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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 151 to 175 out of 765:

  1. That spectral chart is a visual example of what I said - incoming solar energy is primarily in the form of visible & UV light. For your other question, about what makes greenhouse gases so special, you could read the 150 year old research report by Tyndall that I linked to earlier. I apologize for not relinking as this is being tapped out from my phone.
  2. Ah, h-j-m's point (I think) could be stated as follows Since GHG's absorb in the visible (as well as the infra-red), doesn't increasing the concentration mean that the earth receives less energy because the subsequent emission of that radiation scatters some of it into space - back radiation on incoming EM which thus goes into space. Assuming I've understood h-j-m's issue correctly let me offer the following rebutals 1. Absorption of EM radiation, either visible incoming or IR outgoing does not result in all the radiation being emitted, some will be converted into vibrational, rotational and translational energy. Thus increasing absorption of visible incoming EM will, in certain extent warm the upper atmosphere as well as decrease slightly the EM hitting the surface. 2. The areas of the incoming EM spectrum in which H2O (primarily) absorb are not near the peak of frequencies and are at the longer wavelength(lower energy) end. Thus they do not absorb proportional as much as the outgoing earth-light. 3. The Greenhouse effect is measured empirically by comparing the temperature at the top of atmosphere with the ground. The commonly quoted 33deg is therefore the nett effect of IR absorption of Earthlight coming out - Visible absorption of Sunlight coming in. I seem to remember seeing 50deg as a figure for the 1st, but I'll be damned if I can remember where ... Thats quite a long response to a point that I may have misunderstood, and is getting increasingly off-topic for this particular thread... :-(
  3. Re #144 archiesteel You write: "To say they do is either to misunderstand the science, or to disingenuously misrepresent what climatologists believe." Lets look at Trenberth's diagram:- You write:- "The point is not that all of the absorbed photons will go back down. The photon re-emission by GHG molecules happens in a random direction." Trenberth's diagram shows 165Wm^2 going out from the atmosphere, 30Wm^2 from clouds 360Wm^2 going up from the ground and 324Wm^2 back radiation going down. The problem is both the back radiation and the ground radiation. First they are both greater than the input from the Sun, second they are not reflected by the ground or the clouds whereas the Sun's input is reflected by both the clouds and the ground. Third the Suns input is absorbed (67Wm^2) by the atmosphere, why isn't the '324Wm^2 back radiation' similarly absorbed? Since 'back radiation' is emitted by GHGs it does not have a short wave component like sunlight, so a bigger % of the 324Wm^2 is going to be reabsorbed by GHGs. All I was doing in my post #143 was drawing attention to Eli Rabbet's recognition that this IPCC diagram does not represent anything real. Using Eli's explanation there is no substantial downward radiation because the photons emitted by GHGs are largely absorbed locally and certainly never get to the ground at a level comparable to the Sun's input. Likewise Earth's 390Wm^2 surface emission cannot be well over double the 168Wm^2 arriving there from the Sun. The IPCC claims that the GHGs warm the surface by about 33C but there are no numbers on this diagram showing how this happens, even though the various places are shown emitting and absorbing radiation, there are no temperatures showing the basis for explaining the greenhouse effect. Is this the way we plan to change the World fuel economy?
  4. Composer99, the wikipedia diagram as well as a similar diagram (unfortunately only black and white) on the mentioned Science of Doom page show clearly that incoming radiation gets absorbed (by water vapour mostly). The Trenberth et al. diagram I have included in my post #50 shows ~ 20% of incoming energy absorbed by atmosphere. I doubt you could justify neglecting it. Nevertheless I completely failed to find any credible further information on that subject. The rest of your post urges me to some provocative questions. How does the earth measure the incoming radiation? How does the earth measure the outgoing radiation? How can it tell the difference? But if it can do this. Who told the earth that there should be a balance? How can he force the earth to respond? Consider these questions answered. How can the earth know what to do? Does the earth have the means to do what needs to be done? Sorry, somehow that sounds rather non scientific, but I could not help writing it anyway.
  5. Damorbel @ 153 - The problem is both the back radiation and the ground radiation. First they are both greater than the input from the Sun Well, yes both surface and back radiation occur night and day, whereas incoming solar radiation doesn't.
