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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 1176 to 1200 out of 1393:

  1. @1171 muoncounter Fluke is a little rich for my budget. But my instrument reads lower than 30C. Is it accurate at that temperature? I don't know, never sent it out. Maybe all I can say is that my instrument reads at the bottom of it's scale. Still mighty cold. If you want to know what your IR thermometer sees fabricate a tube that gives a D-S ratio appropriate for your instrument. Then look through it. I will reiterate, an IR thermometer doesn't care where the photons come from or how far they travel. If you point it at the clear sky it will see the clear sky. Then it is a matter of it integrating the radiant energy in it's window. If my instrument sees a preponderance of gas at -55F or your Fluke sees a preponderance of gas at -30C it is still far colder than the air in the troposphere intervening. And it is very likely that a simple instrument like this sees a fairly narrow radiation band avoiding the CO2 and H2O absorption bands although the fact that it sees clouds suggests that the water band is included in it's range.
  2. "Generally that only happens in summer and spring when hail forms. Clouds form when convection lifts warm air from the surface and the lapse rate then lowers the temperature of the moisture in the air to below the dew point." No. Wrong, and wrong again. Lowering the temperature of the moisture? And you comment about "GH effect" being a misnomer? How do you lower the temperature of a characteristic of the air? Please. Hail happens when turbulence carries large droplets above the freezing level in cumulonimbus clouds. Above the freezing level, it becomes ice, goes back down where it gathers another layer of liquid water, and up again, where that new layer freezes and so on. In very violent clouds, this cycle can be repeated so many times as to create hailstones the size of a grapefruit, which I personally witnessed in Fort-Worth in 1995. It can also be violent enough to make these stones "pop out" of the cloud and land many miles away, over an area where the storm seems distant enough that you'd be safe, sometimes even on an area overlaid by clear skies. If one finds an intact hailstone and slices it, the layers can sometimes be visible. Clouds do not happen only when air is lifted up and cools down, they form when air temperature is brought to the dew point, whatever the mechanism, and there are many. Contrails are caused by addition of moisture, saturating air that would otherwise not see the formation of clouds because it is too dry. Over the past few nights, where I live there were clear skies and the formation of low altitude clouds, sometimes going all the way to the ground (that's called fog), caused by radiation cooling. These clouds persisted through the rest of the night, and well into the morning; they were composed of water droplets, even though temperature fell below freezing. Cloud formation by addition of moisture is also common over large bodies of water in the fall, when the water remains warm enough to evaporate in cold, dry air. The possible combinations for cloud formations are in fact endless and can be very localized. I am not keeping my instrument rating current around here because the MEAs (google it) are high enough to be above freezing during most of the months when IFR conditions prevail. As a result, I would encounter icing (most likely rime ice, you can google that too) throughout all this time and would need an aircraft approved for flight into known icing conditions, and these are usually too expensive for amateur type of operations. These icing conditions happen precisely because clouds formed of liquid water droplets exist at temperatures below freezing. Then there is also supercooled water, you can google that too. Quite an interesting phenomenon. Although it is unusual, it does happen and will give you an instant coating totalling several hundred pounds to upward of a ton, depending what you're flying. "the fact that it sees clouds suggests that the water band is included in it's range." Isn't the water band for water vapor? Can you share your source for the specific range of IR emission for amospheric liquid water? Isn't it rather that these liquid water clouds only relfect radiation, a totally different process that your instrument can not discriminate? Methinks, you're talking about things that you have no true expertise about and that G&T have you fooled with their wordplay, because that's all their paper really is. Whatever.
