Carbon Dioxide
Atmospheric carbon dioxide naturally comes mainly from the decay of plants, volcanic eruptions and as a waste product of animal respiration. It is removed from the atmosphere by photosynthesis in plants and by dissolving in water, especially on the surface of oceans. Carbon dioxide remains in the atmosphere for about 100 years.
Carbon dioxide is currently the gas most responsible for the 'enhanced greenhouse effect'. Burning fossil fuels to run vehicles, heat homes, power factories, etc releases the carbon dioxide stored millions of years ago and further gas is added with deforestation, which releases the carbon stored in trees and also results in less carbon dioxide being removed from the atmosphere. Further due to Global warming as the oceans get warmer, its ability to absorb Carbon dioxide is reduced. Due to the length of time it remains in the atmosphere, even if we stopped Carbon dioxide emissions immediately, the effects of what we've already done would still influence our weather for years to come.

Methane
Methane is generated naturally by bacteria that break down organic matter, it is found in the guts of termites and other animals and in natural gas deposits. Methane is removed from the atmosphere through a natural process to form water. Methane remains in the atmosphere for about 12 years.
But the recent increase in atmospheric methane is being contributed by an increase in livestock farming and rice growing, extraction of fossil fuels and landfill sites, and the burning of biomass. Even though methane occurs in lower concentrations than carbon dioxide, it produces 20 times as much warming as Carbon dioxide. Methane is the second biggest contributor of the 'enhanced greenhouse effect'.

Nitrous Oxide
Nitrogen is naturally removed from the atmosphere by plants, converted into forms such as ammonia and used, this is called nitrogen fixation. While micro-organisms remove nitrogen from the soil and put it back into the atmosphere, called denitrification and this process produces nitrous oxide. Nitrous oxide also enters the atmosphere from the ocean. Nitrous oxide remains in the atmosphere for about 160 years.Burning fossil fuels and wood, widespread use of nitrogen-base fertilisers and Sewage treatment plants are the various causes for in crease in atmospheric nitrous oxide. Nitrous oxide is 200 to 300 times more effective in trapping heat than carbon dioxide. Due to the long atmospheric life time the nitrous oxide that we release today will still be trapping heat till the end of the next century.

Halocarbons
The best known in this group of gases are CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and the newer substitutes HFCs (hydroflurocarbons). These gases very rarely occur naturally. Halocarbons are 3,000 to 13,000 times more effective in trapping heat than carbon dioxide. CFCs are released into the atmosphere from spray cans (propellents, solvents, cleaners and coolants), discarded or leaking refrigeration, air conditioning equipment and burning plastic foam products. CFCs are lowering the average concentration of ozone in the stratosphere. Chlorofluorocarbons move up to the stratosphere gradually over several decades. Under high energy ultra violet (UV) radiation, they break down and release chlorine atoms, which speed up the breakdown of ozone (O3) into oxygen gas (O2). Once these gases are in the atmosphere, they resist breakdown and don't disappear for many decades and remain in the atmosphere for up to 400 years. Due to their long atmospheric lifetimes the that have been already released will continue trapping heat for centuries to come.
In 1987 many of the world's nations agreed to control the use of CFCs, however the substitute HFCs, while being less damaging to the ozone layer, still trap heat in the atmosphere and are adding to the greenhouse effect.

Water Vapour
Water Vapour is the biggest contributor to the 'natural greenhouse effect' and varies the most in the atmosphere. Cold air can hold little water and so the atmosphere over the polar regions contains very little water vapour. In contrast, air over the tropics is very humid and the atmosphere can contain up to 4% water vapour. So a small increase in global temperature would lead to a rise in global water vapour levels thus further enhancing the greenhouse effect.
Human activities have little direct impact on the level of water vapour in the atmosphere.

Ozone
Everyday Ozone is created and destroyed by ultraviolet light from the Sun, the high energy rays create it while the low energy rays destroy it. Some ozone is man-made by various kinds of air pollution, which then reacts in sunlight.

In conclusion we are already seeing a change in the world around us which could turn into an inhospitable place for the future generations if we as individuals, societies and nations do not try to bring about a change in our environmental consciousness NOW!