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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 26 to 50 out of 846:

  1. Here's a breakdown, h-j-m.
  2. h-j-m, you should definitely check out the post that DSL links to. Note, in particular, this graph: Courtesy Science of Doom What that shows is that there's almost no overlap between the spectral ranges of downwelling solar irradiance and upwelling terrestrial thermal IR radiance. Increasing the concentration of CO2 in the atmosphere reduces the transparency of the atmosphere to longwave radiation, but not to shortwave radiation. This is how the greenhouse effect works, in a nutshell.
  3. I don't get it! Are you saying that solar IR radiation does not heat the earth? If so, then why should visible light be able to do so? Or do you mean that incoming IR radiation does not get absorbed by green house gases? Then the diagrams both of wikipedia as well as from Science of Doom show that as well. So, what's your point guys?
  4. The point is that there pretty much is no solar IR radiation. It's minuscule compared to the visible / near-IR range. Look at the graph from SoD I posted in the previous comment. To go back to the beginning of this discussion, you said "As incoming and outgoing radiation is (more or less) equally effected by the insulation it is quite hard to see how the result could be a warming of the earth. " The point is that incoming and outgoing radiation are in completely different parts of the electromagnetic spectrum, and thus they aren't equally affected by greenhouse gases. Does that explanation help?
  5. Re #19 Daniel Bailey. Energy can only flow from a high temperature place to a lower temperature place. The surface of Earth is always hotter than the atmosphere above. As you say there is a net transfer of energy from the surface to the upper atmosphere, that is to say the upper atmosphere, which loses energy to deep space at 2.7K, is a net gainer of energy from the surface. The energy the upper atmosphere gains energy (net) from the surface preventing the (upper atmosphere) temperature from dropping to the (2.7K) temperature of deep space. The surface, as you may now realise, is a net radiater of heat to the atmosphere via longwwave IR radiation; the surface loses a lot more heat by convection of air and evaporation/condensation of water. To keep the surface temperature (more or less) stable the surface gets heat from the Sun via many different routes. The tropics are where most of the Sun's heat comes in, heating the atmosphere and sea water. This tropical heat is transported to the poles by air and water currents. Some of this heat is radiated directly from the tropics and some at intermediate distances on the way to the poles. Of course some heat also arrives from the Sun away from the tropics. The whole business of heat transport is governed by local and global temperature differences, starting with the 5780K of the Sun and finishing in deep space at 2.7K. There are a number of curiosities; it is posible using a magnifying glass or a mirror to make a local concentration of the Sun's energy on a spot but the maximum temperature you will get is 5780K. If you use an arc lamp with a temperature >5780K to boost this spot and your lens is still focussed on the Sun, the Sun will now be heated, just a little, by your lamp.
  6. h-j-m - Visible light has more energy per photon than IR does. UV has even more. There's plenty of energy in the visible portion of sunlight to heat the earth. And the atmosphere is almost totally transparent to visible light. Thermal IR, on the other hand (5-30 micrometers) does not pass through the atmosphere very well, due to greenhouse gases.
  7. damorbel - Excellent post, very correct in all respects. And at equilibrium the energy radiated to space equals the energy received from the sun. In reference to greenhouse gases - these slow the transport of energy from the surface to space. They effectively reduce the emissivity of the Earth, meaning that for the same energy radiated the Earth has to be at a higher temperature, as per the Stefan–Boltzmann law, where power radiated scales with emissivity (e) and T^4. Power = emissivity * SB constant * area * T^4
  8. More reply to h-j-m, who wrote: Are you saying that solar IR radiation does not heat the earth? If so, then why should visible light be able to do so? The very, very small amount of solar IR radiation does heat the Earth. But it's dwarfed by the much larger amount of visible light. Let's put some numbers to this. Assume we have 100 units of incoming solar radiation, distributed as follows: * 99 units of visible, near-IR, and shortwave IR * 1 unit of longwave IR Outgoing radiation from the Earth is also 100 units (because it's in balance with the incoming radiation from the sun), but distributed as follows: * 0 units of visible, near-IR, and shortwave IR * 100 units of longwave IR Now, let's say you introduce some substance into the atmosphere that absorbs longwave IR but transmits visible, near-IR, and shortwave IR. That will slightly reduce the 1 unit of downwelling solar irradiance, producing a tiny cooling effect. On the other hand, it will also reduce the 100 units of emitted terrestrial longwave radiation, producing a much larger warming effect (about 100 times larger, in fact).
