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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 176 to 200 out of 763:

  1. Phil, my original point was that the mentioned isolation (blanket) analogy is no way valid to explain the facts. How you rephrased this is of no concern to me. Now let me try to give my view on the green house effect which indeed leads to the conclusion that the green house effect does not interfere with the second law of thermodynamics, but for reasons that so far have just been mentioned here but not explained. In my posts #148 and #162 I tried to show that green house gases differ from other gases in their ability to store (trap) heat (falsely hoping nobody would object to that) at a significant higher rate than other gases. From here the argument is simple. Higher concentrations of green house gases in the the atmosphere will allow the more energy to be stored in the atmosphere. Due to the chemical composition of green house gases the bulk of that energy has to be drawn from surface emissions. As a result the energy content of the atmosphere is higher than before. In consequence incoming radiation will be less able to heat the atmosphere and more of it will reach and heat the surface. That in turn may cause more green house gases to be released. This explanation does not rely on back-radiation to heat the surface and therefore does not contradict the second law of thermodynamics.
  2. Just an afterthought. My explanation will no way predict a tropospheric hot spot so you can cease looking for it.
  3. h-j-m, You have successfully described the greenhouse effect. One point of note: That in turn may cause more green house gases to be released. Yes, that is how it happened historically. An initial forcing factor, such as a solar irradiance increase caused by an orbital change (Milankovitch cycles), caused a temperature increase. This temperature increase caused the release of CO2 from the oceans which increased the temperature further and caused more oceanic CO2 release. That isn't what is happening currently. The oceans & terrestrial biomes have been net CO2 absorbers during this recent warming. CO2 is coming from the ocean (argument #87)
  4. Re: h-j-m (177) "My explanation will no way predict a tropospheric hot spot so you can cease looking for it." That's OK, it's already been found here and has been confirmed more recently here (source study here). The Yooper
  5. Bibliovermis, can you please explain how in hell can I can come up with a correct description when everything that led to it was wrong, misinterpreted and misunderstood as far as any comment to it told. By the way, I was deliberately using the term green house gases for not referring to CO2. I had more H2O in mind as well as these semi solid hydrocarbons in the oceans just kept in their state due to a delicate balance of pressure and temperature.
  6. 180: "how ... can I can come up with a correct description" Check back a hundred or so comments. You've basically retold the same story; with or without any of the so-called 'back radiation' you find so distasteful, the result is the same (and we won't let anyone know you're now a believer). "semi solid hydrocarbons in the oceans just kept in their state " Can you explain what that means, where you heard about it and what it has to do with the (now verified) Greenhouse Effect?
  7. muoncounter, yes you are right I don't change arguments (stories) as long as I find them sufficiently backed by facts. But of course, I will never be a believer. I thought this is rather to be a matter of science, not belief. As to hydrocarbons in the oceans I might have meant this.
  8. h-j-m, if you are trying to find a correct description which somehow misses a measurable phenomena like back-radition, then good luck. How do you account for what DLR pyrgeometers actually detect then? People have pointed you at many good resources for getting a correct description - you appear to have rejected all because they dont conform to your incorrect understanding of physics. I suggest that you go to the textbook and read it from there, correcting your misinterpretations of science as you go. eg "Fundamentals of Heat and Mass Transfer, Incropera and DeWitt (2007)". And as for methane - hydrate release would be a disastrous feedback but not likely. We can tell from isotopic composition that bulk of methane going into atmosphere is not from fossil/hydrate sources. I do agree that that "insulator" analogy is poor because most people think in terms of a conductive insulator and its easy to jump to the wrong conclusions. I dont like "heat storage" in atmosphere because the convention use of the term does not strictly apply. This is all about the physics of radiative heat transfer and better understood in those terms rather than by analogy.
  9. #182: "I thought this is rather to be a matter of science," One believes what the science says. As opposed to refusing to believe it, no matter how many times it is demonstrated, referenced, explained, etc. The New Scientist article you cite refers to methane from melting permafrost bubbling out into the Arctic Oceans. There do not appear to be 'semi-solid hydrocarbons' in the oceans, unless you are applying that designation to methane hydrates, colloquially known as 'ice that burns'. The discovery will rekindle fears that global warming might be on the verge of unlocking billions of tonnes of methane from beneath the oceans, which could trigger runaway climate change. ... The team located more than 100 hotspots where methane is leaking from seabed permafrost. Most of the water in the region had methane concentrations more than eight times the normal amount in the Arctic Ocean, and concentrations of the gas in the air above averaged four times the Arctic norm. Yes, methane is a GHG, but this is clearly a response to warming that is already underway. This added methane will indeed make things worse; whether it is 'runaway climate change' or not remains to be seen. Let's hope it's not. The headlines 'Arctic Ocean catches fire' will be too hard for even Watt$ to spin. The problem is now that your argument will break down if you are OK with water vapor as a GHG, but refuse to accept CO2. As yocta explained earlier, they are both molecules with the vibrational modes needed to capture IR radiation from the surface. You can't believe that one is a GHG and the other isn't; that just wouldn't be scientific.
