On a previous post a reader queried me about my position. Taking him to be serious, I prepared a reply with resources that can serve anyone wanting to understand radiative GHG theory and reality. The key is to escape radiation myopia, that is focusing on radiative energy transfers in earth’s climate system to the exclusion of the other transfers. Energy in our world moves by conduction, convection and phase changes of H2O in addition to radiation. And not surprisingly at any place and time, the most active mode is the one with the least resistance.
The post triggering the question was this one:
My Reply to Questioner
Thanks for your response. Your inital question sounded trollish, but I take your comment seriously.
Firstly, you said “I’ve never seen anyone outside of the anti-GHG crowd ever talk about “back-radiation”. Actually references to that notion are readily found since it is the primary way global warming/ climate change is explained to the public. Some examples:
“However, GHGs, unlike other atmospheric gases such as oxygen and nitrogen, are opaque to outgoing infrared radiation. As the concentration of GHGs in the atmosphere increases due to human-caused emissions, energy radiated from the surface becomes trapped in the atmosphere, unable to escape the planet. This energy returns to the surface, where it is reabsorbed.” UNEP
“Greenhouses gases are atmospheric gases such as carbon dioxide (CO2), methane (CH4), and water vapor (H2O) that absorb and re-radiate heat, which warms the lower atmosphere and Earth’s surface. This process of absorption and re-radiation of heat is called the greenhouse effect. Although greenhouse gases only make up a small percentage of the atmosphere, small changes in the amount of greenhouse gases can greatly alter the strength of the greenhouse effect, which in turn, affects the Earth’s average temperature and climate. UCBerkeley
“As CO2 soaks up this infrared energy, it vibrates and re-emits the infrared energy back in all directions. About half of that energy goes out into space, and about half of it returns to Earth as heat, contributing to the ‘greenhouse effect.’ ColumbiaU
The favored term now is “re-radiation” and it is central in the narrative everywhere, including among others, NASA, MIT and of course multiple UN agencies. So it is necessary to debunk the notion.
I know as well as you that back- or re-radiation is a caricature, and climate scientists make a different claim, namely raising the ERL which slows the cooling. That theory is also wrong for different empirical reasons. See:
Secondly, the root issue is the abuse of Stefan-Boltzman law to create a fictious downward energy transfer, such as seen in energy balance cartoons, misleading and not funny. The equation calculates the transfer from the difference in temperature between two bodies in thermal contact, it does not attribute thermal radiation to each of them. Full explanation here:
And regarding the failed energy balance diagrams:
For extra credit and insight, look at a Sabine Hosenfelder video to understand how current GHG theory goes astray. Link includes excerpts and critique.
Summary
“The Earth, a rocky sphere at a distance from the Sun of ~149.6 million kilometers, where the Solar irradiance comes in at 1361.7 W/m2, with a mean global albedo, mostly from clouds, of 0.3 and with an atmosphere surrounding it containing a gaseous mass held in place by the planet’s gravity, producing a surface pressure of ~1013 mb, with an ocean of H2O covering 71% of its surface and with a rotation time around its own axis of ~24h, boasts an average global surface temperature of +15°C (288K).
Why this specific temperature? Because, with an atmosphere weighing down upon us with the particular pressure that ours exerts, this is the temperature level the surface has to reach and stay at for the global convectional engine to be able to pull enough heat away fast enough from it to be able to balance the particular averaged out energy input from the Sun that we experience.
It’s that simple.” E. M. Smith
See Also
Science Matters 
Interesting. I will have to go through this carefully, if I can ever find the time.
Some quick points:
(1) re: E. M. Smith’s remarks – if the atmosphere were composed only of nitrogen, which doesn’t absorb infrared, would the temperature be the same as with the present mixed gas atmosphere? From what Smith says, one would expect that it would. But I wonder if Will Happer would agree.
Now, supposing we swap in CO2 or H2O for nitrogen molecules on a 1:1 basis, I would expect that the adiabatic lapse curve would shift to the right – owing to an added ‘greenhouse effect’. That seems simple enough as well.
(2) the top-of-the-atmosphere curve (which appears in a recent article posted on this site) shows clear evidence of absorption of infrared by water vapor and CO2 – meaning that infrared is being trapped in the atmosphere. I would expect that there will be an increase in temperature on this basis. And I would call this a greenhouse effect (regardless of whether the process is actually the same as in a real greenhouse – as discussed in the same article). To deny this would seem to contradict the First Law of thermodynamics, which doesn’t seem like a good stance to take.
(3) In fact, that same article lays out the case for a ‘greenhouse’ effect, and therefore contradicts the article that started this discussion, as well as the arguments made in the present post.
(4) Concerning this re-radiation business. I presume that no one is arguing that a molecule that has absorbed infrared radiation won’t re-radiate energy at some time afterwards. And I assume that the molecule will also increase its velocity for some time, causing a similar increase in other molecules. And that some of the re-radiated energy will be absorbed by other molecules in the atmosphere. So re-radiation doesn’t strike me as a controversial issue in itself. The point is that all this business should result in some temperature increase. But I don’t think it’s necessary to invoke some sort of net “back radiation” as a critical factor. Maybe this is a semantic issue, since the term “back radiation” seems to imply a net downward flow, as opposed to simply occurring in all directions.
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Thanks for commenting further, rw. On your first point, please read the above experimental study by Thomas Allmendinger. He confirms that a parcel of ordinary air and a parcel of pure CO2 show the same warming from exposure to sunlight and separately exposure to IR.
On your second point, I recommend GHG Theory and the Tests It Fails. There is no “trapping”, only a delay of cooling, which is saturated at present concentrations.
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