Carbon dioxide is a chemical compound with the formula CO₂. Each molecule contains one carbon atom connected to two oxygen atoms. At room temperature, it exists as a gas and has no smell. It is the main source of carbon for living things on Earth through the carbon cycle. In the air, carbon dioxide allows visible light to pass through but traps heat by absorbing infrared radiation, making it a greenhouse gas. It dissolves in water and is found in groundwater, lakes, ice caps, and oceans.

Carbon dioxide is present in Earth’s atmosphere at 428 parts per million (ppm), or about 0.043% (as of July 2025). This level has increased from pre-industrial times, when it was 280 ppm or about 0.028%. Burning fossil fuels is the main reason for this rise, which contributes to climate change.

Before human activity, carbon dioxide levels were controlled by living things and Earth’s natural processes. Plants, algae, and cyanobacteria use sunlight to turn carbon dioxide and water into carbohydrates, releasing oxygen as a byproduct. Oxygen is used by aerobic organisms during respiration, which produces energy and releases carbon dioxide. Carbon dioxide is also released when organic materials decay or burn, such as during wildfires. When carbon dioxide dissolves in water, it forms carbonate and bicarbonate ions, which increase ocean acidity as CO₂ levels rise.

Carbon dioxide is 53% heavier than dry air but mixes thoroughly in the atmosphere. About half of extra CO₂ emissions are absorbed by land and oceans, which act as carbon sinks. These sinks can become full or release CO₂ back into the air due to decay or fires. Over time, carbon dioxide is stored in rocks and organic materials like coal, petroleum, and natural gas.

Most human-produced carbon dioxide enters the atmosphere. Less than 1% is used commercially, mainly in fertilizers, oil and gas industries, food production, metal manufacturing, cooling systems, fire suppression, and greenhouse plant growth.

Chemical and physical properties

The carbon dioxide molecule has a straight, symmetrical shape at its most stable form. The distance between the carbon and oxygen atoms in carbon dioxide is 116.3 pm, which is shorter than the typical 140 pm length of a single C–O bond. Because the molecule is symmetrical around its center, it does not have an electric dipole moment.

As a straight molecule with three atoms, CO₂ has four ways it can vibrate. In two of these vibrations, the atoms move along the same line as the molecule. The other two vibrations are bending movements that occur at the same frequency and energy because of the molecule’s symmetry. When CO₂ interacts with a surface or another molecule, these bending vibrations may have different frequencies. Some vibrations are visible in infrared (IR) light: the antisymmetric stretching vibration appears at 2349 cm⁻¹ (wavelength 4.25 μm), and the two bending vibrations appear at 667 cm⁻¹ (wavelength 15.0 μm). The symmetric stretching vibration does not create an electric dipole and is not visible in IR light but can be seen in Raman spectroscopy at 1388 cm⁻¹ (wavelength 7.20 μm), with a double peak at 1285 cm⁻¹.

In the gas phase, CO₂ molecules move and change shape constantly. However, experiments using Coulomb explosion imaging show that no CO₂ molecules are perfectly straight at any moment. This result is due to the way atomic motion works in molecules that are not diatomic (two-atom).

Carbon dioxide dissolves in water and forms carbonic acid (H₂CO₃), which is a weak acid because it does not fully break apart in water. The equilibrium constant for this reaction at 25°C shows that most CO₂ remains unchanged in water and does not affect the pH.

The amounts of CO₂, H₂CO₃, bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) depend on the pH of the water. In neutral or slightly alkaline water (pH > 6.5), bicarbonate is the most common form (>50%), becoming the dominant form (>95%) in seawater (pH around 8.2–8.5). In very alkaline water (pH > 10.4), carbonate is the most common form. Seawater contains about 120 mg of bicarbonate per liter.

