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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 501 to 525 out of 1393:

  1. damorbel: 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? The diagram was about the flow in energy which is expressed in units of W/m^2. That is: energy (watts) over a specific area (a square meter). Are you suggesting that it should be "degrees" per square meter? 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. Why, because they used the proper of units of measure instead of the ones you suggest which make no sense whatsoever for describing the flow of energy. Also, why are you criticizing the IPCC when the diagram you are talking about is by Trenberth, Fasullo and Kiel (2009)?
    Response: [Muoncounter] Damorbel has asked the same question on prior threads. Do not try to rehash the same thing with him or it will never end.
  2. 498 Stamples -
    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?
    Surely 'elementary' physics asks and answers that question - although expressed more precisely - with the Carnot Cycle etc. No?
  3. Hello, Boy, this is a long thread! Can someone explain me why we are having a discussion about something that has been proved over and over again? - GW is real and it's clear as a daylight that we, humans, are driving it ...
    Response: Welcome to Skeptical Science! Please note this site's Comments Policy. Comments should be applicable to the topic at hand, and should refrain from inflammatory or insulting speech. These policies ensure that the science is communicated effectively.
  4. I happened to be a scientist as well, and claim to understand a thing or two about laws of thermodynamics and planetary energy budgets ... Satellite spectral data overwhelmingly show that earth's atmosphere absorbs strongly in the bands attributed to greenhouse gases ... what can be more proof that the greenhouse effect is real and that heat-absorbing gases are cousing it!!
  5. I've been reading through the postings on this thread, and saw some really nonsensical statements (from a physics point of view) that are backed by no evidence. I think whoever is moderating this blog (if anyone) should be more discriminating and not allow unscientific claims and statements be posted here ...
    Response: Thank you for your concern, but being wrong is not a violation of the Comments Policy. The idea is to educate by encouraging rational discussion, not shut down anyone due to a misunderstanding of the science.
  6. Thanks you! So there is a moderator after all. I totally agree with you.
    Response: There are several moderators in addition to our host John Cook.
  7. To Moderator - Yes, I agree with rational discussion, but what's rational about the crappy 'arguments' put forward by ( -Snip- )
    Response: [Daniel Bailey] Many commenters put forth crappy arguments, but the majority comply with the Comments Policy. As previously stated, dialogue is encouraged; no attempts to steer the debate or stifle free speech will be made here as long as all parties do it peacefully and respectfully. And yes, one can plainly see which are quality arguments and which are..."less so".
  8. Does anyone here agree with PhysSci, or find any credibility in those arguments?
  9. To KR 455 - You make a very good point about SB law when applied to planetary atmospheres: An increase in greenhouse gases directly decreases emissivity by absorption band deepening and widening. This drops emitted energy to space. A lot of scientists do not actually understand that indeed adding more heat-absorbing gases actually REDUCES emissivity of the atmosphere. I only know a few scientists, who realize that.
  10. With regard to the greenhouse effect - I think it works very similar to an actual greenhouse. The atmosphere does act as blanket to reduce heat loss to space and the obvious fact to support this is that we have 397 W m-2 leaving the surface and only 239-240 W m-2 exiting the atmosphere. So, the difference is actually retained by the blanket (our atmosphere). It's a no-brainer ... Also, see this recent article by prof. R. Pierrehumbert from University of Chicago, who explains very well how the greenhouse effect works making the point that the atmosphere is analogous to a house insulation in the way is prevents heat loss. Now that's real physics!
  11. John Cook - looking through the arguments list, I dont find one for the "GHE violates the 1st Law of thermodynamics". This have been dealt with at Science of doom in some detail but perhaps a place-mark article in the arguments is needed for debate on this? Sounds like PhysSci is using this.
    Response: [Not John Cook] Concur.
  12. If think PhysSci was talking about the downward thermal radiation from the atmosphere (rather than outgoing IR from the surface) that is larger than the absorbed solar radiation. Is this really true? If so, do the arguments presented at Science of doom still hold?
