Major Greenhouse Gases

Greenhouse gases are continuously cycled between the atmosphere and the biosphere/earth.
The places where the gases are released are called sources, while the places where they are removed are called sinks.
Greenhouse Gas	Percentage in Atmosphere	Source	Sink
Water Vapour (H2O)	0-4%	Evaporation and transpiration.	Condensation and precipitation.
Carbon Dioxide* (CO2)	0.035%	Burning fossil fuels such as oil, coal and natural gas; and deforestation.	Mainly, the earths oceans and growing plants.
Methane (CH4)	0.0002%	Garbage tips, burning vegetation, ruminant animals (eg cows), rice paddies & wetlands.	The sinks for CH4 are not well understood.
Nitrous Oxide (N2O)	^78% of the atmosphere is nitrogen (N2), and 21% is oxygen (O2).	60% of emissions come from the oceans and soils. 40%from human activities such as fossil fuel combustion.	Upper atmosphere, where sunlight breaks it down.
Chlorofuorocarbons (CFCs)	N/A	Refrigerators, air-conditioners, propellants in aerosol cans, from plastic production and as solvents.	Sink is unknown.
Ozone (O3)	0.000004%	The source for atmospheric O3 is the atmosphere itself.	The sink for atmospheric O3 is the atmosphere.

* Carbon dioxide is the most important gas humans have added to the atmosphere. Human activities add 26 gigatonnes of CO2 to the atmosphere each year!

^ Nitrous Oxide (N2O) breaks down into nitorgen (N2) and oxygen (O2). There are a range of reactions that cause this to occur over time.

For more information vist the Australian Greenhouse Office's Education pages.