CO2, known as carbon dioxide, is a chemical compound composed of one carbon atom and two oxygen atoms.


Carbon dioxide is colourless, odourless, dense  (heavier than air) and very stable. CO2 is a greenhouse gas (GHG) that accounts for about 0.03% of the atmosphere. A greenhouse gas can be defined as “a gas present in the atmosphere, of natural or anthropogenic origin, that absorbs and re-emits infrared rays coming from the surface of the Earth” (translated from OQLF, 2005).

How much is one tonne of CO2?

At a temperature of 0°C and at ambient pressure, one tonne (1,000 kg) of CO2 occupies a volume of about 505 m3. This represents a cube about 8 metres across.

When temperature and pressure conditions exceed the critical point of 31.1°C and 73.8 bars (about 74 times atmospheric pressure), CO2 changes into what is called a “supercritical state”. Supercritical carbon dioxide is dense like a liquid but still behaves like a gas in terms of diffusion. Supercritical conditions for CO2 occur at depths more than 800 metres below the surface. When CO2 is stored in a geological reservoir 1,000 metres underground, one tonne of the pure gas occupies a volume of only 1 to 2 m3, or about 250 to 350 times less than the space it would fill at the surface. It is therefore possible to store large quantities of CO2 in very small volumes, which is a major advantage of the geological storage option.

Comparison of the volume occupied by one tonne of CO2 gas at the surface of the Earth with one tonne of CO2 in a supercritical state at a depth of about 1,000 metres.
Comparison of the volume occupied by one tonne of CO2 gas at the surface of the Earth with one tonne of CO2 in a supercritical state at a depth of about 1,000 metres.

Origin and effects of CO2

CO2 is an inevitable product during the combustion of fossil fuels, such as oil, natural gas or coal. The gas is also emitted during some industrial processes (chemical reactions during cement manufacturing, chemical reactions in aluminum smelters…). According to the IPCC, more than half of the world’s CO2 emissions are produced by concentrated and localized sources referred to as “major emitters” (IPCC, 2005). Examples of major emitters include thermal power plants, cement plants, refineries, iron and steel factories, and aluminum smelters.

Increasing atmospheric concentrations of greenhouse gases, such as CO2, contribute to climate change. Greenhouse gases retain a significant amount of solar radiation even though they comprise less than 1% of the atmosphere. During the past century, atmospheric concentrations of greenhouse gases have risen by 50%; the amount of CO2 alone by 31%.

The carbon cycle

The carbon cycle is particularly complex. It includes all molecular exchanges of carbon (CO2, organic matter, methane, limestone, coal…) between the four great carbon reservoirs of our planet: the atmosphere, biosphere, hydrosphere and lithosphere. These exchanges are the result of numerous chemical, physical, geological and biological processes.

Simplified carbon cycle.
Simplified carbon cycle.

In the atmosphere, carbon is present as CO2 and methane (CH4). Some atmospheric carbon is absorbed by plants during photosynthesis. This biochemical reaction allows plants to remove carbon from the air and form organic molecules. CO2 and/or CH4 are produced and returned into the atmosphere when the biosphere breathes, when carbon-bearing materials burn, and when organic matter decomposes.

An even greater quantity of atmospheric carbon is absorbed by the oceans. Oceans also release a certain amount of carbon back into the atmosphere, but the remainder is absorbed and eventually stored on the ocean bottom in the form of marine sediments.

CO2 emissions caused by human activities—burning fossil fuels for example—create an imbalance in the natural exchanges between the four planetary reservoirs. CO2 then accumulates in the atmosphere, reinforcing the greenhouse effect. In fact, the carbon in hydrocarbon fuels and coal would never return to the atmosphere without humans extracting and burning them to generate energy.

Nevertheless, thanks to the carbon cycle, not all CO2 generated by humans will stay in the atmosphere. It will eventually be divided between the biosphere and the oceans, but this also means it is impossible to accurately predict the lifespan of CO2 in the atmosphere. On average, however, we can estimate it will take about 100 years to establish a new state of equilibrium among the various carbon reservoirs (de Perthuis,C. (2009) Et pour quelques degrés de plus… Nos choix économiques face au risque climatique. Pearson, 320 pages).