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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 401 to 425 out of 727:

  1. damorbel - My post on basic energy exchanges just demonstrated the principle of energy conservation, which your interpretation of the 2nd law of thermodynamics breaks. The more detailed discussion here simply extends that to the sun/Earth/atmosphere/space case. Gas compression is a red herring; irrelevant to the discussion. Temperatures and energy flows are deeply interrelated - more energy flows out of an object (or gas, or liquid) when it is warmer, less when it's cooler. Climate temperatures are the response and feedback to the total energy flows - net flows (summing all directions) as well as individual flows, such as the sun and atmosphere warming the surface. --- I did a bit of research, damorbel, and you have been pushing these incorrect ideas on the 2nd law of thermodynamics for at least 3 years. Given the number of people who have pointed out your errors without your understanding, I suspect you won't get the idea this time either. I would love to be proven incorrect - but educating you on this topic appears to be a Sisyphean task.
  2. Re #401 KR you wrote:- "My post on basic energy exchanges just demonstrated the principle of energy conservation, which your interpretation of the 2nd law of thermodynamics breaks." 1st Law of thermodynamics "Energy can be neither created nor destroyed. It can only change forms." 2nd Law of themodynamics The second law states that spontaneous natural processes increase entropy overall, or in another formulation that heat can spontaneously flow only from a higher-temperature region to a lower-temperature region, but not the other way around. In this case I agree with the Wiki article (apart from 'heat flowing') on 1st and 2nd laws of thermodynamics. PS you have noticed that I contribute to Wiki. But please be aware I personally am not the subject of this thread and I regard such comments as a waste of time and effort.
  3. 397 damorbel: "Would you like to reccommend some passage?" I'm not sure I understand the question nor, indeed, that I need to. Just though some people (clearly you are not excluded) might like some sources for clearing up some of these basic ideas - let alone the more complex ones! Looks like a good list of texts and comments on that. Hoping some knowledge flows in and some errors flow out :) 401 KR - see 367!
  4. damorbel - The issue with the 1st law of thermodynamics and your formulation is that you are ignoring the energy contribution of cooler objects (such as the atmosphere) to warmer objects (the surface), which increases the total energy in the surface and requires a higher temperature to radiate that energy away. Note that as long as the summed energy goes from warmer to cooler, which is true here, the 2nd law of thermodynamics is intact as well. Your claim that 'cooler objects cannot warm warmer objects' ignores that energy contribution, and hence breaks the 1st law - the energy from the cooler object doesn't just vanish. That means your claim is incorrect. My last comment is upon your intransigent position - you have received a great deal of input on this issue over the last 400 comments here and (looking around a bit) from numerous others over several years. Yet you still seem to think the radiative greenhouse effect violates physics.
  5. Dr Roy Spencer, contrarian and topic of this recent thread, posted an item on his blog last summer titled "Yes, Virginia, Cooler Objects Can Make Warmer Objects Even Warmer Still" and I thought it might be useful for both damorbel and KR et al. For damorbel it shows a contrarian showing "well, I’m going to go ahead and say it: THE PRESENCE OF COOLER OBJECTS CAN, AND DO, CAUSE WARMER OBJECTS TO GET EVEN HOTTER (sic)". For KR, and the rest, I thought it might be fun to read, in the comments, Dr Spencer trying to do your 'job' but without any reinforcements.
    Response: [DB] Please refrain from using all-caps. Thanks!
  6. MichaelM - I pointed damorbel to that article here, several months ago. His reply? "He's wrong." Hence my comments about intransigence.
  7. My apologies, I first directed damorbel that article in November, if not earlier.
  8. KR@406 The "he's wrong" post also shows damorbel doesn't understand that the surface is heated directly by the sun, so the "insulation" explanation is perfectly reasonable.
  9. Dikran - I attempted to address that particular misconception here with spectra, but was ignored.
  10. Re #404 KR you wrote:- "Your claim that 'cooler objects cannot warm warmer objects' ignores that energy contribution, and hence breaks the 1st law - the energy from the cooler object doesn't just vanish. That means your claim is incorrect." So are you saying that, if two equal blocks of metal, No1 at 300K and No2 at 320K were put in thermal contact, No2 would be >320K ?
  11. damorbel - Dynamic equilibrium, with energy flowing through the system, not static equilibrium; you have the wrong system in your example. Take instead a block of metal, heated on one side with 1KW of power, sitting on the other side on a huge block of ice. It will reach some dynamic equilibrium temperature, say 100 degrees. Now put a piece of wood between the block and the ice. The wood will reach a temperature between that of the block and the ice (and in fact will have an internal gradient), but the block (because of the slowed energy loss to the ice) will reach a temperature considerably above 100 degrees. A cooler object (wood) has warmed the warmer object (block) by reducing the energy lost, as that loss is only via the energy difference at the block/wood interface - much smaller than a direct block/ice interface. It has reduced energy loss by its presence, and hence warmed the block. Now substitute sun->1KW heater, Earth surface->metal block, GHG atmosphere->piece of wood, and space at 3K->huge chunk of ice.
