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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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.



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Comments 426 to 450 out of 1107:

  1. Finally damorbel came to the right point in his comment #424. He simply needs to add an heat source and he's done. I don't have much hope he'll do, though.
  2. damorbel wrote: "But the atmosphere is also between the Sun and the Earth, just like a blanket with a corpse underneath it is between the (ambient or Sun) heat source." However: (a) The atmosphere absorbs LW IR radiation but is essentially transparent to SW radiation (visible light and ultraviolet) (b) Most of the radiation given off by the Sun is SW radiation not IR. (c) Most of the radiation from the sun is not absorbed by the atmosphere [see (a)], so the Sun does not significantly heat the atmosphere directly. The atmosphere does not significantly insulate the surface from suns radiation. (d) When the surface absorbs most of the suns SW radiation, it heats up and re-radiates LW IR radiation upwards. (e) The atmosphere does absorb some of this outbound IR radiation, and hence it heats up. The atmosphere is not warmed directly by the sun, but indirectly by the IR radiated from the surface. (f) The atmosphere being warmer than space thus insulates the surface from space, causing it to be warmer than it would otherwise be. (g) This does not violate the second law of thermodynamics as the net transfer of heat is from the warmer surface to the cooler atmosphere. (h) The greenhouse effect thus does not violate the second law of thermodynamics, and Roy Spencer (amongst others is perfectly correct on this point and G&T are flat wrong). O.K., I've spelled it out for you. Which of these points do you disagree with? For the sake of clarity, it would be best if you could make a list (a)-(h), saying whether you agree with the point or not, and if not explaining why.
  3. 423 damorbel, You are obfuscating the same point I was trying to make to RW1. In one post he calls it "energy", in another, "power", in another "radiation". The point is a that an object of higher temperature can receive "energy, power or radiation", or what the hell ever you and RW1 want to call it when it is convenient for you, from a lesser temperature object because that higher temperature object cannot discriminate the source of the "energy, power, or radiation" it absorbs.
  4. Re #428 RickG you wrote:- "You are obfuscating the same point I was trying to make to RW1. In one post he calls it "energy", in another, "power", in another "radiation"" 'energy' is measured in Joules 'power' is measured in Watts (Joules per sec.) 'Radiation' is hvwhich is a form of energy dependent on the source frequency 'v'. If you don't get these right then you will become hopelessly confused. Perhaps RW1 didn't get it right, one has to be careful.
  5. damorbel - If the sun heated the Earth via heating the atmosphere (which seems to be what you are arguing), the lapse rate would be reversed and there would be essentially no convection. This is not the case. Sunlight passes through the atmosphere, heats the Earth just as the heater coil in my example did. "If you blocked the Sun off (and you can) the Earth would cool. This is because the Sun is external to the Earth and its atmosphere." - This is a complete red herring. You could turn the heater coil on the block off and on again - it doesn't change the physics of heat transfer. Claiming that is quite the horrid dodge, damorbel - you're really straining there, and I find it appalling. You've painted your argument into a corner. Energy goes from the Sun -> Earth -> Atmosphere -> Space, with the atmosphere acting as an insulator. --- Everyone, this has been quite a discussion. >400 posts on the greenhouse effect, many of which have done a good job looking at the physics. Unfortunately, damorbel, you seem willing to go to extremes to deny the radiative greenhouse effect, including that silly bit about 'turning the sun on and off' to attempt to disprove the fact that the atmosphere is heated by the Earth's surface. At this point I have to consider you to be either (a) a troll, stretching this out to claim controversy on the topic, or (b) fanatically unwilling to listen to reason and abandon your incorrect ideas on thermodynamics. Either way, damorbel, you are no longer worth my time. I believe the current thread has plenty of information for newcomers, who can clearly judge the topic (and posters) based upon what they read. I'm done here.