Carbonic acid can donate two protons, so it has two acid dissociation constants. The first constant describes the breakdown of carbonic acid into bicarbonate. This value is calculated differently depending on whether the dissolved CO₂ is considered as carbonic acid or as separate molecules. A commonly used value (4.16 × 10⁻⁷, or pKa₁ = 6.38) is based on the incorrect assumption that all dissolved CO₂ becomes carbonic acid. The true value is higher because most dissolved CO₂ remains as CO₂ molecules.

The bicarbonate ion can act as an acid or a base depending on the pH. At high pH, bicarbonate breaks down into carbonate. In living organisms, carbonic acid is produced quickly by the enzyme carbonic anhydrase.

Carbon dioxide in water also changes the water’s electrical properties. When CO₂ dissolves in desalinated water, the water’s electrical conductivity increases from below 1 μS/cm to nearly 30 μS/cm. Heating the water reduces this conductivity, especially above 30°C.

Carbon dioxide is a strong electrophile, similar to benzaldehyde or certain carbonyl compounds. However, reactions between CO₂ and nucleophiles are often reversible. This property is used in CO₂ scrubbers and carbon capture processes. Strong nucleophiles like Grignard reagents can react with CO₂ to form carboxylates.

In metal complexes, CO₂ can act as a ligand, helping convert CO₂ into other chemicals. Converting CO₂ to carbon monoxide is difficult but can be done by the enzyme carbon monoxide dehydrogenase.

Plants and cyanobacteria use sunlight to turn CO₂ and water into sugars through photosynthesis.

Carbon dioxide is colorless and has no smell at low concentrations. At high concentrations, it has a sharp, acidic odor. At standard temperature and pressure, CO₂ is about 1.53 times denser than air.

CO₂ does not exist as a liquid below 0.51795 MPa (5.11 atm). At 1 atm, CO₂ changes directly from gas to solid at temperatures below −78.46°C and back to gas above this temperature. Solid CO₂ is called dry ice.

Liquid CO₂ forms only above 0.51795 MPa. The triple point of CO₂ is at −56.56°C and 5.11 atm. The critical point is at 30.98°C and 72.8 atm. At high pressures, CO₂ can form an amorphous glass-like solid called carbonia. This form is unstable at normal pressures and reverts to gas when pressure is released.

Above the critical point, CO₂ behaves as a supercritical fluid.

Tables of thermal and physical properties for saturated liquid CO₂ and CO₂ at atmospheric pressure are available.

Biological role

Carbon dioxide is a final product of cellular respiration in organisms that gain energy by breaking down sugars, fats, and amino acids with oxygen. This includes all plants, algae, animals, and aerobic fungi and bacteria. In vertebrates, carbon dioxide moves through the blood from the body’s tissues to the skin (e.g., amphibians) or gills (e.g., fish), where it dissolves in water, or to the lungs, where it is exhaled. During active photosynthesis, plants can take in more carbon dioxide from the atmosphere than they release through respiration.

Carbon fixation is a process in which plants, algae, and cyanobacteria take in atmospheric carbon dioxide and convert it into energy-rich organic molecules like glucose, creating their own food through photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars, which are used to build other organic compounds, and oxygen is released as a by-product.

The enzyme ribulose-1,5-bisphosphate carboxylase oxygenase, or RuBisCO, is involved in the first major step of carbon fixation. It combines carbon dioxide with ribulose bisphosphate to form two molecules of 3-phosphoglycerate. RuBisCO is believed to be the most common protein on Earth.

Phototrophs use the products of their photosynthesis as food and to make complex organic molecules like polysaccharides, nucleic acids, and proteins. These molecules support their growth and form the basis of food chains and webs for other organisms, including humans. Some phototrophs, such as coccolithophores, create hard calcium carbonate scales. A significant species, Emiliania huxleyi, produces calcite scales that form sedimentary rocks like limestone, storing atmospheric carbon for long periods.

Plants can grow up to 50% faster in air with 1,000 parts per million CO₂ compared to normal air, assuming no changes in climate or nutrient availability. Higher CO₂ levels increase crop yields, with wheat, rice, and soybean showing 12–14% yield increases in experiments. Elevated CO₂ reduces the number of stomata (tiny leaf pores), lowering water use and improving water efficiency. However, studies show reduced micronutrient levels in crops under high CO₂, which may affect herbivores needing to eat more to gain the same protein.

