Idealized greenhouse model

{{see also|Earth's energy budget}}

The model will find the values of T_s and T_a that will allow the outgoing radiative power, escaping the top of the atmosphere, to be equal to the absorbed radiative power of sunlight. When applied to a planet like Earth, the outgoing radiation will be longwave and the sunlight will be shortwave. These two streams of radiation will have distinct emission and absorption characteristics. In the idealized model, we assume the atmosphere is completely transparent to sunlight. The planetary albedo α_P is the fraction of the incoming solar flux that is reflected back to space (since the atmosphere is assumed totally transparent to solar radiation, it does not matter whether this albedo is imagined to be caused by reflection at the surface of the planet or at the top of the atmosphere or a mixture). The flux density of the incoming solar radiation is specified by the solar constant S₀. For application to planet Earth, appropriate values are S₀=1366 W m⁻² and α_P=0.30. Accounting for the fact that the surface area of a sphere is 4 times the area of its intercept (its shadow), the average incoming radiation is

S₀/4.

For longwave radiation, the surface of the Earth is assumed to have an emissivity of 1 (i.e. it is a black body in the infrared, which is realistic). The surface emits a radiative flux density F according to the Stefan–Boltzmann law:

: $$

F=\sigma T^4

where σ is the Stefan–Boltzmann constant. A key to understanding the greenhouse effect is Kirchhoff's law of thermal radiation. At any given wavelength the absorptivity of the atmosphere will be equal to the emissivity. Radiation from the surface could be in a slightly different portion of the infrared spectrum than the radiation emitted by the atmosphere. The model assumes that the average emissivity (absorptivity) is identical for either of these streams of infrared radiation, as they interact with the atmosphere. Thus, for longwave radiation, one symbol ε denotes both the emissivity and absorptivity of the atmosphere, for any stream of infrared radiation.

Image:IdealizedGreenhouseEmissivity78.png

Image:IdealizedGreenhouseEmissivity82.png

Image:IdealizedGreenhouseEmissivity00.png

The infrared flux density out of the top of the atmosphere is computed as:

: $$

F\uparrow =\epsilon \sigma T_a^4 + (1-\epsilon) \sigma T_s^4

In the last term, ε represents the fraction of upward longwave radiation from the surface that is absorbed, the absorptivity of the atmosphere. The remaining fraction (1-ε) is transmitted to space through an atmospheric window. In the first term on the right, ε is the emissivity of the atmosphere, the adjustment of the Stefan–Boltzmann law to account for the fact that the atmosphere is not optically thick. Thus ε plays the role of neatly blending, or averaging, the two streams of radiation in the calculation of the outward flux density.

Zero net radiation leaving the top of the atmosphere requires:

: $$

-\frac{1}{4}S_0(1-\alpha_p)+\epsilon \sigma T_a^4 + (1-\epsilon) \sigma T_s^4= 0

Zero net radiation entering the surface requires:

: $\backslash frac\{1\}\{4\}S\_0(1-\backslash alpha\_p)+\backslash epsilon\; \backslash sigma\; T\_a^4\; -\; \backslash sigma\; T\_s^4\; =\; 0$

Energy equilibrium of the atmosphere can be either derived from the two above equilibrium conditions, or independently deduced:

: $$

2 \epsilon \sigma T_a^4 - \epsilon \sigma T_s^4 = 0

Note the important factor of 2, resulting from the fact that the atmosphere radiates both upward and downward.

Thus the ratio of T_a to T_s is independent of ε:

: $T\_s\; =\; \{2^\{0.25\}T\_a\}$

= { 1.189 T_a }

Thus T_a can be expressed in terms of T_s, and a solution is obtained for

T_s in terms of the model input parameters:

: $$

\frac{1}{4}S_0(1-\alpha_p)= -\epsilon \sigma T_a^4 + \sigma T_s^4=\left( 1-\frac{\epsilon}{2} \right) \sigma T_s^4

or

: $$

T_s=\left[ \frac{S_0(1-\alpha_p)}{4\sigma} \frac{1}{1-{\epsilon \over 2}} \right]^{\frac{1}{4}}

The solution can also be expressed in terms of the effective emission temperature T_e, which is the temperature that characterizes the outgoing infrared flux density F, as if the radiator were a perfect radiator obeying F=σT_e⁴. This is easy to conceptualize in the context of the

model. T_e is also the solution for T_s, for the case of ε=0, or no atmosphere:

: $T\_e\; \backslash equiv\; \backslash left[\; \backslash frac\{\backslash frac\{1\}\{4\}\; S\_0(1-\backslash alpha\_p)\}\{\backslash sigma\}\; \backslash right]^\{\backslash frac\{1\}\{4\}\; \}$

With the definition of T_e:

: $$

T_s= T_e \left[ \frac{1}{1-{\epsilon \over 2}} \right]^{\frac{1}{4} }

For a perfect greenhouse, with no radiation escaping from the surface, or ε=1:

: $$

T_s= 2^{0.25} T_e = 1.189 T_e \qquad T_a=T_e

Using the parameters defined above to be appropriate for Earth,

: $T\_e\; =\; 255\; ~\backslash mathrm\{K\}\; =\; -18\; ~\backslash mathrm\{C\}$

For ε=1:

: $T\_s\; =\; 303\; ~\backslash mathrm\{K\}\; =\; 30\; ~\backslash mathrm\{C\}$

For ε=0.78,

: $T\_s\; =\; 288.3\; ~\backslash mathrm\{K\}\; \backslash qquad\; T\_a\; =\; 242.5\; ~\backslash mathrm\{K\}$.

