Carbon dioxide

• 1. Introduction

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  Carbon dioxide makes up just 428 parts per million of Earth's air, about 0.043% as of July 2025. That is a trace, a rounding error in a breath. Yet this one molecule, a single carbon atom double-bonded to two oxygen atoms, sits at the center of nearly everything alive. It is the carbon source that feeds plants, algae and cyanobacteria. It is the waste gas every aerobic creature exhales. It is transparent to visible light but quietly absorbs infrared, trapping heat as a greenhouse gas. How did a Flemish chemist first catch this invisible substance in a closed jar? Why does it kill animals near a volcano but rise dough in a bakery? And how did a gas that was 280 parts per million before industry climb to where it is now, becoming the primary cause of climate change? The answers run from the inside of a red blood cell to the floor of the Mariana Trench.

• 2. Shape Of The Molecule

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  The carbon-oxygen bond in carbon dioxide measures 116.3 picometers, noticeably shorter than the roughly 140 picometers of a typical single C-O bond. That tightness comes from the double bond. The molecule is linear and centrosymmetric at its equilibrium geometry, which means it carries no electric dipole moment. Its symmetry hides in plain sight.

  Four vibrational modes define how this triatomic molecule trembles. The antisymmetric stretching mode shows up in the infrared spectrum at wavenumber 2349 per centimeter, a wavelength of 4.25 micrometers. The degenerate pair of bending modes appears at 667 per centimeter, or 15.0 micrometers. The symmetric stretch creates no dipole, so infrared cannot see it, yet Raman spectroscopy catches it at 1388 per centimeter with a Fermi resonance doublet at 1285.

  Gas-phase carbon dioxide never holds still. A Coulomb explosion imaging experiment, paired with calculations on an ab initio potential energy surface, reached a counter-intuitive verdict. None of the molecules in the gas phase are ever exactly linear, because the nuclear motion volume element vanishes for linear geometries. The same quirk applies to every molecule except diatomic ones.

• 3. Dissolved In Water

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  When carbon dioxide meets water it reversibly forms carbonic acid, a weak acid whose ionization stays incomplete. Most of the dissolved gas never converts at all. At 25 degrees Celsius the hydration equilibrium leaves the majority as plain molecules, which means it does not touch the pH on its own.

  The Bjerrum plot tells the rest of the story by following pH. In neutral or slightly alkaline water above pH 6.5, bicarbonate predominates, climbing past 95% at the pH of seawater. Above pH 10.4 the carbonate ion takes over. The oceans, mildly alkaline at a typical pH of 8.2 to 8.5, hold about 120 milligrams of bicarbonate per liter. In living organisms the enzyme carbonic anhydrase speeds the production of carbonic acid.

  Dissolved carbon dioxide also rewires water's electrical behavior. In desalinated water the conductivity jumps from below 1 microsiemens per centimeter to nearly 30. Heat the water and that induced conductivity fades, especially once temperatures climb past 30 degrees Celsius. The carbonate ion itself arrives only at high pH, where bicarbonate dissociates with a pKa2 of 10.329.

• 4. Life's Carbon Engine

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  RuBisCO is thought to be the single most abundant protein on Earth, and its job is to grab carbon dioxide. The enzyme drives the first major step of carbon fixation, building two molecules of 3-phosphoglycerate from carbon dioxide and ribulose bisphosphate. From there plants, algae and cyanobacteria assemble glucose, polysaccharides, nucleic acids and proteins, the raw material of every food web that feeds animals like us.

  The coccolithophore Emiliania huxleyi turns this gas into stone. Its calcite scales have formed the basis of sedimentary rocks such as limestone, locking what was once atmospheric carbon away for geological timescales. In the upper ocean, photosynthesis by phytoplankton consumes dissolved carbon dioxide and pulls more down from the air.

  More of the gas can mean faster growth. Plants can grow as much as 50% faster at 1,000 parts per million, assuming no climate change and no shortage of other nutrients. In FACE experiments wheat, rice and soybean all showed yield gains of 12 to 14% under elevated levels. The trade-offs are real. Enrichment thins out the stomata on leaves, cuts micronutrient concentrations in crops, and can shift the balance of secondary metabolites like phenylpropanoids and flavonoids. A mature forest, long thought carbon neutral, can keep accumulating carbon and stay a valuable sink.

• 5. When The Air Turns Deadly

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  Concentrations of 7% to 10% can suffocate a person within minutes to an hour, even with enough oxygen present, bringing dizziness, headache, and unconsciousness. Above 10% the gas may cause convulsions, coma, and death. Beyond 30% it acts in seconds. Because carbon dioxide is 53% denser than dry air, it pools in sheltered hollows below ground level where wind cannot disperse it. Near the city of Goma, emissions from the volcano Mount Nyiragongo have killed children this way. The Swahili term for the phenomenon is mazuku.

