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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 851 to 875 out of 895:

  1. KR 804 I "The climate, on the other hand, is driven by a band-limited solar input which does not match the thermal emissive spectra, is not greatly affected by greenhouse gases, and hence represents a fixed input, not a match to the thermal spectra at all." To further insight lets solve stepwise. The earth SURFACE has a very high emissivity (~.96) solar input to the earth SURFACE of 240 W/m^2 equates to~255K. Q1)Do you agree? Solar IR re-radiation via the earth SURFACE equates to ~ 240 W/m^2 emitted at 255K...this represents the temp maximum via solar radiation. Before the apoplectic visceral post, first consider the following. One litter of gasoline contains 34.8 MJ. No matter how well the losses are retarded or how slow/fast those Joules are consumed, the max possible energy/litter is as defined. Q2)Do you agree? Q3)By "thermal spectra" do you mean "atmospheric forcing"?
  2. L.J. Ryan I'll reply to those points out of order, first noting that you keep stating "240 W/m^2 emitted at 255K" without the very important black body included. Q3 - "By "thermal spectra" do you mean "atmospheric forcing"?" No. I mean the thermal emission spectra of the objects in question. The Sun's and Earth's thermal emission spectra are not identical (#27), and in fact barely overlap. Hence what affects one region (visible, IR, etc.) may or may not affect others. Q1 - " solar input to the earth SURFACE of 240 W/m^2 equates to~255K." Inexact and misleading. The sun provides ~240 W/m^2 via the 70-75% of solar emissive spectra that makes it through to the surface. This amount is at different wavelengths than the Earth thermal emission. And while 240 W/m^2 may be the energy emitted by a black body at 255K, neither solar input or Earth output is an unfiltered black body curve. Q1 - Nonsense. 255K represents the maximum black body temperature, but since solar input is mostly unaffected by GHG's (except for WV/clouds, but the sum effect appears to decrease emission relative to absorption, increasing forcing) - incoming energy is essentially fixed. And in order to radiate 240W/m^2, a temperature of 255K is an absolute minimum temperature for a gray body such as the Earth, as decreasing emissivity reduces power emitted at any particular temperature. To echo your posting - Do you agree?

    [mc] fixed unclosed bold tag

    [DB] KR, I think I guessed correctly as to which comment you were linking to & also included the Figure it seemed likely you had in mind. If I'm wrong, let me know & how to fix it & I will.

  3. KR 852 "To echo your posting - Do you agree?" somewhat Ok, "incoming energy is essentially fixed" i.e. ~240W/m^2, so what temperature should be generated via this, and only this flux. your hyperlink #27 goes nowhere..or is it the included chart
    Response: [DB] See above.
  4. L.J. Ryan - That link to #27 was intended to be the chart; sorry if I was unclear. Given a power to be radiated of 240W/m^2, the Stefan-Boltzmann relationship of Power=emissivity*Area*σ*T^4, where σ is the SB constant, and the [observed, measured!] effective emissivity from ground to space of ~0.612, I expect the surface temperature to be about 287K. A black body could emit 240W/m^2 at a much lower temperature. But: if radiated power remains fixed (by the 1st law of thermodynamics, conservation of energy), any gray body object (or planet) with a lower emissivity must necessarily have a higher temperature than a black body to radiate that power level. Which is the point I have been trying to make for the last several posts.
  5. DB - Thanks, that's where I was trying to point; I was trying to (A) refer to that graph (yes, that's the one) and to (B) make a point that this information been presented since the beginning of this thread. Sorry about the poor typing/tags; there may be more caffeine in my immediate future.
  6. KR 854 You've skipped the SW absorption step. Since you seem to be avoiding my question---Ok, "incoming energy is essentially fixed" i.e. ~240W/m^2, so what temperature should be generated via this, and only this flux?---lets try it this way. Q1)What is the SW radiation emissivity of the earths surface?
  7. L.J. Ryan - I did answer your question: 240 W/m^2 incoming flux is expected to, and does, generate a temperature of about 287K in the Earth climate system. How was I not clear about that? "Q1) What is the SW radiation emissivity of the earths surface": Irrelevant question - it's zero. The Earth is not at a temperature to glow in visible light. The Earth effective absorptivity, on the other hand, is about 70-75% for the solar spectrum. The band-limited input energy that doesn't overlap with the output IR is why input power is essentially fixed.
  8. KR Q2) What is the LW radiation emissivity of the earths surface? Note: I'm not asking about the effective surface to space emissivity but rather the surface emissivity.
