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The greenhouse effect and the 2nd law of thermodynamics

What the science says...

Select a level... Basic Intermediate

The 2nd law of thermodynamics is consistent with the greenhouse effect which is directly observed.

Climate Myth...

2nd law of thermodynamics contradicts greenhouse theory

 

"The atmospheric greenhouse effect, an idea that many 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)

 

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 gases trap some of the escaping heat closer to the Earth's 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 gases 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 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 gases? 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 gases 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.

Basic rebuttal written by Tony Wildish


Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

 


Update October 2017:

Here is a walk-through explanation of the Greenhouse Effect for bunnies, by none other than Eli, over at Rabbit Run.

Last updated on 7 October 2017 by skeptickev. View Archives

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Further reading

  • Most textbooks on climate or atmospheric physics describe the greenhouse effect, and you can easily find these in a university library. Some examples include:
  • The Greenhouse Effect, part of a module on "Cycles of the Earth and Atmosphere" provided for teachers by the University Corporation for Atmospheric Research (UCAR).
  • What is the greenhouse effect?, part of a FAQ provided by the European Environment Agency.

References

Comments

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Comments 726 to 750 out of 1393:

  1. Re #615 Tom Curtis you wrote:- "Do you also agree with me that this simple model does not violate any laws of thermodynamics? " Perhaps not. But what it doesn't do is model the atmosphere or RW1's model in #613 In #619 L.J. Ryan shows the flaw in your model - adding the same energy twice! It doesn't model the atmosphere because GHGs do not reflect light, they absorb and, to a certain extent, re-radiate it. When a photon is reflected its energy remains the same e.g. mirrors do not change the colour of light. When photons are absorbed they may cause re-radiation but this is not necessarily so, e.g. if a chemical change is induced by the absorption of a photon. If the absorbing material does re-radiate photons this may well occur at a much longer i.e. 'without limit' longer wavelength. What cannot happen (2nd Law again) is for a photon to be emitted at a shorter wavelength. More exactly, in a single photon process, a photon more energetic than the incoming photon cannot be emitted. But there are two (or more) photon events that result in the emission of single photons of higher energy than either of the input photons, but the total energy is still conserved. There is no law of conservation for photons, they start with an emission and end with an absorption, even if they travel light years between the two events. Re #616, les your box is just fine, it is what J J Fourier described - it just doesn't describe the atmosphere - as Fourier himself noticed 'the air is not held still in the atmosphere' Re #677 Tom Curtis your box has a major flaw; the energy source is outside the box but once inside it is reflected 100% this doesn't happen, the walls that reflect inside the box must also reflect 100% light coming from outside, there are no one-way mirrors in physics. Further, if you put a lamp inside with N Watts power and the walls were 100% reflecting (thus 100% insulating) the temperature would rise until something was destroyed! But consider, how would you get the N Watts power in? Heat conduction in metals in largely a function of electrons in the conduction band thus the wires would carry much of the heat inside back out to the generator (or battery) where the energy came from in the first place. The wires may get hot in the process but have you ever dealt with high power lights enclosed in a projector? Everything gets terrifyingly hot!
  2. Tom, before you waste too much time on damorbel, you might like to look at page 5,6 etc. Also, note my post here. It seems that damorbel is not willing to be persuaded by experimental evidence on those terms.
  3. Re 727 scaddenup you wrote:- "It seems that damorbel is not willing to be persuaded by experimental evidence on those terms." Having checked your link and scanned the thread, I am not at all sure which experimental evidence you claim 'does not persuade me'. I would appreciate your clarification.
  4. 726 damorbel: "your box is just fine, it is what J J Fourier described - it just doesn't describe the atmosphere - as Fourier himself noticed 'the air is not held still in the atmosphere'" Really, mate, I wasn't asking for your approval of the model! You know, a physicist doesn't need name-dropping to see what is and is not in a model. Clearly I did not include convection etc. It is redundant to point it out - unless your only aim is go take pot-shots at everything for no apparent constructive reason. I don't know. My point was to show how one would move from the model proposed to one closer to something describing radiation etc. Waving hands and naming theories didn't really help develop the model - so, no contribution from you on the physics front. Still, thanks for the remarks, however trivial.
  5. damorbel @720:
    "Yes indeed. And so did Gustav Kirchhoff writing in 1862. From that he concluded that the temperature of a body black or or otherwise, is not affected by its emissivity as long as it has no internal heat source (or heat sink) i.e. it is in thermal equilibrium. This is the basis of his argument that emissivity and absorptivity are the same for any given body."
