Guest Post by Willis Eschenbach
Dr. Judith Curry notes in a posting at her excellent blog Climate Etc. that there are folks out there that claim the poorly named planetary “greenhouse effect” doesn’t exist. And she is right, some folks do think that. I took a shot at explaining that the “greenhouse effect” is a real phenomenon, with my “Steel Greenhouse” post. I’d like to take another shot at clarifying how a planetary “greenhouse effect” works. This is another thought experiment.
Imagine a planet in space with no atmosphere. Surround it with a transparent shell a few kilometres above the surface, as shown in Figure 1.
Figure 1. An imaginary planet surrounded by a thin transparent shell a few kilometres above the surface (vertical scale exaggerated). The top of the transparent shell has been temporarily removed to clarify the physical layout. For our thought experiment, the transparent shell completely encloses the planet, with no holes. There is a vacuum both inside and outside the transparent shell.
To further the thought experiment, imagine that near the planet there is a sun, as bright and as distant from that planet as the Sun is from the Earth.
Next, we have a couple of simplifying assumptions. The first is that the surface areas of the planet and the shell (either the outside surface or the inside surface) are about equal. If the planet is the size of the earth and the transparent shell is say 1 kilometre above the surface, the difference in area is about a tenth of a percent. You can get the same answer by using the exact areas and watts rather than watts per square meter, but the difference is trivial. Assume that the shell is a meter above the surface, or a centimeter. The math is the same. So the simplification is warranted.
The second simplifying assumption is that the planet is a blackbody for longwave (infra-red or “greenhouse”) radiation. In fact the longwave emissivity/absorptivity of the Earth’s surface is generally over 0.95, so the assumption is fine for a first-order understanding. You can include the two factors yourselves if you wish, it makes little difference.
Let’s look at several possibilities using different kinds of shells. First, Fig. 2 shows a section through the planet with a perfectly transparent shell. This shell passes both long and shortwave radiation straight through without absorbing anything:
Figure 2. Section of a planet with a shell which is perfectly transparent to shortwave (solar) and longwave (“greenhouse”) radiation. Note that the distance from the shell to the planet is greatly exaggerated.
With the transparent shell, the planet is at -18°C. Since the shell is transparent and absorbs no energy at all, it is at the temperature of outer space (actually slightly above 0K, usually taken as 0K for ease of calculation). The planet absorbs 240 W/m2 and emits 240 W/m2. The shell emits and absorbs zero W/m2. Thus both the shell and the planet are in equilibrium, with the energy absorbed equal to the energy radiated.
Next, Figure 3 shows what happens when the shell is perfectly opaque to both short and longwave radiation. In this case all radiation is absorbed by the shell.
Figure 3. Planet with a shell which is perfectly opaque to shortwave (solar) and longwave (“greenhouse”) radiation.
The planet stays at the same temperature in Figs. 2 and 3. In Fig. 3, this is because the planet is heated by the radiation from the shell. With the opaque shell in Fig. 3, the shell takes up the same temperature as the planet. Again, energy balance is maintained, with both shell and planet showing 240 W/m2 in and out. The important thing to note here is that the shell radiates both outward and inward.
Finally, Fig. 4 shows the energy balance when the shell is transparent to shortwave (solar) and is opaque to longwave (“greenhouse”) radiation. This, of course, is what the Earth’s atmosphere does.
Here we see a curious thing. At equilibrium, the planetary temperature is much higher than before:
Figure 4. Planet with a shell that is transparent to shortwave (solar) radiation, but is opaque to longwave (“greenhouse”) radiation.
In the situation shown in Fig. 4, the sun directly warms the planet. In addition, the planet is warmed (just as in Fig. 3) by the radiation from the inner surface of the shell. As a result, the planetary surface ends up absorbing (and radiating) 480 W/m2. As a result the temperature of the surface of the planet is much higher than in the previous Figures.
Note that all parts of the system are still in equilibrium. The surface both receives and emits 480 W/m2. The shell receives and emits 240 W/m2. The entire planetary system also emits the amount that it receives. So the system is in balance.
And that’s it. That’s how the “greenhouse effect” works. It doesn’t require CO2. It doesn’t need an atmosphere. It works because a shell has two sides, and it radiates energy from both the inside and the outside.
