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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 876 to 900 out of 1008:

  1. damorbel (RE: 869), "Now be so good as to answer my question:- What % of the heat tranferred to the atmosphere from the ground by radiation:- 14%?......40%?.......90%?" I would but I'm not quite sure exactly what you're asking. How much is transferred kinetically?
  2. damorbel (@872) "Fred, what I like about 'Back Radiation' is that it goes straight into the surface, nothing is reflected, even though most of the surface is water with a refractive index of 1,33; I'm sure Fresnel is weeping in his grave!" Care to explain what the refractive index of a substance has to do with its reflectivity? I think the key quantity you should be focussing on is absorptivity, which by Kirchoff's law is always equal to emissivity at a given wavelength. In the thermal infrared (the pertinent wavelength), the emissivity of water is about 0.95. Source: http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
  3. Re #877 Stu you wrote:- "Care to explain what the refractive index of a substance has to do with its reflectivity?" All materials, transparent or not have a refractive index, it is just that some are so opaque that transmission is dominated by absorption and incoherent scattering. In practical terms the propagation of light in transparent materials is governed by the Fresnel equations you will see on the link that a beam of light arriving at a 'change in refractive index' - normally one thinks of air/glass with RI for air = 1.0 and RI for glass = 1.4 - 1.7. At the surface of the glass some of the light is reflected and some passes into the bulk of the glass. In this case the RI of air is very low so the amount reflected depends on 1/the RI of the glass and 2/the angle the beam is incident on the glass. So, if back radiation really exists, it would be reflected by any surface, liquid or solid, just like the sunlight in Trenberth's diagram. Further you wrote:- " think the key quantity you should be focussing on is absorptivity, which by Kirchoff's law is always equal to emissivity at a given wavelength. In the thermal infrared (the pertinent wavelength), the emissivity of water is about 0.95" The only problem I have is the figure 0.95 you cite for water. Yes, that is what it says in your link. I have no idea how this figure was obtained, it cannot possibly be valid for water, other transparent materials in the list have similar figures. Light incident on material can either be 1/reflected; 2/absorbed; or 3/transmitted. You clearly know that absorptivity(a) = 1 - reflectivity(r), and emissivity(e) = a What is missing here is transparency(t), light can also pass through material (glass etc.) without being absorbed (much) or reflected (much). So how the compilers of the table in your link can give a figure of 0.95 as the emmisivity of pyrex glass, I do not know, it is absurd, it is higher than the figure given for carbon, is someone joking?
  4. Re #876 RW1 you wrote:- "I would but I'm not quite sure exactly what you're asking. How much is transferred kinetically?" That is the question I asked you. But I will accept evaporation and convection as 'kinetic'.
  5. damorbel: "In practical terms the propagation of light in transparent materials..." I presume this might be where you're having trouble... in the relevant part of the spectrum (thermal infrared) water is opaque, not transparent or even translucent. It absorbs all thermal infrared incident upon in within a few tens of microns of the surface (apart from the small fraction that it reflects). Glass is also opaque to thermal infrared. No-one's joking, you just need to be aware that absorptivity is a function of wavelength. I haven't been following your discussion, only jumped in here, but I see you say 'if back radiation really exists...' Well, can you explain measurements of downwelling clear-sky longwave radiation without it being 'back radiation'? If it's not back radiation, what is it? Here's a paper that quantifies downwelling clear-sky longwave radiation as measured during Antarctica's polar night: http://journals.ametsoc.org/doi/pdf/10.1175/JCLI3525.1 I do hope you don't just dismiss these two points out of hand by simply saying 'it can't be valid' or 'it's absurd'.