    Kirchoff certainly relied on the fact that when to bodies are in thermodynamic equilibrium, altering their emissivity cannot change the temperature of either (for doing so would violate the 2nd law of dynamics). From this he then proved that emissivity = absorptivity at every wavelength for every thing. But the issue of an internal heat source or sink is extraneous to his discussion, and your definition of thermodynamic equilibrium as "not having an internal heat source or sink" is false. On the contrary, two bodies are in thermodynamic equilibrium with each other if an only if, when heat can pass freely between them, neither loses heat nor gains it. As noted in the definition below, when two bodies are in thermodynamic equilibrium, they are at the same temperature:
    thermal equilibrium The condition under which two substances in physical contact with each other exchange no heat energy. Two substances in thermal equilibrium are said to be at the same temperature.
    It follows that if they are not at equal temperature, they are not in thermodynamic equilibrium; and if not in thermodynamic equilibrium, Kirchoff's Law does not forbid a change in emissivity resulting in a change of temperature for one or the other. The Earth and the Sun are, of course, not at the same temperature. Therefore the special application of Kirchoff's Law you appeal to does not apply. I am, of course, very happy to concede that were the Earth heated to the same temperature as the Sun, the green house effect would not warm the surface (and likewise if the Sun was cooled to the same temperature as the Earth).
    The meaning of this is clear, the size of the albedo has no affect on the temperature of the Earth, the position of climatologists, that the Earth's equilibrium temperature is lowered by 33K from 288K to 255K has no scientific basis.
    Having rewritten the definition of thermodynamic equilibrium to give yourself the semblance of an argument, you now do the same with the theory you are contesting. Climatologists claim that the equilibrium temperature of the Earth would be approximately 278 degrees K without albedo or greenhouse effects. Because ice and clouds raise the Earth's albedo at wavelengths at which it absorbs light from the sun, but not at wavelengths where it itself radiates, that cools it by about 23 degrees. Because GHG lower the Earth's effective emissivity at wavelengths where it radiates but not at those where it receives light from the sun, that raises the Earth's effective temperature by about 33 degrees.
  6. LJRyan @723, actually I did not specify a rate. What I did specify was that, "In that case, after sufficient time for light to transit the box three times, and with a constant light source providing beam (A), then the box will have the following equalities." A single photon does not qualify as either a "beam" or a "constant light source". By reducing the case to that of a single photon, you are quite clearly trying to avoid discussing the model as specified. Any interested readers should note your evasion, and that you do not feel confident enough to discuss the case on its merits. That should come as no surprise - I certainly would not want to discuss my case on its merits if I held your purported beliefs.
    Response: [DB] It has been noted. :)
  7. LJRyan @724, again you are changing the details of the experiment to avoid refutation. I very carefully specified, "Suppose you have an electrical stove ...". The reason for that is very simple, while the colour of a gas flame is fairly constant with temperature, the colour of an electrical heating element above a certain temperature is not. Therefore, you can see on the electrical element as you cannot see on the gas flame the effects of changes of temperature. Reverting then, to the original specification, if you have a pot full of water on an electrical element which is on, and glowing a dull red; and then you remove the pot, the element will become warmer, and glow a brighter red as a result. Inverting the pot over the element will reduce convective heat transfer, but by so small an amount that the heat difference from simply removing the full pot is unlikely to be detectable by eye.
  8. First, my preceding post should be read as a response to 725, not 724. LJRyan @724, the "lid forcing" is due to reduced heat loss due to convection and latent heat transfer. There-fore-making the lid transparent to IR, a very small source of heat loss in the situation, will make virtually no difference. This does not change the fact that the addition of a cooler object resulted in greater heat in a warmer object that it would have had without the cooler object! Your attempts to distract us from this fact will not work, and nor will we forget that they represent a complete refutation of your claim in 715 the presence of a cooler object cannot result in increased warmth in a warmer object.
  9. Re #730 Tom Curtis you wrote:- "But the issue of an internal heat source or sink is extraneous to his discussion, and your definition of thermodynamic equilibrium as "not having an internal heat source or sink" is false." Sorry but I must point out that an object with an internal heat source can never be in thermal equilibrium because it will always have a temperature gradient of some sort inside it. Further you wrote (1):- "two bodies are in thermodynamic equilibrium with each other if an only if, when heat can pass freely between them" This is not a requirement. If 'heat can pass freely' they will reach the same temperature (the only condition for equilibrium) at the fastest rate possible. And (2):- "As noted in the definition below, when two bodies are in thermodynamic equilibrium, they are at the same temperature" Which appears to contradict (1)and agree with (2) Emissivity is an intrinsic property of the material, it is not a direct function of its