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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 101 to 125 out of 254:

  1. Phil, what is to explain there? The picture clearly shows a heat source (human body) emitting less heat where it is isolated. Unless you consider the earth to be it's own source of heat it has nothing to do with what needs discussed here.
  2. h-j-m, how do you explain the clothes staying cool when there is a warm body underneath them?
  3. Tom Dayton, because the clothes constitute the isolation I mentioned!
  4. Anyone who still doubts that CO2 absorbs infrared radiation should see this demo. A picture ... worth a thousand words. A video ... must be a thousand pictures!
  5. Re #104 muoncounter you wrote:- "Anyone who still doubts that CO2 absorbs infrared radiation should see this demo..." True, I don't know anybody who thinks CO2 doesn't absorb IR radiation. But there is a large number of people who believe they exist; rather like bad fairies, somewhere out there, trying to prove there is no greenhouse effect. I also know a lot of people who doubt CO2 doesn't emit infrared in exact proportion to the amount it absorbs... Now let us all think hard where these really strange people are to be found!
  6. Re #98 muoncounter you wrote:- "However, we know that both objects radiate, albeit at different wavelengths. Some of the cooler object's radiation is absorbed by the warmer; however, more total energy is transferred from warmer to colder." Which is fair enough, it is what your MIT link explains. And then for some bizarre reason you appear to make an exception for GHE radiative transfer :- "The 2nd Law is satisfied AND the greenhouse effect still works." (my emphasis) But the GHE requires that GH gases increase the temperature of the surface that is already hotter than the upper troposphere. (The troposphere can only get warmer than the surface in the exceptional condition of temperature inversion.) To get the surface temperature to increase the GH gases would have to cause, as you put it, 'a net transfer of total energy' from the troposphere to the surface. You also cite this post:- #83 CBD Your belief that this means energy can ONLY flow from 'hot' to 'cold' is simply nonsense, and rejected as such by all but the outermost looney fringe of modern physics. with favour. Surely you notice this post is in complete conflict with your MIT link?
  7. We often hear people say that GHG warms the surface. I understand its use in common language, but it's not to be taken litterally. Indeed they don't, they only reduce the outgoing flux. It's the unbalanced flux from the sun that makes the surface warmer.
  8. Re #107 Riccardo you write:- "I understand its use in common language, but it's not to be taken litterally." I do hope you don't mean this. Either the GHGs cause a warming of the surface or they don't. I have yet to read anything on the GH effect that doesn't posit an actual (literal?) warming of the surface. Since they are found in the atmosphere CO2 and H2O in the atmosphere actually are big players in radiating heat from Earth, you can see how much by looking at IR images of Earth here But the Earth wouldn't bake if there weren't any GHGs in the atmosphere. The average temperature would not be different, only the distribution of temperature would change, given the lack of heat transport from equator to poles by oceans and atmosphere.
  9. darmobel I meant exactly that. It should not come as surprise, often physics and common language share words with slightly different meaning. GHGs cause a change in the balance of the energy fluxes, but do not directly heat the surface. (Note the difference between energy and heat). It would be more correct to say that the greenhouse effect impedes cooling.
  10. h-j-m @101 So areas of the human body that are surrounded by clothes and hair emit less heat than those exposed ? Is that what you're saying ? I'm affraid thats not true. The part of the body under the clothes is emitting just as much heat as the exposed areas (the face, for example). But the clothes are clearly emitting less heat. Whats happened to the rest ?
  11. Re #109 Riccardo, you do not respond about the effect on the Earth's average temperature of removing all the GHGs from the atmosphere, will it go up or down?
  12. Phil, I just answered to your question which was about what a picture showed. The picture itself can only show an image of the radiation that arrived at the camera. Sorry, I thought that to be obvious. To deduce from a picture what it does not show is pure speculation. I try to avoid entering that realm.
  13. damorbel @108 You need to understand the difference between GHGs warms the surface and GHGs cause a warming of the surface These statments are different. My analogy with clothes may help you. Clothes do not warm your body, but they do cause your body to warm.
  14. h-j-m @112 If you refer to my post @98 I suggested you explain the temperature distribution, I did not suggest that you describe it. If you do not wish to try, then thats up to you, but doing so should help you with the errors you are making in your comments.
  15. Phil, I am referring to your posts #98 and #110. In your post #110 you state: "The part of the body under the clothes is emitting just as much heat as the exposed areas" and ask me to comment on this. Unfortunately you fail to provide any evidence to back that assumption. To explain something on the base of pure assumptions is called speculation in contrast to science where explanations are based on facts.
