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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 651 to 675 out of 1089:

  1. les 638 No not Maxwells Demon, but rather Kirchhoff's black body theorisation.
  2. Re #649 les, you wrote:- "damorbel 648 - fine. We agree on so much including, it would seem, that the diagram is not "completely deficient in temperature information". " I think there is a definite problem there. I'm afraid I do not understand just what is it that makes you say:- " the diagram is not "completely deficient in temperature information". ? and:- "It clearly does "present ... useful information for any discussion on climate change " I'm not sure what you mean here. I am of course thinking of 'useful information' in the sense of scientific information, suitable for putting in reports called 'the Scientific Basis', the name of the sections of IPCC reports using this diagram. Trenberth's diagrams are among the few (perhaps only) in IPCC reports showing directional energy effects from greenhouse gases, I cite them because, without temperature, any figures showing energy emissions from the atmosphere, the Earth's surface or anything else have no scientific basis.
  3. LJ@651 >Sounds like a violation of the 1st law...remember perfectly reflective walls. Sounds as if you don't believe your own answer. The full context of that comment was: "... in practise you would not have perfectly reflecting walls," So his assumption with that statement was that in reality you cannot have perfectly reflecting walls. Now stop trying to nitpick and give his example some real thought. >ok lets assign three wavelengths and redo @646 As Tom pointed out this would just be a multiplier i.e. energy per photon X number of photons. To convert wavelength into energy per photon, use the equation h*c/wavelength. You can type this directly into google like so "h * c / 11364 nm". Once you have that number is just a matter of multiplying it by the number of photons, try it yourself. Re: question 1a) The aperture he is referring to is the open hole where the lid used to be. There is no aperture in Tom's original example. Re: question 2a) The lid still holds the same properties it did in the original example, i.e. it transmits 50% and reflects 50%.
  4. 653 damorbel: again, we are in complete agreement in not completely understanding the bits where I quote you. Still, to do what you require follow the advice of others here and read the full papers rather then looking at the pretty pictures. 652 Ryan, well, no actually. Not for that weird selective filter thing. Still, given the arbitrary was bits of physics are being thrown around this thread; have it your way, what the hell!
  5. LJ@651 >So the accumulated "boxed" light would radiate 1W for a 100 hrs, once the second aperture is opened? The wattage of the aperture would be entirely determined by the the size of the hole, the size of the box, and the amount of energy in the box. As the energy escapes from the box the wattage of the aperture decreases. It has no relationship to the original wattage of the flashlight. That value is determined by the flashlight's ability to convert chemical energy into radiant energy. In this example all the energy is already radiant. The box doesn't "remember" the wattage of the original light source.
  6. L.J Ryan@651 "So the accumulated "boxed" light would radiate 1W for a 100 hrs, once the second aperture is opened?" Wouldn't that depend on the size of the aperture? On the photon level wouldn't the "flow rate" out of the aperture change as less light remained in the box? Fewer photons would be available to hit the target/sec. I imagine it would be a bright and rapidly fading light once the hole was opened. Since this is all imaginary...
  7. e654 "So his assumption with that statement was that in reality you cannot have perfectly reflecting walls. Now stop trying to nitpick and give his example some real thought." The perfectly reflective wall was Tom constraint not mine. Violating the 1st law in order to illustrate radiative forcing is not nitpicking, rather it's very fundamental. To say emissivity >0 leads to "light decay" is a convenient concept...does the photon slow down until comes to rest at the bottom of the box? What is light decay? Tom said: "The answer has to be in terms of photon numbers, not energy because the wavelength of the photons has not been specified. If we specify that all photons have the same wavelength, then the multipliers for photons in the answers above can be used for energy." you say: "To convert wavelength into energy per photon, use the equation h*c/wavelength. You can type this directly into google like so "h * c / 11364 nm". Once you have that number is just a matter of multiplying it by the number of photons, try it yourself." You seem to be at odds with Tom. Your suppositions concludes, regardless of wavelength the accumulated ENERGY within the box is twice the input...as detailed by @646
  8. e656 pbjamm657 Make the aperture diameter 2x photon diameter, and shazam you have a flashlight powered photon laser.
  9. LJ> Everything you said in post 658 is false. I will not detail the problems because they are all simple reading comprehension errors. If you're not going to make any effort to read and comprehend the point that is being made then you are not here for intelligent discussion; there's no point in talking to you.
  10. les655 "Ryan, well, no actually. Not for that weird selective filter thing. Still, given the arbitrary was bits of physics are being thrown around this thread; have it your way, what the hell! " I agree sloppy physics, lot fundamental violations. The weird filtering was Tom's not mine. Are you familiar with Kirchhoff's black body cavity theorisation/experiment?