    Response: [muoncounter] KR should be commended for showing terrific patience in going this far. [DB] Seconded. I would add that I found myself learning the subject even more thoroughly as a result, so in that regard it was time not wasted.
  6. @muoncounter Not only should st. KR be commended, he should be canonised - he clearly has the patience of a saint, so he should get the honorific as well ;o) In discussions of climate, again and again we see evasion, pedantry a refusal to stick with thought experiments intended to clarify matters and a refusal to give a direct answer to a direct question (which risks giving a hostage to fortune - but those only interested in the truth are not bothered by that). The, best approach to dealing with this seems tireless patience; it is quickly evident who is there to discuss the science, but it shouldn't really be necessary.
    Response: [muoncounter] Agreed: St. KR it is. Forgive me if I omitted thanking anyone who has stayed with this for the long haul; haste causes carelessness. It is a shame when a thread becomes so long and convoluted that contributing posters finally say 'enough.'
  7. I agree with muoncounter's moderator response above, but would also add commendations to Dikran Marsupial and les - as well as several others who tried their best to impart their knowledge. I certainly found it very educational, if only because you all had to go round and round repeating the same facts again and again, hoping it would all sink in eventually ! It worked with me, anyway... (Strange how none of the self-appointed expert so-called skeptics got inolved, though. No, actually, it's not strange at all, is it ?)
  8. Re: JMurphy (432) Indeed. One was left with the sound of one hand clapping... The Yooper
  9. I've actually found most of the misunderstandings of the greenhouse effect and the 2nd law of thermodynamics are intimately tied to the Fallacy of Division: that what is true of the whole is true of the constituents. "4 is even. 1 and 3 are part of 4, so they must be even too!" Each individual energy transfer - collision, molecular diffusion, photon absorption/emission - can occur in any direction. From a cold nebula to a star, from a candle to a plasma torch, from your hand to the boiling pot you mistakenly grabbed. It's only statistically, in the aggregate, that "Heat" transfer shows up, as the higher energy moves to the lower, based on the fact that there are many more energy transfer events going in that direction. The 2nd law of thermodynamics, and entropy, is a statistical law, a law of summations, not a factor in individual molecular events. It does not, can not, block energy from moving between a cold object and a warm object, adding to its energy. It's just that statistics indicate a greater number going the other way - and with molecular numbers statistics are pretty much a dead certainty. This fallacy is never more clear than with the "colder objects can't heat warmer ones", where I've seen attempted justifications such as "warm objects won't absorb low energy photons!", "destructive interference of cold photons with warm ones keeps them away", and my favorite "They just know, and the warm object doesn't accept them!" When faced with such "logic", it's a tossup whether to laugh, cry, or fetch a strong beer... --- I have to compliment fellow posters for their patience too - Dikran, les, muoncounter, everyone else. I find this website a fantastic forum for civilized discussion; I cannot think of another place where this wouldn't have degenerated into poorly written invective a long time ago.
  10. Re #427 Dikran Marsupial I agree (a)->(e) But then you wrote:- "(f) The atmosphere being warmer than space thus insulates the surface from space, causing it to be warmer than it would otherwise be." Insulate is not really the right word for the atmosphere. The gases of the atmosphere are free to mix and flow, convection is happening all the time, none of these produce the effects of an insulator. "g) This does not violate the second law of thermodynamics as the net transfer of heat is from the warmer surface to the cooler atmosphere." Couldn't agree more! "(h) The greenhouse effect thus does not violate the second law of thermodynamics, and Roy Spencer (amongst others is perfectly correct on this point and G&T are flat wrong)." I'd like to know how you think the GHE effect works. 2nd law of thermodynamics states that energy does not transfer spontaneously from a cold place to a hotter. For example, can energy transfer from a place at 0K to a place at 50K? If this were true I suggest that energy might indeed transfer spontaneously (in the atmosphere) from 250K to 300K but I don't see it happening somehow.
