Carbon Dioxide: The Wonder Molecule that Keeps the Biosphere Alive

But CO2 may also be too much of a good thing.

CO2 is a remarkable molecule. With its carbon atom tightly tucked between two oxygen atoms, it is stable for a long time in the atmosphere. But it is also reactive enough to be a crucial element of the metabolism of living beings. Too little CO2 could kill the biosphere by making the photosynthesis reaction impossible. But too much CO2 could do enormous damage to the biosphere because of extreme global warming, ocean acidification, and metabolic poisoning.

For some reason, the debate on the human-caused CO2 concentration increase in the atmosphere is all focused on global warming. The chemical effects of CO2 are neglected or said to be a good thing because “CO2 is food for plants.” But that’s a narrow view of the problem that neglects the importance of CO2 as a reactive molecule. Living beings are made of carbon-based polymers and every carbon atom in every living being in the biosphere was once a part of a carbon dioxide molecule. It is carbon dioxide that makes life possible on Earth: no CO2, no life. But too much CO2 can have bad effects, too.

CO2 is a stable molecule, with the carbon atom tightly embraced by two oxygen atoms. It doesn’t react so easily with other entities, but when it does, many things change. Here is a summary of how CO2 interacts with the ecosphere.

— CO2 absorbs infrared photons. It does that mainly by having its two oxygen atoms oscillating together like the wings of a butterfly. It is called bending mode, and it occurs at around 667 cm-1, almost exactly at the center of the Earth's infrared emission band. It is the crucial element of “radiative forcing” (aka “greenhouse effect”). It absorbs heat that otherwise would be lost in space and heats the atmosphere. The result is that Earth’s surface is warmer than it would be without CO2. Global warming is mainly a consequence of increasing CO2 concentrations.

— CO2 is an acid. When it interacts with water, it absorbs an H2O molecule, and it becomes carbonic acid: H2CO3. This molecule releases hydrogen ions, H+, and what’s left is called “bicarbonate.” Acid/base properties are a crucial property of all liquid phases that form the biosphere. It includes oceans but also the liquids inside animals, for instance the “plasma” of the blood. The blood's acidity must be carefully controlled; otherwise, various disastrous alterations of the metabolism may ensue.

— CO2 is an electron acceptor (another definition of acid). This property is the core of the fundamental reaction of photosynthesis. First, H2O is split into hydrogen and oxygen atoms using the energy of a solar photon. Then, a highly reactive hydrogen atom cedes an electron to the CO2 molecule, starting the reactions that produce sugars and all sorts of carbon compounds (it is called the “Calvin-Benson” cycle). More CO2 may make this reaction faster, at least for some plant. The result of increasing atmospheric CO2 concentrations is called “global greening.” But the effect peaks at about 600-1000 ppm, then it declines.

— CO2 is the product of the oxidation reaction of the Krebs cycle inside mitochondria. It is the vital source of metabolic energy of all living beings, the ATP molecule. The CO2 produced in mitochondria must be quickly removed, least the Krebs cycle should start going in reverse (CO2 poisoning). It is one of the two tasks of blood circulation, the other being oxygen transport. The transport of CO2 in blood is so important that there is a specific enzyme, carbonic anhydrase, that speeds the dissolution reaction by several orders of magnitude.

All these reactions are affected by the atmospheric CO2 concentration. Too little CO2 means that photosynthesis slows down, and plants may die. The limit is said to be around 150 ppm (currently it is about 420 ppm), but plants have developed a specific photosynthesis mechanism. called “C4.” which allows them to survive at much lower concentrations. Plants may thrive with a little more CO2, but they do so mainly by accumulating water inside their tissues. “Global greening” is mainly a global pandemic of plant obesity.

High CO2 concentrations in the atmosphere strain the capability of the blood circulation system to remove it. For concentrations over ca. 5%, CO2 is immediately lethal to human beings. Without arriving at these extremes, concentrations over 1000 ppm may have several negative metabolic effects, from headaches to pathologies such as inflammation. “Global dumbing,” a worldwide trend of decline of the average brain capabilities of human beings, has been attributed to this effect. Even though the current atmospheric concentration (ca. 420 ppm) is still within the limits believed to be safe, much higher concentrations are normally observed in closed spaces, for instance, in buildings or when people wear face masks.

My colleagues Bierwirth, Huang, and McIntyre and I are preparing a paper on this subject. We are still working on it; for the time being, you can see a draft of mine on “Arxiv,” titled “Carbon Dioxide as a Pollutant.” Here is a table from the paper we are preparing. Everything in this table is approximate; it is there just to give an idea of the situation.

___________________________________________________

References

[1] “Joseph Black and the Discovery of Carbon Dioxide,” Medical Journal of Australia, vol. 1, no. 23, pp. 801–802, 1957, doi: 10.5694/j.1326-5377.1957.tb59881.x.

[2] “The artificial production of carbon dioxide and its influence on temperature - Callendar - 1938 - Quarterly Journal of the Royal Meteorological Society - Wiley Online Library.” Accessed: Nov. 27, 2024. [Online]. Available: https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.49706427503

[3] C. Azar and H. Rodhe, “Targets for Stabilization of Atmospheric CO2,” Science, vol. 276, no. 5320, pp. 1818–1819, Jun. 1997, doi: 10.1126/science.276.5320.1818.

[4] R. Revelle and H. E. Suess, “Carbon Dioxide Exchange Between Atmosphere and Ocean and the Question of an Increase of Atmospheric CO2 during the Past Decades,” Tellus, vol. 9, no. 1, pp. 18–27, 1957, doi: 10.1111/j.2153-3490.1957.tb01849.x.

[5] O. V. Eliseeva, “[ON THE DETERMINATION OF MAXIMUM PERMISSIBLE CARBON DIOXIDE CONCENTRATIONS IN THE AIR OF APARTMENT BUILDINGS AND PUBLIC BUILDINGS],” Gig Sanit, vol. 29, pp. 10–15, Aug. 1964.

[6] C. J. Tucker, I. Y. Fung, C. D. Keeling, and R. H. Gammon, “Relationship between atmospheric CO2 variations and a satellite-derived vegetation index,” Nature, vol. 319, no. 6050, pp. 195–199, Jan. 1986, doi: 10.1038/319195a0.

[7] S. Wang et al., “Recent global decline of CO2 fertilization effects on vegetation photosynthesis,” Science, vol. 370, no. 6522, pp. 1295–1300, Dec. 2020, doi: 10.1126/science.abb7772.

[8] D. Robertson, “Health effects of increase in concentration of carbon dioxide in the atmosphere,” Current Science, 2006, Accessed: Jul. 11, 2024. [Online]. Available: https://www.semanticscholar.org/paper/Health-effects-of-increase-in-concentration-of-in-Robertson/cf7ed5a3445f9d14b406334a0ac4469354a4dc56