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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.

References

Comments

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Comments 276 to 300 out of 727:

  1. I suppose this discussion about the role of albedo on equilibrium temperature should be moved to the thread about albedo. Coincidentally, Rovinpiper just posted a question about exactly that, exactly there. I replied there.
  2. Re #273 CBDunkerson you write:- "Object A reflects 90 units and absorbs 10. That 10 absorption heats up the object until it is emitting 10 units. At that point the 90 units reflected + 10 units emitted equals the 100 units incoming and the object is at equilibrium. Object B reflects 25 units and absorbs 75. That 75 absorption heats up the object until it is emitting 75 units. At that point the 25 units reflected + 75 units emitted equals the 100 units incoming and the object is at equilibrium." Fair enough. But then you write:- "At equilibrium object A is emitting 10 units of energy and object B is emitting 75 units. Object B is thus much hotter than object A. Albedo has a direct and obvious impact on temperature." How so? A has only 10% absorption capacity, B has 75%. Now the absorption capacity is always equal to the emission capacity, after all the reflection part cannot emit as well as reflect, can it? So both objects have the same temperature, any difference would clearly break the 2nd Law of thermodynamics.
  3. Re #275 Ned you cite my post:- "Highly reflective materials (high albedo) heat up slowly and cool down slowly in the absence of input; an example of this is a thermos flask with its highly polished surfaces." Then you write:- "Look, this is just wrong. It really is." So thermos (vacuum) flasks don't work this way? Care to explain how they do?
  4. damorbel, at face value your statement is correct that "the absorption capacity is always equal to the emission capacity." But I think you meant something else--something incorrect. Here is a correct rephrasing: The reflected photons are irrelevant to the absorption and emission of the object, and therefore are irrelevant to the temperature of the object. The only contributors to the temperature of an object are the photons absorbed and the photons emitted. You will get exactly the same temperatures, absorptions, and emissions of objects A and B that you get in the scenario that CBDunkerson described, in this different scenario: (1) Make both objects A and B perfectly absorptive--no reflection, in other words albedos of zero. (2) Isolate A from B. (3) Give object A its own radiation source--a source that sends only 10% of the radiation that CB's original source did. Object A is absorbing 100% of that, so Object A is absorbing the same radiation (and therefore the same energy) that it was getting in CB's original scenario. (4) Give object B its own radiation source--a source that sends only 75% of the radiation that CB's original source did. Object B is absorbing 100% of that, so Object B is absorbing the same radiation (and therefore the same energy) that it was getting in CB's original scenario. (5) The temperature of Object A will be lower than the temperature of Object B.
  5. damorbel, a vacuum flask has an inner chamber inside an outer chamber. If the inner chamber is filled with a hot liquid, emission from that chamber can be reduced by making the inner surface of that inner chamber reflective. But emission from that inner chamber is not reduced by making the outer surface of that inner chamber reflective. Once that radiation has escaped from the inner chamber, it must get through the walls of the outer chamber, which can be reduced by giving the inner-facing walls of that outer chamber a reflective coating; that bounces the radiation back from the outer wall into the gap between the inner and outer chambers.
  6. Re #279 Tom Dayton you wrote:- "1) Make both objects A and B perfectly absorptive--no reflection, in other words albedos of zero." This destroys the whole matter. If you consider the case when albedo is zero there can be no effect due to albedo and there can be no confusion arising from the influence of albedo and 2nd law of thermodynamics. "(2) Isolate A from B." Why? "3) Give object A its own radiation source--a source that sends only 10% of the radiation that CB's original source did" I think you should be more precise and define the source better. I really do not understand why you need two sources to explain these concepts.
  7. damorbel, I posed my alternate scenario so you can see that the temperatures of the objects in that scenario are identical to the temperatures of those objects in CBDunkerson's original scenario. That should help you understand that photons reflected don't contribute to temperatures of the objects. Only the absorbed photons matter.
  8. Consider Tom's example using one source, but objects A & B are spaced so that they receive the amount of energy described and are isolated from each other (i.e. directly opposite each other & fully obscured by the source).
  9. Re #280 Tom Dayton you wrote:- "But emission from that inner chamber is not reduced by making the outer surface of that inner chamber reflective." A polished metal outer surface is an excellent insulator, the old fashioned silver coffee pot is a a good example, the modern chrome model is just as good because it doesn't need polishing so much. Another application of this principle is multilayer insulation Multilayer insulation stacks up reflective surfaces and is extremely effective.
