Gavin Cawley explains the radiative greenhouse effect

Posted: November 2, 2018 by tallbloke in atmosphere, climate, physics, radiative theory, Thermodynamics
Comments
  1. Phoenix44 says:

    I honestly don’t know what point Cawley is making. Imagine I keep adding energy to a system and that energy doesn’t escape. I have the energy I have added. Well yes. True but trivial.

    If I add heat to a thing and the heat doesn’t fully escape, the thing gets warmer.

    Using sciencey phrases like black body and shell really doesn’t make it less trivial.

  2. oldbrew says:

    Does any of this tell us how an increase of a trace gas from 0.03% to 0.04% of the atmosphere could have any effect worth talking about?

  3. nobody says:

    “Now put a thin shell surrounding, but not touching the sphere, of the same blackbody material. The outgoing sphere will heat the shell, and it too will begin to radiate heat until it too reaches thermal equilibrium. Now the shell will radiate equally out into the vacuum and inwards towards the sphere. As the sphere is now receiving 1W from the heat source AND the “back-radiation” from the shell, it must end up at a higher equilibrium temperature than in the absence of the shell.”

    Can someone explain this? Wouldn’t the shell absorb the incoming 1W from the heat source, not both absorb it and pass it through to the sphere? And if the shelled sphere reached a higher temperature than the sphere alone, wouldn’t it radiate out >1W energy? Which would mean the shell was receiving 1W from the source and >1W from the sphere, and the shell would then radiate out >1W.

    It seems to me that an equilibrium shell+sphere must radiate out 1W from the shell to keep with conservation of energy, which would infer a 1W sphere if the shell radiates heat from external and internal sources in both directions equally.

  4. ivan says:

    To answer your question oldbrew – NO.

    It looks like a last ditch effort to try and keep the gravy train running.

  5. EternalOptimist says:

    OldBrew, their theory was that the CO2 would warm the tundra , releasing lots of water vapour which is a more powerful GHG.

  6. tallbloke says:

  7. oldbrew says:

    Re: ‘A successful prediction’…

    Bob Tisdale explained this 10 years ago: Atmospheric CO2 Concentration versus Sea Surface Temperature

    https://tallbloke.wordpress.com/suggestions-36/comment-page-1/#comment-144114

  8. dai davies says:

    Roger, the standard energy balance diagrams are incomplete because they assume “trapping” near the surface.
    Here is an updated version:

  9. dai davies says:

    All the arguments/thought experiments involving metal or black body shells are rubbish. The atmosphere isn’t a bb or a grey body, it’s IR thermoluminescent from collisional excitation of radiative gasses. Totally different dynamics.
    These emit in all directions, but there is a general upward movement of energy because the mean free path of photons is greater upward than downward due to decreasing density of atmosphere.
    Transfer by this mechanism is fast enough to cause insignificant Radiative Delay heating (RDE not GHE. we shouldn’t use their terminology).
    See details in: http://brindabella.id.au/ftp/RadiativeDelayInContext180818.pdf

  10. Brin says:

    > The outgoing sphere will heat the shell, and it too will begin to radiate heat until it too reaches thermal equilibrium. Now the shell will radiate equally out into the (2/n) vacuum and inwards towards the sphere. As the sphere is now receiving 1W from the heat source AND the “back-radiation” from the shell, it must end up at a higher equilibrium temperature than in the absence of the shell.

    I must be misunderstanding this part, because it sounds like he is describing a runaway greenhouse effect: A [the ground] heats B [the 1st shell] until B = A, but then B back-heats A so A = A+1. Okay, I can understand this part, but…

    … due to the first argument about A heating B until thermal equilibrium is reached, this should then result in the now hotter A then further heating B until we also have B = B+1, something that he does not state. Perhaps he skips it because this should then (based on his second argument – quoted above) heat A until A = A + 2, when would then heat B until B = B + 2, which would then heat A until A = A + 3, ad infinitum. Instead he skips this to talk about the next shell (C, which should also be affected by this runaway effect, albeit delayed by the time it takes for heat to transfer from B to C after B is heated by A).

    Also, if B is being heated by A and then radiating in all directions, shouldn’t B only heat A until at most A = A + (B / 2), not A = A + A? This should result in 1 + 0.5 + 0.25 + 0.125 + …. While this would still be a runaway effect, the maximum possible heating of B, C, etc from A would be A * 2. Which would mean the 102 W difference between 342 and 240 would suggest the Earth radiates 51 W. Which also does not make sense to me, since I seem to recall that the Earth radiates less than 1 W / m^2? So I figure I must be misunderstanding something.