temperature. Emissivity can only change if the basic structure changes e.g. diamond has an emissivity different from graphite. You wrote:- "The Earth and the Sun are, of course, not at the same temperature. Therefore the special application of Kirchoff's Law you appeal to does not apply" The Earth absorbs the energy from the Sun that is not reflected by the albedo 'a' , this absorbed heat is radiated by the Earth with an emissivity e = 1 - a (a is the albedo) that is Kirchhoff's law. The law applies because the average temperature of the planet is not changing, it is in equilibrium with the radiation from the Sun. You wrote:- "I am... happy to concede that were the Earth heated to the same temperature as the Sun, the green house effect would not warm the surface" At the Earth, the Sun's radiation density is reduced according to the inverse square law, but the photons it intercepts still have the same energy as when they were emitted, it is just that they are spread over a larger area. If they are re-concentrated e.g. focussed by a mirror, the resultant image can, if it is only losing heat by radiation, reach the same temperature as the Sun. (It can't do this at the Earth's surface because the atmosphere absorbs some of the Sun's energy).
  10. Damorbel @726: Response to 613: As very clearly shown in 676, the radiant energy transfers are isomorphic with the radiant energy transfers in the third model of 676. The third model is an actual example of the GHE. Because of the isomorphism of energy transfers, it follows that the model of 613 is a model of the third model of 613, and hence a model of the greenhouse effect. It is good to see you endorsing LJRyan's answer at 619, however. He claims there that A=B. It follows that as C + D = B (by definition of half mirrored) and C = D (by definition of half mirrored) that C =/= B and hence C =/= A. But C = A by definition of equilibrium. So on LJR (and your) analysis of the box, C both equals and does not equal A. A contradiction that clearly proves your analysis to be false. Re your response to 677, that the lid of the box could not in fact be developed in life (as I mentioned) is irrelevant in what is after all an ideal thought experiment. A work around in real life could easily be developed using a laser. Why then are you concentrating on trivia?
  11. scaddenp @727, it is quite obvious from the way they discuss the topic that neither LJRyan nor Damorbel are interested in resolving the issues being contested here. Rather their intent appears to be to prolong discussion to create the appearance of rational dispute. It is a sham, of course, because their responses to arguments are neither rational, nor responsive. Instead there strategy appears to be merely to deflect and distract from core issues. This raises three issues. First, are they amongst the PR consultants which are known to be paid by some firms to create a haze of spurious disputation around sites that explain the truth about global warming? Second, how should site administrators respond to such evidently troll like behaviour given that they know that at least some such behaviour is paid for verbiage rather than reflecting genuine opinions? And third, as participators at a site, should we ignore their responses given that we recognise that their disputation is strategic rather than genuine? My answer to the third question is that it is better to not leave their responses unanswered, at least until they have exposed themselves as the empty shams they undeniably are.
  12. 736 - Tom Curtis "First, are they amongst the PR consultants which are known to be paid by some firms to create a haze of spurious disputation around sites that explain the truth about global warming?" Come on, get serious - no one would pay good money for contributions like those, surely!
  13. Tom Curtis, you and the others here are doing sterling work but I wonder to what end. This has become like a thread that involves Poptech : circular and pointless. As you say, it is creating an illusion of some sort of debate, which is incomprehensible to the vast majority of people who understand that the greenhouse effect does not break any physical laws. Perhaps it is time to ask and demand answers to certain basic, and on-topic questions, from the so-called skeptics, with anything else being deleted as off-topic and time-wasting ? They would scream censorship, no doubt, but I believe the rest of us would welcome the decline in time-wasting nonsense - as I'm sure you would welcome the ability to concentrate on other matters !
  14. les @737, in PR it is often quantity, not quality that counts. What is more, damorbel and LJRyan seem to repeatedly make claims that anyone who knows the theory behind the GHE or Thermodynamics can clearly see to be false, but which are plausible enough to pass muster with those having only a casual acquaintance with either. They are exactly the sort of contributions likely generate uncertainty in the uninformed.
  15. JMurphy, that might be a suitable solution. Fortunately, though, it is out of my hands to determine if a more robust response is appropriate.
  16. LJRyan #715: "To proclaim star sourced energy can be increase itself by it's own reflection and/or re-radiation is a violation of the 1st law." Presumably parabolic mirrors don't exist in your reality.
    Response: [DB] Or perhaps he meant this law...
  17. JMurphy@738 "Tom Curtis, you and the others here are doing sterling work but I wonder to what end." I would like to second this Tom but let you know that it is not pointless. Not being a physicist myself I have found your explanations very easy to follow and enlightening. The lack of understanding displayed by our resident skeptics is as damaging to their cause as their math.
