Editing Greenhouse effect (section)

=== Metrics for the greenhouse effect ===
[[File:Greenhouse Effect metrics time series.svg|thumb|upright=1.35|Increase in the Earth's greenhouse effect (2000–2022) based on NASA CERES satellite data.]]

The IPCC reports the ''greenhouse effect'', {{mvar|G}}, as being 159 W m{{sup|-2}}, where {{mvar|G}} is the flux of longwave thermal radiation that leaves the surface minus the flux of outgoing longwave radiation that reaches space:<ref name="ipcc-ar6wg1-ch7" />{{rp|968}}<ref name="ravram1" /><ref name="Schmidt2010" /><ref name="Schmidt2010paper" />

:<math>G = \mathrm{SLR} - \mathrm{OLR}\;.</math>

Alternatively, the greenhouse effect can be described using the ''normalized greenhouse effect'', {{mvar|g̃}}, defined as

:<math>\tilde g = G/\mathrm{SLR} = 1 - \mathrm{OLR}/\mathrm{SLR}\;.</math>

The normalized greenhouse effect is ''the fraction of the amount of thermal radiation emitted by the surface that does not reach space''.
Based on the IPCC numbers, {{mvar|g̃}} = 0.40. In other words, 40 percent less thermal radiation reaches space than what leaves the surface.<ref name="ipcc-ar6wg1-ch7">{{cite book |title=Climate Change 2021: The Physical Science Basis |date=2021 |publisher=IPCC |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf |access-date=24 April 2023 |chapter=Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity}}</ref>{{rp|968}}<ref name="ravram1">{{cite journal |last1=Raval |first1=A. |last2=Ramanathan |first2=V. |title=Observational determination of the greenhouse effect |journal=Nature |date=1989 |volume=342 |issue=6251 |pages=758–761 |doi=10.1038/342758a0 |bibcode=1989Natur.342..758R |s2cid=4326910 |url=https://www.nature.com/articles/342758a0}}</ref><ref name="ravram2">{{cite journal |last1=Raval |first1=A. |last2=Ramanathan |first2=V. |title=Observational determination of the greenhouse effect |journal=Global Climate Feedbacks: Proceedings of the Brookhaven National Laboratory Workshop |date=1990 |pages=5–16 |url=https://www.osti.gov/servlets/purl/6440147-fP92Pd/#page=10 |access-date=24 April 2023}}</ref>

Sometimes the greenhouse effect is quantified as a temperature difference. This temperature difference is closely related to the quantities above.

When the greenhouse effect is expressed as a temperature difference, <math>\Delta T_\mathrm{GHE}</math>, this refers to the effective temperature associated with thermal radiation emissions from the surface minus the effective temperature associated with emissions to space:

:<math>\Delta T_\mathrm{GHE} = T_\mathrm{surface,eff} - T_\mathrm{eff}</math>
:<math>\Delta T_\mathrm{GHE} = \left(\mathrm{SLR}/\sigma\right)^{1/4} - \left(\mathrm{OLR}/\sigma\right)^{1/4}</math>

Informal discussions of the greenhouse effect often compare the actual surface temperature to the temperature that the planet would have if there were no greenhouse gases. However, in formal technical discussions, when the size of the greenhouse effect is quantified as a temperature, this is generally done using the above formula. The formula refers to the effective surface temperature rather than the actual surface temperature, and compares the surface with the top of the atmosphere, rather than comparing reality to a hypothetical situation.<ref name="Haberle2013">{{cite journal |last1=Haberle |first1=Robert M. |title=Estimating the power of Mars' greenhouse effect |journal=Icarus |date=2013 |volume=223 |issue=1 |pages=619–620 |doi=10.1016/j.icarus.2012.12.022|bibcode=2013Icar..223..619H }}</ref>

The temperature difference, <math>\Delta T_\mathrm{GHE}</math>, indicates how much warmer a planet's surface is than the planet's overall effective temperature.