High CO₂ can also change the production of secondary metabolites like phenylpropanoids and flavonoids in plants. Plants release CO₂ during respiration, so most plants and algae using C3 photosynthesis absorb more CO₂ during the day than they release. While growing forests absorb large amounts of CO₂ annually, mature forests release as much CO₂ through respiration and decomposition as they take in through photosynthesis. Despite this, mature forests can still store carbon and act as important carbon sinks. Additionally, phytoplankton in oceans absorb dissolved CO₂, helping reduce atmospheric CO₂ levels.

The CO₂ content in fresh air (measured between sea level and about 30 km altitude) ranges from 0.036% (360 ppm) to 0.041% (412 ppm), depending on location.

In humans, exposure to CO₂ above 5% causes hypercapnia (too much CO₂ in the blood) and respiratory acidosis (blood becomes too acidic). Levels of 7–10% may lead to suffocation, with symptoms like dizziness, headaches, and unconsciousness. Levels above 10% can cause convulsions, coma, or death. At over 30%, unconsciousness occurs rapidly.

Because CO₂ is heavier than air, it can accumulate in low-lying areas near volcanic or geothermal activity, suffocating animals and even humans, as seen near Mount Nyiragongo in Goma, where the Swahili term mazuku describes this phenomenon.

Humans adapt to higher CO₂ levels through changes in breathing and kidney function to balance blood acidification. Some studies suggest 2.0% CO₂ exposure in enclosed spaces (e.g., submarines) is safe for short periods, but others show cognitive decline even at lower levels. Prolonged exposure to CO₂ below 1% may harm health, with occupational limits set at 0.5% (5,000 ppm) for eight hours. At this level, astronauts reported headaches, fatigue, and sleep issues. Animal studies show kidney and bone damage after eight weeks of exposure. Human studies also show reduced cognitive function at 1,000 ppm CO₂.

However, research on CO₂’s effects on cognition is limited, with many studies having design flaws or unclear results. For example, studies on CO₂ in motorcycle helmets were criticized for using mannequins instead of real riders and for not accounting for real-world conditions.

Poor ventilation is a major cause of high indoor CO₂ levels, leading to poor air quality. CO₂ concentrations above outdoor levels are sometimes used to estimate ventilation rates. Higher indoor CO₂ levels are linked to health issues, discomfort, and reduced performance. Standards like ASHRAE 62.1–2007 set ventilation guidelines to address these risks.

Human physiology

The human body produces about 2.3 pounds (1.0 kg) of carbon dioxide each day, which contains 0.63 pounds (290 g) of carbon. In humans, this carbon dioxide moves through the veins and is released from the lungs, which lowers the amount of carbon dioxide in the arteries. The amount of carbon dioxide in blood is often described as partial pressure, which means the pressure carbon dioxide would have if it occupied the blood volume alone. Blood carbon dioxide levels are shown in the table next to this text.

Carbon dioxide in blood is carried in three ways. The exact percentages differ between blood in arteries and veins.

• Most (about 70% to 80%) is changed into bicarbonate ions (HCO⁻³) by an enzyme called carbonic anhydrase in red blood cells. This happens through a chemical reaction.
• About 5% to 10% dissolves directly in blood plasma.
• About 5% to 10% attaches to hemoglobin in red blood cells as carbamino compounds.

Hemoglobin, the main molecule in red blood cells that carries oxygen, also carries carbon dioxide. However, carbon dioxide does not bind to the same part of hemoglobin as oxygen. Instead, it connects to the N-terminal groups on the four parts of the hemoglobin molecule. Because of how hemoglobin changes shape when it binds to carbon dioxide, this process reduces the amount of oxygen that hemoglobin can hold for a given oxygen pressure. This is called the Haldane Effect and helps move carbon dioxide from tissues to the lungs. On the other hand, when carbon dioxide levels rise or blood pH drops, oxygen is released from hemoglobin more easily. This is known as the Bohr Effect.