  12. To expand on that - the final temperature the block reaches will be that temperature where 1KW of heat is passing through the wood to the ice. That's when the incoming/outgoing energies balance. The wood (by conduction) will pass some heat to the block, the block (by conduction) will pass a great deal more to the wood, 2nd law duly observed. The final substitution in my example is radiation for conduction.
    Response: [Dikran Marsupial] Added a "be"
  13. damorbel, Two questions. 1) Does an object have to be at a specific temperature in order to emit energy? 2) Is an object receiving energy selective to receiving energy only from objects warmer than them?
  14. "I am most interested in what you write but two lines is just a bit too little to give me a proper grasp of your point." An experiment is proposed. You use your understanding of thermodynamics to calculate a result. Result is also calculated by textbook thermodynamics. Results are compared to what actually is observed. If your method fails, then do you concede that your understanding is flawed?
  15. Re #411 you wrote:- "Take instead a block of metal, heated on one side with 1KW of power, sitting on the other side on a huge block of ice. It will reach some dynamic equilibrium temperature, say 100 degrees." But your system has a 1kW heat source on one side (of a block) and you add some insulator (the wood) to the other side. Of course the temperature will rise. The Sun/Earth arrangement has the heat source (the Sun) outside the Earth/atmosphere system. Although your model is set up with the Earth as a heat source this is not the case. My best model is the blanket. A blanket keeps you warm because it stops your body heat escaping, as a result you are warmer than the bedroom. If you die, your body heat stops and your body (now a corpse) cools down to room temperature. If the room temperature increases (because the Sun is making the room hot) the corpse under the blanket will get warmer too because it follows the room temperature. There will be a little delay in the change of the corpse's temperature because of the insulating effect of the blanket and the thermal inertia of the corpse but after a while thermal equilibrium will be restored. It is the same with the Sun/Earth system. The Sun streams out photons with a mean temperature of 5780K. But, because of the inverse square law, the density of (5780K) photons drops with distance (photons do not lose energy with distance - just the number/m^2 changes with distance), so the (average) temperature at a planet is dependent only on the distance from the Sun. Even if the planet reflects most of the Sun's photons (i.e. it has a very high albedo) that will only slow down the rate of heating by the Sun, the planet will just get to its final temperature more slowly (than a black body planet). PS the high albedo slows down the rate of cooling also. PPS the albedo works like a blanket, and just like MFI (Multilayer Foil Insulation).
  16. damorbel@415 wrote: "The Sun/Earth arrangement has the heat source (the Sun) outside the Earth/atmosphere system." what you don't appear to realise is that the atmosphere is largely transparent to the suns visible and ultraviolet radiation, which directly warms the Earths surface not the atmosphere. The atmosphere is warm not because it absorbs a lot of IR radiation from the sun, but from the IR radiated by the surface that has been heated by absorbing SW radiation from the sun and by conduction/convection. Thus the atmosphere is acting as an insulator, insulating the warm surface from the cold of space. This has been pointed out to you at least twice on this thread.
  17. damorbel - The sun is directly analogous to the 1KW heater - as I posted here the solar spectra passes through the atmosphere to the Earth, affected primarily by Raleigh scattering (not GHG's), and warms the surface. The analogy is completely correct, the energy flow is from the Sun to the surface and out to space through the atmosphere. A small amount of sunlight heats the atmosphere directly (your block heater touches part of the piece of wood in the analogy); that changes only in detail, not in essentials. And the atmosphere is a radiative insulator between the Earth and space. The surface of the Earth has an emissivity of ~.97 to .98 in IR, while the effective emissivity of the Earth and atmosphere to space is ~0.612; the insulation. And that insulation makes the planet warmer than it would be without the greenhouse gas atmosphere.
  18. Re #413 you wrote:- "1) Does an object have to be at a specific temperature in order to emit energy?" I am not being pedantic here! Objects do not emit energy, they emit radiation. The radiation they emit depends on the temperature of the body. Energy may or may not be transferred to other bodies even deep space; dependent on their temperature; energy may be transferred to the body in the paragraph above, again dependent on the temperature of bodies in range (deep space included) it all depends on relative temperature. You wrote:- "2)Is an object receiving energy selective to receiving energy only from objects warmer than them?" Does the first answer work here also? To sumarise: all bodies above 0K emit radiation; all bodies absorb radiation regardless of temperature; energy transfer takes place in the direction high temperature to low temperature - always!
