Greenhouse gases are gaseous components of the atmosphere that contribute to the greenhouse effect. The major natural greenhouse gases are water vapor, which causes about 60% of the greenhouse effect on Earth, carbon dioxide (about 26%), and ozone.
Minor greenhouse gases include methane, nitrous oxide, sulfurhexafluoride (SF6) and halocarbons such as perfluoromethane , freon and other CFCs.
The major atmospheric constituents (N2 and O2) are not greenhouse gases, because homonuclear diatomic molecules (eg N2, O2, H2 ...) do not absorb in the infrared as there is no net change in the dipole moment of these molecules.
Anthropogenic greenhouse gases
Human activity contributes to the greenhouse effect primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible  http://www.grida.no/climate/ipcc_tar/wg1/fig6-6.htm .
The concentrations of several greenhouse gases have increased over time  http://www.grida.no/climate/ipcc_tar/wg1/016.htm 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 recent 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.
The role of water vapour
At least one IPCC TAR chapter lead author (Michael Mann) 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  http://www.commondreams.org/headlines03/0730-03.htm . Water vapor is a natural greenhouse gas and an increase in atmospheric temperature caused by anthropogenic gases will lead to greater evaporation of water from the tropical sea surface and an increase in the water vapor content of the troposphere. Whether the main impact is through heating due to trapping of infrared radiation or through harder-to-determine effects through cloud changes, it is a definite part of the greenhouse gas equation even though not under direct human control.
Increase of greenhouse gases
Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased.
(Source: IPCC radiative forcing report -- 1994 -- ppvm : parts per million 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 (condensation and precipitation 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 the hydroxyl radical, OH, which is naturally present in the atmosphere, to produce CO2 and water vapor (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 form 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 are dissociated in this way which releases Cl and F as free radicals with disasterous effects on ozone (halocarbons 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 : IPCC, table 6.7 http://www.grida.no/climate/ipcc_tar/wg1/248.htm
- Greenhouse gas calculator http://www.hybridcars.com/calculator/ .
Last updated: 02-07-2005 16:18:32