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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 126 to 150 out of 1393:

  1. I believe the substance of my criticism of your position is in a later part of my comment #122, namely:
    It would seem to me that there is not much question as to which of the below alternatives is more likely: - that all this empirical evidence (global cryosphere mass balance decline, sea level rise, changing migration patterns, pest prevalence in increasingly higher latitudes, temperature anomalies, &c &c &c) isn't really pointing to global warming via the atmospheric greenhouse effect - that you and damorbel have managed to misunderstand thermodynamics
    [Emphasis in underlines mine in this comment, not in #122.] That is, the inconsistencies are more the purview of skeptics/contrarians attempting to appeal to thermodynamics than of the scientific evidence supporting the existence of the greenhouse effect. When reviewing this thread, as a non-scientist (I am a musician by training) I have found the posts of the likes of muoncounter, Phil, et al to be clearer and more compelling than the posts of yourself or h-j-m. In particular, your blanket analogy in #117 practically mirrors the analogy used in the original post, save that you draw what appears to be a bizarre conclusion from it.
  2. "Let me jut explain these two worlds which I will just name here and there." h-j-m - what is really going on, is that you have misunderstood physics and created an invalid worldview which means you would incorrectly predict the outcome of experiments. The best approach is learn what it really correct and how they are reconciled (sit down with a text book). Making real world decisions (like voting on measures to deal with climate change) on the basis of an incorrect understanding is a bad thing.
  3. Re #126 Composer99, you wrote:- "your blanket analogy in #117 " Composer99, it isn't an analogy. A blanket is one sort of insulator. Anything that interferes with heat transfer can be an insulator of greater or lesser effectiveness. Your doubts about my competence in thermodynamics would be so much more admirable if you knew about insulation.
  4. "Anything that interferes with heat transfer can be an insulator of greater or lesser effectiveness." Indeed.
  5. Damorbel @128, "Your doubts about my competence in thermodynamics" Actually, your failure to grasp a scientific theory (not hypothesis) that has been around for over 150 years seriously questions your competence in thermodynamics. Have you actually read Roy Spencer's posts on this matter which I and others have directed you to? Talking of Spencer, have you read Spencer Weart's book, The Discovery of Global Warming? I'm thinking not. But then again, why would you-- you clearly believe that you know better than Arrhenius, Fourier, Tyndall and many, many other eminent scientists who get it. Maybe you are a D-K victim after all. Given that you cannot wrap your head around this fundamental theory underlying physics of AGW, I hope not to see you arguing points on any other page. Why argue other points related to climate science and AGW if you cannot grasp the greenhouse effect? We get it, you think AGW is not real, and your behavior here has demonstrated that no one is going to be able to convince you otherwise, so please (pretty please) stop wasting everyone's time.
  6. Composer99, as I picked on the matter of insulation (blanket analogy) first (starting with post #15) I feel obliged to reply to your take on it. You wrote (post # 126) "In particular, your blanket analogy in #117 practically mirrors the analogy used in the original post, save that you draw what appears to be a bizarre conclusion from it." which was in reply to a posting by damorbel. As I share his opinion in this matter let me try to make things more clear. Our conclusion is simple, however effective any insulation is, it can not cause the isolated body to warm unless there is a source of heat inside the insulation. Does this contradict what is said in the lead article? Let's see. It states: "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!" I fail to spot any contradiction, do you?
  7. h-j-m, Your conclusion is indeed simple, and wrong. The solar energy is the heat source. It passes through the atmosphere in one form (visible & ultraviolet light), get absorbed by the planet and re-emitted in another form (infrared light) that the greenhouse gases in the atmosphere insulate against.
  8. Our conclusion is simple, however effective any insulation is, it can not cause the isolated body to warm unless there is a source of heat inside the insulation. There is no claim that the Greenhouse Effect warms the Earth, only that it slows the Earths emission of heat into space, this was explained to you by Riccardo @107, but you seem to have ignored this. In this sense the GHE does behave exactly like clothes or a blanket around a human body. The human body gets its energy through metabolising food, circumnavigating the insulation. The Earth gets its energy from the sun, also circumnavigating the GHE insulation because the frequencies of EM radiation from the sun do not match the emitted ones from Earth which do, sadly, lie in the CO2 absorption bands.
  9. Re: damorbel (125)
    "Clausius was not an ignoramus, he was almost certainly aware of refrigerators since a patent for a refrigerator was granted to Jacob Perkins in 1834, when Clausius was 12."
    Hey, that Clausius guy traveled in rarified circles: hobnobbing with Perkins (who lived in England) while Clausius was growing up in Germany. Or did he read about it in Europe Today? Fascinating, the stuff I learn here... The Yooper
  10. Re #130 Albatross I have have a copy or have read most of the books, articles etc. etc. you refer to, I possess many more. You may have noticed that I have confined my arguments to the subject of this thread which effectively claims that GHGs in the upper troposphere can induce about 33K temperature rise above the 255K said by climatologists to be the equilibrium temperature of Earth without GHGs. This is the position of the IPCC, it is in all their assessment reports which are search-able as PDF files on my computer. What is not identified in the IPCC reports is the temperature of the upper troposphere, it is very cold about -50C at 50km altitude, about -20C at 4km. There is no way the CO2, H2O vapour and whatever other GHGs you find 'up there' will raise the temperature at 0km by anywhere from 23C to 33C. What happens is the GHGs (at -20 to -50C) absorb some of the radiation from the warmer surface (-10C to 25C) on average and a small amount of heat is added to the atmosphere in addition to the vast amount transferred by the evaporation/rainfall heat transfer cycle. All of the heat in the atmosphere is radiated to deep space by the GHGs. Some of the heat in the Earth does not get into the atmosphere because it is radiated directly to deep space. Because heat transfer can only go from a hot place to a cold one, no heat at all can be transferred from the troposphere to the surface because the troposphere is always colder than the surface and, since no heat can be transferred in this direction, there can be no warming effect; it would be like getting a ball to roll up hill completely unaided. The reason the air at altitude is colder than at the surface is fairly simple. Think of a ball thrown vertically, gravity reduces its velocity, this means its kinetic energy =1/2mv^2(KE) is reduced. There is no real difference between the thrown ball and an air molecule except the temperature of a gas is proportional to the KE of its molecules, so in the troposphere the temperature of the air drops linearly with height at 6,5C/km because the KE is transformed into potential energy (PE) = mass x gravity x height (mgh). PE is seen to increase linearly with height which means that the KE/Temperature decreases linearly with height (as observed).
  11. Re #134 Daniel Bailey, you wrote:- "Clausius guy traveled in rarified circles" Just like Bill, he just wouldn't come home.
  12. Agreed CO2 intercepts some of the LWIR from the Earth, but it can only re-radiate just less than 1/2 back to the Earth which, for most of the time is warmer and therefore cannot make use of, or be affected by those low-energy photons.The effect of this gas,at 0.028% of the atmosphere is negligible. Water vapour is the greenhouse gas, except that it doesn't work like a greenhouse. The Earth is the watery planet and its climate can be fully accounted for by the density of the atmosphere and H20 and its three states. C02, were it not an essential component of life which is a special feature of the Blue Planet, could be ignored.I want my children and my children's children to have the same lifestyle as I've had.Sure there may or may not be peak oil. So what. If as many dollars had been put into battery research as have been squandered in this climate stuff, we might have been that bit further down the road to the hydrocarbonless society.
  13. RE# 137 AWoL The effect of this gas,at 0.028% of the atmosphere is negligible. Please offer a citation in peer reviewed literature that claims this is the case. For if it is you would need to explain to the reader how Satellite measurements of outgoing longwave radiation are incorrect.
  14. Damorbel, "What is not identified in the IPCC reports is the temperature of the upper troposphere, it is very cold about -50C at 50km altitude, about -20C at 4km." I'm not going to argue with you. Maybe it was a typo or maybe it was a very revealing slip-- but the troposphere doesn't extend to 50 km. The mid troposphere is typically deemed to be around 500 mb or around 5.5 km where temps. are typically near -20 C. Temperatures are -55 C or so between 11 km and 20 km. At 50 km you are in the stratopause where temps are a balmy -5 C or so. Surely you read that in the many books that you claim to have read. Maybe the wise rabbet can explain all this to you.
  15. Bibliovermis and Phil, Are you really saying that incoming radiation from the sun is not effected by greenhouse gases? Want to bet?
  16. All, Here is an online MODTRAN applet maintained by Dr. Archer. Have fun.
  17. #125: "The death knell of the GHG hypothesis really is the effect of gravity on the atmosphere." Had you disclosed that you were a devotee of that idea sooner, this wouldn't have gone on this long. Rabbett took it down back in May. Are you a student of Jelbring? If so, why didn't you say so -- at least that was a published reference. "Svensmark's cloud hypothesis has far greater traction" Another idea shot down in flames. #135: "no real difference between the thrown ball and an air molecule" Ah, the heat at the surface is due to all that liberated gravitational potential energy, just like a ball that fell to the ground... Except the air molecules in question have not fallen from the top of the atmosphere. Or did you mean that since PV=nRT, the higher surface pressure results in higher temperature? Someone kicked that around here not that long ago, but I don't have the thread handy. So we must choose: Perhaps Clausius knew of a patent issued for an evaporative refrigerator (which depended on reducing the 'caloric' content of fluids -- it's online) and was thus motivated to include the words 'sole result' in thermodynamic theory. And Damorbel is the only one who knows this... but that renders the 2nd Law of Thermodynamics kind of trivial. Or perhaps 'sole result' refers to net energy transfer, elevating the work of Clausius to that of the key discoveries of science. Just like the words net force occupy the central place in Newton's 2nd Law, elevating that to the level of Law. #123: "I try to explain, if I am not understood I try to work out why" Here's a suggestion: Try to listen to what other people are saying and digest what they are asking. Then avoid repeating the same explanation without offering any substantiation.
  18. Re #139 Albatross Yes a typo, -50C at 10km, more strength to my argument, it puts -60C to -80C between the the bottom and top of the troposphere, where then is this GHG warming effect coming from? Your link to Rabbet is very informative, of high altitude GHG emissions (CO2 etc), he says:- "Below that level, energy emitted by a greenhouse gas molecule is soon absorbed by another relatively nearby one. Thus the energy simply cannot be radiated to space to balance the incoming solar energy." which satisfies all observations of molecular absorption and emission. But don't you notice that this kills off the 'back radiation' nonsense? There are plenty of gases, the so-called greenhouse gases, that emit and absorb radiation; Rabbet is acknowledging that adjacent GHG molecules will absorb GHG emitted photons. To a non-climatologist this means that the thermal equilibrium of a gas is not disturbed by the phenomenon of molecular emission/absorption; a climatologist does not notice the absorption part of the phenomenon and claims that the emitted photons are not absorbed in the atmosphere but somehow, contrary to the 2nd law of thermodynamics, bunch together to form a radiative flux downwards to Earth's surface. Thank you for the link. This phenomenon was explained by Einstein in his 1917 paper on emission and absorption, Rabbet is the first person I have come across who recognises this reality in popular literature. Re #142 muoncounter, you also link to Rabbet and further to Jelbring on how heat is distributed in a gravitationally retained atmosphere. In your link to Rabbet you will find a further link to Steve Goddard's thread on the matter of atmospheres, gravitation and temperatures, there are currently over 400 postings and they make interesting reading. Given the vigorous discussion, your claim that "Rabbett took it down" is just absurd. Would you care to say what Rabbet wrote that convinced you? I am not interested who writes something, I need to know the argument used. Please tell me which part of Rabbet's post convinced you, I would like to check it out. The same with Jelbring.