  16. #105: "a lot of people who doubt CO2 doesn't emit infrared in exact proportion to the amount it absorbs" You accept that CO2 absorbs IR, but now posit that it re-emits in 'exact proportion'? If that were true, why does the IR emitted by candle flame disappear when the CO2 is absorbing its IR? If emission were in 'exact proportion' to absorption, where is that emitted IR in the demo? Or does the IR absorbed remain as kinetic energy in the gas, resulting in increased temperature? -- A result completely consistent with all the other grade school level science experiments showing that a bottle with CO2 attains a higher steady state temperature than a bottle of room air. "But the GHE requires that GH gases increase the temperature of the surface" No, the GHE increases the temperature of the atmosphere. Your parsing of words and inserting phrases that aren't in the question is quite tedious; it isn't contributing to the actual debate -- which is, I suspect, your actual purpose in hijacking this thread. "Surely you notice this post (CBD's #83) is in complete conflict with your MIT link?" In what way? Be specific, without drifting into irrelevancies. This isn't a course in rhetoric. #108: "CO2 and H2O in the atmosphere actually are big players in radiating heat from Earth ... the Earth wouldn't bake if there weren't any GHGs in the atmosphere. The average temperature would not be different" Please substantiate these new claims with something more than a weather satellite image -- which doesn't illustrate whatever your point is.
  17. Re #113 Phil You wrote :- "Clothes do not warm your body, but they do cause your body to warm." Um... not true. While you are alive it is your (fuel burning) metabolism that keeps you warm, your clothing is an insulator that reduces the rate of heat transfer to the ambient, should the ambient be below 37C. If the ambient is above above 37C then you have problems because you cannot switch your metabolism off (well you can, but the problem is to switch it back on again) and, in the absence of any action to change the situation your body temperature may well rise to heatstroke levels. If you cannot switch your metabolism back on, your body will slowly settle to ambient temperature, what ever that happens to be. How long it takes to reach ambient depends on how good an insulator your clothes are. If you are put in a large vacuum flask with mirrored walls to reduce the radiative heat transfer then it might take days; without the mirrored walls - a day? The point is the insulator, be it a woolly blanket, a multilayer mylar sheet or a vacuum container, can only slow down the rate of heat transfer and thus the time taken to reach ambient. Your clothes keep you warm because you are continually replacing the lost heat with your metabolic heat - until you die, of course!
  18. Re #116 muoncounter You wrote :- "why does the IR emitted by candle flame disappear when the CO2 is absorbing its IR?" Because the IR from the candle is from a small, high temperature source and the CO2, absorbing the IR, warms only a little because there is much more of it. Given time and a sufficiently sensitive set up, the warming of the CO2 would be detectable - because the CO2 also emits IR and this would show up on an IR imager. A suitably sensitive IR imager may be able to resolve 1/1000K, such devices detect bodies under piles of concrete rubble when looking for earthquake victims. You wrote :- "No, the GHE increases the temperature of the atmosphere." When ever I have read about the GH effect it has been the surface temperature that was the worry. Sorry if I picked you up wrong! You wrote :- " "Surely you notice this post (CBD's #83) is in complete conflict with your MIT link?" In what way? Be specific, without drifting into irrelevancies." In #83 CBDunkerson wrote :- "energy flows into all surrounding objects regardless of their relative temperatures." Whereas the MIT link (section 2) has this :- "No process is possible whose sole result is the transfer of heat from a cooler to a hotter body" Now the reason 'sole result' is included is because heat pumps and refrigerators do transfer heat from a low temperature to a high one but they require another source of energy over and above the 'energy' they are pumping, thus the energy transfer the bring about is not the 'sole result' or sole action if you will. Thus CBDunkerson's statement contradicts your MIT link.
  19. damorbel, Solar energy arrives primary in the frequencies of visible & UV light. This passes through CO2 unimpeded. The planet absorbs the energy and re-emits it largely in the IR range (i.e. heat). CO2 and other GHG gases slow down the emission of these frequencies to outer space. This causes a increase of energy at the planet surface, and a cooling of the stratosphere since the rate of energy loss has slowed, until a new equilibrium is reached.
  20. #118: "Given time and a sufficiently sensitive set up, the warming of the CO2 would be detectable" Then your 'exact proportion' argument from #105 requires a very low constant of proportionality. And you accept that the gas is warmed -- please explain where this excess kinetic energy goes if the proportion emitted is so low. "I have read about the GH effect it has been the surface temperature that was the worry." Please tell us what you've been reading and what surfaces you appear to be worried about. "the reason 'sole result' is included is because heat pumps and refrigerators do transfer heat" Yes, I am sure Clausius had exactly refrigerators and heat pumps in mind. Perhaps you ought to state for the record: Do you accept the idea that greenhouse gases absorb infrared energy radiated from the surface of the earth and oceans, resulting in a warming of the atmosphere? Does CO2 play a major role in this process? If not, what are your specific objections? What published science (as opposed to vague references to individual interpretations of the 2nd Law) can you cite to support your objections?
