There are at least seven important greenhouse gases:
- Carbon dioxide (CO2), mainly from the burning of fossil fuels and agricultural activity
- Methane (CH4), mainly from animals and waste; leakage during fossil fuel extraction; and recently suspected melting of methane hydrates
- Nitrous oxide (N2O), mainly from agriculture
- Hydrofluorocarbons (HFCs), mainly from leaking refrigerants
- Sulphur hexafluoride (SF6), mainly from the electricity industry
- Perfluorocarbons (PFCs), mainly from aluminium production
- Nitrogen trifluoride (NF3), mainly from production of silicon wafers, liquid crystal displays and silicon-based solar cells
The global warming potential of each may be expressed as carbon dioxide equivalents (CO2-e) to be able to compare the various emissions with each other.
For example, the global-warming potential for methane is estimated between 25 and 83 (depending on the time period) and for nitrous oxide is 298 – emitting 1 million tonnes of CH4 and N2O respectively is equivalent to emitting 25 (or 83) and 298 million tonnes of CO2.