A little thing about “Back Radiation” that people forget
As we have discussed GHGs and clouds are overlapped all over the place. “Climate science” largely ignores this and the logical implications to it. Moreover however, GHGs overlap themselves so to say. It is for this reason, that a doubling of CO2 will not double its radiative forcing, but will only cause a gradual increase. Some people call this “saturation”, suggesting CO2 and other GHGs would hardly enhance the GHE with additional quantities. While this is essentially true, there is no innovation in it, as the orthodoxy claims nothing else.
For instance modtran suggests some 33W/m2 of gross, or “single factor addition” GHE of CO2 at 280ppm. This figure is a little too high, as we have discussed previously, because the model overstates surface emissivity. Anyway, doubling CO2 would then only increase this GHE by the well known 3.7W/m2 to 36.7W/m2. As the second batch of 280ppm naturally have the properties as the first batch, it will equally produce 33W/m2 of GHE in this instance. But 89% of it are overlapped with the first batch, and thus will not make a difference in the end. Only the remaining 11% produce a net gain. And of course these figures will shrink by a significant margin, once overlaps with other GHGs and clouds are considered.
The same thing that goes for emissions TOA, is also true for “back radiation” (BR). BR can naturally never exceed the emissions of black body of the same temperature as the atmosphere. As the atmosphere is a grey body with an atmospheric window, it well necessarily emit somewhat less radiation downward. And if there are multiple layers so to say emitting BR, these emissions will not add up and accumulate, but rather are going get absorbed by the atmosphere itself. In other words, there is much more BR then we can measure at the surface, but most of it is re-absorbed within the atmosphere, and never reaches the surface. The question is, how much is there?
With the help modtran1 we can try to find out. If we take the “U.S. Standard Atmosphere”, nullify all GHGs, then there is only 1.658W/m2 of BR. Apparently this comes mainly from N2O, which we can not modify in this model. Now if we set CO2 to 400ppm there is 67.541W/m2 of BR. Assuming that CO2 and N2O are not or hardly overlapped, CO2 alone should provide 67.541– 1.658 = 65.883W/m2.
Now if we reduce CO2 to only 0.4ppm, only a thousandth of the previous concentration, we are still left with 8.657W/m2 of BR. Again we subtract the N2O contribution which leaves us with 6.999W/m2, which I will always do from now on, without further mentioning it. So while the CO2 concentration dropped by a factor of 1000, BR fell not even by a factor of 10. The graph below shows BR at 400 and 0.4ppm. The little peak to the left is N2O btw.
The next thing is a simple logical consideration. A 400ppm of CO2 consists out of 1000 parcels of 0.4ppm. If such a single parcel emits said 6.999W/m2, the other 999 parcels will do so as well. In fact every molecule of CO2 will emit the same radiation (at the same temperature), as the SB law tells us. So there should be 6.999 * 1000 = 6,999W/m2 in total BR, of which only a tiny share ever reaches the surface, while the bulk of it will simply get absorbed by other parcels of CO2. From this alone we can already guess there must be huge amounts of BR within the atmosphere.
The problem is that a parcel size of 0.4ppm is an arbitrary choice. Most likely, even with such a low concentration, there will be re-absorption occurring. Ideally we should choose the smallest parcel size possible to minimize this factor, but this might get us in conflict with the limited resolution of modtran. There is a chance we might run into random numbers.
The other problem is that most BR measured at the surface comes from the lowest atmospheric layers, where it is relatively warm. Higher up the temperatures are lower so that CO2 molecules there will emit less radiation.
Thankfully both problems can be dealt with by minimizing CO2 concentration in the model. The less concentration, the less re-absorption will occur, and the higher up the average emission altitude of BR will be. With the first problem being dominant, we should see ever higher amounts of BR the smaller the modelled CO2 concentration. On top of that, if the model is accurate enough, the results should converge towards a certain magnitude, as we use ever smaller concentrations.
Let us try it out..
Following this procedure towards ever smaller parcel sizes, thereby minimizing inner-parcel re-absorption, and then multiplying the BR by how many parcels fit into the 400ppm base scenario, we get a result that makes sense. These are not crazy, contradicting numbers, but rather we see a very smooth convergence towards about 20.000W/m2. Using even smaller concentrations than 0.001ppm will not produce any reasonable result, but since the convergence has been well established at this point, there is no need to go any further.
So CO2 alone produces about 20.000W/m2 in BR, of which almost the entirety is again re-absorbed by CO2 itself. I guess this is a pretty impressive figure. In some ranges of the spectrum CO2 is so optically thick, that photons would only travel for a couple of centimetres before they get re-absorbed.