Carbon dioxide helps control blood flow to tissues. When carbon dioxide levels are high, blood vessels widen to increase blood flow to the area.

Bicarbonate ions are important for keeping blood pH balanced. Breathing rate affects carbon dioxide levels in the blood. Breathing too slowly or shallowly can cause respiratory acidosis, while breathing too quickly can lead to hyperventilation, which may cause respiratory alkalosis.

Although the body needs oxygen for energy, low oxygen levels usually do not trigger breathing. Instead, breathing is controlled by carbon dioxide levels. This means that breathing in air with very low pressure or no oxygen (such as pure nitrogen) can cause a person to lose consciousness without feeling breathless. This is especially dangerous for high-altitude pilots. It is also why flight attendants tell passengers to put on their own oxygen masks first during a cabin pressure loss before helping others, to avoid losing consciousness.

The brain’s breathing control centers aim to keep the carbon dioxide pressure in arteries at 40 mmHg. If someone intentionally breathes very quickly (hyperventilates), the carbon dioxide level in the blood can drop to 10–20 mmHg, and the urge to breathe decreases. This is why people can hold their breath longer after hyperventilating than without it. However, this can be dangerous because unconsciousness may occur before the body feels the need to breathe, which is why hyperventilation is risky before free diving.

Concentrations and role in the environment

Carbon dioxide (CO₂) is a trace gas in Earth's atmosphere. It plays an important role in the greenhouse effect, the carbon cycle, photosynthesis, and the ocean's carbon cycle. It is one of three main greenhouse gases in Earth's atmosphere. In 2024, the concentration of CO₂ reached 430 parts per million (ppm), or 0.0430% (based on the number of molecules), which represents a mass of 3,364 gigatonnes. This is a 54% increase since the start of the Industrial Revolution, which began in the mid-18th century. Before that time, CO₂ levels were about 280 ppm for 10,000 years. The rise in CO₂ levels is due to human activities.

The main reason for the increase in CO₂ levels is the burning of fossil fuels. Other human activities that release CO₂ include cement production, deforestation, and burning of biomass. Higher levels of CO₂ and other long-lived greenhouse gases, such as methane, increase the amount of infrared radiation absorbed and emitted by the atmosphere. This has caused the average global temperature to rise and ocean acidification to occur. Another effect is the "CO₂ fertilization effect," which influences plant growth and other climate-related changes.

Carbon dioxide is a greenhouse gas. It absorbs and emits infrared radiation at specific wavelengths: 4.26 micrometers (2,347 cm⁻¹) and 14.99 micrometers (667 cm⁻¹). These wavelengths are related to the way CO₂ molecules vibrate. CO₂ contributes to Earth's surface temperature by trapping heat in the atmosphere. Earth's surface emits most of its energy as infrared light, while the Sun emits most of its energy as visible light. When CO₂ absorbs infrared light, it traps heat near Earth's surface, warming the planet and its lower atmosphere. This process also cools the upper atmosphere.

The current level of CO₂ in the atmosphere is the highest in 14 million years. During the Cambrian period, about 500 million years ago, CO₂ levels were as high as 4,000 ppm. In contrast, during the Quaternary glaciation (the last two million years), CO₂ levels were as low as 180 ppm. Reconstructed temperature records show that CO₂ levels peaked at around 2,000 ppm during the Devonian period (400 million years ago) and again during the Triassic period (220–200 million years ago).

When CO₂ dissolves in the ocean, it forms carbonic acid, bicarbonate, and carbonate. The ocean holds about 50 times more dissolved CO₂ than the atmosphere. The ocean acts as a large carbon sink, absorbing about one-third of the CO₂ released by human activities.