The “greenhouse effect” does not violate any known laws of physics. Energy is neither created nor destroyed. All that happens is that a bit of the outgoing energy is returned to the surface of the planet. This leaves the surface warmer than it would be without that extra energy.
So yes, dear friends, the “greenhouse effect” is real, whether it is created by a transparent shell or an atmosphere.
And now, for those that have followed the story this far, a bonus question:
Why is the above diagram of a single-shell planetary “greenhouse” inadequate for explaining the climate system of the earth?
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Willis Eschenbach says on November 28, 2010 at 3:47 pm – In theory there is no limit to how much you can concentrate energy (slow down its loss) in this manner.
Well, there is, as I have shown by arguing in extremis. There has to be some property of the shell (or the walls of the Dewar) that allow mechanisms to operate in a realistic and probabilistic time. For example, your shell will take forever to warm up on the inside if it is hugely massive and it will not warm up much at all if it is so thin that energy transfer is of low probability. Your shell has to have properties that allows it to act as a shell and I suggest that that one property is appropriate mass.
To the extent that mass is relatable to density in a real life situation, I think that this is part of the reason for the decrease of temperature with altitude to the tropopause. We are argiung about how much importance that “part” has.
Steven Mosher says:
……….”The woods experiment is a farce.”………
R W Woods has been often described as the best experimental Physicist that America ever produced.
Yet you describe his experiment and comment as a farce!
His experiment still stands.
It has been verified for much larger structures such as the polyethylene polytunnel.
The IR radiative effects of CO2 and H2O exist but in the context of the troposphere they play an insignificant role.
You can call this insignificant role the “greenhouse effect” if you like.
Leonard Weinstein with William C Gilbert set out a much more coherent explanation of heat transfer in the troposphere on SoDs site.
It involved convection modifying the gravitationally established lapse rate.
and in my opinion this explanation is correct.
The radiative effect becomes more important above the tropopause where it radiates away the much longer end of the IR spectrum.
My point about raising the question of Woods experiment was to underline the fact that the radiative effect is so small, in contrast to the convective heat transfer, that it is almost unnoticeable.
davidmhoffer says:
November 28, 2010 at 7:59 pm
Thanks. Wow, just wow.
The Climate System is CHAOTIC.
Another experiment,
I’ll hang up the sun for 1000 years.
And in the distant future (3010).
The sun will be turned on again.
If AGW theory is correct. November 4010 will be the hottest year in history.
‘davidmhoffer says:
November 28, 2010 at 4:18 pm
Insert Willis.
I send you a check for $240. You balance is $0. My balance is $240
You send Willis a cheque for $240. Your balance is $0, My Balance is $0, Willis is $240
Willis sends Me a cheque for $120 and you a cheque for $120.’ Willis balance is $0 Your and my balance is $120 each , So where is the extra $120
You now have an extra $120, so the next day…
This is a topic that is very frustrating to me. Clearly the atmosphere causes the temperature to be warmer. I was baffled when I initially found people objecting to the GHE as a whole.
I do disagree with the diagrams posted though. I have re-done the Energy Balance by Trenberth (08) and by focusing on the NET transfers I get a solid GHE with the surface transferring 120 W/m2 to the troposphere. That is enough to warm the first km of the troposphere up 9.3C.
The energy balance and the GHE need to have discussion picked up a notch. 🙂
John Kehr
The Inconvenient Skeptic
Vince Causey says:
“But transfering heat energy from “cold” to “warm” violates the second law of thermodynamics.
There is no way around that.”
“There is a lot of misunderstanding surrounding the second law of thermodynamics.”
This is right 😉
“The original formulation of this law says approximately what you attribute to it. This was applied to heat transfers in conducting bodies. The modern formulation is in entropy and states that in a closed system, the total entropy must always increase. The application to heat transfer has been changed to refer to net heat transfers. ”
There is no such thing like “net-heat”. How should a flow of energy know that an other one exists and it has to accept to get netted??? The rule is simple: there is NO transfer of heat from cold to warm. Full stop.
“The implications to conduction of heat have not changed. However, we know that radiative transfers do indeed take place in both directions.”
You know that? WOW !!!
