For introducing a controversial/critical point. The IPCC considers 6 greenhouse gases based on their physical capacity to emit and absorb infrared radiation.
These gases are the well-known carbon dioxide, methane and chlorofluorocarbons but also three less newsworthy ones such as nitrogen oxides, ozone and water vapour.
NASA has calculated that the temperature of the Earth's atmosphere without any of them would be -18°C, with ozone being particularly important, which absorbs ultraviolet radiation, and that without its presence this high-frequency radiation would make tropogenic life unsustainable.