Global Warming Potentials defined on a 100-year horizon (IPPC, 2007)
| Greenhouse Gas | GWP | Main Source | |
|---|---|---|---|
| Water vapour | Not calculated | Evaluation from bodies of water | |
| Carbon dioxide | 1 | Combustion of fossil fuels | |
| Methane | 25 | Agriculture, waste disposal, leakage from the gas distribution system and coal mining | |
| Nitrous oxide | 298 | Agriculture, transport, industrial processes and coal combustion | |
| Ozone | Non calculated | Secondary pollutant formed in the atmosphere by chemical reactions with other pollutants, initiated by sunlight | |
| F-Gases | HFCs | 124-14,800 | Refrigeration and air conditioning, foam blowing, aerosols, electrical switchgear, "cover" gases in metal smelting |
| PFCs | 9,390-12,200 | ||
| SF6 | 22,880 | ||
| NF3 | 17,200 | ||
Greenhouse gases contribute to long term global climate change by a process known as the greenhouse effect.
Light and heat energy (infrared radiation) from the sun is absorbed by the surface of the Earth during the day, and is released again slowly over time. Greenhouse gases in the atmosphere absorb the infrared radiation and re-radiate it in all directions, including back to the Earth's surface. In this way heat is trapped in the lower levels of the troposphere, causing warming and higher temperatures than would be experienced if there were no greenhouse gases.
The direct greenhouse gases have different effectiveness in radiative forcing, which measures the influence a factor has in altering the balance of incoming and outgoing energy in the earth's atmosphere. The Global Warming Potential (GWP) is a means of providing a simple measure of the relative radiative effects of the emissions of the various gases. The index is defined as the cumulative radiative forcing between the present and a future time horizon caused by a unit mass of gas emitted now, expressed relative to that of CO2. It is necessary to define a time horizon because the gases have different lifetimes in the atmosphere. National GHG inventories use a time horizon of 100 years.