Greenhouse gas

Greenhouse gases are gaseous components of the atmosphere that contribute to the greenhouse effect (see also global warming). The major natural greenhouse gases are water vapor, which causes most (about 60%) of the greenhouse effect on Earth, carbon dioxide (about 26%), and ozone. The remaining fraction is caused by minor greenhouse gases which include methane and nitrous oxide. Industrial greenhouse gases include the heavy halocarbons (chlorinated fluorocarbons), CFC, HCFC-22 molecules such as freon and perfluoromethane, and sulphur hexafluoride (SF₆).

Greenhouse gases are transparent to certain wavelengths of the sun's radiant energy, allowing them to penetrate deep into the atmosphere or all the way to the Earth's surface, where they are re-emitted as longer wavelength radiation (chiefly infrared radiation}. Greenhouse gases and clouds prevent some of this radiation from escaping, trapping the heat near the Earth's surface where it warms the lower atmosphere. Alteration of this natural barrier of atmospheric gases can raise or lower the mean global temperature of the Earth.

The concentrations of several greenhouse gases have increased over time due to human activities, such as:

burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations,
cattle and paddy rice farming, land use and wetland changes, and pipeline losses leading to higher methane concentrations,
the use of CFCs in refrigeration and fire suppression systems.

According to the global warming hypothesis, greenhouse gases from industry and agriculture are partly or wholly to blame for global warming. Carbon dioxide is the subject of the proposed Kyoto Protocol. Nitrous oxide and methane are also taken into account in the international agreements, but not ozone.

At least one IPCC TAR chapter lead author considers mention of the effect of water vapor upon the Earth's greenhouse effect to be misleading as water vapor can not be controlled by humans.

Increase of greenhouse gases in the atmosphere

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased.

Carbon dioxide is up 30%, from 278,000 ppvb to 358,000 ppvb
Methane is up from 700 ppvb to 1721 ppvb
Nitrous oxide 15%, from 275 to 311 ppvb
CFC-12 from 0 to 0.503 ppvb
HCFC-22 from 0 to 0.105 ppvb
Perfluoromethane from 0 to 0.070 ppvb
Sulphur hexafluoride from 0 to 0.032 ppvb

(Source: IPCC radiative forcing report -- 1994 -- ppvb : part per billion in volume)

Duration of stay and global warming potential

The greenhouse gases, once in the atmosphere, do not remain there eternally. They can be withdrawn from the atmosphere:

as a consequence of a physical phenomenon (rain, condensation, remove water vapor from the atmosphere)
as a consequence of a chemical phenomenon intervening within the atmosphere. This is the case for methane, which is partly eliminated by reaction with radicals OH naturally present in the atmosphere, to give CO₂ (this effect due to the production of CO₂ is not included in the methane GWP)
as a consequence of a chemical phenomenon intervening at the border between the atmosphere and the other compartments of the planet. This is the case for CO₂, which is reduced by photosynthesis of plants, and which is also dissolved in the ocean to end up giving bicarbonate and carbonate ions (CO₂ is chemically stable in the atmosphere)
as a consequence of a radiative phenomenon. For example the electromagnetic radiation emitted by the sun and cosmic rays break molecular bonds of species in the upper atmosphere. Some halocarbons disappear in this way (they are generally too stable to disappear by chemical reaction in the atmosphere).

The lifetime of an individual molecule of gas in the atmosphere is frequently much shorter than the lifetime of a concentration anomaly of that gas. Thus, because of large (balanced) natural fluxes to and from the biosphere and ocean surface layer, an individual CO₂ molecule may last only a few years in the air, on average; however, the calculated lifetime of an increase in atmospheric CO₂ level is hundreds of years.

Aside from water vapour near the surface, which is has a residence time of few days, the greenhouse gases take a very long time to leave the atmosphere. It is not easy to know with precision how long is necessary, because the atmosphere is a very complex system. However, there are estimates of the duration of stay, i.e. the time which is necessary so that the gas disappears from the atmosphere, for the principal ones.

Duration of stay and warming capability of the different greenhouse gases can be compared:

CO₂ duration stay is variable (approx 200-450 years) and its global warming potential (GWP) is defined as 1.
Methane duration stay is of 12.2 +/- 3 years and a GWP of 22 (meaning that it has 22 times the warming ability of carbon dioxide),
Nitrous oxide has a duration stay of 120 years and a GWP of 310
CFC-12 has a duration stay of 102 years and a GWP between 6200 and 7100
HCFC-22 has a duration stay of 12.1 years and a GWP between 1300 and 1400
Perfluoromethane has a duration stay of 50,000 years and a GWP of 6500
Sulphur hexafluoride has a duration stay of 3 200 years and a GWP of 23900.

Source : GIEC