Editing Greenhouse effect (section)

==Measurement==
[[File:Climate Factor CO₂ (ZDF, Terra X).webm|thumb|How {{CO2}} causes the greenhouse effect.]]
Matter emits [[thermal radiation]] at a rate that [[Stefan-Boltzmann law|is directly proportional to the fourth power of its temperature]]. Some of the radiation emitted by the Earth's surface is absorbed by greenhouse gases and clouds. Without this absorption, Earth's surface would have an average temperature of {{convert|−18|C|F|sigfig=1}}. However, because some of the radiation is absorbed, Earth's average surface temperature is around {{convert|15|C|F}}. Thus, the Earth's greenhouse effect may be measured as a ''temperature change'' of {{convert|33|C-change}}.

Thermal radiation is characterized by how much energy it carries, typically in watts per square meter (W/m{{sup|2}}). Scientists also measure the greenhouse effect based on how much more longwave thermal radiation leaves the Earth's surface than reaches space.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ipcc-ar6wg1-ch7"/>{{rp|934}}<ref name="ravram1"/><ref name="Schmidt2010paper"/><ref name="Schmidt2010">{{cite web |author=Gavin Schmidt |date=1 October 2010 |title=Taking the Measure of the Greenhouse Effect |url=https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/ |url-status=live |archive-url=https://web.archive.org/web/20210421003101/https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/ |archive-date=21 April 2021 |access-date=13 January 2022 |publisher=NASA Goddard Institute for Space Studies - Science Briefs}}</ref> Currently, longwave radiation leaves the surface at an average rate of 398 W/m{{sup|2}}, but only 239 W/m{{sup|2}} reaches space. Thus, the Earth's greenhouse effect can also be measured as an ''energy flow change'' of 159 W/m{{sup|2}}.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ipcc-ar6wg1-ch7"/>{{rp|934}} The greenhouse effect can be expressed as a fraction (0.40) or percentage (40%) of the longwave thermal radiation that leaves Earth's surface but does not reach space.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ravram1"/><ref name="ravram2"/>

Whether the greenhouse effect is expressed as a change in temperature or as a change in longwave thermal radiation, the same effect is being measured.<ref name="ravram1"/>