  3. Oh dear. Sorry DB. Bit of brain fade today.
  4. "You can argue whether it is opaque or just largely opaque. But the Box experiment proves it is not totally opaque to IR." Just exactly what is the difference between "not totally" and "largely"? Sorry but that is just poor rethoric,especially from soneone accusing others to put words in his mouth. About this: "From Table 8 on page 22 G&T come up with 45.2% of the energy arriving from the Sun as infrared. 44.8% is visible light and 10% is UV." No, that's what not what the table says at all. It says that 45.2% of "Sunlight" (from the table's caption) received is within a certain range of wave length, 44.8% in another range and 10% in yet another. Not all wave lengths carry the same amount of energy, so the total amount of energy depends on how much of the spectrum is at what frequency and how much energy that frequency carries. Higher frequency photons are more energetic. If we were receiving 50% IR and 50% UV, guess which 50% would carry the most energy? Another indication of your lack of comprehension in these matters. By the way, is that repartition in table 8 at the top of atmoshere or at the surface? Do you understand why that matters? Do you know what the repartition is at the surface? If any difference exists, what does the difference tell us? I find it suprising that you could interpret G&T's table 8 as a distribution of energy. They don't suggest that themselves, despite their remark that the frequency (wave length) repartition is often overlooked. That remark itself is misleading; what matters in consideration of an atmospheric GH effect is the amount of solar radiation that reaches the surface, of which only a small portion is IR, precisely because of the fact that the atmosphere is largely opaque to IR, so most of the solar IR does not reach the surface. Which begs the question: why is there so much IR to be measured by instruments at the surface? Surely there has to be some work done by scientists to study this, don't you think? Have you looked for it? What is out there? On another note, the instrument to measure IR radiation is a pyrgeometer, different from an IR thermometer. The only thing you are demonstrating with your measurements is that there is indeed no violation of the 2nd law in the atmosphere. Quite a different thing than measuring how much downwelling IR radiation reaches the surface and what the overall energy balance is across the entire spectrum. The question is, when you use your instrument in the way you described, what exactly are you measuring? What physical quantity is represented by these numbers? How does that measurement contradict the existence of downwelling IR radiation from the atmosphere to the surface (as measured by a pyrgeometer)? Your way of approaching science is very much reminiscent of the people on WUWT who could not understand the phase diagram of CO2 and had to partially recreate the diagram before they could finally grasp that carbonic snow was not going to happen on Earth. The fact that you find in yourself the authority to attempt to pontificate on these subjects is truly srange. Nonsense on top of confusion, endless play on words, mangled semantics used as a basis for rethorical argument, it does not get better as time goes by. The more this thread goes on, the more it reveals about D-K effect, rather than atmospheric radiative physics. G&T knew exactly what the public out there is like, so they knew what they were doing and they should be ashamed for doing so much harm. To prove what? That they could play with words? that atmospheric scientists should be more careful with their language?Sheesh.
  5. TOP - I am perfectly capable of reading what G&T said. Are you capable of seeing what is wrong with that statement?
  6. TOP @1172: I retain the numbering of topics from my post 1167: Point 1) I refer you to the global energy balance by Trenberth et al, 2009: Upward IR radiation from the surface is 390 W/m^2. Upward IR radiation from the surface at the TOA is 40 W/m^2. Ergo, approximately 90% of all IR radiation from the surface is absorbed by the atmosphere. If you had a block of quartz which only let through 10% of the light shone into it, I am sure you would say it was "largely opaque". Why you are so determined to apply a different standard to atmospheres is beyond me. Point 2) The incoming solar energy is an irrelevant point. There is clearly less energy being radiated at some bands than at others in the IR spectrum. It follows that in those other bands, they must radiate at a higher intensity than they otherwise would have, and ergo the source of the radiation must be warmer. However, as noted in the diagram above, incoming, unreflected solar radiation is 239 W/m^2. Point 4) I am not going to try an reform the linguistic conventions of an entire language just because fake skeptics attempt to use those conventions to deceive people. Furthermore, I have seen how fake skeptics treat those who do try to reform the language. They take claims saying "greenhouse effect" is an inaccurate term and misquote them as claims that there is no greenhouse effect, ie, that the surface is not warmed by the presence of CO2 in the atmosphere. Everybody can understand that rubber ducks and tin cans are no longer made out of rubber or tin (respectively) but that the name persists. Everybody (almost) can understand that the ancient Greek theory of the four humours is false, and are not confused by people saying they are in "good humour", or that somebody has a phlegmatic personality. Likewise, everybody willing to think can understand that the warming of the surface by CO2 in the atmosphere is called the "greenhouse effects" due to a historical misunderstanding, but that that is consequently its name. They further understand that people trying to argue against a scientifically demonstrated physical effect based on coincidences of linguistic accident are intent on deceiving. Are you in that latter category? And if not, why are you making this an issue?
  7. So we're supposed to be willing to overthrow a well-known scientific principle on the basis of one hotly-disputed paper and a backyard experiment. If valid, from such discoveries come Nobel Prizes. The details of that experiment: - A measuring device costing less than $82 (current price for a Fluke 62). - "Is it accurate at that temperature? I don't know, never sent it out." - "it is very likely that a simple instrument like this sees a fairly narrow radiation band avoiding the CO2 and H2O absorption bands" -- my IR thermometer has a quoted spectral response of 6.5 to 18 microns, which the figure below shows is not 'narrow band.' - the quoted range of such sensors is 6 feet or less -- works fine for checking AC/heating duct air temp. But if this is a credible experiment, tickets to Stockholm are in order. However, I wonder why NASA goes to all this trouble and expense designing and calibrating real narrow band (centered around 10.8 and 12 microns) IR sensors for satellites. Why not just put up a few hundred dollars worth of retail models? Oh, I forgot, they just do all this to boost their funding. Right.
