A What is a Greenhouse Gas?

There are various definitions for a Greenhouse Gas but each carry the same theme.  Examples are:

a gas that contributes to the greenhouse effect by absorbing infrared radiation, e.g. carbon dioxide and chloroflourocarbons.” The Merriam Webster Dictionary™

 “any of the gases whose absorption of solar radiation is responsible for the greenhouse effect, including carbon dioxide, methane, ozone and fluorocarbons.  Dictionary.com™

 “Greenhouse gas, any gas that has the property of absorbing infrared radiation (net heat energy) emitted from Earth’s surface and reradiating it back to Earth’s surface, thus contributing to the greenhouse effect. Carbon dioxide, methane, and water vapour are the most important greenhouse gases.”  The Encyclopedia Britannica™

“Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary greenhouse gases in the Earth’s atmosphere.”  IPCC Definition at ipcc-data.org.

Tropospheric water vapour is the single most important greenhouse gas, but its atmospheric concentration is not significantly influenced by direct anthropogenic emissions. Of the greenhouse gases that are directly affected by human activities, CO2 has the largest radiative effect, followed by the CFCs, CH4, tropospheric O3, and N2O.”  IPCC (1990) pg. 7

All of the definitions above includes a bootstrap clause, i.e. circular limitation.  This means a greenhouse gas is a gas that contributes to the greenhouse effect.  The two dictionary definitions leave out the most abundant infrared absorbing gas i.e. water vapor.  The Encyclopedia Britannica™ mentions water vapor but in third place. As set forth in the Section B below, CO2 has a concentration of around 400 ppmv while water vapor varies between 20,000 to 50,000 ppmv.  That’s 75 times more concentrated plus water vapor has a much wider absorption profile as discussed in Section C below.  Water vapor’s absorptions abilities dwarfs all other “Greenhouse Gases” combined by hundreds of times.

 The IPCC definition is the most accurate if the clause “This property causes the greenhouse effect” is deletedFor purposes of this discussion, Quorvita will adopt the IPCC definition as modified, i.e. Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds.

There is no such thing as a greenhouse gas since the gases in a greenhouse is air.  On the other hand, the expression Greenhouse Gas is a convenient and useful analogy in describing a gas that absorbs and emits infrared radiation within the spectrum emitted by the Earth’s surface.