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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.



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 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.


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 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.


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 551 to 575 out of 974:

  1. Climate_Protector - Please keep in mind that the overly simplified example I gave was a Gedankenexperiment, intended to describe the important of certain aspects of the climate system if everything else is held fixed. If the Earth were at -17C, we would have a "snowball Earth", extremely high albedo from ice sheets, and the final state would be much much colder. That doesn't matter for the point of a GHG reducing effective emissivity. Suffice it to show that we would be hugely colder without it, and that energy levels can be higher at the surface than incoming values depending on how effectively the energy leaves. As to the values I gave: 100% - 80% = 20% passing through without absorption. Half the 80% (40%) radiates up as well, so upward radiation is 20%+40% = 60% of 240, or 144. The difference between incoming and outgoing is 96, so this cannot be a stable state. Given the (toy) numbers I gave, with 60% of surface going out to space, 240 / 60% * 100% = 400. That's a stable number. Again - that was an overly simplified example to demonstrate the principle. Actual calculations are done line by line modeling 50+ layers of the atmosphere, incorporate convection and latent heat, and multiple greenhouse gases with different altitude distributions such as H2O and CO2. It's the difference between a high-school physics experiment discussing spherical frictionless objects, and a numeric integration of a complex function. --- Opacity is a straight blockage of light, which may or may not result in some thermal radiation at possibly different wavelengths - Absorption/Emission are thermal radiation terms, directly related, and absorption/emission spectra are equal at thermal equilibrium. Opacity is the wrong physical term.
  2. Tom Curtis @ 548: Yes, thank you! This explanation is much better (for my level of understanding at least)! A couple more questions, though: 1) Why does the atmosphere radiate 67% more IR energy to the surface as it does to space? My guess is because temperature decreases with height (at least in the troposphere), but then what's causing the temperature to drop with altitude? Is is the decrease in water vapor concentration? (because I think CO2 profile is pretty uniform throughout the troposphere, correct?). Bur water vapor amounts in the air depends on temperature. So what's driving what? 2) What is the actual enhancement of the surface temperature due to presence of GH gases, i.e. the greenhouse effect? This question relates to the comment in my previous posting about the amount of absorbed solar radiation in the absence of atmosphere and clouds. About GHG concentrations, I just found a number about the average H2O vapor concentration in the air - 0.25%. Does this sound right? So that means that the mean concentration of all GH gases is about 0.5% or less, correct? ... It's amazing how this minute amount is able to warm the planet to such an extend!! This means the efficiency of redirecting of IR energy towards the surface by GH gases is quite high!
  3. Tom Curtis (RE: 548), "As it happens, the atmosphere radiates 37.4% (199 w/m^2/532 w/m^2) of its energy to space, and 62.6% to the surface (333 w/m^2/532 w/m^2). That means it radiates 37.4% of the energy it receives by convection to space, and 37.4% of the energy it receives by evapo/transpiration to space, and 37.4% of the energy it absorbs directly from the sun to space, and 37.4% receives as IR radiation from the surface to space. It follows also that 33% (175 w/m^2/ 531 w/m^2) of back radiation comes from energy introduced to the atmosphere other than by absorbing IR radiation from the surface." What? How can the atmosphere radiate 199 W/m^2 to space from thermals and evaporation when it only moves 97 to the atmosphere?
  4. II see that the focus is now on the excess internal energy in the lower troposphere. Does this mean that everyone agrees that solar input alone cannot account for that additional energy?
  5. What's important to realize here is that air temperature is a direct function of the internal energy of the atmosphere at any level, and IR emission is a product (result) of that temperature, not the other way around! So in the chain of cause and effects, the sequence is: Internal energy --> temperature --> IR radiation.
  6. What about the strength of the 'greenhouse' effect? Anybody agree that its 133C increase of surface temperature, and not 20 or 33?
