2010-09-13 02:36:42Basic rebuttal 68: 2nd law of thermodynamics contradicts greenhouse theory


Hi folks,

 fourth attempt! Getting closer???


Skeptics sometimes claim that the explanation for global warming contradicts the second law of thermodynamics. But does it? To answer that, first, we need to know how global warming works. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming.

Global warming, in a nutshell, works like this:

The sun warms the Earth. The Earth and its atmosphere radiate heat away into space. They radiate most of the heat that is received from the sun, so the average temperature of the Earth stays more or less constant. Greenhouse gasses trap some of the escaping heat closer to the Earths surface, making it harder for it to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the greenhouse gasses make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation.

The second law of thermodynamics has been stated in many ways. For us, Rudolf Clausius said it best:

"Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature."

So if you put something hot next to something cold, the hot thing won't get hotter, and the cold thing won't get colder. That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil!

The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth. If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics.

So have the climate-scientists made an elementary mistake? Of course not! The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference.

To see why, consider that blanket that keeps you warm. If your skin feels cold, wrapping yourself in a blanket can make you warmer. Why? Because your body is generating heat, and that heat is escaping from your body into the environment. When you wrap yourself in a blanket, the loss of heat is reduced, some is retained at the surface of your body, and you warm up. You get warmer because the heat that your body is generating cannot escape as fast as before.

If you put the blanket on a tailors dummy, which does not generate heat, it will have no effect. The dummy will not spontaneously get warmer. That's obvious too!

Is using a blanket an accurate model for global warming by greenhouse gasses? Certainly there are differences in how the heat is created and lost, and our body can produce varying amounts of heat, unlike the near-constant heat we receive from the sun. But as far as the second law of thermodynamics goes, where we are only talking about the flow of heat, the comparison is good. The second law says nothing about how the heat is produced, only about how it flows between things.

To summarise: Heat from the sun warms the Earth, as heat from your body keeps you warm. The Earth loses heat to space, and your body loses heat to the environment. Greenhouse gasses slow down the rate of heat-loss from the surface of the Earth, like a blanket that slows down the rate at which your body loses heat. The result is the same in both cases, the surface of the Earth, or of your body, gets warmer.

So global warming does not violate the second law of thermodynamics. And if someone tells you otherwise, just remember that you're a warm human being, and certainly nobody's dummy.

2010-09-13 05:40:03Too long



This explanation is a bit wordy. If you can boil it down to a few brief points, I think it will actually be more easily understood.

I think it goes more or less like this:

- The 2nd law of thermodynamics forbids a process that, in net, transfers heat from a colder to a warmer component, without work being put into the system.

- It does not forbid some transfer of heat from a colder to a warmer component, provided the net transfer is from warmer to colder.

- In the greenhouse effect, the net flow of energy is from the Sun to the Earth (specifically the ground), from the ground to the atmosphere, and from the atmosphere to space. Each of these flows is from warmer to colder.

- There is no problem with some transfer of energy from the atmosphere to the ground, as long as it does not reverse the sense of the net flow. The greenhouse effect does not reverse the net flow.

This explanation may seem a bit abstract. However, what is supposed to be argued is actually abstract: a non-violation of a law that forbids a particular result. I guess the most boiled-down version would just say that the flow of energy is definitely up & out: Otherwise, if the net energy flow were from the atmosphere to the ground, the Earth would be accumulating big time, because none of the heat from the Sun would be able to go anywhere!


2010-09-13 18:34:26
Paul D


Actually this is an ideal subject to use diagrams to explain the textual descriptions.

It might also help you reduce the amount of text!

I have to say that this sort of basic rebuttal is ideal to get a bigger audience. People can relate to these sort of basics and so will be more interested. Thermodynamics effects people in their daily lives.

So I would be keen to see this one completed and lets see some diagrams.
I mean you even take a desk lamp as an example.

You could even have an animation to show what happens.

2010-09-13 20:36:34Diagram
John Cook

If you need a diagram created, just let me know what you need and I'll whip something up for you
2010-09-15 04:56:45


I too think this is a bit wordy but probably only because you're keen to be thoroughly explicit.

I'd emphasize the notion that objects radiate at a certain rate at a given temperature and if supplied with a source of heat derived from whatever source thus have a gain/loss budget. All sources of heat must be accounted for in this budget and thus any energy radiated by a cooler object and absorbed by a warmer object must be accounted for in that warmer object's budget. You do actually work through that case but I think your level of detail obscures the simplicity of the point.

As others suggest, illustrating this is amenable to a graph that could also be tied to a simple numerical example of the notion of a budget.

That leaves the problem of "intelligent photons" but such animism is beyond the scope of this article.  :-)

2010-09-15 06:06:43


thanks for the suggestions so far. I agree it's too verbose, but as Doug says, i wanted to be explicit. I think people will take it on board better if they can understand it in their own terms, rather than just accept that someone tells them it's so. That's why I want to get in an analogy they can think about for themselves.

I'll shorten it, and get the diagrams in. I'm a bit busy this week so it'll probably not happen before the w/e. If that's too slow, someone should speak up, otherwise, I'm very happy to keep on with it.

Thanks again for the input, at least I'm not wildly off-base.

2010-09-20 02:45:35post updated



 i've updated the post, and included a graphic I found. Let me know what you think, please.


2010-09-20 07:16:18Sticking my oar in...
Graham Wayne

Hi Tony,

I don't think you address the mechanism that Neal refers to - where work is done (and the 2nd law is therefore respected). Heat can be measured as kinetic energy (vibration of molecules) or electromagnetic energy (long-wave radiation). Long wave radiation can be re-radiated into space, but the kinetic energy remain in the medium that stores it. This storage process is the 'blanket' of GHGs that keeps the Earth 30 degrees C warmer than it would be otherwise. A blanket delays the movement of heat to cold: greenhouse gasses change the electromagnetic energy into vibrational energy, and delay its escape. In order to do this, work is done in the conversion of EM to kinetic energy.

But CO2 can only absorb so much heat, and will eventually emit a photon of long wave radiation. When it does so, the direction the photon takes is random, and so some heat is scattered back into the atmosphere, while the rest continues into space. CO2 is like a mirror-ball in a dance hall - it scatters heat in all directions.

2010-09-20 07:39:58
Paul D


Oh %$****. I just wrote a detailed post, pressed a key on my keyboard and lost the whole thing!


Your new version is quite messy, I don't feel it addresses the title.

Also my deleted post said something similar to gpwayne regarding energy descriptions.

You need to be consistent with the way you refer to different energies/frequencies, I don't feel you do that.
Reading the new version, I think I am reading about the greenhouse effect and not much else.

2010-09-20 19:24:58
Paul D


I think the problem is probably at what level the 2nd law is applied.
If you think about it even the 'model' that Neal defines, breaks the 2nd law in the literal eyes of a contrarian. eg. It assumes a simple model of Earth and Sun.
eg. energy travels from the Sun into space, which is 'colder', but if the 2nd law is taken literally using a contrarian POV (they often say 'space is 'cold' therefore 'back radiation' defies the 2nd law' or other similar memes) then sunlight effectively defies the second law and goes from the coldness of space to a warmer Earth. Or again from a contrarian view, really the Earth should reach the same temperature as the Sun, because the flow has to be from hot to cold, until the colder object is the same temperature as the hotter object, because the cooler object 'can't' emit back out. But as Neal rightly points out the Earth does emit energy/heat.

From a contrarians view and if someone took the 2nd law literally, you would have intelligent energy that would bend round corners to satisfy the 2nd law.

If I imagine myself as a photon being emitted by any atom/molecule, I couldn't care a monkeys didgeridoo whether I'm heading into space or towards a lump of concrete on the ground. Assuming gravity isn't particularly strong, I'm heading off in a straight line until I hit something else. So in one direction (space) I have left the closed system of the earth and have 'cooled' it a small amount, but in the other direction (towards the ground) I get absorbed and temporarily increase the grounds energy by a tiny amount, until a 'comrade' photon gets emitted back up through the atmosphere and eventually makes it back out to space.

I think wikipedia highlights the issue (my emphasis):
"The second law is an expression of the fact that OVER TIME, differences in temperature, pressure, and chemical potential tend to balance out in an isolated physical system."
The words 'over time' are important, all systems behave correctly 'over time'. They may not do so instantaneously, especially at the quantum level.


Is the 2nd law good science?
Is it a very rough model that is passed it's use by date?

2010-09-20 20:09:10
Paul D


In the original rebuttal by John, he references a rebuttal by Joshua Halpern, which has a useful explanation in the abstract:


Also one paragraph in Johns rebuttal probably sums up what the Basic rebuttal should focus on:

"The net flow of radiant heat is still upwards from the surface to the atmosphere, because the upwards thermal emission is greater than the downwards atmospheric backradiation. This is a simple consequence of the second law of thermodynamics. The magnitude of the net flow of heat is the difference between the radiant energy flowing in each direction. Because of the backradiation, the surface temperature and the upwards thermal radiation is much larger than if there was no greenhouse effect."

I think if you base your Basic rebuttal around these concepts and forget about any detailed description about the greenhouse effect, then you might avoid the complexity. If someone wants to know about the green house effect then offer links to suitable Skeptical Science pages or other sources.

2010-09-24 01:08:41
Paul D


Hi is this image any use??
If it isn't quite correct, or you need numbers on it etc, just say.

It is supposed to show the net flow of energy is into the atmosphere. If I got it wrong I can change it.

2010-09-24 02:42:44


If it's of any help, the blanket analogy has been found via formal research to be resonant and useful in the minds of laypersons.

Analogies don't work against "skeptics" but archetypal skeptics are not the audience for basic rebuttals.  Skeptics insist on ignoring the tool-like properties of analogies, somewhat akin to refusing to acknowledge the utility of a hammer, but analogies actually -are- useful in the ordinary course of cognition, for normal people trying to understand a topic.  

I think you need to stay away from the concept of wavelengths. The nut of the argument you present here:

"Greenhouse gasses, such as CO2, absorb the earths radiation, and in turn re-radiate the energy they absorb. Some of this re-radiated energy is re-absorbed by the earth, meaning that the earth has to radiate even more energy than it would without the greenhouse gasses in the atmosphere. To radiate that extra energy, the earth has to warm up, and voila, you have global warming!"

is just fine as it is. Coupled with your diagram this adequately tells the story. 

2010-09-24 05:40:29
Paul D


Doug The original diagram doesn't show that the ground radiates more energy than it absorbs as backradiation. The arrows imply similar amounts of energy.

That is the main point about the 2nd law John is getting over in the original rebuttal. Ideally any diagram should re-enforce the text.

I would write something like:

"Greenhouse gases, such as CO2, absorb the heat radiated by the Earths surface, and in turn re-radiate the heat they absorb. Some of this re-radiated heat is re-absorbed by the surface, meaning that the surface has to radiate the re-absorbed heat plus the energy it has absorbed from the sun, this total radiated heat energy is more than would be the case without the greenhouse gases in the atmosphere. To radiate the extra heat, the Earths surface has to warm up, and voila, you have global warming!"