  6. Re #876 Stu you wrote:- "Glass is also opaque to thermal infrared. No-one's joking, you just need to be aware that absorptivity is a function of wavelength." True, but the link doesn't say that or give any figures for different wavelengths, so I do not understand what you are driving at. In the case of water, evaporation is by far the dominant heat loss mechanism for incident radiation, even Trenberth's diagram show's this. You can easily discover the figures for yourself by checking world wide rainfall; the heat needed to evaporate water is deposited in the atmosphere when it condenses. Further "Well, can you explain measurements of downwelling clear-sky longwave radiation without it being 'back radiation'? If it's not back radiation, what is it" I have read the paper you linked plus the description of the radiometer used and it merely confirms what I wrote in #875. To summarise, the energy associated with heat is constantly being exchanged between (adjacent) molecules both mechanically by (elastic) collision and electromagnetically by absorption and emission of radiation; somewhere there is a link to a paper by Einstein in this connection. I know this sounds pedantic but it has to be said. What the radiometer used in your paper measures is the radiation from gases that emit electctromagnetic radiation because they have T>0K. What the radiometer doesn't do is measure the 'upwelling' radiation from the same gases. If you could measure the 'upwelling' radiation from the same gases then you would be able to determine how much energy was being transferred and in which direction; only then would you be able to work out what was happening to the temperatures at the various locations of interest. Measuring the 'downwelling radiation' tells you almost nothing because you are far from sure about what kind of material (pressure; density; temperature etc. - for a gas) is emitting this radiation (thus you don't know its emissivity) and where it is. The difference between 'back radiation' and the real world is that, in the real world radiation is emitted and absorbed locally, as explained by Einstein, and the world of back radiation where it comes from 'up there'!
  7. damorbel @881, I find your comment bizarre on two counts. First, the absorption strengths at different frequencies for all common, and most uncommon atmospheric components is well documented in the HITRANdatabase. Because of that, except in those areas of overlapping absorption, the actual molecules emitting the down welling IR radiation is easily determined, as indeed is the average altitude from which the gas emissions originated. So to a significant degree of accuracy, the downwelling IR radiation tells us a great deal about the kind of material - including its temperature (from its brightness temperature) and pressure (from pressure broadening)- that emitted the radiation. Second, you talk as though observations have not been made from above the locations of observed downwelling spectra. Indeed, you do so despite the fact that downwelling and upwelling spectra from the same location are shown in figure 1 of the intermediate version of the article above. These observations have been made not only by satellite, but also by aircraft from a variety of altitudes. Here, for example, are spectra obtained by an aircraft flying at 3.7 km altitude over the North Sea: And a comparison of those spectra with model results: The large spike at the point at which CO2 emissivity is highest is because the cabin temperature was slightly higher than the air at that altitude, as discussed in the paper. It illustrates the point, however, that except at the frequency of greatest emissivity, radiation emitted in the atmosphere will travel hundreds of meters, and even several kilometers before being absorbed (on average). So, "to summarize", energy is constantly being exchanged in the atmosphere between adjacent molecules by collision and radiation, and between very distant molecules at a much slower rate by radiation. In fact, the concept of "back radiation" is not premised on the radiation coming from high in the atmosphere (you are wrong about that); but your claimed refutation is based on an error. This may be a case of an ugly fact wrecking Einstein's beautiful theory, but I suspect the fact is just wrecking your misinterpretation of Einstein rather than the master himself.
  8. "In the case of water, evaporation is by far the dominant heat loss mechanism for incident radiation, even Trenberth's diagram show's this. You can easily discover the figures for yourself by checking world wide rainfall; the heat needed to evaporate water is deposited in the atmosphere when it condenses. Yes evaporation is a big consideration, but I'm not sure why it trumps radiation. Locally (or indeed regionally), the latent heat flux can be massive, especially where cold air flows over warm water, but Trenberth's schematic shows evapotranspiration having an average value of 80 W/m2, compared to 396 W/m2 radiation: Indeed if you take water to have an emissivity of 0.95 at 300 K, then it's emitting 436 W/m2 at that temperature. And regarding that value of emissivity for water: "True, but the link doesn't say that or give any figures for different wavelengths, so I do not understand what you are driving at." The problem then is just that you didn't read the part of the page that says "As a guideline the emmisivities below are based on temperature 300 K" A substance at 300 K will have its peak emissive flux in the thermal infrared. "I have read the paper you linked plus the description of the radiometer used and it merely confirms what I wrote in #875. To summarise, the energy associated with heat is constantly being exchanged between (adjacent) molecules both mechanically by (elastic) collision and electromagnetically by absorption and emission of radiation; somewhere there is a link to a paper by Einstein in this connection. I know this sounds pedantic but it has to be said. What the radiometer used in your paper measures is the radiation from gases that emit electctromagnetic radiation because they have T>0K. What the radiometer doesn't do is measure the 'upwelling' radiation from the same gases. If you could measure the 'upwelling' radiation from the same gases then you would be able to determine how much energy was being transferred and in which direction; only then would you be able to work out what was happening to the temperatures at the various locations of interest." When you say 'energy associated with heat is... exchanged... by absorption and emission of radiation', you need to qualify it with the fact that only greenhouse gases do any absorbing or emitting. Again, it's not just that gases emit because they have T>0K. N2 and O2 do next to no emitting and their energy has to be transferred by collision to a GHG in order to be radiated away. Also, radiation is emitted isotropically; if you measure a certain downward flux, you can be sure that the 'layer' of the atmosphere you're measuring is emitting the same flux upwards. I think Tom Curtis has answered your other points nicely.
