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| In [[climate]] science, '''radiative forcing''' is defined as the difference of [[irradiance|radiant energy]] received by the earth and energy radiated back to space. Typically, radiative forcing is quantified at the [[tropopause]] in units of [[watt]]s per square [[meter]] of earth's surface. A positive forcing (more incoming energy) warms the system, while negative forcing (more outgoing energy) cools it. Causes of radiative forcing include changes in [[insolation]] (incident solar radiation) and in [[concentrations]] of [[greenhouse gas|radiatively active gases]] and [[aerosol]]s.
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| == Radiation balance ==
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| Almost all of the energy which affects Earth's weather is received as radiant energy from the [[Sun]]. The planet and its atmosphere absorb and reflect some of the energy, while [[Outgoing longwave radiation|long-wave energy is radiated back]] into space. The balance between absorbed and radiated energy determines the average temperature. Because the [[atmosphere]] absorbs some of the re-radiated long-wave energy, the [[planet]] is warmer than it would be in the absence of the [[atmosphere]]: see [[greenhouse effect]].
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| The radiation balance is altered by such factors as the intensity of [[solar energy]], reflectivity of clouds or gases, absorption by various [[greenhouse gas]]es or surfaces, emission of heat by various materials. Any such alteration is a radiative forcing, and causes a new balance to be reached. This happens continuously as sunlight hits the surface, clouds and aerosols form, the concentrations of atmospheric gases vary, and seasons alter the ground cover.
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| == IPCC usage ==
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| [[File:Radiative-forcings.svg|thumb|right|250px|2005 radiative forcings as estimated by the IPCC.]]
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| The term "radiative forcing" has been used in the [[IPCC]] Assessments with a specific technical meaning, to denote an externally imposed perturbation in the radiative energy budget of Earth’s climate system, which may lead to changes in climate parameters.<ref>{{cite web|url= http://www.grida.no/climate/ipcc_tar/wg1/212.htm|title= Radiative Forcing of Climate Change}}</ref> The exact definition used is:
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| :The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm<sup>-2</sup>) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.<ref>http://www.grida.no/climate/ipcc_tar/wg1/214.htm#611</ref>
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| In a subsequent report,<ref>http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf</ref> the IPCC defines it as:
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| <blockquote>
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| "Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in Watts per square meter (W/m<sup>2</sup>)."
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| </blockquote>
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| In simple terms, radiative forcing is "...the rate of energy change per unit area of the globe as measured at the top of the atmosphere."<ref name="Rockstrom09">{{cite journal | last1 = Rockström | first1 = Johan |authorlink1= Johan Rockström | last2= Steffen | first2= Will | last3 = Noone | first3 = Kevin | last4 = Persson | first4 = Asa | last5 = Chapin | first5 = F. Stuart | last6= Lambin | first6= Eric F. | last7 = et al. | title = A safe operating space for humanity | journal = [[Nature (journal)|Nature]] | year = 2009 | volume = 461 | pages = 472–475 | pmid = 19779433 | first7 = TM | last8 = Scheffer | first8 = M | last9 = Folke | first9 = C | last10 = Schellnhuber | first10 = Hans Joachim | last11 = Nykvist | first11 = Björn | last12 = De Wit | first12 = Cynthia A. | last13 = Hughes | first13 = Terry | last14 = Van Der Leeuw | first14 = Sander | last15 = Rodhe | first15 = Henning | last16 = Sörlin | first16 = Sverker | last17 = Snyder | first17 = Peter K. | last18 = Costanza | first18 = Robert | last19 = Svedin | first19 = Uno | last20 = Falkenmark | first20 = Malin | last21 = Karlberg | first21 = Louise | last22 = Corell | first22 = Robert W. | last23 = Fabry | first23 = Victoria J. | last24 = Hansen | first24 = James | last25 = Walker | first25 = Brian | last26 = Liverman | first26 = Diana | last27 = Richardson | first27 = Katherine | last28 = Crutzen | first28 = Paul | last29 = Foley | first29 = Jonathan A. | issue = 7263 | doi = 10.1038/461472a|bibcode = 2009Natur.461..472R | display-authors = 8 }}</ref> In the context of [[climate change]], the term "forcing" is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, no surface and tropospheric feedbacks in operation (''i.e.'', no secondary effects induced because of changes in tropospheric motions or its [[thermodynamic state]]), and no dynamically induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms).