  13. I suspect that there is some kind of confusion with these energy fluxes. Yes, they claim these are based on satellite observations but they also admit that the accuracy of these measurements is quite uncertain. So, I doubt that the downward IR flux is in reality larger than solar flux. That does not make much physical sense to me, unless I'm missing something.
  14. Climate_Protector @517 My understanding (and I am not an expert on Climate) is that the fact that the downward flux is greater than the Solar input is due to heat accumulated in the climate system. You can see the results of a simple spreadsheet model of the longwave component only here. The numbers don't relate to Trenbeths diagram, I picked them so the calculation would reach equilibrium fairly quickly. Column A is somewhat badly named - it should be the amount of Solar radiation absorbed and re-emitted as longwave radiation. If you wish to replicate this, the cell formulae are: Cx=Ax+Bx Dx=Cx*0.4 Ex=Cx*0.6 Bx=D(x-1) and so this represents an atmosphere that radiates 40% of its longwave radiation to space and 60% back to the ground. I hope the spreadsheet is clear, towards the end I turn the Sun off, and calculate the time taken for the planet to dissipate the accumulated heat. At the end the energy in does indeed equal the energy out - which proves that I (and OpenOffice) got the sums right and that the model conserves energy. The main point is that, in this model at least, the back-radiation does indeed grow to be greater than the incoming Solar. As I said earlier, I'm not an expert so I would welcome corrections and clarifications !
  15. Apologies I got a cell formula wrong in the previous post; Bx=E(x-1) not D(x-1)
  16. Phil 518 - Yes, this makes sense. The question now is how much energy can the real atmosphere store (accumulate). It has to be quite a bit in order for the downward IR flux to exceed the absorbed solar flux. The atmosphere is pretty deep, so maybe it could store substantial amount of energy? I'm curious what others think?
  17. Climate_Protector - The thermal mass of the atmosphere is pretty tiny; about 4-5% gets stored as warming the land mass, and about 92% goes into warming the oceans. The atmosphere is pretty much a direct in-out of the energy stored, radiating out as much energy as it receives in IR (396 W/m^2), latent heat of evaporation (80), and thermals (17). But even a simple single-layer model will show this effect of high energy in the climate system - it has to be that high in order to put 240 W/m^2 back into space. So will a zero-dimensional emissivity model - surface emissivity is about 0.98 (almost a perfect black body), and to radiate the 240 W/m^2 received back to space would be at a temperature of about -17C. The top of atmosphere (TOA) spectra integrates to an effective emissivity of ~0.612 with all the absorption dips, and to put 240 W/m^2 into space the surface needs to be at about 390 W/m^2.
  18. KR 521 - Thank you. I understand the surface fluxes and the TOA fluxes, but if the heat storage capacity of the atmosphere is tiny as you suggest, how we have such a large downward IR flux (larger than solar)? That's the part I do not quite get and that's why I suspect (personal opinion) that the proposed downward IR flux may be incorrect ...
  19. Climate_Protector @520, the atmosphere "stores" considerable heat both in the form of the kinetic energy of motion (temperature) and as the vibrational and rotational energy of molecules (specific heat). The amount stored as vibrational energy and rotational energy is always proportional to that stored as kinetic energy, ie, as temperature. The amount stored is very small, however, compared to that stored by oceans. The specific heat of dry air is 1.026 Joules per kilogram per degree Kelvin, while that of water is over four times that. Combined with the much greater mass of the ocean, this means the vast majority of thermal energy at the Earth's surface is stored in the Ocean. However, this is largely irrelevant to PhysSci's argument. The reason the atmosphere maintains a very high back radiation is because a very high energy flux into the atmosphere is maintained, both by energy transfers from the Earth (IR, convection and evapo/transpiration) and directly from the Sun. In fact, when one of those (the sun) is removed at night, the back radiation falls very rapidly unless either: 1) The surface IR flux is maintained at high levels by being over ocean; or 2) The humidity is very high (which increases the specific heat of the atmosphere) and there is a low cloud cover (which lowers significantly the average altitude from which the back radiation comes, and significantly increases the heat capacity of the source of the back radiation). Obviously, there is some regional geographic effects, with warm air over sea water at night helping maintain a higher night time back radiation on the coast than in the interior. All of this follows because the back radiation is simply the thermal (IR) emissions of the lowest kilometer or so of the atmosphere; and therefore fluctuates with the temperature of that lowest portion of the atmosphere. Because of this, back radiation can fluctuate by more than 145 watts per meter squared in a single 24 hour period (the calculated night/day variation for winter in Mount Isa, Queensland) even without major changes in weather.