  9. damorbel - I generally do not respond to your postings, as they seem to follow 'trolling' principles (lots of red herrings, for example), but I will on the following howler. I apologize for my tone, but this has gone on far too long. "Measuring the 'downwelling radiation' tells you almost nothing because you are far from sure about what kind of material (pressure; density; temperature etc. - for a gas) is emitting this radiation (thus you don't know its emissivity) and where it is." Measuring downward IR at the surface tells you exactly what you need to know - the amount of energy coming down from the atmosphere at the air-surface interface, and hence the appropriate values for the energy budget. That's 333 W/m^2 downward IR, repeatedly and accurately measured. Your quibbling does not change the data! Your objections are really just red herrings, damorbel, a pattern you have repeated over and over and over again on this thread for multiple months - every time you get corrected, you change the subject. We know the energies, we know (as per Tom Curtis's post) where those come from in the atmosphere, we know the upwelling radiation, and we really really do know what the IR absorptivity/emissivity of water, sand, dry and wet dirt, etc., are. And we know the physics of CO2, EM, and thermodynamics. The radiative greenhouse effect is fully supported by all of this - no other viable explanations have been put forth. ---- Back to the actual thread: The "2nd law" objection to the greenhouse effect is based upon a mistaken notion (As per Gerlich et al) that a cool object cannot add energy to a warmer object, since net energy transfer is in the other direction - a classic Fallacy of Division, as net transfer is a statistical effect, not a restriction on individual photons. Hence the "2nd law" objection is false. Are there any actual issues with this that are on topic? Moderators, might I suggest that topic adherence be strongly enforced, as we're at >850 postings on this topic, many of which are serious digressions?
    Response:

    [DB] "might I suggest that topic adherence be strongly enforced"

    Agreed. In the words of the King:

    "So let it be written, so let it be done!"

  10. Damorbel keeps digging while trying to impress by citing Einstein (in German, for added effect). "every time you get corrected, you change the subject." You reckon KR? I'm still waiting on how the energy of a photon is affected by the temperature of the source. I'm not counting on a quantum theory revolution any time soon. Damorbel knows just enough vocabulary to impress and confuse the gullible but he has repeatedly demonstrated the lack of understanding of his own words. I am unmoved with both him and LJR. So far, in a lot less than the 850 posts contained in this thread, I've seen the howler I cite here, the one you addressed above and LJR asking what the Earth emissivity is in the SW. But they're ready to give lessons to everyone and caution against the bold "hypothesis" of an atmospheric GH effect. What a farce. It's almost as if they are in disguise, on a campaign to completely discredit the ridiculous GH effect skepticism launched by G&T.
  11. Re 885 Philippe Chantreau you write:- "I'm still waiting on how the energy of a photon is affected by the temperature of the source." I think it is sumarised inWien's displacement law
  12. damorbel - Your reference clearly demonstrates that identifying the temperature of an emitting object requires a statistical analysis of a spectra of photons, meaning that an individual photon does not by itself identify the temperature of the source. Which therefore indicates that your assertion here is incorrect. See my earlier post on photon absorption in that regard. Photons, unlike Arizonan citizens, do not carry ID cards indicating their origin. Do you have any comments on the actual issue of this thread, the (observed) exchange of energies between cooler and warmer objects and the implications thereof toward the radiative greenhouse effect?
  13. Moderator/DB - I believe damorbel's last comment actually was on topic, if wildly incorrect in it's implications: multiple assertions have been made that the photons from cooler objects hitting warmer ones do not reach / don't get absorbed / are turned away by a restrictive bar-room bouncer after being carded, in violation of the 1st law of thermodynamics and the physics of absorptivity for individual photons. So while (as I said) quite incorrect, it is somewhat relevant - if only to highlight the errors made by "2nd law" objections.