  10. damorbel writes: So thermos (vacuum) flasks don't work this way? Care to explain how they do? The problem is that planets don't work this way. Read the rest of my comment. The point is that the incoming and outgoing radiation fluxes have different spectral distributions. A change in the visible/NIR albedo doesn't imply a corresponding change in thermal infrared emissivity. A lot of your comments in this thread seem to involve trying to analogize the earth-sun radiation balance to some object, like a thermos or asphalt or a coffee pot. With all due respect, that's not necessarily the best approach.
  11. damorbel, the reflective outer surface of an inner layer of multilayer insulation does not help by reducing that layer's emission. It helps instead by reducing that layer's absorption of the radiation emitted by the next-most-outer layer--radiation that this inner layer emitted, that was returned by the outer layer.
  12. Tom Dayton writes: damorbel, at face value your statement is correct that "the absorption capacity is always equal to the emission capacity." ... at a particular wavelength. Part of the problem with damorbel's argument here is that the incoming solar radiation has a very different spectral distribution from the outgoing longwave radiation. Absorptance in the visible/near-IR is not necessarily equal to thermal infrared emissivity.
  13. damorbel - The last 15-20 postings you have presented have made it increasingly clear that you do not have a firm grasp of the physics involved. That's not an insult - we all start somewhere. I, like Ned, strongly suggest you go check with your local university or other institute of learning, and find out some more of the basics.
  14. damorbel writes: Another application of this principle is multilayer insulation Multilayer insulation stacks up reflective surfaces and is extremely effective. Good grief! Did you even read that wikipedia page you linked to? How could you not have noticed that their explanation of how multilayer insulation works is exactly the process whereby backradiation from CO2 in the atmosphere raises the temperature of the earth above what it would be in the absence of that CO2! The process that you yourself cite as "extremely effective" is the exact same process that you claim violates the second law of thermodynamics!
  15. KR writes: damorbel - The last 15-20 postings you have presented have made it increasingly clear that you do not have a firm grasp of the physics involved. That's not an insult - we all start somewhere. Yes. And I would note that, as we saw with the Evil Waste Heat Thread, the usual SkS "skeptics" are once again standing by on the sidelines. Apparently they're willing to quibble endlessly over things like UHI, who wrote what in a snippet of somebody's email, etc. But they're not willing to speak up and help address the problems with even the most appallingly confused argument coming from the "skeptic" side. As always, it turns out that "climate skepticism" is rather asymmetric around here. The unwritten rule seems to be that "No SkS skeptic shall ever publicly disagree with another SkS skeptic." IMHO that's pretty depressing.
  16. Ned @ 290 - there's always the exception to the rule, albeit very minor, BP waded into Ken Lambert a few weeks back over a "theoretical observed" comment.
  17. And took hits for not observing the Skeptics' "Code Duello" The Yooper
  18. Re #287 Ned you write:- "... at a particular wavelength. Part of the problem with damorbel's argument here is that the incoming solar radiation has a very different spectral distribution from the outgoing longwave radiation." The wavelenth difference is indeed great but what that count for? Sure it indicates that the Sun/Earth system is in considerable disequilibrium. But the only significance of this is the nature of the disequilibrium, which is precisely what we are talking about, the contradiction of AGW/GHE 'science' and the 2nd Law of thermodynamics, exactly the OP topic of this thread. Further you write:- "Absorptance in the visible/near-IR is not necessarily equal to thermal infrared emissivity. A statement like that just confirms what I am arguing. If they weren't equal there wouldn't be an equilibrium temperature of any sort. If emissivity always was different from 1-a (a is albedo) then the temperature would never be stable, rising or falling according to the sign of the difference.
  19. damorbel, I can't get past even your second paragraph, which seems to be gibberish.
  20. Tom, it is gibberish. No "seems" about it. The fourth paragraph I kind of get -- damorbel is confused about the difference between emissivity and absorptance, on the one hand, and emitted energy and absorbed energy, on the other. The first two are unitless fractions, and the latter two are radiant fluxes. That confusion probably explains the seemingly erroneous conclusion about rising or falling temperatures. But the second paragraph? Yeah, it's nonsense. Let's try a few substitutions: "The salmon difference is indeed great but what that count for? Sure it indicates that the Estonia/marshmallow system is in considerable hypothermia. But the only significance of this is the nature of the hypothermia, which is precisely what we are talking about, the contradiction of platypus/unicorn 'badminton' and the 2nd Earl of Ambergris, exactly the OP topic of this thread." Does that make any more or less sense than the original? Hard to say!