    Response: This sounds like a good additional Argument for the Arguments list. John Cook, what do you think?
  20. Climate_Protecter @522, the back radiation is a direct function of the temperature of the lowest part of the atmosphere. Because the lowest part of the atmosphere is very close in temperature to the surface, and the surface is far hotter than it would be if there were no greenhouse effect, the back radiation is also very high. Absent the greenhouse effect, the surface IR radiation would equal the incoming solar radiation. The green house effect lifts that till it is much higher, and therefore the back radiation is higher. It is as simple as that.
  21. I see. So, most heat is storred in the oceans. That makes sense. It sounds like you all are telling me that there is no error in the reported intensities of back radiation, and that it is larger than the absorbed solar flux. Correct? I'm an ecologist and from working with observed met data (to drive my ecosystem models), I noticed that the diurnal variation in back radiation is much smaller than solar radiation. In fact, back radiation is nearly invariant (+- 20W m-2 or so) between day and night. This is true for sites in the NH. I'm bringing this up in response to Tom Curtis's remark that back radiation varies diurnally quite a bit. I've seen the opposite in the data sets I worked with ... Anyway, I understand that the lower atmosphere, where most of the back radiation is coming from, is heavily heated by the surface through surface-atmosphere exchange of various energies. But I'm still not clear what's making the temperature of the surface to increase beyond the black-body temperature. I guess this is the chicken-and-egg question; which is heating which?
  22. Climate_Protector @521, to evaluate your claim that back radiation at your location varies by no more than 40 w/m^2 from day to night, I would need to know the temperature range. However, two factors may be relevant. The closer you are to the coast, the smaller the diurnal temperature range, and hence diurnal range of back radiation. And the colder the climate, the smaller range in back radiation for a given range in temperatures. For instanct, a temperture range of 27 degrees (as in Mount Isa) but with the maximum equaling Mount Isa's minimum (0 degrees C) would only result in a 107 w/m^2 range in back radiation. The reported intensities in the diagram have significant margins of error (around 5% I believe), but are the best available estimates.
  23. Was the air temperature proportional to the kinetic energy of air? I think Tom Curtis said that above. So, since back radiation is also proportional to temperature (somehow I think?), then would that mean that the kinetic energy of air is proportional to back radiation? If that is the case, then the lower atmosphere must contain kinetic energy that is larger than solar radiation, IF back radiation is really larger than the absorbed solar flux. This suggests that the atmosphere must be able to somehow store significant amount of energy. So, we come again to the atmospheric heat storage question. Correct? Something here does not add up - either we have a large atmospheric heat storage or the back radiation numbers must be wrong (or I'm profoundly misunderstand something). Do you agree?
  24. To Tom Curtis 522 - I work in Colorado (far from any coast), and the climate here is relatively dry. The diurnal temperature range in the summer is about 10-13C, and the winter might be much higher or much lower depending on the frontal weather system we get.
  25. To clarify regarding the back radiation I have seen. The flux changes little between day and night ONLY if you look at any particular series of 2-3 days, but it can fluctuate more than 100 W m-2 over the coarse of a season or a year.

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