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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.

Last updated on 22 October 2010 by TonyWildish.

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Related Arguments

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 1401 to 1406 out of 1406:

  1. BernhardB your misunderstanding about radiators has been addressed. Since you ignored my earlier comment, I'll ask you again the question, which was also asked before to other GHE deniers on this thread: if there is no radiative GH effect, where does the downwelling IR measured at the surface come from? No GHE denier has yet come up with a good answer, perhaps you have one.
  2. It is not often I am moved to defend Dr Spencer's reputation, but in this case I can make an exception. What Dr Spencer said, that has been misinterpreted by BernhardB, was:

    "The more you can reduce the rate of energy loss to the cold walls, the hotter the plate will get. Yes, the surrounding objects act to control the rate at which the plate can lose energy. I have no idea what happens if you can keep the plate from losing energy at all….I suspect the heater wire melts "

    I say misinterpreted because Dr Spencer is clearly discussing the case of no heat loss. The only way a convection based heat sink place in a vacuum would lose no heat is if all external surfaces emitted no thermal radiation. Of course, if they did that, then there would be no "back radiation" between adjacent fins of the heat sink so BernhardB's thought experiment would not hold. So BernhardB has taken a correct explanation by Spencer, applied it to a situation that does not satisfy the conditions Spencer specified, and then claimed that Spencer's prediction would fail, and that Spencer was talking nonsense when the prediction fails outside of the conditions in which it applies.

    I suspect Spencer would blow a fuse at this level of misrepresentation.

    Which brings me to the second point. BernardH claims that the only way the wire can be melted is by increasing the voltage. Given the large number of industrial applications for melting wires by increasing the resistance while holding the voltage constant, his claim is simply false. Perhaps the most common of those applications is electrical arc welding (most commonly as MIG welding), but others abound.

    (Image from wikipedia)

    So far BernardH shows that his arguments depend on not just miscomprehending the physics, but a complete failure to understand how common industrial processes and computer components works. It is not the esoteric, but the commonplace that shows BerhardH is ...

    (Image from BernhardB)

    Well, you get the idea.
  3. "I suspect Spencer would blow a fuse at this level of misrepresentation."

    Which is fitting since, as we all know, "blowing a fuse" is what happens when a wire designed to melt under certain conditions meets these conditions... :-)
  4. I wrote on the other thread:

    33. Steve Case at 12:03 PM on 1 April, 2012
    Tom Curftis #31 Wrote
    … Science of Doom has an extensive discussion of the difference of the ocean's response to heating by solar radiation and back radiation …

    I suppose this will be considered nit picking, but back radiation from the cooler atmosphere doesn’t do any heating of the ocean. It does slow the cooling of the ocean by canceling out part of the spectrum, but it’s the sun that does the actual heating and reestablishment of equilibrium. Yes, the effect is the same and it’s perhaps just semantics, but claiming that back radiation heats the ocean leads to erroneous thinking.

    Moderator Response: [DB] Your statement about back radiation is off-topic on this thread. Any who wish to respond to it please do so on a more appropriate thread. Thank you.

    So here I am and I find this right away:

      12. Daniel Bailey at 12:01 PM on 20 September, 2010
      ...The downward radiation adds to the energy received from the sun and heats up the surface of the earth more than if this downward radiation did not occur.
      It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction.

    And it doesn't mean that the colder atmosphere heats up the surface. It doesn't mean that the downward radiation heats up the surface either. It means as I stated in the other thread (see above) that part of the radiation from the surface is cancelled out and the surface cools off at a slower rate and so the sun at nearly 5800K continues to warm it up until that rate is again at equilibrium with the incoming heat energy from the sun. Perhaps this is considered trivial, but I see over and over again, statements that the ocean is being heated by the back radiation and the downward radiation heats up the surface and so on. It's not exactly right, and leads to 2+2=5 thinking.

    And what is that downward radiation that makes me say 2+2=5 is comparable to claiming the downward radiation heats up the surface? It's around 15 microns isn't it? And isn't the temperature of a body that radiates mostly at 15 microns very cold? Around 200K or so which is (-100°F/-73°C) or about as cold as dry ice.

    Having said all that and from what I read, the greenhouse effect without considering feedbacks should warm things up about 1.2°C for a doubling of CO2.
  5. Increased back radiation (from increased levels of greenhouse gases) heats the ocean by altering the thermal gradient in the 'cool skin' layer of the sea surface. See SkS post: How Increasing Carbon Dioxide Heats The Ocean.

    It's true that back radiation doesn't penetrate into and heat the ocean but, by reducing the loss of heat to the atmosphere through conductivity, the oceans store more energy from the sun and therefore become warmer. That's why the ice core records show a strong relationship (correlation) between CO2 and global temperature:

  6. Steve Case @1404, the back radiation comes from a variety of frequencies, mostly associated with H2O emissions. Typically it is close to the surface temperature in brightness temperature. Globally averaged the back radiation has an effective brightness temperatures of 277 degrees K, compared to the globaly averaged effective brightness temperature of 289 degrees K for the upward surface radiation.

