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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 1251 to 1275 out of 1393:

  1. TOP - SoD means not "Subject of Discussion", but a reference to The Science of Doom, a physics oriented website that I'm certain you have been given links to. In particular, I would like to refer you to On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009), a discussion of thermodynamics related to the misconceptions you have demonstrated here. I'm sorry if that was not clear - please follow the links.
  2. 1250 Tom No mysticism. No difficulty explaining the RGHG. 23&npsp;W/m2 added to the atmosphere due to long wave radiation from the surface raises the heat content of the atmosphere. It is far more mystical to me why a gas is shown radiating energy in one direction, down, where it should be shown radiating upwards as well.
  3. 1252, TOP,
    ...why a gas is shown radiating energy in one direction, down, where it should be shown radiating upwards as well.
    What is this a reference to? What are you talking about?
  4. TOP @1252, if you think it is possible to construct a model of the greenhouse effect using net energy flows only, by all means go ahead. For myself, I notice that the people who have dedicated their lives to understanding the green house effect require all of the laws of radiative physics plus a number of others beside. I am not so arrogant as to think that I can do better with less. As it stands, the proof, and indeed the only relevant proof that your truncated "net energy flow" only physics is not mysticism is the ability to predict from that truncated physics the effects of increasing green house gases, etc. Call me when you have your working model.
  5. I've been hesitant to add to the fray, but here goes... Trenberth's diagram (just a recent variant of similar diagrams that have been around for decades) represents global mean net fluxes, and any any interpretation that treats them as absolute fluxes is doomed to failure. All the associated energy fluxes vary in magnitude in space and time, and for a full understanding of global climate you have to look at the fluxes in three dimensions. That's why people make 3-D climate models. As Tom, Sphaerica, and other regulars have pointed out repeatedly: all the simple models (be they descriptive or numerical) while useful for helping explain parts of the system are very limited in looking at the details. Unfortunately, the discussion by people such as TOP and RW1 falls into a couple of unproductive patterns: 1) misunderstandings of the simple models (Trenberth's diagram is one such example of a simple, descriptive model with some numbers attached), in particular what the simplifications are and why they are made. For example, TOP's statement in #1252 about the diagram only showing "radiating energy in one direction", when it is only intended to express the average net result (e.g., amount of IR absorbed at the surface, from the atmosphere, in the case of the 333 W/m^2 back radiation). 2) basic misunderstandings of physics, such as the difference between radiative energy transfer and convective heat transfer, or how energy is transferred as "latent heat". In particular, both of them seem to be missing the concept of absorption of radiation as the driver for atmospheric temperature change, rather than the flux of radiation. Thus, the difference between absorption and emission drives the energy balance (with the added requirement that convective fluxes be considered, too). At any single point, it is the flux divergence that controls heating/cooling rates, not the absolute fluxes themselves. Tom, Sphaerica, other regulars: I strongly disagree with you that TOP and RW1 have a lot to learn. Unfortunately, I think the major problem is that they have a lot of just plain wrong stuff in their heads that they have to unlearn first. Their obstinate view that they have some unrecognized gift to knowledge is a major hurdle. Unless they are willing to sit back and say to themselves "let's start from scratch" and begin to learn physics and climatology from a blank slate, then pointing out their wrongheadedness is futile (except in the goal of making sure that the casual reader sees their foolishness). RW1:, TOP: in comments 1143 and 1145 I challenged Fred Staples to explain some modelling results that relate to how radiation transfer really works. Fred seems to have gone missing completely - do either of you wish to take up the challenge?
  6. "no chance for the 2nd law naysayers to squawk" Thing is, real scientists do not give a hoot about 2nd law naysayers. They really don't. 2nd law naysayers are either so hopelessely confused as to be irrelevant, or, in the case of G&T, trying to have fun by exploiting the hopelessely confused. All of it has no bearing whatsoever on the real science. Has G&T led to proclamation of a revolution in atmospheric physics by AGU, NSF, the major publications? No, it met complete indifference. Only a few involved in fighting climate disinformation have taken up the task of disentagnling the sad confusion generated by this useless piece.
  7. @ Bob Loblaw It was about a year ago that I counseled RW1 that, in order to better be able to combat & overturn climate science with the ideas learned at the knee of George White, he must first start with a clean slate & learn climate science from the basics up. Then and only then would he be able to apply himself to overturning it.
  8. The 2nd reference, "Physical Principles of Meteorology and Environmental Physics", under Further Reading is not free. The Table of Contents and Chapter 1 can apparently be freely downloaded for free.
  9. This is nonsense. There are no CO2 absorption bands at 6-7.5μm :
  10. YOGI - Note the log scale on the X axis in that graph. There is a CO2 absorption band around 4.5μm. [Source]
  11. Yogi, No where in the article do the numbers 6-7.5 μm comes up. The closest will be "Carbon dioxide is the major contributor for emission seen between between about 600 and 750 cm-1". If this is what you are referring to, notice that 600 and 750 cm-1 is in fact the wavenumber, and it corresponds to ~13-16 μm in wavelength (see fig 1a). The absorption spectrum you've linked to indicates that CO2 indeed absorbs around these wavelegnths!
  12. Yes and there is also a nitrous oxide band at 4.5μm which has 298 times the greenhouse effect of CO2. This does not detract from the point that 6.0-7.5μm absorption displayed: cannot be due to CO2 as it does not absorb that band, period.
  13. @IanC, 1000 cm-1 is 10μm. You can also confirm that by looking at the ozone spike at 1000 cm-1 ~ 10μm.
  14. @IanC, no I`ll dtract that, you are correct, the scales run in the opposite directions
  15. YOGI: Wavenumber Wavelength Note the top and bottom X-axes.
  16. Ugh. The 2nd Law thread does things to people.
  17. 600-750 cm-1 does overlap with a water vapour band too. how does one decide the relative effects of each within that band ?
  18. Yogi - you do it like this
  19. So if water vapour is 50% of the greenhouse effect at 0.4% of the atmosphere content, and CO2 is 20% of the greenhouse effect at 0.00039% of the atmosphere content, CO2 would have to have 410 times the greenhouse effect of water vapour. Is that correct ??
  20. more like 2% and 0.039% so that would make CO2 50 times more powerful as a green house gas ?
  21. Yogi, please read the paper that you were pointed to. A less technical account is at realclimate.
  22. YOGI#1270: What makes you think that greenhouse gas 'power' (actually 'global warming potential') is proportional to atmospheric concentration? Where I live, a lot of that water vapor winds up on my car every morning. That might give you a clue about what these numbers mean.
  23. 1266 - DSL. Just be thankful there's no "Advanced" tab, given the law: (confusion of 'skeptics') = α(depth of physics)4
  24. Brilliant les!! That was the equation missing so that one could make sense of all this mess. I understand now...
  25. At the risk of deletion, it could be called the Stuffing-Doltzmann Law or the Law of Diminishing Clarity.

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