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| ==Climate sensitivity==
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| {{main|Climate sensitivity}}
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| Radiative forcing can be used to estimate a subsequent change in equilibrium surface temperature (Δ''T''<sub>s</sub>) arising from that radiative forcing via the equation:
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| : <math>\Delta T_s =~ \lambda~\Delta F</math>
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| where λ is the climate sensitivity, usually with units in K/(W/m<sup>2</sup>), and Δ''F'' is the radiative forcing.<ref>http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/222.htm</ref> A typical value of λ is 0.8 K/(W/m<sup>2</sup>), which gives a warming of 3K for doubling of CO<sub>2</sub>.
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| == Example calculations ==
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| [[File:ModtranRadiativeForcingDoubleCO2.png|thumb|left|250px|Radiative forcing for doubling CO<sub>2</sub>, as calculated by radiative transfer code Modtran. Red lines are [[Planck's law|Planck curves]].]]
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| [[File:ModtranRadiativeForcing8xCH4.png|thumb|left|250px|Radiative forcing for eight times increase of CH<sub>4</sub>, as calculated by radiative transfer code Modtran.]]
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| ===Solar forcing===
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| Radiative forcing (measured in Watts per square meter) can be estimated in different ways for different components. For the case of a change in solar irradiance (''i.e.,'' "solar forcing"), the radiative forcing is simply the change in the average amount of solar energy absorbed per square meter of the Earth's area. Since the cross-sectional area of the [[Earth]] exposed to the Sun (πr<sup>2</sup>) is equal to 1/4 of the surface area of the Earth (4πr<sup>2</sup>), the solar input per unit area is one quarter the change in solar intensity. This must be multiplied by the fraction of incident sunlight that is absorbed, F=(1-R), where R is the reflectivity, or [[albedo]], of the Earth. The albedo of the Earth is approximately 0.3, so F is approximately equal to 0.7. Thus, the solar forcing is the change in the solar intensity divided by 4 and multiplied by 0.7.
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| Likewise, a change in albedo will produce a solar forcing equal to the change in albedo divided by 4 multiplied by the [[solar constant]].
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| ===Forcing due to atmospheric gas===
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| For a greenhouse gas, such as [[carbon dioxide]], radiative transfer codes that examine each spectral line for atmospheric conditions can be used to calculate the change ΔF as a function of changing concentration. These calculations can often be simplified into an algebraic formulation that is specific to that gas.
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| For instance, the simplified first-order approximation expression for [[carbon dioxide]] is:
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| : <math>\Delta F = 5.35 \times \ln {C \over C_0}~\mathrm{W}~\mathrm{m}^{-2} \, </math>
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| where ''C'' is the CO<sub>2</sub> concentration in parts per million by volume and ''C''<sub>0</sub> is the reference concentration.<ref>Myhre et al., [http://www.agu.org/pubs/crossref/1998/98GL01908.shtml New estimates of radiative forcing due to well mixed greenhouse gases], [[Geophysical Research Letters]], Vol 25, No. 14, pp 2715–2718, 1998</ref> The relationship between carbon dioxide and radiative forcing is [[logarithmic scale|logarithmic]], and thus increased concentrations have a progressively smaller warming effect.
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| A different formula applies for some other greenhouse gases such as [[methane]] and [[nitrous oxide|N<sub>2</sub>O]] (square-root dependence) or CFCs (linear), with coefficients that can be found ''e.g.'' in the [[Intergovernmental Panel on Climate Change|IPCC]] reports.<ref>[http://www.grida.no/climate/ipcc_tar/wg1/222.htm IPCC WG-1] report</ref>
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| == Related measures ==
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| Radiative forcing is intended as a useful way to compare different causes of perturbations in a climate system. Other possible tools can be constructed for the same purpose: for example Shine ''et al.''<ref>Shine et al., [http://www.agu.org/pubs/crossref/2003/2003GL018141.shtml An alternative to radiative forcing for estimating the relative importance of climate change mechanisms], Geophysical Research Letters, Vol 30, No. 20, 2047, {{doi|10.1029/2003GL018141}}, 2003</ref> say "...recent experiments indicate that for changes in absorbing aerosols and ozone, the predictive ability of radiative forcing is much worse... we propose an alternative, the 'adjusted troposphere and stratosphere forcing'. We present [[global circulation model|GCM]] calculations showing that it is a significantly more reliable predictor of this GCM's surface temperature change than radiative forcing. It is a candidate to supplement radiative forcing as a metric for comparing different mechanisms...". In this quote, GCM stands for "[[global circulation model]]", and the word "predictive" does not refer to the ability of GCMs to forecast climate change. Instead, it refers to the ability of the alternative tool proposed by the authors to help explain the system response.