  7. RW1 @547: "All I'm saying is an increase in CO2 concentration will increase absorption in the CO2 absorbing bands, yes, but this does not necessarily mean all of the other absorbing bands will remain constant, especially at various levels of the atmosphere where their concentrations can vary (H20 in particular). That increased CO2 will decrease total transmittance through the whole atmosphere not definite by any means - just seemingly probable." Not only probable, but observed, as shown in this plot of the differences in brightness spectra for various wavelengths between 1970 and 1996: So, not only is your proposition extremely improbable, it is known to be false from observation.
  8. Also, think about what might be contributing to the internal energy of the atmosphere (particularly the lower troposphere) besides the Sun? In other words, is there another factor that can enhance the kinetic energy of air beyond the level supported by the absorbed solar solar radiation)?
  9. RW1 @553, the total IR radiation from the atmosphere to space is 199 w/m^2. Of that, 6.4 w/m^2 will have been transported to the atmosphere by thermals, 29.2 would have been absorbed SW radiation, and 29.9 would have been carried into the atmosphere by convection.
  10. Gentlemen, absorption and re-emission can only redistribute available energy, but cannot produce extra heat in the atmosphere needed to explain the near-surface thermal enhancement presently called atmospheric 'greenhouse effect'. That why the key question is the one I asked in 558.
  11. Did you know that convection can offset the potential warming by back radiation. If you try solving the radiative transfer simultaneously with convective heat exchange, you will see that convection can neutralize the entire effect of back radiation!
  12. The key to understanding the GH effect is NOT radiative transfer ...
  13. CP @552: 1) Your guess is correct. The vast majority of the GH effect is caused by CO2 and water vapour. As it happens, concentrations of water vapour fall to a level where IR radiation can escape to space in its most strongly absorbing frequencies at an altitude of about 5 km. For CO2, that altitude is about 8 km. That means the IR radiation escaping to space comes from a source about 30 degrees cooler than the surface for the frequencies where water vapour is a strong absorber of IR radiation, and about 42 degrees colder for frequencies where CO2 is a strong absorber (including in that part where it overlaps with H2O). The reason the troposphere cools with altitude is because of the rate air heats (or cools) due to compression (or expansion) as it falls (or rises) in the atmosphere. Because air is cooled as it rises by expansion, and heats as it falls through compression, this establishes a stable rate of temperature change with altitude (the lapse rate) such that, if the atmosphere lies near that rate, it will not tend to rise or fall, and if it is far from that rate, it will tend to rise or fall until it is close to it. That rate is a function of the gravitational acceleration and the specific heat capacity of the gas involved. 2) The actual enhancement is greater than 33 degrees C, but the exact amount is difficult to determine, for reasons I discussed @464 on this thread. 3)The following is a graph of the H2O concentrations in the atmosphere at 100% humidity at various temperatures: You can see that a typical concentration would be between 0.5 and 1% (approx. 33 to 66% humidity at 15 degrees C) but it can rise much higher and go much lower. You should also note that because the concentration falls sharply with falling temperature, H2O concentrations fall sharply with altitude. That is why its effective altitude for radiation to space is lower than that for CO2. It also means CO2 is relatively speaking a far more important green house gas at the poles than at the equator (where water vapour is more dominant).
  14. Climate_Protector, you indicated that with the low concentrations involved, the GHG must have a very powerful effect. They do not have a powerful effect per molecule, but a lot of molecules are involved. As to who powerful, this is a graph of the absorption of IR by CO2 travelling through 1 meter of atmosphere at sea level at the wavelengths where it is strongest as an absorber of CO2 (courtesy of Science of Doom): And so you can see the same thing with your own eyes: Please ignore the comment at the end of the video about "That's exactly how CO2 works in the atmosphere ..." which, as we discussed above, presents only half of the equation.
