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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.



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Comments 1276 to 1300 out of 1393:

  1. muoncounter I`m more interested in seeing how much changes in clouds and humidity follow changes in global temperature since 1985. see the graphs in the Clouds + Climate section. I cant see any evidence for positive feedbacks on water vapour/clouds either.
  2. YOGI - You might be interested in looking at the difference between relative and absolute humidity. A certain relative humidity is required to form clouds (data on cloud amounts don't show a huge amount of change with temperature - 'tho what kind of cloud is going to be very important). Clouds, through precipitation, form an upper limit on the amount of relative humidity - past a certain relative humidity it simply rains or snows. However, as the air warms, the absolute humidity required to give a particular relative humidity increases - warmer air can hold more total water vapor. Trenberth estimates that we've seen an increase of ~4% to the total amount of atmospheric water vapor since ~1975, or roughly (if I recall correctly) the equivalent of Lake Erie. And all that water vapor acts in feedback as a greenhouse gas.
  3. YOGI, what you're looking for is probably right here, in rebuttal to our star of the month, Lord Viscount Ubermensch Protector of the Realm and the Cure for Cancer, Christopher Monckton.
  4. scaddenp, I meant that CO2 and water overlap the same absorption band at 600 and 750 cm-1 so some of that effect there must be partly due to the water vapour. Water vapour is 1-4% lower down, so if we say 2%, thats 51 times the amount of CO2, and the the paper you linked says 75.5% for clouds and WV, and a mean of 17.5 for CO2. Which imply volume for volume, CO2 has about 12 times the warming potential of water vapour (from those figures).
  5. KR: "data on cloud amounts don't show a huge amount of change with temperature" 4% "Trenberth estimates that we've seen an increase of ~4% to the total amount of atmospheric water vapor since ~1975"
  6. Discussion of cloud and humidity does not belong here. DSL has given you the link to the correct thread - and to what is wrong with the data in your links. You will linking to science instead of misinformation sites a better idea. As to water overlap, the real code (referenced) integrates over the full vertical profile of the atmosphere. You cannot draw accurate conclusions from simplistic constructions.
  7. scaddenp*misinformation sites * is there something wrong with NOAA and ISCCP data ? And how do I know that the data in graph at the head of this article is good data ? The two plots show that in the 600 and 750 cm-1 band, 265Mw radiate downwards, but only 225Mw upwards. How is that possible ?
  8. "is there something wrong with NOAA and ISCCP data ?" No, only with Humlum's representation of it. Read their website instead. (And put "humlum" into the search box here for more examples of stuff from him). And if you look closely, you will see the textbook that the data came from. And below it, the link to Science of Doom which goes into the science behind your questions in considerable detail over a 10 part series. I highly recommend you to look at it. More detail than a blog commentary can give plus links to the textbooks and papers.
  9. The S.O.D. article does answer my question, it just says.. "The atmosphere, once heated up, radiates equally in all directions. Some of this is downward."
  10. typo... The S.O.D. article does NOT answer my question
  11. sorry, another typo, that should have been.. The two plots show that in the 600 to 750 cm-1 band, 265K radiate downwards, but only 225K upwards. How is that possible ?
  12. yogi#1276: "I`m more interested in ..." Sorry, I must have misread your comments here and here. An abrupt change in interest is usually a sign that the given answer was accepted and we're moving on. So the 'water is much more abundant than CO2 and should thus be much more of a GHG' issue is settled. Especially since it's off topic for this thread. Look here.
  13. Yogi 1286, The temperature lines in the graph indicates the emission spectrum of a black body at that temperature. By comparing the irradiance to these curves you can get a sense of which part of the atmosphere the emission is from. The reason why the emission in the downwelling is at 265K is because it will be from CO2 that is relatively close to the surface. Since it is measured in the arctic it sounds about right. On the other hand in the graph at 20km looking down, it will be from CO2 that is fin the upper troposphere/stratosphere, explaining the lower brightness temperature.
  14. IanC, Your lower CO2 should radiate at 265K upwards too apparently. And on the downward view, on regions free of absorption bands, the OLR is the same temp as the DLR in the 600-750 band on the upward view. How can that be ?
  15. Yogi, "Your lower CO2 should radiate at 265K upwards too apparently." Just to make sure I understand you, do you mean the CO2 near the surface radiates at 265K upwards? If this is what you mean, you'll need to remember that the CO2 higher up absorbs at the same wavelength, and hence masks the signal from the surface CO2. As for your question, in the OLR (20km looking down) graph and at regions without atmospheric absoprtion, you will be seeing radiation coming from the surface, which is ~265K. For the DLR in the 600-750cm-1 band, the radiation is coming from CO2 near the surface, which is again close to 265K.
