The abundance of heat-trapping greenhouse gases in our atmosphere reached a new record high last year. According to the World Meteorological Organisation (WMO) Greenhouse Gas Bulletin, carbon dioxide, methane and nitrous oxide concentrations have all reached levels unprecedented in human history.
The relative impact of each gas on global warming is a function of its concentration in the atmosphere and its ability to absorb and radiate energy, known as its global warming potential. The longer a gas remains in the atmosphere, the more powerful its effect.
CO2 is the most abundant of all greenhouse gases and a key contributor to global warming. It is emitted when fossil fuels are burned for electricity, heat and transport. It is also emitted by deforestation, intensive livestock farming, use of synthetic fertilizers and industrial processes. The emissions from these and other sources are estimated by EPA and included in the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
Methane is another important greenhouse gas and has a shorter lifetime in the atmosphere, but still contributes to climate change. It is emitted from natural and human activities including wetlands, tundra and ocean bottom sediments, and by termites and livestock. It is also emitted by the production and transportation of fossil fuels, as well as from landfills and organic waste degradation in soils and forests.
Other long-lived greenhouse gases include the halocarbons, such as CFCs (chlorofluorocarbons) and HCFCs, used in refrigeration and air conditioning, and SF6 (sulphur hexafluoride), which is used in foam plastic. These are largely produced for specific uses, which have been restricted by international agreements.