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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 1026 to 1050 out of 1481:

  1. Tom Curtis (RE: 1025), "For RW1 convenience (and for the umpteenth time) there is no guarantee that energy absorbed by the atmosphere will make its way to the surface, and as most of it is absorbed in the stratosphere, most of it doesn't. In fact, most of it is radiated to space." Not according to the text of the paper (look on the 6th page where they give absorbed solar radiation ASR and NET down data). What you don't realize is the amount absorbed by the atmosphere that is radiated back out to space is included in the albedo of 102 W/m^2. This is why when you look at the amount of outgoing LW from satellites it tends to be about 250 W/m^2 instead of 240 W/m^2. 341 W/m^2 - 250 W/m^2 = 91 W/m^2 not the 102 W/m^2 albedo referenced. The difference of about 10 W/m^2 is the LW emitted back up out to space as part of the albedo. All the energy at and below the surface came from the Sun (excluding an infinitesimal amount from geothermal). In the steady-state, conservation of energy dictates that 100% of the post albedo - in this case 239 W/m^2, gets to the surface one way or another.
  2. Tom Curtis (RE: 1025), "Most importantly, the average 333 Watts/m^2 is not only that which has been calculated using Line by Line and Atmosphere-Ocean Global Circulation Models, it is the back radiation that has actually been observed. Any theory that does not predict it, in other words, is falsified by observation. Of course only greenhouse theories predict that back radiation, or at least they are the only ones that do so without violating the laws of thermodynamics. So and denier of green house theories is left to explain how there can be an average 333 Watts/m^2 back radiation given a 240 Watts/m^2 input energy from the sun, and without the absorption and reradiation of energy by green house gasses." I do not dispute there is downward emitted LW from the atmosphere significantly above the 157 W/m^2 required for the net flux of 396 W/m^2 at the surface (396-239 = 157), though obtaining an accurate global average is impossible without measuring equipment looking up all over the globe. That aside, what you don't seem to understand is that the downward emitted LW from the atmosphere has 3 potential sources. Some if it last originated from surface emitted radiation, some of it last originated from the Sun (yet to reach the surface) and some of it last originated from the kinetic energy (latent heat and thermals) moved from the surface into the atmosphere. The bottom line is that the surface cannot be receiving a net energy flux above 396 W/m^2 in the steady-state. All the energy entering and leaving at the TOA is radiative. Any net energy loss from the surface to the atmosphere from thermals (convection), for example, just offsets the amount of energy that would otherwise be need to be radiated from the surface.
  3. RW1> "Not according to the text of the paper (look on the 6th page where they give absorbed solar radiation ASR and NET down data)." If you are referring to Table 1a, those are TOA measurements, they not distinguish between energy absorbed in the atmosphere and energy absorbed at the surface. It in now way implies that energy absorbed in the atmosphere must make its way to the surface. If you are looking for measurements at the surface specifically, then you should be looking at Table 1b. "Solar down" to the surface is shown to be about 160, exactly as depicted in the diagram. In the steady-state, conservation of energy dictates that 100% of the post albedo - in this case 239 W/m^2, gets to the surface one way or another. No, it dictates that it gets either to the atmosphere or the surface.
  4. Tom Curtis (RE: 1025), Actually, the ASR and NET Down data is on the 8th page of the paper (not the 6th).
  5. RW1 "Actually, the ASR and NET Down data is on the 8th page of the paper (not the 6th). " Both pages show these values, they are for different time periods. Same thing I pointed out earlier goes for tables 2a and 2b, 2a shows TOA measurements, it does not distinguish between surface and atmosphere so it is irrelevant to your claim.
  6. e (RE: 1028), Look at the Global data in table 2a. for the row entitled "this paper". ASR is 239.4 W/m^2, OLR is 238.5 W/m^2, NET Down is 0.9 W/m^2. Are you seriously claiming that this means that of the 239.4 W/m^2 absorbed, only 0.9 W/m^2 gets to the surface and the remainder is radiated out to space without ever reaching the surface?
  7. e (RE: 1028), Furthermore, if you look at the surface components in table 2b, you see 161.2 W/m^2 of "Net Solar" and 78.2 W/m^2 of "Solar absorbed". Is it just a coincidence that 161.2 + 78.2 equals 239.4 W/m^ and this is exactly the same as the ASR at the TOA?
  8. RW1@1031 You are not understanding, please read carefully: TOA measurements do not distinguish between the surface of the earth and the atmosphere. They treat the entire surface/atmosphere system as a black box emitting and absorbing energy. The 239.4 W/m^2 could be absorbed by the surface or it could be absorbed by the atmosphere. The table you are looking at does not tell you how much is absorbed by each. For that you need to take a look at table 1b or 2b which treat the surface separately from the atmosphere. It shows that only about 160 W/m^2 of solar energy is absorbed by the surface. The 0.9 W/m^2 is just the difference between the energy entering the surface/atmosphere system and the amount of energy leaving. Again, it says nothing about where within the earth the energy goes.
