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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 atmosphere of the Earth is less able to absorb shortwave radiation from the Sun than thermal radiation coming from the surface. The effect of this disparity is that thermal radiation escaping to space comes mostly from the cold upper atmosphere, while the surface is maintained at a substantially warmer temperature. This is called the "atmospheric greenhouse effect", and without it the Earth's surface would be much colder.

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)


Most participants in climate debates can agree that the atmosphere's capacity to interact with thermal radiation helps maintain the Earth's surface temperature at a livable level. The Earth's surface is about 33 degrees Celsius warmer than required to radiate back all the absorbed energy from the Sun. 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.

This absorption is due to trace gases which make up only a very small part of the atmosphere. Such gases are opaque to thermal radiation, and are called "greenhouse gases". The most important greenhouse gases on Earth are water vapor and carbon dioxide, with additional contributions from methane, nitrous oxide, ozone, and others. If the atmosphere was simply a dry mix of its major constituents, Oxygen and Nitrogen, the Earth would freeze over completely.

Observing the greenhouse effect in action

The simplest direct observation of the greenhouse effect at work is atmospheric backradiation. Any substance that absorbs thermal radiation will also emit thermal radiation; this is a consequence of Kirchoff's law. The atmosphere absorbs thermal radiation because of the trace greenhouse gases, and also emits thermal radiation, in all directions. This thermal emission can be measured from the surface and also from space. The surface of the Earth actually receives in total more radiation from the atmosphere than it does from the Sun.

The net flow of radiant heat is still upwards from the surface to the atmosphere, because the upwards thermal emission is greater than the downwards atmospheric backradiation. This is a simple consequence of the second law of thermodynamics. The magnitude of the net flow of heat is the difference between the radiant energy flowing in each direction. Because of the backradiation, the surface temperature and the upwards thermal radiation is much larger than if there was no greenhouse effect.

Atmospheric backradiation has been directly measured for over fifty years. The effects of greenhouse gases stand out clearly in modern measurements, which are able to show a complete spectrum.

IR spectrum at the North Pole
Figure 1. Coincident measurements of the infrared emission spectrum of the cloudfree atmosphere at (a) 20km looking downward over the Arctic ice sheet and (b) at the surface looking upwards. (Data courtesy of David Tobin, Space Science and Engineering Center, University of Wisconsin-Madison. Diagram courtesy of Grant Petty, from Petty 2006).

When you look down from aircraft at 20km altitude (Fig 1a), what is "seen" is the thermal radiation from Earth that gets out to that height. Some of that radiation comes from the surface. This is the parts of the spectrum that follow a line corresponding in the diagram to about 268K. Some of that radiation comes from high in the atmosphere, where it is much colder. This is the parts of the spectrum that follow a line of something like 225K. The bites taken out of the spectrum are in those bands where greenhouse gases absorb radiation from the surface, and so the radiation that eventually escapes to space is actually emitted high in the atmosphere.

When you look up from the surface (Fig 1b), what is "seen" is thermal backradiation from the atmosphere. In some frequencies, thermal radiation is blocked very efficiently, and the backradiation shows the temperature of the warm air right near the surface. 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.

The smooth dotted lines in the diagram labeled with temperatures are the curves for a simple blackbody radiating at that temperature. Water vapor has complex absorption spectrum, and it is not well mixed in the atmosphere. The emissions seen below 600 cm-1 are due to water vapor appearing at various altitudes. Carbon dioxide is the major contributor for emission seen between between about 600 and 750 cm-1. The patch of emission just above 1000 cm-1 is due to ozone.

The term "greenhouse"

The term "greenhouse" was coined for this atmospheric effect in the nineteenth century. A glass greenhouse and an atmospheric greenhouse both involve a physical barrier that blocks the flow of heat, leading to a warmer temperature below the barrier. The underlying physics is different, however. A glass greenhouse works primarily by blocking convection, and an atmospheric greenhouse works primarily by blocking thermal radiation, and so the comparison is not exact. This difference is well understood and explained in most introductions to the subject. Where confusion arises, it is usually the glasshouse that is improperly described, rather than the atmospheric greenhouse effect.

The enhanced greenhouse effect

The greenhouse effect itself has always been an important effect on Earth's climate, and it is essential for maintaining a livable environment. Without it, the surface would rapidly freeze.

The existence of a greenhouse effect itself should not be confused with changes to the greenhouse effect. Global warming in the modern era is being driven by increasing concentrations of greenhouse gases in the atmosphere, which leads to an enhanced greenhouse effect. This is covered in more detail as a separate argument: How do we know more CO2 is causing warming?

Intermediate rebuttal written by John Cook

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 1504:

  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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