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Carbon dioxide (CO2) is naturally occurring. People and animals produce it when breathing out whereas plants absorb and metabolise it, releasing oxygen in return.

The level of carbon dioxide in the atmosphere is measured in parts per million (ppm); or simply by weight (kg, tonnes).

Scientists believe carbon dioxide levels have been much higher in the Earth’s distant past. Geological evidence suggests that in the Cretaceous period (65-144 million years ago) it was three to six times higher than today.