Some greenhouse gases occur naturally and enter the atmosphere as a result of both natural processes (such as decomposition of organic matter) and human activity (such as burning fossil fuels and agriculture). Greenhouse gases that occur both naturally and from human activities include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). Other greenhouse gases have essentially no natural sources, but are side products of industrial processes or manufactured for human purposes such as cleaning agents, refrigerants, and electrical insulators. These include the fluorinated gases: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HCFCs), bromofluorocarbons (halons), perfluorocarbons, PFCs, nitrogen trifluoride, NF3, and sulfur hexafluoride, SF6.
These gases contribute to atmospheric warming when they absorb infrared radiation emitted by the solar-warmed Earth and transfer their extra energy to the surrounding atmospheric gases.