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| ==Changes in radiative forcing==
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| The table below shows changes in radiative forcing between 1979 and 2012.<ref name="noaa aggi">
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| {{Include-USGov
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| | agency=NOAA
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| | source={{Cite document| title=THE NOAA ANNUAL GREENHOUSE GAS INDEX (AGGI)
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| | publisher= NOAA/ESRL Global Monitoring Division
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| | url=http://www.esrl.noaa.gov/gmd/aggi/aggi.html
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| | author=Butler, J.H. and S.A. Montzka
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| | date=1 August 2013
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| | postscript= <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}
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| }}
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| }}
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| </ref> The table includes the contribution to radiative forcing from [[carbon dioxide]] ({{CO2}}), [[methane]] ({{chem|CH|4}}), [[nitrous oxide]] ({{chem|N|2|O}}); [[chlorofluorocarbon]]s (CFCs) [[CFC-12|12]] and [[CFC-11|11]]; and fifteen other minor, long-lived, [[halogen]]ated gases.<ref>
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| [[CFC-113]], [[tetrachloromethane]] ({{chem|CCl|4}}), [[trichloromethane]] ({{chem|CH|3|CCl|3}}); hydrochlorofluorocarbons (HCFCs) [[HCFC-22|22]], [[HCFC-141b|141b]] and [[HCFC-142b|142b]]; [[hydrofluorocarbon]]s (HFCs) [[HFC-134a|134a]], [[HFC-152a|152a]], [[HFC-23|23]], [[HFC-143a|143a]], and [[HFC-125|125]]; [[sulfur hexafluoride]] ({{chem|SF|6}}), and halons [[halon 1211|1211]], [[halon 1301|1301]] and [[halon 2402|2402]])
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| </ref> The table includes the contribution to radiative forcing of long-lived greenhouse gases. It does not include other forcings, such as aerosols and changes in solar activity.
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| [[File:Changes in radiative forcing of long-lived greenhouse gases between 1979-2012.png|thumbnail|right|400px|Changes in radiative forcing of long-lived greenhouse gases between 1979 and 2012.]]
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| [[File:Carbon Dioxide radiative forcing.png|thumbnail|right|Radiative forcing, relative to 1750, due to carbon dioxide alone since 1979. The percent change from January 1, 1990 is shown on the right axis.]]
