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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 1351 to 1375 out of 1393:

  1. YOGI... This is the point where I usually suggest to "doubters" that they need to collect and publish their findings, if they can. To everyone else reading this, we accept the past 150+ years of research that makes up the basis for the greenhouse effect. This is established physics and is not in question by... well, by anyone really. You're incomplete understanding, and your desire to find some fault somewhere, is driving you into a circuitous pattern that has no end. If, somehow, someway, you managed to connect all your dots and come up with a theory that was complete it would literally mean re-writing the past 150 years of physics. You'll please excuse me if I suggest that the likelihood of this is extremely low, to the point of being a near impossibility. But if you want to try, knock yourself out. Outside of that you are clearly exhibiting a Dunning-Kruger effect and wasting everyone's time and energy.
  2. YOGI What is the point of bringing up clouds? The blanket analogy is meant to explain the effect of greenhouse gases on loss of heat from the earth's surface. Clouds require a different analogy entirely, one that also accounts for changes to albedo or incoming energy. As Phil notes, in this analogy the absorption of solar energy by the earth's surface is the equivalent of heat released by the human body as a result of metabolism. Both the earth and the human body are open systems thermodynamically, although they receive energy in different forms. From that point of view atmopsheric GHGs and blankets act similarly, both reducing loss of heat generated by absorption of solar energy or metabolism. The analogy does break down eventually if you take it far enough, as all analogies do. For example, the human body actively alters its metabolism to maintain a relatively constant core temperature, while the earth actually appears to amplify variations in solar radiation. That makes calculation of equilibrium surface temepratures different in each case. But the point of the analogy is not to represent energetics of the earth system, but simply to make the GH effect tangible by relating it to common experience. The blanket analogy succeeds on that front.
  3. Mods...Sorry about the triple posts...thread didn't update properly. Delete accordingly.
  4. I have to agree with Rob above. If there was some disagreement between the GH effect and the 2nd law, it would have been noticed by at least one of the thousands of scientists, all steeped in the Laws of Thermodynamics, who have studied some aspect of GHGs over the last 150 years. Instead of arguing about semantics and analogies, the doubters need to produce a reproducible experiment that can withstand peer review. The fact that haven't done so, despite the fact that it would yield a Nobel Prize, is telling.
  5. Stephen Baines The back radiation is less than the heat radiated from the surface, so the net flow of heat is from warmer to cooler, so it evidently does obey the second law of thermodynamics. I suspect that the problem is that the common definition of the second law that crops up pre-dates statistical mechanics and concept of a photon, in which case it is perhaps understandable that it doesn't specify the net flow of heat (implying that there may be an exchange of energy, but that it is biased in the direction from warmer to cooler).
  6. DM Yes, I totally agree. Were the doubters to take measurements and do the actual science, they would find that the GHE is perfectly in line with thermodynamics. But they don't do the measurements and instead engage in endless obfuscation, which indicates that they are not actually serious about the issue. It is downright laughable that physicists would not 1) recognize that the 2nd law was contradicted and 2) would not do anything about it if they did. It's also downright depressing that this thread is going on 28 pages!!!
  7. OK cover the Earth with a blanket (FULL CLOUD COVER) and see how cold it gets Venus?
  8. Sadly, this thread is the happy hunting ground for the wilfully ignorant. It's wasting time trying to teach those who are determined not to let facts undermine their fantasies.
  9. Actually, I think every denier should be forced to read this thread, and then to look in a mirror, and recognize that to a lesser degree they are doing the same thing. It may be easier to argue their position, or to talk themselves into believing their position has substance, but in 99% of cases it comes down to the exact same thing.
  10. How come if 239W/m go in and 239W/m go out, that 396W/m exists within the system ? surely that violates the conservation of energy law ?
  11. Eric (skeptic) Venus has 92 bar surface pressure. Around 53 km up, pressure and temperature are comparable to those at Earth's surface.
  12. While I strongly doubt YOGI is actually interested in understanding this, but Do Trenberth and Kiehl understand the first law of thermodynamics rather exhaustively covers this. Understanding however requires getting your head around the physics not looking for talking points. Claiming the diagram is "wrong" is tricky when those flows are from measurements.
  13. Stephen Baines "What is the point of bringing up clouds?" Its the only fair equivalent to a blanket. Full cloud cover at night is the surest way to keep the heat in. But as the Earth is externally heated, it will cool with total cloud cover, as the albedo will be huge. GHG`s have huge holes so are not exactly a blanket.

    [DB] Way back here, you were asked to succinctly put forth the one objection that you wanted to hang your hat on.  You have made 6 comments on this thread since then and in exactly...none of them have you done so. Failure to do that amounts to a de facto admission that you are here to simply waste the time of others.

    Cease making unsupported assertions (i.e., lacking support in the peer-reviewed literature published in reputable journals) until you can demonstrate that you are able to carry on a science-based discussion in this forum.

