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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 1601 to 1613 out of 1613:

  1. Likeitwarm... "GHE is a hypothesis, not fact."

    This is very base level climate denial. I mean, jeez, we literally measure the greenhouse effect and have been doing so for over a century. You can measure it from your own backyard with relatively simple, inexpensive equipment. 

  2. I was intrigued by the Ward (2016) quote made by Likeitwarm @1590.

    "It is the frequencies and amplitudes of these radiating oscillations that, when absorbed by cooler matter, increase the amplitudes and frequencies of the internal oscillations of the absorbing matter, thereby increasing the absorbing matter’s temperature. It is these frequencies and amplitudes that appear to be reflected, rather than absorbed, by warmer matter [22]. When radiation has lower amplitudes of oscillation at each frequency than the prevailing amplitudes of oscillation within receiving matter, heat cannot flow into the matter by resonance, cooler to hotter. Therefore, by conservation of energy, “colder” radiation must be reflected."

    How do you measure the 'temperature' of a photon to know that it is "colder"? We have no actual knowledge of what a photon is, just its impact on the universe it is emitted from and absorbed/reflected/refracted by. And that shows us the energy of a photon (so presumably some marker of its 'temperature') is solely a function of its wavelength. There is no variation in any 'amplitude'.  So all this guff from Ward about radiation having both 'frequency' and 'amplitude' is a fantasy.

    The reference [22] which Ward uses three times to support the same crackpot contention refers to Evans & Popp (1985) 'Pictet's experiment: the apparent radiation and reflection of cold', an account of a certain Marc-Auguste Pictet's 1784 experiment along with Evans & Popp's explanation of how the 'reflection of cold' was actually measuring a drop in temperature when the "relatively high-temperature radiation** is replaced ... the thermometer now recieves less energy per unit time than previously." [**That is radiation emitted from a relatively warmer source.]

  3. MA Rodger @1602 :

    Dr Ward's novel ideas are certainly intriguing, and may be introducing us to concepts that go completely beyond Quantum Mechanics.  In physics, they could represent a great leap forward (to borrow from the political terminology).   Or a quantum leap (to use the vernacular term).   What a time to be alive!

    The commenter Likeitwarm may be on the cusp of understanding how these advanced ideas are producing a fusion  ~ a fusion of Denialati and Illuminati.   Truly a new Enlightenment.

    Perhaps this new development will give Likeitwarm encouragement and . . . a quantum of solace?

  4. I think you're on to something, Eclectic. That's pretty much what it would take to overturn our modern understanding of the GHE and climate change. It would require a complete restructuring of physics as we understand it today. 

    It's certainly a possibility that Dr. Ward is the person who accomplishes this feat. An infinitessimally small possibility, but still a possibility none-the-less. And if that should come to pass, we will all recognize Likeithot for his keen insight and can re-embark on burning FF's with reckless abandon.

    Until that time, I'm going to stick with the current overwhelming body of scientific research.

  5. A Christmas gift idea for Dr Ward and Likeitwarm:

    Prygeometers

     

    Response:

    [RH] Shortened link

  6. 1600. Rob Honeycutt et al
    I'll wear that name proudly!
    I'm just looking for the most plausible reasons for climate change.
    I have found a number of theories. You will call them all quackery because they are not your theory.

    I like Peter Ward's. Scroll down the page you sent me and read Peter's responses to his challengers. He makes a lot of sense. His challengers did not prove him wrong, only disagreed with him.
    What I find wrong with your version of the science is that you say the small amount(less than 8% of all IR from the surface) re-radiated IR from a colder part of the atmosphere causes warming of the surface per Trenberth chart. That cannot happen. Your radiated photons from all emitting gases carry wave length and amplitude dependent on temperature emitted from. Not enough energy to heat the surface there. Per Ward 2015 colder IR is reflected by warmer object, not destroyed.
    Magically, your chart shows the down welling radiation is greater, almost double, than what the sun supplies. Satellites see 255k for the temperature that is radiated from about 5-6 km altitude, not from the surface. The surface is warmer, not from the GHE, but from gravity doing work on the atmosphere causing adiabatic heating. This is why near surface temperatures are ~33c warmer than Planck equations predict. That makes sense unlike the GHE raising the temperature that much.
    There is no experiment showing co2 warms the atmosphere.
    There is no measurement showing human emissions of co2 cause the recent warming.
    All you have is a correlation that doesn't prove anything.
    The extra UV-B radiation reaching the surface warms the ocean and the warmer ocean emits more co2 per Ward 2015 makes sense and he does have a correlation with ozone levels and temperatures. Read his paper I linked to.