  6. damorbel - "... there is no substantial downward radiation because the photons emitted by GHGs are largely absorbed locally and certainly never get to the ground at a level comparable to the Sun's input." Nuh. Any molecule that can absorb radiation must, by definition, emit radiation. The fact that radiation is absorbed, emitted, absorbed again, emitted again multiple times within the atmosphere before striking the surface (again) or eventually escaping at TOA is what the "greenhouse effect" consists of. All this bouncing around is the evidence of energy staying in the system. When there are more GHGs in the atmosphere, more energy stays in the system longer.
  7. You are aware that backradiation etc is MEASURED? (Look for DLR stations). If your understanding mismatches experimental observations, then your understanding is wrong. As to balance - Planck radiation is the "balancing mechanism". First Law of thermodynamics - that you cant destroy energy - is why you have balance. If a body absorbs energy its temperature rises - temperature is expression of average molecular kinetic and potential energy in the body. It emits radiation in proportion to its temperature. When radiation outgoing matches incoming energy temperature stays constant. Its simple physical law, readily demonstrated a lab. A body "knows" what do in accordance Stefan-Boltzmann, derivable from QM theory - a moving charged particle must irradiate. Incoming and outgoing radiation are measured at TOA by atmosphere. There is problem with the measurements in they have good precision but poor accuracy.
  8. @damobel: the graph simplifies how the heat exchange mechanisms work. You can't look at such a graph and claim it is supposed to accurately represent the path of each photon. Others have explained this clearly. If you still can't understand it at this point, then one of two things must be true: a) this is beyond your intellectual capacity, or b) you're not debating in good faith. There are a lot of trolls and astroturfers here, please don't join their ranks and make a serious effort to read the material on this site before repeating the debunked junk peddled by professional climate deniers...
  9. Bibliovermis wrote: "That spectral chart is a visual example of what I said - incoming solar energy is primarily in the form of visible & UV light." So far so true if you define primarily to be more than 50%. But I would not dare to call 45% negligible. Further you stated that incoming solar radiation is not effected by green house gases but the chart clearly shows the opposite.
  10. Re #158 archiesteel you wrote@ "the graph simplifies how the heat exchange mechanisms work. You can't look at such a graph and claim it is supposed to accurately represent the path of each photon." I take it you mean the diagram in #153? I am not arguing that there aren't better explanations for the GH effect but this diagram is what is used by the IPCC in its Assessment Reports and its Summaries for Policymakers. This is what is used by government agencies like the Met Office when they are advising on energy source policies. The diagram is a principle feature of the IPCC AGW science, it appears in volumes of the various IPCC Assessment Reports called "The Scientific Basis". In thess volumes it is stated that the GHE warms the surface (on average) from 255K to 288K i.e. 33K. If the diagram were improved it would show just how this 33K comes about. As it stands there are no temperatures at all reported on it, this should be possible using themal models of the atmosphere such as the US Standard Atmosphere (which does not recognise the GHE), then perhaps the arguments of the IPCC will become more clear.
  11. Phil, first you say as Bibliovermis did, that incoming solar radiation is not effected by green house gases. Now you state otherwise, but now you claim that the effect on outgoing radiation is greater though you don't say how much as well as to provide any evidence. OK, again that leaves all the work for me. Then I will try evaluate the effects. If I am not mistaken then usually differences between TOA and the earth surface are taken as a measurement of the green house effect. Seems quite reasonable, let me try. I suppose all can agree that I use the numbers provided by the Trenberth et al. diagram I have included in my post #50. It says incoming at TOA 341, reflected 102 and absorbed at surface 161. As the reflected part is not affected by the green house effect it has to be taken out (subtracted from the TOA value) which leaves us with an effective TOA value of 239. So the relation is 239 / 161 = 1.48 Now we have surface radiation 396 Radiation leaving the atmosphere is at 239. Here the relation is 396 / 239 = 1.65. So, yes you are right, the green house house effect is stronger on outgoing radiation though I hardly assume the magnitude of the difference satisfied your strong wording. As to your comment about this being rather off topic I have to disagree for the reason that the whole argument of the lead article rests on the green house effect. So any discussion on this is quite on topic.
