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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 76 to 100 out of 673:

  1. Re #75 muoncounter, you wrote:- "This seems to summarize the universe of the skeptic: It is what they say it is; until they say something else and believe that too -- even if it contradicts their prior position." You seem to have forgotten that AGW is all about a rise in global temperature above some historical average i.e. the whole dispute is about temperature. Thermodynamics is the science of heat of which temperature is a measure of strength, intensity - analogous to pressure in fluids. How much heat - is a quantity measure and, for heat, is specified in Joules (ergs or calories if you like). It is interesting to note that certain posters on this thread have dismissed the science of thermodynamics (2nd Law of Thermodynamics and so-forth) as if it were some kind of fantasy, instead of being the basis of all modern physics that it is.
  2. #76: "seem to have forgotten the whole dispute is about temperature." I most certainly did not forget that; I saw a high over 29C yesterday, two days before Thanksgiving! My point was -- and still is -- that one can see a broader picture of 'the state of skepticism' emerge by looking at several threads here in context and counting the contradictions. "certain posters on this thread have dismissed the science of thermodynamics" Yes, despite the number of times it is shown to be incorrect, there is a recurring skeptic/denialist meme that the enhanced greenhouse effect somehow violates thermodynamics. "2nd Law of Thermodynamics ... being the basis of all modern physics " I'd quibble with that, but its not at all the topic.
  3. Re #77 muoncounter, you wrote:- "there is a recurring skeptic/denialist meme that the enhanced greenhouse effect somehow violates thermodynamics." I haven't found anything in thermodynamics (or statistical mechanics for that matter) about gases (of any sort) causing a change of temperature simply by being there. Do you have a good link?
  4. #78: "about gases (of any sort) causing a change of temperature simply by being there. Do you have a good link?" I don't know what you mean by 'simply by being there', but here's not just one, but two good links. And two more here.
  5. damorbel, your reply again completely misses the point of my comment. I was trying to refute what I thought was your claim that each individual photon carries information about the temperature of its source, and that the target of that photon can use that information to reject a photon that came from a source cooler than the target. If that was not your claim, I apologize for misunderstanding, but then I do not understand why you think the greenhouse gas effect violates the second law of thermodynamics (the topic of this post we are commenting on).
  6. damorbel, like muoncounter and Tom Dayton I'm often at a loss to figure out what you're trying to say. Just as a friendly suggestion, it might be helpful to try to be a bit more straightforward and clear in your comments. For example, this: I haven't found anything in thermodynamics (or statistical mechanics for that matter) about gases (of any sort) causing a change of temperature simply by being there. What do you mean when you say "simply by being there"? Taken literally, that's an absurd suggestion, and one that has nothing whatsoever to do with the greenhouse effect. So I'm sure you didn't mean that to be taken literally. But I have no idea what you did mean by that sentence.
  7. KR, first you say my argument is wrong because any electromagnetic radiation hitting any mater will get absorbed. When I specifically asked tor some evidence for that claim I failed so see any presented. But let me help you out. I assume you have already seen diagrams showing certain types of matter do absorb electromagnetic radiation at different rates depending on the wavelength constituting so called absorption bands. As a rule they show also bands where no absorption takes place. I assume this might constitute some evidence to back my argument. Now you come and accuse me of a logical fallacy saying I am missing necessary technical background. I am ready to accept that if you would be so kind as to explain what is missing in my argument that renders it invalid.
  8. Damorbel and h-j-m, you've had the flaws in your position explained with simple everyday examples to the contrary which even a child could understand. You've been pointed to numerous resources which disprove your ideas at various levels of technical explanation. Yet you stubbornly repeat nonsensical mis-statements of the second law of thermodynamics. Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures. Since the amount of energy released by objects increases with the amount of energy they contain this inherently means that NET energy flows always run from 'hot' to 'cold'. Your belief that this means energy can ONLY flow from 'hot' to 'cold' is simply nonsense, and rejected as such by all but the outermost looney fringe of modern physics. Energy enters the Earth's climate from the Sun. Greenhouse gases delay the escape of this energy from the atmosphere just as insulation on a house delays the escape of heat generated by a furnace. This delay in energy escape means that the steady incoming energy from the Sun/furnace is supplemented by retained energy which has not escaped yet due to the greenhouse gases/insulation. Thus, the Earth/house is warmer with the greenhouse gases/insulation than without. Really. Insulation on your home DOES actually exist. It DOES allow your house to be warmed to higher temperature than it could without the insulation. This is reality... not a violation of the second law of thermodynamics. You have experienced it actually happening... so why cling to a misinterpretation of physics which says it CANNOT happen?
  9. Re #81 Ned you write:- "What do you mean when you say "simply by being there"? Taken literally, that's an absurd suggestion, and one that has nothing whatsoever to do with the greenhouse effect." Take a balloon full of O2/N2 mixture and measure its temperature at various places, top bottom etc.; add some CO2, do you expect any of the temperatures to change? Replace all the O2/N2 mixture with CO2, do you expect a temperature gradient anywhere? If you found one, would you be able to use it to drive an engine? PS1 You can heat the balloon with radiation if you wish. PS2. Temperatures must have stabilised before you do the measurements, just like in the atmosphere.
  10. Re #83 CBDunkerson you write:- "Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures" Tell me something, as the energy flows do the temperatures of the objects change? If the temperatures change (they will!), can you say how and why?
  11. damorbel, The experiment you are describing was first conducted by John Tyndall in 1859 & the results published in 1861. American Institute of Physics: The Carbon Dioxide Greenhouse Effect On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connection of Radiation, Absorption, and Conduction
    Tell me something, as the energy flows do the temperatures of the objects change?
    If there is a net difference in the energy flows, yes. If the net energy flow is zero, no.
