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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 201 to 225 out of 765:

  1. #197: "you'd be in a warm bath of air at 14C, 1/2 surrounded by radiation from the ground at 14C; 1/2 from deep space at 2.7K." Damorbel, did you just say that radiation from space at 2.7K contributes 50% to keeping that 'warm bath of air' at 14C? What would Clausius say to that?
  2. Re #196 scaddenp you wrote :- "continuing to talk about what happens and how the 2nd law works in conductive energy transfer is not helping you understand how it works in radiative energy transfer." Energy transfer by photons is amazingly similar to that in gases, both exchange momentum in collision processes; in gases it is by inelastic collisions and with photons it is by elastic collision.
  3. Damorbel wrote: "The balls will slowly approach a temperature dependent only on the distance of and the power emitted by your UV source." As I understand it: If the source is a star, and both balls are receiving UV energy, then yes--even though the balls will still exchange radiation at equilibrium. However, that's not what I said. The second ball is not receiving UV radiation. It has been given one time heat by some unknown source. It is cooler than the first ball. My argument is that such a ball is an energy source for the first ball, in addition to the UV source, and it will continue to be an energy source because it receives radiation from the first ball. In this analogy, the second ball is the atmosphere. It receives radiation from the surface, and even though it is cooler than the surface, it radiates some of that energy back toward the surface. Eventually, some of the "backradiation" is absorbed by the surface and turned into work, but most of it gets hung up dancing from molecule to molecule in the troposphere, where convection and conduction also bring it into contact with the surface or bring it to the stratosphere. The point, though, is that the upper troposphere can indeed act as an energy source for the surface and for the lower troposphere (where surface temps are measured), even though the upper troposphere is concurrently cooler. GHGs basically redirect certain frequencies of longwave radiation. The more GHGs, the more LWR is redirected, and the more time LWR stays within the system (heating, doing work, being a nuisance, etc.).
  4. Sigh. If you thought greenhouse effect was energy transfer from atmosphere to surface by conduction, then that WOULD be violation of second law. However, this is not what is happening as people repeatedly tell you. No incoming radiation, no GHG effect. You cant take the sun out of it. If you are determined not to learn physics, then we are wasting our time trying to teach you.
  5. #199: "Convection is a bit special because it won't work 'downwards'" Oh, dear. I guess plate tectonics, thermohaline ocean circulation and onshore/offshore breezes, among other things all just stopped because damorbel says 'it won't work downwards'.
  6. Re #198 KR The case for warming due to back radiation has not been made. GHGs are distributed more or less uniformly up to 80km except for water, by far the dominant GHG, which drops to a low concentration above 15-20km. The relevant point about GHG emission is that it takes place throughout the atmosphere, it is highest where the gas density is highest and, most important, because the temperature at any given location does not change quickly, to a first approximation, GHGs are absorbing (locally) just as much IR as they emit. They do not collect radiation together somehow 'at the top of the atmosphere' (TOA) and send it down (or up) as in Trenberth's 'Radiation Balance' diagram. If that 'collecting at TOA' according to Trenberth were possible there might be a better case for surface warming but you are still stuck with the problem that the TOA is extremely cold and can only take heat from the surface, not send heat down to the surface. Notice that I wrote 'not send down heat to the surface', not 'not send radiation down to the surface'. Radiation is not heat, heating (or cooling) arises only when there is an imbalance between absorbed and emitted radiation. In the troposphere radiation is emitted and absorbed primarily in a balanced way, with height being the only exception. The importance of this exception means that heat transfer due to radiation goes only in one direction only, out into deep space. There are two reasons for this, the atmospheric temperature falls steadily with height according to the lapse rate (-6.5K/km), the atmospheric density also falls with height. Thus the lapse rate defines the direction of heat tansfer and the density variation also ensures that there is always less radiation 'downwards' rather than 'upwards' for the simple reason that the amount of radiating gas reduces with height due to the drop in density with height.
  7. Re #204 scaddenp You wrote:- "If you thought greenhouse effect was energy transfer from atmosphere to surface by conduction, then that WOULD be violation of second law." Yes, but no need for emphasis! And:- " However, this is not what is happening as people repeatedly tell you." Tell me? Don't I know it! then you write :- "No incoming radiation, no GHG effect. You cant take the sun out of it." What does that mean? Isn't it the GHGs that are supposed to cause the GH effect? h-j-m has been arguing successfully that the atmosphere is heated directly by GHGs absorbing energy directly from the Sun's radiation, Trenberth's diagram shows it, who is disputing it? The whole planet is heated by the Sun's radiation and very little else, if you have a problem with this could you expand on it? Heat transfer by radiation can only be from a hot body (gas etc.) to a cooler, no different from conduction diffusion or convection.
  8. damorbel wrote: "Microwaves are not 'thermal' like a grill" So... you are arguing that electromagnetic radiation in the range we designate as 'infrared' (or 'thermal' energy) behave differently than all other forms of electromagnetic radiation? While ridiculous on its face... that doesn't pass the everyday reality test either. Most remote controls use infrared signals. By your logic they would not function if the receiver, or any space leading up to it, were even a fraction of a degree warmer than the transmitter. Many pieces of electronic equipment get quite warm when they have been running for a while... yet the receivers in them still pick up infrared signals from the cooler remote. Also: "Oh alright then, not 0K, lets put 0.00000000001K." Close. Multiply that by ten and you've got the lowest temperature ever observed. However, you are missing the point. Your claim that objects at 0K emit no radiation is meaningless because there AREN'T any objects at 0K.
  9. Re #205 muoncounter You wrote:- "all just stopped because damorbel says 'it won't work downwards'." And you inserted a nice diagram showing convection by a fluid between two surfaces, the warmer one being underneath, you can see how it works by the arrows on the convective flow loops. But muoncounter, surely these loops do not have a uniform temperature all the way round? According to me the left hand part of the loop (the 'upward' part) will be warmer than the righthand (the descending) part. I really don't get what you are on about. I suggest you put some indication of the temperature distribution on your diagram, that should help to clarify what is happening energywise. It is an interesting fact that convection can take place with very small temperature differences. Have you ever come accross a heat pipe? Heat pipes have fluid inside them that transfers heat by evaporation and condensation; they are very effective, my computer has one for cooling the video driver chip(s?) You must look at Earth as a sort of giant three dimensional heat pipe that transfers heat from the tropics to the poles and the upper atmosphere by processes rather similar to those in a a heat pipe.
  10. damorbel #206 Trenberth does not say anything even near "collection at TOA"; it's just your (wrong) interpretation of Trenberth's schematic diagram.
  11. Re #208 CBDunkerson You wrote:- "So... you are arguing that electromagnetic radiation in the range we designate as 'infrared' (or 'thermal' energy) behave differently than all other forms of electromagnetic radiation?" What is call 'thermal radiation' is radiation from a thermal source, glowing metal, hot carbon are typical, they give out radiation with a broad spectrum first described by G Kirchhoff as 'blackbody radiation'. The important factor is that the emission is proportional to temperature, the implication is that any substantial body must have the same temperature throughout, that is what is meant by equilibrium. The importance of uniform temperature comes from the fact that, if the temperature is not uniform, the parts with different temperature will emit different amounts of radiation. Also heat will possibly flow by conduction etc. bteween the parts with different temperatures. Ultimately, how can you say a body, whose parts are at different temperatures, has 'one' temperature? But radiation comes in all sorts and sizes from DC(?) to beyond blue light I have heard. Thermal radiation has a characteristic Planckian spectrum that comes from a black body and it is related to the temperature of this 'black body' (yes I know it isn't black if it's radiating!) Radiation from other sources such as lasers, microwave ovens and radio and television transmitters is largely monochromatic they have only one frequency. All these forms of radiation get converted to heat when absorbed; this heat tends to raise the temperature of the irradiated object. This heat tends to be dissipated in the surroundings by any process you care to mention, radiation; convection; conduction etc. or even into chemical energy e.g. plant growth. The important point is that not all sources of radiation have a temperature.
  12. Re #210 Riccardo You wrote:- "Trenberth does not say anything even near "collection at TOA"; it's just your (wrong) interpretation of Trenberth's schematic diagram." It is an expression used all over the place in climatology, Trenberth has it here If it is any comfort to you it is a meaningless concept not least because the TOA is completely undefined, temperature? pressure? altitude? All are unidentified.
  13. damorbel wrote: "What is call 'thermal radiation' is radiation from a thermal source, glowing metal, hot carbon are typical, they give out radiation with a broad spectrum first described by G Kirchhoff as 'blackbody radiation'." So... 'sunlight'. Which is the broad spectrum of radiation given off by a thermal source known as the Sun. Yet sunlight travels from the cold of space to the warmer upper atmosphere to the warmer still lower atmosphere. damorbel wrote: "The important point is that not all sources of radiation have a temperature." Which is an oxymoron. All sources of radiation have a temperature... otherwise they couldn't be sources of radiation. The theoretical 'no temperature' of 0 Kelvin is defined as the point at which matter emits no radiation.
  14. damorbel could you please point me where Trenbrth said such thing? I couldn't find it.
  15. #206: "GHGs are distributed more or less uniformly up to 80km except for water, by far the dominant GHG, which drops to a low concentration above 15-20km. ... the atmospheric temperature falls steadily with height according to the lapse rate (-6.5K/km), the atmospheric density also falls with height." Multiple sources show CO2 and H2O concentrations vary considerably with altitude. Temperature isn't uniformly decreasing with altitude. Density isn't linear with altitude. #209: "surely these loops do not have a uniform temperature all the way round?" Who suggested that they did? You said 'convection won't work downwards'; most people associate 'convection' with some sort of circulation - including the return trip down. There's no subduction of lithospheric plates without it. "I suggest you put some indication of the temperature distribution on your diagram," The 'heat input' on the bottom and 'fluid cools' at the top would be enough for Wikipedia-level readers to get the point. Apparently you require additional notation? "Isn't it the GHGs that are supposed to cause the GH effect?" Duh; but without solar heat input, there's no surface IR radiation for GHGs to absorb. Deflecting the discussion with these irrelevancies was just tedious some hundred comments ago; now it's pointless clutter, but I suspect that's your actual goal here.
  16. Re #213 CBDunkerson you wrote:- "So... 'sunlight'. Which is the broad spectrum of radiation given off by a thermal source known as the Sun. Yet sunlight travels from the cold of space to the warmer upper atmosphere to the warmer still lower atmosphere." Of itself a vacuum contains no material so it can have no temperature, since temperature is a measure of the heat content of molecules and atoms. Electromagnetic (EM) radiation is produced by the vibration (more accurately the acceleration) of electric charge (electrons and protons), so it always is associated with matter how ever far away that matter happens to be. EM radiation starts and finishes with matter, it moves at the speed of light and it cannot be stored or otherwise conserved like energy. Since EM radiation is produced by matter which in turn has a temperature there is a sort of connection between temperature and radiation. But only a connection. If the connection is to be strong the radiation must at least have a spectrum according to the Planck radiation formula. Just having the spectrum is not sufficient, it must have the right intensity also. If the intensity is weakened, say because a star is at a distance then, even though the spectrum remains the same, the temperature is reduced because the intensity is no longer that given by Planck's formula. Further you wrote:- "The important point is that not all sources of radiation have a temperature." Which is an oxymoron. All sources of radiation have a temperature... otherwise they couldn't be sources of radiation." Lasers, radio and television transmitters, microwave ovens are all sources of radiation that does not conform to the Planck spectrum, so the source is not related to matter having a temperature (be careful, the radiation output of a microwave oven etc. is (more or less) independent of its physical temperature). When this 'non-Planckian' radiation is absorbed (by matter) the temperature of the absorbing matter increases - the stored energy is thermal in character, the matter has a temperature.
  17. #213 CBDunkerson at 00:13 AM on 1 December, 2010 All sources of radiation have a temperature... otherwise they couldn't be sources of radiation. The theoretical 'no temperature' of 0 Kelvin is defined as the point at which matter emits no radiation Which is an oxymoron. damorbel is obviously talking about radiation sources very far from thermal equilibrium. Such systems do not have a unique well defined temperature, yet they may emit radiation. Otherwise how would you explain laser cooling? (Heat never moves spontaneously from a cold place to a hot one, so in a sense laser light has to be cold indeed to be able to cool down things to several nanokelvins.) LED lamp is another example. Or is the common glow-worm (Lampyris noctiluca) hot? For that matter neither has OLR (Outgoing Longwave Radiation) at TOA (Top of Atmosphere) a well defined blackbody temperature. Partly because Earth is not a blackbody, partly because due to heavy frequency dependence of atmospheric transparency, layers of very different temperature give contributions to OLR. Again, Earth is a system very far from thermodynamic equilibrium.
  18. damorbel - As I stated before, greenhouse gases reduce cooling of the surface, which has the result of the Earth's surface heating up in order to radiate in balance with the incoming solar energy. The observed backradiation from lower atmosphere GHG's is part of the energy balance, which Trenberth listed in his 2009 paper - it's an energy exchange, part of the balance sheet including incoming solar, outgoing top of atmosphere (which as a point of demarcation is chosen as somewhere above the majority of GHG's), surface IR, back IR, thermals, etc etc. You are obviously familar with EM, heat, energy exchanges, etc. Your description of lapse rates, thermal radiation, etc., seem reasonable, except for your somehow deciding that backradiation doesn't have a role. You have, however, put up repeated strawman and red herring points, such as dying due to lack or H2O in thought experiments, quibbling about monochromatic sources, etc. At this point I consider you to just be objecting for the sake of objecting. If you have actual issues, fine - otherwise I'm leaving this thread.
  19. damorbel, imagine there are two stars, named Miami and Anchorage. Both are 100,000 light years from our Sun. From hottest to coolest, Miami > Sun > Anchorage. Miami and Anchorage's radiation emission curves overlap, and intersect at a wavelength W, so the two stars emit the same number of photons having wavelength W. Simultaneously 100,000 years ago, Miami emitted a photon named Sally, and Anchorage emitted a photon named Greg. Sally and Greg both have wavelength W. 100,000 years later, Sally and Greg arrive at our Sun. Our Sun is cooler than Miami but hotter than Anchorage. All physicists in the world agree that both Sally and Greg are absorbed by the Sun. The Sun has no way of knowing that Sally's source was hotter than the Sun, and that Greg's source was cooler than the Sun, because Sally and Greg have the same wavelength W. I believe that in stark contrast you have been claiming that the second law of thermodynamics requires the Sun to absorb Sally but not absorb Greg. Here is a simple question for you: What happens to Greg?
  20. Berényi - Well written post on non-thermal radiation, thank you. Directly inferring temperature from EM spectra only works when the spectra is sufficiently similar to a blackbody curve, whether it has band-gaps or not. Monochromatic and 'cold-light' sources have an inherent energy, but since they are not thermal emitters that doesn't directly correspond to a temperature. However, when you say that "...Earth is a system very far from thermodynamic equilibrium", I would like to point out that as far as we can tell (again from Trenberth 2009, although I'm sure there are slightly different estimates out there) the balance sheet is currently tipped only about 0.9 W/m^2 from dynamic equilibrium. If we can reduce or prevent further GHG emissions, we can reduce that imbalance, and the resulting shift in global temperatures.
  21. BP, you are arguing (correctly) that not all rectangles are squares. Damorbel is arguing (incorrectly) that not all squares are rectangles. That is... yes, there is such a thing as non-thermal radiation. However, there is NOT such a thing as radiation from a source with no temperature. Two very different arguments. damorbel writes: "EM radiation starts and finishes with matter, it moves at the speed of light and it cannot be stored or otherwise conserved like energy." So... EM radiation is not energy. Fascinating stuff. However, setting that aside... your dodge about the vacuum of space being empty (it isn't) doesn't address the fact that sunlight travels through the Earth's atmosphere. How does sunlight hit gas molecules in the stratosphere, which is very cold, and then continue from there down to the troposphere, which is much warmer? Your argument would make this impossible... yet it obviously happens. Sunlight travels from a cold region of the atmosphere to a warmer one... indisputable fact. The whole 'broad spectrum' bit was nonsense to begin with because there is no reason a wide range of EM emissions should behave differently than a narrow band... and sunlight shows that it doesn't. There are countless examples of EM radiation traveling from cold areas to warmer ones... and it makes no difference whether it is individual wavelengths vs a wide band or the emissions source is thermal or non-thermal. Examples have been provided of ALL of these behaving the same way... there is no magical 'warmth barrier' to radiation. It's pure nonsense and observably so in the everyday world all around us.
  22. Re #218 KR You write:- "greenhouse gases reduce cooling of the surface, which has the result of the Earth's surface heating up in order to radiate in balance with the incoming solar energy." And:- "The observed backradiation from lower atmosphere GHG's is part of the energy balance, which Trenberth listed in his 2009 paper" And crucially:- "except for your somehow deciding that backradiation doesn't have a role" Back radiation would have a role in raising raising the surface temperature if it exceeded the output from other sources. But Trenberth himself has back radiation at 333W/m^2 and the surface sourcing 356W/m^2 to the atmosphere, thus the net upward radiation to the atmosphere is just 26W/m^2 and since it 'is net upward radiation' it is cooling the surface, not heating it! Trenberth has a total of 198W/m^2 going into the atmosphere and 169W/m^2 plus 30W/m^2 = 199W/m^2 leaving to deep space; leaving 1W/m^2 to raise the temperature of the surface by 30K. I may have got some of the figures wrong but without any temperatures on the diagrams it is quite impossible to make any check of the claimed warming effect, so without any question it is a scientifically unsound explanation, I have no idea how you manage to have such faith in it.
  23. damorbel - Aha, I think I see the issue you're having. Without greenhouse gases, the atmosphere would not emit much IR at all - nitrogen and oxygen don't have the structure to emit in the thermal IR bands. So, without the 333 W/m^2 backradiation, the surface of the Earth at current temperatures would still radiate upwards to space at net 356 W/m^2, not net 26 W/m^2. Don't you think this would have a cooling effect? If greenhouse gases were to go away the Earth would rapidly cool towards -18C, where outgoing top of atmosphere IR would be in balance with incoming solar energy, rather than the current +14C average temperature. Of course, that would lead to glaciation, increasing albedo, and reducing the temperature even further - the -18C thought experiment is just a first pass example. Backradiation greatly reduces cooling efficiency of the Earth - it has to be hotter to remain in energy balance with the sun. Backradiation doesn't have to exceed surface radiation in order to change the net heat loss, which you yourself have shown. The whole issue of backradiation and thermal balance is still based upon energy coming in from the sun, net energy flow to space, and the temperature of the Earth. Reduce the net cooling energy flow to space (reduced emissivity from GHG's), and the system is imbalanced until the temperature rises to compensate. I've pointed you to the very straightforward Thermal Radiation writeup on this, and the governing equation P = e * s * A * T^4. To put it bluntly, if you don't understand that, I don't think I can help you.
  24. Re #221 CBDunkerson You write:- "How does sunlight hit gas molecules in the stratosphere, which is very cold, and then continue from there down to the troposphere, which is much warmer? Your argument would make this impossible... yet it obviously happens. Sunlight travels from a cold region of the atmosphere to a warmer one... indisputable fact." When sunlight hits the atmosphere the UV component at 200 microns and below splits the O2 molecules into two O atoms which then join other O2 molecules to form ozone - O3 O3 further absorbs UV at 300 microns and shorter, thus the sunlight proceeds to the surface shorn of its dangerous UV. But what do you mean when you write this:- "Sunlight travels from a cold region of the atmosphere to a warmer one... indisputable fact."? The sunlight that passes through the atmosphere is not affected by it. The absorbed UV heats the stratosphere and generates the Ozone layer. The heating by UV causes a massive temperature inversion which makes the stratosphere very calm in comparison with the troposphere. Oh, and the temperature at the stratopause is not that low, just about freezing, 0C.
  25. Other have covered some of your points BUT "Heat transfer by radiation can only be from a hot body (gas etc.) to a cooler, no different from conduction diffusion or convection. " Missing word in here is NET heat transfer. Energy is transferred from cold to hot - a photon isnt magically not absorbed because the absorption surface is hotter than its source. The energy warming from surface of earth is from sun, the ghg are merely backscattering outgoing radiation.
    Response: Sorry to nag, but please refrain from using all caps.

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