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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 76 to 100 out of 1089:

  1. Re #75 muoncounter, you wrote:- "This seems to summarize the universe of the skeptic: It is what they say it is; until they say something else and believe that too -- even if it contradicts their prior position." You seem to have forgotten that AGW is all about a rise in global temperature above some historical average i.e. the whole dispute is about temperature. Thermodynamics is the science of heat of which temperature is a measure of strength, intensity - analogous to pressure in fluids. How much heat - is a quantity measure and, for heat, is specified in Joules (ergs or calories if you like). It is interesting to note that certain posters on this thread have dismissed the science of thermodynamics (2nd Law of Thermodynamics and so-forth) as if it were some kind of fantasy, instead of being the basis of all modern physics that it is.
  2. #76: "seem to have forgotten the whole dispute is about temperature." I most certainly did not forget that; I saw a high over 29C yesterday, two days before Thanksgiving! My point was -- and still is -- that one can see a broader picture of 'the state of skepticism' emerge by looking at several threads here in context and counting the contradictions. "certain posters on this thread have dismissed the science of thermodynamics" Yes, despite the number of times it is shown to be incorrect, there is a recurring skeptic/denialist meme that the enhanced greenhouse effect somehow violates thermodynamics. "2nd Law of Thermodynamics ... being the basis of all modern physics " I'd quibble with that, but its not at all the topic.
  3. Re #77 muoncounter, you wrote:- "there is a recurring skeptic/denialist meme that the enhanced greenhouse effect somehow violates thermodynamics." I haven't found anything in thermodynamics (or statistical mechanics for that matter) about gases (of any sort) causing a change of temperature simply by being there. Do you have a good link?
  4. #78: "about gases (of any sort) causing a change of temperature simply by being there. Do you have a good link?" I don't know what you mean by 'simply by being there', but here's not just one, but two good links. And two more here.
  5. damorbel, your reply again completely misses the point of my comment. I was trying to refute what I thought was your claim that each individual photon carries information about the temperature of its source, and that the target of that photon can use that information to reject a photon that came from a source cooler than the target. If that was not your claim, I apologize for misunderstanding, but then I do not understand why you think the greenhouse gas effect violates the second law of thermodynamics (the topic of this post we are commenting on).
  6. damorbel, like muoncounter and Tom Dayton I'm often at a loss to figure out what you're trying to say. Just as a friendly suggestion, it might be helpful to try to be a bit more straightforward and clear in your comments. For example, this: I haven't found anything in thermodynamics (or statistical mechanics for that matter) about gases (of any sort) causing a change of temperature simply by being there. What do you mean when you say "simply by being there"? Taken literally, that's an absurd suggestion, and one that has nothing whatsoever to do with the greenhouse effect. So I'm sure you didn't mean that to be taken literally. But I have no idea what you did mean by that sentence.
  7. KR, first you say my argument is wrong because any electromagnetic radiation hitting any mater will get absorbed. When I specifically asked tor some evidence for that claim I failed so see any presented. But let me help you out. I assume you have already seen diagrams showing certain types of matter do absorb electromagnetic radiation at different rates depending on the wavelength constituting so called absorption bands. As a rule they show also bands where no absorption takes place. I assume this might constitute some evidence to back my argument. Now you come and accuse me of a logical fallacy saying I am missing necessary technical background. I am ready to accept that if you would be so kind as to explain what is missing in my argument that renders it invalid.
  8. Damorbel and h-j-m, you've had the flaws in your position explained with simple everyday examples to the contrary which even a child could understand. You've been pointed to numerous resources which disprove your ideas at various levels of technical explanation. Yet you stubbornly repeat nonsensical mis-statements of the second law of thermodynamics. Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures. Since the amount of energy released by objects increases with the amount of energy they contain this inherently means that NET energy flows always run from 'hot' to 'cold'. 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. Energy enters the Earth's climate from the Sun. Greenhouse gases delay the escape of this energy from the atmosphere just as insulation on a house delays the escape of heat generated by a furnace. This delay in energy escape means that the steady incoming energy from the Sun/furnace is supplemented by retained energy which has not escaped yet due to the greenhouse gases/insulation. Thus, the Earth/house is warmer with the greenhouse gases/insulation than without. Really. Insulation on your home DOES actually exist. It DOES allow your house to be warmed to higher temperature than it could without the insulation. This is reality... not a violation of the second law of thermodynamics. You have experienced it actually happening... so why cling to a misinterpretation of physics which says it CANNOT happen?
  9. Re #81 Ned you write:- "What do you mean when you say "simply by being there"? Taken literally, that's an absurd suggestion, and one that has nothing whatsoever to do with the greenhouse effect." Take a balloon full of O2/N2 mixture and measure its temperature at various places, top bottom etc.; add some CO2, do you expect any of the temperatures to change? Replace all the O2/N2 mixture with CO2, do you expect a temperature gradient anywhere? If you found one, would you be able to use it to drive an engine? PS1 You can heat the balloon with radiation if you wish. PS2. Temperatures must have stabilised before you do the measurements, just like in the atmosphere.
  10. Re #83 CBDunkerson you write:- "Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures" Tell me something, as the energy flows do the temperatures of the objects change? If the temperatures change (they will!), can you say how and why?
  11. damorbel, The experiment you are describing was first conducted by John Tyndall in 1859 & the results published in 1861. American Institute of Physics: The Carbon Dioxide Greenhouse Effect On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connection of Radiation, Absorption, and Conduction
    Tell me something, as the energy flows do the temperatures of the objects change?