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| {| class="wikitable"
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| |+ Global radiative forcing, [[carbon dioxide equivalent|{{CO2}}-equivalent]] mixing ratio, and the Annual Greenhouse Gas Index (AGGI) between 1979-2012<ref name="noaa aggi"/>
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| {| summary=Radiative forcing due to long-lived greenhouse gases has increased substantially between 1979 and 2012
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| ! Year !! {{CO2}} !! {{chem|CH|4}} !! {{chem|N|2|O}} !! CFC-12 !! CFC-11 !! 15-minor !! Total !! {{CO2}}-eq<br/>ppm !! AGGI<br/>1990 = 1 !! AGGI<br/>% change
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| | 1979 || 1.027 || 0.419 || 0.104 || 0.092 || 0.039 || 0.031 || 1.712 || 383 || 0.786 ||
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| | 1980 || 1.058 || 0.426 || 0.104 || 0.097 || 0.042 || 0.034 || 1.761 || 386 || 0.808 || 2.8
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| | 1981 || 1.077 || 0.433 || 0.107 || 0.102 || 0.044 || 0.036 || 1.799 || 389 || 0.826 || 2.2
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| | 1982 || 1.089 || 0.440 || 0.111 || 0.108 || 0.046 || 0.038 || 1.831 || 391 || 0.841 || 1.8
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| | 1983 || 1.115 || 0.443 || 0.113 || 0.113 || 0.048 || 0.041 || 1.873 || 395 || 0.860 || 2.2
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| | 1984 || 1.140 || 0.446 || 0.116 || 0.118 || 0.050 || 0.044 || 1.913 || 397 || 0.878 || 2.2
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| | 1985 || 1.162 || 0.451 || 0.118 || 0.123 || 0.053 || 0.047 || 1.953 || 401 || 0.897 || 2.1
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| | 1986 || 1.184 || 0.456 || 0.122 || 0.129 || 0.056 || 0.049 || 1.996 || 404 || 0.916 || 2.2
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| | 1987 || 1.211 || 0.460 || 0.120 || 0.135 || 0.059 || 0.053 || 2.039 || 407 || 0.936 || 2.2
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| | 1988 || 1.250 || 0.464 || 0.123 || 0.143 || 0.062 || 0.057 || 2.099 || 412 || 0.964 || 3.0
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| | 1989 || 1.274 || 0.468 || 0.126 || 0.149 || 0.064 || 0.061 || 2.144 || 415 || 0.984 || 2.1
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| | 1990 || 1.293 || 0.472 || 0.129 || 0.154 || 0.065 || 0.065 || 2.178 || 418 || 1.000 || 1.6
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| | 1991 || 1.313 || 0.476 || 0.131 || 0.158 || 0.067 || 0.069 || 2.213 || 420 || 1.016 || 1.6
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| | 1992 || 1.324 || 0.480 || 0.133 || 0.162 || 0.067 || 0.072 || 2.238 || 422 || 1.027 || 1.1
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| | 1993 || 1.334 || 0.481 || 0.134 || 0.164 || 0.068 || 0.074 || 2.254 || 424 || 1.035 || 0.7
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| | 1994 || 1.356 || 0.483 || 0.134 || 0.166 || 0.068 || 0.075 || 2.282 || 426 || 1.048 || 1.3
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| | 1995 || 1.383 || 0.485 || 0.136 || 0.168 || 0.067 || 0.077 || 2.317 || 429 || 1.064 || 1.5
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| | 1996 || 1.410 || 0.486 || 0.139 || 0.169 || 0.067 || 0.078 || 2.350 || 431 || 1.079 || 1.4
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| | 1997 || 1.426 || 0.487 || 0.142 || 0.171 || 0.067 || 0.079 || 2.372 || 433 || 1.089 || 0.9
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| | 1998 || 1.465 || 0.491 || 0.145 || 0.172 || 0.067 || 0.080 || 2.419 || 437 || 1.111 || 2.0
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| | 1999 || 1.495 || 0.494 || 0.148 || 0.173 || 0.066 || 0.082 || 2.458 || 440 || 1.128 || 1.6
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| | 2000 || 1.513 || 0.494 || 0.151 || 0.173 || 0.066 || 0.083 || 2.481 || 442 || 1.139 || 0.9
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| | 2001 || 1.535 || 0.494 || 0.153 || 0.174 || 0.065 || 0.085 || 2.506 || 444 || 1.150 || 1.0
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| | 2002 || 1.564 || 0.494 || 0.156 || 0.174 || 0.065 || 0.087 || 2.539 || 447 || 1.166 || 1.3
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| | 2003 || 1.601 || 0.496 || 0.158 || 0.174 || 0.064 || 0.088 || 2.580 || 450 || 1.185 || 1.6
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| | 2004 || 1.627 || 0.496 || 0.160 || 0.174 || 0.063 || 0.090 || 2.610 || 453 || 1.198 || 1.1
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| | 2005 || 1.655 || 0.495 || 0.162 || 0.173 || 0.063 || 0.092 || 2.640 || 455 || 1.212 || 1.2
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| | 2006 || 1.685 || 0.495 || 0.165 || 0.173 || 0.062 || 0.095 || 2.675 || 458 || 1.228 || 1.3
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| | 2007 || 1.710 || 0.498 || 0.167 || 0.172 || 0.062 || 0.097 || 2.706 || 461 || 1.242 || 1.1
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| | 2008 || 1.739 || 0.500 || 0.170 || 0.171 || 0.061 || 0.100 || 2.742 || 464 || 1.259 || 1.3