“In order for the second law to be obeyed, the net transfer must always be from warmer to cooler. This should be intuitive – even a cool body emits photons of energy which must travel at the speed of light regardless of the fact that they may impinge on surfaces that are warmer. The net transfer argument should make it plain that more photons will be emitted from the warmer surface. ”
Wron question! Right qestion is: can the photons, emitted by the colder body transfer their energy to the warmer body? Answer: NO
“A modern version of Maxwells demon thought experiment is to imagine a demon sitting between the cool and warm surfaces. The demon is supposed to allow the photons from the cooler surface to reach the warmer surface, but prevent the flow in the opposite direction. The action of the demon would cause the cool surface to become cooler and the warm surface to become warmer. This is clearly in violation of the second law. The Maxwell demon paradox was solved when it was realised that the demon needs information to sort the photons and this prevents the system from being closed.”
The second law applies to open AND closed systems
“In practice, although photons from the cool surface carry energy to the warm surface, a higher flow occurs in the opposite direction. The result is that the warm surface cools and the cool surface warms. Ie, they equilibriate and their entropy increases. Thus, back radiation does not violate the second law.”
This is voodoo-physics
davidmhoffer says:
November 28, 2010 at 7:59 pm
Excellent!
pwl says:
November 28, 2010 at 7:40 pm
“How can the troposphere, the imaginary shell(s), at -65 c warm the surface at +15 c?”
It cannot. But it can slow down how fast the surface can give up heat. The energy transfer is moving from surface to the black of space which sits at 3 degrees above absolute zero. Throw anything in the way which isn’t completely transparent to the infrared frequencies involved and the transfer rate slows.
The problem for the warmists is that things can also be thrown in that speed up the transfer where that “thing” is evaporation and convection which skips around the radiative transfer and mechanically transports a massive amount of energy as latent heat of vaporization in the H2O molecules and where that latent heat is released high in the troposphere or in some stronger convective cells even in the stratosphere.
After this surface energy is mechanically transported thousands of meters above the ground and released the underlying greenhouse gases now make it more difficult for that energy to radiate back towards the surface and thus make the radiative path upwards to space even easier than it would be absent any greenhouse gases.
Heat transfer from surface to space occurs by convection, conduction, evaporation, condensation, and radiation. All must be considered simultaneously because all three happen together. Isolating only the radiative transfer only works in a vacuum and doesn’t tell much of the story on a planet with a thick atmosphere and a surface covered with liquid water. In the end, as long the water cycle is running (temperature above freezing and below boiling), the water cycle is the dominant player in how the earth warms during the day and cools during the night. If the earth is covered by glaciers and sea ice then and only then do non-condensing greenhouse gases like CO2 play a dominant role.
The only important thing CO2 does is raise the average surface temperature above freezing then the water cycle takes over and limits how warm it can get. If more non-condensing greenhouse gases are added the extra energy in the system simply speeds up the water cycle and negates (evidently) about 75% of the potential surface temperature increase of the non-condensing greenhouse gases. It does this, as described above, by mechanically transporting heat away from the surface in the form of latent heat of vaporization in water molecules.
davidmhoffer says:
November 28, 2010 at 7:59 pm (Edit)
Steven Mosher;
If you want to argue about feedbacks, that’s fine. That question is open for debate. If you want to argue that known working physics doesnt work, well then there is no debate.>>
Oh yes there is. There is a huge debate. Massive in fact
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Well said.
old construction worker,
the problem with trying to apply accounting analogies is that they depict an instantaneous position whereas the situation is one of flows. I prefer the following analogy:
Imagine a line of people filing into a building through the in-door, at the rate of 240 people per hour, and filing out the other end through the out-door. This represents the energy flow arriving and leaving the earth. Now suppose that half of the outgoing people pair up with those on the incoming line. You now have 360 people per hour entering the building. These extra people haven’t materialised out of thin air – they came in the original line.
bessokeks says:
November 29, 2010 at 6:24 am
“There is no such thing like “net-heat”. How should a flow of energy know that an other one exists and it has to accept to get netted??? The rule is simple: there is NO transfer of heat from cold to warm. Full stop.”
Not by conduction at any rate. Radiative transfer is an exchange. That’s why we can see a satellite from the ground and the satellite can see us looking at it. Radiation from the satellite is reaching us and radiation from us is reaching the satellite. The satellite might be much colder or hotter than the ground observer yet each can still observe the radiative energy from the other.
