CO2 heats the atmosphere…a counter view
This is a great and easily understood article on radiative energy transfer within the atmosphere. TE and LTE and the Equipartition Theorum. Once the 15 micron surface IR is absorbed there is no more CO2 warming to be had (minor exception described below).
The article ends with two important caveats-
"
Caveat 1
The statement we proved cannot be interpreted as “CO2 has no impact on the dynamics of the Earth-atmosphere system” . What we have proven is that the CO2 cannot heat the atmosphere in the bulk but the whole system cannot be reduced to the bulk of the atmosphere . Indeed there are 2 interfaces – the void on one side and the surface of the Earth on the other side . Neither the former nor the latter is in LTE and the arguments we used are not valid . The dynamics of the system are governed by the lapse rate which is “anchored” to the ground and whose variations are dependent not only on convection , latent heat changes and conduction but also radiative transfer . The concentrations of CO2 (and H2O) play a role in this dynamics but it is not the purpose of this post to examine these much more complex and not well understood aspects .
Caveat 2
You will sometimes read or hear that “the CO2 has not the time to emit IR because the relaxation time is much longer than the mean time between collisions.” We know now that this conclusion is clearly wrong but looks like common sense if one accepts the premises which are true . Where is the problem ?
Well as the collisions are dominating , the CO2 will indeed often relax by a collision process . But with the same token it will also often excite by a collision process . And both processes will happen with an equal rate in LTE as we have seen . As for the emission , we are talking typically about 10ⁿ molecules with n of the order of 20 . Even if the average emission time is longer than the time between collisions , there is still a huge number of excited molecules who had not the opportunity to relax collisionally and who will emit . Not surprisingly this is also what experience shows"
The mean free path of a surface 15 micron IR photon is less than 2 metres, the extinction height is roughly 10 metres. All of the surface 15 micron energy is put into the atmosphere by 10 metres, half by 2 metres. After that it is all passive diffusion as described in the article above. Once the air has thinned enough the 15 micron IR will escape, but only as much as the much cooler air can produce. The difference between what went in near the surface, and what comes out much higher up, is the amount of energy available to warm the atmosphere.
There is another bottleneck at the precipitating cloudtops where latent heat is released but this effect is smaller and at a fuzzy boundary. It still follows the same mechanism.
Other GHGs also follow this mechanism. For the wavelengths that water vapour strongly absorbs the mean free path is close to the surface and that is where the warming takes place. For weakly absorbed wavelengths the mean free path may be kilometres from the surface. For some wavelengths there is no absorption, no warming, and the energy escapes directly to space.
But for CO2 warming it is right near the surface. Increasing CO2 shortens the mean free path. Because surface temperature is actually measured 1.5 metres into the air, most (edit- much) of the CO2 warming is considered to be surface warming.
I hope this clears up some of the poorly worded descriptions in the media of CO2's impact on the GHE and AGW.
This is a great and easily understood article on radiative energy transfer within the atmosphere. TE and LTE and the Equipartition Theorum. Once the 15 micron surface IR is absorbed there is no more CO2 warming to be had (minor exception described below).
The article ends with two important caveats-
"
Caveat 1
The statement we proved cannot be interpreted as “CO2 has no impact on the dynamics of the Earth-atmosphere system” . What we have proven is that the CO2 cannot heat the atmosphere in the bulk but the whole system cannot be reduced to the bulk of the atmosphere . Indeed there are 2 interfaces – the void on one side and the surface of the Earth on the other side . Neither the former nor the latter is in LTE and the arguments we used are not valid . The dynamics of the system are governed by the lapse rate which is “anchored” to the ground and whose variations are dependent not only on convection , latent heat changes and conduction but also radiative transfer . The concentrations of CO2 (and H2O) play a role in this dynamics but it is not the purpose of this post to examine these much more complex and not well understood aspects .
Caveat 2
You will sometimes read or hear that “the CO2 has not the time to emit IR because the relaxation time is much longer than the mean time between collisions.” We know now that this conclusion is clearly wrong but looks like common sense if one accepts the premises which are true . Where is the problem ?
Well as the collisions are dominating , the CO2 will indeed often relax by a collision process . But with the same token it will also often excite by a collision process . And both processes will happen with an equal rate in LTE as we have seen . As for the emission , we are talking typically about 10ⁿ molecules with n of the order of 20 . Even if the average emission time is longer than the time between collisions , there is still a huge number of excited molecules who had not the opportunity to relax collisionally and who will emit . Not surprisingly this is also what experience shows"
The mean free path of a surface 15 micron IR photon is less than 2 metres, the extinction height is roughly 10 metres. All of the surface 15 micron energy is put into the atmosphere by 10 metres, half by 2 metres. After that it is all passive diffusion as described in the article above. Once the air has thinned enough the 15 micron IR will escape, but only as much as the much cooler air can produce. The difference between what went in near the surface, and what comes out much higher up, is the amount of energy available to warm the atmosphere.
There is another bottleneck at the precipitating cloudtops where latent heat is released but this effect is smaller and at a fuzzy boundary. It still follows the same mechanism.
Other GHGs also follow this mechanism. For the wavelengths that water vapour strongly absorbs the mean free path is close to the surface and that is where the warming takes place. For weakly absorbed wavelengths the mean free path may be kilometres from the surface. For some wavelengths there is no absorption, no warming, and the energy escapes directly to space.
But for CO2 warming it is right near the surface. Increasing CO2 shortens the mean free path. Because surface temperature is actually measured 1.5 metres into the air, most (edit- much) of the CO2 warming is considered to be surface warming.
I hope this clears up some of the poorly worded descriptions in the media of CO2's impact on the GHE and AGW.
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