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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 1376 to 1393 out of 1393:

  1. Camburn indeed you can't throw the laws of physics out of the window, and indeed the GHE effect does not.
  2. O.K., we have reached page 6 and there is a point that seems wrong to me, but it may be something I don't understand. "No here's the clincher@ imagine that you take a mirror which reflects infrared light, and you reflect some of the infrared light the blackbody is emitting back onto itself. What happens to the temperature of the blackbody? One might think that because the blackbody is now absorbing more light, even if it is its own infrared light, it should warm up. But in fact it does not warm up; its temperature remains exactly the same [because it is in radiative thermal equilibrium with the light source]" Now my intuition would indeed be that it would warm up. To remain at the same temperature, it would have to be radiating energy at the same rate that it is absorbed. If you increase the amount absorbed using the mirror, the amount emitted must increase as well. However the Stefan-Boltzman law says that the rate at which a blackbody radiates energy is proportional to the fourth power of its temperature, so it can't increase emissions without an increase in temperature. If my intuition is wrong, I would be very happy to have my error explained! Note to YOGI. There are three examples here of how scientific discussion should proceed. Firstly I am happy to admit whan I am not sure I understand something, this is vital in avoiding Dunning-Kruger syndrome. Secondly, rather than just provide minimally informative comments/answers, I have explained as fully as possible what my understanding of the issue actually is. Lastly I am actively happy to have my intuition challenged and corrected.
  3. DM#1374: Presumably there is some other black body universe out there, radiating energy for our universe to absorb. Its the same place all your missing socks go. Your question (where does it come from?) is thus itself a black body, as it answers itself. Reminds me of the first part of Asimov's 'The Gods Themselves,' in which matter is exchanged between universes with disastrous consequences. The title itself is an interesting lesson.
  4. Dikran Marsupial @ 1372, a minor nit pick. Postma does not say that the black body stops absorbing light, only that the light it absorbs stops contributing towards heating the body. However, he appears to be making a bizarre claim, ie, that there is some temperature which constitutes thermal equilibrium such that, if a black body reaches that temperature it will automatically emit all light that it absorbs, where that temperature can be determined without calculating the energy balance. In fact, radiative thermal equilibrium is achieved when energy radiated equals energy absorbed, simpliciter, so he has the explanation backwards. He makes his error in the preceding paragraph where he writes:
    "If the source of light is constant, meaning it shines with the same unchanging brightness all the time, then the blackbody absorbing that light will warm up to some maximum temperature corresponding to the energy in the light, and then warm up no further."
    Here "some maximum temperature corresponding to the energy of the light" is ambiguous. Does he mean the brightness temperature of the light? In that case his claim is false. Or does he mean that it will warm up until energy emitted equals energy absorbed? Well, then what he says is true, but he has taken several paragraphs to say in a very confused way what he could have said clearly with one sentence. Of course, his " bizarre claim" of paragraph 2 of page 5 is not bizarre at all, but merely obscure if we give the second meaning to the quoted ambiguous passage.
  5. Regarding my previous post, I suspect postma is neglecting the fact that the blackbody will have an equilibrium temperature less than that of the lighsource, which means that its temperature can still rise due to the reflected IR. It couldn't become warmer than the light source though. Readling the rest of page six and the first half of seven, I think that Postma is making the same error than YOGI was. The GHE doesn't violate the second law of thermodynamics because the NET flow of heat is always from warmer to cooler, and as a result never makes the warmer object warmer, but it does mean that its equilibrium temperature can be higher if the surface is warmed by something hotter than the atmosphere (which it is).
  6. Tom Curtis Nitpick completely accepted. The first seven pages of the paper give the impression of having been written by a student that doesn't really understand the material, and hence is full of clunky explanation that if not actually wrong, are at least misleading or confusing. muoncounter Nevermind socks, which blackbody universe keeps absorbing my car keys? I'll lookup the Azomov book, haven't read any for years.
  7. Dikran Marsupial @ 1377, your intuition is correct. In setting up his description, Postma says:
    "When a blackbody absorbs the energy from light and there are no other heat or light sources around to warm it, then it will warm up to whatever temperature is possible given the amount of energy coming in from the light being absorbed. If the source of light is constant, meaning it shines with the same unchanging brightness all the time, then the blackbody absorbing that light will warm up to some maximum temperature corresponding to the energy in the light, and then warm up no further. When this state is reached it is called “radiative thermal equilibrium”, which means that the object has reached a stable and constant temperature quilibrated with the amount of radiation it is absorbing from the source of light."
    (My emphasis) He has defined radiative equilibrium relative to a specific light source on the assumption that there are no other heat or light sources available. He then, as a thought experiment introduces another source of light (the mirror) and assumes the radiative equilibrium remains constant even though the presupposition of his definition is now false. To see that he has made an error, imagine that the mirror is angled to reflect the rays of an IR lamp glowing with the same intensity as the black body. Clearly in this instance the black body would warm up further. As the IR photons do not come with labels indicating their origin, it makes no difference in the thought experiment whether we use an actual IR lamp, or save on our budget by using the black body as the IR lamp because, according to the hypothesis, the IR lamp and black body shine with the same intensity.
  8. Thanks Tom, reassuring that my intuition was in the ball park. It is rather telling that an error in Postmas paper can be spotted by someone whose only qualification in Physics is an A-level obtained in the mid 1980s! It is interesting that my intuition "To remain at the same temperature, it would have to be radiating energy at the same rate that it is absorbed." appears to be something called Kirchoff's law, which Postma actually uses later in the paper! ;o)
  9. Postma makes an excellent point at the top of page 12 that the blackbody equivalent temperature of 255k (-18C) is not the temperature of the surface, "but because most of the Earth's thermally emitted radiation comes from high up in the atmopshere and therefore this is the temperature you find up there" Well quite, that is exactly as AGW theory would suggest!
  10. Dikran immagine to have a box made of ideally absorbing material (emissivity=1). An object (a black body itself) inside the box and in thermal equilibrium with it receives from the walls as much energy as it radiates. Now, take a piece of the wall (or the whole of it) away and replace it with a mirror (ideally emissivity=0). In thermal equilibrium the mirror will reflect back to the object as much energy as the absorbing wall was emitting. So, the object will receive exactly the same energy as before.(*) Maybe Postma thinks his example describes a similar situation. Though, in his example the black body is not inside a cavity. He apparently does not note the (foundamental) difference. In doing so, he breaks the First Law of Thermodynamics. Definitely the object will warm more, as you say. Like turning the central heating on while the fireplace is running. (*) The way I described the thought experiment is not strictly correct but (hopefully) gives the idea. This reasoning is not mine, the correct description is part of the work published by Kirchhoff himself when demonstrating his well known law. Good old physics, I'd say. Rerference: Philosophical Magazine, v. XX, n. CXXX, p. 1, 1860, "On the relation between Radiating and Absorbing Powers of different Bodies for Light and Heat"
  11. All very good expanations etc above, but I think YOGI has to accept that all matter is trying to achieve a temperature of absolute zero. It is continuously cooling, and it doesn't care what direction it sheds its emissions. IF he can't accept that basic fact, then he will never understand.
  12. Cheers Riccardo, thought experiments are always very handy in testing out ones intuition about these things, and that one is very neat. As I mentioned earlier, Postma's example violates Kirchoffs laws, but he then goes on to use Kirchoff's laws later in the paper, so it isn't as if he didn't know them. This seems to me to be the sort of lack of self-skepticism that leads to Dunning-Kruger syndrome; presumably once he had an example that he thought he could use to argue the GHE violated the second law of thermodynamics, he didn't stop to consider whether it [his example] violated the first law of thermodynamics!
  13. scaddenp#1362 That`s just a 30kW oven. The radiative transfer is all outwards, there is no energy within the middle PVC wall returning to make the inner surface hotter. Its not much different to say the maximum surface temperature that the Moon or any object in near Earth space can get to, which is 121°C. If you get closer to the Sun it gets hotter, which would be the analogy of a thicker wall on the PVC oven.
  14. 1387, Dikran, I posted a response to you on the proper thread on Postma.
  15. YOGI - as SoD explains, the example is chosen because it is easy to calculate and shows how the insulation doesnt violate 1st Law. That is the point, no more.
  16. Both would be equally convinced... but the competent would have the overwhelming weight of peer reviewed evidence on their side while the incompetent would cite inane drivel from blogs.
  17. CBDunkerson (-Snip-)

