The Greenhouse gas reference article from the English Wikipedia on 24-Jul-2004
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Greenhouse gas

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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.

Table of contents
1 Anthropogenic greenhouse gases
2 Duration of stay and global warming potential
3 See also

Anthropogenic greenhouse gases

Human activity contributes to the greenhouse effect primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible [1].

The concentrations of several greenhouse gases have increased over time [1] 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 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.

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

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 They can be withdrawn from the atmosphere: The lifetime of an individual molecule of gas in the atmosphere is frequently much shorter than the lifetime of a concentration anomaly of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 : IPCC, table 6.7

See also