Of course we have other GHGs and clouds. I will not try to annoy you by applying the same or a similar procedure on them. But it seems obvious that at least WV and clouds should be even more potent emitters and absorbers of radiation. For the sake of argument let us guess there was a total of at least 50.000W/m2 in BR throughout the atmosphere, with almost the entirety getting emitted and re-absorbed there, and only a tiny fraction of about 300W/m2 radiated directly onto the surface. Then what are the implications?
First of all we would need to know if it makes a difference, whether BR is absorbed within the atmosphere or at the surface. For those who believe in a BR caused GHE it is a delicate question. Is only the BR absorbed at the surface relevant? If so, why? What about the plenty of BR re-absorbed by the atmosphere very close to the surface? If all BR caused a GHE, with some 50,000W/m2 of it, should we not be toast?
But there is more. What if we recall those typical Earth energy budgets? Why do these only show the BR radiated onto the surface, and not the abundance of BR throughout the atmosphere? Is it because this would make the diagram incredible complicated, or because it might jeopardize the argument it tries to make? Is it not interesting how “climate science” constantly forgets what does not fit the narrative, while exaggerating everything that does?
There are a lot of people who rightfully deny a “back radiation” driven GHE. Regrettably the arguments brought forward are a) “back radiation” would not exist, b) pyrgeometers were unreliable instruments (which they may be), or c) the second law of thermodynamics. They are all pointless, as “back radiation” of course does exist, although radiation does not care about a direction. Also the argument c) is pointless, as the colder atmosphere, or a positive lapse rate respectively, is indeed what is causing the GHE. In this sense, cold “heats” warm by insulation. But it is not about a flow of energy.
Anyway, the actual problem with “back radiation” is not its inexistence, but its abundance. It is everywhere, it is ubiquitous, and there is plenty of it. If it did anything, climate wise, we would all be doomed! Not some decades into the future, but now, or long ago, whatever.
But this also tells us a lot about the average optical thickness of the atmosphere. The average temperature of the atmosphere (weighted by mass) should be about 250K. A black body at this temperature would emit 221W/m2. Accordingly the average optical thickness of the atmosphere should be at least 200 (50,000 / 221 > 200).
This brings up an interesting perspective on the multi-layer GH model as presented by Manabe, Strickler 19642 and others34 . Per se the idea of splitting the atmosphere in opaque layers seems a bit odd, since partially the atmosphere is perfectly transparent (within the atmospheric window), or extremely optically thick, like at 15µm due to CO2. However, since it is all about BR serving as a heat engine anyway, arithmetic averaging of optically thick and thin parts should be fine.
The model assumes only like 3 atmospheric layers providing moderate amounts of total atmospheric BR, far less than there actually is. Yet it produces a GHE far larger than we observe. Manabe, Strickler name 332.3K for a “pure radiative equilibrium”, so that the GHE was actually 77K (332 – 255). They also explain why thank god Earth is not that hot, as convection would counter this massive GHE. Accordingly the lapse rate would then be a compromise between the radiative GHE and mitigating convection.
Ironically the lapse rate is actually a function of the ideal gas law instead, something the model implicitly denies. Moreover the assumed 3 layers are just a totally unsubstantiated guess to support the model, which hugely overstates the GHE nonetheless. Had they ever looked up the properties of the atmosphere and how much BR there actually is, their model would have gone haywire, producing a GHE of well over 500K. It would have been obvious the model does not work.
However, as such a reality check was never done, the model is still alive and taught in schools. And Mr. Manabe was even awarded with the Nobel Prize..
PS. As before I fully acknowledge the IPCC has dropped “back radiation” from its definition of the GHE with AR5 eventually. I am confident it will not re-emerge with AR6, although the final WG1 report has not been published yet.
Douglas Cottonat 04.11.2022
The Second Law of Thermodynamics can only be applied either to a single process (eg one-way radiation from atmosphere to the surface) or to two or more interacting thermodynamic systems (see Wikipedia "Laws of Thermodynamics"") and Clausius also added that such other processes must be at the same time. Thus back radiation cannot raise the surface temperature at all because there are no other interacting systems. My peer-reviewed paper on this is linked from the website below.
This then leaves everyone (except myself) without an explanation for planetary surface temperatures.
This comment is continued at http://climate-change-theory.com and in the seven linked papers of mine.
Of course a "back radiation" driven GHE was always nonsense. If you look up the glossary in the IPCC WG1 reports, where they define the GHE, you will see "back radiation" was last featured in AR4, but no more in AR5 or AR6. They silently dropped it. Of course you would not make much noise over it, cause it is embarrassing if the "settled science" only just learned how the GHE works.
