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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 1326 to 1350 out of 1393:

  1. No, it just reduces the rate of cooling, it cant warm it up.
  2. YOGI I didn't say that it would warm the surface up, I said it would be warmer than it would if there were no water vapour in the atmosphere (and therefore no back-radiated IR). Do you agree that the back-radiated IR from the water vapour in the atmosphere would cause the surface to be warmer than it would be if the water vapour were not there?
  3. #1325 Solar near IR is forty something % of the heat input from the Sun. You never heard of IR blocking windows ? its the solar thermal IR they are blocking, not DLR.
  4. #1327 No it causes it to cool down slower.
  5. Can I suggest we ignore YOGI's comment above until we have reached the end of the discussion about deserts. The errors in YOGI's post about solar IR are pretty obvious and begging for an answer, but it would be better if the answer were delayed for a while.
  6. YOGI O.K. well lets revisit the original question, which was: "So what happens in a desert at night, does the CO2 back-radiation turn off when the sun goes down ?" We now know the answer, it isn't that the CO2 back-radiation turns off when the sun goes down, it cools more quickly because there is less back-radiation from water vapour because the atmosphere above the desert is drier. In fact the rapid cooling of the desert at night is a common example used to demonstrate the existence of back-radiation. Now your most recent post is essentially just blatant rhetorical evasion. If the back-radiation from water vapour causes the surface to "cool down slower", then at any point in time after nightfall it will be warmer than it would be if the water vapour were not there, precisely because of the difference in the rate of cooling. The reason why you want to evade admitting that the back-radiation causes the surface to be warmer than it would be if the water vapour were not there is obviously because you would then be forced to concede that the surface would be warmer than it would otherwise be if not for the back-radiation from CO2. Frankly getting to this point has been like getting blood out of a stone, and I can't see why you should think anyone will be willing to engage in a scientific discussion with you if you are going to behave in this manner. I am sorry to have to strongly suggest to everybody "DNFTT".
  7. #1325 There are some absorption bands in the solar IR but plenty gets through. And IR penetrates into the surface of land like radio waves do. [Snipped]
  8. YOGI - you can verify some of this by actually comparing the measured backradiation in a desert compared to somewhere humid. Remember this is measurable properties here. Eg look at the SoD articles on DLR. In part one, you see the spectra for incoming versus outgoing. In part two, there is DLR measurements for Alice springs versus Billings OK. Read and understand.
  9. #1331 If I agree that the back-radiation from the water vapour causes the ground to be warmer than it would otherwise be, that is paramount to saying that it heats the ground. It does not and cannot, as it is colder than the ground, it merely slows the rate of cooling, not warms it.
  10. YOGI Sorry, nobody can say I haven't been extreemly patient with you, but again you are playing word games. Nobody has said that back-radiation makes the surface warmer, just that it makes the surface warmer than it would otherwise be. Are you warmer if you cover yourself in a blanket than you would be without the blanket? Yes of course you are, because the blanket back-radiates some of your body heat. Likewise the back radiation from GHGs causes the earth to loose the heat it gains from absorbing visible and UV light from the sun more slowly by returning some of the out-bound IR back to the surface. I have had more than enough of this discussion; had you behaved better I would continue, but life is just too short.
  11. Yogi, Let me state this clearly. You are wrong and confused. You are religiously clinging to a misunderstanding of the science. You do not need Dikran or anyone else to walk you, step by painful step, through the thought process. What you need is to simply say to yourself "gee, maybe I don't understand all this, and I should open my mind, and go read and learn, and then come back when I have a better understanding of things." Trying to convince everyone else that you know better than all of science is a waste of everybody's time.
  12. #1333 S.O.D. article: "Notice that DLR does not drop significantly overnight. This is because of the heat capacity of the atmosphere – it cools down, but not as quickly as the ground." Thats bigger than some of the seasonal variation !
    Response: [JH] Your propensisty to post factoids without context is wearing very thin. It's like someone throwing gobs of paint against a wall and hoping that some will stick. At the end of the day, the wall is a complete mess. Please cease and desist.
  13. #1336 "Are you warmer if you cover yourself in a blanket than you would be without the blanket? Yes of course you are, because the blanket back-radiates some of your body heat." Unlike the Earth I am internally heated so the analogy is not safe, but if the blanket is bright white, it could be handy to reflect the sunshine on a really hot day. But for an externally heated system such as Earth, to have 235W/m go in and out, but to reach 390 watts within the system is impossible: [link]

    [DB] You are advised to read the entire thread above, as your fallacy has been corrected several times already.