  15. PhysSci @561, I am not in general going to respond to your comments, which I consider to be little more than spam. If you want to discuss the issue, mount an argument, don't snipe, then hide behind an as yet unpublished paper. However, in this case you are clearly only partially correct. Specifically, if back radiation tends to heat the surface beyond the temperature dictated by the lapse rate, convection will increase, cooling the surface. As a result, except for short term excursions (ie, over a few hours or in some cases days), back radiation is not very significant in determining average surface temperature. However, back radiation can lift surface temperatures up to the temperature determined by the lapse rate without the heat being dissipated by convection. This is an important effect in temperate zones and the poles where outgoing longwave radiation is greater than incoming short wave radiation. More importantly, this effect is very well known to atmospheric physicists and is part of the full explanation of the greenhouse effect as expounded in elementary textbooks (although it does get passed over in many popular science accounts).
  16. PhysSci "Gentlemen, absorption and re-emission can only redistribute available energy, but cannot produce extra heat in the atmosphere needed to explain the near-surface thermal enhancement presently called atmospheric 'greenhouse effect'." PhysSci, that would be true if the climate were a static, fixed energy system. However, the climate is an open ended energy flow, not a static distribution. Energy will accumulate or dissipate based upon flow rates until input energy equals output energy. Given the limiting throughput of the atmosphere, there is more accumulated energy at the surface than the instantaneous flow rate, but that instantaneous flow rate of 240 W/m^2 enters the system and leaves it. Sorry, Tom Curtis, another analogy. PhysSci, water flows down a river, into a dam, and out the other side. The water is much deeper at the dam than in the upstream river, pressure at the bottom of the dammed lake is much higher than at the bottom of the river, but the flow rate in and out of the dam is equal, or the level at the dam will change until that is true. Your complaint is inappropriate for this situation. Sadly, PhysSci, it is becoming apparent to me that your grasp of the physics involved is not terribly strong. I suggest more reading on your part.
  17. Tom Curtis - Excellent and very clear explanation here at @563; thanks!
  18. It would be very helpful if PhysSci would actually state his position and present his argument. If you are unwilling to do so until after you publish your paper then please wait til then to comment on it.
  19. Tom Curtis @ 563: Thank you for this wonderful explanation! It gave me insight and more food for thought. I knew that air rises when it heats up, but did not fully realize that it also cools at the same time. I did not understand that 'compression' actually heats up the falling air. From what you said: "That [lapse] rate is a function of the gravitational acceleration and the specific heat capacity of the gas involved", I figured out that gravity is somehow responsible for compressing a falling parcel of air. Could you elaborate a bit more how exactly that works? Also, from your explanation, I gather that greenhouse gases are not determining the lapse rate. Is this right? I thought GH gases were either controlling the lapse rate or directly heating the surface, and that the lapse rate was responsible for the higher temperature on the surface. Could you provide a little more clarification on this? This discussion has been really useful for me. I feel like I'm really advancing my understanding in a field I've been taken for granted!
  20. Re #541 RickG, you wrote:- "The diagram was about the flow in energy which is expressed in units of W/m^2. That is: energy (watts) over a specific area (a square meter). Are you suggesting that it should be "degrees" per square meter?" No, I'm not. Degrees per meter i.e. distance, not area. The temperature in the tropsphere (in the diagram) should be shown as -6.5C per km (altitude). Then the author could show how it is affected by CO2, it is the second law of thermodynamics. You also wrote:- "Also, why are you criticizing the IPCC when the diagram you are talking about is by Trenberth, Fasullo and Kiel (2009)? " Because the IPCC use Treberth's diagram in many of its official reports when making the case for governmental action to reduce CO2 emissions Trenberth has updated his diagram a number of times changing the W/m^2 numbers but never showing any temperatures or even temperature gradients. Surely if the IPPC wishes to make the case for CO2 global warming they could have chosen a diagram with temperatures on it so that the warming would be clear to all? It a fundamental of heat transfer, W/m/K Watts per metre per degree Kelvin. You can multiply this by however many m^2 you have to determine the total power being transferred for the given temperature difference, the formula just gives the temperature gradient; in the atmosphere it is -6.5 deg. Kelvin per km.