  11. Re #434 KR you wrote:- "It does not, can not, block energy from moving between a cold object and a warm object," My question is, can the "energy [from] moving between a cold object and a warm object" raise the temperature of the warm object, as described in the GHE? If so how do you calculate it?
  12. Damorbel, The energy from the cold atmosphere is not enough to "raise the temperature" of the warmer Earth. It keeps the Earth from cooling as fast as it used to. The input energy from the sun stays the same. Since the input energy stays the same and the Earth cools slower the equilibrium temperature of the Earth goes up. The net energy flow is from the warm Earth to the cold atmosphere (as required by the second law), but the greenhouse gasses slow the cooling energy transfer. Standing on the Earth surface slower cooling with the same input energy means hotter. The GHE is not to move net energy from the cold atmosphere to the warm Earth. The GHE slows the rate at which the Earth loses heat to space.
  13. damorbel re. (f) you are quibbling about the meaning of words again, rather than addressing the fundamental point. Convection and conduction cause a transfer of heat energy from the near surface to the upper atmosphere, but heat energy can only be lost to space through radiation from the upper atmosphere. Hence the atmosphere as a whole acts as an insulator. Do you agree with the fundamental point that the atmosphere being warmer than outer space means that the surface looses heat energy to space more slowly than it would if the atmosphere were not there? Please can you give an answer to h, I have already explained the basics of how the GHE works in (a)-(f). The only bit I have left out is exactly how the equilibrium temperature is determined, which is not required to refute the assertion that the GHE violates the second law of thermodynamics. If you want clarification of a particular point (a)-(f), then ask specifically. Do you agree that the atmosphere impedes energy, in the form of LW IR, being radiated from the surface from reaching space, but does not substantially impede the SW radiation from the sun reaching the Earth's surface? As to your last point. Consider two black-body objects, A and B placed close to eachother in a vacuum at 0K. Both are fractionally above absolute zero, such that the rate at which they randomly emit photons is very low, but body B is fractionally warmer than body A. We observe both bodies for a period of time that is about the same as the period between IR photons being emmitted by the cooler body. Now in some of those periods of observation, a photon will have been emitted by body B and will be absorbed by body A, making it slightly warmer. In a smaller proportion of observation intervals, a photon will be emitted by object A and absorbed by object B. In that case there has been a transfer of energy from a cooler object to a warmer object. This doesn't violate the second law of thermodynamics however, as on average there will be more intervals where the photon went from warmer to cooler than from cooler to warmer, as the warmer object will be emitting more photons.
  14. Re #438 michael sweet you wrote:- "The energy from the cold atmosphere is not enough to "raise the temperature" of the warmer Earth." But the IPCC says GHE is warming the Planet by 33C - up from 254K to 288K. That is not just a little bit, it's a whacking great amount. Also you wrote:- "It keeps the Earth from cooling as fast as it used to." But radiation from water and CO2 is the way the atmosphere loses heat to deep space. GHE theory says 'more CO2 absorbs more heat' (I thoroughly agree) But doesn't 'more CO2' also radiate more heat into deep space? Seems to me 'AGW due to CO2' has no validity.
  15. damorbel: Seems to me 'AGW due to CO2' has no validity. Then to what do you attribute AGW?
  16. Re #438 Dikran Marsupial you wrote:- "Do you agree with the fundamental point that the atmosphere being warmer than outer space means that the surface looses heat energy to space more slowly than it would if the atmosphere were not there?" Like I said to Mike Sweet, more CO2 in the atmosphere means the atmosphere become a more powerful radiator of heat, don't you agree?
  17. damorbel I see you have gone back to refusing to answer direct questions again and instead are opting for evasion instead. "Like I said to Mike Sweet, more CO2 in the atmosphere means the atmosphere become a more powerful radiator of heat, don't you agree?" Even though your question is evasion, to avoid answering my question, I'll answer it anyway. No, I don't agree. The more CO2 in the atmosphere, the higher in the atmosphere IR has to be emitted before it isn't absorbed by CO2 in the layers above. The lapse rate means this layer will be colder, and hence the less radiation that is emitted and so the atmosphere below and hence the surface will become warmer. If you were familiar with the basic mechanism of the GHE you would know that.
