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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 901 to 925 out of 1393:

  1. Re #894 KR you wrote:- "damorbel - Are you asserting that an individual photon (with a particular energy) can be used to identify the temperature of the object that emitted it, thus affecting it's absorption? " Yes I am, that is the basis of all quantum interactions. Since 'an object' can be an individual particle, I don't see the diffiiculty. Further you wrote:- "Or that the possibility of absorption is not a function of photon energy and absorption spectra? If so, you are sadly mistaken." Does this mean that photon interactions do not need to be quantised? Well I don't accept that photon interactions can occur in a non quantised way; throughout the whole electromagnetic spectrum photon interactions with massive particles are always quantised; both on the macroscopic level (many particles described statistically) and at the individual particle level. NB Photons do not interaction with each other.
  2. darmorbel @897 Wein's displacement law, and the spectrum you allude to apply only to black bodies. Gases (such as CO2 and H2O) are not black bodies. The photons they emit will correspond to some of the frequencies of a black body, but their distribution of intensities will not. It is therefore not appropriate to talk of "temperatures" of photons. The energy of a photon is only related to the quantum states involved in the decay when that photon is emitted. Black bodies are "black" because they have a wide range of quantum states, merging into a continuous band. Otherwise they wouldn't be "black".
  3. Darmorbel @901 "damorbel - Are you asserting that an individual photon (with a particular energy) can be used to identify the temperature of the object that emitted it, thus affecting it's absorption? " Yes I am, that is the basis of all quantum interactions. This is incorrect. A molecule of CO2 (for example) has 4 (one doubly degenerate) vibrational modes. These occur at 2565 cm-1 1480 cm-1 and 526 cm-1 If a molecule is in the 1st level excited bend state it can emit a photon at 526 cm-1 But this reveals nothing about how many molecules are in excited states in the other vibrations or rotational excited states or the translational energy that the molecules have.
  4. damorbel - You have completely misread my question; I have to wonder if it's deliberate. You have two complete strawman arguments misstating what I asked. I asked about a photon and "...the temperature of the object that emitted it...". You replied regarding an individual particle, not an object temperature, strawman. An individual photon does not give the temperature of an emitting object. It doesn't even give the temperature of a single molecule, as there are multiple possible emitting down-transitions in electron orbitals for an excited molecule; it doesn't have to drop to ground state. Your answer is incorrect! "Does this mean that photon interactions do not need to be quantised?" - All photon interactions are quantum interactions, I never stated otherwise, strawman. The issue is whether photons emitted by a cooler object can be absorbed by a warmer object (ensemble temperatures), namely the atmosphere and the surface. Since an individual photon does not indicate it's emitting objects temperature, it's absorption depends only upon the absorptivity of the warmer object (determined by quantum interactions, which are off-topic) and the individual photon energies. You are really reaching here.
  5. Re #898 KR you wrote:- "Actually, the peak of the curve is the mode, not the average; the two are not identical unless the distribution is symmetric. That's a fairly common error. " This is a better explanation of mode The relation of temperature to the peak is, once more, a critical one in quantum theory.
  6. I referred to the Statistical Mode, not emitting, molecular, orbital, or other modes. Mistaking statistical mode for mean (average), or vice versa, is a common error. The rest of your post is rather irrelevant to the discussion.
  7. Re #904 KR you wrote:- "I asked about a photon and "...the temperature of the object that emitted it...". You replied regarding an individual particle, not an object temperature, strawman." I asked you to define 'an object'. Until you do, I don't know what you are talking about. So pleeaaase - do it now! For example you wrote:- "as there are multiple possible emitting down-transitions in electron orbitals for an excited molecule; it doesn't have to drop to ground state." Yes and these transitions can be (almost) anywhere in the EM spectrum where many other kinds of interactions can take place; but please let us stick to thermal matters, i.e. interactions that affect temperature.
    Response: [muoncounter] Pedantic requests, such as the definition of 'an object,' are exactly what have put this thread at over 900 comments, many mere attempt at distraction. If you have difficulty with definitions of everyday language, how can your definitions of scientific terminology be taken seriously?
  8. When the fish is in the boat, it may tend to flop and wiggle violently, desperate to once again command the murky, muddy waters in which it has evolved to thrive.
  9. damorbel - "I asked you to define 'an object'." I cannot find such a request from you. But I'll answer it now that you have asked. I am speaking of relevant objects to the radiative greenhouse effect - the surface of the Earth, and the atmosphere. Both are large, ensemble objects, containing temperatures defined by thermal distributions, with thermal emission and absorption spectra defined by their component molecules. You cannot determine the temperature of such an object from a single photon. Single photons have a single energy, a single wavelength. Absorption likelyhood, the real question at hand, is based upon absorption spectra and individual photon energies. So I will ask again: Are you asserting that the possibility of absorption of a photon is not a function of that photon's energy and the potentially absorbing object's absorptivity spectra? DSL - rotfl.
