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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:

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. (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 They can be withdrawn from the atmosphere: The lifetime of an individual molecule of gas in the atmosphere is frequently much shorter than the lifetime of a concentration anomaly of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:

Source : GIEC

See also: