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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)

 

Skeptics sometimes claim that the explanation for global warming contradicts the second law of thermodynamics. But does it? To answer that, first, we need to know how global warming works. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming. Global warming, in a nutshell, works like this:

The sun warms the Earth. The Earth and its atmosphere radiate heat away into space. They radiate most of the heat that is received from the sun, so the average temperature of the Earth stays more or less constant. Greenhouse gases trap some of the escaping heat closer to the Earth's surface, making it harder for it to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the greenhouse gases make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation.

The second law of thermodynamics has been stated in many ways. For us, Rudolf Clausius said it best:

"Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature."

So if you put something hot next to something cold, the hot thing won't get hotter, and the cold thing won't get colder. That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil!

The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth. If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics.

So have climate scientists made an elementary mistake? Of course not! The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference.

To see why, consider that blanket that keeps you warm. If your skin feels cold, wrapping yourself in a blanket can make you warmer. Why? Because your body is generating heat, and that heat is escaping from your body into the environment. When you wrap yourself in a blanket, the loss of heat is reduced, some is retained at the surface of your body, and you warm up. You get warmer because the heat that your body is generating cannot escape as fast as before.

If you put the blanket on a tailors dummy, which does not generate heat, it will have no effect. The dummy will not spontaneously get warmer. That's obvious too!

Is using a blanket an accurate model for global warming by greenhouse gases? Certainly there are differences in how the heat is created and lost, and our body can produce varying amounts of heat, unlike the near-constant heat we receive from the sun. But as far as the second law of thermodynamics goes, where we are only talking about the flow of heat, the comparison is good. The second law says nothing about how the heat is produced, only about how it flows between things.

To summarise: Heat from the sun warms the Earth, as heat from your body keeps you warm. The Earth loses heat to space, and your body loses heat to the environment. Greenhouse gases slow down the rate of heat-loss from the surface of the Earth, like a blanket that slows down the rate at which your body loses heat. The result is the same in both cases, the surface of the Earth, or of your body, gets warmer.

So global warming does not violate the second law of thermodynamics. And if someone tells you otherwise, just remember that you're a warm human being, and certainly nobody's dummy.

Basic rebuttal written by Tony Wildish


Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

 


Update October 2017:

Here is a walk-through explanation of the Greenhouse Effect for bunnies, by none other than Eli, over at Rabbit Run.

Last updated on 7 October 2017 by skeptickev. View Archives

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Further reading

  • Most textbooks on climate or atmospheric physics describe the greenhouse effect, and you can easily find these in a university library. Some examples include:
  • The Greenhouse Effect, part of a module on "Cycles of the Earth and Atmosphere" provided for teachers by the University Corporation for Atmospheric Research (UCAR).
  • What is the greenhouse effect?, part of a FAQ provided by the European Environment Agency.

References

Comments

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Comments 701 to 725 out of 1393:

  1. Tom Curtis (RE: 635), "RW1 @630, as it is difficult to carry on two discussions at once on the same thread, do you mind holding of on the discussion of the relevance of the light box until we have settled that it does not violate any law of thermodynamics? And to that end, do you agree that the light box does not violate any law of thermodynamics?" I'm not sure why you are asking me this. I do not believe that the GHE effect violates any of the laws of thermodynamics.
  2. Re 701 RickG you wrote:- "Since I asked you to post your revised edition correcting Trenberth's short-comings on how it should appear, I gather your above comment is code for you can't support your claims?" Looking at the Trenberth pdf you cite: on p5 he has:- "At the surface, the outgoing radiation was computed for blackbody emission at 15°C using the Stefan–Boltzmann law R = εσT4, (1) where the emissivity ε was set to 1." Emissivity set to 1? As John McEnroe might have said "he can't be serious" Using such a figure for the Earth's surface inevitabl gives the wrong answer because the Earth's surface does not, by any stretch of the imagination, match the specifications of a black body. Worse still he claims 333W/m^2 from clouds. Clouds are even further from being black bodies. You make no reference to the fact that GHGs absorb and emit radiation only as a function of their temperature, not of their altitude. You must attach some importance to this. These things are the very essence of heat transfer in the atmosphere and anywhere else; I do think they should be examined by climatologists.
  3. damorbel #703 Just a reference about emissivity values. ε=1 is not that bad an assumption at all.
  4. damorbel @ 703 Your basic misconception concerning Trenberth's schematic appears to be about what the schematic is not about. It is not depicting the Stefan–Boltzmann Law and black body radiation. The schematic is based on actual instrumental data showing how energy is distributed globally. Once again, please read the Trenberth et al paper. Earth's Global Energy Budget.
  5. damorbel (RE: 700), "Trenberth's diagram is deficient in so many ways it is beyond revision. The so-called 'back radiation' has the concept behind it that there is a place in the atmosphere from where 'back radiation' comes; but even John Tyndall knew that this is not the case. He measured both the emission and absorption by GHGs and found that their emitted radiation was completely absorbed by gases at a lower temperature." Yes, the Trenberth diagram is a confusing and misrepresents many things. What Trenberth refers to as 'back radiation' is really mostly downward emitted radiation - some of which last originated from the Sun, some of which last originated from the surface emitted, and some of which last originated from the kinetic energy (latent heat and thermals) moved from the surface into the atmosphere. As I was trying to explain earlier in this thread, the proper definition of 'back radiation' is the downward emitted radiation from the atmosphere that last originated from the surface emitted radiation. "This last means radiation emitted by GHGs is immediately absorbed and re-emitted by adjacent GHGs. This is so when the pressure and temperature gradient are zero; in the atmosphere the density reduces with altitude so the upwardly emitted radiation is not completely reabsorbed, an increasing %age gets ever higher until it escapes completely; that is the mechanism for heat radiation from the Earth." The key thing to note is that the 239 W/m^2 of post albedo energy entering becomes about 390 W/m^2 at the surface. The absorption and isotropic re-emission of the outgoing surface emitted infrared by GHGs and clouds is slows down the rate at which energy can leave, causing a 'back up' of energy at the surface. Essentially, what this means is it takes a 'back up' of 390 W/m^2 at the surface to allow 239 W/m^2 to leave the system, offsetting the 239 W/m^2 coming in from the Sun.
  6. RW1 & damorbel, Well guys, there is no doubt that neither of you are the slightest bit interested in discussing any science with the intent of sharing information and understanding. Both of you take a perfectly legitimate and superbly illustrated schematic showing 'Earth's Global Energy Budget' and purposely misrepresent it out of context, completely ignoring the paper that describes it in detail. If you are still troubled with Trenberth's schematic and paper, I suggest you contact him personally. His email address, fax and phone number are listed prominently the NCAR site to which I have previously linked. As for me, I am through with your hand waving and obfuscation.