    Response: [muoncounter] Oddly enough, damorbel used to agree:

    We are all familiar with the Planck spectrum, the amplitude of which is a function of the temperature, But taking one photon (with energy a function of frequency), or even one spectral component, does not represent the entire spectrum thus the temperature is not defined. Although a single photon has energy it does not have a temperature.

    See comment#70, this very thread.

    One could only describe this behavior as 'doublethink.'

  10. damorbel, you seem insistent that the following 'fact' is of great import: - the energy of a photon can tell you the 'temperature' of the particle that emitted it Firstly, a single particle doesn't have a temperature, which is a statistical measure. It has an energy. This seems to have been explained to you before. However we are discussing gases, liquids and solids that, as has been explained, emit photons with a range of energies. These spectra are clearly the applicable measures when discussing radiative energy transfer. Next, from what people have said, it seems you believe that a photon emitted by a cooler object cannot be absorbed by a warmer object. Is this an accurate description of your stance?
  11. damorbel, "What % of the heat tranferred to the atmosphere from the ground by radiation:- 14%?......40%?.......90%?" "That is the question I asked you. But I will accept evaporation and convection as 'kinetic'." I'm not sure what you're getting at here or why this matters, but I'll give it a stab. If over 90% of the thermal mass of the planet is in the oceans and about 5% is in the land mass, that leaves maybe a percent or two in the atmosphere? Are you looking for an actual number? Again though, all the kinetic energy flows from the surface to the atmosphere and back to the surface, relative to the radiative budget, are zero. They have to be because all the energy leaving at the top of the atmosphere is radiative. Also, I haven't read this entire thread, but I'm not sure I understand you're fundamental objection here. I don't see how the greenhouse effect violates the second law because it's not about energy going from cold to hot in conduction process. Are you claiming that a photon cannot travel from the colder atmosphere toward the warmer surface?
  12. Damorbel if you keep digging at that rate, you will soon find yourself in molten iron. Wein's law says nothing like what you imply. It says what is the most likely frequency of photons emitted by a source according to the temperature of that source, provided it is a blackbody. If there are several sources emitting at the same time with overlapping spectra, all it tells you is how likely it is that one frequency originated with a source rather than another. If I'm in a room with some light coming from outside (i.e. from the sun) where a light bulb is on, Wein's law can not allow me to determine what was the source of a given photon. Only the relative probability. To go back to the origin of this discussion, it must be made clear that a photon coming from the sun at a given frequency and one coming fron the light bulb at the same frequency will have the same energy. Exactly the same. You previously argued that it was not the case. Wein's law does not allow you to defend that either. Photons do not carry ID cards, no matter how badly your confused mind wants it. The energy of a photon depends only on its frequency. There is no way to tell where an individual photon originated only by examining its frequency. You could say where it could not have originated, that's all. I must say that throwing the photoelectric effect in the mix was yet one of your funniest moves in that strange display of yours.
  13. muoncounter at 909. I disagree. It can also be just double talk, or talk adapted to the need of the moment when conducting an argument with no other function than sowing confusion. Or could it simply be that Damorbel's own confusion is so thorough by now that he is completely lost in the whole thing?
    Response: [muoncounter] Doublethink includes the ability "to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw it back from oblivion for just so long as it is needed."
  14. 884 KR " The "2nd law" objection to the greenhouse effect is based upon a mistaken notion.." I don't disagree with you. However I'd like to suggest there is another problem. In the post, the statement of the 2nd law has missed out the phrase: "whose sole result". This is a statement that the 2nd law only applies to a closed system. For practical purposes, the system which consists of: the sun, outer space, the solid earth and the earths atmosphere is not a closed system. Outer space is, for practical purposes, an infinite sink. The sun is, again for practical purposes, an infinite source of energy. No one can deny that energy from the sun reaches the surface of the earth - at least, nor that radiated energy which isn't reabsorbed somewhere leaves the system... This is implicit in the text - body heat is in effect a source of energy external to the heat exchange system which is moderated with blankets. So long as some source continues to pump out energy irrespective of the destination of that energy, we're free to build an engine which uses that energy to, for example, concentrate it up to any temperature we can manage. How that engine works (photons, gases, cogs, whatever) is immaterial - the beauty of things like the statistical and termodynmaies is that they are defined for an abstract engine, which applies to all real engines...
