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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.



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 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.


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 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.


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 1276 to 1300 out of 1613:

  1. muoncounter I`m more interested in seeing how much changes in clouds and humidity follow changes in global temperature since 1985. see the graphs in the Clouds + Climate section. I cant see any evidence for positive feedbacks on water vapour/clouds either.
  2. YOGI - You might be interested in looking at the difference between relative and absolute humidity. A certain relative humidity is required to form clouds (data on cloud amounts don't show a huge amount of change with temperature - 'tho what kind of cloud is going to be very important). Clouds, through precipitation, form an upper limit on the amount of relative humidity - past a certain relative humidity it simply rains or snows. However, as the air warms, the absolute humidity required to give a particular relative humidity increases - warmer air can hold more total water vapor. Trenberth estimates that we've seen an increase of ~4% to the total amount of atmospheric water vapor since ~1975, or roughly (if I recall correctly) the equivalent of Lake Erie. And all that water vapor acts in feedback as a greenhouse gas.
  3. YOGI, what you're looking for is probably right here, in rebuttal to our star of the month, Lord Viscount Ubermensch Protector of the Realm and the Cure for Cancer, Christopher Monckton.
  4. scaddenp, I meant that CO2 and water overlap the same absorption band at 600 and 750 cm-1 so some of that effect there must be partly due to the water vapour. Water vapour is 1-4% lower down, so if we say 2%, thats 51 times the amount of CO2, and the the paper you linked says 75.5% for clouds and WV, and a mean of 17.5 for CO2. Which imply volume for volume, CO2 has about 12 times the warming potential of water vapour (from those figures).
  5. KR: "data on cloud amounts don't show a huge amount of change with temperature" 4% "Trenberth estimates that we've seen an increase of ~4% to the total amount of atmospheric water vapor since ~1975"
  6. Discussion of cloud and humidity does not belong here. DSL has given you the link to the correct thread - and to what is wrong with the data in your links. You will linking to science instead of misinformation sites a better idea. As to water overlap, the real code (referenced) integrates over the full vertical profile of the atmosphere. You cannot draw accurate conclusions from simplistic constructions.
  7. scaddenp*misinformation sites * is there something wrong with NOAA and ISCCP data ? And how do I know that the data in graph at the head of this article is good data ? The two plots show that in the 600 and 750 cm-1 band, 265Mw radiate downwards, but only 225Mw upwards. How is that possible ?
  8. "is there something wrong with NOAA and ISCCP data ?" No, only with Humlum's representation of it. Read their website instead. (And put "humlum" into the search box here for more examples of stuff from him). And if you look closely, you will see the textbook that the data came from. And below it, the link to Science of Doom which goes into the science behind your questions in considerable detail over a 10 part series. I highly recommend you to look at it. More detail than a blog commentary can give plus links to the textbooks and papers.
  9. The S.O.D. article does answer my question, it just says.. "The atmosphere, once heated up, radiates equally in all directions. Some of this is downward."
  10. typo... The S.O.D. article does NOT answer my question
  11. sorry, another typo, that should have been.. The two plots show that in the 600 to 750 cm-1 band, 265K radiate downwards, but only 225K upwards. How is that possible ?
  12. yogi#1276: "I`m more interested in ..." Sorry, I must have misread your comments here and here. An abrupt change in interest is usually a sign that the given answer was accepted and we're moving on. So the 'water is much more abundant than CO2 and should thus be much more of a GHG' issue is settled. Especially since it's off topic for this thread. Look here.
  13. Yogi 1286, The temperature lines in the graph indicates the emission spectrum of a black body at that temperature. By comparing the irradiance to these curves you can get a sense of which part of the atmosphere the emission is from. The reason why the emission in the downwelling is at 265K is because it will be from CO2 that is relatively close to the surface. Since it is measured in the arctic it sounds about right. On the other hand in the graph at 20km looking down, it will be from CO2 that is fin the upper troposphere/stratosphere, explaining the lower brightness temperature.
  14. IanC, Your lower CO2 should radiate at 265K upwards too apparently. And on the downward view, on regions free of absorption bands, the OLR is the same temp as the DLR in the 600-750 band on the upward view. How can that be ?
  15. Yogi, "Your lower CO2 should radiate at 265K upwards too apparently." Just to make sure I understand you, do you mean the CO2 near the surface radiates at 265K upwards? If this is what you mean, you'll need to remember that the CO2 higher up absorbs at the same wavelength, and hence masks the signal from the surface CO2. As for your question, in the OLR (20km looking down) graph and at regions without atmospheric absoprtion, you will be seeing radiation coming from the surface, which is ~265K. For the DLR in the 600-750cm-1 band, the radiation is coming from CO2 near the surface, which is again close to 265K.
