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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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 476 to 500 out of 1107:

  1. Tom Curtis @ 471: It appears to me that you are confusing the mechanisms responsibly for a planetary average temperature with those determining with the average temperature distribution across a planet. These two types of mechanisms are very different. Let me explain: The average near-surface temperature on a planet is a linear function of the total internal energy in the lower atmosphere. Therefore, that temperature can only be changed by increasing or decreasing the total internal energy. Winds, thermal inertia, axial rotational speed and other heat-transfer mechanisms on the surface of a planet only serve to redistribute that total energy, but do not change its overall amount. As a result these mechanisms can only affect the degree of uniformity of the equilibrium temperature field, but cannot change the planetary mean temperature. This follows from the law of conservation of energy (First Law of thermodynamics), for any change of average temperature requires a net change in total internal energy. I'm working as fast as I can to complete my manuscript and submit it for publication. But it takes time to explain in a clear yet bullet-proof way the new concept, simply because there has been at least one whole generation of scientists indoctrinated into the wrong paradigm. The situation is analogous to that described in this legend, where American Indians could not see at first Columbus's ships on the horizon, because they had no mental concept of what a sail ship is. It wasn't until the local shaman explained to them the new 'event' in terms they could understand that they were able to see the ships (I think this legend was portrayed in the movie "What the Bleep Do We Know"). In other words, what we believe in and are accustomed to (i.e. our worldview) determines to large extend our ability to see or not certain things. In no way am I implying that I'm the 'shaman' here and everybody else represents the 'unenlightened Indians'! The new concept I'm presenting in the paper is actually quite simple (since it's rooted in physical principles that are 150 years old). BUT it does require a shift in perception in order to fully grasp it. That is in part why it constitutes a new paradigm... :) NO, I'm not planning to submit it to E&E, although this journal has published a number of articles that have brought some important valid points to the climate debate. I can bring up a similar objection with respect to Nature as well, since that journal has published the hockey-stick temperature paper by Michael Mann & Co in 1998, which has been since discredited for its [snipped] statistical analysis both in the peer-reviewed literature and at Congressional hearings (Note that IPCC is longer using the hockey-stick graph).
    Response: Your claims about the hockey stick are incorrect. Regarding your claim that the hockey stick has been discredited, see the Argument "Hockey stick is broken." Regarding your claim that the IPCC is no longer using the hockey stick graph, you can see the hockey stick just by looking in the 2007 IPCC report itself!
  2. "(Note that IPCC is longer using the hockey-stick graph)." You may have to add a few pages to your monograph to elaborate on your claims about this aspect of the science. It will be a lot of pages in fact. Lotta hockey sticks here
  3. scaddenp @ 474: The short answer to your question is YES - my new theory fully accounts for known observational results AND is in full compliance with standard thermodynamics theory, which is not the case with the current GH concept as I explained in #458. It also perfectly predicts the observed temperatures on hard-surface planets in the solar system! This is actually one of the main strengths of the new theory ... Ladies & Gents, I will be leaving you now for a while, since I've got work to do and have already stirred enough the pot for couple days ... Keep thinking about this discussion in an open-minded way and wait till my paper is published ... We will continue the discussion then. Good luck to all of you!
  4. adelady @ 477: Sorry for the missing word in that sentence. What I meant was "(Note that IPCC is no longer using the hockey-stick graph)" .. The problem of perception in scientific inquiry is a psychological issue and a topic of another discussion ...
    Response: Your change in phrasing doesn't make your claim correct. You have been pointed to the Skeptical Science thread where your claim is rebutted, and where you must post any further comments on that particular topic.
  5. I hope the answer is still YES to my actual question not your rephrasing of it. ie "do you also accept the principle that your claims must account for empirical results? Ie if the textbook interpretation of thermodynamics accounts for observation results and yours do not, then perhaps the textbook is correct and you need to do more reading? " You have untold empirical mountains to climb and excuse my extreme skepticism until that is presented. My money is on you not being able to get published given what you said to date.
  6. PhysSci (RE: 458), "If the GH effect were due to absorption and re-emission of IR energy by greenhouse gases ultimately traceable to solar input as claimed by the current theory, then how is it possible that the down-welling thermal flux exceeds the total solar input by 44% (343/239 = 1.44)." This is not correct. The downward emitted flux is only about 150 W/m^2. The surface on average emits about 390 W/m^2. About 90 W/m^2 passes through unabsorbed and goes straight out to space. About 300 W/m^2 is absorbed and re-emitted by the atmosphere, of which half goes up and half goes down (150 W/m^2 up and 150 W/m^2 down).