    [DB] You chose to "hang your hat" on the Postma paper you linked to.  You were then challenged to defend a particularly egregious distortion of physics Postma makes, here.  You cannot through dereliction run away from your defense of this paper, as it is your chosen field of play. 

    A failure to follow through on your self-assumed duty will have consequences.

    Off-topic snipped.

  18. YOGI writes: "...but observations indicate a negative feedback." Which, of course, is complete nonsense. The net of observed feedbacks on greenhouse warming is clearly positive. No, peer review does not guarantee that something is true... but decades of peer reviewed research consistently finding the same result serve as a slightly better indicator than 'some story I just made up on the spot'.

    [DB] You are very correct, CBD.  Please note that Yogi has been tasked to defend an assertion he made earlier, which he then chose "hang his hat on" (see the response to his comment above to which you refer).

    Until he follows through on that he will not be allowed to divert any other threads.

  19. BernardB @9:30 AM, March 30th on the Sun Cycle Length page asks why heat sinks on electronic components work if back radiation warms the surface. Supposedly the "back radiation" between opposing fins in the heat sink would result heat simply being recycled in the unit. At a minimum, BernardB's reasoning is specious on the minimal grounds that the heat sink will still radiate thermal energy away from itself. It is true that the effective surface area for a heat sink relying purely on thermal radiation for cooling would be no larger than that of a solid box of the same dimensions, but that surface area is still much larger than that of the CPU (or other electronic component) the heat sink is designed to cool, and the the emissivity of the heat sink is potentially much higher than that of the chip. Consequently a heat sink provides significant gains in cooling relative to the computer chip by itself even if forced to rely exclusively on thermal radiation. In space, that cooling by thermal radiation would be more efficient provided it is not exposed to direct sunlight. That is because on Earth, within the computer casing the heat sink is exposed to back radiation of approx 390 W/m^2 in all directions from bodies at the ambient surface temperature. In space the "back radiation" when not exposed to direct sunlight is effectively at the temperature of the cosmic background radiation of 2.7 degrees K, or about 3 millionth of a Watt per meter squared. In practice that means the heat sink would radiate heat away at 390 W/m^2 faster than would an equivalent heat sink on the Earth's surface. More fundamentally BernardB is neglecting the fact that heat sinks work be convection. The air (or other fluid medium) between the fins is heated up primarily by contact with the fins. Because the it is then warmer, it then rises carrying the heat away far more efficiently than would radiative transfer. Because the initial transfer of heat is by conduction, the greater the surface area the greater the heat, hence the fins, which are always (or nearly always) oriented vertically for improved convective flow. With large modern PCs, even this process is insufficient and fans are placed above the heat sink to force the airflow greatly increasing cooling efficiency. The presence of the fan noise you can almost certainly hear as you read this is proof that BerarnB's understanding of the operation of heat sinks is faulty. An exception to the use of fans is found in some modern PCs which are filled with oil. The greater heat capacity of oil relative to air allows convection to continue to cool the heat sinks effectively, thereby eliminating noise and saving on power (and CO2 emissions). For more on the operation of heat sinks, see here and here. For more on oil filled computers, see here.
  20. BernardB also seems to think backradiation leads to a runaway (meltdown). Positive feedback != runaway. Moving the cooler to vacuum (so convection taken away - as well as most of the effectiveness of the heatsink), and you have a situation close to the one discussed at Science of Doom. Ie two stars side by side. However Science of Doom does the maths. You could do the same for the heatsink and see how much difference the backradiation makes.
  21. No Sir it`s not me that made the assertion of a "run-away melt down". Roy Spencer is the one who claims that the "back radiation" would eventually heat the heated plate to a point where the heater wire would burn out. Please do read his (350) responses and You shall find him saying so. I mentioned it because I thought it`s rather humorous, that Roy Spencer did not realize that in any resistive heating wire the resistance (Ohms) increases with the temperature and drop the current, ergo limits the amperes that can possibly flow through his heater wire preventing the wire from melting. The only way to heat the wire to a higher temperature would be to increase the Voltage ! That does however speak volumes how poor Spencer`s understanding of power expressed in watts is. I also want to point out, that satellites are not pressurized with any gas and even if You were to mount the consequentially superfluous cooling fan as shown in the PC power supply pictures chosen by Tom Curtis You would not have any convection helping to cool the components on a heat sink. Also all electronic components on modern satellites are modular & "plug in boxed" and not mounted anywhere on or near the vehicle shell where any such heat sink could radiate directly to the outside (into space). Heat sinks have been thoroughly researched and engineered for maximum efficiency, especially so for space exploration, BECAUSE there is no convection available to help cool high power components ! If "back radiation" from a colder to a hotter body were indeed a problem, then You would not find a single heat sink where the fins are arranged perfectly parallel to "mirror" heat at each other.
  22. One more example of a denier giving "skeptics" a bad name, to borrow Fred Singer's wording. "It is surprising that this simplistic argument is used by physicists, and even by professors who teach thermodynamics" and indeed the 2nd law of thermodynamics is maybe the most misunderstood law of physics, probably because it's only apparently simple but its consequences go far beyond the too common superficial (mis)understanding.