However, this is just one of many blunders in "the science". You probably think you found the ONE mistake and so you are excited over it. But trust me, there is so much more.
2. It's the temperature of the air that produces the insulation effect. The higher temp, the greater the insulation.
3. The more insulation the air provides, the higher the surface temperature must be (to "push through" the greater insulation effect)
4. GHGs both absorb IR energy and release it [almost immediately] as either conduction to nearby molecules in the dense air near the surface, thereby warming them, or [at altitude] as photons which are too far away from the surface to have any surface warming effect. Indeed at altitude GHG's provide a net cooling effect.
5. Thus the GHE should be 99% an artifact of the non-GHGs as they have 99% of the thermal mass of the atmosphere, and thus provide 99% of the insulation effect.
The so-called back radiation is not an additional source of energy input to the surface, but rather a measure of this insulation effect.
While extra GHGs will have some marginal impact on the mass of the atmosphere, it is de-minimus because all the NCGHGs are a tiny percentage of the total.
The non-GHGs can absorb energy, but by themselves cannot release it other than by collisions, which is to say they cannot release it to outer space. The net effect of the gases in the atmosphere is that the GHGs allow earth's outgoing IR to be absorbed by the atmosphere but they immediately release it either to nearby molecules or to outer space. The non-GHG molecules only distribute absorbed IR energy around the atmosphere but not to shed it to outer space.
The ugly truth then is that GHG's actually are anti-GHGs, because net/net they serve to cool the planet. Conduction and latent heat are what warm the air and thus provide the insulation effect that warms the surface.
amazing blog. Still reading all post carefully to fully understand them.
Just an observation. If all this BR is “stored” in higher layers of atmosphere, as models supposedly predict, why observations don’t match? According to satellite observations, at tropopause observations gives around 0.17dC/decade, while CMIPv6 models predict an average of 0.40dC/decade.
Then the questions is, where is going all this radiation? Obviously calcs are wrong, but where?
In this thought expirement your dough-body would instantly cool down to ambient temperature. In reality our "surface" is only about 1.8m2 of skin. If it had 310K vs. an ambient temperature of 293K (20°C), we might lose some 190Watt (=(310^4 - 293^4)*5.67e-8*1.8)) net. That is still too much, unless we excercise. We only produce about 100Watt plus we transpire. Thus the skin temperature is somewhat less, we wear clothes, or set room temperature to over 293K.
As the example shows, there is plenty of "back- and forth radiation" within your body, which goes nowhere. If it escaped, you could not maintain your body temperature. If it heated anything, you'd be toast. Thankfully it goes nowhere and does nothing! And it is exactly the same thing with back- and forth radiation anywhere else, like within the atmosphere. And if BR increases, that is predominantly because of rising temperatures.
Thanks a lot for your answer. My main question was about the difference between models and observations at lower pressure atmospheric layers. For example, at the equator mid & upper tropospheric layer, models predict higher temps as a consequence of GHG BR, but reality is providing half, what’s the reason? Why GHGs are not creating as much temperature as theory is claiming?
Thanks in advance.
The problem is NOT models overstating warming, but that the given warming is barely due to GHGs. Most of the warming will be rather due to aviation induced cirrus.
thanks for your answers.
As far as i researched, there is only one paper published around 2014 by a NASA scientist were he proposed aviation induced cirrus as one possible cause, but there is nothing more about it. Do you have any more information about that?
thanks in advance.
I already quoted Patrick Minnis from NASA..
"This result shows the increased cirrus coverage, attributable to air traffic, could account for nearly all of the warming observed over the United States for nearly 20 years starting in 1975"
Another interesting recent find is this presenation by Charles Long (NOAA).
at the end of last year i discovered a NOAA monitoring program SURFRAD webpage:
They have 8 stations in United States where they can monitor Surface Radiation Budget. Its really interesting since you can plot lots of different variables, like Downwelling Infrared radiation, or Back Radiation, as you call it here.
The most surprising thing about it is that, wherever is the day or location, if you plot this variables
Infrared Net Radiation
you can see that the firsts two get canceled, and total net is always under 0 W/m2, while solar net is, of course, above 0 W/m2 when sun sunshines. Or in other words, there is always more infrared leaving surface than reaching surface, so in net terms, its always below cero watts/m2.
Could you please elaborate a little bit more about it? i would appreciate, because its like a puzzle that is still not finished in my head, and i suspect that this charts are related to your back radiation article.