  7. PhysSci (RE: 459), "As I said in my previous posting, the lower troposphere contains more energy than supplied by the Sun. Where is that energy coming from?" It's coming back from the atmosphere. The rate at which the energy is coming in from the Sun is faster than the rate at which it's able to leave to planet. In essence it takes 1.6 W/m^2 at the surface to allow each 1 W/m^2 to leave the planet, offsetting each 1 W/m^2 entering the system from the Sun. (390 W/m^2 at the surface divided by 239 W/m^2 from the Sun = 1.6).
  8. PhysSci, BTW, I agree that Conservation of Energy is the biggest Achilles heel of the entire CO2/AGW theory but not for the reasons you claim.
  9. PhysSci, "Why do variations in global temperature over the past 27 years correlate much better with observed changes in cloud albedo than with those in GH gases?" My guess is because the temperatures are tied mostly to the available energy in the system (i.e. the amount of post albedo energy from the Sun), rather than GHG concentrations or atmospheric opacity.
  10. Response to RW1 @ 481: Forgive me, but comments like yours make me smile and really wonder what's the level of scientific expertise on this blog?... A global down-welling thermal flux of over 320 W m-2 has been extensively measured and confirmed by both satellite and surface observations for 15 years now! The actual estimates vary between 324 and 348 W m-2. This is now considered a basic information about Earth's radiation budget!! Here are some references (in chronological order) to help you update your knowledge base on this: Rossow, W. B. and Zhang, Y.C. 1995. Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets, 2, Validation and first results, J. Geophys. Res., 100, 1167–1197. Trenberth, K. E. 1997. Using atmospheric budgets as a constraint on surface fluxes. J. Climate, 10, 2796–2809. Gupta, S. K., Ritchey, N. A., Wilber, A. C., and Whitlock, C. A. 1999. A Climatology of Surface Radiation Budget Derived from Satellite Data, J. Climate, 12, 2691–2710. Pavlakis, K. G., D. Hatzidimitriou, C. Matsoukas, E. Drakakis, N. Hatzianastassiou, and I. Vardavas. 2003. Ten-year global distribution of downwelling longwave radiation. Atmos. Chem. Phys. Discuss., 3:5099-5137. Trenberth, K.E., J.T. Fasullo, and J. Kiehl. 2009. Earth’s global energy budget. BAMS, March:311-323 Also, look at the NASA's Surface Radiation Budget (SRB) project page, where you can get global maps of LW and solar fluxes on a monthly basis for 24 years, where you'll clearly see that LW fluxes at the surface exceed incoming SW fluxes by a large margin:
  11. PhysSci, "Forgive me, but comments like yours make me smile and really wonder what's the level of scientific expertise on this blog?... A global down-welling thermal flux of over 320 W m-2 has been extensively measured and confirmed by both satellite and surface observations for 15 years now! The actual estimates vary between 324 and 348 W m-2. This is now considered a basic information about Earth's radiation budget!!" I'm not referring to the total downward flux, but the downward amount that last originated from the surface emitted. I think you're forgetting that a good amount of incoming solar energy is absorbed by the atmosphere/clouds and emitted down toward the surface. Conservation of Energy dictates that the downward emitted amount that last originated from the surface cannot be 300+ W/m^2. If you are claiming this is so, show me the power in = power out calculations that prove it.
  12. General Response to RW1: RW1, I suggest you stop embarrassing yourself with comments like the ones you made in 482 to 484 ... I'm not even going to respond to a such a profound lack of understanding. You are totally excused, if you are not a scientist, but if you are, then you've got a BIG problem with basic physics ... Thank you for participating and good luck!
  13. PhysSci, Sorry, clarification to my own #481, I meant 'back radiation'- meaning the downward emitted that last originated from the surface.
  14. PhysSci, "RW1, I suggest you stop embarrassing yourself with comments like the ones you made in 482 to 484 ... I'm not even going to respond to a such a profound lack of understanding. You are totally excused, if you are not a scientist, but if you are, then you've got a BIG problem with basic physics ..." Sorry, you're going to have to do better than that.
  15. PhysSci, "RW1, I suggest you stop embarrassing yourself with comments like the ones you made in 482 to 484" #484 I was guessing. #482 - are you claiming that the post albedo energy of 239 W/m^2 does NOT become about 390 W/m^2 at the surface? Are you claiming that 390/239 is NOT about 1.6? Are you claiming that this does NOT mean that it takes 390 W/m^2 at the surface to allow 239 W/m^2 to leave the planet, offsetting the 239 W/m^2 entering? If yes, please explain why in detail and then we'll discuss via give and take.