    Additionally, you would be wise to read this guest post by Dr. Trenberth for yet further exposition into the subject.  It contains this updated version of the graphic you link:

    Click to enlarge

    [RH] Hotlinked url that was breaking page format.
  14. Response: [JH] Your propensisty to post factoids without context...... It was not out of context. I was commenting on the SOD link at #1333. And the data shows that daily min/max soil temp`s range less than the atmosphere.
    Response: [JH] You are skating on very thin ice. Please cease and desist.
  15. "[DB] You are advised to read the entire thread above, as your fallacy has been corrected several times already." Nutshell it for me now if you can, unless you can link me to the relevant comment, I`m not going to read all the comments for it.

    [DB] If you cannot be bothered to read work already done, there for inspection, then why should anyone here engage you?  Perhaps if you succinctly narrow down your objection to the one thing you want to hang your hat on then someone here will be able to help you.

    Unless your goal is to simply waste the time of others.  As also has been evidenced on this thread.

  16. Yogi 1338, If, hypothetically speaking, the source of heat on earth is internally generated, would you, in this case, agree that the earth will be warmer with water vapour present compared to the case without?
    Response: [JH] Please do not feed the troll.
  17. I will read the Trenberth guest post and comment later, thank you.
  18. Ahem... Um, is this the Dunning-Kruger thread?
  19. YOGI wrote: "Unlike the Earth I am internally heated so the [blanket] analogy is not safe" which clearly shows that YOGI has not the slightest familiarity with how the greenhouse effect works, which is not a good position from which to be questioning it. The atmosphere is largely transparent to visible and UV light, which makes up the majority of the Sun's radiation. Thus most of the Suns radiation is absorbed by the surface, which heats up as a result, and re-radiates this energy as IR radiation. Thus as far as the atmosphere is concerned, it is being heated from below by the surface, which is why the blanket analogy is perfectly reasonable, provided it is not extended beyond this most basic point. Now anybody who has made the slightest effort to understand the way in which the greenhouse effect works will know that already. Sadly the WWW is full of Dunning-Kruger sufferers who think that a few things they have read on climate blogs means they know more than scientists who work on this for a living. Generally all they manage is to make themselves look silly, as YOGI has done on this thread (he/she is not the first and I suspect will not be the last). The basic idea of the greenhouse effect goes back to the 1820s at the very least, so it would be extremely unusual for a physical theory to have survived this long if it contradicted a fundamental law of thermodynamics! Of course it doesn't. The error is to assume that the second law says that heat cannot flow from a cooler body to a warmer one. It doesn't, essentially the net flow of heat must be from the warmer body to the cooler one, and even then it is only true in a statistical sense after the exchange of a sufficiently large number of IR photons. To see this is true, consider a thought experiment, where there are two identical bodies, separated by a small gap, both of which are only slightly above absolute zero. One is warm enough to emit photons of IR at random at an average rate of one per year (365 days). The other is a little cooler, so that it radiates a photon at an average rate of one per 366 days. If you observed them for a month, then assuming one photon was emitted by one body and absorbed by the other in that month, it is almost as likely to have gone from the cooler body to the warmer body, thus making the warmer body a little warmer. However this is not in contradiction of the second law of thermodynamics. If you observe the bodies for long enough, you will find that slightly more IR photons from the warmer body are absorbed by the cooler body than vice versa, and hence the net flow of heat is from warmer to cooler. Anyway, I hope my exchange with YOGI has demonstrated that he/she is not here for rational discussion of the science, and is not worth the effort. DNFTT.
  20. 1343, Rob, No, this is the Galileo thread. The Dunning-Kruger thread overloaded and exploded.
  21. There's always the Poptech thread...the Valhalla of mythic D-K.
  22. With regard to the Blanket analogy, it is worth pointing out that the heat humans (and other organisms) produce is derived from the respiration of foodstuffs, and so is not an internal source of energy and typically pass through a blanket unimpeded (unless you happen to have it over your head :-) ). Incoming Solar radiation is thus analogous to food in the Blanket analogy.
  23. Dikran Marsupial OK cover the Earth with a blanket (FULL CLOUD COVER) and see how cold it gets. I really did mean your blanket analogy was not safe, and if you want to continue pushing it, I have no problem with de-bunking it at every step. Solar IR makes up a significant proportion of insolation. Your thought experiment proves that beyond a single photon or so, the second thermo law must also apply between the Earth`s atmosphere and surface.
  24. Phil#1347 We create our body heat internally from the calorific value of the food we eat. Excess heat may be lost through respiration or evaporation of perspiration, and body heat is continually lost in IR radiation from the body surface (when the surrounding air temperature is lower than the body temperature). Now as you know a blanket will reduce the IR emmission from your body in cold air by insulating it, and enable you to maintain your body temperature easier.
  25. Daniel @ 1346... Actually, I think that's officially called the "He who must not be named" thread.

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