 BTW technically 'earth' is a name, so it should be Earth.

Added: You really have to be careful how you refer to different things.
Some people would understand Earth as including the atmosphere, so you need to be clear that it is the surface that is being referred to.

2010-09-24 06:05:04


sorry I haven't been more responsive here, but I get very little free time to work on this. Thanks for all the comments and feedback, it's certainly got me thinking clearer, and I'll revise the text again at the w/e.

2010-09-24 10:30:33

Agree w/ The Ville that the diagram should unambiguously reflect the notion of budget. 
2010-09-27 00:10:52third draft


I've just put up the third draft.

I've completely dropped the idea of talking about component flows of radiation in the atmosphere because, I realised, they are not actually the relevant point here. The component flows are relevant to energy conservation, but do not in themselves explain why the second law is not violated. The key there is actually the fact that the flows are not spontaneous, because they are driven by the sun. So I've re-worked it around that theme.

Although I don't say it in this basic rebuttal, in fact, the reason the second law is not violated is that by considering the Earth and the GHGs, we're only considering part of the system. The 2nd law will apply to the system as a whole, but not to subsets of it which have an external forcing from another component.

 If people are happier with this version then I will make a diagram that shows the heat-flows in a fridge, with the mechanical forcing of the heat-flow, and compares it with the heat-flows in the atmosphere, with the sun in the driving seat.

2010-09-27 00:25:49

I hate to say it, Tony, but I think introducing a heat pump into the explanation makes it more opaque. You start w/a blanket analogy; why not show how that analogy is appropriate by explaining the radiative properties of a blanket w/ regard to the energy budget of a human body? In this case, body heat liberated by metabolism is "the sun."
2010-09-27 01:05:40


Couple of problems:

- HEAT does not flow from cooler to warmer; but ENERGY can move from cooler to warmer, as long as it does not result in a net flow of HEAT (without external work).

- It is OK for RADIANT energy to go from anywhere to anywhere; but the net transfer of radiant energy will still be from warmer to cooler. But this is not HEAT FLOW.

I think the problem here is that it is not clear what point is to be made. You are trying to prove that the 2nd law is not being violated; but the meaning of the 2nd law is not that straightforward to state; so the explanation that it IS NOT VIOLATED is even more difficult to make clear.

So what is the actual complaint? "The greenhouse effect only makes sense if heat is flowing from the cool atmosphere to the warm ground."

This is incorrect, because:

- The greenhouse effect is a mechanism that operates in the transfer of radiant energy and heat from the ground to the upper reaches of the atmosphere: No net transfer of energy, or heat, from cooler to hotter is suggested.

-  The mechanism by which it works is to slow down the escape of radiant energy from the upper reaches of the atmosphere into space. Because radiant energy is "on the move", what doesn't escape right away bounces around, sometimes heading downward. But it doesn't change the fact that the net flow of radiant energy is up and out.

- Analogy: You get a shotgun and fire up into the air. If the sky above you is not blocked by trees, the shot go up and out. But if you're standing under a leafy tree when you fire up, a lot of the shot will be bounced back down at you, although a lot will escape. Despite the backflow, the net transport of shot will be up and out. The fact that shot goes from the gun to the sky is not ultimately affected, even if some of it is blocked. And in the case of infrared radiation, it is as if all the bounced-back shot were collected up and put into new shotgun shells to be fired off again. Ultimately, every shot will be launched.


2010-09-27 06:36:05


Doug, the energy-budget is not the real issue here. That just explains conservation of energy, and does not explain why the second law is not violated. The second law is not violated because considering only energy transferred between the Earth and the atmosphere does not constitute a closed system. It's being externally driven by the sun.

I agree, I could go with the blanket analogy, but I personally find the fridge analogy more useful.

Neal, you are using the term 'net transfer' to refer to components of energy transfer in the system. Energy transferred each way between the Earth and the atmosphere are only components of the energy flow of the whole system, which also includes the sun and space itself. The net transfer of energy between the (Earth+atmosphere) and (sun+space) is zero. I am not suggesting that there is a net transfer of energy from the atmosphere to the Earth, but there is a component which transfers energy in that direction.

The fact that the radiant energy from the Earth+atmosphere actually escapes into space from the top of the atmosphere is not important. Were there less CO2, but still some, such that the atmosphere absorbed part of the IR radiation from the surface, but not all of it, there would still be a greenhouse effect.

The key point to answer is, "why is the 2nd law not violated?" The answer is that by considering only the atmosphere and the Earth, you are not considering an isolated system. The 2nd law does not apply separately to components of a system (that's what 'without external work' means), so is not being violated.

2010-09-27 08:12:45



It is not really appropriate to add the Sun plus space to make one system, and to contrast to that the Earth plus atmosphere: The overall flow is from the Sun out into space, with the Earth plus atmosphere as a way-station along the way.

2010-09-27 17:25:16

the second law applies only to closed systems. The earth+atmosphere is not a closed system, because it does not include the source (sun) and sink (space) for the energy. The fact that you are conserving energy by matching incoming and outgoing flux does not matter, you still have energy flowing across your boundary. It is wrong to apply the second law to the Earth+atmosphere alone.
2010-09-27 17:40:01

It's not at all easy to explain the 2nd law in simple terms.
One source of confusion is the use of heat, energy and thermal energy interchangeably. And indeed starting from maybe middle school kids are taught that heat IS energy. For example, we often read that Stefan-Boltzman law states that a body releases heat as σ*T^4, which is not true. That's energy, not heat. Heat is a net flux of energy between two bodies due to a temperature difference, or the energy exachanged between them.
It is strange that this problem arises only with radiative transfer. The fridge analogy, i.e. heat conduction, should rise the same question. The thermal insulator is at a temperature somewhere between the low and high temperature sides; how come that a warmer object makes the fridge cool more?
Maybe you could try to begin with the distinction between heat and energy, then the fridge analogy, highlighting that the warmer thermal insulator makes the fridge cool more by reducing the transfer of heat. Not sure it will work but I think it's worth a try.
In any case, try to keep the terminology clear and consistent.

2010-09-27 18:18:10



The reason a refrigerator works is because work is done on part of the system: Just as a work-producing engine takes energy from a hotter source, dumps energy at a cooler sink, and takes the difference in work, a refrigerator takes energy from the cooler source + work and dumps the result as energy at the hotter sink. A refrigerator is an engine run backwards.

I think the fundamental problem with this issue is that it's not terribly clear what we are trying to convince the reader of. The claim to be rebutted is, "The greenhouse effect violates the 2nd law of thermodynamics." The question is, What does the reader/complainant think that this statement means? In fact, the GHE doesn't violate the 2nd law of thermodynamics; but it's hard to make that point clearly if the reader doesn't have a clear idea of what s/he means by the 2nd law anyway. And how much of the reader's attention will it take to become educated about what the 2nd law does and does not mean? 

We are discussing it here using terminology that is not very precise, and that is part of why we are having problems communicating about how best to write a Basic rebuttal. I think the topic is not so simple, and the right way to proceed is to write the argument fully and correctly for the Advanced level, where you are not constrained to overly simple language and reasoning; and then see if it's possible to make a Basic rebuttal from that. 

2010-09-27 19:59:26



i know how a refrigerator works but as for the analogy, it doesn't matter if the source/sink of heat takes the form of an engine or whatever. The point of the analogy is the reduction of heat conduction through the insulator. The parallel with the GHE should be clear.

The 2nd law in simple terms is usually stated as "heat spontaneously only flows from the hot to the cold object". The apparent contradiction of this law by the GHE is that it is interpreted as a flow of heat from the colder object, the atmosphere, to the warmer, earth surface. My point was that the cause of this error is the confusion between heat and energy.

Anyway, I agree that a better procedure would probably be to write the advanced version first.

2010-09-27 20:33:13
Paul D


Thanks Riccardo for a simple explanation about heat and energy:

"That's energy, not heat. Heat is a net flux of energy between two bodies due to a temperature difference, or the energy exchanged between them."

I confess I probably mixed the two up myself. My earlier description should have referred to IR radiation rather than heat radiated.
However I think it is important that words used are understandable outside the physics community. I'm not sure what the best approach is.

All this talk of fridges, reminds me of a recent fridge invention that works by heating the device up on an open fire. It's designed for places like Africa, where you might not have access to electricity. It's a small cannistor, I think when you remove it from the fire, it cools inside and you can store vaccines etc. for hours or days.

2010-09-28 03:04:46


the Ville:

Technically, one should explain it w/o simplification, to see what it takes to get the explanation done properly. 

Then you can see what you can afford to cut out w/o doing violence to the essential point of the explanation.

wrt you open-flame refrigerator: I guess a dumb way to do it would be to run a DC refrigerator off the stored heat. I don't know of a smart way to do it.



I agree with you that the point is that it is conducted heat flow that is forbidden, and not radiant heat transfer. 

The problem is that anyone who can understand that distinction is never going to get caught up in this false paradox anyway.

For people who don't see that point, all you're likely to get back is:

"Oh, you're saying the 2nd law doesn't apply to this situation? Uh, OK." 


"Oh, you're saying the 2nd law doesn't apply to this situation? Why should I believe you?"


The same problem applies to the open-system/closed-system distinction: The difficulty is not in understanding the answer, the difficulty is in explaining what the question means.

2010-09-28 06:23:49
Paul D


"wrt you open-flame refrigerator: I guess a dumb way to do it would be to run a DC refrigerator off the stored heat. I don't know of a smart way to do it."


It doesn't work quite how I remember it.

Arrgh, this editor is rubbish, I just lost a load of text again!

2010-09-29 08:31:37
Paul D


Is one of the authors here also the author of Science of Doom??

Because Science of Doom has posted a detailed article on the 2nd Law of Thermo:


"Let’s avoid a semantic argument about the correct or incorrect use of the word “heat”. "

2010-09-29 12:51:48


I confess I'm experiencing a complete disconnect with the cognitive gulf here. I'm no genius; I just don't understand what's so difficult about

"If an object is radiating energy upwards and that radiation encounters nothing that will absorb some of the upwardly radiated energy and reradiate it downwards to be reabsorbed by the radiating object, the radiating object will be cooler than otherwise." 

Anything else requires smart photons, photons that ignoring their wavelength will selectively permit their own absorption, or not.  

Even counting "breathing causes global warming" I've never before encountered such stubborn obduracy. Spencer tried to sort this out; the man is endlessly patient, explained it myriad ways, to no avail. 

Don't bother trying to make this airtight, it's a cognitive psychology problem pointing straight to "denial" in the technical sense.  

