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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 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)

 

At a glance

Although this topic may have a highly technical feel to it, thermodynamics is a big part of all our everyday lives. So while you are reading, do remember that there are glossary entries available for all thinly underlined terms - just hover your mouse cursor over them for the entry to appear.

Thermodynamics is the branch of physics that describes how energy interacts within systems. That interaction determines, for example, how we stay cosy or freeze to death. You wear less clothing in very hot weather and layer-up or add extra blankets to your bed when it's cold because such things control how energy interacts with your own body and therefore your degree of comfort and, in extreme cases, safety.

The human body and its surroundings and energy transfer between them make up one such system with which we are all familiar. But let's go a lot bigger here and think about heat energy and its transfer between the Sun, Earth's land/ocean surfaces, the atmosphere and the cosmos.

Sunshine hits the top of our atmosphere and some of it makes it down to the surface, where it heats up the ground and the oceans alike. These in turn give off heat in the form of invisible but warming infra-red radiation. But you can see the effects of that radiation - think of the heat-shimmer you see over a tarmac road-surface on a hot sunny day.

A proportion of that radiation goes back up through the atmosphere and escapes to space. But another proportion of it is absorbed by greenhouse gas molecules, such as water vapour, carbon dioxide and methane.  Heating up themselves, those molecules then re-emit that heat energy in all directions including downwards. Due to the greenhouse effect, the total loss of that outgoing radiation is avoided and the cooling of Earth's surface is thereby inhibited. Without that extra blanket, Earth's average temperature would be more than thirty degrees Celsius cooler than is currently the case.

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one form to another it can be neither created nor destroyed. The Second Law does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. That qualifier term, 'net', is the important one here. The Earth alone is not a "closed system", but is part of a constant, net energy flow from the Sun, to Earth and back out to space. Greenhouse gases simply inhibit part of that net flow, by returning some of the outgoing energy back towards Earth's surface.

The myth that the greenhouse effect is contrary to the second law of thermodynamics is mostly based on a very long 2009 paper by two German scientists (not climate scientists), Gerlich and Tscheuschner (G&T). In its title, the paper claimed to take down the theory that heat being trapped by our atmosphere keeps us warm. That's a huge claim to make – akin to stating there is no gravity.

The G&T paper has been the subject of many detailed rebuttals over the years since its publication. That's because one thing that makes the scientific community sit up and take notice is when something making big claims is published but which is so blatantly incorrect. To fully deal with every mistake contained in the paper, this rebuttal would have to be thousands of words long. A shorter riposte, posted in a discussion on the topic at the Quora website, was as follows: “...I might add that if G&T were correct they used dozens of rambling pages to prove that blankets can’t keep you warm at night."

If the Second Law of Thermodynamics is true - something we can safely assume – then, “blankets can’t keep you warm at night”, must be false. And - as you'll know from your own experiences - that is of course the case!

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!


Further details

Among the junk-science themes promoted by climate science deniers is the claim that the explanation for global warming contradicts the second law of thermodynamics. Does it? Of course not (Halpern et al. 2010), but let's explore. Firstly, we need to know how thermal energy transfer works with particular regard to Earth's atmosphere. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming.

Thermal energy is transferred through systems in five main ways: conduction, convection, advection, latent heat and, last but not least, radiation. We'll take them one by one.

Conduction is important in some solids – think of how a cold metal spoon placed in a pot of boiling water can become too hot to touch. In many fluids and gases, conduction is much less important. There are a few exceptions, such as mercury, a metal whose melting point is so low it exists as a liquid above -38 degrees Celsius, making it a handy temperature-marker in thermometers. But air's thermal conductivity is so low we can more or less count it out from this discussion.

Convection

Convection

Figure 1: Severe thunderstorm developing over the Welsh countryside one evening in August 2020. This excellent example of convection had strong enough updraughts to produce hail up to 2.5 cm in diameter. (Source: John Mason)

Hot air rises – that's why hot air balloons work, because warm air is less dense than its colder surroundings, making the artificially heated air in the balloon more buoyant and thereby creating a convective current. The same principle applies in nature: convection is the upward transfer of heat in a fluid or a gas. 

Convection is highly important in Earth's atmosphere and especially in its lower part, where most of our weather goes on. On a nice day, convection may be noticed as birds soar and spiral upwards on thermals, gaining height with the help of that rising warm air-current. On other days, mass-ascent of warm, moist air can result in any type of convective weather from showers to severe thunderstorms with their attendant hazards. In the most extreme examples like supercells, that convective ascent or updraught can reach speeds getting on for a hundred miles per hour. Such powerful convective currents can keep hailstones held high in the storm-cloud for long enough to grow to golfball size or larger.