  16. Yogi, You are having trouble because you are applying a grossly oversimplified model to a complex situation. More specifically, you are treating the atmosphere as one, homogeneous slab, simplifying emissions to "up and down," and honing in on particular wavelengths. In reality, the atmosphere is a body of continuously varying density and makeup (for example, CO2 concentrations are relatively, proportionally consistent throughout the atmosphere, but water is not). As such, the radiation at 1 km differs from 2 km differs from 10 km or 1.5 km or 1.25 km. One cannot simply treat the entire thing as a solid, homogeneous block. Radiation is emitted in all directions, so you must consider geometry, which affects how much goes up and down, and how much atmosphere each particular photon must navigate before being observed, absorbed, or escaping to space. So at every conceivable altitude the emissions are affected by the density, temperature and makeup of the atmosphere at that altitude. In addition, between you as an observer (whether on the surface of the earth, up in space, or in a weather balloon in between) and the emitting layer under investigation, emissions may be absorbed or not by intervening layers (again, dependent on density and makeup), so what you see has some radiation filtered by intervening layers, some passing through, and some radiation added to it by intervening layers. To get a hint at some of the complexity involved, play around with this page, which uses a complex computer program to band by band, altitude by altitude, go through computing what is probably happening in order to project the probable observed emission spectrum given an observation point and specific atmospheric conditions.
  17. OK in the absorption bands, ingoing and outgoing IR is absorbed, radio telescopes can`t see out through them. Quote; your article: "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." IR is emitted in the "infrared window",the 20kn downward view shows it emitting 268K in the window (away from the absorption bands). And the atmosphere emits IR brightly from 7.5 to 40 microns: "The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter). So the best view of the infrared universe, from ground based telescopes, are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground."
  18. YOGI#1292 IR telescopes not radio telescopes
  19. Sphaerica#1291 Interesting so in the sub-Arctic it has no effect on OLR when surface temp`s are -30C or lower.
  20. Yogi, The graphs in the SkS article is data from arctic, where water vapour content is relatively low. Between 8-13μm water does aborb IR slightly, so if you are in a moist atmosphere it will alter the picture. Below is a comparison between DLR at arctic vs tropics. You can see from the curve for the tropics there is actually significant emission between 8-13 μm due to water vapour. You said: And the atmosphere emits IR brightly from 7.5 to 40 microns Now brightness is relative, so it obviously depends on the application. What is dim in atmospheric science may not be dim for astronomy. From your source, they classify the emission at 3-4 micron as low. If you extrapolate the black body curves all the way out to 4 micron, or 2500cm-1, you'll see that the radiance between 8-13 micron is in fact quite high in the case of a tropical atmosphere. Infrared windows do exist, but that depends on how dry the atmosphere is, and what application you have in mind.
  21. 1294, Yogi, in the sub-Arctic it has no effect on OLR when surface temp`s are -30C or lower.
    I have no idea what this means. What is the "it" in "it has no effect..."? Greenhouse gases? A basic element of radiation, as evidenced by Stefan-Boltzmann, is that temperature plays a huge role. So for a few months of the year, temperatures are around 243˚K, radiation is only 198 W/m2. Conversely, when the sun is above the horizon, temperatures are around 273˚K and radiation is about 315 W/m2. While there is a huge difference between summer and winter, and between poles and tropics, I don't think anything qualifies for the statement "has no effect." But the spherical nature of the planet influences climate in a variety of other ways, as well, such as the spread of incoming sunlight over a larger area near the poles, the circulation of heat and moisture from the tropics through advection in the atmosphere, and other factors. It's not simple. It's not impossible to untangle, but not simple.
  22. IanC#1290 "As for your question, in the OLR (20km looking down) graph and at regions without atmospheric absoprtion, you will be seeing radiation coming from the surface, which is ~265K. For the DLR in the 600-750cm-1 band, the radiation is coming from CO2 near the surface, which is again close to 265K." But if I look down from 3km, the 600-750cm-1 band barely shows up, and looking down from 1km its not there at all, that does agree not with most of this being due to near surface CO2. And looking up from the surface with heavy cloud/rain, all bands are radiating, but when looking down from 20km, the difference between clouds/no clouds is minor in comparison.
  23. Sphaerica#1296 "I have no idea what this means. What is the "it" in "it has no effect..."? Greenhouse gases?" I`m saying on the 20km looking down, you cannot see the 600-750 band any more when the sub-Arctic reaches -30C. But more interesting is that OLR from 20km is always the same regardless of location and temperature on this model:
  24. #1298..But more interesting is that OLR (in the 600-750 band) from 20km is always the same regardless of location and temperature on this model:
  25. 1299, Yogi, Why is that, do you think?

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