2010-09-30 06:29:59


let me see if I can try to break this down a bit:

1) 2nd law only applies to closed systems, not to systems that are open

2) a closed system is one that includes all the sinks and sources

3) the Earth+atmosphere taken together, on their own, does not make a closed system. The heat-source (sun) and heat-sink (space) are not included

4) therefore the 2nd law does not apply to considerations involving only Earth+atmosphere

5) therefore 2nd law is not violated, and the skeptic is debunked

So far, so good. Now on to the fridge analogy and how I am trying to explain the non-violation

6) apart from the sign of the temperature differential, the situation with the fridge & the Earth is identical: Heat-pump (sun or fridge-mechanics), target-medium (surface of earth, inner volume of fridge), heat-sink (outer space, the room), and something between target-medium and heat-sink (GHGs, fridge insulation).

7) explaining that the fridge stays cold because heat is pumped out, and that the insulation determines how cold it gets for a given rate of pumping, is directly equivalent to explaining that the Earth is warm because heat is pumped in, and that the GHGs make it warmer still.

8) because this explanation doesn't involve the distinction between heat and energy, I make no attempt to explain that distinction. As people have noted, it's not a trivial thing to explain, and certainly not easy to explain to your granny. They'll think you're trying to hoodwink them, so it's best left out if possible.

9) since the debunking relies only on the non-closure of the system, let me say explicitly, radiative properties of anything are not relevant.

10) also explicitly, the wavelength of the radiation is not relevant to the explanation, nor is the altitude it occurs from, nor are photons, of any sort.

11) as Doug observed, the blanket analogy would also serve, and in fact would probably be better. The polarity is at the same, and it's just as easy to grasp.

I'm willing to re-write to use the blanket analogy if people think that would be superior, but I really think people need to understand what this is about before I try again.

I realise my tone here comes across as terse or abrupt, please don't take it that way. I try to be brief deliberately, to minimise risk of ambiguity. I do thank you for the input you've given, and I thank you again in advance for whatever comments you have now.

2010-09-30 07:21:27I hate to be negative, but


does it really make sense for us to try to teach someone thermodynamics in a brief article?

Your points 1 - 5 are true enough (for one statement of 2nd law; but another statement is that entropy always increases, and that applies in general, not just to closed systems): But are they going to mean anything to the fellow who has to have it explained to him?

The whole issue with comparing the Earth to a refrigerator will be hard to keep straight, because the analogies are being made between a component being warmed and a component being cooled.

Maybe the best approach is to say that the atmosphere is just like a blanket: It slows down loss of heat. Leave out the radiation/conduction distinction, and hope no one asks about the radiation bounced back downward from the atmosphere.

Joke: A seeker of truth climbs a difficult trail to find a hermit. Presenting an offering, he asks: "What is the meaning of life?" The guru replies, "The atmosphere is like a blanket." The seeker meditates on this for three days, but cannot make sense of it. He presents another offering, and asks: "WHY is the atmosphere like a blanket?" The guru, surprised, responds, "You mean it's not?"

Non-joke: A professor I knew at Berkeley once told the story of a Famous Physics Professor at University of Chicago, who was asked: "If I seal off a room, but leave inside it a refrigerator, with its door open, plugged into the electricity, will the room get hotter or colder?" The FPPaUoC gave the wrong answer. 




2010-09-30 09:12:44


Tony, some things cannot be "simplified" without introducing ambiguity, leaving important parts unexplained. As Neal suggests, providing the fundamental education necessary to see how the 2nd law is irrelevant here is beyond the scope of "basic." 

From the perspective of eliminating ambiguity and going beyond reliance on trust, a proper explanation of many of the things we speak of here (not just this single topic) would require to be shown in equations. 

You don't come across as terse, and besides this matter would test the patience of a saint, the closest examples of which we have are Roy Spencer and the author of Science of Doom.  

I amend my sentence above to:

"At any given instant, if an object is radiating energy upwards and that radiation encounters nothing that will absorb some of the upwardly radiated energy and reradiate it downwards to be reabsorbed by the radiating object, the radiating object will be cooler than otherwise."  

Without going into mathematical treatment or an air-tight experiment or both, I think that's the essence of the matter. In the case of Spencer and SoD, mathematical demonstrations were provided, and that's where the hard psychological problem of technical denial reveals itself. Can't be solved in this context. 

If you do go with the blanket analogy, you might eliminate the popular intellectual short-circuit of convection by using spacecraft thermal blanketing as the example. Blankets plus bimetal-controlled louvers were an early and popular choice for thermal control of the innards of spacecraft; louvers open or close to permit more or less energy to be radiated past the blankets. Stretching the analogy, the louvers are akin to IR windows of varying efficacy. Come to think of it, the lovers are exactly variable IR windows!

2010-10-01 03:20:19

Maybe using the blanket analogy and net fluxes instead of energy/heat will be physically sound and easier to explain.
2010-10-01 05:08:41


thanks for the feedback guys, it looks like we're converging on a 'blanket analogy' approach, which is good.

Neal, I don't find your comments negative, I appreciate them. I agree we can't explain thermodynamics in a brief article, but we are obliged to do something in that direction, since thermodynamics is at the heart of the skeptics complaint. The rebuttal has to address the second law, or it falls short. I think this can be done without any jargon.

BTW, the second law does only apply to closed systems, in all its forms. The entropy of an open system can decrease, the classic example being the existence of life on Earth, if you only take the Earth into account and not the sun pumping in energy.

I can fully appreciate the Professor getting it wrong, I've known a few myself who were far happier in 11 dimensions than in 3. That's why we need this peer-review process, and I'd say it's working.

Doug, I fully agree with your statement about radiation, but it's important to realise that that is not strictly relevant here. That's the mechanism of heat-transfer, but it's not the reason the 2nd law is not violated. The reason is that the system is open. If there were no sun, that mechanism would continue to operate, but the Earth would cool. It's the non-closure of the system that matters, the fact that heat is being pumped into it.

The blanket analogy is physically sound, but probably more accessible to the non-expert than the fridge. I'll rework the text again around that theme. The aim will be to show how (body-heat) + (body) + (blanket) + (cooler air) parallels the system of (solar heat) + (Earth) + (GHGs) + (outer space). That ought to do the trick!

2010-10-01 13:55:11

Gerlich and Tscheuschner are headed for a specially cold and dark place.  Between this, SoD, Spencer, RC, myriad other explanations, just imagine how many hours have been dissipated-- talk about maximum entropy!
2010-10-07 07:30:14
Pete Murphy


Is there a simple way of explaining why  the Second Law of Thermodynamics isn't being violated by examining  a  real greenhouse.

Typically the temperature in a greenhouse is higher than the atmosphere around it  due to the properties of glass .

Heat  apparently  flows from the outside cooler atmosphere  to the warmer greenhouse atmosphere. 

I'm not violating the second law of thermodynamics every time I grow tomatoes . I simply forgot to consider that there is a heat source called the sun furnishing me with energy via radiative transfer.

The Second Law of thermodynamics is expressible interms of describing the entropy of a system across all of its components. It's not as simple as saying heat flows from hotter to colder if I miss part of the system out . 

2010-10-07 08:51:20

Pete unfortunately while it has intuitive appeal, a greenhouse is a a fairly poor model for analogizing w/GHGs because a greenhouse traps heat via different means than do GHGs (pardon me if I'm telling you something you already know). Thus the term has evolved into a big, bouncy trampoline for skeptics; employ the word "greenhouse" in a basic description of how GHGs work in the presence of skeptics and anything further you say will be drowned out in a chant of "R.W. Wood! R.W. Wood!" It came into use back in an age of innocence, before the fossil fuel industry was threatened and is now a decayed conceptual albatross hung 'round our necks.  
2010-10-07 09:32:28
Pete Murphy


Fair enough -  I was focusing too much on the hotter to colder aspect when the surroundings are cooler .  I wasn't  thinking about the details of the internal convection enabled by the structure . Obviously too simplistic.

Wondering about the blanket analogy . What takes the role of the sun providing heat energy ?  Or would a glass of whisky help raise the internal temperature .


2010-10-07 12:32:03


Not to blow my horn but rather to reply to Pete's thinking on blankets, I offered a blanket analogy employing thermal control for spacecraft a little earlier in this thread. I like it because it's a fairly tight analogy but neatly slices out convection. 

The blanket analogy has actually tested pretty well in studies of GHG cognition in the lay public.

2010-10-07 12:33:32


Not to blow my horn but rather to reply to Pete's thinking on blankets, I offered a blanket analogy employing thermal control for spacecraft a little earlier in this thread. I like it because it's a fairly close model but neatly slices out convection. 

The blanket analogy has actually tested pretty well in surveys of GHG cognition in the lay public.

2010-10-07 20:30:09


The skeptiks questioning of the analogies really piss me off. Greanhouses, blankets, thermal insulation, perspiration, they are all good ones if one is mentally free. Despite the different heat transport mechanisms they all end up in a lowering of the outgoing heat and hence in warming.

Given that we learn these things in high school, this is one of the main reasons I think that no science will ever convince those guys because there is something else at play. On the contrary, it's easy to explain it with any of the anologies above to people willing to learn.

Maybe we should ignore the skeptics point of view on this.


2010-10-08 06:51:46
Shirley Pulawski

I have a bit of contention with the semantics, and I'm being pedantic here, but I think this is important. I think you need to distinguish between the Earth and the atmosphere. My suggested changes in bold, non-italic:

The sun warms the Earth. The Earth radiates heat away into the atmosphere and space. It radiates exactly as much heat as it receives, so it stays at a (more or less) constant temperature. Greenhouse gasses trap the escaping heat, making it harder for the Earth to shed that heat, so the Earth atmosphere warms up in order to radiate the heat more effectively. So the greenhouse gasses make the Earth's atmosphere warmer - like a blanket conserving body heat by keeping the heat radiated from your body close to you instead of lost to the space around you - and voila, you have global warming. 

The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth. If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics, but it's not the Earth's surface that is getting warmer, it's the Earth warming the atmosphere around it.


My thinking is that clarifying the difference between the Earth and atmosphere makes a stronger case against the violation of 2nd law argument. Honestly, I think the rest of it is kind of pointless, because the Earth surface is warmed by short wave radiation, emits it as long wave, which is what heats the atmosphere. The atmosphere isn't making the Earth  warmer, the insulative properties of the upper atmosphere are allowing the lower atmosphere to retain more heat. I don't like the refrigerator analogy, either, for mostly the same reason: it talks about the Earth heating, not the atmosphere.



2010-10-08 09:21:54
Pete Murphy
I like this explanation but I would suggest one modification in the phrasing of " ... so the -Earth- atmosphere warms up in order to radiate the heat more effectively ..." I would prefer something like "...the Earth's atmosphere warms up until it reaches the point where it can radiate away more heat..." (My nitpick is that the previous phrasing makes it sound like  the atmosphere has decision making powers )
2010-10-09 06:17:16
Paul D


Just how important is it to the 'greenhouse effect' theory that the ground is shown to heat up as a result of greenhouse gas 'back' radiation??

Was this aspect described by Fourier or Tyndall?

I think most people think of the theory as heating up the atmosphere and keeping the ground warmer than would be the case if there were less greenhouse gas.

2010-10-09 07:00:47


...just getting back to this after a week away, sorry for being slow with an update.