Advection

Advection is the quasi-horizontal transport of a fluid or gas with its attendant properties. Here are a couple of examples. In the Northern Hemisphere, southerly winds bring mild to warm air from the tropics northwards. During the rapid transition from a cold spell to a warm southerly over Europe in early December 2022, the temperatures over parts of the UK leapt from around -10C to +14C in one weekend, due to warm air advection. Advection can also lead to certain specific phenomena such as sea-fogs – when warm air inland is transported over the surrounding cold seas, causing rapid condensation of water vapour near the air-sea interface.

Advection

Figure 2: Advection fog completely obscures Cardigan Bay, off the west coast of Wales, on an April afternoon in 2015, Air warmed over the land was advected seawards, where its moisture promptly condensed over the much colder sea surface.

Latent heat

Latent heat is the thermal energy released or absorbed during a substance's transition from solid to liquid, liquid to vapour or vice-versa. To fuse, or melt, a solid or to boil a liquid, it is necessary to add thermal energy to a system, whereas when a vapour condenses or a liquid freezes, energy is released. The amount of energy involved varies from one substance to another: to melt iron you need a furnace but with an ice cube you only need to leave it at room-temperature for a while. Such variations from one substance to another are expressed as specific latent heats of fusion or vapourisation, measured in amount of energy (KiloJoules) per kilogram. In the case of Earth's atmosphere, the only substance of major importance with regard to latent heat is water, because at the range of temperatures present, it's the only component that is both abundant and constantly transitioning between solid, liquid and vapour phases.

Radiation

Radiation is the transfer of energy as electromagnetic rays, emitted by any heated surface. Electromagnetic radiation runs from long-wave - radio waves, microwaves, infra-red (IR), through the visible-light spectrum, down to short-wave – ultra-violet (UV), x-rays and gamma-rays. Although you cannot see IR radiation, you can feel it warming you when you sit by a fire. Indeed, the visible part of the spectrum used to be called “luminous heat” and the invisible IR radiation “non-luminous heat”, back in the 1800s when such things were slowly being figured-out.

Sunshine is an example of radiation. Unlike conduction and convection, radiation has the distinction of being able to travel from its source straight through the vacuum of space. Thus, Solar radiation travels through that vacuum for some 150 million kilometres, to reach our planet at a near-constant rate. Some Solar radiation, especially short-wave UV light, is absorbed by our atmosphere. Some is reflected straight back to space by cloud-tops. The rest makes it all the way down to the ground, where it is reflected from lighter surfaces or absorbed by darker ones. That's why black tarmac road surfaces can heat up until they melt on a bright summer's day.

Radiation

Figure 3: Heat haze above a warmed road-surface, Lincoln Way in San Francisco, California. May 2007. Image: Wikimedia Commons.

Energy balance

What has all of the above got to do with global warming? Well, through its radiation-flux, the Sun heats the atmosphere, the surfaces of land and oceans. The surfaces heated by solar radiation in turn emit infrared radiation, some of which can escape directly into space, but some of which is absorbed by the greenhouse gases in the atmosphere, mostly carbon dioxide, water vapour, and methane. Greenhouse gases not only slow down the loss of energy from the surface, but also re-radiate that energy, some of which is directed back down towards the surface, increasing the surface temperature and increasing how much energy is radiated from the surface. Overall, this process leads to a state where the surface is warmer than it would be in the absence of an atmosphere with greenhouse gases. On average, the amount of energy radiated back into space matches the amount of energy being received from the Sun, but there's a slight imbalance that we'll come to.

If this system was severely out of balance either way, the planet would have either frozen or overheated millions of years ago. Instead the planet's climate is (or at least was) stable, broadly speaking. Its temperatures generally stay within bounds that allow life to thrive. It's all about energy balance. Figure 4 shows the numbers.

Energy Budget AR6 WGI Figure 7_2

Figure 4: Schematic representation of the global mean energy budget of the Earth (upper panel), and its equivalent without considerations of cloud effects (lower panel). Numbers indicate best estimates for the magnitudes of the globally averaged energy balance components in W m–2 together with their uncertainty ranges in parentheses (5–95% confidence range), representing climate conditions at the beginning of the 21st century. Figure adapted for IPCC AR6 WG1 Chapter 7, from Wild et al. (2015).