    Seeing we are into nitpicks at the moment, in some circumstances the overlying atmosphere is warmer than the surface so that it does warm the surface even in your use of the term.

    More importantly, the IR radiation from the atmosphere is absorbed at the surface causing an increase vibrational or translational motion in the absorbing molecule, which vibrational and translational motion is called heat. In the popular vocabulary, that means the atmosphere heats the surface. It is true that the surface radiates energy, and hence cools faster than the atmosphere can heat it, but that is almost irrelevant to the choice of terms.

    It is only "almost irrelevant" because some physicists have defined "heat" to mean "the net transfer of thermal energy" by which definition "heat" can only mover from the hotter to the colder body, and having moved ceases to exist (although the thermal energy doesn't) because heat only exists when thermal energy is being transferred. In so doing they have defined the term so that it is strictly inconsistent with popular usage of the term (causing endless confussion), and indeed, strictly inconsistent with the usage of the term by the greats of thermodynamics including Lord Kelvine, Rankine, Clausius etc.

    Any "2+2=5 thinking" as you put it, can be avoided by being aware that in the popular meaning of the term "to heat", the second law of thermodynamics must be stated as, "Net heat flow can only proceed from a warmer to a cooler body".
  7. @ RW1 at 11:08 AM on 16 December, 2011

    "There is no violation of the second law with the Greenhouse Effect, because it's not about energy going from cold to warm through a conduction process."

    You are confusing radiation (electromagnetic waves) with creation of thermal energy (heat). Heat is a process involving transfer of energy based on temperature - as opposed to generic radiation of photons. It follows that thermal radiation (a process creating heat) from a cold to a hot body (i.e. from the atmosphere to the Earth) is a physical impossibility. The Second Law prevents this because otherwise it would be possible to obtain work from transfer of heat into the atmosphere; i.e power station cooling towers. Essentially, we could then reuse the energy and build a perpetual motion machine. AGW is predicated on a misunderstanding of the Second Law which is thoroughly debunked by eminent German physicists (as opposed to climate scientists who are generally not professional physicists) Gerlich and Tscheuschner.
    Response: TC: Link to RW1's post added.
  8. Not this again!


    There is an enormous body of theoretical, experimental, and most importantly empirical evidence showing that the atmospheric greenhouse effect, however misnamed, is fact.

    It does not violate any law of thermodynamics, it does not allow any sort of perpetual motion machine.

    Gerlich & Tscheuschner are, simply put, wrong, wrong, wrong.

    For more in-depth information you can check out the blog Science of Doom which has some reviews of the G&T paper here. You can also check out some other important Science of Doom posts regarding the relevant physics here and here. Science of Doom relies heavily on the actual maths of the situation and basic radiative physics known for decades.
    Response: [DB] In addition to the valuable links provided by Composer99 above, please see this SkS post, Has the greenhouse effect been falsified?.
  9. Silas - I would suggest, as Composer99 recommended, to look at the Science of Doom site (search there for "Gerlich"), including such gems as On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009) and Radiation Basics and the Imaginary Second Law of Thermodynamics.

    Quite frankly, I cannot think of another paper in the field that has been so definitively and repeatedly shown to be dreck. G&T's work is absolutely horrible...
  10. KR at 05:24 AM on 24 November, 2010

    Heat flow is the unidirectional and is based upon the need for thermal equilibrium between bodies. Extremely basic physics. It is a process not a summation and therefore cannot be "negative" - an egregious nonsense. (-Snip-)

    [DB] Please familiarize yourself with this site's Comments Policy before composing future comments.

    Egregious inflammatory snipped.

  11. @Composer99 at 05:27 AM on 27 July, 2012

    I have read the G&T paper and have a background in engineering thermodynamics.

    It is incumbant upon proponents of the GHE to demonstrate their theory through (-Snip-) prediction of events in nature - i.e. unexpected and cataclysmic consequences of global warming. Confirmations do not count in science as such 'evidence' is always easy to find. (-Snip-)

    Response: [DB] Multiple examples of unsupported sloganeering snipped.
  12. Silas, then can I suggest then that you read the Science of Doom articles to which you have been pointed to and if that is unconvincing, then try the physics textbooks from which the author draws his points? There is nothing contradicting basic physics/thermodynamics here, just a misunderstanding of the physics at work. If G&T were right, then how would explain the MEASURED back-radiation at the surface or the drop in the energy band as measured at the TOA? (among the many experimental confirmations of normal physical theory). The experimental evidence is with conventional physical theory, not with G&Ts strange interpretation of LTE.

