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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 51 to 75 out of 1119:

  1. h-j-m - your part 1 is very much in error. The temperature of an object does not affect absorbance, and when the object is at equilibrium (incoming = outgoing), the absorbance and emissivity are equal. At equilibrium the greenhouse gases in the atmosphere are emitting as much as they receive, but ~50% (as spherically distributed incoherent radiation) heads back down to the ground. If an object receives a 10 micron IR photon, it gains that amount of energy. Photons do not carry ID cards indicating where they came from (unlike Arizonans)! It doesn't matter whether that photon came from the inside of an icebox (a few photons) or a plasma torch (a lot more), it's a photon. Objects cannot reject photons based upon their source. Hence your argument does not hold up - it violates physics. Temperature changes are caused when incoming and outgoing energies are not equal (heat flows). But photons are flying in all directions in some numbers. If you read the Trenberth article, there is about ~0.9 W/m^2 inequality heating the planet.
  2. h-j-m - I have seen your argument in many places before. It reflects a confusion between energy movement and heat flow. Energies move in all directions - up, down, and sideways. Heat flow is the sum of energy movements, and can be positive, negative, or at equilibrium based upon the magnitudes of the various energies. This is the failure at the core of the G&T paper that sparked this thread - it is a mistake to conflate the two.
  3. h-j-m, just stop and THINK about what you are arguing. You are claiming that infrared and other EM radiation cannot travel from substance A to substance B if B is already warmer than A... instead you say it must be "reflected". So... if we were to fire a laser at a block of iron in a cold room the laser would hit the block and warm it up slightly. At which point the block is warmer than the air adjacent to it... so the laser can no longer strike the block. Instead it must reflect off. Thus our laser can never cause the block to get significantly hotter than the room around it. The laser most slowly and uniformly warm the entire room because otherwise it reflects off the warmest part and thus cannot make it any warmer. This is all clearly not the case. Ditto sunlight... if it could not pass from the cold of space to the warmth of planet Earth then we would all live in perpetual darkness (which would prevent the planet from being warm). Even a cursory examination of the world around you disproves everything you are saying.
  4. #53: "Ditto sunlight..." Also to this point, sunlight must indeed pass through the cool atmosphere to the warmer ground and ocean surface. Or else there is no such thing as weather...
  5. I think that what the arguments so far show is that if you have a simplistic understanding of 2nd Law and a simplistic understanding of greenhouse effect, then you can easily make 2+2 = 5. Is 2nd Law the most misunderstood of common physics? I can only recommend, as other have done, the excellent series at Science of Doom. Of course, if someone only wants excuses to ignore science rather than understanding, then they would run a mile from this.
  6. Re #47 CBDunkerson wrote:- "How exactly do you explain sunlight traveling from space (very cold) to the Earth (much warmer) in your world?" Taking space as a vacuum, it really does not have a temperature since heat, (measured by temperature) arises from the microscopic motions of atoms and molecules. There are of course always a few molecules knocking about in space but not normally enough to have much influence on a substantial body like a spaceship. I say not normally but every now and then the Sun has indigestion and belches out energetic particles with energy in the multi MeV (million electron volts) region. You can think of these particles as having a temperature and it would be well over 10^11K. But such temperatures are largely irrelevant since the more devastating effect of the particles is the ionisation of the atoms in your body! In space there is always some radiation energy in the form of photons which comes (mostly) from hot bodies like stars 3500K-100000K. Much less intense are the photons from planets like Earth 150K-350K. Finally there is Cosmic Background Radiation (CMB) at 2.7K, this is the 'cold of space' you are probably thinking of. If you are near a star you get hot because you intercept a large number of very hot photons. At a distance from a star, like the Earth, you still intercept very hot photons but many fewer, so you receive much less energy in total. Since the energy from the photons that you receive from the star heats you up, as your temperature rises above 0K (lets start at the bottom!) you begin to radiate heat also. Your temperature stabilises when it rises far enough for you to emit enough photons of sufficient energy to match the total energy of the incoming (hot) photons from the star. An important point is the fact that the same power (W/m^2) of light (same as electromagnetic) radiation may be a few high energy photons from the hot star or a lot of lower energy photons from a much cooler planet, so when you see an 'energy balance diagram' covered in numbers with 'W/m^2' attached like the one on this page it really doesn't mean very much because there is no mention of any temperatures - anywhere, not of the Earth's surface, the atmosphere nor even the Sun.
  7. damorbel - "...it really doesn't mean very much because there is no mention of any temperatures": Directly, no, indirectly, absolutely yes. The 396 W/m^2 radiated from the Earth is the power emitted from the near-blackbody (average emissivity almost 1.0) of the Earth at a temperature of 14C, including temperature variations (an earlier paper estimated 390, but with insufficient attention to local variations).
  8. Re #55 scaddenp I checked your link to 'Science of doom' and this is (some ) of what I find there:- "It’s possible that the imaginary second law has taken a strong hold because anyone who does look it up finds statements like dS/dt>=0, where S is entropy. Wow. Clever people. What’s entropy? How does this relate to candles? Candles can’t warm the sun, so I guess the second law has just proved the “greenhouse” effect wrong.. According to Wikipedia, Clausius expressed the second law (validly) like this: 'Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature' Again, that seems right and it doesn’t have any entropy involved in the description. I never did like entropy. It never seemed real." To me the writer appears to accept that the 2nd Law of Themodynamics may even disprove the GH effect but does not seem to be very well informed on the matter. Lets face it, if he is only happy if entropy is excluded, he must be leading a very restricted (thermodynamical) life!
  9. #56: "an 'energy balance diagram' covered in numbers with 'W/m^2' ... it really doesn't mean very much because there is no mention of any temperatures " That diagram (the familiar IPCC global radiation budget) clearly indicates the incoming radiation is solar -- and therefore has the solar energy spectrum. Hence the temperatures are known. Same for earth surface and atmosphere. So what do you mean when you say 'it doesn't mean very much'?
  10. Re: damorbel (58) If you consider Science Of Doom to not "be very well informed on the matter" then perhaps (read: no uncertainty whatsoever) you should go back to the drawing board: Learn the basics of climate science and then build on that more solid foundation rather than to spout off on that which you don't even know what you don't know. No offense. The Yooper
  11. damorbel, I think your comment 56 is okay up to the next to last paragraph where you wrote "...incoming (hot) photons from the star," and the last paragraph's "... a few high energy photons from the hot star or a lot of lower energy photons from a much cooler planet...." You might have a misconception that all the photons coming from a source have the same energy, and that single energy is higher when the source is hot than when the source is cold. Instead, the radiation from a blackbody includes photons of low energy, high energy, and shades in between. That's why each blackbody radiation "curve" is a curve rather than single vertical line at a single energy. The temperature of the source determines the relative numbers of photons at those different energies. That distribution of photons' energies is the sole extent of the relevance of the source's temperature. Each photon has no memory of the temperature of its source. (We can calculate the probability of that photon having come from a hot source versus a cold source, but that's the limit of our knowledge, and the photon doesn't know even that.) Your next to last paragraph is correct if you simply leave out the phrases "(hot)" and "from the star." The correct paragraph would be "Your temperature stabilizes when it rises far enough for you to emit enough photons of sufficient energy to match the total energy of the incoming photons from all sources." The photons don't know where they came from. Your body can't tell where the photons came from; your body accepts them all.
  12. Damorbel clearly does not believe the physics when explained to him by "warmists", so maybe Dr. Roy Spencer (a "skeptic") will be able to convince him... Please go here and here.
  13. damerol - You said According to Wikipedia, Clausius expressed the second law (validly) like this: 'Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature'. That is NOT what wikipedia quotes. Their translation of Clausius is: "No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature". This statement is correct within the context for which he made it. Again, that seems right and it doesn’t have any entropy involved in the description. I never did like entropy. It never seemed real. What do mean by "seems right"? And entropy has very precise definition from Clausius (as does 2nd law) in mathematics. You cant go drawing wild conclusions from imprecise english statements excerpted from context and claim this overturns application of a very precise mathematical framework from which the statement is derived. As I said, half-grasped ideas just lead to 2+2=5
  14. damorbel - Your last post left me a bit stunned. If you honestly feel that SoD "does not seem to be very well informed on the matter", then you are suffering from what is known as the Dunning-Kruger effect. You need to go back and review the basics - you've certainly been directed to them repeatedly. Until you do, the points you raise won't even be wrong.
  15. KR, sorry, I did not know that meanwhile our technology is advanced enough to observe directly what happens when photons hit matter or are emitted by it. I'd really like to see that. Then I will gladly accept that your notions about emissivity and absorptivity. That a photon is a photon regardless of it's origin is outright wrong unless they are at the same energy level (wavelength). But the wavelength of a photon depends on the temperature of their source (hence different black body radiation curves for different temperatures) and yes, so to speak, they do carry their ID cards (sort of). No, I do not confuse between energy movement and heat flow, I just state that temperature plays a dominant role. While you seemingly argue that taking temperature out of the game will leave no room for the violation of thermodynamic laws. I just looked up the science of doom page that scaddenp referred to and noticed the same trick. But a bit bolder as the author eliminates the term temperature and speaks of amounts of energy instead. Unfortunately temperature is not a measurement of energy amounts. It is a measurement of energy intensity. CBDunkerson, if you would read more carefully and reply to what I wrote you might be able to get a point. And yes sunlight can never reach the earth if its source is cold and empty space, but if it's source is the far hotter sun, I think that might change things a bit.
  16. h-j-m, I'm glad you wrote that two photons are equals if their wavelengths are the same. Now you need to understand that a photon of a given wavelength can come from a range of different-temperature sources. Imagine those two identical-wavelength photons hitting their target. The target absorbs them identically, because as you wrote, they are effectively identical. Which means the temperature difference between the source and the target is irrelevant.
  17. I just looked up the science of doom page that scaddenp referred to and noticed the same trick. But a bit bolder as the author eliminates the term temperature and speaks of amounts of energy instead. Unfortunately temperature is not a measurement of energy amounts. It is a measurement of energy intensity. Firstly, SoD is text-book stuff. Secondly, I am sorry but I fail to understand your comments on temperature. Temperature of say a gas is linearly related to average kinetic energy of the gas particles. Energy in those diagrams isnt measured - its derived from temperature. Can you express what you mean by "energy intensity" in mathematical terms and relate it to temperature please?
  18. h-j-m - I read your last post, and spent some time thinking about it. I think you are approaching the issues with a great deal of common sense, but not much technical background. That is a reasonable first approach, but leads to the Common Sense logical error - applying day-to-day reasonable responses to problem domains outside that experience. The Stefan–Boltzmann law (also here) is one of the more established properties of thermal radiation - it applies to all objects with a temperature above absolute zero. But it's not intuitive - it required detailed spectroscopy to establish this basic behavior. In direct response to your post, temperature sets the amount of thermal radiation, as per P=e*s*A*T^4 (Power, emissivity as a ratio to a theoretic black body [always 1 or less], Area, and Temperature). The thermal mass, and hence the total energy, are set by the particular object in question. But the amount of radiation is set by emissivity, area, and temperature. Nothing else. That's why everyone talks about temperatures in regard to climate. Thermal mass and total energy affect how fast temperatures change. But temperatures and emissivity differentials (primarily temperatures) affect how total energy changes - at whatever rate. And the direction of change is directly dependent on energy emission/absorption, not total heat content. We really worry about the directions, although we're also interested in the rate of change. I hope these comments are helpful. I would suggest looking into the Science of Doom site as a resource - search on "greenhouse", "2nd law of thermodynamics", etc. He has a good way with explaining these issues. Also look at Dr. Roy Spencer (noted 'skeptic'), here and here
  19. Oh, and 's' is the Stephen-Boltzmann constant, which scales this relationship. Sorry about that...
  20. Re #66 Tom Dayton You wrote:- "Now you need to understand that a photon of a given wavelength can come from a range of different-temperature sources." That is true but misses an important factor, a thermal source has a broad spectrum that is determined by the temperature. We are all familiar with the Planck spectrum, the amplitude of which is a function of the temperature, But taking one photon (with energy a function of frequency), or even one spectral component, does not represent the entire spectrum thus the temperature is not defined. Although a single photon has energy it does not have a temperature. You can say the same for a laser, a laser's beam may contain a great deal of energy which is all squashed into one frequency, all its photons have the same energy. If the laser beam is absorbed its single frequency energy is converted into thermal energy with its characteristic temperature dependent Maxwell-Boltzmann energy distribution.
  21. damorbel, you completely missed my point. And your reply seems to be gobbledygook.
  22. Re #60 Daniel Bailey You wrote:- "you should go back to the drawing board: Learn the basics of climate science" But 'Science of Doom' write weird things like:- "What’s entropy? How does this relate to candles? Candles can’t warm the sun, so I guess the second law has just proved the “greenhouse” effect wrong.." Of course a candle can warm the Sun if the candle's temperature is high enough, difficult to achieve I know but not impossible. It won't warm it much because the energy available from any reasonable candle is rather small compared with the Sun. But if the candle is hot enough and you have a large enough (very large!) number of them there will be a visible effect! Perhaps this is not found in the 'basics of climate science' but nevertheless it is the 2nd Law of Thermodynamics.
  23. Re #71 Tom Dayton "And your reply seems to be gobbledygook." The bit about lasers having photons with the same energy? Or something else?
  24. Re #61 Tom Dayton you wrote:- "I think your comment 56 is okay up to the next to last paragraph where you wrote "...incoming (hot) photons from the star," and the last paragraph's "... a few high energy photons from the hot star or a lot of lower energy photons from a much cooler planet...." " And then:- "You might have a misconception that all the photons coming from a source have the same energy, and that single energy is higher when the source is hot than when the source is cold." No misconception here, the average energy of the photons from a hot source is higher than the average energy of those from a cooler source. But you must recognise that the power (W/m^2 or J/s/m^2) in a stream of photons is proportional to the number of photons/s times hv, their energy. For example the photons from the Sun come from a high temperature source (5780K) these have enough hv to split O2 molecules and thus allow the formation of ozone. The power in this stream of photons from the Sun is 1370W/m^2, which averages to 342.5W/m^2 over the surface. This average power, when converted to heat, produces a temperature in the region of 280K. The Earth then radiates photons with an average energy based on this 280K, keeping the Earth's temperature stable while being bombarded with photons from a high energy photon source at 5780K. The photons radiated from the Earth also have a thermal (Planck) distribution which also allows a very small probability of splitting O2 molecules but the proportion of photons emitted by the Earth at 280K with sufficient energy to do this is very very small, many times smaller than the proportion in sunlight
  25. Did anyone notice we now have a case of skeptical whiplash? In this thread, #65: "I do not confuse between energy movement and heat flow, I just state that temperature plays a dominant role." In a comment on another thread we read, "Temperature is not a useless metric in given circumstances. It, by itself tho, is a useless metric when talking about climate. ... When talking about climate one must think in terms of heat content. " This seems to summarize the universe of the skeptic: It is what they say it is; until they say something else and believe that too -- even if it contradicts their prior position.

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