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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.



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Comments 601 to 625 out of 1481:

  1. each time a collission occurs, there is a net loss of energy to gravity What?? gravity gains energy? I dont understand this comment at all.
  2. Camburn @598, the atmosphere does not cause the Earth to accelerate in space, so there is no loss of energy to gravity. In fact, the paths of all molecules are accelerated to towards the surface, but because the atmosphere is evenly distributed over the Earth (to a reasonable approximation), the Earth is accelerated in all directions equally by this interaction, and hence not accelerated at all. The energy gained by gravitational acceleration of particles in the atmosphere is returned by elastic collisions at the surface. Further, collisions do not expend heat energy. You are assuming the statistical properties of a group of molecules must also be the properties of individual particles. The energy involved in the collision of particles is the kinetic energy of the particles, plus their individual rotational and vibrational energies. Collisions can result in the redistribution of energy amongst the particles, and the rotational/vibrational states, and can result in the emission of IR radiation. But there is no "heat energy" as a distinct property from the kinetic energy, and rotational/vibrational energy of the particles; and sum of the energies of the particles (including photons) involved is always conserved.
  3. A loss of energy to gravity???!!??? What in the world is that?
  4. Tom C@602: 1. Conservation of energy. Gravity is a force. Everything in our atmosphere/earth sphere is affected by gravity. It consumes energy. Without it, we would all float etc. Yes, kinetic energy is kinetic energy. But even kinetic is the result of heat and can be converted to heat. Friction is an example. When two molecules, even tho of minisucle weight, collide, energy is not only transferred, it is expended because they do not bond, rather they collide and go a different direction and they will go in a different direction at a slightly slower speed. That energy is absorbed by gravity. Even our atmosphere, while of atomic weights, requires an expenditure of energy to stay aloft. This energy comes from the sun as I know of no other source. A physics prof was trying to get this into my mind in college. Not sure I ever really totally grasped what he was trying to show, but rattling my old cobwebs I am trying to understand it still. It gets back to including all energy in any equation talking about energy. Maybe he was all wet, but the older I get the more I think he understood something quit clearly that I didn't.
  5. Tom Curtis (RE:597), "1) The energy absorbed at the surface is the Incoming Solar Radiation absorbed at the Surface (approx 161 w/m^2) plus the Back Radiation absorbed at the surface (approx 333 w/m^2)." OK, here is my question then to you: If you agree that the atmosphere cannot create any energy of its own, and 78 W/m^2 of the 239 W/m^2 (239 - 161 = 78) entering the system never reaches the surface, then where is the 333 W/m^2 of back radiation coming from if 239 W/m^2 is also leaving the system? 40 + 78 + 121 = 239 W/m^2 leaving. 396 W/m^2 - 239 W/m^2 = 157 W/m^2 emitted down to the surface. 161 + 157 = 318 W/m^2 at the surface (396 W/m^2 required). Also, 157 W/m^2 + 97 W/m^2 = 254 W/m^2 (333 W/m^2 required).
  6. RickG @ 591 "Specifically, what is it that you guys don't understand about this diagram? You are trying to make it into something that it is not." I'm not trying to make this diagram into anything. I question the basis by which the energy is stored in the atmosphere...if not temperature. So I ask again how do you know is there...can it be measured? And by what means is the atmospheric energy stored?
  7. Sorry Camburn, it still makes no sense. You say "Gravity is a force." Then you say "That energy is absorbed by gravity." So energy is absorbed by a force. Please elaborate on the physical process there, I'm at a loss.
  8. Tom Curtis (RE: 597), "Saying that 239 w/m^2 becomes 396 w/m^2 is to directly assert the non-conservation of energy." How do you figure? I said the 239 W/m^2 entering the system becomes 396 W/m^2 at the surface. The amount leaving is still 239 W/m^2. Power in = power out = Conservation of Energy.
    Response: [DB] Fixed unclosed html tag.
  9. Tom Curtis (RE: 597), "5) The surface radiation is a function of temperature and emissivity, which is not 1 at any location, though very close to 1 at most." OK yes, temperature and emissivity, which for all practical purposes is 1 because the surface is a near perfect black body radiator.
  10. Philippe@607: Now you are understanding why I didn't understand. I was trying to explain his reasoning, which I did a poor job at doing. His lectures etc on that still tickle my brain tho. I know there is something there that I didn't understand.