  12. Concerning my post #148: Seemingly no one contradicted my postulations concerning the behaviour of gases. So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases. This of cause leaves the green house gases off the hook when subjects like back-radiation are concerned, as it should be clear that the whole atmosphere plays a part in that game. That of cause, as Bibliovermis has correctly pointed out when he referred to Tyndall, points to the green house gasses speciality being able to trap (meaning store) heat and this way delay its further transmission. But considering this the most significant data with respect to global warming should be the specific thermal capacity of green house gases and I wonder why I can not recall it being mentioned. Someone willing to offer further information on this?
  13. h-j-m "Seemingly no one contradicted my postulations concerning the behaviour of gases. So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases" I can see one more possibility, people gave up trying to explain if you don't even bother to check this two century old physics.
  14. #162: "So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases." You can say whatever you like; whether what you say is correct or not might matter to some. Look at these lecture notes for some further basics, including a model of how gas molecules absorb energy.
  15. #160: "If the diagram were improved it would show just how this 33K comes about." That would be called reinventing the wheel. Look here, particularly the paragraph beginning "If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth,"
  16. @damorbel: the graph serves its purpose. It is not misleading to anyone with any kind of base scientific knowledge. I'm sorry, but it really sound as if you're grasping at straws, here. The greenhouse effect is real, a fact the majority of climate change skeptics recognize. Heck, I even had skeptics here assure me that "no one disputes the greenhouse effect"...yet it seems that this is exactly what you're (unsuccessfully) attempting here. Is this really a wise tactic on your part?
  17. RE#148 h-j-m. Drawing on these texts [**] I’m going to attempt to answer this question: Why is CO2 a greenhouse gas? Introduction... Gas molecules whether they be CO2, N2, O2, CH4, CO, H2, He, Ar etc will all interact with light at specific frequencies. So for example if a single photon is absorbed by one of these gas molecules the absorption or emission of a photon will be accompanied by a change in the internal energy state of the molecule. This is a consequence of Quantum Mechanics that a molecule can only take on values drawn from a finite set of possible energy states. The distribution of which is determined by the structure of the molecule. The energy states involved in infrared absorption and emission are connected with displacement of the nuclei in the molecule, and take the form of vibrations or rotations. So how does the number of atoms in a molecule and its geometry effect this? The noble gases like He, and Ar are have one atom (monatomic) and have only electron transitions, so are not active in the infrared. And indeed QM calculations and lab experiments verify this. A molecule with two atoms (a diatomic molecule) eg CO, O2, N2 amongst others has a set of energy levels associated with the oscillation caused by pulling the nuclei apart and allowing them to spring back and forth. Now triatomic molecules (like CO2 or H2O) have an even richer set of vibrations and rotations, especially if their equilibrium state is bent rather than linear. What specifically then, makes one type of gas molecule more infrared active than another... For a molecule to be a good infrared absorber and emitter, it is not enough that it have transitions whose energy corresponds to the infrared spectrum. In order for a photon to be absorbed or emitted, the associated molecular motions must also couple strongly to the electromagnetic field. You can classically think of the infrared light as providing a large scale fluctuating electromagnetic field which alters the environment in which the molecule finds itself in, and, exerts a force on the constituent parts of the molecule. This force displaces the nuclei and electron cloud, and excites vibration or rotation. The strongest interaction that will happen between an electromagnetic field and a particle is one where the particle has a net charge. A charged particle will experience a net force when subjected to an electric field, which will cause the particle to accelerate. In relation to Earth's atmosphere... Ions are extremely rare in the atmosphere. Thus molecules involved in determining a planet’s energy balance are almost invariably electrically neutral. So where does this leave us? Ok we have now elimated charged particles…so what’s the next best physical property of a molecule that will make it a strong infrared active one? Why molecules that have a dipole moment! (This is when we have a disproportionate part of a molecule’s negatively charged electron cloud bunched up on one side, while a compensating excess of positive charged nuclei are at the other side.) Does our atmosphere have a molecule that fits this criteria? Yes! Good old CO2! CO2 is a linear molecule with the two oxygens symmetrically lying about the central carbon. Whilst a uniform stretch of such a molecule does not create a dipole moment, a vibrational