  9. e (RE: 1028), What do you think a NET Down of 0.9 W/m^2 means? It's showing a positive energy imbalance at the surface of 0.9 W/m^2 - meaning more energy is entering the surface from the Sun than is leaving at the TOA as OLR (239.4 - 238.5 = 0.9 W/m^2). It's quite apparent to me that few people here actually understand the data in that paper and the constraints Conservation of Energy puts on the boundary between the surface and the TOA. Part of the problem is the diagram itself, which is only loosely connected to the text and details presented in the paper.
  10. e, You do know that the atmosphere cannot create any energy of its own, right? If, of the 396 W/m^2 emitted at the surface, 70 goes straight to space, then 326 W/m^2 is the amount absorbed by the atmosphere (396 - 70 = 326). Are you saying that this energy never leaves because there is 333 W/m^2 of 'back radiation' from the atmosphere? Where does the difference of 7 W/m^2 go? Where is the 169 W/m^2 emitted to space from the atmosphere coming from then?
  11. e, How can the surface be receiving 161 W/m^2 from the Sun and 333 W/m^2 of 'back radiation' from the atmosphere when it's only emitting 396 W/m^2? The surface cannot be receiving more than a net flux of 396 W/m^2 unless it is warming, but we are referring to the system in the steady-state (or at least an imbalance less than 1 W/m^2).
  12. RW1, I've explained this to you before. You are ignoring the 80 W/m2 from evapotranspiration and 17 W/m2 from thermals, or 97 W/m2 more. 396 W/m2 + 97 W/m2 = 493 out. 161 W/m2 + 333 W/m2 = (surprise) 494 in. They balance. Minus, of course, the net 0.9 which is being absorbed by the planet and thus increasing its temperature. You can't just ignore the thermals and evapotranspiration/latent heat because they are not in the form of radiation. They still represent energy transfer.
  13. e, Table 2b in the paper does not define 'back radiation' as the downward emitted LW from the atmosphere that last originated from surface emitted. It just defines it as "LW downward radiation to the surface". The problem is as I said in post #1027, not all of this is 'back radiation' as defined as that which last originated from surface emitted radiation. Some of it is LW 'forward radiation' from the Sun that has yet to reach the surface. What Trenberth does in the diagram is lump the 78 W/m^2 absorbed by the atmosphere from the Sun and the 97 W/m^2 of latent heat and thermals all in the same return path of 333 W/m^2 designated as 'back radiation'. This is highly misleading and why everyone is so confused. 157 W/m^2 from surface emitted (396 - 239 = 157) + 78 W/m^2 from the Sun designated as "absorbed by the atmosphere" + 97 from latent heat and thermals = 332 W/m^2 all lumped in the return path as 'back radiation'. Trenberth has an extra watt in there for at total of 333 W/m^2 to account for the NET Down of 0.9 W/m^2.
  14. RW1, You are tying yourself in knots making this more difficult than it needs to be. Before we move on, please answer the question posed by this simple analogy: Suppose I have two bank accounts. Now suppose you pay me $240, and I in turn spend $239 dollars, leaving $1 in bank account #2. From this information alone, can you tell me what the deposit amounts will be for each bank account? (Assumptions: I cannot create or destroy money, and nobody is paying me except you.).
  15. RW1, as e has pointed out, the correct values are listed in the paper on tables 2a and 2b under "this paper" for the TOA and surface respectively, with the solar radiation absorbed in the atmosphere listed with the surface values for convenience. Your apparent inability to read the paper or distinguish between TOA and surface values is not a problem with the paper. If you read my 1025 (sections (3b) and (4) I clearly do include atmospheric absorbed solar radiation, thermals and evapo/transpiration as sources of energy which is later reradiated to the surface as back radiation. Further, I included them as specific terms in the slab atmosphere model I mention in my section (4). Your insistence on attributing to me a view that would involve non-conservation of energy despite the evidence to the contrary is again your problem, not mine. Frankly your claim that, "the surface cannot be receiving a net energy flux above 396 W/m^2 in the steady-state" makes no sense. In the steady state (no change in the energy stored in the climate system), the net surface energy flux must be zero (ie, energy in - energy out = zero). Trenberth et al are claiming the climate system is not in a steady state. If your claim is about total energy flux, the downward energy flux at the surface by best estimate (excluding reflected solar, which self cancels) is 494 Watts/m^2 which almost exactly matches the net upward flux (excluding reflected solar) of 493 Watts/m^2. Your 1031 is an even more bizzare misunderstanding. Frankly your style of analysis seems to consist of taking a figure at random from what somebody writes and simply asserting a random falsehood about it, then attributing that falsehood to your opponent. I do not have the time to continuously rebut such inane ramblings. Nor should I need to as it is an obvious trolling strategy. I will not feed the troll, but request that the moderators also no longer permit you to troll this site.