  9. L.J. Ryan "What is the LW radiation emissivity of the earths surface?" About 0.98 on average. This is known data; I've stated that as well in this thread. Effective emissivity from the surface to the top of the atmosphere, however, is still 0.612, as shown as the integral of the Earth emission spectra. Incidentally, I would appreciate something longer than three sentence postings in this discussion, rather, a post long enough to have some content worth discussing, perhaps a statement of what your hypothesis is rather than a drawn out, multiple day, QA cycle. This is especially important in this case - if you feel that the radiative greenhouse effect actually violates thermodynamics, then please point out what effect does not, in a way consistent with the very large amount of evidence from multiple investigatory pathways that supports the greenhouse effect. The surface of the Earth averages 14C, and without the GHE it would run at about -20C. What alternative to the GHE do you propose that accounts for that?
  10. damorbel (RE: 780), "What you say doesn't just apply to a car, it is the same for a greenhouse or any surface exposed directly to the Sun's output. It's well known that, in a desert, the Sun can heat a surface well above 100C, enough to fry an egg. But even the arguments for the GH effect agree that it is the average temperature that is inportant, so they account for this by saying the Sun's output (the solar constant ) is not the measured 1370W/m^2 (@5780K if they include the temperature of the photons) but 342.5W/m^2 this latter would give an average temperature of about 279K, an average taken over the entire planet - summer and winter; pole to pole." But the temperature is about 288K - not 279K. How is that so? Here are more questions for you: Do you agree that all of the Sun's emitted energy is radiative? Do you agree that the Sun's emitted energy is transparent through space to the Earth? Do you agree that the Sun's energy is mostly transparent through Earth's atmosphere? Do you agree that space is colder than than the Earth's atmosphere? Do you agree that the atmosphere of the Earth is colder than the surface of the Earth? Do you agree that of the roughly 390 W/m^2 emitted at the Earth's surface, all of it is radiative? Do you agree that the emitted 390 W/m^2 is a result of the Earth's surface temperature and nothing else? Do you agree that the emitted radiation from the surface is mostly NOT transparent to the atmosphere? Do you agree that a lot of the surface emitted radiation is absorbed and re-emitted isotropically by the atmosphere?
  11. damorbel, Do you know that the 2nd law does not apply to photons? Thermal energy by definition is kinetic and not radiative. The kinetic energy in the atmosphere is not what's heating the surface. It cannot as the 2nd law dictates. It's the photons emitted from the surface and re-emitted isotropically by the atmosphere that is heating the surface. The net effect the kinetic energy in the atmosphere has on the radiative budget is zero, as I explained earlier in this thread. It seems to be a significant source of confusion in a multitude of issues - not this one.
  12. At the end of 861, I mean to say "not just this one."
  13. LJR: "Q1)What is the SW radiation emissivity of the earths surface?" What? Why would one even ask such a question is baffling.
  14. Phillippe @863 - Didn't you know - Global Warming is caused by glow-worms. :-)
  15. Re #861 RW1 you wrote:- "Do you know that the 2nd law does not apply to photons? Thermal energy by definition is kinetic and not radiative" I suggest a few moments contemplation will reveal that it does. Anything that radiates heat also absorbs heat and, if two bodies are near each other, they are thermally linked by the exchange of photons, in the absence of other heat sources (and heat sinks) the two will arrive at a common temperature, purely by means of radiation. However radiation is not heat, it is a way of transmitting energy that does not involve mechanical contact, so to that extent you are correct.
  16. Re #865 I wrote:- "they are thermally linked by the exchange of photons" Which is true. But the link does't have to have a thermal spectrum according to the Planck law; even if there was a filter that allowed only part of the spectrum to pass between the two bodies (there normally is a filter of some sort) they would eventually reach the same temperature, the filter just slows things down; it slows them down a great deal if the filter is a highly reflective mirror.
  17. Attractive hypothesis Phil, and as plausible as much of the stuff that has been thrown around on this thread...
  18. Philippe, Phil - My personal suspicion is that global warming is due to an overabundance of hotheaded naked ice borers reducing polar ice, increasing albedo. Obviously in this crisis we should organize large hunting parties to reduce this overpopulation of dangerous animals. Oh, and /sarcasm...
