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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 1301 to 1325 out of 1393:

  1. @1297 Yogi, When I talked about the OLR graph in #1290, I said "the radiation coming from the surface", not radiation coming from CO2 near the surface. When you are looking down in the 600-750 band, you are seeing the emission from CO2 at the level near the detector. Using a standard lapse rate of 6.5 degrees, at 3 km, the brightness temperature should be about 19.5 lower than the surface temperature, so the dip will be relatively small. Using the standard atmosphere at 3km looking down, that is about right (it'll be clearer if you turn off water vapour, which tends to distort the top of the spectra). The closer you get to the surface, the less temperature contrast between the surface and the air temperature near the "instrument", and so the dip will be smaller. Now for clouds it appears to radiate as a black body. If you are on the surface looking up, you are effectively seeing the bottom, and if you are using heavy cloud/rain, the cumulus base is at 0.66km. The temperature on the surface is 288K, so lapse rate of 6 degree puts you at 284k at the base. that looks about right from the spectra. If you look closely at the 600-750 band you'll see a slight bump, since there you are actually detaching radiation from CO2 near the ground, which is warmer. When you are looking down at 20km, having cloud is effectively like having a surface that is cooler. If you compare the case without cloud, and altostratus at cloud top at 3km, you'll expect a change in brightness temperature in the 800-1000 band by about 20 degrees, and that is exactly what you'll get.
  2. 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." But the surface is at 287.7K so must be looking at something colder further up. "The closer you get to the surface, the less temperature contrast between the surface and the air temperature near the "instrument", and so the dip will be smaller." #1301 The closer you get to the surface the less absorption there is from the 600-750 band, and the majority of the effect is above 3km.
  3. So what happens in a desert at night, does the CO2 back-radiation turn off when the sun goes down ? and what is warming the ground up quicker through the next morning, the Sun or the back-radiation ?
  4. YOGI (i) Why choose a desert rather than say a rain forest? (ii) the sun of course, the back radiation is only there because GHGs in the atmosphere absorbs IR radiated from the surface that is heated by absorbing largely visible and UV light from the sun. N.B. the surface doesn't stop radiating IR when the sun goes down, as it has considerable thermal inertia.
  5. "(i) Why choose a desert rather than say a rain forest?" its drier so has more temperature range. (ii) has considerable thermal inertia. yes, but the desert cools rapidly at night.
  6. YOGI Yes, but *why* does dryness mean there is a bigger temperature range?
  7. Now if I had a greenhouse with glass that was coated with a highly IR reflective layer, it should heat up considerably slower than one with panes made from a material which IR can pass through, according to this flow chart:
  8. Dikran Marsupial moisture moderates extremes.
  9. YOGI Perhaps it would be better if you finished discussing the previous example you raised before initiating another? So *why* does moisture moderate extremes?
  10. Earth`s average surface temperature is maintained by Ocean heat not 33C of back-radiation.
  11. YOGI Please answer my question, why does moisture moderate extremes (explaining how the dryness of a desert means there is a greater temperature range)?
  12. By slowing the loss of heat at night (unless clouds/air that are warmer move in, then that can radiate and warm a cooler surface below), and in daytime by reducing sunlight, and increasing convection due to the low density of water vapour.
  13. YOGI O.K. so *how* does water vapour slow the loss of heat at night (we can move onto the day later)?
  14. Insulation, it cant warm the ground though unless its warmer than the ground.
  15. YOGI O.K. so how does it insulate the ground? IR is radiated from the surface, then what happens to it?
  16. If I take my cup of tea out in the cold night air it will cool quicker. Same for the ground, but if there is more water vapour above it will absorb IR from the ground, warm up, and reduce the rate at which the ground can then radiate.
  17. YOGI O.K. so the water vapour absorbs the IR radiated from the ground and then warms up. What happens to the heat energy that the water vapour has acquired in this way? n.b. insulators do not work by reducing the rate at which something radiates photons of IR (that depends on its temperature)
  18. YOGI 1302, Our discussion up to post 1290 is related to the figures in the article. If you read the caption that came with the figure, you'll notice that 265K, instead of 287K, will be the more sensible surface temperature. "The closer you get to the surface the less absorption there is from the 600-750 band, and the majority of the effect is above 3km." No. Absorption increases with density. Since the density is the greatest near the surface, the absorption is the greatest there too.
  19. IanC#1318 So from the IR intensity Earth looks a lot colder from 20km up than it really is on the surface. Did you like my greenhouse in #1307 ?
  20. IanC can I suggest we leave YOGIs greenhouse until we get to the conclusion of the discussion of why the desert cools rapidly at night. We won't make any progress in this discussion if it keeps branching out onto new thought experiments before the earlier ones have been resolved.
  21. Dikran Marsupial "O.K. so the water vapour absorbs the IR radiated from the ground and then warms up. What happens to the heat energy that the water vapour has acquired in this way?" It will want to radiate it towards somewhere colder. But while it is warmer, it will mean the ground will lose heat slower, remember my cup of tea ? ~ insulation. "insulators do not work by reducing the rate at which something radiates photons of IR" Paint your roof white and it will do exactly that.
  22. YOGI wrote "It will want to radiate it towards somewhere colder." O.K. so when a molecule of water vapour in the atmosphere emits a photon of IR, how does it know not to emit it in the direction of a warmer body?
  23. #1322 ok there may an exchange but it cant get cool off in the direction of the warmer body, as the warmer body is warming it faster than it can cool off.
  24. YOGI O.K., so you agree that the water vapour re-radiates some IR photons back to the surface. Would you agree that these back-radiated IR photons cause the surface to be warmer than it would be if those molecules of water vapour were not there?
  25. "Insulation, it cant warm the ground though unless its warmer than the ground." The "insulation" effect is radiative. A quick look at the thermal properties of moist air (conductivity, heat capacity) cf dry air, would tell you that conductive insulation isnt at work. Do the math. As to your "greenhouse". Incoming radiation from sun isnt in IR range (that is whole point) so no, it would heat up slower. If you have really read the SoD articles explaining the science here, you are showing little evidence of it.

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