Editing Greenhouse effect (section)

=== Greenhouse gases ===
{{Main|Greenhouse gas}}A greenhouse gas (GHG) is a gas which contributes to the trapping of heat by impeding the flow of longwave radiation out of a planet's atmosphere. Greenhouse gases contribute most of the greenhouse effect in [[Earth's energy budget]].<ref name="IPCC" />

==== Infrared active gases ====
Gases which can absorb and emit longwave radiation are said to be ''infrared active''<ref name="Archer2011Ch4">{{cite book |last1=Archer |first1=David |title=Global Warming: Understanding the Forecast, Chapter 4: Greenhouse Gases |date=2011 |publisher=Wiley |isbn=978-0470943410 |edition=2 |url=http://forecast.uchicago.edu/chapter4.pdf |access-date=14 June 2023}}</ref> and act as greenhouse gases.

Most gases whose molecules have two different atoms (such as carbon monoxide, {{chem|CO}}), and all gases with three or more atoms (including {{H2O-nl}} and {{CO2}}), are infrared active and act as greenhouse gases. (Technically, this is because when these molecules [[molecular vibration|vibrate]], those vibrations modify the molecular [[Bond dipole moment|dipole moment]], or asymmetry in the distribution of electrical charge.   See [[Infrared spectroscopy#Number of vibrational modes|Infrared spectroscopy]].)<ref name="IPCC" />

Gases with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, {{chem|N|2}}, and oxygen, {{chem|O|2}}) are not infrared active. They are transparent to longwave radiation, and, for practical purposes, do not absorb or emit longwave radiation. (This is because their molecules are symmetrical and so do not have a dipole moment.) Such gases make up more than 99% of the dry atmosphere.<ref name="IPCC" />

==== Absorption and emission ====
[[File:Longwave Absorption Coefficients of H2O and CO2.svg|thumb|upright=1.35|[[Outgoing longwave radiation|Longwave]] [[absorption coefficient]]s of water vapor and carbon dioxide. For wavelengths near 15 microns (15 ''μ''m in top scale), where Earth's surface emits strongly, CO<sub>2</sub> is a much stronger absorber than water vapor.]]Greenhouse gases absorb and emit longwave radiation within specific ranges of wavelengths (organized as [[spectral line]]s or [[absorption band|bands]]).<ref name="IPCC" />

When greenhouse gases absorb radiation, they distribute the acquired energy to the surrounding air as thermal energy (i.e., kinetic energy of gas molecules). Energy is transferred from greenhouse gas molecules to other molecules via [[elastic collision|molecular collisions]].<ref name="uaATMO336eeb" />

Contrary to what is sometimes said, greenhouse gases do not "re-emit" photons after they are absorbed. Because each molecule experiences billions of collisions per second, any energy a greenhouse gas molecule receives by absorbing a photon will be redistributed to other molecules before there is a chance for a new photon to be emitted.<ref name="uaATMO336eeb" />

In a separate process, greenhouse gases emit longwave radiation, at a rate determined by the air temperature. This thermal energy is either absorbed by other greenhouse gas molecules or leaves the atmosphere, cooling it.<ref name="uaATMO336eeb" />

==== Radiative effects ====
''Effect on air:'' Air is warmed by [[latent heat]] ([[virtual temperature|buoyant water vapor]] condensing into water droplets and releasing heat), [[thermal]]s (warm air rising from below), and by sunlight being absorbed in the atmosphere.<ref name="budget" /> Air is cooled radiatively, by greenhouse gases and clouds emitting longwave thermal radiation. Within the [[troposphere]], greenhouse gases typically have a net cooling effect on air, emitting more thermal radiation than they absorb. Warming and cooling of air are well balanced, on average, so that the atmosphere maintains a roughly stable average temperature.<ref name="Wallace2006">{{cite book |last1=Wallace |first1=J. M. |last2=Hobbs |first2=P. V. |title=Atmospheric Science |date=2006 |publisher=Academic Press |isbn=978-0-12-732951-2 |edition=2}}</ref>{{rp|139}}<ref name="Manabe1964">{{cite journal |last1=Manabe |first1=S. |last2=Strickler |first2=R. F. |title=Thermal Equilibrium of the Atmosphere with a Convective Adjustment |journal=J. Atmos. Sci. |date=1964 |volume=21 |issue=4 |pages=361–385 |doi=10.1175/1520-0469(1964)021<0361:TEOTAW>2.0.CO;2|doi-access=free |bibcode=1964JAtS...21..361M }}</ref>

''Effect on surface cooling:'' Longwave radiation flows both upward and downward due to absorption and emission in the atmosphere. These canceling energy flows reduce radiative surface cooling (net upward radiative energy flow). Latent heat transport and thermals provide non-radiative surface cooling which partially compensates for this reduction, but there is still a net reduction in surface cooling, for a given surface temperature.<ref name="Wallace2006" />{{rp|139}}<ref name="Manabe1964" />

''Effect on TOA energy balance:'' Greenhouse gases impact the top-of-atmosphere (TOA) energy budget by reducing the flux of longwave radiation emitted to space, for a given surface temperature. Thus, greenhouse gases alter the energy balance at TOA. This means that the surface temperature needs to be higher (than the planet's ''effective temperature'', i.e., the temperature associated with emissions to space), in order for the outgoing energy emitted to space to balance the incoming energy from sunlight.<ref name="Wallace2006" />{{rp|139}}<ref name="Manabe1964" /> It is important to focus on the top-of-atmosphere (TOA) energy budget (rather than the surface energy budget) when reasoning about the warming effect of greenhouse gases.<ref name="PierrehumbertTextbook" />{{rp|414}}
[[File:Atmospheric heat flow profile.svg|thumb|upright=2.5|Flow of heat in Earth's atmosphere, showing (a) upward radiation heat flow and up/down radiation fluxes, (b) upward non-radiative heat flow ([[latent heat]] and [[thermals]]), (c) the balance between atmospheric heating and cooling at each altitude, and (d) the atmosphere's temperature profile.]]