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What I don't understand: the IR that's being blocked by the GHGs is energy which has already been absorbed by the Earth. So this shouldn't change the Earth's overall absorbivity, and I don't see how it could change the Earth's temperature.
Twitter may not be the best medium for this(??) But if someone who understands this well can point me to a good explanation, I'd appreciate it. Thanks!
@AndrewDohertyQu @dabacon @worrydream @patrickc Do you know?
I omitted the technical details, but it's this: the temperature should be set by: incoming energy flux = epsilon sigma T^4, where epsilon is the emissivity, sigma is the Stefan-Boltzmann constant, and T is the temperature.
Since anything absorbed by the GHGs has already been absorbed by the Earth, the absorptivity (and thus the emissivity) shouldn't be changed by the GHGs, and so I don't see how T can be changed by the GHGs.
I should have said earlier in the thread, but the key thing I'm worried about: why is epsilon in the Stefan-Boltzmann relation changed, since net absorptivity apparently isn't? Or is S-B the wrong way to be thinking?
Update: I believe @PESimeon has isolated the source of my confusion. A summary (more or less) can be found here:https://twitter.com/michael_nielsen/status/1096990267259809793 …
Thankyou for all the comments and the links. It's very much appreciated, and has clarified matters greatly for me, especially (though certainly not just) in the part of the thread linked in the last tweet.
Earth gains energy from spectra that are in non-IR wavelengths, loses comparatively more energy through IR wavelengths (it's converted when it hits earth from e.g. visible to IR black body radiation).
Greenhouse gases capture IR but NOT visible spectra, so they mostly let through the IN-energy, but trap OUT-energy.
In many books geared towards younger readers, this isn't addressed at all but is a totally reasonable question - ie., if it's 'stopping' something from leaving, why isn't it also stopping it from coming in?
That I understand - the atmosphere is mostly transparent at optical & UV frequencies (most solar radiation), but GHGs make it more absorptive at IR frequencies. At the Earth's temperature most re-emitted grey body radiation is at IR frequencies, so can be absorbed by the GHGs.
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