Greenhouse gasses which occur naturally in our environment include water vapor(H₂0), Carbon dioxide (CO₂), Methane (CH₄), Oxides of Nitrogen (NO_x) and ozone (O₃).

Naturally occurring greenhouse gases are essential to life on earth considering the important role they play in the earth’s greenhouse effect. Since they slow the transference of the earth’s thermal radiation into space, these gases help keep the earth approximately 33°C warmer than it would otherwise be without them.

Unlike naturally occurring greenhouse gases, anthropomorphic (man made) greenhouse gases are believed to have caused an over accumulation of greenhouses gases in our atmosphere. This is believed to have affected an observed increase in the earth’s average surface temperature, commonly referred to as global warming.

Common anthropomorphic greenhouse gases include Carbon dioxide (CO₂), Methane (CH₄), Dinotrogen monoxide (N₂O, a form of "NO_x") and ozone (O₃). The anthropomorphic production of these gases is mainly attributable to the burring of fossil fuels such as coal, petroleum and natural gas, and deforestation and agricultural ativities.

Several halogenat ed substances such as Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and bromofluorocarons are also greenhouse gases, but not in the strictest sense. Nearly all of these substances are byproducts of industrial activities, but unlike CO₂, CH₄, N₂O and O₃, most CFCs and HCFCs do not directly slow the earth’s outward transference of thermal (infra red) radiation. Rather, they are substances which deplete stratospheric ozone, which consequently decreases the rate at which the earth’s atmosphere can filter incoming ultra violet solar radiation. Conversely perfluorocarbons (PFCs) and sulfur hexafluoride (SF₆) do not deplete the ozone layer, but are potent isolative greenhouse gases.

Finally, there are several gases which no not have direct effects on global warming, but which directly or indirectly affect terrestrial and/or solar radiation absorption by influencing the formation or destruction of greenhouse gases and/or troposphere and stratospheric ozone. These gases include carbon monoxide (CO) and non-CH₄ volatile organic compounds.

The following graphs provide slightly varying accounts of atmospheric percentages of greenhouse gas compositions:

Water Vapor

Water Vapor is the most abundant greenhouse gas in the atmosphere. Changes in its concentration are considered to be a result of climate feedbacks related to the warming of the atmosphere, rather than a direct result of industrialization.

As explained by the US Oceanic and Atmospheric Administration (NOAA), as the temperature of the atmosphere rises, more water is evaporated from ground storage, i.e. soils, rivers, lakes, oceans, etc. Because the air is warmer, relative humidity can be higher. In essence, the air is able to “hold” more water when it’s warmer. This leads to the presence of more water vapor in the atmosphere. Whereas water vapor is a greenhouse gas, an atmosphere with higher concentrations of water vapor is able to absorb increased amounts of thermal IR energy radiated from the earth. This absorption causes the atmosphere to warm. The warmer atmosphere can then hold more water vapor, and so on. This is referred to as a "positive feedback loop”.

Carbon dioxide (CO₂)

Carbon dioxide is a colorless, odorless gas that is an essential component of the respiratory and photosynthetic processes which make life on earth possible.

The unnatural accumulation of CO₂ in the earth’s atmosphere is a cause of concern due to the fact that CO₂ is very efficient at slowing the transference of earth’s infrared radiation into space, an effect which is believed to contribute to recently observed increases in the earth’s average surface temperature.

On a worldwide basis, anthropogenic emissions ofCO₂ are known to be relatively small. The Energy Information Administration (EIA) estimates global anthropogenic production of CO₂ to be approximately 25 billion tones, (50 trillion pounds) in 2003. Although this is an enormous number, when comparison with the gross fluxes of carbon from natural systems, this number represents only a fraction (approximately 2%) of total global emissions. The important thing to remember however, is that this number is perceived to account for most of the observed, accumulated CO₂ in the atmosphere.

Methane (CH₄)

Methane is an odorless hydrocarbonic gas with the chemical formula CH₄.

Methane is an extremely effective absorber of the earth’s infrared radiation. The USEPA estimates that methane is 20 times more effective at trapping heat in the atmosphere than carbon dioxide. Nonetheless, concentrations of atmospheric methane are less than concentratons of CO₂ , and methane’s lifetime in the atmosphere is relatively short when compared to other greenhouse gases such as CO₂ , NO_x and CFCs.

Methane has both natural and anthropogenic sources and it is released as part of the biological processes in low oxygen environments, such as in swamplands or in rice production. Over the last 50 years, human activities such as rice growing , cattle farming, using natural gas, buring ever increasing volumes of petroleum, and coal mining have added to the atmospheric concentration of methane.

NO_x, N2O & Nitrous oxide

When referring to Dinitrogen monoxide (N₂O) as an anthropomorphic greenhouse gas, most sources refer to it as “Nitrous oxide”. Use of the description "Nitrous oxide" however, to specifically refer to N₂O, is imprecise because the term "Nitrous oxide" can be properly used to refer to any oxides of nitrogen, or any mixture(s) of them.

Conversely, “NO_x” is a good generic term for all the various forms of nitrogen oxides that are produced during combustion. These include:

Nitric Oxide (NO)
Nitrogen dioxide (N₂O)
Dinitrogen monoxide (N₂O) (“Nitrous oxide”)
Dinitrogen trioxide (N₂O₃)
Dinitrogen tetroxide (N₂O₄)
Dinitrogen pentoxide (N₂O₅)

Nitric Oxide (NO)	Nitrogen dioxide (NO2)	Dinitrogen monoxide (N2O)
Dinitrogen trioxide (N2O3)	Dinitrogen tetroxide (N2O4)	Dinitrogen pentoxide (N2O)

Chlorofluorocarbons (CFCs) &Hydrofluorcarbons (HFCs)

CFCs are "indirect" greenhouse gases, in that, instaead of having direct insulative qualities, they reduce stratospheric ozone. When stratospheric ozone is reduced, more ultraviolet radiation reaches the earth's surface, thereby generating increased levels of IR radiation that becomes trapped by “direct” greenhouse gases such as CO₂, CH₄ and NO_x.

CFCs have no natural source. Rather, they are entirely synthesized for uses such as refrigerants, aerosol propellants and cleaning solvents. Since their creation in the late 1920’ ;s, concentrations of CFCs in the atmosphere have risen steadily.

After the discovery that the production of CFCs was affecting dramatic reductions in stratospheric ozone, a successful global effort to halt their production was undertaken. Current measurements indicate that atmospheric concentrations of CFCs have leveled, and in some cases, have even begun to decline.

Even with recent abatements in their production, because of their long atmospheric lifetimes, relatively high concentration of the CFCs are expected to remain in the atmosphere for the next hundred years.

Hydrofluorcarbons HFCs are similar to CFCs, although they are less stable in the atmosphere. Because of this, they are beleivd to have less potential environmental impact than their longer lived CFC cousins.