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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 1051 to 1075 out of 1481:

  1. Fairly clear evidence that RW1 is trolling rather than debating: Me @1048:
    "The difference is 1.2 rather than 0.9, but Trenberth et al use 0.9 because: a) The difference between 1.2 and 0.9 is well within experimental error; b) The TOA balance has smaller experimental errors (+/-3% for individual components), and hence is considered more accurate than the surface balance (+/-5% for individual components except for Surface Radiation and Back Radiation which are +/-10%); and because c) If the surface was absorbing 0.3 Watts/m^2 more than was the planet (TOA) over a five year period, the excess energy would need to come from the atmosphere, plummeting atmospheric temperatures by about 24 degrees C over that period, whereas atmospheric temperatures increased over that period."
    RW1 @1050:
    "how is it that the 'NET Down" in the surface components table 2b and the TOA components table 2a is exactly the same (0.9 W/m^2?)? Are you saying that 'Net Down' means something different in each table?"
    So, in the post to which RW1 is responding I indicate that Trenberth et al use the Net Down calculated from the TOA at the surface rather than that calculated at the surface. I give sound reasons for that decision. In response RW1 accuses me of saying the Net Down means something different for the TOA and Surface tables, and suggests the identity of the values is unexplained. Either RW1 is deliberately misrepresenting the content of my (and e, and Sphaerica, and whoever else has been mad enough to try and clear up his "confusion" in this 1050 post thread) and of Trenberth et al; or he is terminally stupid; or he simply does not bother reading the responses in any event. All of RW1s confusions have been cleared up multiple times before, including by myself in the last 24 hours. If he really wants to understand, he can reread those posts and try to understand them.
    Response:

    [DB] When dealing with RW1, remember his own words:

    "I appreciate that you seem to be interested in helping me, but I'm not really interested in being helped per say. I'm a staunch skeptic of AGW, so my purpose here is to present contradictory evidence and logic that disputes the theory. That's what I'm doing."

    We deal with a closed-minded individual who is here for the sole purpose of wasting as much of as many people's time as possible.

    Solution

    Ignore him.  DNFTT.

