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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 1 to 25 out of 1516:

  1. I'm not a scientist so forgive me if I'm off topic There is a lay explanation of the physics underlying climate alarmism. KE Research, a German public policy consultancy firm, prepared the report based on interviews and editing assistance from noted German theoretical physicists Ralf D. Tscheuschner & Gerhard Gerlich, authors of the peer-reviewed paper Falsification of the Atmospheric CO2 Greenhouse Effects within the Frame of Physics, and numerous other climatologists, physicists, and scientists at Conclusions of the report include: The terms “greenhouse effect” and “greenhouse gas” are misnomers and obstruct understanding of the real world. Earth has a natural “cooling system”. If the planet warms, it will automatically raise its cooling power. An increase of earth temperatures is only achievable if the heating power is stepped up: first to “load” matter with more energy (i.e. to raise temperatures) and then to compensate for the increasing cooling, which results from the increase of IR radiation into space. CO2 and other IR-active gases cannot supply any additional heating power to the earth. Therefore, they cannot be a cause of “global warming”. This fact alone disproves the greenhouse doctrine. The “natural greenhouse effect” (increase of earth temperatures by 33°C) is a myth. IR-active gases do not act “like a blanket” but rather “like a sunshade”. They keep a part of the solar energy away from the earth’s surface. IR-active gases cool the earth: 70% of the entire cooling power originates from these molecules. Without these gases in the air, the surface and the air immediately above the ground would heat up more. The notion that a concentration increase of IR-active gases would impede earth’s cooling is impossible given the true mechanisms explained above. As a consequence the very foundation of the “Green Tower of Climate Dogma” crumbles. Computer models alleging to forecast warming based on “greenhouse effects” are worthless, and any speculation about the “impact of climate change” accordingly dispensable. Since the greenhouse hypothesis has been disproven by the laws of physics, it is only a matter of time until the truth becomes public opinion. Does anyone have any comments on the contents of that report?
  2. val majkus, you're not at all "off topic" -- this thread is basically explaining why Gerlich and Tscheuschner are completely wrong. The report you cite mostly just repeats the same errors and misinformation from G & T's original claims.
  3. Val, the responses to those incorrect claims are in this page above your comment, including the items referenced by the post. If you have questions after reading those, then please do ask here. But please read those first.
  4. "noted German theoretical physicists" - noted for an imaginary second law? Note also posts on Science of doom and Halpern et al above for a formal response.
  5. Fascinating to see Roy Spencer getting annoyed at denialists who won't allow radiation from the atmosphere to warm the surface of the earth. Click...
  6. To whom it may concern, "This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere." Are you stating that the primary method of transferring energy from the surface to the Troposphere is via radiation absorption rather than via conduction? When you say 'colder', are you referring to a region or a temperature?
  7. To whom it may concern, "This absorption is due to trace gases which make up only a very small part of the atmosphere." So are you saying that IR from the surface heats the GHG's and then they transfer the energy to the primary gases N2 and O2? If so, how is that transfer made? Conduction from molecules colliding?
    Response: Yep.
  8. To whom it may concern, Forgive me, but this post is ripe with errors, from my understanding of physics. Would you prefer I ask questions regarding the supposed errors, or would you prefer I explain my reasoning outright?
    Response: Whichever is shorter.
  9. Regarding molecular collisions and heat distributions: At surface temps and pressure each air molecule (CO2, O2, N2, argon, etc.) collides with another molecule roughly one billion times per second (thanks, Ned). The relaxation time for an energized CO2 molecule is 100ns or more, depending on the vibrational state. That means that an IR energized CO2 molecule has on average a minimum of 100 collisions with other molecules before it has a chance to emit IR. CO2 _will_ maintain thermal equilibrium with the rest of the air mass, whether the air mass as a whole is cooling or heating by IR. (Or conduction, convection, latent heat changes, etc.)
