The Kyoto Protocol covers emissions of five gases beside carbon dioxide: methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). In addition, water vapor is also a greenhouse gas, but its presence in the atmosphere is not directly affected by human activity.
Gases with natural and significant human sources
Methane is the second biggest contributing greenhouse gas, and is responsible for 20 percent of the enhanced (human caused) greenhouse effect. It's about 23 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of roughly 12 years.
Sources of methane include decomposing organic waste (in nature and in garbage dumps), and the raising of livestock. It's also emitted during the production and transport of coal and natural gas. Although natural sources exist, human activities are significantly contributing to the amount of methane in the atmosphere. Globally, atmospheric concentrations of methane have increased by about 150 percent since 1750, and are now at higher levels than in the last 400,000 years. Once in the atmosphere, methane decays into carbon dioxide over a period of a few years.
Nitrous oxide (N2O)
Nitrous oxide is 296 times more powerful a greenhouse gas than carbon dioxide, and remains in the atmosphere for 114 years. It is naturally emitted from oceans and soils, but human driven sources are increasing its atmospheric concentrations.
Uses include some agricultural (mostly nitrogen fertilization) and industrial activities, and it is created during combustion of fossil fuels and other organic matter. Nitrous oxide also has a variety of direct uses - including as an aerosol propellant and as an anaesthetic (i.e. "laughing gas").
Artificial gases with very high global warming potential
HFCs make up only a small portion of greenhouse gas emissions, but they are extremely potent greenhouse gases.
Depending on the exact type of HFC, they are up to 20,000 times more powerful greenhouse gasses than carbon dioxide, and have atmospheric lifetimes of up to 260 years.
Some uses of HFCs are in refrigeration (both commercial and domestic), in air-conditioning (homes, cars, offices etc), and they are also used as foam blowing agents, solvents, fire fighting agents and aerosol propellants.
HFC use and production surged after they were actively promoted as replacement refrigerants when a phase out of the ozone depleting chlorofluorocarbons (CFCs) was mandated by the Montreal Protocol. This is despite Greenpeace's successful Greenfreeze project, which proved that more natural and benign alternatives are commercially viable for refrigeration. In fact, safer alternatives exist for almost every use of HFCs - making them a good target for emission reductions.
June 2004: Unilever, Coca Cola and McDonalds ditch climate-wrecking refrigeration.
PFCs are from 5,700 to 10,000 times more powerful greenhouse gasses (depending on the exact type) than carbon dioxide, and have an atmospheric lifetime of up to 50,000 years. PFCs are by-products of aluminium smelting. They are also used in semi-conductor manufacture, and as substitutes for ozone depleting chemicals. Emissions of PFCs are small even compared to HFCs. However, given their potency, long lifetimes and availability of alternatives already on the market, PFCs should be urgently phased out.
Sulphur Hexafluoride (SF6)
Sulphur Hexafluoride is the most potent greenhouse gas evaluated by the Intergovernmental Panel on Climate Change. It is 23,900 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of 3,200 years.
It has a number of uses including in Nike Air shoes, car tires, for electrical insulation, semiconductor manufacture, and in the magnesium industry.
Like PFCs, the effects of Sulphur Hexafluoride to date are fairly small. However, since it is a very persistent and potent greenhouse gas, there is concern about its continuing build up in the atmosphere. Given its potency, long lifetime and availability of alternatives already on the market, Sulphur Hexafluoride should be urgently phased out.
Water and ozone
Ozone occurs both naturally, and from human activities. It is present both in the upper atmosphere, where it forms the ozone layer shielding us from harmful levels of ultraviolet solar radiation, and in the lower atmosphere, where it is the main component of smog.
Some people confuse the issue of ozone depletion with climate change. In reality, they are separate but related. The man made chemicals that destroy the ozone layer are greenhouse gases, as are some of the chemicals that are replacing them. Also, as the Earth's lower atmosphere warms and traps more heat, the upper atmosphere (where the ozone layer is) becomes colder, which facilitates the chemical reactions that damage the ozone layer.
Water vapor (H2O)
Water vapor is the most abundant greenhouse gas. The direct effect of human activity on global water vapor concentrations is thought to be negligible. However, water vapor is important for climate change because of an important feedback effect. Warmer air can hold more moisture, enhancing climate change. The exact size of this important feedback remains to be determined by scientists.
Editor's note: Gases are commonly compared to one another according to their Global Warming Potential (GWP), which refers to their warming effect over a set time compared to the same amount (by weight) of carbon dioxide.
Comparing GWPs is useful because it takes into account both the warming potential of each molecule of a gas, and its atmospheric lifetime (how long it stays in the air). Carbon dioxide is the commonly accepted point of reference (with a GWP of 1) because it is the most significant greenhouse gas from human activities.
For simplicity, this page refers to the warming potential of each gas relative to carbon dioxide over a 100-year period. This is the same as its GWP with a hundred year benchmark. Thus, a kilogram of carbon dioxide emission has a GWP of one, while a kilogram of nitrous oxide has a GWP of 310 - which we have expressed here as "nitrous oxide is 310 times more powerful a greenhouse gas than carbon dioxide".
However, it is worth noting that since some gases will stay in the atmosphere much longer than 100 years, their total greenhouse effect over time is actually greater than expressed here.
Atmospheric lifetime = How long the gas stays in the atmosphere.