A measure that combines the global warming effect of the 6 greenhouse gases listed in Annex A of the Kyoto Protocol – carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride (SF6) – into a single meaningful number. Specifically, CO2-e represents the carbon dioxide emissions that would cause the same heating of the atmosphere as a particular mass of Annex A greenhouse gases.