While the flow in and out of our atmosphere from or to space is essentially the same, the atmosphere is inhibiting the cooling of the Earth, storing that energy mostly near its surface. If it were simply a case of sunshine straight in, infra-red straight back out, which would occur if the atmosphere was transparent to infra-red (it isn't) – or indeed if there was no atmosphere, Earth would have a similar temperature-range to the essentially airless Moon. On the Lunar equator, daytime heating can raise the temperature to a searing 120OC, but unimpeded radiative cooling means that at night, it gets down to around -130OC. No atmosphere as such, no greenhouse effect.

Clearly, the concentrations of greenhouse gases determine their energy storage capacity and therefore the greenhouse effect's strength. This is particularly the case for those gases that are non-condensing at atmospheric temperatures. Of those non-condensing gases, carbon dioxide is the most important. Because it only exists as vapour, the main way it is removed is as a weak solution of carbonic acid in rainwater – indeed the old name for carbon dioxide was 'carbonic acid gas'. That means once it's up there, it has a long 'atmospheric residency', meaning it takes a long time to be removed. 

Earth’s temperature can be stable over long periods of time, but to make that possible, incoming energy and outgoing energy have to be exactly the same, in a state of balance known as ‘radiative equilibrium’. That equilibrium can be disturbed by changing the forcing caused by any components of the system. Thus, for example, as the concentration of carbon dioxide has fluctuated over geological time, mostly on gradual time-scales but in some cases abruptly, so has the planet's energy storage capacity. Such fluctuations have in turn determined Earth's climate state, Hothouse or Icehouse – the latter defined as having Polar ice-caps present, of whatever size. Currently, Earth’s energy budget imbalance averages out at just under +1 watt per square metre - that’s global warming. 

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one component to another it can be neither created nor destroyed. Self-evidently, the "isolated" part of the law must require that the sun and the cosmos be included. They are both components of the system: without the Sun as the prime energy generator, Earth would be frozen and lifeless; with the Sun but without Earth's emitted energy dispersing out into space, the planet would cook, Just thinking about Earth's surface and atmosphere in isolation is to ignore two of this system's most important components.

The Second Law of Thermodynamics does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. To reiterate, the qualifier term, 'net', is the important one here. In the case of the Earth-Sun system, it is again necessary to consider all of the components and their interactions: the sunshine, the warmed surface giving off IR radiation into the cooler atmosphere, the greenhouse gases re-emitting that radiation in all directions and finally the radiation emitted from the top of our atmosphere, to disperse out into the cold depths of space. That energy is not destroyed – it just disperses in all directions into the cold vastness out there. Some of it even heads towards the Sun too - since infra-red radiation has no way of determining that it is heading towards a much hotter body than the Earth,

Earth’s energy budget makes sure that all portions of the system are accounted for and this is routinely done in climate models. No violations exist. Greenhouse gases return some of the energy back towards Earth's surface but the net flow is still out into space. John Tyndall, in a lecture to the Royal Institution in 1859, recognised this. He said:

Tyndall 1859

As long as carbon emissions continue to rise, so will that planetary energy imbalance. Therefore, the only way to take the situation back towards stability is to reduce those emissions.


Update June 2023:

For additional links to relevant blog posts, please look at the "Further Reading" box, below.

Last updated on 29 June 2023 by John Mason. View Archives

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Comments 1351 to 1375 out of 1393:

  1. YOGI... This is the point where I usually suggest to "doubters" that they need to collect and publish their findings, if they can. To everyone else reading this, we accept the past 150+ years of research that makes up the basis for the greenhouse effect. This is established physics and is not in question by... well, by anyone really. You're incomplete understanding, and your desire to find some fault somewhere, is driving you into a circuitous pattern that has no end. If, somehow, someway, you managed to connect all your dots and come up with a theory that was complete it would literally mean re-writing the past 150 years of physics. You'll please excuse me if I suggest that the likelihood of this is extremely low, to the point of being a near impossibility. But if you want to try, knock yourself out. Outside of that you are clearly exhibiting a Dunning-Kruger effect and wasting everyone's time and energy.
  2. YOGI What is the point of bringing up clouds? The blanket analogy is meant to explain the effect of greenhouse gases on loss of heat from the earth's surface. Clouds require a different analogy entirely, one that also accounts for changes to albedo or incoming energy. As Phil notes, in this analogy the absorption of solar energy by the earth's surface is the equivalent of heat released by the human body as a result of metabolism. Both the earth and the human body are open systems thermodynamically, although they receive energy in different forms. From that point of view atmopsheric GHGs and blankets act similarly, both reducing loss of heat generated by absorption of solar energy or metabolism. The analogy does break down eventually if you take it far enough, as all analogies do. For example, the human body actively alters its metabolism to maintain a relatively constant core temperature, while the earth actually appears to amplify variations in solar radiation. That makes calculation of equilibrium surface temepratures different in each case. But the point of the analogy is not to represent energetics of the earth system, but simply to make the GH effect tangible by relating it to common experience. The blanket analogy succeeds on that front.
  3. Mods...Sorry about the triple posts...thread didn't update properly. Delete accordingly.
  4. I have to agree with Rob above. If there was some disagreement between the GH effect and the 2nd law, it would have been noticed by at least one of the thousands of scientists, all steeped in the Laws of Thermodynamics, who have studied some aspect of GHGs over the last 150 years. Instead of arguing about semantics and analogies, the doubters need to produce a reproducible experiment that can withstand peer review. The fact that haven't done so, despite the fact that it would yield a Nobel Prize, is telling.
  5. Stephen Baines The back radiation is less than the heat radiated from the surface, so the net flow of heat is from warmer to cooler, so it evidently does obey the second law of thermodynamics. I suspect that the problem is that the common definition of the second law that crops up pre-dates statistical mechanics and concept of a photon, in which case it is perhaps understandable that it doesn't specify the net flow of heat (implying that there may be an exchange of energy, but that it is biased in the direction from warmer to cooler).
  6. DM Yes, I totally agree. Were the doubters to take measurements and do the actual science, they would find that the GHE is perfectly in line with thermodynamics. But they don't do the measurements and instead engage in endless obfuscation, which indicates that they are not actually serious about the issue. It is downright laughable that physicists would not 1) recognize that the 2nd law was contradicted and 2) would not do anything about it if they did. It's also downright depressing that this thread is going on 28 pages!!!
  7. OK cover the Earth with a blanket (FULL CLOUD COVER) and see how cold it gets Venus?
  8. Sadly, this thread is the happy hunting ground for the wilfully ignorant. It's wasting time trying to teach those who are determined not to let facts undermine their fantasies.
  9. Actually, I think every denier should be forced to read this thread, and then to look in a mirror, and recognize that to a lesser degree they are doing the same thing. It may be easier to argue their position, or to talk themselves into believing their position has substance, but in 99% of cases it comes down to the exact same thing.
  10. How come if 239W/m go in and 239W/m go out, that 396W/m exists within the system ? surely that violates the conservation of energy law ?
  11. Eric (skeptic) Venus has 92 bar surface pressure. Around 53 km up, pressure and temperature are comparable to those at Earth's surface.
  12. While I strongly doubt YOGI is actually interested in understanding this, but Do Trenberth and Kiehl understand the first law of thermodynamics rather exhaustively covers this. Understanding however requires getting your head around the physics not looking for talking points. Claiming the diagram is "wrong" is tricky when those flows are from measurements.
  13. Stephen Baines "What is the point of bringing up clouds?" Its the only fair equivalent to a blanket. Full cloud cover at night is the surest way to keep the heat in. But as the Earth is externally heated, it will cool with total cloud cover, as the albedo will be huge. GHG`s have huge holes so are not exactly a blanket.
    Response:

    [DB] Way back here, you were asked to succinctly put forth the one objection that you wanted to hang your hat on.  You have made 6 comments on this thread since then and in exactly...none of them have you done so. Failure to do that amounts to a de facto admission that you are here to simply waste the time of others.

    Cease making unsupported assertions (i.e., lacking support in the peer-reviewed literature published in reputable journals) until you can demonstrate that you are able to carry on a science-based discussion in this forum.

