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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 151 to 175 out of 405:

  1. That spectral chart is a visual example of what I said - incoming solar energy is primarily in the form of visible & UV light. For your other question, about what makes greenhouse gases so special, you could read the 150 year old research report by Tyndall that I linked to earlier. I apologize for not relinking as this is being tapped out from my phone.
  2. Ah, h-j-m's point (I think) could be stated as follows Since GHG's absorb in the visible (as well as the infra-red), doesn't increasing the concentration mean that the earth receives less energy because the subsequent emission of that radiation scatters some of it into space - back radiation on incoming EM which thus goes into space. Assuming I've understood h-j-m's issue correctly let me offer the following rebutals 1. Absorption of EM radiation, either visible incoming or IR outgoing does not result in all the radiation being emitted, some will be converted into vibrational, rotational and translational energy. Thus increasing absorption of visible incoming EM will, in certain extent warm the upper atmosphere as well as decrease slightly the EM hitting the surface. 2. The areas of the incoming EM spectrum in which H2O (primarily) absorb are not near the peak of frequencies and are at the longer wavelength(lower energy) end. Thus they do not absorb proportional as much as the outgoing earth-light. 3. The Greenhouse effect is measured empirically by comparing the temperature at the top of atmosphere with the ground. The commonly quoted 33deg is therefore the nett effect of IR absorption of Earthlight coming out - Visible absorption of Sunlight coming in. I seem to remember seeing 50deg as a figure for the 1st, but I'll be damned if I can remember where ... Thats quite a long response to a point that I may have misunderstood, and is getting increasingly off-topic for this particular thread... :-(
  3. Re #144 archiesteel You write: "To say they do is either to misunderstand the science, or to disingenuously misrepresent what climatologists believe." Lets look at Trenberth's diagram:- You write:- "The point is not that all of the absorbed photons will go back down. The photon re-emission by GHG molecules happens in a random direction." Trenberth's diagram shows 165Wm^2 going out from the atmosphere, 30Wm^2 from clouds 360Wm^2 going up from the ground and 324Wm^2 back radiation going down. The problem is both the back radiation and the ground radiation. First they are both greater than the input from the Sun, second they are not reflected by the ground or the clouds whereas the Sun's input is reflected by both the clouds and the ground. Third the Suns input is absorbed (67Wm^2) by the atmosphere, why isn't the '324Wm^2 back radiation' similarly absorbed? Since 'back radiation' is emitted by GHGs it does not have a short wave component like sunlight, so a bigger % of the 324Wm^2 is going to be reabsorbed by GHGs. All I was doing in my post #143 was drawing attention to Eli Rabbet's recognition that this IPCC diagram does not represent anything real. Using Eli's explanation there is no substantial downward radiation because the photons emitted by GHGs are largely absorbed locally and certainly never get to the ground at a level comparable to the Sun's input. Likewise Earth's 390Wm^2 surface emission cannot be well over double the 168Wm^2 arriving there from the Sun. The IPCC claims that the GHGs warm the surface by about 33C but there are no numbers on this diagram showing how this happens, even though the various places are shown emitting and absorbing radiation, there are no temperatures showing the basis for explaining the greenhouse effect. Is this the way we plan to change the World fuel economy?
  4. Composer99, the wikipedia diagram as well as a similar diagram (unfortunately only black and white) on the mentioned Science of Doom page show clearly that incoming radiation gets absorbed (by water vapour mostly). The Trenberth et al. diagram I have included in my post #50 shows ~ 20% of incoming energy absorbed by atmosphere. I doubt you could justify neglecting it. Nevertheless I completely failed to find any credible further information on that subject. The rest of your post urges me to some provocative questions. How does the earth measure the incoming radiation? How does the earth measure the outgoing radiation? How can it tell the difference? But if it can do this. Who told the earth that there should be a balance? How can he force the earth to respond? Consider these questions answered. How can the earth know what to do? Does the earth have the means to do what needs to be done? Sorry, somehow that sounds rather non scientific, but I could not help writing it anyway.
  5. Damorbel @ 153 - The problem is both the back radiation and the ground radiation. First they are both greater than the input from the Sun Well, yes both surface and back radiation occur night and day, whereas incoming solar radiation doesn't.