  The danger scales down to ordinary rooms. The United States set an occupational limit of 0.5%, or 5,000 parts per million, over eight hours. International Space Station crew at that level reported headaches, lethargy, mental slowness, and disrupted sleep. One study found significant cognitive effects at concentrations as low as 0.1%, likely from increased cerebral blood flow, though a later review found only a small effect on high-level decision making below 5,000 parts per million.

  Miners gave the hazard its own vocabulary. They called mixtures of carbon dioxide and nitrogen blackdamp, choke damp, or stythe, and carried caged canaries that would stop singing and fall from the perch as the air went bad. The Davy lamp burned less brightly in blackdamp, which sinks and gathers near the floor. Dry ice has proven just as lethal in modern life. In February 2020 three people died from suffocation at a Moscow party after dry ice was dropped into a swimming pool to cool it.

• 6. Carried By The Blood

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  A person produces roughly 2.3 pounds of carbon dioxide per day, holding 0.63 pounds of carbon, and breathes nearly all of it out through the lungs. The respiratory centers work to hold an arterial partial pressure near 40 millimeters of mercury. Breathing too slowly or shallowly tips a person into respiratory acidosis, while breathing too fast causes hyperventilation and respiratory alkalosis.

  Three routes move the gas through the blood. Most of it, about 70 to 80%, becomes bicarbonate ions through carbonic anhydrase inside red blood cells. Another 5 to 10% dissolves in plasma, and a similar share binds to hemoglobin as carbamino compounds. That binding does not compete with oxygen's site; it attaches to the N-terminal groups on the four globin chains. Through allosteric effects this is the Haldane Effect, and the related Bohr effect offloads oxygen when partial pressure rises or pH falls.

  The body breathes for carbon dioxide, not oxygen. Low oxygen normally does not stimulate breathing; higher carbon dioxide does. That is why breathing pure nitrogen can drop a person unconscious without any sense of air hunger, a peril for high-altitude fighter pilots. Hyperventilating before a free dive lowers arterial pressure to 10 to 20 millimeters of mercury and dulls the urge to breathe, which is exactly why it is so dangerous in the water.

• 7. Made And Used

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  Around 230 million tonnes of carbon dioxide are used each year, far more of it from the fertiliser industry than anywhere else. Urea production accounts for 130 million tonnes, while the oil and gas industry takes 70 to 80 million tonnes for enhanced oil recovery. Injected underground under supercritical conditions, it grows miscible with crude oil, cutting viscosity and lifting recovery by 7 to 23% beyond primary extraction.

  Industry has many ways to make it. Burning fossil fuels for energy produces 36.8 billion tonnes per year as of 2023, with nearly all of it going to the atmosphere. Fermentation yields it as yeast turns sugar into carbon dioxide and ethanol. Decomposition in landfills releases it as about 40 to 45% of landfill gas, the rest mostly methane. Heating limestone to about 850 degrees Celsius in the manufacture of quicklime drives it off, and acids liberate it from most metal carbonates with a telltale foaming.

  Its uses run from the kitchen to the server room. As food additive E290 it works as a propellant and acidity regulator, and the candy Pop Rocks is pressurized with it at about 4,000 kilopascals. Carbon dioxide carbonates soft drinks and beer, with the sour taste coming from carbonic anhydrase 4 rather than the bursting bubbles. Winemakers chill grape clusters with dry ice during the cold soak to block wild yeast. Fire extinguishers flood a flame and starve it of oxygen, though one review counted 51 incidents between 1975 and 2000, causing 72 deaths and 145 injuries. As refrigerant R744 it carries a global warming potential of 1, non-ozone depleting and non-flammable, a possible successor to the HFCs in cars and supermarkets.

• 8. First Gas Named

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  Around 1640 the Flemish chemist Jan Baptist van Helmont burned charcoal in a closed vessel and noticed the ash weighed far less than the charcoal he started with. He decided the missing matter had become an invisible substance, and he coined a word for it from the Greek for chaos. He called it a gas, or a wild spirit, spiritus sylvestris. Carbon dioxide became the first gas ever described as a discrete substance.

  In the 1750s the Scottish physician Joseph Black pinned down its properties and named it fixed air. He heated limestone, or treated it with acids, to release a gas denser than air that supported neither flame nor animal life. When he bubbled it through limewater it precipitated calcium carbonate, which let him show that respiration and microbial fermentation produce the same gas. In 1772 the English chemist Joseph Priestley dripped sulfuric acid on chalk and dissolved the result into water in his paper Impregnating Water with Fixed Air.

  The gas was tamed into liquid and solid within a few decades. Humphry Davy and Michael Faraday first liquefied it under elevated pressure in 1823. In 1835 the French inventor Adrien-Jean-Pierre Thilorier opened a pressurized container of the liquid and watched rapid evaporation produce a snow of solid carbon dioxide. That snow is the dry ice that still cools ice cream, decaffeinates coffee, and on rare days kills the people who handle it carelessly.