Whether or not there is any net transfer of energy it will be from the more energetic source to the lesser. If they are both exactly the same temperature the exchange is exactly even but in no case does the radiation from one not fall upon the other.
bessokeks says:
“There is no such thing like “net-heat”. How should a flow of energy know that an other one exists and it has to accept to get netted???”
It does not need to know. The term net energy flow means that more energy will flow from the warmer body to the cooler body than the other way round. This is a simple consequence of the warmer body having a higher radiative flux as per Stefan Boltzman.
“You know that? WOW !!!”
Yes.
“Wron question! Right qestion is: can the photons, emitted by the colder body transfer their energy to the warmer body? Answer: NO”
Let us consider the question in stages. Can a cooler body emit a photon? Clearly yes. Can that photon impact a cooler body? Obviously yes. Can it also impact a warmer body? Yes. Does a photon add energy on impact? Yes – a photon absorbed by an atom will raise an electron to a higher energy state. Therefore, ergo demonstratum, the body has gained energy.
“The second law applies to open AND closed systems.”
Yes, but it makes no difference to the argument.
“This is voodoo-physics.”
Sorry to disappoint. The modern form of the second law is about entropy, that must increase. Any closed system in which entropy decreased would be in violation of the second law. As long as the cooler body gets warmer and the warmer body gets cooler as they approach the same temperature, then entropy has increased to a maximum.
@Vince Causey says:
“Does a photon add energy on impact? Yes – a photon absorbed by an atom will raise an electron to a higher energy state. Therefore, ergo demonstratum, the body has gained energy.”
This is an often found mistake: raising an elektron has nothing, absolutely nothing to do with transfer of heat. When, in our case, heat is transferred, the molecules will raise the frequenzy of the intermolecular binding. The energy to raise electrons is much higher than what we get from our infrared radiation.
Please also be aware, that the second law ONLY applies on heat, not on energy in general. The frequencies which make you “see” the satellite are far from the infrared spectrum emitted by a warm body
@Vince Causey says: (2)
“The term net energy flow means that more energy will flow from the warmer body to the cooler body than the other way round. This is a simple consequence of the warmer body having a higher radiative flux as per Stefan Boltzman”
Imagine a small, warm body allmost completely surrounded by a big, rel. colder body.
The amount of energy radiated from the colder body to the warmer body will be much bigger than the amount of energy that is radiated by the warmer body to the colder one. If the energy of the colder body would be absorbed by the wormer one, it would heat up. But it does not…
@Dave Springer says:
““How can the troposphere, the imaginary shell(s), at -65 c warm the surface at +15 c?”
It cannot. But it can slow down how fast the surface can give up heat.”
NO! Nothing can influence the surface to give up heat. It only a function of the temperature of that surface and its factor of emissivity
Well Willis, your model does what it claims to do; demonstrate that a “Greenhouse effect” as we understand that term (which is not the way greenhouses work) can raise the temperature of a glass enclosed planet.
Too bad your model is of an infinitely conductive thermally earth that does not rotate.
Your glasss earth does not explain how climate works; but it does show that a greenhouse effect is possible.
bessokeks, Vince Causey,Dave Springer
There seems to be a great deal of confusion when the word “heat” is used.
There is a huge difference between the vernacular use of “heat” and the thermodynamic use of “heat”
Vernacular use tries to convey the idea of “hotness” and “temperature” and even the sensation of taste of a curry.
Thermodynamic use of “heat” is very precise and it deals with the process by which thermal energy is transferred from a higher temperature object to a lower temperature object.
Heat always travels from hot to cold never the reverse.
Thermal energy can be radiated and absorbed by both hot and cold objects.
Heat (thermodynamic use) has the capacity to do WORK which again had a precise meaning.
If you are in any doubt about whether “heat” is involved use the “WORK” test it always sorts out the confusion.
Have a look in a thermodynamics text book particularly the chapters dealing with the Carnot cycle and the second law, there any loose use of the word “heat” would render the chapters uninteligable.
Vince Causey and bessokeks:
The waters surrounding the issue of a second law violation by the “back radiation” are muddied by a widespread misuse of the word “heat” by climatologists. The radiative energy flux that is incident upon a surface at a space point is not a heat flux; it is properly referenced as a “vector irradiance.” The radiative energy flux that is reflected, emitted or transmitted through the surface at the same space point is not a heat flux; it is properly referenced as a “vector radiosity.” Climatologists err in referencing the vector irradiance and vector radiosity as “heat fluxes.” As neither is a heat flux, neither the vector irradiance nor the vector irradiance is bound by the second law.