    Response: [Muoncounter] You've raised this point before. If you have an objection of substance, perhaps you can take it up directly with Dr. Trenberth and report back.
  21. It is amazing that someone who does not even understand the units thinks that professional scientists have been wrong about the 2nd law for 150 years. And they are the only ones who can figure it out! degrees/km, it would be funny except people like this keep us from taking any action about AGW.
  22. Tom Curtis @ 565 I just read your comment to PhysSci. "As a result, except for short term excursions (ie, over a few hours or in some cases days), back radiation is not very significant in determining average surface temperature. However, back radiation can lift surface temperatures up to the temperature determined by the lapse rate without the heat being dissipated by convection." I thought that back-radiation was the primary factor controlling the global surface temperature. That has been my impression all along from the popular literature describing the greenhouse effect. However, as someone has pointed out on this thread before, one cannot learn good physics from popular literature and analogies ... So, should I understand that back radiation is only marginally important for surface temperature globally because of the presence of convection? Is this another way of saying that the lapse rate is the main factor determining surface temperature? Thanks again!
  23. Protector @569, very happy you are finding it useful. The air heats as it falls because it is compressed by the higher pressure air around it. It cools as it rises because it expands because of the lower pressure air around it. This is an indirect effect of gravity, which is of course the reason the air becomes more dense, and under higher pressure the lower you get in the atmosphere. GHG also impose a lapse rate on the atmosphere, however, in the troposphere, the effect of convection overwhelms that of GHG in determining the lapse rate because it takes much less time for convection to move energy than it does radiation. Radiation counter intuitively takes a long time to move energy because it only travels a short distance before being absorbed. It will then take considerable time before it is re-emitted. So, while convection carries energy quite slowly, and radiation carries it very fast (at the speed of light), convection carries it in one continuous motion, while the radiation makes a series of short journeys with very long delays in between. The tortoise and the hare come to mind. At higher altitudes, because the molecules are greatly seperated radiation becomes the main means of carrying energy. But because the molecules a greatly seperated, very little of the Earth's radiation to space comes from those altitudes, so they can be effectively ignored. So, in the troposphere, convection determines the lapse rate. What GHG do is determine the temperature in the upper troposphere. By determining that temperture, they also determine the temperture at the surface because the two are related by the lapse rate. And to avoid one common confusion, this is not a case of the upper troposphere warming the surface. The sun warms the surface. The lapse rate and GHG determine how efficiently the energy from the sun can escape the surface, and hence how much the surface is warmed by the sun.
  24. Michael Sweet @571, damorbel is using the correct units, and the approximate value of the environmental lapse rate, which is critical to the greenhouse effect. (See my comments at 563 and 573.) Where he is wrong is in supposing that Trenberth's diagram from the IPCC is a model of the greenhouse effect. It is not, except in the most rudimentary terms. It is only what it claims to be, the Earth's energy budget, ie, a tabulation of what comes in and what goes out. His demand about the proper presentation of the table amounts to a demand that every diagram related to a theory should explain every feature of the theory, which is absurd.
  25. Protector @572, back radiation is relatively unimportant in determining global temperatures, although situations arise where it is very important in determining the local temperature for a period of time. However, if back radiaton raises surface temperatures to far, this will raise temperatures at the top of the troposphere by convection. The raised upper tropospheric temperatures will result in more radiation escaping to space, thus cooling the Earth. In consequence, the temperture at the surface in the long term is set by: 1) The Lapse rate; and 2) The balance of IR radiation to space from the upper troposphere, as determined by GHG. You need both factors. If you consider a number line representing the surface tempertures, then the lapse rate is a sloped line intersecting the number line. The point of intersection will determine the surface temperature. However, if we just know the slope, we do not know the point of intersection. We also need to know the location of at least one other point on the slope, and that is determined by the GHG in the atmosphere.

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