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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 651 to 675 out of 895:

  1. les 638 No not Maxwells Demon, but rather Kirchhoff's black body theorisation.
  2. Re #649 les, you wrote:- "damorbel 648 - fine. We agree on so much including, it would seem, that the diagram is not "completely deficient in temperature information". " I think there is a definite problem there. I'm afraid I do not understand just what is it that makes you say:- " the diagram is not "completely deficient in temperature information". ? and:- "It clearly does "present ... useful information for any discussion on climate change " I'm not sure what you mean here. I am of course thinking of 'useful information' in the sense of scientific information, suitable for putting in reports called 'the Scientific Basis', the name of the sections of IPCC reports using this diagram. Trenberth's diagrams are among the few (perhaps only) in IPCC reports showing directional energy effects from greenhouse gases, I cite them because, without temperature, any figures showing energy emissions from the atmosphere, the Earth's surface or anything else have no scientific basis.
  3. LJ@651 >Sounds like a violation of the 1st law...remember perfectly reflective walls. Sounds as if you don't believe your own answer. The full context of that comment was: "... in practise you would not have perfectly reflecting walls," So his assumption with that statement was that in reality you cannot have perfectly reflecting walls. Now stop trying to nitpick and give his example some real thought. >ok lets assign three wavelengths and redo @646 As Tom pointed out this would just be a multiplier i.e. energy per photon X number of photons. To convert wavelength into energy per photon, use the equation h*c/wavelength. You can type this directly into google like so "h * c / 11364 nm". Once you have that number is just a matter of multiplying it by the number of photons, try it yourself. Re: question 1a) The aperture he is referring to is the open hole where the lid used to be. There is no aperture in Tom's original example. Re: question 2a) The lid still holds the same properties it did in the original example, i.e. it transmits 50% and reflects 50%.
  4. 653 damorbel: again, we are in complete agreement in not completely understanding the bits where I quote you. Still, to do what you require follow the advice of others here and read the full papers rather then looking at the pretty pictures. 652 Ryan, well, no actually. Not for that weird selective filter thing. Still, given the arbitrary was bits of physics are being thrown around this thread; have it your way, what the hell!
  5. LJ@651 >So the accumulated "boxed" light would radiate 1W for a 100 hrs, once the second aperture is opened? The wattage of the aperture would be entirely determined by the the size of the hole, the size of the box, and the amount of energy in the box. As the energy escapes from the box the wattage of the aperture decreases. It has no relationship to the original wattage of the flashlight. That value is determined by the flashlight's ability to convert chemical energy into radiant energy. In this example all the energy is already radiant. The box doesn't "remember" the wattage of the original light source.
  6. L.J Ryan@651 "So the accumulated "boxed" light would radiate 1W for a 100 hrs, once the second aperture is opened?" Wouldn't that depend on the size of the aperture? On the photon level wouldn't the "flow rate" out of the aperture change as less light remained in the box? Fewer photons would be available to hit the target/sec. I imagine it would be a bright and rapidly fading light once the hole was opened. Since this is all imaginary...
  7. e654 "So his assumption with that statement was that in reality you cannot have perfectly reflecting walls. Now stop trying to nitpick and give his example some real thought." The perfectly reflective wall was Tom constraint not mine. Violating the 1st law in order to illustrate radiative forcing is not nitpicking, rather it's very fundamental. To say emissivity >0 leads to "light decay" is a convenient concept...does the photon slow down until comes to rest at the bottom of the box? What is light decay? Tom said: "The answer has to be in terms of photon numbers, not energy because the wavelength of the photons has not been specified. If we specify that all photons have the same wavelength, then the multipliers for photons in the answers above can be used for energy." you say: "To convert wavelength into energy per photon, use the equation h*c/wavelength. You can type this directly into google like so "h * c / 11364 nm". Once you have that number is just a matter of multiplying it by the number of photons, try it yourself." You seem to be at odds with Tom. Your suppositions concludes, regardless of wavelength the accumulated ENERGY within the box is twice the input...as detailed by @646
  8. e656 pbjamm657 Make the aperture diameter 2x photon diameter, and shazam you have a flashlight powered photon laser.
  9. LJ> Everything you said in post 658 is false. I will not detail the problems because they are all simple reading comprehension errors. If you're not going to make any effort to read and comprehend the point that is being made then you are not here for intelligent discussion; there's no point in talking to you.
  10. les655 "Ryan, well, no actually. Not for that weird selective filter thing. Still, given the arbitrary was bits of physics are being thrown around this thread; have it your way, what the hell! " I agree sloppy physics, lot fundamental violations. The weird filtering was Tom's not mine. Are you familiar with Kirchhoff's black body cavity theorisation/experiment?
