Greenhouse gases (GHG) are molecules that absorb and radiate energy in the infrared spectrum, causing heat to be trapped on Earth. Their concentration is increasing due to human activities, and their effects are observed as global warming.
There are many naturally occurring greenhouse gases in the atmosphere, such as water vapour, carbon dioxide, methane and nitrous oxide. There are also synthetic or human-made GHGs, such as chlorine and bromine, hydrochlorofluorocarbons, perfluorocarbons, and sulphur hexafluoride, which are found in aerosol sprays, air conditioning and refrigeration, and electronics.
Most scientists agree that the increase in greenhouse gas concentrations is due to human activities. They are primarily caused by the burning of fossil fuels, which releases large amounts of carbon dioxide and other GHGs into the atmosphere. These GHGs remain in the atmosphere for long periods of time, which contributes to the current climate change.
GHGs can be measured at a wide range of locations worldwide using sophisticated instruments. The data are sent to the WMO World Data Centre for Greenhouse Gases (WDCGG) in Japan, which collects and distributes the information globally.
Carbon dioxide is the most important GHG for the enhanced greenhouse effect, followed by methane and nitrous oxide. Methane is a longer-lived GHG with a warming potential of about 18 times that of carbon dioxide. It is emitted by natural and human sources, such as wetlands, landfills, rice agriculture, and the production, transport and use of fossil fuels. Its atmospheric lifetime is about a decade. Nitrous oxide is a very potent greenhouse gas and ozone depleting chemical that has about 300 times the global warming potential of CO2. It is emitted from human activities, such as fertiliser use.