  14. YOGI... Look, people are not trying to pile on, even though it may seem like it. What is frustrating is that you're clearly confusing your lack of knowledge on this subject with there being something wrong with greenhouse theory. It's a very complex subject. But rather than assuming that 150 years of research has somehow produced an error that has somehow slipped past 10's if not 100's of thousands of scientists, how about just acknowledging that maybe you need to be better trained to even begin to understand this subject. It's great that you are trying to propose questions but the ones you're asking are really pretty easily answered if you take some time to better understand the subject matter.
  15. YOGI writes: "How come if 239W/m go in and 239W/m go out, that 396W/m exists within the system ? surely that violates the conservation of energy law ?" Really? So, on your world, if the stream feeding into a lake carries 239 m^3/s and the stream flowing out of the lake also carries 239 m^3/s then the lake cannot possibly hold 396 m^3 of water? How very sad for you and your world where clouds do not precipitate out of the atmosphere when the temperature decreases. Here on our Earth planet things work differently.
  16. [DB] I`ll hang my hat on this:

    [DB] You are thus relatively off-topic on this thread.  The more appropriate thread for your discursion is Postma disproved the greenhouse effect.  Everyone please respond there.

    Yogi:  Please read the above thread and the following threads in their entirety

  17. DB, actually the PDF specified by YOGI appears to be a different screed by Postma wherein he insists that the "Greenhouse Theory" contradicts the laws of thermodynamics. Including alot of the usual 'energy cannot flow from a colder area to a hotter one' idiocy. So, complete nonsense... but on topic nonsense.

    [DB] Noted.  But as Dikran points out, Postma commits a great number of grievous errors in this new piece.  Perhaps a continuing series on Postma, with this one comprising "Joseph Postma is Wrong About _________, Pt. 4"?

  18. I think YOGI has to understand that all mass exibits energy, and that all mass is continuously cooling by emitting energy. What slows that cooling is the absorbtion of energy. The mass doesn't care if the energy came from a colder object or a warmer object. When mass absorbs a photon from a colder object, all that happens is the RATE of cooling slows a bit. The whole crux is that the Sun adds energy, which will result in warmth, but matter is still doing its thing by trying to cool.
  19. Well I got as far as page 3 before spotting the first scientific error (mainly becuase the first two and a half pages are rhetorical posturing and essentially free of any scientific content). Postma writes "Does back-scattered infrared radiative transfer act like a blanket upon, and explain the temperature of, the surface of the Earth, analogeous to the way a greenhouse building works..." Immediate fail. Greenhouses work principally by preventing convection. Nobody claims that the Greenhouse effect works in the same way an actual greenhouse, even Fourier knew this.
  20. Next error, page 4 "A blackbody is simply exactly what it sounds like: an object that is completely black. The reason why it is black is because it absorbs 100% of the light that strikes it, and doesn't reflect any of it back. Therefore it appears black" The sun is a blackbody, doesn't appear very black to me. The middle sentence is sort of true, a blackbody does absorb all the light that falls on it, but that doesn't mean that it appears to be black. I know that is only a fairly trivial point, but it doesn't encourage confidence when it comes to less trivial issues. Note this is the second scientific point in the paper so far and Postma has made errors in both of them.
  21. Next problem, first paragraph of page 5 tries to explain "radiative thermal equilibrium" without mentioning that the blackbody is constantly radiating heat. Not strictly speaking an error, but again it doesn't inspire confidence.
  22. Catastrophic error - second paragraph of page 5 When a blackbody has reached thermal equilibrium, it can no longer absorb more light for heating and therefore has to reemit just as much energy as it is absorbing" This is factually incorrect; the blackbody never stops absorbing light - it is a blackbody, that is what they do. Likewise if it is above absolute zero it also never stops emitting radiation. It reaches radiative equilibrium when its emissions (which depend on temperature) rise suffuciently high that emission matches absorption. If Postma can't even explain radiative equilibrium or what a blackbody does correctly, I think we can safely say that YOGI needs to pick his/her has up off the floor.
  23. Bottom of page 5 "For example if the object absorbs visible light, then it will reemit infrared light, which we can't see, and therefore it still appears black" Postma half-corrects his initial definition of a blackbody here (would have been better just to get it right in the first place), but this statement is obviously incorrect. As an example, consider solar power plants which can be used to make objects glow white hot (and thus be producing visible light) Virtually every scientific statement Postma has made by the end of page 5 is wrong.
  24. top of page 6 "As amazing as this sounds, the only thing that really does seem to perfectly resemble a black body is the entire universe itself" So this light that the entire universe is absorbing. Were is it coming from exactly?
  25. Dikran Marsupial: YOGI also needs to understand that what you a describing is basic physics, and has nothing to do with the GHE. Matter behaves in predictable ways. How one wants to apply it to the GHE is up to each individual, but in that just CANNOT throw basic physics out the window and expect conclusions that have any merit.

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