    Response: [DB] Apologies then. My initial read found context for his linked reference lacking in his comment. Sometimes one because conditioned to expect certain things...
  14. To fill out my last post: Any argument that photons from a cool object will not add to the internal energy of a warmer object they impinge upon due to their origin are specious; there is no such information encoded in the energy of an individual photon, absorption is based solely upon that energy and the absorption spectra of the impinged object. Hence any such "2d law" energy transfer objections are clearly wrong, and based upon a misunderstanding of the physics involved.
  15. Re #887 (& 849) you wrote:- "identifying the temperature of an emitting object requires a statistical analysis of a spectra of photons" Temperature is the property of an individual particle since it is the measure of the energy in that individual particle. I have mentioned this before and nothing about statistics is necessary to establish this. What you are thinking of is the temperature of an ensemble of many particles that are interacting strongly with each other (in equilibrium) so that the particles have a distribution of energies, the distribution is called the Boltzmann distribution. Your #849 directly contradicts the basis of quantum theory. You write:- "An object, at 20C, has an 80% absorptivity for 6 micron photons" For a start - absorbing takes place at the particle level. Further, to be absorbed a photon has to encounter the absorbing particle in the correct phase etc. But, according to quantum laws, the photon cannot be absorbed at all unless it has sufficient energy E (E=hv, 'h' is Planck's constant and 'v' is the characteristic frequency) This was the basis of Einstein's 1905 paper on the Photo-electric effect It is an experimentally observed fact that photons must have sufficient energy i.e. a sufficiently high equivalent temperature (discovered from the Wien displacement law) before they can be absorbed and cause an electron to be emitted. This applies in all areas of physics and chemistry, ozone is formed when O2 is split into 2 x O atoms, with light; but this only happens with ultraviolet light (UV) shorter than 0.2microns
  16. damorbel - Quite a kettle of fish in that post, which is to say red herrings. I note that by comparing it to facts: Absorption does not require photoelectric emission of an electron; that's a completely different question - Herring. Individual photons have energies, not temperatures - False statement. Estimates of object thermal emission temperatures require a spectra (statistical knowledge) and some idea of the emission spectra. An individual photon is incapable of supplying sufficient information - Incorrect statement and herring. "...absorbing takes place at the particle level. Further, to be absorbed a photon has to encounter the absorbing particle in the correct phase etc." - Herring. My statement in this post regarding absorption points out that if the absorption spectra of an object has a value of 0.8 for 6 micron photons, it has an 80% chance of absorbing any 6 micron photon, no matter the origin. - Your post is flatly, completely, wrong.
  17. Re #889 KR you wrote:- "Any argument that photons from a cool object will not add to the internal energy of a warmer object they impinge upon due to their origin are specious; there is no such information encoded in the energy of an individual photon" But you are making much of the statistical distribution of energies in a given sample of particles with a given temperature. To start with, to have a defined temperature the particles must be in equilibrium, the average energy must be steady. In this condition the average energy is the total energy of all particles in the sample divided by the number of particles in the sample. But there is no need to have a certain number of particles to make a sample, so one particle with the same energy as the average energy of all the particles also has the same temperature as the whole sample.
  18. Wein's displacement Law: 1. A black body emits photons at range of frequencies. 2. The number of photons emitted (otherwise known as the intensity) at any particular frequency will vary. The plot of the intensity versus the frequency is the "black body spectrum" 3. The temperature of the black body can be calculated using the frequency of the most common (or more intense) photons. This is notated as υmax indicating the frequency at which there is maximum intensity. 4. The photons of this frequency do not have this, or any other, temperature.
  19. damorbel - "one particle with the same energy as the average energy of all the particles also has the same temperature as the whole sample. " (Emphasis added) Flatly wrong. That photon (the subject of the discussion) will have the average energy of the whole sample. Any individual photon can come from anywhere in the emission spectra of the object (high to low), and from any individual photon there is no fixed determination of temperature. damorbel - Are you asserting that an individual photon (with a particular energy) can be used to identify the temperature of the object that emitted it, thus affecting it's absorption? Or that the possibility of absorption is not a function of photon energy and absorption spectra? If so, you are sadly mistaken. Boltzmann thermal distributions are off-topic distractions. Energy transfer from cooler to warmer objects is the issue raised in this thread.