  10. Again I am getting accused of denying the existence of back-radiation, saying CO2 is no green house gas and for my incorrect understanding of physics as well as refusing to accept science. I thought of having gained some patience during 60 years, but now it is wearing really thin. So far I have taken on any concrete counter argument and refuted it with evidence. So far I did not recognize that someone offered evidence for his counter arguments which makes the whole event somewhat lopsided. Nevertheless I tried and hopefully managed to keep calm and polite. But having to deal any other reply with, as it meanwhile seems, wilfully misreadings or misinterpretations of what I wrote is getting at my nerves. Then there are explanations like that of yocta, really helpful. It is as if you go to your car mechanic and ask him why the cylinder-head of your vehicle broke and he answers with a detailed description on the metallurgic composition of the cylinder-head. I'm too old for this crap. Good bye.
  11. Re #167 yocta you explain that CO2 etc, the greenhouse gases (GHGs) absorb radiation in the IR band. I have never heard this disputed, do you think that those who question the GH effect actually question this? I certainly don't. Do you know that GHGs also radiate IR? The most common explanation for the GH effect is that this radiation causes the surface to get warmer somehow; this thread is about how GHGs which are between cold and very cold (-50C) in the upper atmosphere can warm Earth's surface which is normally between +30C and -30C. It is rather like claiming that if you are cold (+10C) you can get warm by taking your clothes off and hugging a snowman at -3C but much, much worse! The snowman hugger gets cold because his body heat, at 10C transfers to the colder (-3C) snowman, melting part of the snowman meanwhile making the hugger quite a few degrees colder. This is what the 2nd Law of Thermodynamics is all about. The explanations of the IPCC claim that GHGs in the troposphere warm Earth's surface at 255K by 33K to 288K, that's a lot of warming!
  12. damorbel - IR at greenhouse frequencies gets absorbed and re-emitted within about 100 meters. That means the surface is facing an atmospheric IR emitter at 14C, not -50C. The -50C is reached through atmospheric lapse rate temperature drop, until the altitude where lowering pressure reduces IR absorption enough to radiate to space. Now, realize that without the GHG absorption and emission at 14C we would instead be radiating those bands directly from the surface to space, which is (if you include microwave background radiation) at -269C? I suggest you think about sitting (a) in a room at 14C, then try (b) sitting in a cryo-fridge at -269C, and consider what kind of body temperature you could maintain in those two circumstances. Wear warm socks.
  13. Re #166 archiesteel you wrote :- "@damorbel: the graph serves its purpose. It is not misleading to anyone with any kind of base scientific knowledge." A diagram without any temperatures? Telling us that the surface is warmed? This is not scientific, the thread is about the 2nd Law of themodynamics which is about how heat moves between places with different temperatures; I am curious to know how you find a diagram without any temperatures on it AT ALL "serves a [useful] purpose" in this regard.
  14. Re #174 Tom Dayton you wrote :- "wavelengths of radiation that are plentifully emitted by the Earth but only weakly emitted by the Sun, thereby acting as a partially closed valve that traps energy below the top of the atmosphere" A (partially) closed valve? The absorbed insolation is converted to heat, it warms the soil, water, atmosphere etc. A (partially) closed valve is not an idea that applies to radiation, even the IPCC doesn't mention this idea! Now this soil, water, atmosphere etc. only emits radiation when it is above 0K, whereas it absorbs radiation regardless of its temperature. For example Earth can absorb microwave radiation very efficiently but the whole point of microwave ovens is that their radiation does not have thermal properties, it is "monochromatic" with a wavelength many orders of magnitude longer than IR. Starting from 0K the temperature of material receiving radiation (of any sort) can only rise and as it rises it starts to emit radiation, eventually reaching a temperatures at which it emits as much radiation as it absorbs - it has nothing to do with the wavelength of the incoming radiation. However if you want to assign a temperature to your source then you must choose a source with a thermal spectrum, a spectrum that follows Planck's radiation law; it must not be monochromatic like a laser, a microwave oven or any other non-Planckian spectrum, only then can you speak of a temperature.
  15. damorbel "It is rather like claiming that if you are cold (+10C) you can get warm by taking your clothes off and hugging a snowman at -3C but much, much worse!" No, it's rather like claiming that 10 °C is better than -3 °C. Your clothes are colder than your body on average, at almost the same temperature inside and colder outside. But they reduce the heat fluxes due to both conduction and convection and you feel warmer. The point here is that whatever reduces heat dispersion (heat flux) makes you feel "warmer", even if it is colder than yourself. And even if the prevailing mechanism is radiative instead of conductive or convective. Please note the flux from the atmosphere to the surface is not a heat flux but an energy flux, the former being the net energy flux at the surface. So, the heat flux does not revert upon increasing the greenhouse effect as required by the 2nd law of thermodynamics.
