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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 1251 to 1254 out of 1254:

  1. TOP - SoD means not "Subject of Discussion", but a reference to The Science of Doom, a physics oriented website that I'm certain you have been given links to. In particular, I would like to refer you to On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009), a discussion of thermodynamics related to the misconceptions you have demonstrated here. I'm sorry if that was not clear - please follow the links.
  2. 1250 Tom No mysticism. No difficulty explaining the RGHG. 23&npsp;W/m2 added to the atmosphere due to long wave radiation from the surface raises the heat content of the atmosphere. It is far more mystical to me why a gas is shown radiating energy in one direction, down, where it should be shown radiating upwards as well.
  3. 1252, TOP,
    ...why a gas is shown radiating energy in one direction, down, where it should be shown radiating upwards as well.
    What is this a reference to? What are you talking about?
  4. TOP @1252, if you think it is possible to construct a model of the greenhouse effect using net energy flows only, by all means go ahead. For myself, I notice that the people who have dedicated their lives to understanding the green house effect require all of the laws of radiative physics plus a number of others beside. I am not so arrogant as to think that I can do better with less. As it stands, the proof, and indeed the only relevant proof that your truncated "net energy flow" only physics is not mysticism is the ability to predict from that truncated physics the effects of increasing green house gases, etc. Call me when you have your working model.
  5. I've been hesitant to add to the fray, but here goes... Trenberth's diagram (just a recent variant of similar diagrams that have been around for decades) represents global mean net fluxes, and any any interpretation that treats them as absolute fluxes is doomed to failure. All the associated energy fluxes vary in magnitude in space and time, and for a full understanding of global climate you have to look at the fluxes in three dimensions. That's why people make 3-D climate models. As Tom, Sphaerica, and other regulars have pointed out repeatedly: all the simple models (be they descriptive or numerical) while useful for helping explain parts of the system are very limited in looking at the details. Unfortunately, the discussion by people such as TOP and RW1 falls into a couple of unproductive patterns: 1) misunderstandings of the simple models (Trenberth's diagram is one such example of a simple, descriptive model with some numbers attached), in particular what the simplifications are and why they are made. For example, TOP's statement in #1252 about the diagram only showing "radiating energy in one direction", when it is only intended to express the average net result (e.g., amount of IR absorbed at the surface, from the atmosphere, in the case of the 333 W/m^2 back radiation). 2) basic misunderstandings of physics, such as the difference between radiative energy transfer and convective heat transfer, or how energy is transferred as "latent heat". In particular, both of them seem to be missing the concept of absorption of radiation as the driver for atmospheric temperature change, rather than the flux of radiation. Thus, the difference between absorption and emission drives the energy balance (with the added requirement that convective fluxes be considered, too). At any single point, it is the flux divergence that controls heating/cooling rates, not the absolute fluxes themselves. Tom, Sphaerica, other regulars: I strongly disagree with you that TOP and RW1 have a lot to learn. Unfortunately, I think the major problem is that they have a lot of just plain wrong stuff in their heads that they have to unlearn first. Their obstinate view that they have some unrecognized gift to knowledge is a major hurdle. Unless they are willing to sit back and say to themselves "let's start from scratch" and begin to learn physics and climatology from a blank slate, then pointing out their wrongheadedness is futile (except in the goal of making sure that the casual reader sees their foolishness). RW1:, TOP: in comments 1143 and 1145 I challenged Fred Staples to explain some modelling results that relate to how radiation transfer really works. Fred seems to have gone missing completely - do either of you wish to take up the challenge?
  6. "no chance for the 2nd law naysayers to squawk" Thing is, real scientists do not give a hoot about 2nd law naysayers. They really don't. 2nd law naysayers are either so hopelessely confused as to be irrelevant, or, in the case of G&T, trying to have fun by exploiting the hopelessely confused. All of it has no bearing whatsoever on the real science. Has G&T led to proclamation of a revolution in atmospheric physics by AGU, NSF, the major publications? No, it met complete indifference. Only a few involved in fighting climate disinformation have taken up the task of disentagnling the sad confusion generated by this useless piece.
  7. @ Bob Loblaw It was about a year ago that I counseled RW1 that, in order to better be able to combat & overturn climate science with the ideas learned at the knee of George White, he must first start with a clean slate & learn climate science from the basics up. Then and only then would he be able to apply himself to overturning it.
  8. The 2nd reference, "Physical Principles of Meteorology and Environmental Physics", under Further Reading is not free. The Table of Contents and Chapter 1 can apparently be freely downloaded for free.
  9. This is nonsense. There are no CO2 absorption bands at 6-7.5μm :
  10. YOGI - Note the log scale on the X axis in that graph. There is a CO2 absorption band around 4.5μm. [Source]
  11. Yogi, No where in the article do the numbers 6-7.5 μm comes up. The closest will be "Carbon dioxide is the major contributor for emission seen between between about 600 and 750 cm-1". If this is what you are referring to, notice that 600 and 750 cm-1 is in fact the wavenumber, and it corresponds to ~13-16 μm in wavelength (see fig 1a). The absorption spectrum you've linked to indicates that CO2 indeed absorbs around these wavelegnths!
  12. Yes and there is also a nitrous oxide band at 4.5μm which has 298 times the greenhouse effect of CO2. This does not detract from the point that 6.0-7.5μm absorption displayed: cannot be due to CO2 as it does not absorb that band, period.
  13. @IanC, 1000 cm-1 is 10μm. You can also confirm that by looking at the ozone spike at 1000 cm-1 ~ 10μm.
  14. @IanC, no I`ll dtract that, you are correct, the scales run in the opposite directions
  15. YOGI: Wavenumber Wavelength Note the top and bottom X-axes.
  16. Ugh. The 2nd Law thread does things to people.
  17. 600-750 cm-1 does overlap with a water vapour band too. how does one decide the relative effects of each within that band ?
  18. Yogi - you do it like this
  19. So if water vapour is 50% of the greenhouse effect at 0.4% of the atmosphere content, and CO2 is 20% of the greenhouse effect at 0.00039% of the atmosphere content, CO2 would have to have 410 times the greenhouse effect of water vapour. Is that correct ??
  20. more like 2% and 0.039% so that would make CO2 50 times more powerful as a green house gas ?
  21. Yogi, please read the paper that you were pointed to. A less technical account is at realclimate.
  22. YOGI#1270: What makes you think that greenhouse gas 'power' (actually 'global warming potential') is proportional to atmospheric concentration? Where I live, a lot of that water vapor winds up on my car every morning. That might give you a clue about what these numbers mean.
  23. 1266 - DSL. Just be thankful there's no "Advanced" tab, given the law: (confusion of 'skeptics') = α(depth of physics)4
  24. Brilliant les!! That was the equation missing so that one could make sense of all this mess. I understand now...
  25. At the risk of deletion, it could be called the Stuffing-Doltzmann Law or the Law of Diminishing Clarity.

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