    If there is a net difference in the energy flows, yes. If the net energy flow is zero, no.
  12. Re #86 Bibliovermis I have no problem with Tyndall's observation that H2O vapour and CO2 absorb and emit radiation and, as you write:- "If there is a net difference in the energy flows, yes. If the net energy flow is zero, no." But in #83 CBDunkerson wrote:- "Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures." You can't both be correct.
  13. Yes, both statements are correct. Every object in existence emits energy is all directions and the energy flows into all surrounding objects regardless of their temperature. When the net energy flow is zero, there is no temperature change. When there is a difference in the energy flows, the temperature changes. Did you not know about Tyndall's work before I referenced it?
  14. damorbel, when Bibliovermis wrote "net energy flow is zero," the word "net" means the difference between the incoming and outgoing energy for each object. Let's say object A has 100 units of energy coming in and 80 units going out. Object A's net energy gain/loss is a gain of 100-80=20, so object A warms.
  15. Re #88 Bibliovermis you write:- "Every object in existence emits energy is all directions and the energy flows into all surrounding objects regardless of their temperature." So the T^4 Stefan Boltzmann thermal radiation law is quite mistaken? I suggest you are on dodgy ground here! The second law of thermodynamics says heat always flows in the direction of the colder place, it doesn't matter whether it is by conduction, convection or radiation that's what the experiments (and common experience) show.
  16. Please explain how that quoted comment contradicts SB. The second law of thermodynamics says net heat always flows in the direction of the colder place.
  17. damorbel, you are being confused by the term "heat." That is a common confusion, because that term is used in multiple ways, sometimes loosely. In the case of the second law, that term means the net flow of energy. The second law says nothing about the constituent two flows of energy in the two directions, only about the final result.
  18. #84: "Take a balloon full of O2/N2 mixture and measure its temperature ... add some CO2," Those experiments are done, although usually with bottles; you can see them on youtube if you look. When you put both an 'air' bottle and a bottle with extra CO2 under a source of external radiation, the CO2 containing bottle attains a higher temperature. "Replace all the O2/N2 mixture with CO2, do you expect a temperature gradient anywhere?" That's just nonsense, as your PS2 required temperature equilibrium.
  19. Re #89 Tom Dayton, between two places there is no other kind of energy flow than net energy. You write:- "Object A's net energy gain/loss is a gain of 100-80=20, so object A warms" But you do not mention where the 'in flowing energy' comes from, I do not wish to suggest you think it comes from an object B or place B at a lower temperature, which of course would conflict with the 2nd Law
  20. damorbel, It does not matter where the energy flows in from. Energy is energy, regardless of the temperature of the emitting object. Energy flowing from cooler object B to warmer object A does not contradict the 2nd law of thermodynamics, because more energy flows from warmer object A to cooler object B. That is what "net" means - more energy flows to relatively cooler objects than flows from them. I am still wondering if you had never heard of Tyndall's work before I referenced it here.
  21. damorbel, I see that I made a mistake on this object A vs. object B point. Object A would be warming (i.e. receiving a net flow of energy) from energy flowing from a relatively warmer object, object C. It would be receiving energy from the cooler object B, but would be emitting more energy to it. Again, that is what "net" means. Do you seriously not understand the basic concept of net change?
  22. Re #96 Bibliovermis, you wrote:- "Do you seriously not understand the basic concept of net change?" I just don't recognise any other. BTW I have had a copy of Tyndall's articles for some years now and I see no fault in them, although there have been many developments in thermal physics since he wrote them. In particular the roles of kinetic theory, quantum theory and thermodynamics in thermal physics; all three of these emerged because older theories did not explain experimental observations. Kinetic theory resolved the behaviour of heat in gases and lead to more efficient heat engines; thermodynamics not only extended kinetic theory to the general problems of heat in solids and liquids but also to chemical reactions. None of these resolved the observations of radiative heat transfer, it was Max Planck who opened that door, it is best explained these days by quantum electrodynamics (QED). You will need to understand QED if you wish to get a grip of the efficiency of energy processes in lasers.
  23. #97: "'seriously not understand the basic concept of net change?'" I just don't recognise any other." Perhaps that's the problem. Here is a summary of 2nd Law statements from a class at MIT. Note the figure below, which appears under the statement: No process is possible whose sole result is the transfer of heat from a cooler to a hotter body. The key word is sole, which appears in bold in the original for good reason. The caption states for T1 less than T2 this is not possible. 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. The 2nd Law is satisfied and the greenhouse effect still works. As for the rest, you can talk QED if you like, but that will not help you answer any of CBD's excellent points.
  24. h-j-m and damorbel. You might like to explain the temperature distribution illustrated here
  25. damorbel, previously I was explaining to you merely that "net" means "difference." Now here is what the second law says for objects A and B, complementing the picture muoncounter provided. "Net" in this case means the difference in the flows between the two objects: Given: HeatFlow(from A to B) - HeatFlow(from B to A) = Net Heat Flow, If A initially was hotter than B, then Net Heat Flow has a positive sign. (A cools and B warms.) If B initially was hotter than A, then Net Heat Flow has a negative sign. (B cools and A warms.) If you want to nitpick about the word "heat," substitute "Energy." Example: A Flow To B = 100 units B Flow to A = 80 units Net Flow = 100-80 = 20 units from A to B. From A's perspective, A gets 80 units and emits 100 units, so A ends up with 80 - 100 = -20 units relative to its initial state. From B's perspective, B gets 100 units and emits 80 units, so B ends up with 100 - 80 = +20 units relative to its initial state.

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