  16. "It is rather like claiming that if you are cold (+10C) you can get warm by taking your clothes off and hugging a snowman at -3C but much, much worse!" Good grief, damorbel: you really don't get it, do you? Warmer objects can indeed absorb radiation from cooler objects. If you shine a steady UV light on a spinning steel ball, the ball will eventually reach a specific temperature. True? If you then place another steel ball nearby--a steel ball below the temp of the first one but above 0K--then I'll bet you lunch that the first steel ball's surface temperature will increase slightly until it once again reaches an "equilibrium" temp. Allow this to take place in an atmosphere that does not support convection or conduction--only radiation. The total incoming radiation for the first ball will have increased due to the second (cooler) ball's radiation (and subsequent re-radiation of the first ball's radiation). In your model, does the radiation from the second ball just "bounce off" the first ball? Or does the second ball magically know not to radiate toward the first ball? Your snowman example is not good, because A) you're working primarily with conduction and convection and B) the naked person has an internal engine. There is still radiative transfer, though, between the snowman and the naked person.
  17. damorbel wrote: "Do you know that GHGs also radiate IR? The most common explanation for the GH effect is that this radiation causes the surface to get warmer somehow" So, you accept that GHGs absorb and then re-emit IR. Yet you insist that this re-emitted radiation can't possibly warm the planet. So... what exactly do you think happens to it? It somehow 'knows' the relative temperatures of the matter it was emitted from and the matter it is about to impact and 'changes course' to avoid any matter which is warmer than the previous? How do you imagine microwave ovens work? After all, as the food gets warmer the microwave photons can't possibly travel from the cool walls of the microwave to the warm food... they must be repulsed away from anything warmer. Therefor, a frozen dinner might be warmed up to room temperature, but a microwave oven could never make anything warmer than room temperature because the radiation can only travel into colder objects. Ditto sunlight, lasers, radio and television broadcasts, remote controls, and dozens of other aspects of everyday life. All of which demonstrate that your position is gibberish. Seriously. How can you not see that you are spouting completely ludicrous nonsense?
  18. damorbel wrote: "Now this soil, water, atmosphere etc. only emits radiation when it is above 0K, whereas it absorbs radiation regardless of its temperature." BTW... it should be pointed out that 0K has never been observed. It's a theoretical minimum. Nothing that cold is actually known to exist. Therefor your contrast between one thing which 'only' happens above 0K and another which happens regardless of temperature is really two things which happen regardless of temperature.
  19. @damorbel: "A diagram without any temperatures?" Yes. It's a diagram about energy flow. it serves its purpose, no matter how much *you* misunderstand it. "I am curious to know how you find a diagram without any temperatures on it AT ALL "serves a [useful] purpose" in this regard." Because it shows energy transfers. What's your problem with it, apart from the fact that you don't understand what it's used for? It seems that, like many deniers here, you are consciously trying to muddy the waters and create confusion about AGW science. Too bad (for you) the level of knowledge on this site is so high...
  20. h-j-m, well patience of both is tried. Your "evidence" and "counter-examples" simply revealed a flawed understanding of the physics. People have responded by trying to help you understand the physics.
  21. damorbel - continuing to talk about what happens and how the 2nd law works in conductive energy transfer is not helping you understand how it works in radiative energy transfer. People are trying to help you understand this. As a matter of interest what do understand the relationship of temperature to energy to be?
  22. Re #187 KR you wrote :- "IR at greenhouse frequencies gets absorbed and re-emitted within about 100 meters. That means the surface is facing an atmospheric IR emitter at 14C, not -50C. The -50C is reached through atmospheric lapse rate temperature drop, until the altitude where lowering pressure reduces IR absorption enough to radiate to space" To a considerable extent you are correct. On Earth the density of atmospheric GHGs is very low and low level absorption of IR is a very small % of the thermal input to the atmosphere. Much more atmospheric energy comes from the evaporation of water, water heated by the direct input of the Sun's radiation. The atmosphere is also heated by direct convection from the surface. Water evaporation becomes spectacular in hurricanes, violent air convection is the corresponding phenomenon over land, sometimes called tornados; neither extreme form is required for convection to take place. You then wrote:- "Now, realize that without the GHG absorption and emission at 14C we would instead be radiating those bands directly from the surface to space, which is (if you include microwave background radiation) at -269C?" Well? Is this going to change the average temperature? The answer depends on how thick the atmosphere is. Atmospheres are held in place by gravity, the effect of this is to make a temperature profile that increases (at the "lapse rate") with depth. Such an increase gets very high with the very deep atmospheres of planets like Jupiter; relly massive gas objects like stars reach nuclear fusion temperatures in their core, that is where their energy comes from. Earth has a surface pressure of 1 bar and its surface temperature is not much above the equilibrium temperature of 279K. Venus has a much higher surface pressure about 92bar and a surface temperature of 735K. You wrote:- "without the GHG absorption and emission at 14C we would instead be radiating those bands directly from the surface to space, which is (if you include microwave background radiation) at -269C? " Erm. no you wouldn't, you'd be in a warm bath of air at 14C, 1/2 surrounded by radiation from the ground at 14C; 1/2 from deep space at 2.7K. The thing that would finish you off would be the complete absence of H2O, you would be dead before you knew. With water and no CO2 you would only die of hunger because without CO2 nothing would grow, there would be no plant life.
