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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.

References

Comments

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Comments 751 to 775 out of 846:

  1. For myself, Tom, I kinda like your box idea. It could be called a path lengthening device, increasing the number of photons in the system at a given time. Time is often the missing idea when considering GH effect. At equilibrium, energy in and energy out at TOA are equal. Increase IR opacity and energy out at TOA becomes less than in, until the surface temp increases to radiate enough out through the increased opacity. There is no violation of thermodynamics at all.
  2. damorbel - I didnt describe any experiment. I proposed that an experiment be designed such that normal understanding of physics and your understanding would calculate a different result. This is normal way to test scientific arguments. I asked if the experiment didn't go your way, whether you would be prepared to abandon your view and read the textbook. (ie, behave like a scientist). Got a yes/no? In fact, have you got an experiment that you think validates your views over mainstream physics?
  3. CBDunkerson750 You said: "In that both cause an area to be warmer due to redirected electromagnetic energy it isn't a matter of belief, but rather observed reality." Focusing a large radiative input to a specific point is NOT what proponents argue. Redirecting radiation is not a point of contention. What is disputed...a cold atmosphere warms the warmer earth. Google solar cooker. The principal works by redirecting radiation, (from a large area) to a small focal area. When facing the sun, the focal area (the small area) gets hot. When facing away from the sun the focal area (small area) gets cold. How can this be? Maybe you should understand what you proclaim to know.
  4. What is disputed...a cold atmosphere warms the warmer earth. No-one believes the cold atmosphere warms the warmer earth. The Greenhouse effect slows the rate at which heat escapes from the earth. Period.
  5. Just to make one thing clear. Photons from a 'hot' (5780K) source like the Sun can be refocussed at any distance (if the mirror is big enough) to recreate the surface temperature (5780K) of the source, but no higher. No amount of refocussing etc., etc., of photons from a cold (255K) source, like the upper troposphere, can produce a temperature higher than 255K... anywhere. It's the photons you see, they don't have enough energy. They may have all the power (W/m^2) you can imagine* but no amount of refocussing, adding together, accummulating in reflecting cavities etc., etc. will raise their energy. Or, in other words, increase the temperature of.... anything. That is what the 2nd law is all about. It why quantum theory holds sway. And why the 'GH' effect is not a radiation effect... from... GH gases. * Power is not energy. Power is W(atts); energy is J(oules). A surface emitting power has two options it can emit relatively few 'hot' photons to get power 'P'. Or it can emit a large number of low energy photons to get the same power 'P'. There is a lot of grief on this thread cause by contributors not distinguishing between power balance and energy balance by defining both as 'W/m^2', that is a mistake.
  6. Ryan I have read the entire thread, in case you've missed the numerous references to comments from November and December. You apparently have also missed the point about 'at the same frequency'; IR photons have considerably less energy than visible light photons. CBD's point here is quite valid, with or without the parabolic geometry (which is a mere artifact of your solar cooker analogy). You've not proved anything with it, except how much you are willing to argue for the sake of argument.
  7. Re #756 muoncounter That's exactly what #755 explains. In #750 CBD writes :- "In that both cause an area to be warmer due to redirected electromagnetic energy it isn't a matter of belief, but rather observed reality." No amount of redirecting, refocussing adding together etc.,etc., of photons can increase their energy: warmer the Earth's surface may well be but it is not a radiation effect*. *It is actually a gravity effect.
  8. CBDunkerson750 You said: "In that both cause an area to be warmer due to redirected electromagnetic energy it isn't a matter of belief, but rather observed reality." Focusing a large radiative input to a specific point is NOT what proponents argue. Redirecting radiation is not a point of contention. I say cold atmosphere radiation can not shorten terrestrial radiation, GHG proponents say otherwise. Google solar cooker. The principal works by redirecting and focusing, from a large hot area, radiation to a small target area. When facing the sun (hot),the focal area (the small area) gets very hot. When facing away from the sun (the cold atmosphere) the focal area (small area) gets cold. So the small target area looses energy via the large area to the cold atmosphere. See this process (unlike the magic box) abides the 2nd law, Hot to Cold. The atmospheric LW does NOT accumulate within the target area. The target area looses energy to the radiating atmosphere. GHG physics concludes conversely.
  9. mods - I really do think that damorbel, and particularly 755 & 757 violates the comments policy. Although not exactly an ad hominem attack, it is an attack on the work of many, many physicists through the ages. On behalf of those upon who's shoulders most of us stand - bun on who's feet damorbel seems to be stepping, I pray you, make it stop!
  10. Ugh, LJ -- not the alleged solar cooker evidence. It is proof of nothing. The reflective surface focuses incoming radiation. That radiation could be coming from the atmosphere. Your argument is that we can't differentiate between radiative transfer and conduction/convective transfer. In other words, why doesn't the solar cooker cool to near absolute zero when pointed at the open night sky? Something must be heating it. (expected response) Ok, then put it in a clear vacuum box and point it at the night sky. With no internal heat source, and no way to receive energy (according to your physical model), it should cool down at its rate of emission (and internal conduction) until it reaches near absolute zero. I'll wager that it won't, though. I'll wager that it cools more slowly, because even if we warmed it up to 50C before we put it in the box it's still receiving infrared radiation from the atmosphere. Photons can't choose their paths.