  11. Tom Curtis (RE: 599), "RW1 @595, all energy "emitted" from the surface is radiative only because we do not talk about "emitting" convection, or evapo/transpiration." OK, this is the crux. Is 396 W/m^2 radiated from the surface or not? "Not all energy flux from the surface, however, is radiative. In fact, only 80% of it is." Agreed. "And some of the energy flux carried by convection and evapo/transpiration makes its way to space. Do you deny that?" No. As I said, trade offs do occur but an equal and opposite amount less is then returned to the surface. Less energy returned to the surface than what initially left in kinetic form will cool the surface, which reduces the amount of emitted radiation by an equal and opposite amount.
  12. Camburn @604: 1) First and most importantly, the action of a force on an object never consumes energy. It only ever changes its form, and its location. This concept is actually very hard to grasp for most people because it is counter intuitive based on our everyday experience. In our everyday experience, a body in motion will come to rest unless an external force acts on it. This is the experience fairly well captured by Aristotle's laws of motion, but it is an illusion based on not taking into account the effects of friction and air resistance. In space (or by careful experiment) we can see that Newton's laws reign supreme, and that: a) A body at rest or in a state of steady motion will remain at rest or in a state of steady motion unless an external force acts on it; b) Force equals mass by acceleration; and c) For every action, there is an equal and opposite reaction. So, if we consider a gas molecule heading towards space. Gravity indeed acts on it, and it decelerates, losing energy in the process. But gravity equally, and oppositely acts on the Earth at the same time, accelerating it so that it has more energy. In fact, it gains exactly as much energy as the molecule loses. If we follow the path of the molecule under gravity, and ignore all the other collisions (which cancel out in effect), eventually the molecule will collide with Earth, resulting in another exchange of energy that cancels out the exchanges that took place under gravity. (To tell this story completely accurately, I would need to include gravitational potential energy, which shows up in General Relativity as very small changes in mass; but this is just a comment on a blog so you'll have to settle with the short and dirty version). Anyway, gravity does play a crucial role in the atmosphere. It is because of gravity that the atmosphere is dense near the surface, and thin away from the surface. A secondary consequence of this is that molecules move rapidly (the gas is warm) near the surface and slowly (the gas is cool) away from it; and it is possible to predict those temperature relationships using Newtons theory of gravity. But it does not result in a loss of energy, because all forces only result in the exchange of energy, never its loss. 2) Heat that is generated by friction is just kinetic the energy of the molecules that make up the substance being warmed as they vibrate in position; or in the case of a gas, move around in their container. But because that heat is just the motion of those small particles, the small particles themselves do not lose energy to friction. 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, they slow down, and will eventually not be able to escape the surface (which will mean the gas has condensed as a liquid or solid). However, they have this energy, the do not need to expend energy because any forces just rearrange the energy, not dissipate it. Further, they do not have frictional energy losses. The do lose energy by the radiation of IR gases, which needs to be replaced directly or indirectly from the sun - but for practical purposes they do not lose energy over and above that.
  13. Tom Curtis (RE: 599), "And some of the energy flux carried by convection and evapo/transpiration makes its way to space. Do you deny that?" Let me try to explain this a little better. Yes, some of the kinetic energy flux into the atmosphere from the surface by thermals and latent heat can end up being radiated out to space. However, if this occurs, it also must result in an equal amount of energy less being returned to the surface. If less energy is returned to the surface than the amount of energy that left the surface, the surface will cool. As a result, the cooler surface will radiate an equal amount less than the amount radiated out to space from latent heat and thermals. Thus, the net effect of latent heat and thermals on the radiative budget is zero, as Conservation of Energy dictates, because all the energy leaving at the top of the atmosphere is radiative.
  14. Tom@612. I'm saving that one. Your physics explanations are always good, but this one has bells, bows, balloons and buzzers. Outstanding.
  15. RW1, would you consider the following simple model: The model is a simple box with mirrored back and sides. We will assume the mirrors are 100% efficient, and reflect all light. The box is covered with a material that is completely transparent to all light coming from outside the box. However, it is half mirrored on the inside, reflecting exactly 50% of all light from the inside of the box, and transmitting without loss the remainder. The box is not a model of the greenhouse effect; but it does have the virtue that any thermodynamic issues raised by the greenhouse effect are also raised by this box, but in a simplified form. In this box, we have the following equalities: 1) Incoming light (A) = Outgoing light (C) (by virtue of conservation of energy). 2) Light reflected from the lid (D) = light transmitted by the lid (C) = Outgoing light (by virtue of the defined half mirrored property of the lid). 3) Light reflected from the back = light striking the underside of the lid (B) = light transmitted by the lid (C) plus light reflected by the lid (D) (by virtue of conservation of energy). Therefore 4) Light reflected from the back of the box (B) = light reflected from the back of the lid (D) plus Incoming light (A) = 2 x A (again, by conservation of energy. By simplifying the situation, ie, by getting rid of any concerns about convection and light absorbed by the atmosphere etc, we should be able to raise any issues you have with the consistency of the GHE with the laws of thermodynamics without getting hung up on trivia. Do you agree? Do you also agree with me that this simple model does not violate any laws of thermodynamics?