    Response: [muoncounter] Please be aware that this is not necessarily the best forum for such give and take. Much of this detail has already been the subject of numerous comments, both on this thread and climate sensitivity threads.
  16. From Trenberth, Fasullo and Kiel (2009): Globally averaged incoming SW radiation at the Earth's surface is shown to be 184 w/m^2, of which 23 w/m^2 are relfected back to space. Total backradiation (all sky downward long wave flux) globally averaged is 333 w/m^2. Total upwelling engergy from the Earth's surface, globally averaged is 17 w/m^2 (thermals) plus 80 w/m^2 (evapotranspiration) plus 396 w/m^2 (upwelling long wave radiation), or 493 w/m^2. Downward = SW + LW = 161 + 333 = 494, the difference between them being the net absorbed. So, no contradiction of conservation of energy involved. That wasn't so hard, was it?
  17. "Globally averaged incoming SW radiation at the Earth's surface is shown to be 184 w/m^2, of which 23 w/m^2 are relfected back to space. Total backradiation (all sky downward long wave flux) globally averaged is 333 w/m^2. Total upwelling engergy from the Earth's surface, globally averaged is 17 w/m^2 (thermals) plus 80 w/m^2 (evapotranspiration) plus 396 w/m^2 (upwelling long wave radiation), or 493 w/m^2. Downward = SW + LW = 161 + 333 = 494, the difference between them being the net absorbed. So, no contradiction of conservation of energy involved." But latent heat and thermals are kinetic (not radiative) - meaning their energy moved into the atmosphere didn't come from from surface emitted radiation. So even assuming all the latent heat and thermal energy is ultimately returned to the surface as downward emitted radiation (highly unlikely), the net effect is zero relative to surface emitted radiation. It has to be for Conservation of Energy to be met. The bottom line is only about 150 W/m^2 of the surface emitted radiation can be returned as 'back radiation'. By 'back radiation' I specifically mean radiation that last originated from surface emitted (this is a key distinction, especially since all the energy ultimately originated from the Sun). You do know that the surface emitted radiation of about 390 W/m^2 is directly due to its temperature and nothing else, right?
  18. If you notice, even Trenberth is showing 169 W/m^2 emitted by the atmosphere and 70 W/m^2 passing through completely unabsorbed (40 W/m^2 through the atmosphere, 30 W/m^2 through the clouds) for 239 W/m^2 leaving. This means that of the 396 W/m^2 emitted by the surface, using Trenberth's numbers at least, 157 W/m^2 of it has to be 'back radiation' from the atmosphere. He's obfuscating this by absorbing some of the post albedo energy by the atmosphere and returning it as 'back radiation' when it's really 'forward radiation' that last originated from the Sun - not the surface. He's then returning the latent heat and thermals as 'back radiation' to come up with at total downward emitted of 333 W/m^2, which he incorrectly designates as being all 'back radiation'. It's a mess.
  19. 161 W/m^2 + 78 W/m^2 = 239 W/m^2 post albedo at the surface. 239 W/m^2 from the Sun + 157 W/m^2 downward emitted from the atmosphere = 396 W/m^2 emitted by the surface.
  20. The moderator of this website has an agenda to prevent a free and open discussion on important subjects. He deletes postings that are totally relevant to the subject at hand but do not conform to his ideology or agenda.
    Response: Discussion of hockey sticks and the little ice age are not relevant to this thread. You have already been pointed to the appropriate threads for discussion of these topics, feel free to post your comments there.

    In addition, your latest comments have been peppered with inflammatory invective. This will not be tolerated. You are a guest on this site, and as such you are expected to abide by this site's comment policy. Please try and behave like a mature adult if you want to be taken seriously.
  21. The moderators, plural, at this site ensure that the contributions from all of us conform to the Comments Policy... This might mean that this comment disappears along with your #495 but that doesn't matter. To comply with the policy, your statements must avoid politics, accusations of dishonesty, against anyone, and your scientific statements have to have some backing or reference to the scientific literature. You've set yourself a difficult task in trying to overturn 150 years of physics and nearly as many years of observations from glaciology, biology and the rest. But if your science is sound, you should be able to point out scientific references that support whatever limited statements you do make about your new theory. There is an agenda. "Stick to the science" best describes it. There are some exceptions but this thread isn't one of them.
