Description:
A table of the main greenhouse gases and their attributes, sources and concentration levels from 1998. Naturally occurring greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases that are not naturally occurring include hydro-fl uorocarbons (HFCs), perfl uorocarbons (PFCs), and sulphur hexafl uoride (SF6), which are generated in a variety of industrial processes. Water vapour is the most abundant greenhouse gas. However, human activities have little direct impact on its concentration in the atmosphere. In contrast, we have a large impact on the concentrations of carbon dioxide, methane and nitrous oxide. In order to be able to compare how different gases contribute to the greenhouse effect, a method has been developed to estimate their global warming potentials (GWP). GWPs depend on the capacity of greenhouse gas molecules to absorb or trap heat and the time the molecules remain in the atmosphere before being removed or broken down. The GWP of carbon dioxide is 1 (constant for all time periods) and the GWPs of other greenhouse gases are measured relative to it. Even though methane and nitrous oxide have much higher GWPs than carbon dioxide, because their concentration in the atmosphere is much lower, carbon dioxide remains the most important greenhouse gas, contributing about 60% to the enhancement of the greenhouse effect.