Carbon dioxide (CO₂) is a naturally occurring colourless and odourless gas that is a normal part of the atmosphere.    Each molecule contains one carbon atom and two oxygen atoms bonded together.

IUPAC Name: Carbon dioxide Molecular Formula: CO2 Molecular Mass: 44

CO₂ is exhaled by humans and animals as a result of respiration.  It is absorbed by plants during photosynthesis and by bodies of water, particularly the sea.  As such a natural cycle, called the carbon cycle, is formed.  As a result carbon dioxide is present in low concentrations in the air we breathe.

Carbon dioxide is also a greenhouse gas, it can trap heat in the atmosphere.  Whilst levels of carbon dioxide in the atmosphere naturally vary over time, they can also be affected directly by human activities.  The major source of human carbon dioxide emissions is fossil fuel combustion.  CO₂ emissions are also a product of forest clearing, biomass burning, and other industrial processes like cement production.  Carbon dioxide is used as the baseline reference to compare the effect of all other greenhouse gases (carbon dioxide equivalent).