  16. Sphaerica (RE: 1037), "I've explained this to you before. You are ignoring the 80 W/m2 from evapotranspiration and 17 W/m2 from thermals, or 97 W/m2 more. 396 W/m2 + 97 W/m2 = 493 out. 161 W/m2 + 333 W/m2 = (surprise) 494 in." I have not ignored the 97 W/m^2 from latent heat and thermals. It's a net zero flux at the surface. The diagram has 97 W/m^2 leaving the surface and 97 W/m^2 coming back as part of the 333 W/m^2 designated as 'back radiation' as explain in my post # 1038. Subtract 97 from 493 and you get a net flux of 396 W/m^2 - the amount emitted at the surface.
  17. RW1, Your post @1041 illustrates continued misunderstanding about how these numbers should add up. It is really much much simpler than you are making it. Since physical explanations are failing to make this clear to you, I think it might help if you thought of the diagram as illustrating the gross flow of money between three accounts: sun, atmosphere, and surface. You can start by answering my question @1039.
  18. Tom Curtis (RE: 1040), "RW1, as e has pointed out, the correct values are listed in the paper on tables 2a and 2b under "this paper" for the TOA and surface respectively, with the solar radiation absorbed in the atmosphere listed with the surface values for convenience. Your apparent inability to read the paper or distinguish between TOA and surface values is not a problem with the paper." From table 2b "Surface components of the annual mean energy budget for the globe", show me how numbers from the row of "this paper" yield a 'NET Down' of 0.9 W/m^2?
  19. Tom Curtis (RE: 1040) Also, is it just a coincidence the 'NET Down" in the surface components table 2b and the TOA components table 2a is exactly the same (0.9 W/m^2?).
  20. RW1>show me how numbers from the row of "this paper" yield a 'NET Down' of 0.9 W/m^2? Seriously, think of energy absorbed at the surface as "gross income", and energy emitted or transferred via latent heat as "gross expenditures", and the answer to this will be obvious. The first column applies to the atmosphere not to the surface and is simply shown for reference, so we won't add that in. We will also ignore "Solar reflected" as that is neither absorbed nor emitted. Our "gross income" comes from "Net solar", and "Back radiation". Our "gross expenditures" come from "LH evaporation", "SH", and "Radiation up". From here the math is easy: "net income" = "gross income" - "gross expenditures" = ("Net solar" + "Back radiation) - ("LH evaporation" + "SH" + "Radiation up") = (161.2 + 333) - (80.0 + 17 + 396) = 1.2. The .3 difference comes from measurement uncertainty. Also note that the "Net LW" field is just "Back radiation" - "Radiation up" which we already accounted for in the equation.
  21. RW1> is it just a coincidence the 'NET Down" in the surface components table 2b and the TOA components table 2a is exactly the same (0.9 W/m^2?). No it is not, but consider my question @1039 to see why this does not support the claim you are making.
  22. e (RE: 1039) "You are tying yourself in knots making this more difficult than it needs to be. Before we move on, please answer the question posed by this simple analogy: Suppose I have two bank accounts. Now suppose you pay me $240, and I in turn spend $239 dollars, leaving $1 in bank account #2. From this information alone, can you tell me what the deposit amounts will be for each bank account? (Assumptions: I cannot create or destroy money, and nobody is paying me except you.)." I know the diagram is depicting an energy imbalance of about 1 W/m^2. This is not related to the issues of COE in the diagram that I am addressing.
  23. RW1 @1043, for casual readers who may be confused by RW1's trolling: The columns of the table (and values in brackets) are: "Solar Absorbed" which is solar energy absorbed in the atmosphere, and hence not part of the surface balance (78.2); "Net Solar" which is the solar energy absorbed by the surface (161.2); "Solar Reflected" which is the solar energy reflected at the surface (23.1); "LH evaporation" which is the latent heat carried into the atmoshere by by evaporation or transpiration (80); "SH" or sensible heat, which is given as Thermals in the diagram (17); "Radiation Up" which is the Long Wave radiation from the surface, or Surface Radiation (396); "Back Radiation" which is, unsurprisingly, the Back Radiation (333); "Net LW Radiation" which is the Radiation Up - Back Radiation (63); "Net down" which is the total increase in energy at the surface per second per square meter (0.9) The casual reader should now be able to match these values to the surface components of the diagram in post 1019. They will therefore recognise that I have already met RW1's challenge in section (3b) of post 1025. But seeing as how RW1 presents himself as struggling with simple arithmetic and reading comprehension, the balance on the table is: Net Solar = 161.2 =~= 17 + 80 + 63 = SH + LH evaporation + Net LW. 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.
  24. Also, here again is the Trenberth paper we keep referring to.
  25. Tom Curtis (RE: 1048), ""Solar Absorbed" which is solar energy absorbed in the atmosphere, and hence not part of the surface balance (78.2)" Not directly, no. It gets there indirectly, as I explained in #1038. Why even include it in the table? If the 78.2 W/m^2 does not get to the surface as you claim, 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? Is it a coincidence that 161.2 + 78.2 = 239.4 W/m^2 and this is exactly the same as the ASR in table 2a? All I'm saying is that 239.4 W/m^2 from the Sun has to get to the surface one way or another if energy is to be conserved. Maybe you agree with this and we are just talking past each other, but it doesn't sound like it to me.

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