PeteM, the agent that takes the place of the sun warming the Earth is the internal body-heat of the human wearing the blanket. It's generated chemically, instead of delivered by radiation, but it's still a source of heat that is not taken into account if we only consider the body+blanket. Otherwise, a tailors dummy would also warm up when the blanket were placed on them, which clearly does not happen. The skeptic is thinking like the tailors dummy, not like a living human being.

Also, the 2nd law, as stated by Clausius, is valid. There are many statements of the 2nd law that are equivalent to statements that refer to entropy, some are more conceptually straightforward than others. I don't see the point in discussing entropy in a basic rebuttal if I don't need to, I think it's OK to choose a widely-accepted statement of the 2nd law that conceptually fits the situation at hand.

Shirley, the Earth itself is getting warmer. You can see this most clearly in upper-ocean temperatures, which show a clear indication. The atmosphere warms too, yes, but the warming of the Earth is real. I don't know that it's worth the extra complication of including the atmosphere in the warming, I'll see what I can do.

2010-10-09 10:20:02
Paul D


Here is a nice description by 'Frank':


2010-10-12 08:15:21
Bruce Worden


I think the refrigerator analogy is far too complicated for a basic audience. I'd suggest sticking with the blanket.

The basic-level global warming argument is:

The earth constantly radiates heat to space. But GHGs in the atmosphere block that heat and send some of it back to the planet. Thus, GHGs act to SLOW the rate that the earth loses heat to space. Because the sun constantly warms the earth, slowing the loss of heat from the earth to space makes the earth warmer. This is much like the way a blanket keeps you warm by slowing the loss of your body heat to the environment -- it doesn't add heat, it just keeps the heat around longer.

Then you can state the 2nd law and the skeptic argument. Then all you need is something like this:

But the skeptics making this argument misunderstand the 2nd law, which only applies to the net flow of heat on a macroscopic scale. On a microscopic scale, heat CAN flow from a cooler molecule to a warmer molecule, and this happens all the time, but it happens less often than the reverse, so the NET effect over a large number of molecules is that heat flows from warmer to cooler, in keeping with the 2nd law.  With GHGs in the atmosphere, the net flow of heat is still from the warmer earth to the cooler atmosphere (and from the cool atmosphere to frigid space). GHGs merely slow the flow of heat, not reverse it, and therefore don't violate the 2nd law any more than you do when you pull on an extra blanket on a cold night.

Edit: I see you've dropped the refrigerator from the most recent version, so ignore my first comment. -- CBW

2010-10-12 09:04:25suggestions


"The sun warms the Earth. The Earth radiates heat away into space. It radiates exactly as much heat as it receives, so it stays at a (more or less) constant temperature. Greenhouse gasses trap the escaping heat, making it harder for the Earth to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the greenhouse gasses make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation."

=> "The sun warms the Earth. The Earth radiates heat away into space. It radiates exactly as much heat as it receives, so it stays at a (more or less) constant temperature. But additional greenhouse gasses trap the escaping heat, making it harder for the Earth to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the added greenhouse gasses make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation."

"nobodies dummy"

=> "nobody's dummy"

2010-10-12 17:55:14


CBW, I prefer to state the second law up-front. It's better for the audience to know where you're going with something, rather than launch into the explanation first. It gives them a "How's he going to get out of that?" feeling, which makes it more interesting for them. The more interesting it is, the more likely they are to remember it.

 Also, there's no need to invoke molecules or statistical fluctuations, especially for a basic rebuttal. I think that will only cause a glazing-over of eyeballs and a loss of audience.


Neal, I don't think we need specify 'additional' GHGs. Any GHGs at all will warm the globe, as indeed they do, so the argument holds whatever the concentration of GHGs. Thanks for the typo-correction!

2010-10-12 18:09:11



 The reason I added the word "additional" was to prevent collision with your earlier statement that "The Earth radiates heat away into space. It radiates exactly as much heat as it receives".

My point is that the Earth radiated exactly as much heat as it received BEFORE additional GHGs: the Earth was in stable state with the pre-industrial level. 

Once additional CO2 has been added in the industrial period (up to the present day), the radiative balance has been disturbed, and THEN the Earth starts warming.

This distinction - between  radiative steady state in the pre-industrial period and AGW in the industrial period - is why the impact of additional CO2 is generally called the "Enhanced Greenhouse Effect".

2010-10-13 02:39:45


Neal, I don't think the distinction between pre-industrial and now is worth making in a basic rebuttal. The argument I present holds true regardless of any change of concentrations of GHGs, so why go into it?

My intent is to give someone new to the matter a simple model that is a) physically valid and b) easy to remember, so they can use it themselves in case the situation arises. Yes, I ignore many details and complications, but none of them, I feel, change the fundamentals of the rebuttal, or invalidate it in any way.

2010-10-13 02:54:03



 The point is that the statement, "It radiates exactly as much heat as it receives" is currently wrong.

2010-10-13 21:04:41


Neal, that's not exactly true. Radiative equilibrium is established very quickly, certainly when compared to the other timescales involved.

Tthe continued surface-warming is a result of increasing GHGs and of the mechanisms of diffusing that trapped heat throughout the earth. The earth+atmosphere remain in radiative equilibrium with the sun to a very high degree of accuracy.

What changes is the layer the radiation escapes from. More radiation is trapped near the ground, warming it, but more then escapes from the upper atmosphere, maintaining the radiative balance. There's an explanation at http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/index.html that might help.

I agree my text is loose in that it refers to 'Earth' instead of 'Earth + atmosphere'. I've tried to tighten that up, but it doesn't read so cleanly now, which is a shame. Still, let's see if people are happier with that.

2010-10-14 04:23:10
Bruce Worden


The problem as I see it is that your explanation has several incorrect or misleading statements caused by oversimplification. While it is understandable and possibly convincing for a non-expert audience, you have to remember that it is going to be put up on the web and attacked by deniers. To the extent that the deniers can legitimately criticize your rebuttal, they will undermine the credibility of the science. 

"They radiate exactly as much heat as is received from the sun, so the Earth stays at a (more or less) constant temperature."

This is incorrect on two counts. First, as Neal (I believe) has pointed out, there is a current radiative imbalance that is the source of "warming in the pipeline" discussed in other posts on SkS. Your statement contradicts those posts, creating an exploitable inconsistency. Second, the earth is not at a constant temperature, and anyone living on the planet will tell you -- you don't even need a skeptic around. It was much cooler last night than it is today. The earth (were it in radiative equilibrium) would have a constant amount of "heat" or "thermal energy" but the temperature would be all over the place both spatially and temporally.

 "The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference."

This is incorrect and does not address the skeptical argument. The processes involved would obtain whether or not the sun was involved. You are saying that GHGs act like a blanket. The skeptics are saying that the MECHANISM by which your blanket works is impossible. You respond (effectively) that your blanket makes the planet warmer because it slows the loss of heat gained from the sun. You never address the contention that your blanket can't work.

"If your body did not generate heat, putting on a blanket would make no difference."

This is utterly untrue. If your body did not generate heat, adding the blanket would still slow the loss of heat to the environment (or the gain of heat from the environment) until equilibrium was reached. The mechanism by which these processes work is unaffected by the presence or absence of a heat source. If you need proof, put some hot coffee in a thermos and a wine glass and see which cools faster.

"Is using a blanket an accurate model for global warming by greenhouse gasses? Certainly there are differences in how the heat is created and lost, and our body can produce varying amounts of heat, unlike the near-constant heat we receive from the sun."

Again, this mischaracterizes the issue. The source of heat is irrelevant. A person loses heat to the environment primarily via conduction with cooler air. A blanket traps a layer of air close to the body. That small amount of air warms and, to the extent that it stays in contact with the body, slows the conduction. No skeptic will deny that.

But the earth loses heat to space almost entirely by radiation. GHGs block and re-radiate photons of long-wave IR radiation. It's a completely different mechanism than a blanket. The skeptics are saying that the GHG mechanism cannot return those IR photons to the earth because it violates the 2nd law. But you never address that argument, so the skeptics can characterize your entire rebuttal as a red herring intended to divert attention from their argument.

The usefulness of the blanket analogy is that if you take a system in thermal equilibrium and then slow the rate of heat loss, the system has to warm up to return to equilibrium. But GHGs don't work like a blanket in any other way, so blankets cannot address the skeptics' 2nd law argument.

2010-10-15 00:40:14

OK, if the expression I use contradicts other SkS posts, that's not good, so I've changed that. However, getting into discussions of daytime vs. nighttime temperature is just a waste of time, it's not relevant to the rebuttal. I think having 'more or less' in brackets is a sufficiently accurate way of stating that the temperature is not strictly constant, but that the variance is not significant to the argument.

The skeptics claim, as I understand it, is that the (cooler) atmosphere cannot cause the (warmer) earth to become warmer still. I.e. that a cold body causing a warm one to become warmer would violate the 2nd law. The correct reply to that is that the 2nd law applies only to closed systems, and that ignoring the external heat source (the sun) means the system is not closed. Game over.

Of course, that's not clear and simple enough for a basic rebuttal, hence the need to explore the analogy. However, the mechanism of heat transfer is not important. Only the temperatures and heat-flow are important.

As for the statement about putting on a blanket making no difference, it's implicit that the blanket is being put on something that is already in thermal equilibrium. Hence the comment immediately following, about the tailors dummy. I assume the reader will understand I am not talking about putting a blanket on a fresh corpse.

I'd like to be clear about the purpose of this rebuttal. It is never going to silence skeptics, nor is that my intent. I hope to convince people who might listen to the skeptics, so they will reject the skeptics for themselves. Surely the purpose of having basic, intermediate, and advanced rebuttals is to make the facts accessible to the population at large, with whatever degree of expertise they have? Or are we trying to convince the skeptics three times? The skeptics will attack everything, and no amount of rigour will save us from that. The only sensible goal is to deny them their audience by convincing that audience in ways that they themselves can believe.
2010-10-15 01:00:58



The danger in simplifying an argument is that you can put your head through a noose that someone can tug on. That's why it's hard to write a basic-level explanation of something that is actually technical: You have to find something that gets across the point, is simple, and also NOT FALSE. 

This was always going to be a tough one to write, because the target audience has to have some sort of idea about what is meant by the 2nd law, but evidently not enough of an understanding to be able to see through the incorrect argument that is being rebutted.

As noted previously, Spencer spent a lot of time trying to explain why it was not a contradiction, and basically couldn't get buy in - and he's the skeptics' favorite scientist. 

In fact, I don't think there is a good strategy to writing a knock-down/drag-out rebuttal, because the individual readers are all going to start out confused about different things. Probably it really has to be handled on a skeptic-by-skeptic basis, so that each individual's misconception about the 2nd law can be treated individually. You can go much further into detail with someone who has a definite question than with someone who is vaguely curious.

2010-10-15 02:30:08


I'm going to make so bold as to give this a thumbs up. We can predict quibbling about convection w/the blanket, I still think a vacuum example is cleaner but of course that does add complication. Most "reasonable" people are going to easily understand this, probably already do, and I even will go so far as to say you'll hear from very few reasonable people on comments pertaining to this topic. 