  6. damorbel - "... there is no substantial downward radiation because the photons emitted by GHGs are largely absorbed locally and certainly never get to the ground at a level comparable to the Sun's input." Nuh. Any molecule that can absorb radiation must, by definition, emit radiation. The fact that radiation is absorbed, emitted, absorbed again, emitted again multiple times within the atmosphere before striking the surface (again) or eventually escaping at TOA is what the "greenhouse effect" consists of. All this bouncing around is the evidence of energy staying in the system. When there are more GHGs in the atmosphere, more energy stays in the system longer.
  7. You are aware that backradiation etc is MEASURED? (Look for DLR stations). If your understanding mismatches experimental observations, then your understanding is wrong. As to balance - Planck radiation is the "balancing mechanism". First Law of thermodynamics - that you cant destroy energy - is why you have balance. If a body absorbs energy its temperature rises - temperature is expression of average molecular kinetic and potential energy in the body. It emits radiation in proportion to its temperature. When radiation outgoing matches incoming energy temperature stays constant. Its simple physical law, readily demonstrated a lab. A body "knows" what do in accordance Stefan-Boltzmann, derivable from QM theory - a moving charged particle must irradiate. Incoming and outgoing radiation are measured at TOA by atmosphere. There is problem with the measurements in they have good precision but poor accuracy.
  8. @damobel: the graph simplifies how the heat exchange mechanisms work. You can't look at such a graph and claim it is supposed to accurately represent the path of each photon. Others have explained this clearly. If you still can't understand it at this point, then one of two things must be true: a) this is beyond your intellectual capacity, or b) you're not debating in good faith. There are a lot of trolls and astroturfers here, please don't join their ranks and make a serious effort to read the material on this site before repeating the debunked junk peddled by professional climate deniers...
  9. Bibliovermis wrote: "That spectral chart is a visual example of what I said - incoming solar energy is primarily in the form of visible & UV light." So far so true if you define primarily to be more than 50%. But I would not dare to call 45% negligible. Further you stated that incoming solar radiation is not effected by green house gases but the chart clearly shows the opposite.
  10. Re #158 archiesteel you wrote@ "the graph simplifies how the heat exchange mechanisms work. You can't look at such a graph and claim it is supposed to accurately represent the path of each photon." I take it you mean the diagram in #153? I am not arguing that there aren't better explanations for the GH effect but this diagram is what is used by the IPCC in its Assessment Reports and its Summaries for Policymakers. This is what is used by government agencies like the Met Office when they are advising on energy source policies. The diagram is a principle feature of the IPCC AGW science, it appears in volumes of the various IPCC Assessment Reports called "The Scientific Basis". In thess volumes it is stated that the GHE warms the surface (on average) from 255K to 288K i.e. 33K. If the diagram were improved it would show just how this 33K comes about. As it stands there are no temperatures at all reported on it, this should be possible using themal models of the atmosphere such as the US Standard Atmosphere (which does not recognise the GHE), then perhaps the arguments of the IPCC will become more clear.
  11. Phil, first you say as Bibliovermis did, that incoming solar radiation is not effected by green house gases. Now you state otherwise, but now you claim that the effect on outgoing radiation is greater though you don't say how much as well as to provide any evidence. OK, again that leaves all the work for me. Then I will try evaluate the effects. If I am not mistaken then usually differences between TOA and the earth surface are taken as a measurement of the green house effect. Seems quite reasonable, let me try. I suppose all can agree that I use the numbers provided by the Trenberth et al. diagram I have included in my post #50. It says incoming at TOA 341, reflected 102 and absorbed at surface 161. As the reflected part is not affected by the green house effect it has to be taken out (subtracted from the TOA value) which leaves us with an effective TOA value of 239. So the relation is 239 / 161 = 1.48 Now we have surface radiation 396 Radiation leaving the atmosphere is at 239. Here the relation is 396 / 239 = 1.65. So, yes you are right, the green house house effect is stronger on outgoing radiation though I hardly assume the magnitude of the difference satisfied your strong wording. As to your comment about this being rather off topic I have to disagree for the reason that the whole argument of the lead article rests on the green house effect. So any discussion on this is quite on topic.