  2. RW1@everywhere Are you trying to learn about a subject you do not understand or do you think you understand it better than anyone else?
  3. Tom Curtis (RE: 1051), "In response RW1 accuses me of saying the Net Down means something different for the TOA and Surface tables, and suggests the identity of the values is unexplained." I'm not accusing, just asking for clarification because I'm not sure I understand what you're saying. If you agree that "NET Down" means the same thing in both tables, why not just acknowledge it? I specifically asked about the data in the table of 0.9 W/m^2, not the 0.3 W/m^2 discrepancy relative to the numbers in the diagram, which I am aware of. I'll ask one more time. What does "NET Down" mean in tables 2a and 2b? If they mean the same thing, there is only one possible answer.
  4. pbjamm (RE: 1052), "Are you trying to learn about a subject you do not understand or do you think you understand it better than anyone else?" I admit I'm not here specifically to 'learn' per say, but I am fully capable of changing my mind on things when evidence dictates. I even changed my mind on something here due to evidence presented by Tom Curtis in regards to insolation in the Artic. I couldn't deny the evidence he presented to the contrary and acknowledged I was wrong. It is my view that the overwhelming majority of people at this site do not understand the information in tables and diagram from Trenberth's 2009 paper, nor do they understand the constraints COE puts on the boundary between the surface and the TOA, so I'm presenting evidence and logic in support of these things. Everyone here is free to make up their own mind, of course. If Tom does not want to continue this discussion - that's fine with me, but I can't help but to interpret his self removal as defeat. But again, everyone should make up their own mind.
  5. RW1 - "If Tom does not want to continue this discussion - that's fine with me, but I can't help but to interpret his self removal as defeat." Actually, if Tom decides he doesn't want to continue the discussion with you, I would congratulate him. You have consistently and repeatedly dismissed/ignored proven physics, cycled over and over on ideas that have been notably contradicted by actual measurements, and stated that: "...I'm not really interested in being helped per say. I'm a staunch skeptic of AGW, so my purpose here is to present contradictory evidence and logic that disputes the theory. That's what I'm doing." In my eyes, RW1, that makes you a troll, not someone actually interested in the science. Your arguments (and conclusions) are driven by your position, which is exactly backwards from how the scientific method works. And your comments on these posts illustrate that clearly to the unbiased reader - a self correcting issue. Folks, DNFTT.
  6. Tom Curtis (RE: 1048, 1050), ""The difference is 1.2 rather than 0.9, but Trenberth et al use 0.9 because: a) The difference between 1.2 and 0.9 is well within experimental error;" So what you're saying is Trenberth lists 0.9 as the "NET Down" in table 2b because it's arbitrarily within 'experimental error' of 1.2 W/m^2 and not because it means the same thing as "NET Down" in table 2a? OK, I'm perfectly willing to let this stand against what I've presented and everyone can make up their own mind.
  7. RW1,
    It is my view that the overwhelming majority of people at this site do not understand the information in tables and diagram from Trenberth's 2009 paper, nor do they understand the constraints COE puts on the boundary between the surface and the TOA, so I'm presenting evidence and logic in support of these things.
    Here's a homework assignment for you to work out entirely on your own, without assistance. This is a fairly simple assignment. I'm pretty sure just about everyone in my town middle school (6th to 8th grade) could get it right. The Trenberth energy budget has three layers: space, atmosphere, ground. It has 6 distinct paths of energy flow; space/sun to atmosphere, space/sun to ground, atmosphere to space, atmosphere to ground, ground to atmosphere, and ground to space. Please identify the components and individual and sum values for each of these elements (meaning in/out for each layer [3 pairs of values, in and out], and in/out for each interface between layers [6 pairs of values, in and out] ), identify which balance, and where you would expect the system, based on these numbers, to get out of balance. This is not a post that requires any response other than the answers. Until you arrive at these answers on your own, no one has any reason to listen or respond to you.
  8. Sphaerica (RE: 1059), I don't understand the assignment as you've laid it out. Thanks for the interest though.
  9. RW1@1054: "I admit I'm not here specifically to 'learn' per say..." And that about says it all wrt you. " If Tom does not want to continue this discussion - that's fine with me, but I can't help but to interpret his self removal as defeat." Yes, much like refusing to engage the raving derelict on the street corner is an admission of defeat and that his conspiracy is Truth.
  10. 1058, RW1,
    sun (A)<-6---1->atmosphere (B)<-5---2->surface (C)
    <-4-------------------------------------3->
    3 layers: sun/space (A), atmosphere (B), surface (C) 6 paths: sun to atmosphere (1), atmosphere to surface (2), sun directly to surface (3), surface directly to space (4), surface to atmosphere (5) and atmosphere to space (6). For each path (1, 2, 3, 4, 5, 6) identify the total energy flow, and list the contributing components and values (e.g. "thermals, 17"). For each layer (A, B, C) identify to the total in and out in each direction (up, down), as well as the total in/out for the layer, and the net (i.e. in minus out). For extra credit, identify the separate amounts of energy absorbed, reflected, and emitted by each layer. When you have worked through these numbers, and can see that everything balances and why, then you will be ready to actually start discussing any meaning behind the numbers and how they were determined.
  11. Sphaerica (RE: 1060), Even if I or anyone else could actually determine all these numbers (virtually impossible), what would be the point of any such exercise? To discover some new physical law? To discover that Conservation of Energy does not hold at the boundary between the surface and the TOA? If you're trying imply that all of these specific quantities need to be known in order to understand the contraints COE puts on the system, then I suggest you take some time to think about this a bit more. It's not that complicated.
  12. RW1 - I believe that Sphaerica is attempting to determine if you have actually understood the Trenberth diagrams. So far, it is not evident that you have. And hence (so far) your disagreements have not been particularly relevant, insofar as they have been understandable.
  13. KR (1062), I'm not saying the Trenberth diagram is entirely 'wrong' per say - it's just very misleading and has been largely misinterpreted by virtually everyone here. Let me ask you this, where in the diagram is the return path of latent heat in the form of precipitation? Surely, you agree evaporative latent heat of water and precipitation is a major surface -> atmosphere -> surface circulation current, right?
  14. 1061, RW1,
    Even if I or anyone else could actually determine all these numbers (virtually impossible)...
    It is more than possible, it's a trivial task. A twelve year old could do it. Tackle the assignment and see where it leads you. If you are not capable of doing the assignment, or choose not to, then you are clearly simply ignorant of the most basic aspects of Trenberth's diagrams, and communication with you is simply not possible. BIn taking that course, you would then abdicate any right whatsoever to offer any criticism or supposed insight on the subject. Why should anyone listen to you if you not only can't accomplish such a simple, five minute, analysis, but also think it is impossible? Gain some credibility. Perform the assigned task.
  15. 1063, RW1,
    ...where in the diagram is the return path of latent heat in the form of precipitation?
    Precipitation does not transport energy back to the surface. That only happens one way. Water vapor gains heat when evaporating at the surface, then releases that heat when condensing in the atmosphere. This moves the heat from the surface into the atmosphere, and that's all. It is clearly marked on the diagram as 80 W/m2 for "evapotranspiration" and "latent heat". There is no reverse mechanism. Do the assigned task. Until then you are complaining and criticizing without the most basic grasp of the diagram.
  16. 1063, RW1,
    it's just very misleading and has been largely misinterpreted by virtually everyone here.
    RW1, I'm sorry, but this is a laughable comment given how poorly you understand the diagram yourself. You are in no position to make such criticisms of others. Readers are asked to recognize this attitude on RW1's part, and take all of his comments with the grain of salt he has earned.
  17. Grain?
  18. The amount of energy coming into the ground is 184 watts from space(23 of which is reflected), and 333 watts from the atmosphere. The grand total of which is 517 watts. This is very close to the 516 watts being emitted from the ground through reflection (23), thermals (17), evapotranspiration (80) and surface radiation (396). The amount of energy coming into the atmosphere is 17 watts from thermals, 80 watts from evapotranspiration, 356 from surface radiation, 157 watts from space (79 of which is reflected). The grand total being 610 watts. This is very close to the 611 watts exiting through reflection (79), back radiation (333), and simply being emitted (199). The amount of energy coming into space is 102 watts through reflection (23 from the surface, 79 from the atmosphere), and 239 watts from outgoing longwave radiation (40 watts from the surface, 199 from the atmosphere). The grand total being 341 watts. This is the same amount of energy being released through incoming solar radiation. I'm 14.
  19. RW1@1063> Let me ask you this, where in the diagram is the return path of latent heat in the form of precipitation? Please read up on the basics of latent heat before commenting further. This is high school level material, until you understand it you cannot pretend to know what you're talking about. As Sphaerica described, latent heat is only released when a substance goes from vapor -> liquid or liquid -> solid. Since water vapor condenses in the atmosphere, it cannot release that same energy back to the ground as it is already a liquid. The "thermals" portion of Trenberth's diagram refers to sensible heat. In other words, it represents the conduction of heat from the warm water molecules into the cooler air molecules, resulting in the water molecules being cooled and the atmosphere heated. This will always be net positive from surface to atmosphere per the 2nd law of thermodynamics, since the atmosphere is cooler than the surface. The only mechanism by which cold falling rain can heat the warm surface is via the friction generated when the rain hits the ground and makes its way to the ocean. Is this the mechanism you are suggesting that negates energy transfer from latent heat? That would certainly be a "unique" claim.
  20. 1068, p. Curtis :D !!!! I was just going to scold you for giving RW1 the answers (don't you know that exchanging answers is academic misconduct?). Then I saw "I'm 14." Thank you. You made my day.
    Response:

    [DB] That one is precocious indeed.  I could not have done similarly at that age (but it was the mid-70s).

  21. RW1 - "Let me ask you this, where in the diagram is the return path of latent heat in the form of precipitation? Surely, you agree evaporative latent heat of water and precipitation is a major surface -> atmosphere -> surface circulation current, right?" You don't understand latent heat transport? The fact that it's one-way? I'm, well, I'm appalled. Learn some high-school physics before critiquing those who have spent their careers with this material. P. Curtis - My compliments. Folks whose ages are significant integer multiples of yours seem strangely unable to understand what you have done so clearly.
  22. Thanks for the compliments. I feel very happy that those of intelligent status think I am doing well :) Science is my greatest passion so it's always good to know I am succeeding in it.
    Response:

    [DB] Just keep working hard at it if that's what you enjoy.  The only thing separating you from anyone here is time and effort.  Good job!

  23. I don't want to revive a dead thread, but I've noticed that the responses on 2-nd law violation tend to be a bit um... dry. So I thought of an analogy that might...err resonate. Consider pushing a child in a swing. You aren't strong enough to push the swing very far in one push. But each cycle of the swing you can push a bit more and the higher the child goes. The kinetic energy of the swing can greatly exceed what you've put in in any one push. However, eventually you reach a point where the energy you put into the swing is completely dissipated on any cycle.... the swing goes no higher. This isn't obviously an exact analogy, but reason you can have larger values of back radiation and surface emitted radiation than TSI is somewhat analogous to pumping a swing. Whether this will help people caught up in inventing their own versions of the 2nd law I dont know.
  24. Will someone debunk this argument please? http://wattsupwiththat.com/2011/02/17/regarding-thermodynamics-and-heat-transfer-why-al-gore’s-comments-to-bill-o’reilly-at-fox-news-are-wrong/#more-34175
  25. Your claim that the greenhouse warming at the Earth's surface is 33K is plain wrong. This is because for an opaque [to IR] atmosphere, the -18°C equilibrium with space is in the upper atmosphere, about 5 km up. The surface temperature is then set by the rise in temperature from the lapse rate, c. 6.5K/km. So, the Earth's surface temperature is c. 33K higher than the upper atmosphere's radiative equilibrium 'temperature'**. If you take out the GHGs, the IR radiation from the Earth's surface is then not absorbed in the atmosphere, so it cools. However, your claim that the earth's surface would fall to -18°C is plain wrong because you still have the lapse rate, a consequence of gravitational potential energy. Because only a small proportion of heat is directly radiated from the Earth's surface, most is convected away and because the IR emissivity of N2/O2 is very low, that heat remains as sensible heat. The real GHG warming of the earth's surface is a bit less than 10K. If you still believe it's 33K then you have to go back to your basic education. **To calculate that you have to do a Hottel analysis. [-snipped-]

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