One thing  that could stand some better tuning is this:

So if you put something hot next to something cold, the hot thing won't get hotter, and the cold thing won't get colder. That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil! 

probably because it's talking about a familiar example where the system objective is that of cooling, the opposite of the case we're speaking of here and thus is a distraction. As well, it's hinged on convection/conduction, not radiation. 

2010-10-15 02:32:46


Oops, forgot my promise. 

The discussion on this nicely illustrates why skeptics have gotten so much mileage out of this misconception... 

2010-10-15 02:55:07



 I agree there should be nothing false, but there's a fine line whenever an analogy is used. An analogy, by definition, is not accurate in every detail. The key is that the analogy must be valid in the points that are relevant. I firmly believe the blanket analogy qualifies under that definition.

I also agree this was always going to be a tough one. If I'd known how tough it would be, I might not have picked it. You guys have made me work hard! :-)


thanks for the thumbs-up (yay!). I chose that example because it illustrates that something cold will cool down something warm, apparently confirming the skeptics view that cooler air would not cause warmer ground to heat up. I'm open to suggestions for a better example? I'll see if I can come up with any.

2010-10-15 03:10:15

OK, you got rid of the "exactly as much heat".
2010-10-15 04:33:36
Bruce Worden


How about saying that the earth's "average" temperature is more or less constant? That should head off any criticism or confusion.

In general, I'm not suggesting that you try to convince the deniers. That's impossible. But like Neal says, if we put our heads through the noose, they're definitely going to yank on it. And when they can do that legitimately, it hurts the cause. They're always going to find new lies and BS to spew, but there's no sense in giving them real ammunition. That's why I made the point that your argument was probably fine for a lay audience. But I don't think it will hold up for a lay audience while you are being heckled by deniers.

The second law is not dependent on open or closed systems, it applies to both. Closed systems are only invoked as a convenience in expressing the law. If you put two objects together and then constantly heat one of them, the second law still applies: the net heat flow will always be from the warmer object to the cooler one. Even without the sun, the earth would be an "open" system by your definition because space acts like a semi-infinite heat sink ("semi-infinite" here in the physics sense, meaning that it is infinite in one direction, which is to say it has a boundary, namely the earth/space interface; so no matter how much heat the earth dumps to space, space never gets any warmer.) Even if the earth stopped receiving heat from the sun, the greenhouse effect would still function (for as long as the GHGs stayed in the atmosphere and the earth continued to emit the relevant frequencies). But to suggest that the laws of thermodynamics don't apply to the sun-earth-space system because it is "open" is incorrect. The second law as you've expressed it states that heat won't *spontaneously* flow from cool to warm. It applies to all systems, no matter the configuration. (The reason a refrigerator functions is because you use energy to do work in moving heat from cool to warm (i.e., it is not spontaneous), but the second law still applies everywhere in that system.) This is why I am saying that your rebuttal does not address the skeptical argument. The open/closed distinction is a red herring.

Also, I don't think it's implicit that the body in you example is at thermal equilibrium. I certainly didn't find it so. Bodies are warm, and you've been talking about warm bodies for the whole rebuttal. If you want to use that analogy, I'd suggest dropping the non-heated body, and just sticking with the dummy or a rock or something that is naturally at the temperature of the environment. But, again, the distinction you are trying to analogize is not relevant to the skeptical argument.


2010-10-15 06:44:00


Neal, thanks for the thumb!

CBW, I have re-worded the sentence to include 'average' as you propose. Hope you like it. I have also re-worded the part about the tailors dummy to be more explicit.

And no, the 2nd law cannot be applied to an open system. I do not suggest that the 2nd law does not apply to the sun-earth-space system, quite the contrary. I state that the second law doesn't apply unless you include the sun. i.e it does not apply to the earth-space system alone. As you describe, without the sun, the earth would rapidly cool to the temperature of space, so it's not in equilibrium with space on its own.

A closed system is one in which all the sources and sinks of energy are included. Your fridge is not a closed system until and unless you include the source of power that drives the heat-pump, i.e. the electricity it uses. That leads you to the power-station, which in turn requires an energy source, and so on. It's extremely difficult to find real-world examples of closed systems that do not encompass the entire universe! Even our simple Earth-GHG model requires the infinite heat-sink of deep space. The open/closed distinction is not a red herring, it is the essence of the rebuttal.

(Neal, Doug, I hope my recent edits haven't invalidated your thumbs?)

2010-10-15 08:49:12

A lot of effort and discussions, but now it looks good to me.
2010-10-15 16:12:41
Rob Painting
Whew, finally got there in the end!. Good job.
2010-10-15 21:36:31

I like this version. It's clear and the blanket analogy properly stated.
2010-10-15 21:37:20

2010-10-15 21:41:08
Paul D


It does look good, but can we have a diagram as well?

Remember your audience has different ways of assimilating information.

2010-10-15 21:42:20


Two votes from me? :D

For some reason I could not see my comment and vote and I posted again. Now I see both. :P

2010-10-15 23:28:13

Ah yes, a diagram. I guess I know what I'm doing on Sunday then :-)
2010-10-16 04:38:23
Bruce Worden

Tony, you're missing the point. The laws of thermodynamics operate whether or not you can mathematically balance the books. Nothing in the skeptical argument assumes either the presence or absence of a heat source. If your rebuttal actually answered the skeptical argument, it would imply that without a heat source the greenhouse effect wouldn't function, which is absolutely untrue. We all know that the greenhouse effect continues to function at night, when the heat source is gone.

Heat flux depends upon the instantaneous temperature of the elements of the system. The only role a heat source plays is to affect the time-dependent behavior of the system (i.e., it affects what the temperatures are as a function of time). 

The skeptical argument is: By the 2nd law the heat flux must be from the earth to the atmosphere, but greenhouse theory requires heat flux from the atmosphere to the earth, therefore the greenhouse effect is physically impossible. It's a stupid argument, but nothing in your rebuttal actually addresses it. Adding a heat source to the earth only changes the temperature of the earth, not the direction of the heat flux.

2010-10-16 19:05:47


CBW, when you go shopping, do you use a credit card? Do you ignore the fact that there has to be a source of money to balance the books at the end of the month? Or do you just spend, and ignore the fact that the money has to come from somewhere?

The second law of thermodynamics, like all physical laws, is precisely about balancing the books. Conservation of energy. Conservation of momentum. This can never exceed that, and so on. But you can't balance the books without counting all the contributions, just like you can't balance your budget if you ignore spending money on your credit card. In thermodynamics, taking account of all the contributions means you need a closed system. An open system, by definition, is one with unaccounted contributions, and your budget will not balance.

You are right that the skeptic argues in a manner that doesn't consider the presence or absence of the heat source. That is precisely why their argument is wrong.

2010-10-17 04:37:19
Bruce Worden

Tony, you continue to miss the point. When I buy something, money goes from me to the merchant, never the other way around.

A few posts back you wrote:

"However, the mechanism of heat transfer is not important. Only the temperatures and heat-flow are important."

The second sentence is true. Assume the earth is at temperature T1 and the atmosphere at T2. Then the net rate of radiant heat flow from the earth to the atmosphere is given by:

dQ/dt = e * s * A * (T1^4 - T2^4),

where e is the emissivity of the earth, s is the Stefan-Boltzmann constant, A is the surface area of the earth, and '^' signifies exponentiation.

Q: Where is the term for the heat source?

A: There isn't one. The rate of heat flow depends only on the temperatures.

Adding a heat source to the earth can only affect the time-dependent behavior of T1, but the rate of heat flow is an instantaneous quantity. As long as T1 is greater than T2, heat flows from the earth to the atmosphere and never from the atmosphere to the earth (notice how the above equation encapsulates the 2nd law). In the case of the earth, on average, T1 is always greater than T2, so heat always flows from the earth to the atmosphere. The only thing adding a heat source to the earth does is to change the total amount of heat that moves from the earth to the atmosphere over some period of time, it never affects the direction. The skeptics are saying that since the heat has to flow from the earth to the atmosphere, GHGs in the atmosphere cannot heat the earth.

Your heat source and open vs. closed arguments are 1) incorrect, and 2) don't address the skeptical argument.

2010-10-17 07:24:48


CBW, the earth you describe is destined to cool to the temperature of space, and GHGs will only slow that rate of cooling to some degree. That's not the issue here.

What we have to explain is this: the earth has a (more or less) constant (average) temperature. With more GHGs in the atmosphere, that temperature will be higher. How is it that the GHGs can cause this if they themselves are cooler than the earth?

The answer is to look at the heat-flows. All of them Heat is delivered to the earth by the sun. Heat is radiated away from the earth. The GHGs in the atmosphere slow down the rate at which the heat can be lost. That causes heat to build up in the earth and bingo, it gets warmer.

You do not need to invoke heat-transfer between GHGs and the earth to make this work. You need only consider the total heat flow of the whole system.

Let me say that explicitly: You can't explain the warming of the earth by GHGs without considering the incoming heat

2010-10-17 10:45:58


I think that, in general, the real situations are easier to understand. Idelized examples often require a level of abstraction that people do not use so often. For a basic version we should not assume a science student on the other side and whenever possible explain the real thing.

2010-10-17 11:40:01I hate to wade back into this, but


I believe that CBW has a couple of good points. I don't agree with him completely, however.

On the issue of open systems vs. closed systems:

- On the one hand, I don't know why we have to argue about this, since the "open vs. closed" argument is never made in the text of the Basic-level explanation. This is all side discussion.

- On the other hand, I don't think the distinction is very important anyway: The only closed system is the Universe, and my understanding of the 2nd law in this context is, "The total entropy of the universe is increasing." 

On the adequacy of the explanation, I agree with CBW that the "paradox" that you seem to be addressing at the beginning of the rebuttal is "heat is going from the colder atmosphere to the warmer Earth; the 2nd law says that can't happen". But the explanation that you end up providing, "heat is flowing from the Sun to the Earth to the atmosphere to space", which doesn't explicitly address the radiation flow from the atmosphere to the Earth.

So, if I were possessed by this misunderstanding of the 2nd law, I would feel that the rebuttal missed the target.

As I understand the situation: The 2nd law mandates that the NET RESULT of a cycle of actions cannot be the transferral of heat from a warmer to a colder body, without other work or transformations (all of which is hidden in the word "spontaneously"). However, this does NOT mean that there can be no transfer of radiant energy from the colder to the warmer body - as long as the NET result is still from warmer to colder.

For example: Float a chunk (#1) of metal at temperature T_1 in space. It will just radiate away. Now float near it another chunk (#2) of metal at T_2; this will also be radiating away. Some of the radiation emitted from #1 will be absorbed by #2, and and some of the radiation emitted from #2 will be absorbed by #1. Now, if T_1 = T_2, it turns out that the amount of radiant energy emitted by #1 and absorbed by #2 equals exactly the radiant energy emitted by #2 and absorbed by #1. So there is no NET radiant energy transfer (and therefore no NET heat transfer) if T_1 = T_2.