  11. e 660 I asked a question...What is light decay? You are not contradicting Tom,suit yourself.
  12. LJ> In that specific comment, Tom clearly indicated he was talking about a "real-world" box that would not be perfectly reflective. In that case "decay" would be the light escaping the box. I have not contradicted a single thing Tom said, you just aren't reading what's being written.
  13. The energy quality I described in 498 is obviously related to the inverse of entropy, but people struggle with the latter concept. It is easier to see that quality is related to useful energy – that is, energy that can actually do something, such as producing useful work or raising temperature. In any spontaneous transaction involving energy transfer, quality will diminish. What this means is that the energy cannot go backwards, and some of the useful energy will be lost. (I said I would not mention the second law, but that is what it is really about). Incidentally, 499, real life energy losses through friction, etc, make the situation worse. Energy quantity as well as quality is lost to the system Here is another elementary example. An insulated vessel contains gas at a high temperature, and is separated from a vacuum, within the vessel, by a membrane. If the membrane is punctured, the gas flows into the vacuum, and its pressure drops. No work is done, no heat is lost, so the temperature remains the same. The first law says that energy has been conserved. But the gas is obviously able to do less work, starting from the lower pressure. It is also obvious that (Maxwell's demon apart) the gas cannot go back. What has happened is that the quality of the energy has fallen. It turns out (as the pundits say) that it is this elusive characteristic of quality (strictly entropy) that drives all spontaneous transactions – literally everything from chemistry, biology, energy transfer and (fancifully) the tidiness of your desk. It is the quality of energy, not the quantity, that makes something happen. Here are two well known examples. Suppose that a single gas flame operates at a temperature below the melting point of a steel plate. The steel will not melt because the quality of the flame energy is too low. Now apply ten more similar flames. Still the steel will not melt. Another example is Einstein’s experiment to eject electrons (I forget from what) with a beam of incident radiation. Below a certain frequency (energy quality) nothing happened, no matter what the intensity (energy quantity). Above that frequency, electrons were ejected, and quantum mechanics was born. Sadly, however, my definition of quality (available energy) is too simplistic. It is the relative quality that matters. Switching the argument to a power generator, a source with a high temperature can generate work by transferring energy to a sink at a lower temperature. The available energy is high. If the sink is at the same temperature as the source, nothing will happen. There is no available energy. Likewise, if the sink energy can be connected to a second sink with a lower temperature, space heating is possible. Otherwise the waste energy will be ejected to the atmosphere through cooling towers. So, if I have persuaded anyone that the crucial elements of energy transfer are the qualities of the energies concerned, and that energy can’t go backwards without the performance of extraneous work, we can move on to an even more elusive concept. Heat.
  14. Fred Staples - That's a fairly reasonable (if wordy) description of Entropy. You could have just linked to an existing definition and saved much typing. I think I see where you're going - to an argument that the high levels of IR at the surface somehow violate entropy considerations. You might find my comment here relevant in that regard. We aren't dealing with a closed system, but rather a very open one, where the important issues are rate of energy flow, energy differentials, and internal temperatures and energy levels required to maintain a dynamic equilibrium. Not moving a fixed amount of energy around a closed system. Entropy is increasing as sunlight radiates out into the 3K void of space. Local conditions regarding the conversion of that visible light into thermal IR provide a pinch-point, much like the dam in my analogy, one that includes a local collection of energy in order to have an energy differential sufficient to radiate the IR to space.
  15. L.J. Ryan - back to the original light box. We agree that the set up does NOT violate 1st law? No where in the system is energy being created. The thing that seems counter-intuitive apparently is that energy-fluxes appears to double. Whoa! energy creation! No. This illustrates that care has be taken in inferring system energy from energy flux, because in this case, with reflection, the same energy gets counted twice. This is no violation of 1st law going on in Tom's example - nor in Trenberth's diagram for same reason. Just an illustration about care in use of energy flux.
  16. Re 650 RickG :- "For what Trenberth is demonstrating temperature is neither necessary or relevant in that diagram. I have no problem understanding the diagram myself." It is difficult to believe that a diagram showing emission of thermal radiation (W/^m2) can be considered useful when no indication is given of the temperature of the emitting body; why else would the Stefan-Boltzmann equation (E= rhoT^4) be so widely deployed in thermal physics?
    Response: [Muoncounter] you raised this identical 'objection' in November, 600 comments up this very thread. The same replies you received then still apply now. Insistence on mere repetition demonstrates that your argument ran its course.
