Editing Radiative forcing (section)

{{short description|Concept for changes to the energy flows through a planetary atmosphere}}
[[File:Physical Drivers of climate change.svg|thumb|300px|The assessment of radiative forcing and [[climate sensitivity]] shows which physical parameters are contributing to temperature changes.<ref name=":1">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32, doi:10.1017/9781009157896.001.</ref>{{rp|7}} Parameters shown with orange bars lead to a temperature increase (due to positive radiative forcings), whereas parameters shown with blue bars lead to a temperature decrease (due to negative radiative forcing). |alt=Warming contributions of various GHGs, agents, factors [name the year that the contributions pertain to] [*correct reference given under the 'Talk' tab*]. Plus, the figure is inaccurate; at least wrt. to methane.]]'''Radiative forcing''' (or '''climate forcing'''<ref>{{cite web|url=https://earthobservatory.nasa.gov/Features/EnergyBalance/page7.php|title=Climate and Earth's Energy Budget: Feature Articles|first=Lindsey|last=Rebecca|date=14 January 2009|website=earthobservatory.nasa.gov|access-date=3 April 2018|archive-date=10 April 2020|archive-url=https://web.archive.org/web/20200410141354/https://earthobservatory.nasa.gov/features/EnergyBalance/page7.php|url-status=live}}</ref>) is a concept used to quantify a change to the [[Earth's energy budget|balance of energy]] flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of [[Greenhouse gas|greenhouse gases]] and [[Aerosol|aerosols]], and changes in [[albedo|surface albedo]] and [[solar irradiance]]. In more technical terms, it is defined as "the change in the net, downward minus upward, [[radiative flux]] (expressed in W/m<sup>2</sup>) due to a change in an external driver of climate change."<ref name=":0" />{{rp|2245}} These external drivers are distinguished from [[climate change feedback|feedbacks]] and [[Climate variability and change|variability]] that are internal to the [[climate system]], and that further influence the direction and magnitude of [[Earth's energy imbalance|imbalance]]. Radiative forcing on Earth is meaningfully evaluated at the [[tropopause]] and at the top of the [[stratosphere]]. It is quantified in units of [[watt]]s per square [[meter]], and often summarized as an average over the total surface area of the globe.

A planet in radiative equilibrium with its parent star and the rest of space can be characterized by net zero radiative forcing and by a [[planetary equilibrium temperature]].<ref>{{Cite book|title=Fundamental planetary science: physics, chemistry, and habitability|last1=Lissauer |first1=Jack Jonathan|last2=De Pater |first2=Imke |isbn=9780521853309|location=New York City|oclc=808009225|date = 16 September 2013}}</ref>

Radiative forcing is not ''a thing'' in the sense that a single instrument can independently measure it. Rather it is a scientific concept and entity whose strength can be estimated from more fundamental [[Outline of physics#General concepts|physics principles]]. Scientists use measurements of changes in atmospheric parameters to calculate the radiative forcing.<ref name="nrcrf">{{cite book |title=Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties |author=National Research Council |publisher=The National Academic Press |year=2005 |doi=10.17226/11175|isbn=978-0-309-09506-8 }}</ref>{{rp|1–4}}

The IPCC summarized the current [[Scientific consensus on climate change|scientific consensus about radiative forcing]] changes as follows: "Human-caused radiative forcing of 2.72 W/m<sup>2</sup> in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations".<ref name=":1" />{{rp|11}}

The atmospheric burden of greenhouse gases due to human activity has grown especially rapidly during the last several decades (since about year 1950). For [[carbon dioxide]], the 50% increase (''C/C<sub>0</sub>'' = 1.5) realized as of year 2020 since 1750 corresponds to a cumulative radiative forcing change (ΔF) of +2.17 W/m<sup>2</sup>.<ref name="noaa aggi" />  Assuming no change in the emissions growth path, a doubling of concentrations (''C/C<sub>0</sub>'' = 2) within the next several decades would correspond to a cumulative radiative forcing change (ΔF) of +3.71 W/m<sup>2</sup>.

Radiative forcing can be a useful way to compare the growing warming influence of different anthropogenic greenhouse gases over time. The radiative forcing of long-lived and well-mixed greenhouse gases have been increasing in earth's atmosphere since the industrial revolution.<ref name="noaa aggi" /> Carbon dioxide has the biggest impact on total forcing, while [[methane]] and [[chlorofluorocarbons]] (CFCs) play smaller roles as time goes on.<ref name="noaa aggi" /> The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor [[halogenated]] gases.
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