  16. Yogi, You are having trouble because you are applying a grossly oversimplified model to a complex situation. More specifically, you are treating the atmosphere as one, homogeneous slab, simplifying emissions to "up and down," and honing in on particular wavelengths. In reality, the atmosphere is a body of continuously varying density and makeup (for example, CO2 concentrations are relatively, proportionally consistent throughout the atmosphere, but water is not). As such, the radiation at 1 km differs from 2 km differs from 10 km or 1.5 km or 1.25 km. One cannot simply treat the entire thing as a solid, homogeneous block. Radiation is emitted in all directions, so you must consider geometry, which affects how much goes up and down, and how much atmosphere each particular photon must navigate before being observed, absorbed, or escaping to space. So at every conceivable altitude the emissions are affected by the density, temperature and makeup of the atmosphere at that altitude. In addition, between you as an observer (whether on the surface of the earth, up in space, or in a weather balloon in between) and the emitting layer under investigation, emissions may be absorbed or not by intervening layers (again, dependent on density and makeup), so what you see has some radiation filtered by intervening layers, some passing through, and some radiation added to it by intervening layers. To get a hint at some of the complexity involved, play around with this page, which uses a complex computer program to band by band, altitude by altitude, go through computing what is probably happening in order to project the probable observed emission spectrum given an observation point and specific atmospheric conditions.
  17. OK in the absorption bands, ingoing and outgoing IR is absorbed, radio telescopes can`t see out through them. Quote; your article: "In the "infrared window" of the atmosphere, the atmosphere is transparent. In these frequencies, no radiation is absorbed, no radiation is emitted, and here is where IR telescopes and microwave sounding satellites can look out to space, and down to the surface, respectively." IR is emitted in the "infrared window",the 20kn downward view shows it emitting 268K in the window (away from the absorption bands). And the atmosphere emits IR brightly from 7.5 to 40 microns: "The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter). So the best view of the infrared universe, from ground based telescopes, are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground."
  18. YOGI#1292 IR telescopes not radio telescopes
  19. Sphaerica#1291 Interesting so in the sub-Arctic it has no effect on OLR when surface temp`s are -30C or lower.
  20. Yogi, The graphs in the SkS article is data from arctic, where water vapour content is relatively low. Between 8-13μm water does aborb IR slightly, so if you are in a moist atmosphere it will alter the picture. Below is a comparison between DLR at arctic vs tropics. You can see from the curve for the tropics there is actually significant emission between 8-13 μm due to water vapour. You said: And the atmosphere emits IR brightly from 7.5 to 40 microns Now brightness is relative, so it obviously depends on the application. What is dim in atmospheric science may not be dim for astronomy. From your source, they classify the emission at 3-4 micron as low. If you extrapolate the black body curves all the way out to 4 micron, or 2500cm-1, you'll see that the radiance between 8-13 micron is in fact quite high in the case of a tropical atmosphere. Infrared windows do exist, but that depends on how dry the atmosphere is, and what application you have in mind.
  21. 1294, Yogi, in the sub-Arctic it has no effect on OLR when surface temp`s are -30C or lower.
    I have no idea what this means. What is the "it" in "it has no effect..."? Greenhouse gases? A basic element of radiation, as evidenced by Stefan-Boltzmann, is that temperature plays a huge role. So for a few months of the year, temperatures are around 243˚K, radiation is only 198 W/m2. Conversely, when the sun is above the horizon, temperatures are around 273˚K and radiation is about 315 W/m2. While there is a huge difference between summer and winter, and between poles and tropics, I don't think anything qualifies for the statement "has no effect." But the spherical nature of the planet influences climate in a variety of other ways, as well, such as the spread of incoming sunlight over a larger area near the poles, the circulation of heat and moisture from the tropics through advection in the atmosphere, and other factors. It's not simple. It's not impossible to untangle, but not simple.
  22. IanC#1290 "As for your question, in the OLR (20km looking down) graph and at regions without atmospheric absoprtion, you will be seeing radiation coming from the surface, which is ~265K. For the DLR in the 600-750cm-1 band, the radiation is coming from CO2 near the surface, which is again close to 265K." But if I look down from 3km, the 600-750cm-1 band barely shows up, and looking down from 1km its not there at all, that does agree not with most of this being due to near surface CO2. And looking up from the surface with heavy cloud/rain, all bands are radiating, but when looking down from 20km, the difference between clouds/no clouds is minor in comparison.
  23. Sphaerica#1296 "I have no idea what this means. What is the "it" in "it has no effect..."? Greenhouse gases?" I`m saying on the 20km looking down, you cannot see the 600-750 band any more when the sub-Arctic reaches -30C. But more interesting is that OLR from 20km is always the same regardless of location and temperature on this model:
  24. #1298..But more interesting is that OLR (in the 600-750 band) from 20km is always the same regardless of location and temperature on this model:
  25. 1299, Yogi, Why is that, do you think?

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