  8. I'll add to what TC and Muon just said and make one bold statement: the measurements you obtain, considering how your instrument works, are entirely consistent with everything known of atmospheric radiative physics. They do not contradict the GH effect at all. In fact, knowing all the applicable local conditions at the time of measurement, they could be predicted from the physics. I'll leave it to you figure out why and how. You write here with the pretention to demonstrate that current understanding of atmospheric radiative physics is deeply flawed, there is then no doubt that you have the abilities to do that work. Then, you can explain exactly where the flaws are in the process used for the prediction.
  9. TOP - I would like to point out that every 'objection' you have raised has been discussed ad nauseum, and shown incorrect, in the previous thousand comments. G&T is a horrible paper, incredibly flawed, and the various "2nd Law of Thermodyamics" objections to the radiative greenhouse effect are simply not valid. At this point I consider the very fact that someone raises such objections to be a clear indicator that the proponent (a) lacks a sufficient education in physics, and (b) will grab onto anything that might even plausibly provide an objection to the science, regardless of validity. It's not (IMO) a promising sign. Please - read the Opening Post (OP), read through the thread a bit, go look at examinations of this topic such as the excellent work at Science of Doom (who has multiple threads on this topic). I think you might find a deeper understanding of this topic worthwhile.
  10. KR: Having followed the multiple, lengthy exchanges between TOP and you and other SkS authors, I am firmly convinced that TOP's sole purpose is to litter this comment thread with excerpts from the G&T paper. In my opinion, this behavior ought to be against SkS Comment Policy. If it were my call, I would ban TOP from posting on SkS and delete all of his comments.
  11. John Hartz @1185, I do not think merely presenting a view, however flawed, should be grounds for banning or deletion of posts. Continuous and repetitive presentation of the same point again and again should be grounds for deletion of further repetitions as of topic, but I do not believe TOP has reached that point, yet.
  12. @1171 Moderator I viewed the BBC experiment. Thanks for the notes. Among other problems: 1. Lights repositioned between 1:14 and 1:18 so that the CO2 light is more direct. 2. No control over the positioning of the lights. 3. The left bottle had .04% by volume CO2 while the right bottle probably had 90% by volume CO2 proving that CO2 absorbs more IR than air if the experiment actually represented equal impingement of IR on the bottles. All this would prove is that CO2 absorbs IR which nobody is disputing or perhaps that it takes an almost pure CO2 atmosphere to raise the temperature a few degrees. 4. The right bottle had an object behind it that may have reflected energy back into the bottle. 5. There was no control, no Design of Experiment and no statistically relevant reduction of data. This was just a snake oil presentation. This experiment simply proves, if anything, that CO2 absorbs IR, it has nothing to do with explaining why greenhouses warm in the sun. (-snip-).

    [DB] Moderation complaints snipped.

  13. @1186 Tom Curtis Thank you Tom. I am trying real hard to make a reasoned point in a tough forum.
  14. TOP@1187 "There was no control, no Design of Experiment and no statistically relevant reduction of data. This was just a snake oil presentation." Are you serious? This was not a graduate thesis, this was a 6th grade level science demonstration. If you really want to *prove* that it is fabricated nonsense perform the experiment yourself. It should not take more than an hour start to finish and you probably have everything you need in our kitchen. Please post a youtube link when you are done. I eagerly await your results.
  15. @1185 John Hartz This is a discussion about G&T. Their paper is long and I saw little specific discussion of the points that they made that could be traced to specific parts of that paper. So I quote it. Frankly I don't think that a lot of the discussion on this whole thread has attempted to address G&T. And even when quoting G&T the discussion seemed to diverge from the topic of the post rather rapidly. I will say that I haven't found much of anything in the respondents to my comments here that convince me of the error of G&T's ways. I learn a lot but I see a lot of regurgitation of information found in other papers and books. Most of it just comes down on me for word choice or "rhetoric". Guess I'll have to take remedial English or writing. But then this is a blog and I would expect to be cut some slack on form and style like I cut slack for others. And if banning me is the only way to win the argument that speaks loads for the argument. G&T have made some headway. In 1184 KR uses the term "radiative greenhouse effect" which to me is acceptable in place of the "convective greenhouse effect" which is what happens in real greenhouses. And I will note that G&T wanted to use the term "atmosphere effect" in place of "greenhouse effect" when talking about warming the atmosphere by radiation.