  22. RW1 @492-5, "First of all, what is “back-radiation” ? It’s the radiation emitted by the atmosphere which is incident on the earth’s surface. It is also more correctly known as downward longwave radiation – or DLR" There is nothing in that definition that depends on the original source, or penultimate source of the energy in the Downward Longwave Radiation, and with good reason. If you want to use some different concept other than DLR, then find your own term and define it clearly. Don't lazilly misuse an already defined technical term and then accuse people who are using it correctly of doing so incorrectly. Not only is it lazy, it is bound to cause confusion for other when they try to understand you; and yourself when you do the same. The very good reason why back radiation is DLR only is that, when a GHG emits IR radiation, it does not have a phycical state correlating to the penultimate source of the energy it is about to emit. It only has the energy itself. Therefore it can make no difference to the properties of that emitted radiation whether that energy was originally carried into the atmosphere by evaporation, or convetion, or IR radiaton from the ground, of from volcanic emissions, or what ever. Indeed, as most energy transfers in the astmosphere are by collision, it is doubtfull you can say sensibly of any parcel of energy what its penultimate source was. Therefore, energy carried into the atmosphere by evapotranspiration will be radiated to the surface, or to space in the exact ratios as energy carried into the atmosphere by IR radiation, or energy absorbed from SW radiation from the sun. Trying to treat it as a discrete entity after it has been carried into the atmosphere is, scientifically, gobbledy-gook. To finish, Trenberth, Khiel and Fasullo are not obfusticating by indicating some SW radiation is absorbed in the atmosphere. They are describing an indirect emperical result, and one that is more easily determined than, for example the proportion of SW light reflected from clouds, or from the surface. The method is to measure downward SWR at the Top of the Atmosphere, upward SWR at the TOA, and subtract the later from the former. You then measure downward SWR at the surface, and subtract that result from the difference; giving you the amount of SWR absorbed. (Clearly the measurements need to be made at a large number of points and times to determine a global average.) T,K, & F (2009) list a summary of such mesurements on table 2b. It is a telling indictment of your "science" that you cannot use standard definitions correctly, and have to dismiss observational results as "obfustications".
  23. To demonstrate the equivalence of work and energy, elementary physics uses the example of a paddle wheel in a gas filled, insulated container, driven by a falling weight. (Page 51 of Schaum’s Themodynamics for Engineers, since this blog likes references) The potential energy of the weight is converted to kinetic energy as the weight falls, and the rotation of the paddle works on the gas content, increasing its temperature. Neglecting frictional losses, the energy content of the gas (its temperature increase) is equal to the potential energy of the weight. The questions that really elementary Physics does not ask, are these: Is it possible to use the additional energy in the gas to restore the weight to its original position? If not, why not? It is not possible because the quality of the energy has changed. It has been dispersed throughout the gas and, unlike the energy of the weight, its possible use is very limited. To recover the energy, even in principle, we would need to treat the gas as a source of energy, and connect it to a sink at a lower temperature. Energy could then be transferred, and some work could be done to raise the weight, but we would have to discharge most of the additional gas energy into the sink, so the weight would not rise very far. If this concept of energy quality is acceptable to advocates of atmospheric back-warming, we can go on to debate that proposition.
  24. Fred Staples @499, contrary to your claim in 498, your "proof" of the second law of thermodynamics is nothing but hand waving. It seems plausible, and we believe it to be true; but we do so because we are assuming heat loss to the environment through friction, air resistance and imperfect insulation. Absent those assumptions, ie, in an ideal system, it is not clear that the increased pressure and temperature could not be converted to work and raise the weight to its original position. Your merely asserting this is not the case does not constitute proof, and is, contrary to your claim, hand waving. Having said that, why not simply appeal to the second law of thermodynamics:
    "When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at some time allowed to interact, breaking the isolation that separates the two systems, and they exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of the final exchanging systems. In the process of reaching a new thermodynamic equilibrium, entropy has increased, or at least has not decreased."
  25. How can anybody learn anything from the supposed heat transfer shown in the diagram in #491? The author who drew this failed to provide any temperature information anywhere. How can he possibly know the energy tranfers (all over the diagram in W/m^2) without indicating the temperatures? Whoever drew this diagram obviously hadn't the slightest knowledge of the 2nd Law of Thermodynamics. Who pays money for 'stuff' like this? It is the appearance of dagrams like this in IPCC reports that makes one instinctively mistrust all of 'global temperature increase' figures produced by the IPCC.

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