However, if T_1 > T_2, it can easily be seen that the flow of radiant energy from #1 to #2 exceeds the flow in the opposite direction. So the NET flow of radiant energy, and thus of heat, is from #1 to #2. And the fact that there exists a flow from #2 to #1 does not contradict that.

And that is the aspect of the energy exchange that addresses the "paradox".



2010-10-17 12:34:08
Bruce Worden

Tony, I think I finally see where you are coming from. So I've said it before and I'll say it again: Your argument requires that GHGs act like a blanket. The skeptics are saying that your blanket cannot work because its proposed mechanism violates the 2nd law. You never address their argument. You explain why wrapping a blanket around a heat source will increase the internal temperature. That was never in dispute with anyone, anywhere, ever. You're rebutting an argument no one has made.

"CBW, the earth you describe is destined to cool to the temperature of space, and GHGs will only slow that rate of cooling to some degree. That's not the issue here."

It's precisely the issue. The skeptics are saying that the GHGs could not slow the heat loss because the mechanism proposed violates physical law. If GHGs can't slow the loss of heat in my example, adding a heat source won't change that fact. The heat source is a red herring vis a vis the skeptics argument.

You write:

What we have to explain is this: the earth has a (more or less) constant (average) temperature. With more GHGs in the atmosphere, that temperature will be higher. How is it that the GHGs can cause this if they themselves are cooler than the earth?

The answer is to look at the heat-flows. All of them Heat is delivered to the earth by the sun. Heat is radiated away from the earth. The GHGs in the atmosphere slow down the rate at which the heat can be lost. That causes heat to build up in the earth and bingo, it gets warmer.

Your argument here is circular. You assume the answer in your explanation. You assume that GHGs "slow down the rate at which the heat can be lost." That's the very thing that the skeptics deny and that you need to prove.

The proposed mechanism for Greenhouse Theory is that some of the longwave IR radiation emitted by GHGs warms the earth. But the atmosphere is cooler than the earth, so the Theory appears to violate the 2nd law. The question you need to answer is: How can GHGs slow the rate at which heat is lost if they themselves are cooler than the earth? It's a stupid question, but it's the one the skeptics asked.

2010-10-17 21:27:56
Paul D


I think CBW is exaggerating the problem.

What probably needs to be clearer is that the Suns input is averaged out over a period of days. It doesn't matter if it goes away for a while over a 24 hour cycle. Skeptics come up with all sorts of issues regarding the 2nd Law.

I think one of the main issues is that they think the greenhouse effect means greenhouse gases need to be a (primary) source of energy/heat for the greenhouse effect to be true. Or rather it has to 'create' heat.

I'm wondering whether it is more useful to use a practical example for comparison. Lets say one heats up a cube of steel suspended in a vacuum and have an identical cold cube of steel say a few inches away. As the heated cube warms up it will radiate energy and some will be absorbed by the cooler cube. The cooler cube gets warmer, it will then start radiating at a lower level and some of that energy will be directed back to the warmer cube. The net flow over time will be towards the cooler, but there is also a flow towards the warmer cube.

Such an example is exactly the same as the greenhouse effect, but it is an example that everyone would be familiar with.

It is impossible for the cooler cube not to radiate any energy as it gets warmer.
Is there any experiment (and results) like the cube one that shows the warmer cube, stays warmer as a result of having the cooler cube present (added: and radiant heat/energy between the two).

If you had a diagram of the cubes (or similar) and a similar diagram of the Earth and the atmosphere next to each other, it would make the point quite clearly, eg. there is no difference in principle.


The other point is of course that the ambiguity and complexity of the logic in the 2nd law (in the eyes of an average person) means that it has been picked out as a target for motivated skeptics. one can conceive that some skeptics have no problem with GHE and the 2nd law, but still raise issues about it because they know people will be confused.

Another point is don't tackle the skeptics head on. The rebuttal should be primarily education of those that might be convinced by a skeptic. Just show precisely why a skeptic is wrong with a very simple and obvious comparison.

2010-10-18 01:06:07

Riccardo, are you saying I should not go with the blanket analogy? Are you retracting one of your thumbs? :-)

Neal, the open/closed thing is important. I avoid referring to it in the discussion as such, because the terminology would be confusing. By discussing the difference between a warm human being and a tailors dummy, I illustrate the difference without having to use jargon. At least, that is my aim.

The total entropy of the universe is increasing, yes. But the entropy in specific parts of it may decrease, provided that there is a compensatory increase elsewhere to maintain the overall increase of entropy, universe-wide. In other words, for the second law to apply, you have to consider the entire system, not just parts of it. You cannot apply the second law to individual components of the system, that's simply not valid physics.

CBW, I do see your point too. But I still hold to my position, though I clearly have to explain it better.

- The mechanism of radiative transfer is utterly irrelevant to the second law. The second law talks about heat-transfer (in the Clausius statement of it), not about radiation.
- So if the skeptic is complaining about the radiative transfer of energy, they are not complaining about the second law.
- So the correct rebuttal is not to show that the mechanism can work, it's to show how the second law works when applied to the entire system.
- The way I choose to do that is to take an analogous situation which has identical heat-flow properties, and which is familiar to everyone, and show how absurd it is to claim that it doesn't work. Note that it's the heat-flow properties that are identical, not the details of the heat-transfer mechanisms, because, again, the second-law doesn't apply to the mechanisms, so they are not relevant.

That's why I don't go into detail about radiative transfer. Skeptics can and will invent numerous fallacies to cling to their objections, so I don't attempt to address all their possible errors. That simply isn't possible in a basic rebuttal about something as hard to grasp as the second law. I've seen plenty of examples where they accept that GHGs can slow the rate of cooling, but still cling to denying that it can cause warming.

Personally, I feel that if we add something about radiative transfer, it would complicate things way more than necessary. As The Ville says, don't tackle the skeptic head-on. Rather take away their audience by educating them with an obvious example. I'm sure many of the fallacies will come up in the comments, and they can be dealt with there, but why cloud the issue for other people when we don't need to?
2010-10-18 01:16:07

Tony, if you're not willing to separate out radiative transfer from total heat transfer, then I have to come down on CBS's side: Your explanation simply does not address the point of confusion that you've stated you're trying to answer.
2010-10-18 03:25:16


Neal, if we separate out radiative transfer from total heat transfer, then we're not talking about the second law because we're no longer talking about a closed system.

I really don't want to complicate the rebuttal with a discussion of radiative transfer when the basic premise that radiative transfer could violate the second law is not only wrong, it's entirely inapplicable.

If skeptics want to attack it on the basis of their misunderstanding of radiative transfer/second law, I can handle that, but I say again, a pre-emptive explanation of that does not belong in the basic rebuttal.

I think we've reached an impasse here. I really appreciate all the help and discussion we've had on this subject, you've all helped me enormously since the first draft, but I think it's now going backwards. I guess both you and Riccardo have effectively withdrawn your thumbs, so I'm losing votes, not gaining them. I don't think I can make my points any clearer than I have, and it's not working, so it's time for something different.

If someone else wants to take over this rebuttal, please speak up. I'll leave a day or so for other comments, and then unclaim it and hand it over.

2010-10-18 03:41:24
Bruce Worden

The Ville writes:

I'm wondering whether it is more useful to use a practical example for comparison. Lets say one heats up a cube of steel suspended in a vacuum and have an identical cold cube of steel say a few inches away. As the heated cube warms up it will radiate energy and some will be absorbed by the cooler cube. The cooler cube gets warmer, it will then start radiating at a lower level and some of that energy will be directed back to the warmer cube. The net flow over time will be towards the cooler, but there is also a flow towards the warmer cube.

This is an excellent example, and well stated. It, or something like it, is what is needed to address the skeptical argument. The 2nd law doesn't say that heat doesn't flow in both directions, it says that the net heat flow is from warm to cool. That's why the heat flow equation I used above has the (T1^4 - T2^4) term -- if the receiving end gets warmer, heat flow slows down. The reason it slows down on the macroscopic level is the temperature gradient decreases, on the microscopic level it is because photons are going both ways.

Tony, you write:

 The mechanism of radiative transfer is utterly irrelevant to the second law. The second law talks about heat-transfer (in the Clausius statement of it), not about radiation.

I'm sorry, but that's just completely wrong. The second law applies to all* natural heat transfer mechanisms -- radiative, conductive, and convective -- both individually and severally.

*I'm leaving out mass transfer (like an iceberg floating south) because, like the refrigerator, an energy source is needed to move the mass, and it doesn't really apply to GHGs anyway.

2010-10-18 03:54:07
Bruce Worden

Sorry, Tony, I didn't mean to pile on there. You posted again while I was composing.

I really don't think you should let it go. Your explanation of Greenhouse theory and the 2nd law are good. I think you just need to state the skeptical argument, then refute it with something like The Ville's example. I don't think you can avoid talking about radiative transfer because the skeptic argument depends on it. If people understand the skeptical argument, they'll understand your rebuttal.

I hope you'll take another stab at it.

2010-10-18 07:51:42



I meant just the opposite.  I like your blanket analogy more than other idealized situations that people may find harder to grasp. I stand by my comment on 15 Oct 2010, 9:36 PM, as a basic rebuttal for me it's ready to go. A more physically accurate explanation would be necessary for higher levels.

I already gave you two thumbs by mistake, I may add a third :)

2010-10-18 22:07:03
Paul D



This is an excellent example, and well stated. It, or something like it, is what is needed to address the skeptical argument.


Thanks CBW. I think I prefer it to the other ideas and I don't think anyone would be confused by it.
I might have a go at creating a diagram.

The key problem to this IMO is that many people don't understand the radiative case. They can understand that if two pieces of metal are put together that the warmer one transfers energy through vibrations to the cooler one. It's a physical thing, they understand physical things. But radiation is a bit weird for most people.

2010-10-19 02:50:48
Bruce Worden

The Ville,

I agree that you can lose people with some discussions, but I think people have an intuitive feel for at least the effects of thermal radiation. There are a number of examples to explain various processes that are experiential: All of the heat that comes to the earth gets here by radiation, and you can feel it when you are in the sun vs. the shade. Or the way light bulbs get hot even though there is (often) nothing between the filament and the glass. The earth radiates invisible heat to space, like you can't see the heat coming off the heating element of an electric stove, but you can feel it. (The electric stove is good for discussing the range of blackbody radiation, too: turn it up high enough and it starts to glow...)

I'm not saying any of these particular examples are necessary or appropriate for this particular rebuttal -- I'm just saying that it's possible to communicate some complex processes in a way that people can understand intuitively, if you do it in a step-by-step manner. Your example is a good start: 1) people understand that if you warm something up, it gives off heat, and 2) people understand that if you put a hot thing next to a cool thing, the cool thing gets warm. So it's just one gentle step further to get them to see that even though the heat is flowing into the warm thing (per the 2nd law), the warm thing is sending out it's own heat, and some of that goes to the hot thing.

