Carbon dioxide (chemical formula CO₂) is a colourless gas. It consists of a carbon atom covalently bonded to two oxygen atoms. It is about 60% denser than air. CO₂ is soluble in water, in which it forms carbonic acid (H₂CO₃), a weak acid. At low temperatures (below -78.5°C, or -109.3°F) it turns directly into a solid, commonly called dry ice. This turns back into a gas, in a process called sublimation, at temperatures higher than -78.5°C. CO₂ occurs naturally in the Earth's atmosphere in tiny quantities, but these are rising rapidly.

Sources

Natural sources of CO₂ include volcanoes, geysers and hot springs. CO₂ also occurs naturally dissolved in water. It is also present in oil and natural gas, and it can be formed from carbonate rocks such as chalk and limestone when the rocks are dissolved in water.

CO₂ is produced by all living things when they convert nutrients in the food they take in into energy through respiration and is returned either to the air or, in the case of aquatic animals that use gills to breathe, the water. CO₂ is also naturally produced during the process of decay of organic materials. CO₂ is the product of combustion of wood and fossil fuels such as coal, oil and natural gas (which consists mostly of methane).

Greenhouse gas

Like methane and water vapour, CO₂ is a greenhouse gas: it traps part of the Sun's heat from being radiated back into space, warming the Earth's surface. Since the Industrial Revolution, both the burning of fossil fuels and large-scale deforestation (resulting in fewer plants to take in CO₂ from the air) have rapidly increased the concentration of CO₂ in the atmosphere. Current levels are about 0.04% or 428.5 parts per million (ppm) by volume (as of January 2026), having risen from levels of 280 ppm before industrialization began.

Carbon sinks

Plants on both land and in the sea (plankton) absorb (take in) CO₂ as part of their natural cycle. CO₂ is also dissolved in the ocean waters. Oceans absorb more than a quarter of the CO₂ that is added to the air by human activity. Both vegetation and oceans act as what are called carbon sinks. Dense concentrations of trees, such as rainforests, are particularly effective carbon sinks. Soil is also an important carbon sink: it contains more carbon than all vegetation and the atmosphere combined. The process by which carbon sinks remove CO₂ from the atmosphere is known as carbon sequestration.

Deforestation—cutting down trees and not replacing them—diminishes the carbon sinks. It also speeds up the rate at which the organic matter in the soil turns to CO₂. While the oceans continue to absorb CO₂ from the air, this causes acidification, with harmful consequences for marine life and coral reefs, such as coral bleaching.

Uses

CO₂ is used as a gas in welding and fire extinguishers. It is added to drinking water and carbonated drinks to add effervescence (fizziness), and is formed when some leavening agents, such as baking soda, break down. These agents cause dough and cake mixtures to rise. The frozen solid form of CO₂, dry ice, is used as a refrigerant and as an abrasive in blast cleaning.