  23. BernhardB @1396 makes a rhetorical point about the superfluous nature of fans in space, clearly ignoring the fact that the illustration in question was related to the operation of heat sinks in atmospheres. He also makes several unsubstantiated claims about the design of satellites. This appears designed to evade discussion of the operation of heat sinks at the Earth's surface on which he makes no comment. Frankly, I find BernhardB's discussion of satellite design dubious at best. The multi-finned heat sinks used on Earth bound electronic components would constitute so much waste mass in space, and as low mass is critical to keeping launch costs down, I doubt any aerospace engineer would be so negligent as to use them. Rather, they are likely to use heat pipes, and axially grooved heat pipes (such as those shown below) to conduct excess heat to external radiators: Heat Pipes Axially Grooved Heat Pipes (cross section) Heat pipes work by being partially filled by a volatile liquid. Heat evaporates the liquid which then quickly carries the heat the external radiator where it cools and condenses. Surface tension keeps the resulting liquid in contact with the walls of the pipe, and thereby transports it back to the heat source. In axially grooved pipes, the grooves introduce a capillary effect, thereby improving the transport of the fluid back to the heat source. Note the single fin on the right hand Grooved Heat Pipe, which by increasing surface area improves radiative cooling. With regard to the internal or external deployment of heat sinks, some may well be internal. Many satellites operate on surprisingly little power so that heat accumulation is not a problem and an internal heat sink (or no heat sink at all) may be adequate. However, some require more robust solutions:
    "Dissipation of the heat generated by increasingly powerful satellite electronics presents inherent challenges. Today’s satellite applications, especially in the military sector, demand increasingly powerful functionality and a wider variety of electronics, which must be accommodated within a limited space. The drawback of increasing the number and power of electronics components is the generation of increasing amounts of heat while the available exterior surface area of the satellite —— through which the heat is rejected to space —— remains constant at best. Satellite designers and engineers rarely if ever have the luxury of increasing the exterior surface area of a satellite to improve heat rejection; and in many cases, any such increases would quickly be overtaken by increasing heat created by next generation electronics. As heat increases, the thermal devices used to dissipate the heat must transfer the heat effectively in any orientation and in the absence of gravity. Finally, satellite thermal solutions must operate under conditions in which maintenance and repairs are not possible, making flawless reliability a critical factor. To meet these challenges, thermal engineers are turning to deployable radiators. These occupy minimal space on the satellite surface until deployment in orbit, to create increased surface area for heat dissipation."
    (Source) So, to summarize, BernhardH makes false assumptions about the nature of heat sinks in space. In assuming that they have the same design as those used in earthbound electronic instruments, he makes a similar mistake to somebody who assumes that heat sinks in computers must be full of small tubes through which water is pumped just because they serve the same function as radiators in cars. He also falsely assumes that waste heat is radiated into the interior of satellites, whereas in fact, if heat is a significant problem it is radiated to space. Based on these two false assumptions he assumes that the actual design features of heat sinks are designed to work in a vacuum, despite obvious facts to the contrary (see my previous post). From this chain of errors he unsurprisingly comes up with false conclusions.
  24. BernhardB writes "Then I would like to know why the fins on power transistor heat sinks don`t "back radiate" each other into a China Syndrome melt down." I would have thought that was pretty obvious. Assuming the fins are identical and adjacent fins on the heatsink will be at approximately the same temperature, their radiation will be identical. In the worst case, all of the energy radiated by a fin will be absorbed by a neighbouring fin. In this case, fin C will absorb half the radiation emitted by fin B and half emitted by fin D. However this sums of this incoming radiation equals the energy radiated from fin C in the first place. As there is no net gain in radiative energy then fin C stays at the same temperature. However, in practice, not all of the energy is absorbed by the neighbouring fins, a lot of it is radiated away into space, which is why heatsinks are used to cool things. As others have already pointed out, heatsinks still work in space without convection by increasing surface area and emissivity. This isn't exactly rocket science, just a simple bit of accounting.
  25. BernhardB @1396: I thought it`s rather humorous, that Roy Spencer did not realize that in any resistive heating wire the resistance (Ohms) increases with the temperature and drop the current, ergo limits the amperes that can possibly flow through his heater wire preventing the wire from melting. The only way to heat the wire to a higher temperature would be to increase the Voltage ! does however speak volumes how poor Spencer`s understanding of power expressed in watts is. Just to be clear, Spencer's misunderstanding is not "of power expressed in watts" but a misunderstanding of the properties of metals. Changes in conductivity with temperature depend very much on the material being heated, as a simple Google search will inform. Obviously this is just one (of many) places where Spencer's analogy breaks down - but of course all analogies break down if you push them far enough.

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