Halocarbon compounds are chemicals in which one or more carbon atoms are linked with one or more halogen atoms: fluorine, chlorine, bromine or iodine. The best known in this group of gases are CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and the newer substitutes HFCs (hydroflurocarbons). While the concentration of halocarbons are much lower than those of the other greenhouse gases, the warming effect that they produce ranges from 3000 to 13000 times that of carbon dioxide.

These gases very rarely occur naturally. A few halocarbons, including methyl chloride, are produced in large amounts by natural interactions between halogen salts and debris from plants and animals, but most are created in anything more than minuscule traces only through human efforts. A large amount of the naturally occurring halocarbons are created by wood fire, dioxine for example, or vulcanic activities. A second large source are marine algae.

CFCs were used as spray can propellents, solvents, cleaners and coolants until the mid 1970s. Many of the world's nations agreed to control the use of CFCs in 1987 when they signed the Montreal Protocol on Substances that depleted the ozone layer. The substitute HFCs, while less damaging to the ozone layer, still trap heat in the atmosphere and are adding to the greenhouse effect.

Once these gases are in the atmosphere, they resist breakdown and do not disappear for many decades. They can remain in the atmosphere for up to 400 years.

While the concentration of CFCs is stabilizing due to the emission controls mentioned above, levels of the longer lasting gases are increasing

Some halocarbons that are effective in trapping heat are not restricted under the Montreal Protocol neither are the CFC substitutes. Due to their long atmospheric lifetimes they will continue trapping heat for centuries to come.