  20. Re #891 KR you wrote:- "Absorption does not require photoelectric emission of an electron; that's a completely different question - Herring." That was the point of departure for quantum physics in 1905, something I made quite clear. But it is now generally accepted that all electromagnetic interactions take place in the domain covered by quantum physics - it is called quantum electrodynamics (QED) http://en.wikipedia.org/wiki/Quantum_electrodynamics But you are making much of the statistical distribution of energies in a given sample of particles with a given temperature. To start with, to have a defined temperature the particles must be in equilibrium, the average energy must be steady. In this condition the average energy is the total energy of all particles in the sample divided by the number of particles in the sample. But there is no need to have a certain number of particles to make a sample, so one particle with the same energy as the average energy of all the particles also has the same temperature as the whole sample. you wrote:- "Individual photons have energies, not temperatures - False statement" We have been here before, the energy of a photon is available for electromagnetic particle interactions which are governed by quantum laws just the same, so it doesn't matter if the particle energy is defined in electronvolts or temperature, my use of the word 'temperature' to define the energy of a photon is common and justifiable. (In single particle interactions as in high energy physics individual particle energy is frequently just given in Joules. You wrote "Estimates of object thermal emission temperatures require a spectra (statistical knowledge) and some idea of the emission spectra. An individual photon is incapable of supplying sufficient information - Incorrect statement and herring" What do you mean by 'object'? (First line above.) Is a particle not an 'object'?
  21. damorbel - I am quite frankly appalled by your last post - I suspect I am not alone. The photoelectric effect (and in fact your entire post on it here) is a complete red herring, irrelevant to absorption of thermal energies. QED is the basis of absorption spectra determination, but that's a different question - also irrelevant. You are misusing "temperature" - particles have energies and velocities, photons have energies, an ensemble of particles have temperature, an ensemble of photons have spectra, the last of which can (with some idea of the emitting object spectra) be used to identify emitting object temperature. Your personal re-definition and misuse of a term is not in any way a compelling argument against thermodynamics. Do you have any answer to the two questions I posed at the end of my last post? The ones actually relevant to energy exchanges between cooler and warmer objects (atmosphere and surface)? Questions on the actual subject of this thread?
  22. Re #893 Phil you wrote:- " 3. The temperature of the black body can be calculated using the frequency of the most common (or more intense) photons. This is notated as υmax indicating the frequency at which there is maximum intensity. 4. The photons of this frequency do not have this, or any other, temperature." The diagram on this page Wien's displacement law (top right) has a temperature for each peak of the Planck curve - that is the temperature associated with the average energy of the photons.
  23. damorbel - "The diagram on this page Wien's displacement law (top right) has a temperature for each peak of the Planck curve - that is the temperature associated with the average energy of the photons. " Actually, the peak of the curve is the mode, not the average; the two are not identical unless the distribution is symmetric. That's a fairly common error. However, both mode and average are statistical values. You have now contradicted yourself; individual photons do not convey the temperature of the emitting object, as that requires a statistical ensemble.
  24. darmorbel @895 We have been here before, the energy of a photon is available for electromagnetic particle interactions which are governed by quantum laws just the same, so it doesn't matter if the particle energy is defined in electronvolts or temperature, my use of the word 'temperature' to define the energy of a photon is common and justifiable. (In single particle interactions as in high energy physics individual particle energy is frequently just given in Joules. So you agree that photons have an energy, which is related to their frequency, and you agree that the "back radiation" is emitted from molecules that have previously absorbed surface IR radiation (which must of the same frequency). So you agree that your previous notion of surface IR radiation of having "warmer photons than the back radiation" is wrong !
  25. Phil - "...the "back radiation" is emitted from molecules that have previously absorbed surface IR radiation (which must of the same frequency)." Actually, Phil, that's not correct. By my estimates each absorbing molecule at sea level collides with an absolute minimum of 1000 other molecules before it has a chance to emit, each modifying it's internal energy. (10^9 collisions per second, 10^-6 seconds minimum before emission) Emission spectra of the atmosphere depends on the temperature and makeup of the emitting gas - the varied energy level drops resulting in EM emission reflect the varied energies of the emitting molecules and their possible transitions through a radiating level. On average the photons from the surface will be of higher energy (shorter wavelength) than those from the air. But that's irrelevant to the fact that photons from the air are indeed absorbed by the surface, and that this absorption (by the 1st law of thermodynamics, conservation of energy) affects and slows the total, net energy transfer to the atmosphere and hence to space.

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