About greenhouse gases

Carbon dioxide (CO₂) is the primary greenhouse gas contributing to the enhanced greenhouse effect. While CO₂ has a large variety of natural sources and sinks, human activities are responsible for the considerable increase of CO₂ in the atmosphere occurred since the industrial revolution.

CO₂ occurs naturally in the atmosphere as part of the Earth's carbon cycle – the natural circulation of carbon among the air, water and ecosystems. Human activities are altering the carbon cycle either by adding CO₂ to the atmosphere or by influencing the ability of natural sinks to remove CO₂ from the atmosphere. Burning of fossil fuels (coal, oil, natural gas) and wood are the primary source of human-caused CO₂ emissions. Natural sinks are affected by deforestation and other land use changes.

According to IPCC, human-caused greenhouse gas emissions have increased since the pre-industrial era and are now higher than ever. Between 1750 and 2011, about half of the emissions have occurred in the last 40 years.

About 40% of the human-caused emissions have remained in the atmosphere. The rest has been removed from the atmosphere and stored on land in plants and soils, and in the oceans. The oceans have absorbed about 30% of CO₂, with negative side effects such as ocean acidification. However, it is not clear how effectively these CO₂ sinks will operate in the future under a changing climate and increasing human impacts.

Methane (CH₄) is the second most important GHG for the enhanced greenhouse effect after carbon dioxide (CO₂). Methane is emitted by natural and human sources. The main natural sources of methane include wetlands, tundra, oceans and their bottom sediments, and termites. Natural sources make about 36% of methane emissions. Important human sources include landfills, livestock farming (especially enteric fermentation in farm animals), rice farming, biomass burning, as well as the production, transportation and use of fossil fuels. Human-caused sources create the majority of methane emissions, accounting for about 64% of the total emissions.

Nitrous oxide (N₂O) is the third most important GHG for the enhanced greenhouse effect after carbon dioxide (CO₂) and methane (CH₄). Although there is a relatively small amount of N₂O in the atmosphere, its lifetime is long, about 120 years, which makes it very important for the total amount of global greenhouse gases. N₂O has nearly 300 times the global warming potential of carbon dioxide. The nitrous oxide levels are now higher than ever in the past 800,000 years. Since the industrial revolution, the amount of nitrous oxide in the atmosphere has increased by 16%.

Nitrous oxide emissions are produced by both natural and human sources. The major natural sources include soils under natural vegetation, tundra and the oceans. Important human sources come from agriculture (nitrous oxide fertilizers, soil cultivation), livestock manure, biomass or fossil fuels combustion and industrial processes. In total, over one third of the emissions are estimated to come from human actions.

Water can take the form of an invisible gas called water vapour (H₂O). Water vapour is the most substantial natural greenhouse gas in the atmosphere and has a strong effect on water and climate.

Changes in water vapour concentration have resulted from warming of the atmosphere rather than a direct result of human activities. As the temperature of the atmosphere rises, more water is evaporated from ground storages such as rivers, oceans, reservoirs, and soil. As water vapour is a greenhouse gas, more water vapour in the atmosphere leads to even more warming. This positive feedback loop in which water is involved is critically important to projecting future climate change.

Ozone plays two different roles in the atmosphere. At ground level of the Earth's atmosphere, tropospheric ozone can act both as a direct, warming greenhouse gas, and as an indirect controller of greenhouse gases' lifetimes. In the second layer, stratospheric ozone has a cooling effect because it is acting as a shield which filters out most of the ultraviolet light from the Sun.

Ozone is created and destroyed by ultraviolet light from the Sun. It is created from oxygen by high energy rays, while low energy rays destroy it. Some ozone is human-caused by various kinds of air pollution (traffic emissions, biomass combustion), which then reacts in sunlight.

Tropospheric ozone is estimated to have caused around one third of all the direct greenhouse gas related warming seen since the industrial revolution. Tropospheric ozone is a particularly difficult greenhouse gas to keep track of because of its short lifespan in the atmosphere and the vast variations in its regional concentrations.

Tropospheric ozone can affect the lifetime of some greenhouse gases. The breakdown of tropospheric ozone in sunlight leads to the production of hydroxyl (OH) radicals. These radicals help to diminish some other greenhouse gases, like methane, and hence lessen their global warming potential.

The synthetic greenhouse gases are long lasting and absorb the solar radiation extremely efficiently. The synthetic greenhouse gases include halocarbons, such as CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), and HFCs (hydrofluorocarbons). Other synthetic greenhouse gases are e.g. perfluorocarbons (PFCs) and sulphur hexafluoride (SF6).

Halocarbons are used, for example, for propellants, refrigeration devices, air conditioning, certain types of heat pumps, and for foam plastic. Halocarbons are long lasting and powerful greenhouse gases. For example, CFC can remain in the atmosphere for over 102 years and has 3800 times more powerful warming effect that carbon dioxide (CO₂) molecule. However, international regulations have effectively limited the emissions these gases, and the concentrations of most of these gases are now declining or at least levelling off.

HFCs are used in the refrigeration and air-conditioning, aerosol, fire protection and foam-blowing industries. PFCs are created in the making of aluminium and magnesium and in enriching uranium, and they are being used also in eye surgery. SF6 is used in power plants as an insulation gas, gas-insulated switchgear and circuit breaker equipment and in scientific applications.