  18. Re #440 RickG you wrote:- "Then to what do you attribute AGW?" I am much happier with 'Climate Change' or even better 'Climate Variability' Most of the Sun's heat comes in at the tropics, the atmosphere and the oceans spread this over the globe by various currents that do not follow stable routes; the re is a great tendency to turbulence in fluid flow, even on a global scale. The most obvious example of this is the El Nino current which is highly unstable but there are many, many other currents all of which have powerful heat carrying capacities. Monitoring current flows would go a little way to explaining climate changes. Also Svensmark's cloud generating cosmic rays would have a powerful effect on heat transport in the atmosphere, quite enough to explain ice ages.
  19. Re #442 Dikran Marsupial you wrote:- "If you were familiar with the basic mechanism of the GHE you would know that" I am familiar with a variety of GHE explanations including the "higher altitude radiation" one you cite which is quite different from the "backradiation" one; the one that busts the 2nd law of thermodynamics. None of these AGW - CO2 models stand up to serious examination. But I agree that there are rather too many understandings of how thermodynamics works; even simple matters like the definition of temperature are wildly misunderstood and wrongly taught, particularly at universities.
  20. damorbel The "higher altitude radiation" and "backradiation" are both parts of the same theory (neither violate the second law of thermodynamics). Again you are demonstrating that you do not understand the mechanism of the greenhouse effect. I'm also not going to get drawn into a discussion of the definition of temperature, that is just another attempt at to derail the discussion. You are still avoiding answering the questions I posed in earlier posts.
  21. I think I should have stayed out of this conversation, but I will answer your direct question. More CO2 does not mean more radiation into space. The increase in CO2 raises the level in the atmosphere where radiation escapes into space (see post 442 above). Since it is colder higher in the atmosphere less radiation is emitted into space and the planet warms. You are defining what is "warming" and what is "heat transfer" incorrectly. CO2 warms the lower atmosphere by reducing heat loss. This is consistant with the second law. The IPCC says the greenhouse effect "warms" the lower atmosphere 33C by reducing heat loss to space. The CO2 does not contribute energy to the system. My blanket warms me 10-15C every night by reducing heat loss from my body to my room. Where is the problem? I will leave this to the others who are trying to explain it to you, I don't think I have anything to add that they have not said before.
  22. damorbel: "Svensmark's cloud generating cosmic rays" Wow, you think that thermodynamics is 'wildly misunderstood,' but you accept Svensmark as a 'powerful effect'? The GCR->clouds->cooling idea fails rigorous scrutiny; even the CERN CLOUD experiment, tailor made by Kirksby to test the idea, is producing "underwhelming" results. See Its cosmic rays. Are we on ABC territory here? Anything but CO2?
  23. I'm still waiting to see whether damobel would change his mind if an experiment showed a result that was incompatible with his application of 2nd Law (and compatiable with standard radiative physics textbooks).
  24. Folks - we've now had 'natural cycles', 'cosmic rays', 'heat of compression', 'it's the sun' and 'thermodynamics and temperature are poorly defined'. And that's just in the last three days. Before that there was 'albedo', multi-layer insulation, 'sunlight can't make it out of the water', 'constant disequilibrium', 'elastic collision of photons', and the lovely bit of obfuscation, "All materials, even gases, have a refractive index >1, consequently no material substance can behave according to the definition of a black body". And without really defending any one position - just moving to the next whenever getting nailed down in contradictions - not the behavior of someone who actually deeply believed anything they were stating. damorbel is, I believe, a troll. His sole purpose here appears to avoid any answers or conclusions, rather than to reach one. I suggest we not feed the troll...
  25. @449, Agreed, especially with your last comment.

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