What are the greenhouse gases?
Greenhouse gases are a natural part of the atmosphere. It is the increase in the amounts of these gases through human activity that causes global warming. Human activity such as land clearing and burning fossil fuels have increased the concentration of these gases. Humans have had most impact on the enhanced greenhouse effect through increases in the amounts of carbon dioxide, methane and nitrous oxide.

Water vapour
Current state-of-the-art climate models include fully interactive clouds[11]. They show that an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapour content of the troposphere, with approximately constant relative humidity. The increased water vapour in turn leads to an increase in the greenhouse effect and thus a further increase in temperature; the increase in temperature leads to still further increase in atmospheric water vapour; and the feedback cycle continues until equilibrium is reached. Thus water vapour acts as a positive feedback to the forcing provided by human-released greenhouse gases such as CO2.
Water vapour is the most important greenhouse gas, but human activity has little direct impact on the amount in the atmosphere.

Carbon dioxide (CO2)
·CO2 has a variable atmospheric lifetime, and cannot be specified precisely[20]. Recent work indicates that recovery from a large input of atmospheric CO2 from burning fossil fuels will result in an effective lifetime of tens of thousands of years.[21][22] Carbon dioxide is defined to have a GWP of 1 over all time periods.
The amount of carbon dioxide in the atmosphere is about 30% higher now than 200 years ago. The main causes of this increase are:
-the burning of fossil fuels such as oil, coal and natural gas to create energy to make electricity and to produce fuel for transport, and
- the clearing and burning of vegetation.
This gas is the biggest contributor to the enhanced greenhouse effect (about 70%)

Methane (CH4)
·Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 62 over 20 years, 23 over 100 years and 7 over 500 years. The decrease in GWP associated with longer times is associated with the fact that the methane is degraded to water and CO2 by chemical reactions in the atmosphere.
The amount of methane in the atmosphere is about 145% higher now than 200 years ago. The main causes of this increase are:
-digestive processes of cattle and sheep (ie their burps!)
-cultivation of rice
-escape of natural gas
-decomposing waste in garbage dumps or landfills.
This gas is the second biggest contributor to the enhanced greenhouse effect (about 20%)

Nitrous oxide (N2O)
·Nitrous oxide has an atmospheric lifetime of 120 years and a GWP of 296 over 100 years.
The amount of nitrous oxide in the atmosphere is about 15% higher now than 200 years ago. The main causes of this increase are:
-burning of vegetation
-emissions from industries
-the effects of agriculture on the soil (using nitrogenous fertilisers)

Halocarbons
These greenhouse gases have been reduced since the phasing out of chlorofluorocarbons (CFCs) to protect the ozone layer. However, other halocarbons effecting the atmosphere include perfluorocarbons (PFCs) emitted during aluminium production.
·CFC-12 has an atmospheric lifetime of 100 years and a GWP(100) of 10600.
·HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP(100) of 1700.
·Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP(100) of 5700.
·Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP(100) of 22000.