  10. To whom it may concern, Well put, thank you :)
  11. So is CO2 a good reflector while a poor absorber or is it a good absorber but a poor reflector ? It seemed to me, that the 2nd law put the final nail in the coffin of this (CO2) debate. Yet, it's still alive. How is it possible ? The theory that man made CO2 is the cause of Global Warming has had so meny holes punched through it - I don't understand how it's survived this long. It's been de-bunked by simple sciance on so meny levels, it should have been dead long ago. All the while, people far smarter than myself keep pushing it along. What is it that I'am not understanding? I'm a simple man, thats probably a bit more inquisitive than most, looking for answers. Which is what lead me here.
    Response: Please don't post the same comment repeatedly. I deleted your second one.
  12. Re: KnuckleDragger (11) I'm glad you're inquisitive and I'm glad you're here.
    "It seemed to me, that the 2nd law put the final nail in the coffin of this (CO2) debate. Yet, it's still alive. How is it possible ?"
    This is a common objection from people who do not understand the greenhouse effect (or the 2nd law of thermodynamics, for that matter). A simple understanding of the greenhouse effect:
    Longwave radiation from the earth’s surface is absorbed by many trace gases, including water vapor and CO2. The absorption causes these gases to heat up and energy is radiated back out – both up and down. The upward radiation is effectively “no change”. The downward radiation adds to the energy received from the sun and heats up the surface of the earth more than if this downward radiation did not occur.
    The 2nd "law", simply put:
    "Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature"
    What this means is this:
    No net energy can flow from a cold body to a hot body. In the case of the real “greenhouse” effect and the real 2nd law of thermodynamics, net energy is flowing from the earth to the atmosphere. But this doesn’t mean no energy can flow from the colder atmosphere to the warmer ground."
    It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction. Sources: Here and here.
    "The theory that man made CO2 is the cause of Global Warming has had so meny holes punched through it - I don't understand how it's survived this long...What is it that I'am not understanding?"
    The first part of your statement is wrong on every level, but I can understand the confusion you must feel. If it's so wrong, why does every scientific body in the world support it? Why does Shell Oil, of all things, support it? KnuckleDragger, I'm a simple man too. If someone is telling you that CO2 doesn't warm the Earth or that it's the sun or that it's cooling, or it's a natural cycle, then you have 2 possible answers:
    1. They don't understand quite a bit about science, physics, the greenhouse effect or pay attention to developments in the natural world... OR... 2. They're lying to you...
    The greenhouse effect is quite well understood. Here's a quick backgrounder on the GHE, CO2 and AGW (the important bit is the response to Question 1 & the 8 steps outlined). Basically, it come down to this: No-one has been able to come up with a physics-based alternative to the observed & predicted effects of CO2 and GHG's that explains what we can see and measure that ALSO explains why CO2 derived from fossil fuels DOESN'T act as a GHG. At this point, being an inquisitive man, you probably will have more questions. Feel free to ask; the kind people here will be glad to help you gain an objective understanding. The Yooper
  13. KnuckleDragger, to explicitly address your first question on absorption/reflection, thinking in this terms could be confusing. In everyday life we call reflection the "bouncing back" of light from a solid or liquid surface. In a gas there's no surface and it could be hard to understand how "reflection" may occur. I think it's easier to think in terms radiation absorption/emission. Quoting from the post: "Any substance that absorbs thermal radiation will also emit thermal radiation; [...]. The atmosphere absorbs thermal radiation because of the trace greenhouse gases, and also emits thermal radiation, in all directions." The backward emitted radiation is what you (and others) call reflection. So, the answer to your question is that CO2 at high concentration is a good absorber and reflector.
  14. KnuckleDragger, it's best to not use the term "reflect" at all, even as a convenient shorthand, because as Riccardo wrote it has a particular meaning that is different from absorption-emission. CO2 does effectively zero reflection at the wavelengths we are concerned with here, at CO2's concentrations in our atmosphere.