  19. @damorbel: " a climatologist does not notice the absorption part of the phenomenon and claims that the emitted photons are not absorbed in the atmosphere but somehow, contrary to the 2nd law of thermodynamics, bunch together to form a radiative flux downwards to Earth's surface." ...except that's not *at all* what climatologists claim. To say they do is either to misunderstand the science, or to disingenuously misrepresent what climatologists believe. The point is not that all of the absorbed photons will go back down. The photon re-emission by GHG molecules happens in a random direction. The point, however, is that without this absorbtion/emission cycle, the heat would go directly into space. Now, it stays in the atmosphere longer, and some of it does make its way back to the surface. This is why temperatures have been going up, and it in no way violates the 2nd law of thermodynamics. It would be nice if you stopped repeating false information. Please consider it.
  20. @AWoL: "The effect of this gas,at 0.028% of the atmosphere is negligible." No, it isn't. See CO2 effect is weak and CO2 is not the only driver of climate. "I want my children and my children's children to have the same lifestyle as I've had." They won't if we don't take care of AGW. In fact, their lifestyle will positively suck. You seem to believe there's no money to be made in developing renewable energy and other green technolgies. Perhaps you should tell that to China, they're taking the leadership in these areas while the propaganda spread by Big Oil is causing the US miss the boat on these opportunities. Think about it.
  21. #143: "the so-called greenhouse gases, that emit and absorb radiation; Rabbet is acknowledging that adjacent GHG molecules will absorb GHG emitted photons." Your logic continues to spiral downwards. GHG molecules absorbing photons increases the temperature of the atmosphere -- and hence, warming. "and further to Jelbring" I have no particular interest in Jelbring; I assumed that was the source of your gravity=temperature gradient. If not Jelbring, perhaps you are a follower of Landscheidt, then? "you will find a further link to Steve Goddard's thread ... over 400 postings ... the vigorous discussion ... is just absurd." Any credibility you've tried so valiantly to establish here, now gone poof. As far as the number of postings on one of $G's threads being an indicator of substance, value or thought: Really? Here is what Dr. Roy Spencer posted on a similar thread in Watt$land: But it’s when that volume is exposed to outside influences — like IR radiation from the solar-heated surface of the Earth passing through that volume — that a temperature change can occur as a result of adding more CO2 to the volume. -- emphasis added Absurd is as absurd does.
  22. h-j-m: Rather than cryptic wager challenges, why not share a link or other citation to peer-reviewed literature showing the magnitude of greenhouse gas effects on incoming solar EM radiation?
  23. As several posters here accuse me of misunderstanding, misinterpret and general lack sufficient knowledge of physics and thermodynamics I will gladly admit all of that. Nevertheless, as posters doing so fail to provide any evidence in the end it just constitutes an ad hominem comment. Being aware of my poor knowledge I check facts before writing, double-check them while writing and triple-check them before posting. Now I will try to do it an other way. Is it true that matter absorbing incoming energy will result in either 1.a phase change (solid to liquid, liquid to gas, gas to plasma) and/or 2. taking up more volume and/or 3. emitting radiation? If we observe gases absorbing incoming energy at surface temperatures, is it true we will observe them gaining volume and emitting radiation (getting hotter)? If you answered no to at least one of the questions please tell my why, if not then please tell me what specifically makes green house gases so special.
  24. Composer99, for the simple reason that I have already done that in post #23. You can also cross-check with the link DSL provided in post #27.
  25. h-j-m: You mean the Wikipedia graph? That appears to be taken into account by the graph provided by others (from Science of Doom) several comments down: DSL/Ned (comments #26/27). I'm not sure I follow. Surely, if greenhouse gasses in the atmosphere can alter the flow of radiation coming in from the Sun, if that radiation is at the correct wavelenghts, then they can affect radiation coming up from the surface of the Earth, particularly since a very large part of that radiation occurs in the wavelengths most vulnerable to greenhouse gas effects. Leaving out the constant heating from the Sun, the net flow of heat energy, per the Second Law of Thermodynamics, should be from the Earth, through the atmosphere, into space. That is what we see. Greenhouse gasses in the atmosphere delay this flow of heat energy and (bringing incoming Solar radiation back into the picture) force the Earth to increase its surface temperature to bring it back into radiative equilibrium. Again, we see this in empirical observations.

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