<p>Greenhouse gas molecules absorb radiation in a wavelength where most of the radiation comes from below (namely, the earth surface), but re-emit in a random direction. As a consequence, a photon that would have gone out to space is re-emitted in random directions and so has a probability of 1/2 to get back to Earth. In other words, it <em>reduces</em> the outbound energy flux and increases the inbound energy flux, compared to not having (or having less) greenhouse gases around.</p>

CC BY-SA 4.0
2024-10-25T19:48:36.377Z
2024-10-25T19:48:36.377Z
Wolfgang Bangerth
<p>I am reading chapter 8 of the IPCC AR5 [Myhre et al. 2013] and am confused about the instantaneous radiative forcing [IRF].</p>
<p>From my understanding, the idea is that a greenhouse gas is introduced into the atmosphere, and its impact on the radiative flux at the tropopause is found via two radiative transfer calculations.</p>
<p>I understand the idea of radiative forcing = change in radiative flux, but how exactly does the greenhouse gas achieve this? I think each greenhouse gas molecule can absorb and emit radiation at given wavenumbers. So at a given altitude at a given atmospheric temperature [which is fixed since this is the IRF] the molecules should absorb and emit the same amount[*] hence there should be no change to the radiative flux at that altitude. Hence no IRF.</p>
<p>[*] I may be wrong about this point.</p>

<climate-change><radiative-transfer>
How does the instantaneous radiative forcing occur?
VinalV
# How does the instantaneous radiative forcing occur?

(Asked by: username_0 on 2024-10-25)

I am reading chapter 8 of the IPCC AR5 [Myhre et al. 2013] and am confused about the instantaneous radiative forcing [IRF].

From my understanding, the idea is that a greenhouse gas is introduced into the atmosphere, and its impact on the radiative flux at the tropopause is found via two radiative transfer calculations.

I understand the idea of radiative forcing = change in radiative flux, but how exactly does the greenhouse gas achieve this? I think each greenhouse gas molecule can absorb and emit radiation at given wavenumbers. So at a given altitude at a given atmospheric temperature [which is fixed since this is the IRF] the molecules should absorb and emit the same amount[\*] hence there should be no change to the radiative flux at that altitude. Hence no IRF.

[\*] I may be wrong about this point.

**Tags:** climate-change, radiative-transfer


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## 1 answers

### Answer by username_1 on 2024-10-25. Score: 3 | Accepted answer

Greenhouse gas molecules absorb radiation in a wavelength where most of the radiation comes from below (namely, the earth surface), but re-emit in a random direction. As a consequence, a photon that would have gone out to space is re-emitted in random directions and so has a probability of 1/2 to get back to Earth. In other words, it *reduces* the outbound energy flux and increases the inbound energy flux, compared to not having (or having less) greenhouse gases around.


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