  16. 615 Tom Curtis I like, understand and agree with your model, but..
    The box is not a model of the greenhouse effect; but it does have the virtue that any thermodynamic issues raised by the greenhouse effect are also raised by this box, but in a simplified form.
    it doesn't do the thermodynamics justice. (For fun) To do that the 'mirrored' walls should be perfectly black, perfectly insulated and with infinite heat capacity; the radiation hitting them would be perfectly converted into heat and the walls would black-body radiate. Then if the half-mirrored front was, instead, transparent at wavelength λ (give or take), and opaque else where: the box would heat up until radiance from the walls reaching the window at λ equaled the radiance coming in... With a little more messing around (2nd window, transparent at a different wavelength, etc.) you'd have a green house 'box'.
  17. les @616, with some minor amendments, and one major one, your variant box would much better model the greenhouse effect. However, it is not clear that it raises any issues of thermodynamics not raised with my simpler box. So, unless it is clear that we cannot get agreement that no additional thermodynamic arise in the simple model, than I would rather stick with that. If it becomes clear that we cannot get that agreement, we should introduce one variation at a time until we can isolate the actual issue in dispute. The one major disagreement, by the way is the infinite heat capacity. An infinite heat capacity would imply the walls of the box would neither heat nor cool, no matter who much radiation they absorbed or emitted, which would itself violate the laws of thermodynamics (I think) and certainly not accurately model any real physical process.
  18. Tom@617 ... absolutely, hence the "for fun" remark - it was only that, sometimes, one good model deserves another. as for the heat capacity - I hesitated long and... then well, didn't have time to think that one out. Indeed the heat capacity would determine how long it would take to "heat up until" - and an infinite heat capacity would result in it taking an infinite time to heat up. Boundary conditions, hu? always a good way of doing a sanity check on a model - which is the point of your model; so back to that.
  19. Tom Curtis@615 1. A = C ok 2. C can not equal D without violating the 2nd law. Otherwise you have doubled your light/energy with a mirror and a filter. Light can not brighten due to it's own reflection. If your box was fully enclosed such all surfaces are reflective save two small aperture. One aperture to receive light the second to radiate light, do you really think the light/energy will increase beyond it's input? Now change your perfectly reflective interior to one with an emissivity of 1 (black body). Do you believe C will be grater then A. Again no. Black body emission represent the maximum conferred energy for light input. Therefore, a surface with emissivity less then one will NEVER radiate more then it's black body equivalent...regardless of it's own reflection or it' own re-radiation (back radiation).
  20. 619 Ryan - hint: the walls are reflective. All the photons which do D (i.e. don't escape when they hit the front window) will bounce around (lets say B') till they hit the window again and are either C' or D', then B''/C''/D'', B'''/C'''/D''' etc. till they do.
  21. "till they do." - oops, sorry should be "till the sum of Ds = A"... Or do you think it's possible that a significant number of photons will bounce around the box for ever without leaving?
  22. les@621 Like I said C can equal A, but C cannot equal D.
  23. 622 Ryan.. Yes, and I'm pointing out that, really - they are (all) integrals. That may be a problem of notation rather then violation of a fundamental physical property.
  24. L.J.Ryan C will equal D. This scenario is very similar to the spreadsheet I posted here. You can model Tom's box using a 5 column spreadsheet thus; In row 1 type A, blank, B, C, D (to represent the quantities on Tom's diagram) In row 3 type 100, 0, =A3+B3, =C3*0.5, =C3*0.5 In row 4 type 100, =D3, =A4+B4, =C4*0.5, =C4*0.5 Copy row 4 into the next 30 lines of the table. You will find equilibrium reached after about 17 iterations and that Tom's calculations match in every detail. To check the conservation of energy you must let the accumulated energy in the box dissipate. To do this copy row 30 into 31 and set cell A31 to 0. Copy row 31 to the next 15 or so cells. If you sum column A and D (don't forget, column D represents Tom's arrow C) you will find they are equal.
  25. Tom, I really like this. Same idea as Science of Doom example but much simpler.

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