  19. Re #865 RW1 you wrote:- 1/"But the temperature is about 288K - not 279K. How is that so?" Apart from the fact that this 'average' temperature is not accurately known (NASA have a webpage somewhere with +-5K on some median temperature) - all gravitatioally bound concentrations of gas have a temeperature that rises towards the (gravitational) centre, in a star it reaches somewhere between 10^10K and 10^15K, hot enough for thermonuclear ignition. Even the paltry 85km of the Earth's atmosphere shows a temperature profile due to the planet's gravity. It is interesting to note that this gravitational temperature increase is a feature of very deep mines where, in addition to the increasing heat from the Earth (which is very variable with depth) a mine of 3km depth can have additional temperature rise of between 10 and 18K purely due to the higher air pressure at that depth. 2/"Do you agree that all of the Sun's emitted energy is radiative?" - Both electromagnetic and particulate radiation. 3/"Do you agree that the Sun's emitted energy is transparent through space to the Earth?" - I think the nswer is yes. 4/"Do you agree that the Sun's emitted energy is transparent through space to the Earth?" - No. the Earth's atmosphere absorbs all UV <0.3 microns, about 10% of the Sun's output and I believe about 30% in the infrared. 5/"Do you agree that space is colder than than the Earth's atmosphere?" - Space does not have a temperature. Different locations get heat from stars in varying amounts. At night the sky without the Sun has an apparent temperature of 2.7K, this is called the Cosmic Microwave Background or CMB for short. 6/"Do you agree that the atmosphere of the Earth is colder than the surface of the Earth?" - Over a given location - most of the time (except in the stratosphere). (Havn't I seen this stuff before somewhere?) 7/"Do you agree that the atmosphere of the Earth is colder than the surface of the Earth?" - No. 8/"Do you agree that the emitted 390 W/m^2 is a result of the Earth's surface temperature and nothing else?" - See 7. 9/"Do you agree that the emitted radiation from the surface is mostly NOT transparent to the atmosphere?" - See above. 10/"Do you agree that a lot of the surface emitted radiation is absorbed and re-emitted isotropically by the atmosphere?" - Yes Now be so good as to answer my question:- What % of the heat tranferred to the atmosphere from the ground by radiation:- 14%?......40%?.......90%? (You can make your own suggestion if you like.)
  20. Heat. It is much easier to say what heat is not, than what it is. It is not energy, although it is measured in the same units. Because it is not energy, it cannot be trapped, stored or transferred. The internal energy of a system can be defined and measured; it is a property of a system. Heat is not a property of a system. There is no such thing as the internal heat of a system. Most of the confusion in climate science explanations arises because heat and energy are treated as synonymous. Unfortunately, it is conventional to speak of heat transfer when we mean net energy transfer. It works if we keep the definition in mind. Heat is a process. It is energy in transit. It is the net energy transferred across the boundary of system due to a difference in temperature between a system and the surroundings of a system. The idea of net energy transfer is crucial. At the microscopic level, energy can travel in both directions between a higher and a lower temperature. The rate of transfer will be greater from the higher to the lower, so the net transfer will be from the higher to the lower and, risking confusion, we can call this heat transfer. By definition, therefore, it is uni-directional. Spontaneously, heat transfer will cool the higher temperature system and warm the lower temperature. Eventually, all systems in thermal contact will come to equilibrium at the same temperature. This does not necessarily stop energy transfer. It does stop heat transfer. That is why the “back-radiation” explanations of AGW are misleading at best, or wrong. All the explanations which claim direct warming of the surface from back-radiation violate the second law. Heat, net energy, travels only from a source to a sink; from a higher to a lower temperature. This is the only way that raw energy can actually do anything, either generate useful work or warm something. Heat (useful energy) cannot go backwards. For global warming, the first source is the sun, and the earth is the sink. For the atmosphere, the earth is the source and the atmosphere (at a lower temperature) is the sink. Finally, the atmosphere becomes a source and space (close to absolute zero) is the ultimate sink. Quite simple, indeed obvious. No-one would contradict that, surely. Have a look at the version of the Trenberth diagram at post 50, which is quoted all over the blogosphere: Outgoing Radiation: 396 watts per square meter Direct to Space : 40 watts per square meter Back Radiation : 333 Watts per square meter, absorbed by surface (it says) Quite absurd. The correct version is at page 6 of A First Course in Atmospheric Radiation by Grant W Petty. The net outgoing radiation is 21% of the solar energy entering the atmosphere, or 71.7 watts per square meter. This is, of course, a surface cooling effect, alongside conduction and convection (23.9 watts per square meter), and evaporation (78.5 watts per square meter). All these combined are atmospheric warming effects. So have climate scientists made an elementary mistake, as this thread suggests in the introduction? G and T are undoubtedly correct as far as most journalistic, political, and blogosphere posts are concerned. However, G and T did not address the interaction between outgoing radiation to space and the lapse rate, which is the favoured explanation of some of the climate scientists and the RC blog. Before we try to decide, we will have to examine the lapse rate.
