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The proportion of carbon dioxide in the atmosphere is usually expressed as parts per million by volume (ppmv); this tells us the concentration of carbon dioxide in numbers of molecules per million molecules of air.

Emissions can also be measured by: the weight of carbon dioxide (carbon plus two oxygen atoms) – in grammes, kilogrammes and tonnes – over a period of time. High levels of emissions are measured in millions of tonnes of carbon dioxide or by the weight of carbon alone. One tonne of carbon is equal to 3.67 tonnes of carbon dioxide.

Other greenhouse gases are sometimes expressed in million tonnes of the carbon dioxide equivalent (Mt CO2e) – in other words, the amount of carbon dioxide that would have the same effect.