  12. Re #86 Bibliovermis I have no problem with Tyndall's observation that H2O vapour and CO2 absorb and emit radiation and, as you write:- "If there is a net difference in the energy flows, yes. If the net energy flow is zero, no." But in #83 CBDunkerson wrote:- "Every object in existence emits energy in all directions and this energy flows into all surrounding objects regardless of their relative temperatures." You can't both be correct.
  13. Yes, both statements are correct. Every object in existence emits energy is all directions and the energy flows into all surrounding objects regardless of their temperature. When the net energy flow is zero, there is no temperature change. When there is a difference in the energy flows, the temperature changes. Did you not know about Tyndall's work before I referenced it?
  14. damorbel, when Bibliovermis wrote "net energy flow is zero," the word "net" means the difference between the incoming and outgoing energy for each object. Let's say object A has 100 units of energy coming in and 80 units going out. Object A's net energy gain/loss is a gain of 100-80=20, so object A warms.
  15. Re #88 Bibliovermis you write:- "Every object in existence emits energy is all directions and the energy flows into all surrounding objects regardless of their temperature." So the T^4 Stefan Boltzmann thermal radiation law is quite mistaken? I suggest you are on dodgy ground here! The second law of thermodynamics says heat always flows in the direction of the colder place, it doesn't matter whether it is by conduction, convection or radiation that's what the experiments (and common experience) show.
  16. Please explain how that quoted comment contradicts SB. The second law of thermodynamics says net heat always flows in the direction of the colder place.
  17. damorbel, you are being confused by the term "heat." That is a common confusion, because that term is used in multiple ways, sometimes loosely. In the case of the second law, that term means the net flow of energy. The second law says nothing about the constituent two flows of energy in the two directions, only about the final result.
  18. #84: "Take a balloon full of O2/N2 mixture and measure its temperature ... add some CO2," Those experiments are done, although usually with bottles; you can see them on youtube if you look. When you put both an 'air' bottle and a bottle with extra CO2 under a source of external radiation, the CO2 containing bottle attains a higher temperature. "Replace all the O2/N2 mixture with CO2, do you expect a temperature gradient anywhere?" That's just nonsense, as your PS2 required temperature equilibrium.
  19. Re #89 Tom Dayton, between two places there is no other kind of energy flow than net energy. You write:- "Object A's net energy gain/loss is a gain of 100-80=20, so object A warms" But you do not mention where the 'in flowing energy' comes from, I do not wish to suggest you think it comes from an object B or place B at a lower temperature, which of course would conflict with the 2nd Law
  20. damorbel, It does not matter where the energy flows in from. Energy is energy, regardless of the temperature of the emitting object. Energy flowing from cooler object B to warmer object A does not contradict the 2nd law of thermodynamics, because more energy flows from warmer object A to cooler object B. That is what "net" means - more energy flows to relatively cooler objects than flows from them. I am still wondering if you had never heard of Tyndall's work before I referenced it here.
  21. damorbel, I see that I made a mistake on this object A vs. object B point. Object A would be warming (i.e. receiving a net flow of energy) from energy flowing from a relatively warmer object, object C. It would be receiving energy from the cooler object B, but would be emitting more energy to it. Again, that is what "net" means. Do you seriously not understand the basic concept of net change?
  22. Re #96 Bibliovermis, you wrote:- "Do you seriously not understand the basic concept of net change?" I just don't recognise any other. BTW I have had a copy of Tyndall's articles for some years now and I see no fault in them, although there have been many developments in thermal physics since he wrote them. In particular the roles of kinetic theory, quantum theory and thermodynamics in thermal physics; all three of these emerged because older theories did not explain experimental observations. Kinetic theory resolved the behaviour of heat in gases and lead to more efficient heat engines; thermodynamics not only extended kinetic theory to the general problems of heat in solids and liquids but also to chemical reactions. None of these resolved the observations of radiative heat transfer, it was Max Planck who opened that door, it is best explained these days by quantum electrodynamics (QED). You will need to understand QED if you wish to get a grip of the efficiency of energy processes in lasers.
  23. #97: "'seriously not understand the basic concept of net change?'" I just don't recognise any other." Perhaps that's the problem. Here is a summary of 2nd Law statements from a class at MIT. Note the figure below, which appears under the statement: No process is possible whose sole result is the transfer of heat from a cooler to a hotter body. The key word is sole, which appears in bold in the original for good reason. The caption states for T1 less than T2 this is not possible. However, we know that both objects radiate, albeit at different wavelengths. Some of the cooler object's radiation is absorbed by the warmer; however, more total energy is transferred from warmer to colder. The 2nd Law is satisfied and the greenhouse effect still works. As for the rest, you can talk QED if you like, but that will not help you answer any of CBD's excellent points.
  24. h-j-m and damorbel. You might like to explain the temperature distribution illustrated here
  25. damorbel, previously I was explaining to you merely that "net" means "difference." Now here is what the second law says for objects A and B, complementing the picture muoncounter provided. "Net" in this case means the difference in the flows between the two objects: Given: HeatFlow(from A to B) - HeatFlow(from B to A) = Net Heat Flow, If A initially was hotter than B, then Net Heat Flow has a positive sign. (A cools and B warms.) If B initially was hotter than A, then Net Heat Flow has a negative sign. (B cools and A warms.) If you want to nitpick about the word "heat," substitute "Energy." Example: A Flow To B = 100 units B Flow to A = 80 units Net Flow = 100-80 = 20 units from A to B. From A's perspective, A gets 80 units and emits 100 units, so A ends up with 80 - 100 = -20 units relative to its initial state. From B's perspective, B gets 100 units and emits 80 units, so B ends up with 100 - 80 = +20 units relative to its initial state.

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