  19. Re #416 KR you wrote:- "And the atmosphere is a radiative insulator between the Earth and space." Yes KR that is true. But the atmosphere is also between the Sun and the Earth, just like a blanket with a corpse underneath it is between the (ambient or Sun) heat source. If the Earth was itself a heat source then putting a (partially) reflecting layer round it would, like for a living body, keep the heat in and the temperature would rise. But the same (partially) reflecting layer would keep some of the Sun's radiation out. The temperature of the Earth is thus not changed by the albedo, just the rate of heating and cooling.
  20. damorbel - The vast majority of solar energy (shortwave) passes right through the atmosphere and warms the Earth. As Dikran Marsupial and I have both said, the atmosphere is warmed by the Earth, and hence the heater/block/wood/ice analogy holds, not your warming of a room through a blanket. The atmosphere is basically transparent to SW radiation, emitted by the sun based on it's temperature. The IR radiation emitted by the Earth, on the other hand, is almost completely blocked by the atmosphere. The atmosphere does not block sunlight from the surface, and for the purpose of discussion the sunlight could be coming from underground to warm the surface. To take it back to the analogy - the heater wires could run though the piece of wood, but they don't interact with it. Energy flows from Sun/surface/atmosphere/space, in that order.
  21. #418, Electromagnetic radiation is a form of energy. Trying to dance around words does not help your argument. Here's my point. We agree that all bodies emit and absorb radiation. If a source of a specific temperature emits radiation, what is there to prevent another source of higher temperature to absorb that radiation?
  22. damorbel - We've demonstrated how the atmosphere acts like insulation, to which you've completely agreed except for your objection about sunlight going through the atmosphere. We've then shown you how that isn't an issue with SW radiation, and that the heat goes from sun -> Earth -> atmosphere -> space. In other words, you have agreed that the atmosphere acts as insulation, raising the temperature of the Earth. Not to mention there have been multiple demonstrations the greenhouse effect via line by line integrations, energy balance models, energy conservation, and basic radiative and spectral physics. This includes a couple of simple models you have implemented yourself based on the text here. Since you've agreed with every step of the energy flow discussion here - are you still objecting to the greenhouse effect?
  23. Re 421 RickG you wrote:- "Electromagnetic radiation is a form of energy. Trying to dance around words does not help your argument." Rick, the 2nd law of themodynamics is about the direction energy transfer; radiation is the means of transport, not the transport itself. Radiation is specified by its amplitude, frequency and direction, this is insufficient to measure energy transfer. To find out about how much energy is transferred through a given surface a mathematician would integrate all radiation passing through it over time. What I disagree with is taking the different radiation components passing in one direction and calling that energy transfer. For thermal energy, to qualify as energy change it would need to cause a temperature change derived from the thermal capacity and amount of energy; tthat is the hole in the greenhouse argument. If you examine the GHE argument carefully you will find it claims a temperature rise as consequence of loss of energy, the fact that it is a loss is frequently hidden away with the phrase 'net energy' transfer, as good an example of 'dance around words' as you will find.
  24. Re 422 KR you wrote:- "In other words, you have agreed that the atmosphere acts as insulation, raising the temperature of the Earth." What is it about insulation that will 'raise the temperature' of anything? Sure when you have something with a temperature elevated above its environment, wrapping it in a first layer of insulation will slow down the rate of cooling but it won't increase the temperature. Adding a 2nd layer of insulation will slow down the rate of cooling further and the outer surface of the 1st insulation layer will become warmer but the final temperature will remain the same as the environment. If your 'something with a temperature elevated above its environment' is also a heat source, when you add a second layer of insulation the surface of the 1st layer of insulation will become warmer and the heat source itself might increase in temperature, depending on how it works. This is not very exciting stuff, perfectly normal common experience. I think your problem arises because gas compressed in a gravitational field has a temperature gradient, thus is the source of your so-called greenhouse effect. Now that really is counter intuitive and, since it involves gravitational energy, it isn't generally understood.
  25. Re 420 KR you wrote:- "the atmosphere is warmed by the Earth, and hence the heater/block/wood/ice analogy holds, not your warming of a room through a blanket." In #420 you said it yourself "solar energy (shortwave) passes right through the atmosphere and warms the Earth" If you blocked the Sun off (and you can) the Earth would cool. This is because the Sun is external to the Earth and its atmosphere. If instead the same amount of heat as given to Earth by the Sun was generated inside the planet, then changing the atmosphere, the emissivity etc. would affect the planetary temperature just the same as changing the number of blankets on your bed.

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