Ocean acidification is the gradual decrease in the pH of Earth's oceans. Between 1950 and 2020, the average pH of ocean surface water dropped from about 8.15 to 8.05. Human-produced CO₂ is the main cause of this change. When CO₂ dissolves in seawater, it forms carbonic acid, which releases hydrogen ions. These ions lower the ocean's pH, making it more acidic. However, seawater remains alkaline, with a pH higher than 8. Marine organisms that rely on calcium carbonate to build shells and skeletons, such as corals and mollusks, are especially affected by this change.

A change in pH by 0.1 means a 26% increase in hydrogen ions in the ocean (the pH scale is logarithmic, so a change of one pH unit equals a tenfold change in hydrogen ions). Ocean pH and carbonate levels vary depending on depth and location. Colder, higher latitude waters absorb more CO₂, which can increase acidity and lower carbonate levels in those areas. Other factors influencing ocean acidification include ocean currents, upwelling zones, proximity to rivers, sea ice, and atmospheric interactions with nitrogen and sulfur from human activities.

Changes in ocean chemistry can affect marine life and their habitats. Many marine organisms, such as coccolithophores, foraminifera, crustaceans, and mollusks, rely on calcium carbonate for shell formation, a process called calcification. These structures are vulnerable to dissolving if seawater lacks enough carbonate ions.

Most of the CO₂ that enters the ocean does not stay as dissolved gas. Instead, it breaks down into bicarbonate and hydrogen ions. The increase in hydrogen ions disrupts the chemical balance in the ocean, causing carbonate ions to combine with hydrogen ions to form more bicarbonate. This reduces the amount of carbonate ions available for marine organisms to build shells.

Carbon dioxide also enters the ocean through hydrothermal vents. The Champagne hydrothermal vent, located at the Northwest Eifuku volcano in the Mariana Trench, produces nearly pure liquid CO₂. This is one of only two known sites of liquid CO₂ in the ocean, the other being in the Okinawa Trough. A submarine lake of liquid CO₂ was discovered in the Okinawa Trough in 2006.

Commercial uses

Each year, about 230 million tonnes of carbon dioxide (CO₂) are used, mainly in the fertilizer industry to make urea (130 million tonnes) and in the oil and gas industry to help extract more oil (70 to 80 million tonnes). Other uses include food and drinks, metal manufacturing, cooling systems, fire suppression, and helping plants grow in greenhouses.

Technology exists to capture CO₂ from factory emissions or directly from the air. Scientists are studying ways to use captured CO₂ in products, and some methods are already used in businesses. However, the amount of CO₂ that can be used in products is much smaller than the total amount that could be captured. Most captured CO₂ is treated as waste and stored deep underground in rock formations.

In the chemical industry, CO₂ is mostly used to make urea, with smaller amounts used to create methanol and other products. Some chemicals, like sodium salicylate, are made using CO₂ in a process called the Kolbe–Schmitt reaction.

Captured CO₂ can be used to make methanol or electrofuels. For these fuels to be carbon-neutral, the CO₂ must come from burning plants (bioenergy) or directly from the air.

CO₂ is used in enhanced oil recovery, where it is injected into oil wells under high pressure. This makes the oil mix with CO₂, reducing its thickness and helping it flow more easily to the surface. This method can increase oil recovery by 7–23% beyond what is normally possible.

Most CO₂ used in enhanced oil recovery comes from natural underground deposits. Some CO₂ is captured from factories, like natural gas processing plants, and sent to oil fields through pipelines.

Plants need CO₂ for photosynthesis. In large greenhouses, extra CO₂ is added to help plants grow faster. However, very high levels of CO₂ (10,000 ppm or more) can be harmful to animals, so it is sometimes used to kill pests like whiteflies and spider mites. Some plants grow better with more CO₂, which can change plant communities over time.

CO₂ is used as a food additive in the food industry. It is approved for use in the European Union (E290), the United States, Australia, and New Zealand (INS 290). It helps create bubbles in drinks and keeps food acidic.