  21. damorbel #277: "How so? A has only 10% absorption capacity, B has 75%. Now the absorption capacity is always equal to the emission capacity, after all the reflection part cannot emit as well as reflect, can it? So both objects have the same temperature, any difference would clearly break the 2nd Law of thermodynamics." I'm sorry, but what part of 75 units of energy is greater than 10 units of energy don't you understand? Yes, an object cannot emit more energy than it absorbs. Ergo, if the more reflective object is only absorbing 10 units of energy it can only emit 10 units of energy. Those 10 plus the 90 reflected equal the 100 total incoming and thus incoming and outgoing energy are in balance. Ditto the less reflective object except that it is absorbing and emitting 75 units of energy. 75 > 10. It has absorbed and is emitting more energy. Higher energy absorption and emissions equals higher temperature. For the 90% reflective object to be the same temperature it would have to be emitting the same 75 units of energy... which added to the 90 units reflected would be 165 units total... which runs afoul of the law of conservation of energy... an extra 65 units of energy can't just spontaneously appear from nothing. You seem to be arguing that absorption and emission are the same for all objects... rather than that they are the same for each object. That clearly isn't the case because an object can't absorb energy it has reflected away. Taking Tom's example of a theoretically 100% reflective object it is clear that it would absorb no energy... and thus would be at absolute zero. The more energy an object reflects the less it absorbs and the colder it is.
  22. Re #294 Tom Dayton you wrote:- " I can't get past even your second paragraph, which seems to be gibberish." My 2nd para. goes like this:- 'Sun/Earth system is in considerable disequilibrium.' You may not be familiar with the thermodynamic meaning of the term 'equilibrium'; a thermal system is out of equilibrium when there is a temperature difference inside the system. This means that the entropy is below the maximum and there will be energy transport within the system according to the 2nd law of thermodynamics. The reason why the wavelength of incoming radiation is of no great importance is fairly simple; incoming radiation is either scattered (the albedo or reflected, if you like) or absorbed; the third possibility, transmitted, is not generally considered in planetary physics for reasons that should be self-evident. By definition the absorption does not affect the scattering, it is the scattering that affects the absorption. However it remains true that the scattering that gives the albedo its characteristic wavelength function i.e. its spectral characteristic. From this you will realise that the total scattering depends only on the amount of scatteing material present and the magnitude of the scattering is independent of the direction of arrival of the scattered wave; meaning the material that causes the albedo (scattered solar radiation) will have the same total effect on the emitted radiation, even though the response is in a different part of the spectrum. It is this that makes the emissivity and absoptivity the same in terms of power, even if not at the same frequency.
  23. No, damorbel, you are incorrect that "the total scattering depends only on the amount of scattering material." Scattering does depend on frequency of the radiation and the size of the reflecting matter. But your obsession with scattering is not relevant to absorption, which is the problematic behavior of greenhouse gases. Just to get you off of your reflection obsession, let's assume that you are correct that the same amount of radiation emitted by the atmosphere, water, and land toward space are reflected back, as the amount of radiation coming from the Sun that is reflected by all those. As that emitted radiation is on its way toward space, before it is reflected back down, some of it is absorbed by greenhouse gases. The absorbed radiation's energy can't be reflected, because it's not in the form of radiation any more. Only some of that energy immediately is turned back into radiation. So right there you've got a greenhouse gas trap of radiation and therefore a trap of energy, completely in addition to any reflection. Even if you were correct about reflection (you're not), the greenhouse gas absorption effect would exist, so increasing greenhouse gases would trap more energy.
  24. OK. So, damorbel recently wrote this gem: The wavelenth difference is indeed great but what that count for? Sure it indicates that the Sun/Earth system is in considerable disequilibrium. But the only significance of this is the nature of the disequilibrium, which is precisely what we are talking about, the contradiction of AGW/GHE 'science' and the 2nd Law of thermodynamics, exactly the OP topic of this thread. Now, he/she tries to explain it, but the only explanation is: (1) The difference in the wavelengths of radiation emitted by the sun vs. by the earth means that the sun and the earth are not at the same temperature. (2) This temperature difference means that heat will flow from one to the other. It should be obvious that this contributes nothing whatsoever of value. None of this justifies damorbel's nonsensical claim that planetary albedo is irrelevant to temperature ... and none of it has anything to do with AGW, let alone proving a "contradiction" between AGW and the 2nd law of thermodynamics. Damorbel, did you ever read the last paragraph of this comment? Did you understand it? I'd also note that damorbel has still not explained why he/she approvingly cites an explanation at wikipedia that explicitly relies on the exact same mechanism that he/she thinks violates the 2nd law of thermodynamics.
  25. #299: "nonsensical claim that planetary albedo is irrelevant to temperature" We went through a week or so of back-and-forth on the Chaos theory and global warming thread over 'climate calculators' that show specifically how albedo influences temperature. Seemed like a no-brainer at the time.

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