  21. damorbel, you wrote: "If the ambient is above above 37C then you have problems because you cannot switch your metabolism off (well you can, but the problem is to switch it back on again) and, in the absence of any action to change the situation your body temperature may well rise to heatstroke levels." Of cause that is true in principle. But to my knowledge other actions take place as the body will shed water to increase the heat transport and in consequence the dehydration will lead faster to terminal conditions then the rising temperature. Anyway, I think this will be my last post here, The kind of reasoning here seems to give me headaches. I just happen to live in one world of which I assume it exists and is real. I have to confess that assuming an other world to exist and being real as well seems already too much for me. Switching between this two worlds seemingly according to which one fits the argument better makes it even worse. Let me jut explain these two worlds which I will just name here and there. Here matter absorbs radiation only at certain frequencies and is either transparent or reflects or scatters the rest due to its chemical composition. Here matter emits radiation at certain frequencies (producing something called colour at the visible range) again due to its chemical composition. Here I can feel the warmth of direct sunlight though that should be dwarfed by the earth's surface radiation not to mention back-radiation. Here temperature differences cause winds to blow, currents to flow, water to evaporate and in essence everything needed to sustain life or even to allow for its development. There all matter absorbs all incoming radiation which means that it acts effectively as a black-body. Consequently it also emits radiation where the according black-body spectrum, which only depends on the temperature of the black-body, determinates wavelength range and intensities. There temperature differences are the result of imbalanced energy flows which are caused by black-bodies emitting energy depending on the temperature of the black-body which again is caused by imbalanced energy flows etc. etc. ad infinitum. There is a world not only hard to understand but even harder to accept as real.
  22. h-j-m: Given: (1) that the atmospheric greenhouse effect is well-documented, empirically, as discussed on this website, other websites, and in the peer-reviewed literature; and (2) that the consequences of enhancing this greenhouse effect via anthropogenic emissions of greenhouse gasses - that is to say, global warming - also has a plentitude of empirical evidence to support it, as described on this website, other websites, and in the peer-reviewed literature; It would seem to me that there is not much question as to which of the below alternatives is more likely: - that all this empirical evidence (global cryosphere mass balance decline, sea level rise, changing migration patterns, pest prevalence in increasingly higher latitudes, temperature anomalies, &c &c &c) isn't really pointing to global warming via the atmospheric greenhouse effect - that you and damorbel have managed to misunderstand thermodynamics
  23. Re #121 h-j-m wrote:- "But to my knowledge other actions take place " A feature of this kind of discussion is the tendency not to see the bigger picture. My vision of man sized vacuum flasks was rather far fetched and any working version would probably stop any of your "other actions" so effectively that the death of the person inside would not be long delayed, thus illustrating the bodies return to equilibrium with the ambient temperature! I enjoy your contributions, you can express yourself on thermodynamics without difficulty. The trouble is few of your readers are able to appreciate what you are saying. For my part I try to explain, if I am not understood I try to work out why, because I would like to better explain these matters, especially for non experts. This is a serious ambition because there is a limited future in just talking to experts. There are plenty who will never accept that the AGW hypothesis does not hold together but others, who are genuinely interested, need to be supported by well crafted explanations, that is what I am trying to do here!
  24. Re #122 Composer99 wrote:- "the atmospheric greenhouse effect is well-documented, empirically " This thread is about inconsistencies in the well documented explanations of the greenhouse effect.
    Response: [Daniel Bailey] Just to be clear, you mean supposed "inconsistencies", with you being the supposer.
  25. Re #120 muoncounter, you wrote:- "Yes, I am sure Clausius had exactly refrigerators and heat pumps in mind." Clausius was not an ignoramus, he was almost certainly aware of refrigerators since a patent for a refrigerator was granted to Jacob Perkins in 1834, when Clausius was 12. you wrote:- "Do you accept the idea that greenhouse gases absorb infrared energy radiated from the surface of the earth and oceans, resulting in a warming of the atmosphere?" Perhaps 20% of atmospheric energy gets there by radiative transfer, of which CO2 accounts for perhaps 7%, another 20% by convection from the warm surface and 60% is due to water evaporating. In contrast about 80% leaves the Earth by radiation from atmospheric gases, of which 25% is due to CO2 and the remaining 20% by direct radiation from the surface. OK? Most of these figure come from the well known diagram of of K Trenberth, (I am having difficulty with the link - here's hoping!) they are the credible part of his diagram. The 'back radiation' idea is misconceived and certainly does not change the temperature distribution. Since the presence of H2O and CO2 etc. whose weak effects cannot distort the heat distribution, a distribution which is determined by much more powerful gravitational and cosine effects, exactly how the how the Sun's energy is absorbed is not of the first relevance. It is curious to note that Svensmark's cloud hypothesis has far greater traction than the GHG one. The death knell of the GHG hypothesis really is the effect of gravity on the atmosphere. The troposphere has a vertical temperature gradient which is a uniform 6.5K/km from the equator to the poles. This corresponds to the change of gravitational potential energy with height, it leaves no room for any radiative GH effect; it can even account for the rather strange temperature profile of the Stratosphere.

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