A candy called Pop Rocks is filled with CO₂ gas at very high pressure (about 4,000 kPa). When eaten, the gas is released in small bursts, creating a popping sound.

Leavening agents, like yeast or baking powder, make dough rise by producing CO₂. Yeast uses sugar in the dough to create CO₂ through fermentation, while baking powder releases CO₂ when heated or mixed with acids.

CO₂ is used to make carbonated drinks, such as soda water and soft drinks. Some drinks are carbonated using CO₂ from the fermentation process, while others use recycled CO₂. In pubs, CO₂ is often used to move beer from storage to taps.

The sour taste in carbonated drinks comes from dissolved CO₂, which turns into carbonic acid. This acid creates a sour flavor, and the dissolved CO₂ also triggers a tingling sensation on the tongue.

In winemaking, dry ice (solid CO₂) is used to cool grapes quickly after harvesting, preventing unwanted fermentation. Dry ice is preferred over water ice because it does not add water to the grapes, which could lower the sugar content in the final wine. CO₂ is also used in a process called carbonic maceration to make Beaujolais wine.

CO₂ is sometimes used to fill wine bottles or barrels to stop air from getting in, which can cause oxidation. However, CO₂ can dissolve into the wine, making it slightly fizzy. For this reason, other gases like nitrogen or argon are often used instead.

CO₂ is sometimes used to temporarily stop animals before they are slaughtered. This process may cause the animals discomfort because they are not immediately unconscious.

CO₂ is widely used in tools that rely on pressurized gas, such as pneumatic systems. It is also used in welding, though it can make welds more brittle than those made with other gases. In automotive welding, CO₂ is sometimes called MAG welding (Metal Active Gas). It creates a hotter weld, which can help with certain materials but may not be ideal for all uses.

CO₂ is used in many consumer products because it is safe, not flammable, and can be stored efficiently. It changes from gas to liquid at high pressure, allowing more CO₂ to fit in containers. CO₂ is used in life jackets, air guns, bicycle tire inflators, and carbonation devices. It is also used to kill pests and in processes like supercritical drying of food and materials.

CO₂ can put out fires by removing oxygen from the area around the flame. Fire extinguishers that use CO₂ are good for electrical fires and small liquid fires but are not effective for fires involving regular materials because they do not cool the fire enough. They are often used in server rooms to protect equipment.

History of discovery

Carbon dioxide was the first gas identified as a separate substance. Around 1640, the Flemish chemist Jan Baptist van Helmont noticed that when he burned charcoal in a closed container, the remaining ash weighed much less than the original charcoal. He believed the missing mass had changed into an invisible substance he called "gas" (from the Greek word "chaos") or "wild spirit" (spiritus sylvestris).

In the 1750s, the Scottish physician Joseph Black studied carbon dioxide further. He discovered that heating limestone (calcium carbonate) or treating it with acids produced a gas he named "fixed air." He observed that this gas was heavier than air and could not support fire or life. Black also found that when the gas was passed through limewater (a solution of calcium hydroxide in water), it formed calcium carbonate, a white solid. He used this reaction to show that carbon dioxide is produced during animal breathing and fermentation. In 1772, the English chemist Joseph Priestley described a method to create carbon dioxide by pouring sulfuric acid (called "oil of vitriol" at the time) onto chalk and dissolving the gas in water by stirring.

Carbon dioxide was first turned into a liquid in 1823 by Humphry Davy and Michael Faraday using high pressure. The first description of solid carbon dioxide (called "dry ice") was made in 1835 by the French inventor Adrien-Jean-Pierre Thilorier. He discovered that when liquid carbon dioxide was released from a pressurized container, the rapid evaporation of the liquid caused it to form a "snow" of solid CO₂.

For many years, carbon dioxide mixed with nitrogen was known by names like Blackdamp, stythe, or choke damp. This mixture was found in mining and drilling operations. It formed when oxygen was slowly used up by burning coal or biological processes, leaving behind a dangerous mix of nitrogen and carbon dioxide that could suffocate people.