Carbon dioxide (CO2) is the main greenhouse gas of concern. This is because of the carbon cycle. A given amount of carbon is stored within the earth's oceans, land, living matter and the atmosphere. Natural processes transfer this carbon from one place to another over time, as the carbon atoms are recycled again and again in different forms.
For example, plants and trees absorb carbon dioxide from the atmosphere and use it for the process of photosynthesis to grow. Then animals eat the plants, and use the carbon atoms themselves to build their own tissues. They then return the carbon to the atmosphere as they breathe (carbon dioxide). This carbon is then quickly taken up again by new plants for photosynthesis, completeing the cycle.
As well as returning carbon relatively quickly back to the atmosphere, plants and animals also leave some carbon in the soil when they die and decompose. These carbon atoms can remain stored in the earth for very long periods of time and may only be recycled extremely slowly. For example, over geological time, carbon atoms in the soil may form oil, gas or coal and only be released to the atmosphere in the rare event of a natural fire.
These natural recycling processes mean that some carbon moves in and out of the atmosphere quickly, whilst some takes millions of years to be transferred. But the overall amount of carbon in the atmosphere remains roughly in balance with the earth's other carbon stores.
However when human activities result in, for example, the burning of fossil fuels (oil, gas and coal) or the removal of trees and vegetation on a large scale, much more carbon is released into the atmosphere much more quickly than would happen naturally. This unbalances the natural carbon cycle by increasing the natural concentrations of carbon in the atmosphere (adding to the greenhouse effect).