  15. If I get it right the greenhouse effect works as an insulator and diminishes the heat loss of the earth. So far I am unaware that any unidirectional insulator exists or is even possible. Therefore the greenhouse effect has also to diminish the incoming energy from the sun which heats the earth. As a portion of the incoming energy gets converted into forms of energy that are not radiative (kinetic, chemical, electric) that is not trivial the logical conclusion is that the incoming radiative energy needs to exceed it's outgoing counterpart. As incoming and outgoing radiation is (more or less) equally effected by the insulation it is quite hard to see how the result could be a warming of the earth.
  16. h-j-m, almost all incoming solar irradiance is at short wavelengths where the atmosphere is (mostly) transparent. In contrast, almost all outgoing emitted radiation is at longer wavelengths, portions of which are absorbed by CO2, water vapor, CH4, and other greenhouse gases. This difference between incoming and outgoing radiation is essential to understanding how the greenhouse effect works. If you want to know more, a good place to start is Science of Doom, which has an excellent series of posts explaining the fundamentals of the greenhouse effect.
  17. "So far I am unaware that any unidirectional insulator exists or is even possible." (sound of a vinyl record scratching to a halt) 1. Infrared radiation is emitted by the surface. 2. The radiation--or, rather, radiation at certain frequencies--eventually makes it into the stratosphere and is absorbed and emitted in all directions by molecules of CO2, H20, and CH4. 3. Some of the radiation is eventually emitted into space, because that's one direction. 4. Other directions include all versions of "sideways"--and perhaps right into another molecule of CO2. 5. Down is also another direction. The radiation eventually reaches the top of the atmosphere and is emitted into space (the only way it can leave). The visual, though not physical, analogy is the pinball machine. The atmosphere is a huge pinball machine, and GHG are bumpers (gravity has little effect in this machine). The more bumpers the machine has, the longer, on average, the ball takes to reach the boundary. Another analogy is the dam. The atmosphere is a dam. It doesn't block water, because the water eventually reaches the top and flows over, but when the water reaches the point of flowing over, the same amount of water that flows into the lake behind the dam equals the same amount of water that flows over the top. Yet there is the fact of the lake. And if we build the dam higher, then the lake gets deeper, but eventually the same amount of water will once again begin to flow over the top. We live in the lake. CO2 does not absorb UV radiation, so incoming solar radiation isn't slowed by it.
  18. Re #12 Daniel Bailey, as you say "No net energy can flow from a cold body to a hot body." And "It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction." It is this "net energy flow" from hotter place to colder one that means that it is losing energy in and cooling down, not just 2nd Law but 1st Law also. Just incase you thinking of saying that CO2 slows down the heat transfer rate like an insulator (which it isn't); you can have any insulator you like but it cannot reverse the direction of energy tranferring from a warm surface to colder one.
  19. Re damorbel: You have me a bit mystified; reading over my comment at 12 and then yours at 18 I fail to see what point you're trying to make. If you have one, please rephrase it so that my slow gray matter can understand it. Thanks!
  20. Damorbel #18: " can have any insulator you like but it cannot reverse the direction of energy tranferring from a warm surface to colder one." Which would only be relevant if there wasn't this thing called the Sun constantly transferring energy to the planet's surface. Decrease the rate at which that energy leaves the system (by adding greenhouse gases) and you get incoming energy + retained energy... which is obviously greater than incoming energy alone.
  21. Ned and DSL, Before writing my post I checked the irradiative composition of sunlight. I would advise you to do the same before posting a reply.
  22. h-j-m, only a tiny fraction of exoatmospheric solar irradiance is in the thermal infrared range. The vast majority of it is visible and near-IR. I have no idea what you think you're seeing, but if it differs from what I just said, then you're probably misunderstanding something.
  23. Ned, what I am seeing is this: Solar Spectrum Though I don't know what you are looking at.
  24. h-j-m, that graph doesn't even show the longwave infrared region -- it only goes to 2.5 micrometers. The wavelengths corresponding to emitted thermal radiation from the Earth are in the 8-14 micrometer range.
  25. Ah, perhaps you were confused by the label "infrared" on that graph? It's referring to the near-IR and shortwave IR range. Not the thermal part of the spectrum where the Earth and its atmosphere emit radiation.

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