Greenhouse gases are compounds that allow solar radiation to pass through the atmosphere, but trap some of it and re-emit it back towards Earth. This causes the warming we see on Earth, which is called the greenhouse effect.
The global average concentration of greenhouse gases in the atmosphere has increased over the past 150 years. This increase has been caused by human activities such as burning fossil fuels, agriculture and deforestation. These gases have a long atmospheric lifetime, meaning that they can remain in the atmosphere for decades or centuries.
Some of these gases are natural, such as carbon dioxide and methane, while others are man-made. The man-made gases include chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6). These are used in a range of applications including in aerosol sprays, air conditioning and refrigerants.
Methane is the second most important GHG for the enhanced greenhouse effect. It is emitted by both natural and human sources, but the majority of it comes from human activity such as livestock farming, rice cultivation and landfills. Methane is a colourless gas, and its emissions also come from the production, transport and use of oil, natural gas and coal.
Nitrous oxide is the third most important GHG for the enhanced greenhouse effect. While it has a lower GWP than both CO2 and methane, its high atmospheric concentration has contributed to significant warming over the past 150 years. This is mainly due to human activities such as fertilizer use, the production and transport of fossil fuels and land use change. This is the main reason why reducing N2O is such a key climate policy goal.