    I know you like labels, but get the label right.
    It's "CO2 causes climate change science denier" not "climate science denier".

  7. Likeitwarm @1606 :

    Please make up your mind ~ are you arguing that "gravity" is causing the 288K temperature of the Earth's surface . . . or is the 288K the result of Dr Ward's good vibrations  of non-photons ?

  8. Likeitwarm... "I'm just looking for the most plausible reasons for climate change."

    This is quite obviously untrue because clearly the most plausible reasons for climate change come from the overwhelming body of research demonstrating that CO2 is the primary cause. 

    I think you just don't like that conclusion and are cherry picking your way into "alternative facts," as they say.

  9. Let's put it this way: It is wholly implausible that the past 200 years of science, consisting of 100's of thousands of research papers, is getting this wrong in any fundamental sense.

  10. Likeitwarm, I usually don't comment on the thermodynamic thread, but you have it completely backward.  Scientists have never argued that CO2 causes warming because they measured warming and then said CO2 was the cause.

    In fact, scientists measured the IR absorbtion spectrum of CO2 (in 1850) and then predicted in advance that increasing CO2 would cause the atmosphere to increase in temperature.  Your claim that "All you have is a correlation that doesn't prove anything." is false.  Scientists predicted 150 years in advance that increasing CO2 would cause the atmosphere to heat up.

    A prediction in advance that is correct is very strong evidence in support of a theory.  Can you provide an example of a prediction that your data sourse has made that was correct???

  11. Likeitwarm... "The surface is warmer, not from the GHE, but from gravity doing work on the atmosphere causing adiabatic heating."

    If this was true, how do you get these shifts in the thermal gradient of the atmosphere? Inquiring minds want to know if you think gravitity is fluctuating through the vertical profile.

  12. "I'm just looking for the most plausible reasons for climate change""

    And yet you somehow pick on the most screaming laughable, unphysical explanations because they suit your preference. You cant just cherry pick bits of physics to back up an explanation. All the laws of physics apply, apply the time. Frankly no one like the idea that we have to stop to emitting GHG but scientists are too honest to ignore it. Invoking "reflecting photons" and other fairie dust doesnt cut it.

  13. Yes, the "explanations" that Likeitwarm prefers are indeed "the  most screamingly laughable, unphysical explanations".

    In comment 1606, Likeitwarm brings us back to the laughable paper by Peter Ward. Likeitwarm says:

    Your radiated photons from all emitting gases carry wave length and amplitude dependent on temperature emitted from.

    This repeats the laughable statements from Peter Ward, that attribute "amplitude" as a characteristic of radiation. Radiation does not have "amplitude". It has been over 100 years since physics addressed the issue of particle-wave duality in radiation. Radiation has attributes that can be described or explained by treating it as a particle, and radiation has attributes that can be described or explained by treating it as a wave - but in none of those scenarios does "amplitude" show up.

    When Planck's Law calculates more energy at a specific wavelength or frequency from hotter objects, the increased energy comes from more photons, not higher amplitudes of waves. Every single photon that ever existed at a specific wavelength had exactly the same energy, regardless of when, where, or at what temperature the emission occurred. Any talk about "cold" and "hot" photons of the same wavelength is crank physics.

    The "explanations" that Likeitwarm prefers are not simply "CO2 causes climate change science" denial, or even "climate science" denial - they are basic physics denial.

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