  11. e 660 I asked a question...What is light decay? You are not contradicting Tom,suit yourself.
  12. LJ> In that specific comment, Tom clearly indicated he was talking about a "real-world" box that would not be perfectly reflective. In that case "decay" would be the light escaping the box. I have not contradicted a single thing Tom said, you just aren't reading what's being written.
  13. The energy quality I described in 498 is obviously related to the inverse of entropy, but people struggle with the latter concept. It is easier to see that quality is related to useful energy – that is, energy that can actually do something, such as producing useful work or raising temperature. In any spontaneous transaction involving energy transfer, quality will diminish. What this means is that the energy cannot go backwards, and some of the useful energy will be lost. (I said I would not mention the second law, but that is what it is really about). Incidentally, 499, real life energy losses through friction, etc, make the situation worse. Energy quantity as well as quality is lost to the system Here is another elementary example. An insulated vessel contains gas at a high temperature, and is separated from a vacuum, within the vessel, by a membrane. If the membrane is punctured, the gas flows into the vacuum, and its pressure drops. No work is done, no heat is lost, so the temperature remains the same. The first law says that energy has been conserved. But the gas is obviously able to do less work, starting from the lower pressure. It is also obvious that (Maxwell's demon apart) the gas cannot go back. What has happened is that the quality of the energy has fallen. It turns out (as the pundits say) that it is this elusive characteristic of quality (strictly entropy) that drives all spontaneous transactions – literally everything from chemistry, biology, energy transfer and (fancifully) the tidiness of your desk. It is the quality of energy, not the quantity, that makes something happen. Here are two well known examples. Suppose that a single gas flame operates at a temperature below the melting point of a steel plate. The steel will not melt because the quality of the flame energy is too low. Now apply ten more similar flames. Still the steel will not melt. Another example is Einstein’s experiment to eject electrons (I forget from what) with a beam of incident radiation. Below a certain frequency (energy quality) nothing happened, no matter what the intensity (energy quantity). Above that frequency, electrons were ejected, and quantum mechanics was born. Sadly, however, my definition of quality (available energy) is too simplistic. It is the relative quality that matters. Switching the argument to a power generator, a source with a high temperature can generate work by transferring energy to a sink at a lower temperature. The available energy is high. If the sink is at the same temperature as the source, nothing will happen. There is no available energy. Likewise, if the sink energy can be connected to a second sink with a lower temperature, space heating is possible. Otherwise the waste energy will be ejected to the atmosphere through cooling towers. So, if I have persuaded anyone that the crucial elements of energy transfer are the qualities of the energies concerned, and that energy can’t go backwards without the performance of extraneous work, we can move on to an even more elusive concept. Heat.
  14. Fred Staples - That's a fairly reasonable (if wordy) description of Entropy. You could have just linked to an existing definition and saved much typing. I think I see where you're going - to an argument that the high levels of IR at the surface somehow violate entropy considerations. You might find my comment here relevant in that regard. We aren't dealing with a closed system, but rather a very open one, where the important issues are rate of energy flow, energy differentials, and internal temperatures and energy levels required to maintain a dynamic equilibrium. Not moving a fixed amount of energy around a closed system. Entropy is increasing as sunlight radiates out into the 3K void of space. Local conditions regarding the conversion of that visible light into thermal IR provide a pinch-point, much like the dam in my analogy, one that includes a local collection of energy in order to have an energy differential sufficient to radiate the IR to space.
  15. L.J. Ryan - back to the original light box. We agree that the set up does NOT violate 1st law? No where in the system is energy being created. The thing that seems counter-intuitive apparently is that energy-fluxes appears to double. Whoa! energy creation! No. This illustrates that care has be taken in inferring system energy from energy flux, because in this case, with reflection, the same energy gets counted twice. This is no violation of 1st law going on in Tom's example - nor in Trenberth's diagram for same reason. Just an illustration about care in use of energy flux.
  16. Re 650 RickG :- "For what Trenberth is demonstrating temperature is neither necessary or relevant in that diagram. I have no problem understanding the diagram myself." It is difficult to believe that a diagram showing emission of thermal radiation (W/^m2) can be considered useful when no indication is given of the temperature of the emitting body; why else would the Stefan-Boltzmann equation (E= rhoT^4) be so widely deployed in thermal physics?
    Response: [Muoncounter] you raised this identical 'objection' in November, 600 comments up this very thread. The same replies you received then still apply now. Insistence on mere repetition demonstrates that your argument ran its course.