  14. YOGI... Look, people are not trying to pile on, even though it may seem like it. What is frustrating is that you're clearly confusing your lack of knowledge on this subject with there being something wrong with greenhouse theory. It's a very complex subject. But rather than assuming that 150 years of research has somehow produced an error that has somehow slipped past 10's if not 100's of thousands of scientists, how about just acknowledging that maybe you need to be better trained to even begin to understand this subject. It's great that you are trying to propose questions but the ones you're asking are really pretty easily answered if you take some time to better understand the subject matter.
  15. YOGI writes: "How come if 239W/m go in and 239W/m go out, that 396W/m exists within the system ? surely that violates the conservation of energy law ?" Really? So, on your world, if the stream feeding into a lake carries 239 m^3/s and the stream flowing out of the lake also carries 239 m^3/s then the lake cannot possibly hold 396 m^3 of water? How very sad for you and your world where clouds do not precipitate out of the atmosphere when the temperature decreases. Here on our Earth planet things work differently.
  16. [DB] I`ll hang my hat on this: http://www.tech-know.eu/uploads/Understanding_the_Atmosphere_Effect.pdf
    Response:

    [DB] You are thus relatively off-topic on this thread.  The more appropriate thread for your discursion is Postma disproved the greenhouse effect.  Everyone please respond there.

    Yogi:  Please read the above thread and the following threads in their entirety

  17. DB, actually the PDF specified by YOGI appears to be a different screed by Postma wherein he insists that the "Greenhouse Theory" contradicts the laws of thermodynamics. Including alot of the usual 'energy cannot flow from a colder area to a hotter one' idiocy. So, complete nonsense... but on topic nonsense.
    Response:

    [DB] Noted.  But as Dikran points out, Postma commits a great number of grievous errors in this new piece.  Perhaps a continuing series on Postma, with this one comprising "Joseph Postma is Wrong About _________, Pt. 4"?

  18. I think YOGI has to understand that all mass exibits energy, and that all mass is continuously cooling by emitting energy. What slows that cooling is the absorbtion of energy. The mass doesn't care if the energy came from a colder object or a warmer object. When mass absorbs a photon from a colder object, all that happens is the RATE of cooling slows a bit. The whole crux is that the Sun adds energy, which will result in warmth, but matter is still doing its thing by trying to cool.
  19. Well I got as far as page 3 before spotting the first scientific error (mainly becuase the first two and a half pages are rhetorical posturing and essentially free of any scientific content). Postma writes "Does back-scattered infrared radiative transfer act like a blanket upon, and explain the temperature of, the surface of the Earth, analogeous to the way a greenhouse building works..." Immediate fail. Greenhouses work principally by preventing convection. Nobody claims that the Greenhouse effect works in the same way an actual greenhouse, even Fourier knew this.
  20. Next error, page 4 "A blackbody is simply exactly what it sounds like: an object that is completely black. The reason why it is black is because it absorbs 100% of the light that strikes it, and doesn't reflect any of it back. Therefore it appears black" The sun is a blackbody, doesn't appear very black to me. The middle sentence is sort of true, a blackbody does absorb all the light that falls on it, but that doesn't mean that it appears to be black. I know that is only a fairly trivial point, but it doesn't encourage confidence when it comes to less trivial issues. Note this is the second scientific point in the paper so far and Postma has made errors in both of them.
  21. Next problem, first paragraph of page 5 tries to explain "radiative thermal equilibrium" without mentioning that the blackbody is constantly radiating heat. Not strictly speaking an error, but again it doesn't inspire confidence.
  22. Catastrophic error - second paragraph of page 5 When a blackbody has reached thermal equilibrium, it can no longer absorb more light for heating and therefore has to reemit just as much energy as it is absorbing" This is factually incorrect; the blackbody never stops absorbing light - it is a blackbody, that is what they do. Likewise if it is above absolute zero it also never stops emitting radiation. It reaches radiative equilibrium when its emissions (which depend on temperature) rise suffuciently high that emission matches absorption. If Postma can't even explain radiative equilibrium or what a blackbody does correctly, I think we can safely say that YOGI needs to pick his/her has up off the floor.
  23. Bottom of page 5 "For example if the object absorbs visible light, then it will reemit infrared light, which we can't see, and therefore it still appears black" Postma half-corrects his initial definition of a blackbody here (would have been better just to get it right in the first place), but this statement is obviously incorrect. As an example, consider solar power plants which can be used to make objects glow white hot (and thus be producing visible light) Virtually every scientific statement Postma has made by the end of page 5 is wrong.
  24. top of page 6 "As amazing as this sounds, the only thing that really does seem to perfectly resemble a black body is the entire universe itself" So this light that the entire universe is absorbing. Were is it coming from exactly?
  25. Dikran Marsupial: YOGI also needs to understand that what you a describing is basic physics, and has nothing to do with the GHE. Matter behaves in predictable ways. How one wants to apply it to the GHE is up to each individual, but in that application......you just CANNOT throw basic physics out the window and expect conclusions that have any merit.

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