  7. Re #705 RickG you wrote: "Your basic misconception concerning Trenberth's schematic appears to be about what the schematic is not about. It is not depicting the Stefan–Boltzmann Law and black body radiation." I suggest that you examine the document as closely as I have. All the power figures (W/m^2) e.g. 'Surface Radiation 390W/m^2' and 'Back Radiation 333W/m^2' appear to be derived by applying Stefan's formula (with an emissivity =1) to an estimate of the local temperature. Trenberth mentions 'real' temperatures (°C or K) only seven times and then only to explain how the figures were manipulated to make them more acceptable (see box on p315) The box also contains this unused information:- "The surface emissivity is not unity, except perhaps in snow and ice regions" which beggars belief. Snow certainly has a very low emissivity (and the corresponding low absorptivity) because that is why it takes a long time to melt as ground cover. It is only when the (absorptive) dark earth underneath begins to appear that radiation has a real effect melting the snow. You wrote further:- "The schematic is based on actual instrumental data showing how energy is distributed globally." I don't think you are right here. The only measurements that Trenberth uses are satellite measurments that even he admits are unreliable (because of instrument failure) and disagree violently e.g. compare the 'Solar reflected' (p316) for KT97(Trenberth) at 107W/m^2 and JRA(Japanese re-analysis) at 95.2W/m^2. Trenberth then goes on to use the difference of these measurements to calculate the extra heating due to radiation and the figure vary from positive to negative, depending on who is doing the measurements. Of course it isn't as simple as that; no, many of the figures used have all been re-analysed (aka - they didn't meet requirements) e.g. JRA aka Japanese re-analysis; NRA aka NCEP–NCAR re-analysis and ERA-40 aka 40-yr ECWMF Re-Analysis (p316). This kind of stuff reminds me strongly of [- snip -]
    Response: [muoncounter] Edited due to political accusations in violation of Comments Policy.
  8. Rick G, "Both of you take a perfectly legitimate and superbly illustrated schematic showing 'Earth's Global Energy Budget' and purposely misrepresent it out of context, completely ignoring the paper that describes it in detail." I've read the Trenberth 2009 paper, and yes, it misrepresents the most crucial aspect of the entire GHE. That is how much of the emitted surface radiation is from 'back radiation' from the atmosphere and how much is passing through unabsorbed and going straight out to space. The paper and diagram makes it look like of the 396 W/m^2 emitted at the surface, 333 W/m^2 of it is coming back from the atmosphere. This is incorrect. Using Trenberth's numbers, only 157 W/m^2 of surface emitted radiation is from 'back radiation'. The diagram also obscures what percentage of the surface emitted radiation absorbed and re-emitted by the atmosphere is downward emitted and what percentage is upwards emitted out to space. Using his numbers, 157 W/m^2 is downward emitted toward the surface and 169 W/m^2 is upward emitted, with 40 W/m^2 passing through the clear sky atmosphere and 30 W/m^2 passing through the cloudy sky atmosphere. The total transmittance of 70 W/m^2 is not referenced in the paper and seems to only be a rough estimate or guess.
  9. 707 RickG. I completely share your sentiments - although I hope not sufficiently as to violate the comments policy ;) There's been a lot of words attacking GHE... but no physics that I can discern. I'd suggest that those who feel there's a flaw in the GHE argument construct a clear, physics based, derivation - or find a flaw in explanations such as SoDs - until which time, IMHO, it's probably best to give it a rest.
  10. Re #612 Tom Curtis you wrote:- "3) The molecules in a gas do need to have significant kinetic energy to stay aloft. That is the energy of motion that they have because of the temperature of the gas. If the gas cools" You are so close to an important fact about atmospheres in general. You write "if tha gas cools" - as a molecule rises in the atmosphere it loses kinetic energy(KE) to gravitational potential energy (GPE). But the KE of a gas (well, degree of freedom, DOF; actually) is its temperature, so when it loses KE to GPE it must cool. This is so important because it establishes the basic lapse rate of 6.5C/km(altitude).
  11. 710 les Thanks les, it is a challenge to respond to such foolishness and still keep one's cool enough to remain withing the comments policy. After writing a response I usually walk away for a while before submitting it. I almost always find myself toning my initial reply's down. It doesn't always work but for the most part it does. Contrary to contrarians beliefs, I do get posts deleted from time to time. With contrarians it is quite often difficult to figure out if they really don't understand the science or if they are just up to mischievous. I hope I'm wrong but I fear the latter is too often true.