  17. damorbel given that you know how to calculate temperature from the energy flux, why are you asking Trenberth to report it in a summary graph? You really look polemical here, it adds really nothing to the discussion or to the undersdtanding of the energy budget.
  18. L.J Ryan@559 No you would not have a laser, the beam would not be directional since photons would reach the aperture at a variety of angles and travel out that way too. From a photon=particle bouncing around standpoint I think the brightness of that beam would diminish over time as the number of photons hitting the aperture/sec would diminish along with the number in the box. "Light Decay" in the context of the discussion is light losing energy to the imperfect mirrors. As for Tom Curtis' original diagram @615 I think the missing element in most of this discussion is time. Over time A=C but not for every photon interaction. We need to be clear if we are talking about an instantaneous measurement or the totals over time. Same goes for the Trenberth Diagram. note: I am not a physicist nor mathematician. I reserve the right to be wrong.
  19. scaddenp666 Oh...Tom and Trenberth count the photon/flux twice. So the vector sums are zero, no atmospheric forcing...no AGW. You have confirmed my position.
  20. L.J. Ryan - Since the Trenberth numbers are an energy budget, they should add up and cancel out for an unforced climate. If, however, you carefully add up the Trenberth numbers without rounding you get an imbalance of about 0.9 W/m^2 less leaving than arriving. That's the forcing.
  21. 661 Rysn good try at a twist. I'm afraid that it's transparent to all here that you really are not "doing physics" in your argumentation. At thus stage, really, we're just playing with you. Dont take your arguments so seriously - no one else is.
  22. damorbel, The diagram, or schematic as Trenberth calls it, is from one of his many PowerPoint presentations. In other words the schematic is meant to be presented with discussion in context with his presentation. The schematic as many of us have pointed out to you is about incoming solar energy and how it is distributed throughout the climate system taking different forms of energy. In describing this, not only by Trenberth, but all scientists, use the proper units of measure which is watts per square meter, not temperature. But since no one seems to be able to convince you of that, watch Trenberth describe that very schematic himself in this video.
  23. " So the vector sums are zero, no atmospheric forcing...no AGW. You have confirmed my position." Umm, this is about whether the GHE is consistent with thermodynamics. If it is, the adding CO2 will create forcing as KR has pointed out. (and is measured at TOA). Now the numbers on Trenberth are derived from measurement and the flux has to be consistent with temperatures. The light box discussion is about understanding why these fluxes are not a violation of 1st law.
  24. Fred Staples @664: Considering the example of the steel beam, the flames will not melt the steal beam because they will (eventually if it is well insulated) heat the steal to their own temperature. At that point the black body radiation from the steel will carry the same energy as the flames preventing further warming. Applying that example to my light box model, the photons leaving the box will never have a shorter wavelength (= higher temperature) then the photons leaving it. Nor will there be more of them on average, thus conserving energy. Applying that insight to the Greenhouse effect, that means the the surface of the Earth will never be hotter than the surface of the sun (ie, the temperature of the source of the energy that warms it), and the outgoing radiation will never have a shorter wavelength than the incoming solar radiation. You will struggle to find a prohibition against the greenhouse effect from these two facts.
  25. Tom Curtis 675 "the Earth will never be hotter than the surface of the sun" The Earth receives 240W/m2 from the sun. The blackbody temp resulting from this flux represents the maximum temperature...not the sun's surface. That is, regardless of reflection and or re-radiation, 255K is the pinnacle temp for radiation alone. It is this fact by which blackbody was derived...trapping light in order to discover it's maximum thermal energy. ----clipped from another blog: The idea of trapping light is intriguing, and Gustav Kirchhoff (1824-1887) conceived a solution: A hole in a cave. A beam of light could enter this hole but the walls inside would absorb any reflections and prevent the light from escaping. Thus, by confining incoming radiation, the thermal energy which light confers could be shown to its maximum advantage. Kirchhoff's scheme was superior to selectively transmitting glass because a cave absorbs and traps all wavelengths of light,thus creating a complete radiative imbalance. At least theoretically. Well, so what was found by cavity experiments? That a perfectly absorptive ("black") body rises to a temperature a bit higher than an actual black body that’s free to radiate to its surroundings. A theoretical blackbody thereby defines the upper limit of temperature vs radiant absorption. Try to grasp the implication, then. A blackbody cavity mimics the radiative restriction that"greenhouse gases" are said to induce. Indeed, virtually none of the thermal radiation generated inside this cavity is allowed to escape. It "re-circulates" instead, and is sampled through a tiny hole. Does this confinement lead to a runaway greenhouse effect, though? No, it only sets an upper temperature limit — the SAME limit that’s applied to the earth in the first place, for its estimated temperature is based on a blackbody

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