By its definition in thermodynamics, the “heat” is the energy that crosses a boundary. It is the heat flux that is bound by the second law. A component of the heat flux may be the radiative heat flux. At a space point, the radiative heat flux is the vector difference of the vector radiosity and vector irradiance.
The entity which, in a Kiehl-Trenberth diagram, is labelled as the “back radiation” should be relabelled as a “vector irradiance” with specified spectral characteristics. Relabelling it would help to eliminate the appearance of a second law violation.
The Maxwell demon paradox was solved when it was realised that the demon needs information to sort the photons and this prevents the system from being closed.
Then what prevents information from playing a role in all energy transfers? Why would information only become important in this transfer? And what is the decision making process of the energy when given this information?
Remember. The increase of entropy is added to the demon.
Well, this link you can play at Maxwell’s demon
http://www.imsc.res.in/~sitabhra/research/persistence/maxwell.html
bessokeks says:
November 29, 2010 at 10:06 am
@Dave Springer says:
““How can the troposphere, the imaginary shell(s), at -65 c warm the surface at +15 c?”
It cannot. But it can slow down how fast the surface can give up heat.”
NO! Nothing can influence the surface to give up heat. It only a function of the temperature of that surface and its factor of emissivity
By the same token, nothing can influence how well a surfaced absorbs photons except the emissivity of the surface. So if there are lots of incoming photons, the surface will absorb more energy; if there are few incoming photons, the surface will absorb less energy. This extra absorbed energy will slow the cooling of surface.
Consider a 303 K rock in outer space (and assume the rock is a perfect black body & it is far from any stars). The outgoing IR radiation is 480 W/m^2. The incoming radiation from the 2.7 K cosmic background is going to be around 0.000005 W/m^2 = 0 W/m^2 for all practical purposes. The net loss is 480 W/m^2
Put the same rock (in a vacuum container to avoid conduction & convection) in a freezer at 255 K. The rock still radiates 480 W^m2. But the container radiates 240 W/m^2 back to the rock (again assuming the container is a blackbody). The net loss is 240 W/m^2. The rock will cool half as fast as before (at least initially).
Put the same rock (in a vacuum container again) in a 303 K room. The rock still radiates 480 W/m^2. But the room radiates 480 W/m^2 back to the rock. The net loss is 0 W/m^2. The rock will stay 303 K.
However you want to say it, the net result is that the cooling rate of the rock is slower based on the surrounding environment.
bessokeks says: November 29, 2010 at 10:01 am
Imagine a small, warm body allmost completely surrounded by a big, rel. colder body.
The amount of energy radiated from the colder body to the warmer body will be much bigger than the amount of energy that is radiated by the warmer body to the colder one. If the energy of the colder body would be absorbed by the wormer one, it would heat up. But it does not…
Sorry, but you are wrong again. The real consideration is the solid angle subtended by the radiating source. If the cooler container completely surrounds the hot object, then the rate that energy arrives at the hot object is the same, whether the container is a little larger or much larger.
Willis’ model is ridiculous because planets rotate, allowing a heated side to radiate to space at night. His model would only apply to a body that had zero rotation and had a surface that constantly faced the sun.
Bassokeks must have a very inefficient air conditioning system if it stops working as soon as the temperature inside the house is less than that outside.
The second part of this quote (from Vince Causey) is correct except that an photon in the infra-red region excites the molecular vibrations, not the electronic levels, which means the nuclei move with increased kinetic energy, which means ‘hotter’ at the molecular level. And work has to be done to achieve this, so it is heat by the classical thermodynamic definition as well.
so Bessokeks reply to this
is in fact, nonsense. There is absolutely nothing to stop a photon from a colder object being absorbed by a warmer object, it is just that there are more going in the opposite direction. By the way, this can apply to conduction as well – if you think carefully about what happens when you allow two gases at different temperatures to mix, you will realize that some of the flow of both molecules and energy has to be from what was ‘colder’ towards what was ‘hotter’. Again it is just the net flow that is hot to cold.