2010-10-19 06:16:20Wrong tree?


As I've argued before, part of the problem with this article has been trying to decide exactly what misconception it was that was being debunked. I finally got around to looking at the original starting point, which is:

2nd law of thermodynamics contradicts greenhouse theory

'The atmospheric greenhouse effect, an idea that authors trace back to the traditional works of Fourier 1824, Tyndall 1861, and Arrhenius 1896, and which is still supported in global climatology, essentially describes a fictitious mechanism, in which a planetary atmosphere acts as a heat pump driven by an environment that is radiatively interacting with but radiatively equilibrated to the atmospheric system. According to the second law of thermodynamics such a planetary machine can never exist.' (Gerhard Gerlich)

This is a quote from the infamous paper, "Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics," by Gerhard Gerlich & Ralf D. Tscheuschner. The paper can be obtained here: http://xxx.lanl.gov/find/physics/1/au:+Gerlich_G/0/1/0/all/0/1.

Unfortunately, when you look at this article, you find a much more educated and sophisticated level of nonsense than what we've been trying to deal with up 'til now. 

Not to say that the example of the two cubes of metal is not useful to someone (for example, it's essentially the same as the argument Spencer was presenting to his questioners); and by the way, it's the same point I was making above with T_1 and T_2. But it doesn't seem to have anything to do with the misconceptions of G&T, which have been addressed in detail by better physicists than they.

John Cook's Intermediate-level explanation is a run-through of the greenhouse effect; so it doesn't actually try to point out the logical flaws of their arguments.

So, if the intention of this article is to actually respond to G&T, it should be addressed against the specific nonsense that is promoted in this paragraph above. I am not really sure this is worthwhile: It would take sometime to go through their paper to find out which specific misunderstanding was indicated in the abstract; and then to write back an answer at the appropriate level.

Alternatively, we can write up the article along the lines that it already has - but change the argument to be rebutted. But has this argument really been promoted by others than G&T?



2010-10-19 07:15:38


CBW, thanks for the vote of confidence, I'll give it another go.

BTW, you say that I am wrong about the mechanism of radiative transfer being irrelevant to the second law, then go on to say:

 "The second law applies to all* natural heat transfer mechanisms -- radiative, conductive, and convective -- both individually and severally"

 That's precisely what I mean when I say that the mechanism is irrelevant. I could have put it better, the fact that the transfer is radiative is irrelevant, the second law applies identically, however the heat is transferred. Sorry for the loose language.

Riccardo, thanks for the clarification.

I have to say that I don't think the idea of discussing chunks of metal floating in space very appealing for a basic rebuttal. I really don't think many people would have a clue what to expect there. I think we need something much more related to everyday life, but I have no obvious ideas in mind. Any more 'down to earth' examples that anyone can come up with would be more than welcome!

2010-10-19 17:20:12
Bruce Worden

Tony, thanks for the clarification. Now I understand your point. And, of course, you're right -- any heat transfer mechanism makes the same point because they're all subject to the same physical laws. So maybe this can be done without using the words "radiation" or "radiative."

The main point that I think needs to be illustrated comes from the Halpern et al. paper that Neal cited. In the abstract they say:

They claim that radiative heat transfer from a colder atmosphere to a warmer surface is forbidden, ignoring the larger transfer in the other direction which makes the complete process allowed.

I think many examples could work. One just needs to show that heat flows both ways, and the 2nd law is satisfied as long as the NET heat flow is from warm to cool. Along the lines: We know warm things give off heat -- you can feel the heat coming off a hot object. And we know that warm things heat up cool things -- that's why you don't put the lasagna next to the ice cream in the picnic basket. But just because a warm thing is next to a hotter thing doesn't mean the warm thing stops giving off heat of its own...

Like that, only, you know, better.

I think it is important to get this right. In the recent SkS rebuttal on the falsification of the greenhouse effect someone posted that a google search turned up 31,000 pages on this second law BS.

And, for the record, I take it as a personal and professional embarrassment that the Gerlich and Tscheuschner paper was published. As a reviewer and reviewee, I can tell you that both the reviewers and the editors of that journal committed an epic FAIL in letting that thing into print. Normally that's not a huge issue -- science is fairly self-correcting -- but when the denier morons get a hold on something like this, they're like dogs with a bone...

2010-10-19 19:17:06


CBW & TonyWildish,

Sorry, but there IS an important distinction between heat transferred by radiation and by conduction:

- Radiation CAN transfer energy (and thus heat) from a colder to a hotter body; HOWEVER, the same mechanism will work in the opposite direction, and the hotter body will always lose heat to the colder, on a net-transfer basis. There will be two streams of radiation, one will be stronger.

- Conduction CANNOT transfer heat from a colder to a hotter body: Heat conduction travels against the direction of the temperature gradient. There is only one "stream" of heat flow.

2010-10-20 01:15:14


Neal, I'm afraid you're wrong about that. Conduction can convey energy both ways. Conduction is essentially the transfer of energy by collisions between molecules or atoms at an interface. Normally those molecules or atoms with more energy impart some of that energy to those with less (hence the energy flow follows the gradient).

But any substance will contain molecules (or atoms) with a distribution of energies, and any individual collision 'across the divide' between hot and cold substances may transfer energy either way, depending on where in the respective distributions each participant in the collision comes from. So a given individual collision may transfer energy either way.

So again, there are two components, one in each direction, though the net flow is hot to cold.

2010-10-20 05:30:59
Bruce Worden

I think the difference here is the microscopic vs. macroscopic behavior. But heat transferred by conduction is proportional to the temperature difference, just like radiative transfer (proportional to the difference of the fourth power of the temperatures), so as the receiver warms, the process slows, just like radiative transfer.

My point was mostly that I think Tony could find an example with two objects near one another in which he could say that heat is moving both ways, without getting into the details of radiative or conductive heat transfer. 

2010-10-20 08:38:13



No, it doesn't make sense to apply the concept of temperature or heat to a single molecule or atom: It's a statistical property of a collection of atoms/molecules. If you're talking about heat transfer, you're talking about bulk properties of matter.

As CBW hints, the relevant equation is Fourier's law:

heat flow = - (thermal conductivity) * gradient(Temperature)

It goes one way.



2010-10-20 18:53:58


neal, I didn't apply the concept of temperature to an an atom or molecule, I applied the concept of energy. Individual atomic/molecular collisions can transfer energy either way, regardless of the temperature of the bodies the atoms/molecules come from.

This is the basis of the statistical mechanical formulation of the laws of thermodynamics.

To paraphrase your words, conduction, therefore, "CAN transfer energy (and thus heat) from a colder to a hotter body;" And yes, again, the net flow is hot -> cold, we all know that.

2010-10-20 20:23:37

Tony, I'm sorry: but you're wrong.
2010-10-21 06:18:31


Neal, i don't really want to get into a long discussion of conduction here. I suggest you look up the 'Boltzmann distribution' and think how that applies to, e.g. a perfect fluid. How does a particle of this perfect fluid end up in one of the higher-energy states? Does it stay there or does it change to other states? Do other particles in the fluid occasionally move into and out of the higher-energy state?

Then think what it means to have two perfect fluids that do not mix, in contact with each other. If they are of different temperatures, they have different Boltzmann distributions. What does that imply about collisions between pairs of particles at the fluid-interface?

2010-10-21 08:05:48

2010-10-22 00:24:29



Heat Conduction: 

When we are talking about "heat conduction", we are talking about the realm of macroscopic phenomena. If you put something hot right in contact with something cold, OF COURSE there will be microscopic bumping back and forth and some energy goes both ways. But:

a) The overwhelming direction of the energy transfer will be from the hotter to the colder body; and

b) This entire (fairly one-sided) exchange is what we mean by the term "heat conduction":  I know of no text on thermodynamics, statistical mechanics or mechanical engineering that would describe this as "heat being conducted back & forth between the hotter and colder bodies." This is simply described as "heat being conducted from the hotter to the colder body." Heat conduction is microscopically explained in terms of these exchanges; but these exchanges are not explained as back & forth acts of heat conduction. "Heat conduction" is a term of macroscopic science, that has been used in physics and engineering since before Fourier, and its meaning doesn't even depend on the hypothesis that atoms exist.

Perfect Fluid / Boltzmann:

I have no idea why you are bringing perfect fluids into this at all:

- As used in the context of general relativity, perfect fluids do not support viscosity or heat conduction. Referring to the constituent atoms of a perfect fluid seems to be without point.

- If you are referring to Eulerian or ideal fluids, the relevance is about the same: no viscosity or heat conduction. As far as I know, they're good for producing solvable mathematical problems, but not of much use in describing real liquids. Inviscid hydrodynamics has been described as "the mathematics of dry water." The relevance of atoms is again questionable.

- If you are referring to "nearly perfect fluids" conjectured with regard to quark-gluon plasmas or ultra-cold atoms, it seems like over-kill to explicate a phenomenon that has been studied in depth since at least 1801.

In any case, it does not change the fact that the meaning of the term "heat conduction" has been stable for a very long time, and does not encompass microscopic exchanges of kinetic energy.


The broader point I've been trying to make clear:

- It seems that you have decided the key point of the original wrong skeptical argument is that "heat can't flow from the colder atmosphere to the warmer ground."

- But then the rebuttal you've written and are defending doesn't really explain why it apparently can: Because what is really forbidden is a net transfer of heat (without compensating Carnot-cycle work blah blah). If the medium of transfer were conduction and not radiation, the skeptical argument would be correct: If the greenhouse effect depended upon heat oozing from the cooler atmosphere into the ground, it would be in violation of the 2nd law. But it doesn't.

- The fact is that it is very easy to measure the IR flux upward and the IR flux downward. As long as the upward flux is greater, there's no cause for 2nd-law concern. THAT'S the resolution that addresses the skeptical argument as you have interpreted it.

- But the rebuttal you've written doesn't make this point. In that case, if you're not going to address the main spring of the skeptical concern, why bother writing it at all?


In fact, if we look at the starting point for what this article is supposed to be rebutting, it is the G&T paper. If you want to de-bunk the entire paper, feel free: Lots of people have provided material for that (e.g. http://rabett.blogspot.com/2009/03/burrow-project-gerlich-and-t-have.html ).

Or if you just want to de-bunk the cited section, "... a fictitious mechanism, in which a planetary atmosphere acts as a heat pump driven by an environment that is radiatively iinteracting with but radiatively equilibrated to the atmospheric system. According to the second law of thermodynamics such a planetary machine can never exist."

Or if you want to change the starting point to be something you have actually rebutted, that's another approach.

But in order to provide a satisfactory article, your rebuttal must in fact rebut the skeptical argument that is placed at the beginning of the article as the target of the rebuttal. Otherwise, it's not a rebuttal.

