Greenhouse Gases

Greenhouse gases (GHG) are gaseous components of the atmosphere that contribute to the greenhouse effect. Like greenhouse glass, greenhouse gases are transparent only to some wavelengths of light. When sunlight hits the Earth, some is absorbed and re-emitted at longer wavelengths for which the greenhouse gas is opaque, hindering emission back out into space. This warms the Earth; although it is not the primary mechanism that warms greenhouses.

The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds), but that can be converted to rain by nature; carbon dioxide, which causes between 9-26%; methane, which causes 4-9%, and ozone, which causes between 3-7%. (note that it is not really possible to assert that such-and-such a gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive.) The higher ends of the ranges quoted are for the gas alone; the lower end, for the gas counting overlaps).

Other greenhouse gases include, but are not limited to: nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons - see IPCC list of greenhouse gases.

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 range.

Anthropogenic greenhouse gases

Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible.

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,
• livestock and paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations, many of the newer style fully vented septic systems that enhance and target the fermentation process also are major sources of atmospheric methane.
• the use of CFCs in refrigeration systems. The use of CFCs and halons in fire suppression systems and various manufacturing processes.

According to the global warming hypothesis, greenhouse gases from industry and agriculture have played a major role in the recently observed global warming. Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases are the subject of the Kyoto Protocol, which entered into force in 2005. Methane, nitrous oxide and ozone depleting gases are also taken into account in the international agreements, but not ozone.

To read this article in it's entirety visit http://en.wikipedia.org/wiki/Greenhouse_gas .