  12. damorbel @711, almost right. The temperature of a gas is its mean kinetic energy. That can be expressed in terms of the sum of its kinetic energies for each of its external degrees of freedom, but that does not include internal degrees of freedom, ie, rotational or vibrational energy. The heat capacity ratio of an ideal gas, on the other hand, does depend on the internal degrees of freedom as well, so that the heat capacity ratio is just (f+2)/f where f is the degrees of freedom. The heat capacity at constant pressure and the heat capacity at constant volumes are then just simple functions of the heat capacity ratio. The interesting thing about energy in the form of external kinetic energy an energy in the form of vibrations and rotations within the molecule is that they tend to equalize, so that the energy in each degree of freedom is, on average, the same. That means that when kinetic energy in the axis perpendicular to the surface is lost due to gravity, it is partially replaced by energy from the two axis parallel to the surface, plus from rotational and vibrational energy as a result of collisions. Likewise, a molecule gaining kinetic energy perpendicular to the surface will tend to redistribute it to the other degrees of freedom as the result of collisions. So, it is not gravity alone that determines the lapse rate, but gravity and the redistribution of energy form the various degrees of freedom of the gas molecules, ie, gravity and the heat capacity of the gas. Hence, the lapse rate equals the negative gravitational acceleration divided by the specific heat for constant pressure. All of which is very interesting, but as has been pointed out before, it does not determine surface temperature. It only determines the relationship between surface temperatures and the temperatures at given altitudes. In other words, these fact could all be true about the atmosphere and the Earth's global mean surface temperature be 255 degrees K. It is the greenhouse effect which explains why it is not.
    Response: [muoncounter] Lapse rate is another favorite drum to bang; see 30 November, 525 comments up thread.
  13. RW1 @702, do you mean that you have been muddying the waters by insisting on using non-standard definitions of back radiation, and treating all radiation to space as having originated either directly or indirectly from the surface as a matter of definition for no purpose? The topic of discussion in this thread is the Greenhouse effect and the 2nd law of thermodynamics. If you do not think there is a contradiction, and are not arguing against those who think their is, you have nothing to add to this thread because you are of topic.
  14. Tom Curtis 699 You said: "I get 3.4249 * 10^-19 Joules per photon myself, and hence four times that energy contained in the box." How does a photon of 3.4249 * 10^-19 Joules outside the box increase it's energy to 1.36996E-018 Joule inside the box? And now having a wavelength 145 nm inside the magic box, the photon (as a stipulation of the filter lid) can escape to the vacuum..where it instantaneously lengthens to 580 nm. This most improbable supposition is the basis of your models and GHG theory...shown here to clearly violate the 1st law.
  15. scaddenp647 KR671 KR "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. " scaddenp "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)." Low temperature, (lower energy) atmosphere adding radiative heat to the warmer surface is a violation of the 2nd law. To proclaim star sourced energy can be increase itself by it's own reflection and/or re-radiation is a violation of the 1st law. "Like conduction, thermal energy is in harmony with the second law of thermodynamics such that, in the absence of work, thermal energy is radiated spontaneously from higher temperature to lower temperature matter." M. Quinn Brewster Thermal Radiative Transfer and Properties
  16. Damorbel @703, the sentence you quote is clearly intended to describe the procedure in Kiehl and Trenberth 1997. Following that quote, Trenberth, Fasullo and Kiehl then go on to discuss the changes in method for T, F & K 2009. The most important of these is that they factor in the difference in temperatures due to latitude, which results in a significant increase in the calculated surface radiation. To achieve greater precision, they take the mean of surface radiation in a model which correlates well with surface radiation measurements around the globe. They then discuss differences in emissivity. They note that low emissivities are found in regions with high surface temperatures, ie deserts, the two factors tending to cancel each other out. The also note that low emissivity means not only reduced surface radiation, but reduced absorption of back radiation, factors which again tend to cancel each other out. T, F, & K note (from Wilber et al 1999) that differences in emissivity can result in up to a 6 w/m^2 reduction in surface radiation. That represents just 1.5% of the average surface radiation. A 1.5% reduction in back radiation absorbed would be 5 w/m^2, making a difference of just 1 w/m^2 over deserts. The total area of deserts on Earth (excluding the