO2 levels, it's alarming that recent human activities (deforestation and fossil fuel burning) have increased the CO2 concentration from 0.028% (the stable average of the last 12,000 years) to 0.043%; way higher than any level seen in over a million years. And the increase has been sudden, mainly happening since the industrial revolution around 1850. Does the man-made increase in CO2 constitute pollution? I.e., does it cause any of the harms described at the beginning of this post? Yes. Here are some of the harms it causes:
1. Harm to plants and animals- Increasing CO2 alters photosynthetic processes in ways that favor some plants, disfavor others, and alter plant nutrition, messing up natural systems as well as crop production. Some of these effects were reviewed in a recent, high-profile review in the scientific journal Stress Biology - https://doi.org/10.1007/s44154-025-00217-w Plants and animals are also harmed by CO2 effects on ocean chemistry. CO2 reacts with water to form carbonic acid (H2O + CO2 = H2CO3), which increases the acidity of the ocean and impairs the ability of organisms like coral, plankton, and oysters to make shells and skeletons and carry out their normal life processes. Finally, the global warming caused by increasing atmospheric CO2 has a multitude of harms to species, from simply making it too hot for them to survive, to changing rainfall or other aspects of climate that organisms depend on. Clearly CO2 meets the "harm to plants and animals" criterion for being pollution.
2. Harm to human health- CO2 at high concentrations has direct negative effects on humans. At 0.1% concentration it starts to impair cognitive function, and at 4% concentration it can knock you unconscious. Right now those direct harms to human health are more of an indoor concern, like if you're in a poorly ventilated space with a lot of people exhaling or machinery running. But if we keep putting CO2 into the atmosphere at the rate we have been, the outdoor concentrations could also get to 0.1% cognitive impairment level in just 100 years or so. The more immediate human health impacts of CO2 pollution are the indirect health impacts of climate change and ocean acidification on humans. Like, we're more likely to experience starvation, malnutrition etc. as climate change and ocean acidification distrupt crop production, fisheries, etc. Just because an effect is indirect doesn't mean it's not strong and real.
3. Hindrance of human activities- Too hot to go outside, no snow to ski on, no fish to catch, etc. You get the picture.
4. Reducing ecosystem functionality- Excess CO2 definitely impairs ecosystem functions, as evidenced by a mountain of all sorts of different scientific studies of climate change, ocean acidification, and plant physiology disruption. Here is just one of many papers reviewing these studies- https://doi.org/10.3390/environments10040066
This is a figure from an introductory oceanography textbook that illustrates some of the harmful impacts of CO2 pollution the marine environment, specifically. It really is a huge cascade of harms.
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