  12. Concerning my post #148: Seemingly no one contradicted my postulations concerning the behaviour of gases. So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases. This of cause leaves the green house gases off the hook when subjects like back-radiation are concerned, as it should be clear that the whole atmosphere plays a part in that game. That of cause, as Bibliovermis has correctly pointed out when he referred to Tyndall, points to the green house gasses speciality being able to trap (meaning store) heat and this way delay its further transmission. But considering this the most significant data with respect to global warming should be the specific thermal capacity of green house gases and I wonder why I can not recall it being mentioned. Someone willing to offer further information on this?
  13. h-j-m "Seemingly no one contradicted my postulations concerning the behaviour of gases. So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases" I can see one more possibility, people gave up trying to explain if you don't even bother to check this two century old physics.
  14. #162: "So I can surely say that the emitting of radiation as a result of absorbing energy can be attributed to all gases." You can say whatever you like; whether what you say is correct or not might matter to some. Look at these lecture notes for some further basics, including a model of how gas molecules absorb energy.
  15. #160: "If the diagram were improved it would show just how this 33K comes about." That would be called reinventing the wheel. Look here, particularly the paragraph beginning "If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth,"
  16. @damorbel: the graph serves its purpose. It is not misleading to anyone with any kind of base scientific knowledge. I'm sorry, but it really sound as if you're grasping at straws, here. The greenhouse effect is real, a fact the majority of climate change skeptics recognize. Heck, I even had skeptics here assure me that "no one disputes the greenhouse effect"...yet it seems that this is exactly what you're (unsuccessfully) attempting here. Is this really a wise tactic on your part?
  17. RE#148 h-j-m. Drawing on these texts [**] I’m going to attempt to answer this question: Why is CO2 a greenhouse gas? Introduction... Gas molecules whether they be CO2, N2, O2, CH4, CO, H2, He, Ar etc will all interact with light at specific frequencies. So for example if a single photon is absorbed by one of these gas molecules the absorption or emission of a photon will be accompanied by a change in the internal energy state of the molecule. This is a consequence of Quantum Mechanics that a molecule can only take on values drawn from a finite set of possible energy states. The distribution of which is determined by the structure of the molecule. The energy states involved in infrared absorption and emission are connected with displacement of the nuclei in the molecule, and take the form of vibrations or rotations. So how does the number of atoms in a molecule and its geometry effect this? The noble gases like He, and Ar are have one atom (monatomic) and have only electron transitions, so are not active in the infrared. And indeed QM calculations and lab experiments verify this. A molecule with two atoms (a diatomic molecule) eg CO, O2, N2 amongst others has a set of energy levels associated with the oscillation caused by pulling the nuclei apart and allowing them to spring back and forth. Now triatomic molecules (like CO2 or H2O) have an even richer set of vibrations and rotations, especially if their equilibrium state is bent rather than linear. What specifically then, makes one type of gas molecule more infrared active than another... For a molecule to be a good infrared absorber and emitter, it is not enough that it have transitions whose energy corresponds to the infrared spectrum. In order for a photon to be absorbed or emitted, the associated molecular motions must also couple strongly to the electromagnetic field. You can classically think of the infrared light as providing a large scale fluctuating electromagnetic field which alters the environment in which the molecule finds itself in, and, exerts a force on the constituent parts of the molecule. This force displaces the nuclei and electron cloud, and excites vibration or rotation. The strongest interaction that will happen between an electromagnetic field and a particle is one where the particle has a net charge. A charged particle will experience a net force when subjected to an electric field, which will cause the particle to accelerate. In relation to Earth's atmosphere... Ions are extremely rare in the atmosphere. Thus molecules involved in determining a planet’s energy balance are almost invariably electrically neutral. So where does this leave us? Ok we have now elimated charged particles…so what’s the next best physical property of a molecule that will make it a strong infrared active one? Why molecules that have a dipole moment! (This is when we have a disproportionate part of a molecule’s negatively charged electron cloud bunched up on one side, while a compensating excess of positive charged nuclei are at the other side.) Does our atmosphere have a molecule that fits this criteria? Yes! Good old CO2! CO2 is a linear molecule with the two oxygens symmetrically lying about the central carbon. Whilst a uniform stretch of such a molecule does not create