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| | 2009 || 1.760 || 0.502 || 0.172 || 0.171 || 0.061 || 0.103 || 2.768 || 466 || 1.271 || 1.0
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| | 2010 || 1.791 || 0.504 || 0.174 || 0.170 || 0.060 || 0.106 || 2.805 || 470 || 1.288 || 1.3
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| | 2011 || 1.818 || 0.505 || 0.178 || 0.169 || 0.060 || 0.109 || 2.838 || 473 || 1.303 || 1.2
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| | 2012 || 1.846 || 0.507 || 0.181 || 0.168 || 0.059 || 0.111 || 2.873 || 476 || 1.319 || 1.2
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| |}
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| The table shows that {{CO2}} dominates the total forcing, with methane and the CFCs becoming relatively smaller contributors to the total forcing over time.<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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| The table also includes an "Annual Greenhouse Gas Index" (AGGI), which is defined as the ratio of the total direct radiative forcing due to long-lived greenhouse gases for any year for which adequate global measurements exist to that which was present in 1990.<ref name="noaa aggi"/> 1990 was chosen because it is the baseline year for the [[Kyoto Protocol]]. This index is a measure of the inter-annual changes in conditions that affect carbon dioxide emission and uptake, methane and nitrous oxide sources and sinks, the decline in the atmospheric abundance of [[ozone depletion|ozone-depleting]] chemicals related to the [[Montreal Protocol]]. and the increase in their substitutes (HCFCs and HFCs). Most of this increase is related to {{CO2}}. For 2012, the AGGI was 1.32 (representing an increase in total direct radiative forcing of 32% since 1990). The increase in {{CO2}} forcing alone since 1990 was about 41%. The decline in the CFCs has tempered the increase in net radiative forcing considerably.
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| == See also ==
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| {{portal|energy|global warming}}
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| * [[Climate sensitivity]]
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| * [[Anthropogenic heat]]
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| * [[Emission standard]]
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| * [[Global warming potential]]
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| * [[Sulfate]]
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| == References ==
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| {{reflist}}
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| * IPCC glossary http://www.ipcc.ch/pdf/glossary/ar4-wg1.pdf
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| == External links ==
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| * [http://www.giss.nasa.gov/research/briefs/lacis_01/ CO2: The Thermostat that Controls Earth's Temperature] by [[NASA]]'s [[Goddard Institute for Space Studies]], October, 2010, Forcing vs. Feedbacks
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| * [[Intergovernmental Panel on Climate Change]]’s [[IPCC Fourth Assessment Report|Fourth Assessment Report]] (2007), Chapter 2, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf "Changes in Atmospheric Constituents and Radiative Forcing,"] pp. 133–134 (PDF, 8.6 MB, 106 pp.).
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| * [[Environmental Protection Agency|U.S. EPA]] (2009), [http://www.epa.gov/climatechange/science/recentac.html Climate Change – Science]. Explanation of climate change topics including radiative forcing.
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| * [[United States National Research Council]] (2005), ''[http://www.nap.edu/openbook/0309095069/html/ Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties]'', Board on Atmospheric Sciences and Climate
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| *[http://www.sciencenews.org/view/generic/id/332612/title/Small_volcanoes_add_up_to_cooler_climate Small volcanoes add up to cooler climate; Airborne particles help explain why temperatures rose less last decade] August 13, 2011; Vol.180 #4 (p. 5) ''[[Science News]]''
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| *[http://www.sciencenews.org/view/generic/id/332152/title/Sulfur_stalls_surface_temperature_rise_ Sulfur stalls surface temperature rise; Findings explain decade without warming] July 30, 2011; Vol.180 #3 (p. 17) ''Science News''
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| {{Global warming}}
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| {{DEFAULTSORT:Radiative Forcing (Calculation and Measurement)}}
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| [[Category:Climate forcing]]
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| [[Category:Atmospheric radiation]]
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