  21. #869. Stunning. Please show us your sums for calculating the average surface temperature and lapse rate for say earth, venus, mars by this incredible piece of physics. I am really interested. After all this can be done by normal climate science. Any alternative explanation that you think avoids such "errors as violation of 2nd Law" has to do so as well. Physci claim he/she could though we are still waiting.
  22. Re #870 Fred Staples you wrote:- 1/"It is much easier to say what heat is not, than what it is." - Heat is what is measured by temperature, don't you think? 2/"Back Radiation : 333 Watts per square meter, absorbed by surface (it says)....Quite absurd" - Fred, what I like about 'Back Radiation' is that it goes straight into the surface, nothing is reflected, even though most of the surface is water with a refractive index of 1,33; I'm sure Fresnel is weeping in his grave!
  23. Fred, The back-radiation in Trenberth's original diagram illustrates the gross flow of energy from atmosphere to surface. If you want to see the net energy flow (heat transfer), just subtract it from the gross flow of energy from surface to atmosphere: 356 - 333 = 23 W/m^2 net radiative energy transfer from surface to atmosphere. Where exactly do you see a violation of the 2nd law? Oh and the "correct" version you cited is derived by doing this exact type of math on Trenberth's results. It is the same information, not a corrected version. All they did was sum the gross inputs/outputs into the atmosphere. Also don't forget that Trenberth's diagram documents measured values, it is not a theoretical whimsy. If you doubt that back-radiation exists, go buy an infrared thermometer, point it at space on a clear night, and prove your point.
  24. damorbel @872: 1) Surely you mean that Heat is what is measured by temperature multiplied by heat capacity. "Heat capacity"? How about that. It appears to be implicit to the language of physics (as it has always been to natural languages) that heat is something that can be stored. I think the revisionist physicists who want to outlaw the use of "heat" to describe internal energy are merely sowing confusion for themselves and their students. 2) Any back radiation that is reflected from the surface is simply part of the Surface Radiation. This is because, fairly obviously, IR detectors cannot distinguish between IR radiation emitted and reflected at the same frequency. The amount reflected is, of course, very small, and probably not large enough to include even if you designed instruments that could reliably distinguish it. Therefore there is no problem in treating emitted surface radiation plus reflected back radiation as a single bulk quantity. In this case you are scoffing at ant hills while swallowing mountains of denier nonsense.
  25. Re #874Tom Curtis you wrote:- "1) Surely you mean that Heat is what is measured by temperature multiplied by heat capacity" That would be energy - Joules. Heat capacity is 'C' (Cp; Cv) which is in Joules/Kelvin (J/K) The 2nd law requires for equilibrium, that T is uniform, i.e. no temperature diffences, no teperature gradients. Further you wrote:- "I think the revisionist physicists who want to outlaw the use of "heat" to describe internal energy are merely sowing confusion for themselves and their students." I have noticed this too but it isn't consistent. Since heat is only a function of temperature, it is only a function of molecular motion, the harder they vibrate the hotter things are! The devil in the matter seems to be latent heat which really is a misnomer. Let us take ice; when ice is formed the operative change is the loss of vibrational energy as the molecules line up to form crystals. Crystals have energy stored in the the lattice bonds. This is of course is 'energy in Joules' but it does not affect the temperature as ice forms/melts so it is not really heat but it is part of the internal energy of a water/ice thermal system. I suspect you knew all of this before reading it here. But there seem to be a number of contributors who need reminding! You wrote:- " Any back radiation that is reflected from the surface is simply part of the Surface Radiation." In kinetic theory molecules exchange energy by collision, thus on any (imaginary) surface introduced in a volume of gas the molecules on either side of this imaginary surface exchange energy (momentum, really), (T>0) all the time; so, if you were being pedantic, you could describe this energy exchange as 'forward' energy and 'back' energy and it is going on all the time, at every imaginable 'surface' in the volume. Energy exchange by radiation is not much different, but in this case the collisions are replaced by the emission and absorption of photons. Just like kinetic theory where the 'collisions' are between adjacent molecules. This is where the concept of 'back radiation' fails because it relies on radiation acting over large distances whereas it is actually very local, just the same as kinetic energy exchange. I have referred to this in previous posts, at the time I mentioned Einstein's paper 'Strahlungs-Emission und Absorption nach der Quantentheorie (german 1916) It is available on line in english and it is fairly easy to read and it firmly disposes of the idea that radiating gases can make any difference to the distribution of energy in a gas.

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