Artic and Antarctic) is about 20 million square kilometers, or about 4% of the Earth's surface. Therefore, correcting for the low emissivity of deserts would have altered the final figure by just 0.04 w/m^2, which given the margin of error in the calculations is to small an effect to by worried about. In addition, your claim that the atmosphere is treated as having an emissivity (and hence absorptivity) of 1 is plainly refuted by the fact that some energy escapes from the surface to space in the diagram. McEnroe's histrionics where not even amusing in his time; in the era of Hawkeye, they would have just make him look foolish. He would have been forced to win matches on skill rather than on gamesmanship. On this forum we have something better than Hawkeye. We can read the original papers, and we can think.
  17. LJRyan @715, the photon does not increase its energy. Rather, if you feed in one photon per time interval, at equilibrium there will be four photons in the box, each with the same energy, and hence the total radiant energy in the box will be four times that which is fed in in each time interval. You cannot, apparently, manage even simple reading comprehension, yet you purport to lecture the world's atmospheric physicists about the relation between thermodynamics and the greenhouse effect.
  18. LJRyan @ 716 - you assert that "Low temperature, (lower energy) atmosphere adding radiative heat to the warmer surface is a violation of the 2nd law" That's actually not correct. The 2nd law only talks about NET heat transfer. You can have heat radiating from an object at 500º to an object at 1000º. Obviously, the warmer object will be radiating *more* heat back the other way, but it's still receiving heat from the cooler object. Anyway, don't take our word for it. Ask Dr Roy Spencer, one of the most prominent opponents of human-caused global warming. He discusses it here, and again in more detail here. Dr Spencer may hold views on the causes of global warming that are at odds with the vast majority of climate scientists, but he certainly understands radiative heat transfer (and that, to be honest, makes it really puzzling why he so strongly disagrees with the consensus view on this matter).
  19. LJRyan @716:
    "Low temperature, (lower energy) atmosphere adding radiative heat to the warmer surface is a violation of the 2nd law."
    The energy increase comes from the sun, not the atmosphere. All the GHG in the atmosphere do is decrease the efficiency with which the energy is radiated away. There is a very simple model of this. Suppose you have an electrical stove with a pot of water on it. The pot has no lid. You heat the pot until it is gently simmering, and stably so. In this situation, the heating element will be glowing slightly red, showing a temperature of about 500 degrees C. The water will be just of the boil, indicating a temperature of about 100 degrees C. We now place a lid on the pot, leaving only a small gap. Even though the we do not adjust the heating element, the water will commence to boil vigorously and may even boil over. It you look at the inside of the lid, however, you will see water condensing on it, showing clearly that it is below 100 degrees C in temperature. So, addition of a cooler part, the lid, has caused a hotter part (the water) to gain heat. In thermodynamic terms, the analogy between this and the greenhouse effect is exact. So, anytime anyone on Earth boils some rice, they prove you wrong about thermodynamics.
    "To proclaim star sourced energy can be increase itself by it's own reflection and/or re-radiation is a violation of the 1st law."
    See 718
    ""Like conduction, thermal energy is in harmony with the second law of thermodynamics such that, in the absence of work, thermal energy is radiated spontaneously from higher temperature to lower temperature matter." M. Quinn Brewster Thermal Radiative Transfer and Properties"
    This quote is quiet accurate, but refers to the net energy transfer. It is plainly not true that a cooler body cannot radiate energy towards a warmer body. What it cannot do is radiate more energy that is absorbed than does the warmer body toward it. Ie, radiated energy from the warmer body - radiated energy from the cooler body is always positive. That it is not simply prohibiting any radiation from the cooler body is shown simply, and aptly by the actual measurement of back radiation. The atmosphere is, in nearly all cases, cooler than the surface. Despite this radiation from the atmosphere to the surface has been measured many times. Here is one example: From Science of Doom. See also this and this.