  15. damorbel - I think you've been called out; "By thine own words shalt thou be condemned." To wit: "We are all familiar with the Planck spectrum, the amplitude of which is a function of the temperature, But taking one photon (with energy a function of frequency), or even one spectral component, does not represent the entire spectrum thus the temperature is not defined. Although a single photon has energy it does not have a temperature." - damorbel @70, this thread, 24/11/10 (thanks for pointing this out, muoncounter) versus: "But there is no need to have a certain number of particles to make a sample, so one particle with the same energy as the average energy of all the particles also has the same temperature as the whole sample." - damorbel @892, this thread, 31/3/11 Reductio ad absurdum - by contradiction you have disproven your own arguments. You are a troll - willing to say anything, even contradict yourself, in order to prolong an argument. Nothing you have written can be taken seriously, as you are not engaged in a scientific discussion. I have no idea as to your motivations. Perhaps you just like to argue - in that case I consider you a ( -snip- ). Perhaps you are arguing points you don't believe in for ideological reasons - in that case I consider you an ( -snip- ). Or perhaps you do this because it's your job? I'm familiar with that last case; my brother spent years as a denialist of second hand smoke dangers for a major tobacco company. In that case I would ask you the question I asked him - "How much does a soul go for these days?" Overall, I'm disgusted. Everyone - I would encourage you to consider this demonstrated behavior when evaluating anything that damorbel writes, whether here, or on his multiple attempts to redefine the Wiki page on thermodynamics.
    Response: [DB] I completely agree with you, word for word, but I have a role to fulfill. Sorry for the snips.
  16. A bit of summary here, then. I'll put it in the form of a proof for clarity. - Individual photons have energies, but these do not represent the temperatures of the objects that emitted them. You can say what objects could not have emitted that photon based on temperature, but not which one has. - Absorption of a photon by an object (warmer or colder than the emitting object) has a likelyhood based upon the absorption spectra and the energy of the individual photon; not the temperature of the emitting object. - The Earth's surface has an emissivity and absorptivity of ~0.98 in the IR spectra, so 98% of those photons impinging will be absorbed. - 98% of surface impinging atmospheric thermal radiation (aka "backradiation") will be absorbed by the Earth, as per the Earth absorptivity and atmospheric emissivity spectras. - Each photon absorbed, by the first law of thermodynamics, adds to the internal energy and hence temperature of the absorbing object. - The emitting mass of the atmosphere (due to the lapse rate) is colder than the Earth's surface. - Hence a colder object raises the temperature of a warmer object by it's presence. - Therefore: The assertion by Gerlich and Tscheuschner that a cooler object heating a warmer object violates the 2nd law of thermodynamics is categorically false. Q.E.D - Quod erat demonstrandum. --- I don't think that I need to say anything more on this topic. Adieu.
  17. DB - I understand the moderation role, not a problem. I would encourage everyone to form their own opinions of each poster's contributions in light of their content, and act accordingly.
  18. I would, KR, but if the light is coming from a cooler object, I won't be able to see the contribution.
  19. Perhaps, then, DSL, 'the cool shall rule' by virtue of superior vision?
  20. DSL and KR @ 918, 919. ROFL Thus the troll is reduced to its initial insignificance. Reality and the reality-based have prevailed. Yeah!
  21. Re #900 KR You wrote: "and that this absorption (by the 1st law of thermodynamics, conservation of energy) affects and slows the total, net energy transfer to the atmosphere and hence to space." You write about the 1st Law and radiation as if these were the only two energy processes involved - you take into account radiation only. But for a thermodynamic analysis you must include all forms of energy involved in the whole thermodynamic system that comprises the atmosphere. By confining your consideration to radiation only you may well get the answer you are seeking but that is hardly science! As I have mentioned before, you must also account for the gravitational energy of the gas that makes up the atmosphere; it is, after all, the gravitaional component that gives the troposphere its temperature profile (lapse rate) of -6.5K/km. Any attempt to may an 'energy balance' that doesn't include gravitational energy is not going to give an accurate picture.
  22. damorbel - I'm quite surprised to see you back. Have you read my posting here? Evaporation, convection, and the adiabatic lapse rate have all been covered in tedious detail on this thread; if you're interested, look it up. But (personal opinion) I do not consider it worthwhile to debate with someone who (like you) is willing to contradict your own posts in order to prolong an argument - that is trolling, not science.
    Response: While these topics have indirect relevance to the 2nd law and its relationship to the GHE, this thread is not intended as a substitute for a college level physics course. As you pointed out, these topics have already been covered here in excruciating detail. Future off-topic or repetitive comments will be deleted.
  23. Damorbel, you didn't need anyone's help to cover yourself with ridicule. It is glaringly obvious that, not only you haven't the fuzziest idea about these matters, but you are willing to contradict yourself for the sake of argument. This is rather amusing: in the instance noted above, you later adopted an incorrect position, opposite to your originally stated one, which was correct. And you defended the latter with all your rethorical might. Really, that is comical. Cut your losses.