  17. damorbel given that you know how to calculate temperature from the energy flux, why are you asking Trenberth to report it in a summary graph? You really look polemical here, it adds really nothing to the discussion or to the undersdtanding of the energy budget.
  18. L.J Ryan@559 No you would not have a laser, the beam would not be directional since photons would reach the aperture at a variety of angles and travel out that way too. From a photon=particle bouncing around standpoint I think the brightness of that beam would diminish over time as the number of photons hitting the aperture/sec would diminish along with the number in the box. "Light Decay" in the context of the discussion is light losing energy to the imperfect mirrors. As for Tom Curtis' original diagram @615 I think the missing element in most of this discussion is time. Over time A=C but not for every photon interaction. We need to be clear if we are talking about an instantaneous measurement or the totals over time. Same goes for the Trenberth Diagram. note: I am not a physicist nor mathematician. I reserve the right to be wrong.
  19. scaddenp666 Oh...Tom and Trenberth count the photon/flux twice. So the vector sums are zero, no atmospheric forcing...no AGW. You have confirmed my position.
  20. L.J. Ryan - Since the Trenberth numbers are an energy budget, they should add up and cancel out for an unforced climate. If, however, you carefully add up the Trenberth numbers without rounding you get an imbalance of about 0.9 W/m^2 less leaving than arriving. That's the forcing.
  21. 661 Rysn good try at a twist. I'm afraid that it's transparent to all here that you really are not "doing physics" in your argumentation. At thus stage, really, we're just playing with you. Dont take your arguments so seriously - no one else is.
  22. damorbel, The diagram, or schematic as Trenberth calls it, is from one of his many PowerPoint presentations. In other words the schematic is meant to be presented with discussion in context with his presentation. The schematic as many of us have pointed out to you is about incoming solar energy and how it is distributed throughout the climate system taking different forms of energy. In describing this, not only by Trenberth, but all scientists, use the proper units of measure which is watts per square meter, not temperature. But since no one seems to be able to convince you of that, watch Trenberth describe that very schematic himself in this video.
  23. " So the vector sums are zero, no atmospheric forcing...no AGW. You have confirmed my position." Umm, this is about whether the GHE is consistent with thermodynamics. If it is, the adding CO2 will create forcing as KR has pointed out. (and is measured at TOA). Now the numbers on Trenberth are derived from measurement and the flux has to be consistent with temperatures. The light box discussion is about understanding why these fluxes are not a violation of 1st law.
  24. Fred Staples @664: Considering the example of the steel beam, the flames will not melt the steal beam because they will (eventually if it is well insulated) heat the steal to their own temperature. At that point the black body radiation from the steel will carry the same energy as the flames preventing further warming. Applying that example to my light box model, the photons leaving the box will never have a shorter wavelength (= higher temperature) then the photons leaving it. Nor will there be more of them on average, thus conserving energy. Applying that insight to the Greenhouse effect, that means the the surface of the Earth will never be hotter than the surface of the sun (ie, the temperature of the source of the energy that warms it), and the outgoing radiation will never have a shorter wavelength than the incoming solar radiation. You will struggle to find a prohibition against the greenhouse effect from these two facts.
  25. Tom Curtis 675 "the Earth will never be hotter than the surface of the sun" The Earth receives 240W/m2 from the sun. The blackbody temp resulting from this flux represents the maximum temperature...not the sun's surface. That is, regardless of reflection and or re-radiation, 255K is the pinnacle temp for radiation alone. It is this fact by which blackbody was derived...trapping light in order to discover it's maximum thermal energy. ----clipped from another blog: The idea of trapping light is intriguing, and Gustav Kirchhoff (1824-1887) conceived a solution: A hole in a cave. A beam of light could enter this hole but the walls inside would absorb any reflections and prevent the light from escaping. Thus, by confining incoming radiation, the thermal energy which light confers could be shown to its maximum advantage. Kirchhoff's scheme was superior to selectively transmitting glass because a cave absorbs and traps all wavelengths of light,thus creating a complete radiative imbalance. At least theoretically. Well, so what was found by cavity experiments? That a perfectly absorptive ("black") body rises to a temperature a bit higher than an actual black body that’s free to radiate to its surroundings. A theoretical blackbody thereby defines the upper limit of temperature vs radiant absorption. Try to grasp the implication, then. A blackbody cavity mimics the radiative restriction that"greenhouse gases" are said to induce. Indeed, virtually none of the thermal radiation generated inside this cavity is allowed to escape. It "re-circulates" instead, and is sampled through a tiny hole. Does this confinement lead to a runaway greenhouse effect, though? No, it only sets an upper temperature limit — the SAME limit that’s applied to the earth in the first place, for its estimated temperature is based on a blackbody

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