  18. Tom Curtis 731 You said: " "In that case, after sufficient time for light to transit the box three times, and with a constant light source providing beam (A), then the box will have the following equalities." A single photon does not qualify as either a "beam" or a "constant light source". By reducing the case to that of a single photon, you are quite clearly trying to avoid discussing the model as specified. Any interested readers should note your evasion, and that you do not feel confident enough to discuss the case on its merits. That should come as no surprise - I certainly would not want to discuss my case on its merits if I held your purported beliefs. " Establishing constraints is not avoiding the discussion. Since a "beam" is not scientifically defined, and furthermore "light source providing beam" suggests visible light, I'm trying to establish a specific minimum for your box to work as supposed. If I specify, as you suggested (698) 1 photon per second, A single photon will transverse the box and/or absorbed and re-radiated countless times within a second...so why no increase in energy? I will ask again. Is there a minimum energy for your box? Any interested readers should note your evasion, and that you do not feel confident enough to discuss the case on its merits.
  19. Tom Curtis 732 733 The nonsense of your electric stove analogy is profound. To imply a lid over a pot of near boiling water demonstrates atmosphere forcing, or to refute heat energy flows spontaneously from hot to cold is obfuscation at that least and outright igno....well you seem very intelligent, just wrong. You came to realize however, "the "lid forcing" is due to reduced heat loss due to convection and latent heat transfer. There-fore-making the lid transparent to IR, a very small source of heat loss in the situation, will make virtually no difference." And the lid temperature is NOT relevant The larger pot inverted over the burner, bright red or dull red, should get hotter via re-radiation. Should get much hotter via re-radiation IF GHG theory physics is correct. Or since the burner represents a constant source of light (maybe a beam) shouldn't the pot contain 4x the light entering the pot. Can a pot be a box? Any interested readers should reference the thread starting @676 Any interested readers should also note your evasion, and that you do not feel confident enough to discuss the case on its merits.
  20. CBDunkerson 741, moderator No CB, I believe light can be focused and directed. Are you suggesting the cold atmosphere is a parabolic dish...reflecting focused LW to the earths surface? Way to stay objective moderator.
    Response: [DB] As one who has experienced this thread in all its 700+ comment glory, keeping one's objectivity mandates a sense of humor. Like keeping an open mind also mandates one to not let one's brains fall out. If you've taken offense at my sense of humor, I apologize.
  21. LJ, can't you see that the mechanism is the same? In both cases we have electromagnetic radiation being redirected and resulting in the area of accumulation receiving more energy than if the EMR had not been redirected there. The greenhouse effect works by redirecting 'infrared light' just as a parabolic mirror works by redirecting 'visible light'... in both cases you have concentrated a greater amount of electromagnetic radiation in a given area and thus produced a higher temperature. Your claim that this violates the 1st law of thermodynamics is thus obviously false. No 'extra' energy is being created... already existing energy is being concentrated within an area and thereby causing higher temperatures in that area. Ditto with the nonsense about the 2nd law of thermodynamics... the EMR flows just fine from the cold surface of the mirror to the much hotter focal point. If the greenhouse effect violates either of these laws of thermodynamics then so do parabolic mirrors... yet both keep on working despite illogical beliefs that they cannot.
  22. #743: "If I specify ... 1 photon per second, A single photon will transverse the box and/or absorbed and re-radiated countless times within a second...so why no increase in energy?" Are you suggesting that successive absorption and re-emission of photons at the same frequency increases energy? In what way? If you have this figured out, congratulations, you better get a plane ticket to Stockholm.
  23. muoncounter 747 "Are you suggesting that successive absorption and re-emission of photons at the same frequency increases energy?" That's my point...it can't. Go back and read the entire thread.
  24. CBDunkerson746 "can't you see that the mechanism is the same? In both cases we have electromagnetic radiation being redirected and resulting in the area of accumulation receiving more energy than if the EMR had not been redirected there. The greenhouse effect works by redirecting 'infrared light' just as a parabolic mirror works by redirecting 'visible light'... in both cases you have concentrated a greater amount of electromagnetic radiation in a given area and thus produced a higher temperature." The atmosphere acts like a parabolic dish...you don't really believe that...do you? Hey CB, read up on solar cookers...when not cooking can be turned away from the sun, and COOL the focused contents. How can this happen...how, with intensified hot radiation form the dish atmosphere? Hint: the sky is cooler.
  25. LJ: "The atmosphere acts like a parabolic dish...you don't really believe that...do you?" In that both cause an area to be warmer due to redirected electromagnetic energy it isn't a matter of belief, but rather observed reality. "Hey CB, read up on solar cookers...when not cooking can be turned away from the sun, and COOL the focused contents. How can this happen...how, with intensified hot radiation form the dish atmosphere?" Congratulations... I cannot discern an argument coherent enough to refute.

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