  24. KR When investigating complicated system it's important to approach the mechanism front-wise and linearly. Starting at the start (solar input) avoids confusion as introduction of secondary input/variables (forcing) manifest. If in fact, all the tenets of GHG theory are valid then such a stepwise approach will only hone their formulation. Let's start with what we undoubtedly agree: (e1)emissivity + reflectivity =1 earth's albedo =.3 (e2)emissivity = absorbed energy/ incident energy---or stated continuously--- emissivity = absorbed power/ incident power (e3) σTe4= S/4 * (1-A) flux density emitted via blackbody earth = flux density absorbed via blackbody earth (note to muoncounter, blackbody equivalent) So σTe4= 240 W/m2, represents the theoretical maximum power emitted and adsorbed, via SW, by the surface. This theoretical max flux can be used to calculate actual max flux absorbed by earths surface. Intuitively it make sense, a surface must absorb energy before radiating said energy. So properly, this must be calculated prior to surface to atmosphere emissivity consideration. Also, immediately jumping to actual surface temp and backing out flux, will as said earlier lead to erroneous conclusions. (e4) Because 240 W/m2 is max blackbody absorption it is equal to max incident power. (e5) Earths ε = .98. Using (e2) earths actual absorbed power =.98 * incident power = .98*240 W/ m2 = 235 W/ m2 (e6)Because 235 W/ m2 represents the true absorbed SW radiation value it also represents earth's maximum gray body emissions due to SW. (e7) When surface LW emission flux is equal to SW solar absorption the earth's system is in equilibrium; 235 W/ m2 equates to 254K. Because the 1st law must be upheld this represents the temp maximum via solar radiation. Any additional temperature increase must come from non-radiative energy input. I suspect you are screaming "What about forcing". Ok, by adding radiative reflection/re-radiation flux to solar input, with white, black and gray atmosphere emissivities, LW forcing is easily evaluated. The following section (e8),(e9), (e10), demonstrates GHG physics. Specifically, by adding flux regardless of quantitative magnitude and/or vector magnitude. (e8) Atmosphere ε=0 Teq=time to equilibrium (e7) (235 W/m2) White Note: As Time approaches 2Teqthe Surface flux approaches infinity. Also, as Time approaches 2Teq the atmosphere becomes transparent to visible surface emission...when visible emissions = 240 W/m2 TOA equilibrium is at hand. According blackbody emissions this equates to ~1200K. (e9) Atmosphere ε=1 Teq=time to equilibrium as defined by (e8) Blackbody
    Note: As time approaches 4Teqthe Surface flux approaches infinity. Also, at 3Teq the atmosphere radiates 235 W/m2 plus the 5 W/m2 originally reflected by the surface (e5) give the required 240 W/m2 TOA...302K. (e10) Because(e8) confers the maximum temperature (~1200 K) for TOA equilibrium and (e9) confers the minimum temperature (302K)for TOA, radiative forcing is shown to be a false mechanism. That is, since actual temperature (288 K) is well below the minimum temperature established by blackbody atmosphere, ε < 1, will generate a temperature higher then 302K. (e11) If gray body ε=.612 then, according to (e1), gray body reflectivity = .388 (e12) If gray body ε=.612 then, according to (e2), gray body absorbed = incident flux* ε. (e13) Atmosphere ε=.612 Teq=time to equilibrium as defined by (e8) Gray Note: TOA is achieved when surface radiates 768 W/m2...341K. (e14)As demonstrated, a body's emission can not be increased by it's own reflection,re-radiation, or insulation. As demonstrate lower energy does not increase higher energy, low light does not make more luminous a brighter surface. As demonstrated atmospheric forcing, GHG physics is a false mechanism which in fact violates the 2nd Law. Choosing to ignore this fundamental law leads to fallacious results. Fallacious result such as 341K with an atmosphere emissivity of .612. But just as fallacious misapplication of physics which leads to ε=.612 equating to 288K. Notice I did not say atmospheric radiation does not exist...I did not say the downward radiation does not exist. Atmospheric radiation is isotropic however, lower energy atmospheric radiation can not increase the higher energy surface. Solar input, assuming .3 albedo, can only account for 240 W/m2 flux and therefore delta T between solar input and actual temperature must be a result of non-radiative input.
  25. 924 L.J. Ryan. Nice embedded link to the lecture notes of some Professor Jin-Yi Yu - who has some other nice explanatory material for those finding this all a bit hard. Where did the rest of the material come from (it is only polite to reference sources, after all)?

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