This value of T_s happens to be close to the published 287.2 K of the average global "surface temperature" based on measurements.{{cite journal

|title=Surface air temperature and its changes over the past 150 years

|journal=Reviews of Geophysics

|volume=37

|issue=2

|pages=173–199

|year=1999|doi=10.1029/1999RG900002

|last1 = Jones|first1 = P. D.|last2=New

|first2=M.

|last3=Parker

|first3=D. E.

|last4=Martin

|first4=S.

|last5=Rigor

|first5=I. G.

|bibcode=1999RvGeo..37..173J

|doi-access=free

}} ε=0.78 implies 22% of the surface radiation escapes directly to space, consistent with the statement of 15% to 30% escaping in the greenhouse effect.

The radiative forcing for doubling carbon dioxide is 3.71 W m⁻², in a simple parameterization. This is also the value endorsed by the IPCC.

From the equation for $F\backslash uparrow$,

: $\backslash Delta\; F\backslash uparrow\; =\; \backslash Delta\backslash epsilon\; \backslash left(\; \backslash sigma\; T\_a^4\; -\backslash sigma\; T\_s^4\; \backslash right)$

Using the values of T_s and T_a for ε=0.78 allows for $\backslash Delta\; F\backslash uparrow$= -3.71 W m⁻² with Δε=.019. Thus a change of ε from 0.78 to 0.80 is consistent with the radiative forcing from

a doubling of carbon dioxide. For ε=0.80,

: $T\_s\; =\; 289.5\; ~\backslash mathrm\{K\}$

Thus this model predicts a global warming of ΔT_s = 1.2 K for a doubling of carbon dioxide. A typical prediction from a GCM is 3 K surface warming, primarily because the GCM allows for positive feedback, notably from increased water vapor. A simple surrogate for including this feedback process is to posit an additional increase of Δε=.02, for a total Δε=.04, to approximate the effect of the increase in water vapor that would be associated with an increase in temperature.{{cite journal

|title=Attribution of the present-day total greenhouse effect

|journal=J. Geophys. Res.

|volume=115 |issue=D20106 |year=2010

|last1=Schmidt |first1=Gavin A.

|last2=Ruedy |first2=Reto A.

|last3=Miller |first3=Ron L.

|last4=Lacis |first4=Andy A.

|doi=10.1029/2010JD014287 |doi-access=free

}} This idealized model then predicts a global warming of ΔT_s = 2.4 K for a doubling of carbon dioxide, roughly consistent with the IPCC.

The one-level atmospheric model can be readily extended to a multiple-layer atmosphere.{{cite web |url=https://www.acs.org/content/acs/en/climatescience/atmosphericwarming/multilayermodel.html |title=ACS Climate Science Toolkit - Atmospheric Warming - A Multi-Layer Atmosphere Model |publisher=American Chemical Society |accessdate=2 October 2022}}{{cite web |title=Energy Recirculation and a Layered Radiative Atmosphere Model |url=https://climatepuzzles.org/2021/04/recirculation-layered-radiative-atmosphere/ |website=Climate Puzzles |date=17 April 2021 |access-date=1 June 2023}} In this case the equations for the temperatures become a series of coupled equations. These simple energy-balance models always predict a decreasing temperature away from the surface, and all levels increase in temperature as "greenhouse gases are added". Neither of these effects are fully realistic: in the real atmosphere temperatures increase above the tropopause, and temperatures in that layer are predicted (and observed) to decrease as GHG's are added.{{cite journal |last1=Manabe |first1=Syukuro |author1-link=Syukuro Manabe |last2=Wetherald |first2=Richard T. |title=Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity |journal=Journal of the Atmospheric Sciences |volume=24 |issue=3 |pages=241–259 |date=1 May 1967 |bibcode=1967JAtS...24..241M |doi=10.1175/1520-0469(1967)024<0241:TEOTAW>2.0.CO;2 |doi-access=free}} This is directly related to the non-greyness of the real atmosphere.

An interactive version of a model with 2 atmospheric layers, and which accounts for convection, is available online.{{cite web |title=2-Layer Atmosphere with Solar, Longwave, & Convection |url=https://biocycle.atmos.colostate.edu/shiny/2layer/ |publisher=Denning Research Group, Colorado State University |access-date=1 June 2023}}

• Atmospheric model
• Climate model
• Planetary equilibrium temperature

{{reflist}}

• {{cite book |author1=Bohren, Craig F. |author2=Clothiaux, Eugene E. |title=Fundamentals of Atmospheric Radiation |publisher=John Wiley & Sons |location=Chichester |year=2006 |pages=31–41 |isbn=978-3-527-40503-9 |chapter=1.6 Emissivity and Global Warming}}
• {{cite book |author=Petty, Grant W. |chapter=6.4.3 Simple Radiative Models of the Atmosphere |title=A First Course in Atmospheric Radiation |publisher=Sundog Pub |location=Madison, Wisconsin |year=2006 |pages=139–143 |edition=2nd |isbn=978-0-9729033-1-8 }}

{{Wikiversity|Climate change}}

{{Wikibooks|Climate Change}}

• {{Commonscatinline|Idealized greenhouse model}}
• [https://climateblab.github.io/ideal-greenhouse/ Computing wikipedia's idealized greenhouse model]

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