    You might also like to check out AP Smith rebuttal here.
  13. Silas - I would strongly recommend you read through some of the posts referred to above, some of the >1400 comments on this thread, actual reviews of what G&T presented - before deciding that all the people who have looked at the G&T claims are idiots. Science of Doom in particular is sourced by a physicist - and you seem to have prejudged his expertise.

    I would also point you to the Real Climate collection on this, which links to various commentaries and a peer reviewed comment - all rebutting the G&T nonsense. You might also look at Dr. Fred Singer's (a rather notorious skeptic of just about anything - ozone holes, 2nd hand smoking, climate change) characterizing 2nd Law objections as unsupportable and embarrassing 'denial'.

    "It is incumbant upon proponents of the GHE..."

    No, it is not; that work has already been done. The radiative greenhouse effect is supported by multiple lines of evidence, physics, observations, etc. G&T (and you, apparently) feel that all this data is incorrect - that's an extraordinary claim, and requires evidence supporting that isolated view to be taken seriously. The burden of (dis)proof is on you.
  14. Silas - I think one of the best summaries of the G&T paper comes from Gavin Schmidt, who had some of the same reactions to it that I did:

    It's garbage. A ragbag of irrelevant physics strung together incoherently. For instance, apparently energy balance diagrams are wrong because they don't look like Feynman diagrams and GCMs are wrong because they don't solve Maxwell's equations. Not even the most hardened contrarians are pushing this one....
  15. Silas, you wrote:

    "You are confusing radiation (electromagnetic waves) with creation of thermal energy (heat). Heat is a process involving transfer of energy based on temperature - as opposed to generic radiation of photons. It follows that thermal radiation (a process creating heat) from a cold to a hot body (i.e. from the atmosphere to the Earth) is a physical impossibility."

    It is difficult to interpret this in any other way than a prediction that there is no thermal (IR) radiation originating in the atmosphere and being absorbed by the Earth's surface. I like that. It is a risky prediction that is easily checked by empirical means.

    You later write:
    "It is incumbant upon proponents of the GHE to demonstrate their theory through ... prediction of events in nature"

    Presumably you therefore think it is incumbent on you, since you have made a risky prediction to actually check the data to see if your risky prediction is verified, or falsified by the data. Fortunately, climate scientists believe the same thing. They have predicted the existence of downward IR radiation from the atmosphere, and have checked. Indeed, here is a comparison of some of their predictions with observations:


    I don't want you to notice the very good correlation between AGW predicted and observed Downward IR Radiation. I want you to notice that the downward IR radiation exists, in direct contradiction of your prediction. If you follow the link to the source of the diagram (Science of Doom), you will find many other examples of observations of this radiation you claim cannot exist.

    Indeed, even the noted "skeptic" Roy Spencer is not so foolish as to deny the existence of downward IR radiation (back radiation). In fact, he has measured it himself:

    "For instance, last night I drove around pointing this thing straight up though my sunroof at a cloud-free sky. I live in hilly territory, the ambient air temperature was about 81 F, and at my house (an elevation of 1,000 feet), I was reading about 34 deg. F for an effective sky temperature.

    If the device was perfectly calibrated, and there was NO greenhouse effect, it would measure an effective sky temperature near absolute zero (-460 deg. F) rather than +34 deg. F, and nighttime cooling of the surface would have been so strong that everything would be frozen by morning. Not very likely in Alabama in August.

    What was amazing was that driving down in elevation from my house caused the sky temperature reading to increase by about 3 deg. F for a 300 foot drop in elevation. My car thermometer was showing virtually no change. This pattern was repeated as I went up and down hills.

    The IR thermometer was measuring different strengths of the greenhouse effect, by definition the warming of a surface by downward IR emission by greenhouse gases in the sky. This reduces the rate of cooling of the Earth’s surface (and lower atmosphere) to space, and makes the surface warmer than it otherwise would be."

    (Source, this link should not be interpreted in any way as agreement with Spencer's views on other subjects.)

    So, to the extent that you have predicted that there is no back radiation, you are wrong. Perhaps you would like to show your commitment to the principles of science by stating clearly that you are wrong. If you are willing to do so, we may be able to progress in resolving your conundrum.
  16. A note to the unwary.

    'Silas' is Girma Orssengo, who has in the past displayed an astonishingly blinkered misunderstanding of science. Arguing with this ardent Ayn Rand acolyte will get one nowhere, very fast.

    On the matter of the claim that 'cool' cannot radiate to 'warm', I'd invite Silas/Orssengo to visit

    where such nonsense might be kicked around the park, as was done with Tim Curtin, and thus save clogging the thread here. And as Orssengo is apparently a functioning engineer, I would invite him to explain somewhere in his discourse how energy moves through the lumen* of a Dyson sphere.