2010-10-22 09:31:48

I think that this discussion has been going on for too long, kind of waste of time and resources.
As I said a while ago, it is the distinction of energy and heat that causes confusion. But maybe this is already at a little higher level. At the very basic level, the blanket analogy is good and pretty obvious to anyone.
In my opinion, the choice is between explaining what heat really is or relying on the blanket analogy and the reduced heat loss.

2010-10-22 10:47:17



If the argument to be rebutted was, "There's no self-consistent way to think about the greenhouse effect," the blanket analogy would be OK.

But the targeted argument is, "The greenhouse effect is in contradiction to the 2nd law of thermodynamics because heat is coming from the cooler atmosphere to the warmer ground." Simply talking about an analogy to a blanket doesn't explain the apparent "paradox", it answers a different question.

See for example: http://rabett.blogspot.com/2009/03/second-law-and-its-criminal-misuse-as.html

2010-10-22 18:34:43


True, it does not explain why, but you can always prove something wrong by showing that it leads to unacceptable conclusions (the blanket wouldn't warm you) that the often quoted barmaid or 8th grade school teenager easily understand.

The task of explaining why, could be appropiate for an intermediate or advanced level rebuttal. It might be worth to write and publish both together, and in the basic version explicitly refer to the advanced.

2010-10-22 19:08:09


In that case, the right approach is to first develop an Advanced version that does the job; and then develop the Intermediate & Basic versions that are simplifications of that. It's easier to develop an Advanced version, because you don't have to be as concerned about "hitting the ceiling" with regards to length, language, and especially math. Then you have a better idea of what you need to say, and what you can skip, at the less-sophisticated levels.

That was also how I worked out a 3-version explanation before: They were released as a package, but I developed them in order: Advanced, Intermediate & Basic; and waited until comments had ceased on all 3 versions, before release.



2010-10-22 22:13:42Published
John Cook

Thanks, Tony, great work.
2010-10-23 03:02:11


folks, we have definitely been getting mired in distractions here. The rebuttal as written doesn't address conduction, so there's no point in us arguing the night away about it.

I had agreed to look for a way to bring radiative transfer into this, and have been working on that in what time I have available. I realise now I hadn't stated that much explicitly here, my mistake, and I apologise for that.

In any case, I see John has gone ahead and posted the article, despite it not having the right number of thumbs (2 from Riccardo shouldn't count!), and despite Neal's unresolved objection. That's a little unfortunate, but there it is.

I will continue to try to work in an explanation of radiative transfer into the rebuttal, to satisfy Neal and others, and maybe we can update the post later if this receives more approval. I realise I still have another thumb to earn.

Anyway, this has been a long hard slog, and I want to thank all of you for your contributions. I know we haven't agreed on everything, and that some of you still dont, but I think the peer-review process has worked fairly well overall. I'm not the only one who has put in a lot of time on this, and it's a lot better than it was originally.

John, I believe this is the longest thread in SkS to date. Is there a prize for that? :-)

2010-10-23 07:15:20
Bruce Worden


That's a good link. I'll quote part of it here:

There appears to be confusion about whether the Clausius statement applies to net heat flow or simply any flows of heat. Qualitatively one can make a simple argument about interchange of thermal energy between two bodies. Consider two perfectly absorbing disks in a vacuum at temperatures TA and TB, with TA > TB. If B is isolated, it will emit thermal energy at a rate given by the Stefan-Boltzmann Law. If the Clausius statement referred to any flow of heat when the two disks were placed opposite each other B would have to stop radiating towards A because if it did not, heat would be transferred between a body at lower temperature to a body at higher temperature. This is obviously absurd. The ability of either disk to radiate does not depend on the presence of another disk that absorbs the emitted radiation. Further it is not necessary to restrict the heat transfer mechanism to radiation, the same argument holds when energy is transferred by molecular motion, or electrons. Thus, the Clausius statement clearly must apply only to net heat flow, and one must consider all heat flows when applying the second law and not just selected flows in isolation from the others.

This is more or less what I've been saying, and some example like this is needed. (Note, Neal, that the stuff in bold appears to contradict what you were saying about this argument not applying to conduction.)

The example with the two shells around the sphere is very good, but I think it is too complex for a basic rebuttal. Something like the disks in the above quote should suffice and is simple enough for most people to grasp (IMHO). An object doesn't stop emitting heat just because something warmer comes near it. I'll say that again because I think it is the essence of the proper rebuttal to the skeptics' 2nd law argument:

 An object doesn't stop emitting heat just because something warmer comes near it.

2010-10-23 10:24:28



In my view, conduction is when the energy transferred by molecular/electron motion is predominantly in one direction. I have never heard of two objects sitting next to each other described as "conducting heat back & forth". Instead, when you have a temperature difference between the two, the transfers of molecular or electronic kinetic energy PREDOMINATE in one direction: that very predominance IS heat conduction. Hence the applicability of Fourier's equation, relating the heat flux to the temperature gradient.

As I said before, heat conduction has been studied since at least 1801. Fourier's equation describes the relationship between measured temperature difference and measured heat flux. If Fourier was trying to describe a "net" heat flux that was the postulated vector sum of a transfer in one direction with a transfer in the other, how did he measure the individual vector transfers? How could he determine the amount of random kinetic energy transferred in one direction, from the colder to the hotter object? He would have no way to do that; indeed, it was well into Planck's career nearly 100 years later that many physicists regarded even explanations of phenomena in terms of atoms as rather suspect. You can describe the heat flux in the equation as "net" from the point of view of hindsight, but that cannot be the way anyone thought of it: Indeed, have you ever seen a calculation of the two individual "heat flows"? It's not a practical concept; and thermodynamics, with its roots in engine studies, is a highly practical subject. 

The only possible exception that I have seen proves the rule: the concept of "second sound" (http://en.wikipedia.org/wiki/Second_sound). But this is a phenomenon that occurs in specialized situations as quantum superfluids, and can have no possible relevance to room-temperature situations.


2010-10-23 12:04:51
Bruce Worden

Neal, the link you posted to Rabett's blog provides a pretty convincing argument that the 2nd law was formulated to apply to net heat flows, not to each of the individual transfers that comprise the net. As for Fourier, there is no shortage of examples of scientist formulating empirical relationships without understanding the specific mechanisms by which the phenomena in question operated. Maxwell's equations would be another example. But I'm not sure why you feel that Fourier's understanding (or lack thereof) has anything to do with whether or not at the microscopic level energy is moving in both directions. If two objects of the same temperature were pressed together, giving a net heat flow of zero between them (satisfying Fourier), would you say that no interaction whatsoever between the molecules across the boundary? That would seem implausible, at best. What physical mechanism do you think is responsible for the heat flow being proportional to the gradient instead of simply the temperature of the hotter object?

In any event, I don't think we need to resolve this specific issue. You and I are in basic agreement that a sound rebuttal needs to address the issue of one-way vs. net heat flow between the earth and the atmosphere. I think most people could grasp the idea that if you have a warm thing radiating heat, it doesn't stop radiating heat when something warmer comes near.

2010-10-23 19:03:02


neal, you really are arguing about things that we all agree on now. Your own words show that:

"the transfers of molecular or electronic kinetic energy PREDOMINATE in one direction"

 'transfers', plural, i.e. more than one. Yes, there is transfer of energy in more than one direction

 "that very predominance IS heat conduction"


Nothing I have said is different to this. If you think anything in the text of the rebuttal says otherwise then again, please point it out. If you think anything in the replies says otherwise, all I can say is, at worst, that represents a poor choice of language on my part.

2010-10-23 21:20:31


This is basically a question of terminology.

As Tony has already agreed to incorporate an explanation of the distinction between one-way flows and net heat flow, this whole discussion, which evolved out of discussion to justify this need, is all behind-the-scenes semi-philosophical stuff.

However, the point behind this was to identify the conceptual point at which G&T go off the rails: Their interpretation that radiant heat transfer must be un-analysable in the same way as heat conduction (that cannot really be separated into two different macroscopic processes, in the absence of a "Maxwell's demon").

To speak to this point, an analogy: When you run a business in a competitive industry, not every deal is profitable, and not every day is profitable: some days you make money, some days you lose; money is transferred in, money is transferred out. But we do NOT say that "the business is successful on the profitable (transfer-in) days and unsuccessful on the unprofitable (transfer-out) days." We say that "the business is successful" if the profitable days predominate. 

In the same way, when you put a hot object in contact with a cold object, there is exchange in both directions of molecular/atomic kinetic energy: at some instants the transfer is from the atoms associated with the hotter body, at some instants the transfer is from those associated with the colder body. But we do NOT say that "heat is being conducted back & forth between the two bodies." Because the hot-to-cold transfers predominate, we say "heat is conducted from the hotter to the colder." This is how the term is used. Thus, I regard the addition of the adjective "net" in this situation as redundant and somewhat misleading. It would be appropriate to discuss "net heat transfer" if you have two separate actions of heat transfer, as in a Carnot cycle. But only a Maxwell's demon can separate out the two directions of molecular-level kinetic-energy transfer due to physical contact.

I don't know if we're going to come to an agreement on this point, which is semi-philosophical and only aimed at trying to pin down the watch spring (the failure of intuition) of G&T's error; and ends up being a question of language usage. However, the real work that we have to do is to determine how to incorporate the distinction between net heat transfer and constituent radiant transfers into the current text. I guess the question is: Can we do this in the context of the blanket? Or do we have to create a separate example, as in the two pieces of metal, to illustrate the point? Or should we go all the way back to a more general (and more flexible) explanation of the 2nd law in terms of net entropy production (closed system = the universe)?

If we keep the blanket analogy, we can only use it as a backdrop against which we still have to separate out the two directions of radiant heat transfer. I know this is supposed to be a Basic-level explanation, but do we need this blanket? If the only source of the skeptical argument is G&T, then I point out that they specifically go after the net radiant energy transfer: If a budding skeptic is prepared to be outraged at this, do we need a blanket to make him feel better, or should we assume that he's physics-ly mature enough to deal with one-way radiant transfers?   

The last approach would make the argument easier, but would require re-casting the G&T concept of the 2nd-law violation specifically in terms of entropy. 

Looking at the comments on the posting, to date, I find that most of it is kibbutzing by people who already accept the main point and are arguing about how it could be better expressed (Rather like this discussion, actually!). The only input from someone who seems to be trying to understand the point (maybe): Someone (#14) liked the analogy rephrased (#3):

warm body - colder blanket - cold room

warm ground - colder atmosphere - really cold space

It's encouraging that SOMEONE finds the blanket kind of useful, I guess.

2010-10-24 20:50:35



as John asked us, we should consider writing blog posts instead of long and comprehensive comments. I think you've already written a good part of an advanced rebuttal. :)

2010-10-24 21:21:59



It is actually a lot easier to write comments than to compose a careful rebuttal, because you know EXACTLY whom you're writing for. And if it gets a little bit technical and he gets blown away, that's unfortunate - but he asked for it, didn't he?

Also, it gives you the satisfaction of hopefully nailing at least one specific individual's misunderstanding.

Seriously, the time I'm spending on SkS are moments stolen from some urgent tasks from which I seem to be distracting myself.