  11. Re #760 DSL you wrote:- "Ok, then put it in a clear vacuum box and point it at the night sky. With no internal heat source, and no way to receive energy (according to your physical model), it should cool down at its rate of emission (and internal conduction) until it reaches near absolute zero" There is a comparable effect known as 'clear sky' frost; the condition when surface frost appears e.g. on car windshields, even when the air teperature is not below freezing. In this situation radiation through the intervention of GHGs is exchanged with the higher levels of the troposphere which are below freezing. Some of the xchange might well be with deep space, but only a small part; which of course cannot easily be separated out.
  12. #754. Thanks Phil Glad that everyone's agreed on that one!
  13. damorbel@761 "In this situation radiation through the intervention of GHGs is exchanged with the higher levels of the troposphere which are below freezing. Some of the xchange might well be with deep space, but only a small part; which of course cannot easily be separated out." Would you please expand on this using some science? I can not make sense of what you are trying to explain.
  14. Damorbel - still havent got a yes/no to #752. Are you prepared to have nature be the arbitrator?
  15. Phil 754 "No-one believes the cold atmosphere warms the warmer earth. The Greenhouse effect slows the rate at which heat escapes from the earth. Period. " Earths blackbody temperature is 255K. This represents the theoretical maximum attainable temperature via the suns radiation. Kirchhoff's cavity experiment and subsequent law and Planks law were both derived by this pressies. So regardless of re-radiation and reflection, the atmosphere can NOT warm the warmer Earth...Period. Yet GHG proponents claim slowing the rate at which heat escapes the 255K earth some how warms the earth surface to 288K. So Phil, how does a body at 255K which is loosing heat increase it's temperature simply by slowing it's cooling....remember blackbody temperature is the theoretical maximum temp.
  16. damorbel Your lecture over the distinction between watts and Joules explains nothing; if I shine a 100 watt light bulb on your lecture notes, each second requires 100 Joules of electrical energy. If that energy is constrained from leaving the immediate environment by whatever means, the environment, including your lecture, heats up. If I direct a 125 milliWatt laser pointer on the wick of a candle in the vicinity of those notes, I can quickly set them aflame. Those applications are radiation effects. Your gravitational heating was ably dismissed on a number of occasions; the most recent by Tom C. No one is buying this brand of soap today.
  17. LJ Ryan: "regardless of re-radiation and reflection, the atmosphere can NOT warm the warmer Earth...Period." So the world you live in has an average temperature of -18C? How's that working out? Or did you forget that the blackbody 'theoretical maximum' you cling to refers to an object in equilibrium? Do you suggest that is the case for the earth? If so, do you not recall that we have an atmosphere, which traps energy that would otherwise escape? Even died-in-the-wool deniers accept that much. It is tedious to hear these repetitious 'objections' to basic physics that should be well-understood by now. Read and learn, my friend. If nothing else, the quality of your questions will improve.
  18. Tedious indeed. If you got past the idea that the atmosphere is warming the planet and bothered to learn how the GHE effects, then you would get some progress. Remember nature is the atmosphere. Your version of physics doesnt account for the observed world. The textbook version does. Is that a hint?
  19. damorbel (RE: 755), "And why the 'GH' effect is not a radiation effect... from... GH gases." OK, then what is the primary mechanism for the greenhouse effect? "No amount of refocussing etc., etc., of photons from a cold (255K) source, like the upper troposphere, can produce a temperature higher than 255K... anywhere. It's the photons you see, they don't have enough energy. They may have all the power (W/m^2) you can imagine* but no amount of refocussing, adding together, accummulating in reflecting cavities etc., etc. will raise their energy. Or, in other words, increase the temperature of.... anything. That is what the 2nd law is all about." Not really. The second law primarily states that heat can only flow from warm to cold - not the other way around. The atmosphere is largely transparent to the energy coming in from the Sun. The bottom line is the rate the incoming energy can leave the system from the surface is slower than the rate it is coming into the surface. The effect in principle is not much different than the interior of a car heating up inside on cold day from sitting out in the Sun. The Sun's energy is mostly transparent through the windows. It's then absorbed and re-radiated by the interior car components. The rate at which the energy is entering the interior is faster than the rate the re-radiated energy can leave the interior; therefore, the interior has to heat up. Ultimately, when the rate of energy entering something is faster than rate it can leave, the something has to heat up. That's the GHE.
  20. L.J. Ryan - "Earths blackbody temperature is 255K. This represents the theoretical maximum attainable temperature via the suns radiation." To be succinct - Bull - a completely unsupported statement. The surface of the Earth radiates at a temperature of 14C, or 287.15K, with an overall emissivity of 0.98. Upper layers of the troposphere (due to lapse rate) radiate at much higher altitudes, colder temperatures, rendering the planetary emissivity at ~0.612, regulating the amount of energy radiated at any temperature, and driving the surface temperature to the aforementioned 14C. Please - read some physics, get a clue. --- Folks, L.J.Ryan has shown either a complete lack of physics understanding, or a deliberate set of obfuscations. None of his objections has made sense. Damorbel is (in my opinion) a complete troll - he hasn't defended a single assertion, but keeps raising red herrings whenever pinned down, as he has done for over 4 months on this thread. He is now (as others have noted) circling back to previous disproved notions from several months ago, presented as fresh to people who have joined the thread. We are being trolled - do not feed the trolls. Moderators, John Cook - I'll join with previous posters; this cycle of denial and red herrings has gone on long enough. Are there any options in this regard? Perhaps deleting postings repeating junk from months ago? For some reason this particular thread attracts people who think that they either understand thermodynamics better than the previous 150 years of physicists, or perhaps feel that deliberate obfuscation and "common sense" arguments will draw out the discussion. 16 pages, >770 posts, with every skeptic argument repeated multiple times. Gah...
    Response:

    [DB] "We are being trolled - do not feed the trolls."

    I will join you out on that limb.  Short of a comment directly violating the Comments Policy, what we can do is this: Merely repeating a previously expressed position without adding anything new is indeed grounds for moderation.  So if anyone notices a comment being lifted from a previous one without anything new being added, bring it to the Moderator's attention. 

    700+ posts is a lot; the Mods are human & can't be expected to remember each and every post.  So help each other out:

    1. DNFTT (no matter how much you want to, Do Not Engage)
    2. Be on the watch for recycled comments by those obviously clogging up the threads
    3. Let the Mods know when you see those recycled comments

    Trolls live for attention & generating controversy; deny them the pleasure.  Those that abuse the priviledge of participating in this community may lose that priviledge.

    Thanks for your time.

  21. KR, John Cook, Tom Curtis Please explain why, seriously, why Kirchhoff's cavity experiment does not represent the theoretical BB maximum temp. I realize it doesn't explain the 33K delta but why is wrong? Maybe your superior physics can explain how BB maximums should be determined. Don't insult me or say I'm wrong without explanation of what is wrong.
  22. "how does a body at 255K which is loosing heat increase it's temperature simply by slowing it's cooling....remember blackbody temperature is the theoretical maximum temp." Considering that the body in question is continuously receiving energy at a more or less constant rate, it does not seem so difficult to imagine that its temperature will increase if the rate at which it can radiate that energy is decreased. In fact, I would have a hard time to understand if it did not warm up under these conditions.
  23. L.J. Ryan - Blackbody temperature is important as a theoretic limit. However, the Earth in the real universe is not a blackbody. The Stefan-Boltzmann relationship holds for the Earth, and the power radiated to space (which must match power coming in for dynamic equilibrium) is determined by both the temperature and the emissivity of the planet. Any body that is not a blackbody (with an emissivity < 1.0) must have a higher temperature to radiate the same power as that blackbody. The blackbody is the theoretic best case; everything else is less efficient. With an effective (surface to space) emissivity of 0.612, including clouds, GHG's, tropospheric effects, lapse rates, etc., a surface temp of ~287 radiates ~239 W/m^2 to space, almost equal to the sunlight coming in. The difference of 0.9 W/m^2 is the forcing, heating the climate and moving back towards dynamic equilibrium. So - you are incorrect because the Earth is not a blackbody. It has a limited emissivity to space, and the resulting temperature needed to radiate 240 W/m^2 is around 14-15C.
  24. L.J. Ryan - From your reference: "That a perfectly absorptive ("black") body rises to a temperature a bit higher than an actual black body that’s free to radiate to its surroundings. A theoretical blackbody thereby defines the upper limit of temperature vs radiant absorption." And there is the error. The Earth is not a black body, but is a "gray body" with ~60% the emissive efficiency of a black body. And hence a higher temperature than a perfectly efficient black body radiator. Your reference is in quite serious error to argue against the greenhouse effect in these terms - it's a rather horrid miscasting of the situation. It's only an upper limit for a black body - anything with a lower emissivity will by necessity have a higher temperature for the same power output. I would not trust that source, given the example you presented.
  25. Sorry to break this into multiple posts - it's late and I'm tired. L.J. Ryan - Incoming energy is set by spectral absorptivity, albedo, and the spectra of the sun in visible light. Greenhouse gases have little effect on that - the most important elements there are ozone absorption and Rayleigh scattering. Given a fixed input energy, the black body temperature represents a minimum temperature for the radiating gray body, the Earth. Any lowering of emissivity (such as increasing greenhouse gases) must be balanced by an increase in temperature above that of the black body case to match input power.

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