    [*Yes, it was deliberate...]
  17. Thanks Bernard. I've seen Girma's discourses on various forums so I wont waste my time. Just about anyone who jumps into this thread has to be viewed with some suspicion.
  18. @1407: "...eminent German physicists..."

    This must be a joke. A bad one.

    My turn:
    Italy once had one Gallileo. Germany must be truely blessed for it has got at least two.
  19. KR says
    ...... Science of Doom site (search there for "Gerlich"), including such gems as On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009) and Radiation Basics and the Imaginary Second Law of Thermodynamics.

    The Science of Doom site is an evolving platform.
    Leonard Weinstein has a recent guest post there.

    How the “Greenhouse” Effect Works – A Guest Post and Discussion.

    SoD agrees with the broad outlines of the post.

    Leonard Weinstein would agree with Silas that it is technically incorrect to say heat moves spontaneously from a lower temperature object to a higher temperature one.

    Why do some in climate science take issue with the technical language of thermodynamics?

    There is no debate in physics about whether or not heat can flow spontaneously from a lower to a higher temperature object.
    Thousands of physics textbooks and thousands of physics departments unanimously agree with Clausius that it cannot.
    To argue otherwise is to peddle pseudo science.

    To redirect folk to SoDs site may not have the result that KR intends.
    Response: [DB] Being argumentative, repetitive and pedantic constitutes sloganeering and is in violation of the Comments Policy. FYI.
  20. suibhne - I would refer you to the reference Chris Ho-Stuart pointed you to (his peer-reviewed reply) the last time you argued in support of G&T.

    I would note that suibhne has been repeatedly pointed to the errors in his physics (here, here), and stand by my recommendation
  21. The point of redirection to SoD is to help you understand the physics - that AGW does not in fact involve a mechanism that violates 2nd Law nor that it proposes a novel definition of 2nd law. Keep reading - try to understand what is actually happening.
  22. Soooo..did "PhysSci" ever get his paper published? We *could* have us an 'a-ha' moment from it!

  23. At the beginning of this thread there was b-j-m insisting that CO2 must block incoming and outgoing energy equally because both contain IR components, even after being shown that the emission spectrum of the sun and the Earth are nearly disjoint.

    Here, 1400 posts later, is suibhne, whom I remember from the the G&T debates years ago, still treating heat and energy as synonyms despite the many times that physicists and others have pointed out, with numerous familiar examples, that energy freely travels from cold to hot (and in every other direction).

    In between are numerous other examples of the same phenomenon -- simple, indisputable refutations of claims being rejected out of hand, ignored, or otherwise having no effect on the claimant, who simply repeats the claim in the same or a different form. All of the claims that the greenhouse effect is a violation of the 2LOT were already refuted in the original article, yet numerous people have simply repeated the claim. (And we even have at least participant here claiming in another thread that the assertion that people deny the greenhouse effect is a strawman.)

    People have patiently explained at length the errors in these claims, to no effect. Something can surely be learned from this, some lesson about pedagogy or psychology, but other than bad news I'm at a loss as to what can be taken from it ... how we can use this knowledge to improve our situation. Anyone?
  24. jibal - Appalling, isn't it? The lesson I take from this kind of discussion is simply this:

    There are, and always will be, idjits (IMO, apologies if strongly stated) who cannot be convinced (suibhne, Damorbel, Doug Cotton, others), who have blocks against a rational discussion, who are driven more by their personal worldview than facts that might contradict those. But clearly explaining their errors, even if they themselves cannot accept the data, provides the vast majority of reasonable people rational support they (I sincerely hope) appreciate for judging the issues.

    Most people can look at a discussion and recognize who is speaking from the data, from reason, and who is denying reality. While there is an element, as Friedrich Nietzsche said, of "At times one remains faithful to a cause only because its opponents do not cease to be insipid", I honestly believe that discussing the issues in a rational fashion can only assist those in the majority who might not have the time or training or inclination to personally dig into the for/against data issues.

    While I discuss matters with the deniers who raise objections, I try (insofar as as possible) to speak to the rather more silent majority. There are those who will never be convinced, but most people can clearly distinguish (given enough context, enough of an exchange) between a presentation of facts, and someone speaking from their nether regions.
  25. There are numerous thought experiments on 2nd law argued in this thread. Here is an actual experiment for those who think the 2nd law is broken to chew on.

  26. I want to post here my admiration of the persistence of those who have kept up with almost 1000 posts rebutting a rather obviously flawed argument, that starts from failing to observe that you can’t apply the 2nd law of thermodynamics unless you have a closed system. There is a continuous influx of energy from the sun, so the ground and atmosphere aren’t a closed system. Case closed.

    If Gerlich really is physics professor at an apparently good university who has real students, they should demand a refund if this is the quality of his understanding.

    BTW there are a few dead links in the Notes:


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