  16. 1189 pbjamm Read my response. I don't have a problem with the science. CO2 does absorb IR. Look at muon's graphs and many others posted here. Those are actual measurements taken with expensive instruments. I do have a problem with calling it proof of the "radiative greenhouse effect" as applied to real greenhouses when real greenhouses don't typically have a 90% CO2 atmosphere. Perhaps that experiment should be called the "absorptive greenhouse effect". Mr. Wizard did a much better job of controlling his 6th grade experiments in the '60s.
  17. TOP@1191: I do have a problem with calling it proof of the "radiative greenhouse effect" as applied to real greenhouses when real greenhouses don't typically have a 90% CO2 atmosphere ======= Is this a purely semantic argument that 'greenhouse effect' is an inaccurate description? At this point I am not clear what you are arguing for or against.
  18. TOP - as has been said before, great hunks of G&T are telling us what everyone knows full well but possibly they and definitely you, seem to think contradicts how we understand atmospheric physics to work. The skeptic point to take down in their paper was their idea that 2nd law is being violated. It is not. Muon has pointed you at the spectral observations from both top and looking up. Both agree with model calculations which could not be valid if G&T were right. There is no observational evidence to contradict the GHE and a very great deal to validate it. Now, have you figured out what was wrong with the G&T statement I quoted earlier?
  19. Wow, almost 1200 comments in and we have consensus: It shouldn't be called the greenhouse effect because 'real greenhouses don't work that way.' In what way do these pedantics change the physical science involved - or the outcome? TOP#1187: Very insightful critiques. Now do the same with your 'I measured the temp of the stratosphere with my handheld IR thermometer experiment.'
  20. @1192 pbjamm It isn't semantics. The good BBC professor purported to demonstrate the "radiative greenhouse effect" by comparing temperatures in a) a bottle filled with air (0.04% CO2) and b) a bottle filled with CO2. The experiment did not demonstrate what was purported even a little. Nor did it invalidate Wood's experiment in 1909. You have read G&T haven't you? They have a big problem with the term "greenhouse effect", if for no other reason than it is not well defined, at least for a mathematical physicist's purposes and it is not an "effect". There are 14 subsections in their paper that find flaws in published definitions of the term in, I would hope, respected literature on AGW. G&T are arguing the "semantics" because without defining terms, what exactly is being discussed? What I say is of little consequence. And just a note of style, if you quote, put it in quotes or indent. As everyone knows, I am easily confused.
  21. TOP - we have no problem with idea that greenhouse effect is badly named. Its just not news. This in no way invalidates the fact the atmospheric greenhouse gases warm the planets. That IS the only point of substance for climate. SoD assumed G&T just skipped 100 years of literature; I think it more likely that they were trying to sow seeds of doubt about GHE with that long preamble. Looks like it worked. Are you arguing that IPCC science doesnt understand GHE. (eg as described here). Or that Ramanathan and Coakley 1978 (the basis for current calculations) have got it wrong?
  22. TOP, You still have not answered the question "How can an IR-opaque atmosphere possibly lead to global cooling?" I only ask again (for the third time) because I think that this may be at the heart of at least some of your problems. If you believe this, then there is something seriously wrong with your understanding of the physics involved. Please explain yourself.
  23. @1163, 1193 scadenp Well, I thought I did in 1174, but Philippe in 1179 pointed out that I incorrectly used the term energy for "Sunlight". It just all depends on what Blambda means when integrated as in equation 30 on page 22. It could just be power (energy per unit time) integrated over a portion of the spectrum. [I am being a bit imprecise here. You can look up Blambda elsewhere.] That table in G&T is referring to the black body radiation of the sun before it passes through the atmosphere. So of course the gases in the atmosphere are going to take their "cut" of the radiant energy on the way down. Wood demonstrated, however, that a significant and measurable portion of IR does in fact reach the ground. But muon's charts support this finding. Just out of pure curiosity, how does RGHE deal with the warming of CO2 from the sunlight? And why does muon's graph show such a low temperature in the CO2 notch looking down from above if there is so much radiant energy available to heat it? I have a thought on that but I am wondering what your opinion is.
  24. @1197 Sphaerica I think I just answered part of that in 1198. In the Wood (1909) experiment, there was a small discrepancy in how fast the two boxes heated. The box with the IR transparent cover heated faster. So I jumped to the conclusion that if that was so, and if placing an IR opaque filter in front of the box caused the temperature rise to drop, then the same would be true if the atmosphere were IR opaque (which it isn't, see muon's charts.)
  25. I genuinely have no idea what any of this has to do with violations of the 2nd Law of Thermodynamics.

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