  23. damorbel - And once more, you miss the point in several respects. You've been pointed to Trenberth 2009 several times. Convection and evaporation together represent only 1/4 the energy involved in IR from the ground. The backradiation at 333 W/m^2 is twice the energy of incoming sunlight. The greenhouse effect does not heat the Earth; it slows cooling, by providing a warmer background than outer space, reducing the ability of Earth to dump the energy from incoming sunlight. This is basic radiative energy balance - the Earth radiates to space with P = e*s*A*T^4, and when greenhouse gases decrease emissivity 'e', as per The greenhouse effect and the 2nd law of thermodynamics (intermediate), with an emission spectra like this: Notches in graph A show greenhouse gas bands, where IR is sent back to the ground, as seen in graph B then there is an energy imbalance (more coming in than going out), and the temperature will rise until 'P', the energy radiated to space, equals the sunlight coming in. At this point, damorbel, I'm coming to the conclusion that you are deliberately misunderstanding the point. You've ignored repeated pointers to the physics involved, and brought up multiple straw-man arguments. I don't believe it's worth debating with you unless you are willing to engage in an actual discussion of the science.
  24. Re #191 DSL you wrote :- "Warmer objects can indeed absorb radiation from cooler objects." Look at it this way. Emitted power is proportional to T^4, thus the warmer object emits most power. Both objects absorb power indpendent of temperature thus the warm object cools down and the cool one warms up, they are in thermal contact as if they were touching each other if the two objects are isolated they will slowly arrive at the same temperature, somewhere between the two original temperatures. Further you wrote:- "If you shine a steady UV light... ... once again reaches an "equilibrium" temp." Let us assume these balls are planets and the UV source is a star. Wavelength is unimportant. The balls will slowly approach a temperature dependent only on the distance of and the power emitted by your UV source. The temperature they reach is not dependent on how shiny they are, that only affects the rate they approach this 'equlibrium' temperature You wrote:- "Your snowman example is not good, because A) you're working primarily with conduction and convection and B) the naked person has an internal engine. There is still radiative transfer, though, between the snowman and the naked person." Temperature rules in all thermal tranfers, be it conduction radiation or convection. Convection is a bit special because it won't work 'downwards' but both radiation and conduction will tend to equalise the temperature in an isolated convective (e.g. gravitational) system. Radiative transfer is not influenced by gravity.
  25. Re #192 CBDunkerson you wrote :- "Warmer objects can indeed absorb radiation from cooler objectsYet you insist that this re-emitted radiation can't possibly warm the planet." If it is cooler than the planet, yes. Then you wrote :- "So... what exactly do you think happens to it? It somehow 'knows' the relative temperatures of the matter it was emitted from and the matter it is about to impact and 'changes course' to avoid any matter which is warmer than the previous?" In #200 I wrote:- "Look at it this way. Emitted power is proportional to T^4, thus the warmer object emits most power. Both objects absorb power indpendent of temperature thus the warm object cools down and the cool one warms up, they are in thermal contact as if they were touching each other if the two objects are isolated they will slowly arrive at the same temperature, somewhere between the two original temperatures." Which explains why the cold troposphere cannot raise the temperature of the Earth's surface. You wrote :- "How do you imagine microwave ovens work?" Read #189 3rd para. Microwaves are not 'thermal' like a grill; they have a magnetron inside wich makes single frequency (monochromatic) radiofrequency (RF) power at about 2450MHz, this power is absorbed by water molecules which get hot in consequence. This is quite different from a 'thermal' oven which uses thermal radiation to grill and hot air to bake. In #193 you wrote: "it should be pointed out that 0K has never been observed. It's a theoretical minimum. Nothing that cold is actually known to exist. Therefor your contrast between one thing which 'only' happens above 0K and another which happens regardless of temperature is really two things which happen regardless of temperature." Oh alright then, not 0K, lets put 0.00000000001K.

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