a dipole moment, a vibrational mode which displaces the central atom from one side to the other does. Addionally, the bending modes of CO2 have a fluctuating dipole moment, which can in turn be further influenced by rotation. Modes of this sort make CO2 a very good greenhouse gas. Here you might ask, but the atmosphere is full of O2 and N2 and there is only ppm concentrations of CO2? Many common atmospheric molecules have no dipole moment in their unperturbed equilibrium state. Such nonpolar molecules can nonetheless couple strongly to the electromagnetic field. They do so because vibration and rotation can lead to a dipole moment through distortion of the equilibrium positions of the electron cloud and the nucleii. Diatomic molecules made of two identical atoms, do not acquire a dipole moment under the action of either rotation or stretching. Symmetric diatomic molecules, such as N2, O2 and H2 in fact have plenty of rotational and vibrational transitions that are in the infrared range. However, because the associated molecular distortions have no dipole moment, these gases are essentially transparent to infrared unless they are strongly perturbed by frequent collisions. This is why N2 and O2, the most common gases in Earth’s atmosphere do not contribute to Earth’s greenhouse effect. However, it is important to recognize that situations exist in which diatomic molecules become good greenhouse gases are in fact quite common in planetary atmospheres. When there are frequent collisions, such as on planets with high density atmospheres like Titan and on all the giant planets, diatomic molecules will acquire enough of a dipole moment during the time collisions that are taking place ,and the electromagnetic field can indeed interact with their transitions quite strongly. This makes N2 and H2 the most important greenhouse gases on Titan, and H2 a very important greenhouse gas on all the gas giant planets. I don't think I even scratched the surface, but hooray for physics! [**] Principles of Planetary Climate, R. T. Pierrehumbert Molecular Quantum Mechanics P. W. Atkins (Author), R. S. Friedman An Introduction to Statistical Thermodynamics. T. Hill
  18. Wow, yocta, that was the best explanation I've ever read! Thanks!
  19. addendum to yocta @167 Its also true that the asymmetric isotopic variants of O2 and N2 absorb ever-so slightly in the IR: N14-N15 for example. This is because the stretching vibration becomes ever so slightly asymmetric because of the differing weights of the two nuclei. Because the dipole moment change is so small, and the proportion of isotopes so small, and the frequencies at which these vibrations occur is outside the range of "earthlight" their contribution to the GHE is effectively zero. Nevertheless there was one contributor to this site trying to argue the case a few months back :-(
  20. "Wow, yocta" Double-plus good job! Here are some illustrations of the CO2 molecule's vibrational modes.
  21. h-j-m @161 Your calculations are not correct because they assume that scattering of incoming UV-visible EM and outgoing IR are done by the same molecules in the atmosphere. The largest contributor to scattering UV-visible light is, in fact, Ozone (O3) which is contributing a substantial proportion of your 1.48 figure. You are, in effect, comparing apples with oranges.
  22. yocta, in my post #148 my question was "what specifically makes green house gases so special". Which means: What effects do green house gases produce that other gases don't? I am sorry and apologize if my initial phrasing led to any misunderstanding. When you state at the beginning of your post you are going to answer the question "Why is CO2 a greenhouse gas?" clearly indicates some sort of misunderstanding must have taken place.
  23. Phil, my calculations assume nothing except what I wrote they assume which is 1. the assumption that differences between TOA and surface provide a measurement for the green house effect and 2. that the numbers from Trenberth's diagram are reliably correct. Following your argument the first assumption should be incorrect but then I am the wrong man to point your critique at.
  24. h-j-m, greenhouse gases absorb wavelengths of radiation that are plentifully emitted by the Earth but only weakly emitted by the Sun, thereby acting as a partially closed valve that traps energy below the top of the atmosphere.
  25. h-j-m @173 Your original point was, as I paraphrased at @152: Since GHG's absorb in the visible (as well as the infra-red), doesn't increasing the concentration mean that the earth receives less energy because the subsequent emission of that radiation scatters some of it into space - back radiation on incoming EM which thus goes into space. To answer to this in @161 you derived two numbers that measured the total visible absorption by the atmosphere of incoming EM by all gases in the atmosphere and the absorption of outgoing IR radiation by only GHGs. Since the numbers the first number was slightly smaller than the first, you then concluded that the magnitude of the difference was small: [Quote from @161] So, yes you are right, the green house house effect is stronger on outgoing radiation though I hardly assume the magnitude of the difference satisfied your strong wording. But the "magnitude of the difference" is not valid because you are not comparing like for like.

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