greenhouse gas
any gas that absorbs heat in the atmosphere and re-emits it back toward Earth, causing a warming effect. The greenhouse gases regulated by the United Nations Framework Convention on Climate Change (UNFCCC) are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride. Water vapor and ozone are also greenhouse gases but are not covered by the UNFCCC.