Probably the most misunderstood perception of mine and many others in the Climate Change debate is the concept of Down Welling radiation or back radiation caused by the greenhouse effect. Per the second law of thermodynamics, heat flows from warm to cold. In truth, NET heat flows from warm to cold, so a colder body cannot physically warm a warmer body. The colder body can reduce the rate of cooling of the warmer body so that it would be warmer than if the colder body were not there.
Some people tend to get carried away trying to tweak the second law by stating that a photon traveling in a random direction can be absorbed by a warmer body after being emitted by a colder body. Quite true, but in the process, the colder body would absorb more photons from the warmer body, because the warmer body is emitting more directionally random photons. Net flow will be from warmer to colder, period.
On the surface of the Earth, photons from the colder sky do impact the surface on rare occasion. More frequently they impact molecules closer to their physical location and temperature. Since the sky has a temperature, it emits photons and a large percentage of those photons travel toward the warmer surface. That direction of travel is called Down Welling Long Wave Radiation (DWLR), but where that DWLR impacts changes with atmospheric conditions.
Why many disagree or misunderstand my perception of DWLR, is the result of my choice of a thermodynamic frame of reference. I live on the surface, the surface is my frame of reference. So how can this be controversial?
The base line for determining the magnitude of Greenhouse Effect (GHE) is an Earth with no Greenhouse Gases (GHGs). That thought experiment Earth still has an atmosphere and still has an albedo, or reflection of incoming solar energy. My visualization of that Earth is a semi-solid sphere surrounded by a more fluid atmosphere. Since there is no GHE, both the surface and the atmosphere would emit radiation, only less of the surface radiation would be absorbed by the atmosphere. The atmosphere, which would have a high viscosity since its rate of radiant cooling would be lower than the surface, would be nearly isothermal or approximately the same temperature at every level due to conductive heat transfer from the surface to the atmosphere.
Standing on the surface of the no GHG Earth, you would measure the same temperature in all directions. That temperature would be approximately 255K degrees, which would emit approximately 240Wm-2 in all directions. That is the no GHE zero DWLR value. If you prefer, the 240Wm-2 would be the background radiation value. That is unique to my frame of reference. A Top of the Atmosphere (TOA) reference would make assumptions that the thickness of the no GHG Earth atmosphere is negligibly small. I disagree.
With GHGs, the surface temperature on average is about 288K degrees with an energy flux of approximately 390Wm-2. The surface impact of the GHE would be 390Wm-2 minus 240Wm-2 or 160Wm-2. Since the source of that DWLR is not the surface, but some point in the atmosphere, the source value of DWLR would be greater than 160Wm-2, if it is indeed a true source of reflected energy averaged over the entire atmosphere. Depending on the altitude of that source of DWLR, the value would vary.
Since the source is likely in the lower atmosphere, near the average mass of the atmosphere, I estimate the average DWLR magnitude as approximately 220Wm-2. That value is based on my estimate of the average emissivity between the surface and that point, 160/220 equals 0.73 or the average emissivity from the DWLR source to the surface. There is no perfect energy transfer, so there will be energy lost in the transfer, if DWLR is a true source of energy.
Since we are comparing a real world to a thought experiment world, the real world values would have to accurately compare to the thought experiment values or there is apples and oranges being mixed.
Many chose a TOA frame of reference. From that perspective, the emissivity is approximately 0.61 which would require, 160/0.61 = 260Wm-2 of GHG produced DWLR or as some seem to think, a nifty power series manipulation to arrive at approximately 330Wm-2 at some point near the TOA.
As long as the demands of the choice of frame of reference are carefully met, either can produce accurate results. Mine, IMHO, is easier to maintain, more flexible and provides more information for the actual surface.
Part of that information is the tropopause temperature. In order to warm the surface, the GHGs would have to cool the tropopause. Increasing the surface temperature by 33C from 255K to 288K would decrease the tropopause from 255K to 222K, which would be approximately -51 C degrees. Which is about the range of the average tropopause temperature after allowing for all the approximations. In order to meet the requirements of conservation of energy, 240Wm-2 of DWLR would be the maximum limit of the GHE. That would be the equivalent of perfect insulation by all GHGs. In order to exceed that limit, the tropopause would have to start warming the stratosphere. While that is possible, the amount of additional GHGs available appear to limit that possibility. That I am working on, what is the realistic limit?