  20. Re #717 Tom Curtis you wrote:- "The[y]also note that low emissivity means not only reduced surface radiation, but reduced absorption of back radiation, factors which again tend to cancel each other out." Yes indeed. And so did Gustav Kirchhoff writing in 1862. From that he concluded that the temperature of a body black or or otherwise, is not affected by its emissivity as long as it has no internal heat source (or heat sink) i.e. it is in thermal equilibrium. This is the basis of his argument that emissivity and absorptivity are the same for any given body. The only internal heat source* the Earth has is the radioactivity in is rocks and (possibly) some residual heat from the formation of the planet. Measurements have shown these internal heat sources contribute not more than 0.1W/m^2 to the outgoing radiation, a negligible amount. The meaning of this is clear, the size of the albedo has no affect on the temperature of the Earth, the position of climatologists, that the Earth's equilibrium temperature is lowered by 33K from 288K to 255K has no scientific basis. What the albedo does is reduce the rate of change of temperature, this follows because a high albedo reduces both the absorptivity and the emissivity by exactly the same amount. Common experience shows the reality of this; an every day vacuum flask has a highly reflective coating that produces this effect exactly. I apologise to muoncounter if I have mentioned this before but he should take note that, as yet, the argument about the vacuum flask has not been countered by anyone and the comparisin with a planet heated entirely by energy from the Sun is 100% valid. And, incase you are wondering, Kircchoff's proof of this is based entirely on the 2nd Law of Thermodynamics. * The Earth has a heat sink, chemical change. Some of the Sun's energy is converted into chemical energy, but not very much.
  21. L.J. Ryan. We have measures (ie real world) of 341W/m2 incoming at TOA and 396W/m2 being irradiated from surface. Wow, 1st law violation? Are you trying to say nature is breaking the 1st law? The measurement must wrong? Well no. Tom and others have very patiently been explaining to what is really happening. Ditto, this whole thread on 2nd law where you are jumping on the premise that atmosphere is heating the planet. Nope. Thermodynamics is not flawed. Understanding is.
  22. 712 RickG I guess there are several issues. The one I have with this form of argumentation (not particularly this one, but ones like it) is that it's very often to appear to knock down an argument using broad bush-strokes, appeal to "common experience", focusing on missing details etc. What is harder, and rarely done, is to build up an argument - and specifically to build an argument up to the level to be usable for policy... that is the role, e.g. of the IPCC. Now, "do nothing" is a policy. It just isn't good enough to base this policy - which is potentially very very expensive and life threatening - on a level of analysis which is no more than waffle-words. Something much stronger must be built to attack the science as shown on a site like this... ... you just cannot cut steal with chewing-gum. The tools need to be sharper and harder than the thing you're attacking;you need maths, detail, consistency etc. And these things are, by and large, the unknown-unknowns of a lot of "denilists" - all the stuff along the road that ends in a diagram, a graph or a couple of numbers.
  23. Tom Curtis 718 You said: " the photon does not increase its energy. Rather, if you feed in one photon per time interval, at equilibrium there will be four photons in the box, each with the same energy, and hence the total radiant energy in the box will be four times that which is fed in in each time interval." I did not specify a rate, you did. A single photon will transverse the box and/or absorbed and re-radiated countless times within a second...so why no increase in energy? Is there a minimum energy for your box?
  24. Tom Curtis 720 You said: "In this situation, the heating element will be glowing slightly red, showing a temperature of about 500 degrees C." Do you really think the water boils because of "lid" forcing. If you made the lid transparent to IR from 100C to 500C, would the water still boil? I say yes. But before I endeavor to explain this scientifically sound principle, I'll let you reconsider your latest GHG analogy. hint...pressure cooker. What if we removed the pot of water and simply inverted a larger empty pot over the red hot burner, i.e. no conduction. By your supposition, the burner should get hotter...correct? What is the temperature of the top (actual bottom) of the pot? What if we directed the burner toward your magic box. Would the burner get hotter then 500C?
  25. 725 Tom Curtis 720 partial re-post What if we removed the pot of water and simply inverted a larger empty pot over the red hot burner, i.e. no conduction. By your supposition, the burner should get hotter...correct? What is the temperature of the top (actual bottom) of the pot? Would the burner get hotter then 500C? What if we directed the burner toward your magic box. Would the outside of the box, insulation pulled back for sampling, be hotter then 500C

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