The six greenhouse gases specified in the Kyoto Protocol are:

• Carbon dioxide (CO2)
• Methane (CH4)
• Nitrous oxide (N20)
• Hydrofluorocarbons (HFCs)
• Perfluorocarbons (PFCs)
• Sulphur hexafluoride (SF6)

Approximately 25 other gases, such as chloroform and carbon monoxide, qualify as climate-changing greenhouse gases, but only the above mentioned six are released in sufficient quantities to justify regulation under Kyoto. Water vapour is a very important greenhouse gas, but is not controllable by human intervention.

Carbon dioxide (CO2)
Carbon dioxide comes from the decay of materials, respiration of plant and animal life, volcanic and thermal venting, and the natural and human-induced combustion of materials and fuels. It is removed from the atmosphere through photosynthesis and ocean absorption.

Methane (CH4)
Methane is a more effective heat-trapping gas. It comes from the anaerobic (without oxygen) decay of matter. Primary sources include wetlands, rice paddies, animal digestive processes, fossil fuel extraction, pulp and paper processing and decaying garbage.

Nitrous oxide (N2O)
Soils and oceans are the primary natural source of nitrous oxide. Humans contribute through soil cultivation and use of nitrogen fertilizers, nylon production, and the burning of organic material and fossil fuels.

Halocarbons (HFC and PFC)
Halocarbons are human-produced chemical compounds containing members of the halogen family (bromine, chlorine, and fluorine) and carbon. They are some of the most effective heat trapping greenhouse gases of all; however, most of them are already regulated under the Montreal Protocol (a treaty for the protection of the ozone layer). These newer gases are regulated by the Kyoto Protocol because although they are ozone friendly, they are greenhouse unfriendly.

Sulphur hexafluoride (SF6)
Sulphur hexafluoride is emitted by the electric power industry in circuit breakers, gas-insulated substations and switchgear. That industry uses a significant percentage of the 6,500 to 7,500 metric tonnes produced worldwide each year.

Each greenhouse gas has a different capacity to cause global warming, depending on its radiative properties, its molecular weight and its lifetime in the atmosphere. Its global warming potential (GWP) defines as the warming influence over a set time period of a gas relative to that of carbon dioxide.

A 100-year time horizon is used in the Kyoto Protocol. When the warming effect of current greenhouse gas emissions over the next 100 years is calculated, it indicates that carbon dioxide will be responsible for about two thirds of the expected future warming.