Editing Greenhouse effect (section)

===Radiative balance===
{{Further|Earth's energy budget}}
[[File:Earth Energy Budget with GHE.svg|thumb|upright=1.35|The greenhouse effect can be understood as a decrease in the efficiency of planetary cooling. The greenhouse effect is quantified as the portion of the radiation flux emitted by the surface minus that doesn't reach space, i.e., 40% or 159 W/m<sup>2</sup>. Some emitted radiation is effectively cancelled out by downwelling radiation and so does not [[Heat transfer#Radiation|transfer heat]]. Evaporation and convection partially compensate for this reduction in surface cooling. Low temperatures at high altitudes limit the rate of thermal emissions to space.]]
Earth's top-of-atmosphere (TOA) [[Earth's energy budget|energy imbalance]] (EEI) is the amount by which the power of incoming radiation exceeds the power of outgoing radiation:<ref name="UNeei">{{cite web |title=The Earth's Energy Imbalance: Where does the energy go? |url=https://unfccc.int/sites/default/files/resource/EID%20Pres%20T1%20Mercator%20EEI.pdf |publisher=United Nations Climate Change |access-date=14 June 2023}}</ref>

:<math>\mathrm{EEI} = \mathrm{ASR} -\mathrm{OLR}</math>

where ASR is the mean flux of absorbed solar radiation. ASR may be expanded as

:<math>\mathrm{ASR} = (1-A) \,\mathrm{MSI}</math>

where <math>A</math> is the [[albedo]] (reflectivity) of the planet and MSI is the [[solar irradiance|mean solar irradiance]] incoming at the top of the atmosphere.

The [[Planetary equilibrium temperature|radiative equilibrium temperature]] of a planet can be expressed as

:<math>T_\mathrm{radeq} = (\mathrm{ASR}/\sigma)^{1/4} = \left[(1-A)\,\mathrm{MSI}/\sigma  \right]^{1/4} \;.</math>
 
A planet's temperature will tend to shift towards a state of radiative equilibrium, in which the TOA energy imbalance is zero, i.e., <math>\mathrm{EEI} = 0</math>. When the planet is in radiative equilibrium, the overall effective temperature of the planet is given by

:<math>T_\mathrm{eff} = T_\mathrm{radeq}\;.</math>

Thus, the concept of radiative equilibrium is important because it indicates what effective temperature a planet will tend towards having.<ref name="ACSPredPlanTemp">{{cite web |title=Predicted Planetary Temperatures |url=https://www.acs.org/climatescience/energybalance/predictedplanetarytemperatures.html |website=ACS Climate Science Toolkit |publisher=American Chemical Society |archive-url=https://web.archive.org/web/20230326030348/https://www.acs.org/climatescience/energybalance/planetarytemperatures.html| archive-date=26 March 2023|access-date=14 June 2023}}</ref><ref name="rrtmeeb" />

If, in addition to knowing the effective temperature, <math>T_\mathrm{eff}</math>, we know the value of the greenhouse effect, then we know the mean (average) surface temperature of the planet.

This is why the quantity known as the greenhouse effect is important: it is one of the few quantities that go into determining the planet's mean surface temperature.