mode which displaces the central atom from one side to the other does. Addionally, the bending modes of CO2 have a fluctuating dipole moment, which can in turn be further influenced by rotation. Modes of this sort make CO2 a very good greenhouse gas. Here you might ask, but the atmosphere is full of O2 and N2 and there is only ppm concentrations of CO2? Many common atmospheric molecules have no dipole moment in their unperturbed equilibrium state. Such nonpolar molecules can nonetheless couple strongly to the electromagnetic field. They do so because vibration and rotation can lead to a dipole moment through distortion of the equilibrium positions of the electron cloud and the nucleii. Diatomic molecules made of two identical atoms, do not acquire a dipole moment under the action of either rotation or stretching. Symmetric diatomic molecules, such as N2, O2 and H2 in fact have plenty of rotational and vibrational transitions that are in the infrared range. However, because the associated molecular distortions have no dipole moment, these gases are essentially transparent to infrared unless they are strongly perturbed by frequent collisions. This is why N2 and O2, the most common gases in Earth’s atmosphere do not contribute to Earth’s greenhouse effect. However, it is important to recognize that situations exist in which diatomic molecules become good greenhouse gases are in fact quite common in planetary atmospheres. When there are frequent collisions, such as on planets with high density atmospheres like Titan and on all the giant planets, diatomic molecules will acquire enough of a dipole moment during the time collisions that are taking place ,and the electromagnetic field can indeed interact with their transitions quite strongly. This makes N2 and H2 the most important greenhouse gases on Titan, and H2 a very important greenhouse gas on all the gas giant planets. I don't think I even scratched the surface, but hooray for physics! [**] Principles of Planetary Climate, R. T. Pierrehumbert Molecular Quantum Mechanics P. W. Atkins (Author), R. S. Friedman An Introduction to Statistical Thermodynamics. T. Hill
  18. Wow, yocta, that was the best explanation I've ever read! Thanks!
  19. addendum to yocta @167 Its also true that the asymmetric isotopic variants of O2 and N2 absorb ever-so slightly in the IR: N14-N15 for example. This is because the stretching vibration becomes ever so slightly asymmetric because of the differing weights of the two nuclei. Because the dipole moment change is so small, and the proportion of isotopes so small, and the frequencies at which these vibrations occur is outside the range of "earthlight" their contribution to the GHE is effectively zero. Nevertheless there was one contributor to this site trying to argue the case a few months back :-(
  20. "Wow, yocta" Double-plus good job! Here are some illustrations of the CO2 molecule's vibrational modes.
  21. h-j-m @161 Your calculations are not correct because they assume that scattering of incoming UV-visible EM and outgoing IR are done by the same molecules in the atmosphere. The largest contributor to scattering UV-visible light is, in fact, Ozone (O3) which is contributing a substantial proportion of your 1.48 figure. You are, in effect, comparing apples with oranges.
  22. yocta, in my post #148 my question was "what specifically makes green house gases so special". Which means: What effects do green house gases produce that other gases don't? I am sorry and apologize if my initial phrasing led to any misunderstanding. When you state at the beginning of your post you are going to answer the question "Why is CO2 a greenhouse gas?" clearly indicates some sort of misunderstanding must have taken place.
  23. Phil, my calculations assume nothing except what I wrote they assume which is 1. the assumption that differences between TOA and surface provide a measurement for the green house effect and 2. that the numbers from Trenberth's diagram are reliably correct. Following your argument the first assumption should be incorrect but then I am the wrong man to point your critique at.
  24. h-j-m, greenhouse gases absorb wavelengths of radiation that are plentifully emitted by the Earth but only weakly emitted by the Sun, thereby acting as a partially closed valve that traps energy below the top of the atmosphere.
  25. h-j-m @173 Your original point was, as I paraphrased at @152: Since GHG's absorb in the visible (as well as the infra-red), doesn't increasing the concentration mean that the earth receives less energy because the subsequent emission of that radiation scatters some of it into space - back radiation on incoming EM which thus goes into space. To answer to this in @161 you derived two numbers that measured the total visible absorption by the atmosphere of incoming EM by all gases in the atmosphere and the absorption of outgoing IR radiation by only GHGs. Since the numbers the first number was slightly smaller than the first, you then concluded that the magnitude of the difference was small: [Quote from @161] So, yes you are right, the green house house effect is stronger on outgoing radiation though I hardly assume the magnitude of the difference satisfied your strong wording. But the "magnitude of the difference" is not valid because you are not comparing like for like.

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