  11. But the actual radiation window is 66 w/m2 as agreed by Trenberth see here https://cementafriend.wordpress.com/2015/09/25/radiation-window/

  12. oldbrew says:

    Non-CO2 climate change…

    The Medieval Climate Anomaly in South America
    Available online 31 October 2018

    The most likely key drivers for the medieval climate change are multi-centennial Pacific and Atlantic ocean cycles, probably linked to solar forcing.

    https://www.sciencedirect.com/science/article/pii/S1040618218308322

  13. nobody says:

    Extending my prior comment:

    “You can make this radiation [between the sphere and the shell] arbitrarily higher by adding more and more shells.”

    I’m pretty sure this is also false. Since the inside surface of the shell is concave, some radiation will go from the shell back to the shell. I think this would mean the inside sphere (or inner shells) would be slightly cooler than it would otherwise be at equilibrium.

    Additionally, the outer shells would be cooler than the sphere/shell inside of them. Given the same amount of total radiation emitted, a larger surface area means a lower temperature. So (ignoring the concave issue) even though a shell is receiving 2W total (1W from outside, 1W from inside), it would have over two times the surface area of the inner sphere/shell, and therefore be at a lower average temperature.

  14. arfurbryant says:

    Gavin Cawley ‘thought experiment’

    [“Sadly in my experience, this is what happens pretty much every time I use a thought experiment in discussing climate on-line.”]

    Well, in this case, that’s because you have chosen an inappropriate thought experiment which has nothing to do with the subject of CO2 induced cAGW and your so-called ‘radiative greenhouse effect’…

    1. [“As the sphere is now receiving 1W from the heat source AND the “back-radiation” from the shell, it must end up at a higher equilibrium temperature than in the absence of the shell.”]
    Wrong premise. The back-radiation is irrelevant. The sphere will end up at a higher temperature because the shell is insulating against the sphere’s heat loss. If you put hot coffee into a vacuum flask and then sneak in an electrical heating element, the coffee will get hotter. Insulation alone will not cause the coffee to heat up. This is where your experiment is inappropriate as a means of explanation of the so-called rGHE. You are describing a completely different system. Furthermore, in regards to the rGHE, by what mechanism does the – in your experiment – ‘back-radiation’ cause the sphere to heat further? The back-radiation is at a longer (average) wavelength than that which was emitted by the sphere originally. There is no mechanism by which this longer (average) wavelength radiation can heat an object which is already capable of emitting shorter (average) wavelength radiation. It is this process which describes why the Sun heats the Earth. The atmosphere cannot radiatively heat the Earth as the atmosphere possesses no energy source capable of doing so, apart from those very (very) few localised temporary locations where the atmosphere is warmer than the Earth surface.

    2. [“There is now an exchange of radiation between the sphere and the shell, where the outbound radiation is in excess of 1W, because it also has to radiate away the energy received as back-radiation from the shell.”]
    Wrong conclusion. The energy it received as back-radiation did not increase the internal energy state of the sphere. The restriction of heat loss did that by means of insulation (the physical shell). Are you really saying a system which has 1W input can now output greater than 1W? Seriously? The only energy in the system is from a 1W source. If you are serious, patent it, quickly.

    3. [“Imaging [sic] a black body in a vacuum containing a 1W constant power heat source. Eventually it will reach an equilibrium temperature at which it radiates 1W out into the vacuum. Now put a thin shell surrounding, but not touching the sphere, of the same blackbody material.”]
    Earth does not contain an internal constant power source. You try to explain this error of thought by stating the Sun acts as one but, let’s face it, the Sun doesn’t. If you put shells around the Earth the short wave radiation from the Sun will not heat the Earth, so your thought experiment – as an explanation – fails at the first step.

    4. [“In this thought experiment, the 1W heat source corresponds to the visible/UV radiation from the sun, to which the “shell” of the atmosphere is transparent.”]
    Then you have just invalidated the thought experiment. In the thought experiment you have a physical shell and a sphere and a power source. If the shell is a complete shell then it cannot let in energy from an outside source. You can’t eat your cake and have it. Either the thought experiment stands as is, or you have to re-think it!

    5. [“…purpose of a thought experiment, which is to present a very simple explanation of one aspect of the system under consideration, without extraneous detail, so that one aspect can be understood.”]
    Then respect your thought experiment. Stating the Sun’s radiation can pierce the physical shell which surrounds the sphere is (exactly) an extraneous detail – which you imply you cannot allow. This is a problem for people who want to use a thought experiment without thinking about it.

    6.[“It is almost as if someone is actively trying to avoid acknowledging the single aspect illustrated by the thought experiment!”]
    Or, B, someone is trying to avoid stating exactly what the illustrated single aspect is. Your thought experiment is actually about insulation, not insolation!

    7. [“Do you agree that the physics of the thought experiment, as given, are correct?”]
    No. The physics of the thought experiment is based on an invalid assumption.

    Reality check for warmists:
    A. Radiation is not heat.
    B. Heat only flows from hot to cold.
    C. If back-radiation works in a thermal sense, how many cold objects do you have to surround a hot object before the hot object gets even hotter?

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