  9. Re damorbel: On the cell right now, so I'll leave you with this to chew on for now: How then, when in the freezer section of a grocery store, can one see the energy from the lights coming from the cold interior of the display cases? Also, Google back radiation (Hint: Science of Doom website or over at Chris Colosse's place).
  10. Re #34 Daniel Bailey. "the energy from the lights coming from the cold interior". Nice try! What kind of lights? Oil lamps, LEDs, lasers, gas discharge, gas incandescent, electric incandescent, fluorescent, quartz halogen? You will have to be a bit more specific!
  11. Are you attempting to be funny or do you honestly think that there are some lights that you can't see because of the temperature surrounding the bulb?
  12. KR, thank you for mentioning the energy level, it just comes in handy. Ned, if you don't agree to the widely accepted definition of IR radiation then I don't know how talk to you. Now, if you look at the Science of Doom page you will find: As a proportion of total solar irradiance # Total energy from 0 – 0.75μm 54% – all energy up to infra-red # Total energy from 0 – 4μm 99% – all “shortwave” Now that leaves us 99% - 54% = 45% of total solar irradiance in the infra-red range. I would hardly call that minuscule. If you think that you can not compare radiation in this range with that of thermal infra-red then you are perfectly right. The main difference is, as KR has pointed out that the energy of a particle gets higher the shorter the wavelength. So you can figure out what's the difference between a infra-red photon at 1500 nm trapped by water vapour and one at 10000 nm trapped by CO2.
  13. Re #31 #32 KR "Thermal IR, on the other hand (5-30 micrometers) does not pass through the atmosphere very well, due to greenhouse gases." In a sense you are correct. Very little heat gets into the atmosphere by radiation from the surface because the temperature difference between the surface and the atmosphere is not very great, not only that, transfer of heat into gasses by radiation depends not only on temperature difference but the type of gas (all gasses absorb and emit some radiation) but the density is important also, more gas, more absorption and emission. Most heat gets into the atmosphere by evaporation from the sea, a lesser amount by convection over land and sea. Once in the atmosphere most heat is radiated into deep space by CO2 and H2O. Some heat is radiated directly from the surface into deep space via the 'windows' in the combined spectra of CO2 and H2O. All the heat leaving the planet goes by these 'radiation into deep space' processes, there is no other way! The temperature difference between the atmosphere of planet Earth and deep space is very large, about 200K and given that the heat tranfer is proportional to T^4 then radiation becomes very effective.
  14. Re #36 "Are you attempting to be funny?" Give the guy in #34 a break! Perhaps he is thinking of a candle in a deep freeze, you might find a candle in a deep freeze was too hard to get it lit!
  15. h-j-m, forget about the terminology, which is just confusing you. Here's what you originally wrote: "As incoming and outgoing radiation is (more or less) equally effected by the insulation it is quite hard to see how the result could be a warming of the earth. " But incoming and outgoing radiation are in completely different wavelength ranges. CO2 absorption affects one of these ranges, but not the other. Thus, your assumption that they must be "equally affected" is understandable but wrong. OK?
  16. damorbel - In regards to energy magnitudes of IR, evaporation, and convection, you are unfortunately incorrect. It's a common misconception, though. Please take a look at Trenberth 2009, "Earth's Global Energy Budget", in particular Figure 1. Surface IR runs at about 396 W/m^2, evaporation/latent heat at 80 W/m^2, thermals at 17 W/m^2, averaged over the globe. IR is the primary avenue of energy leaving the surface. Now, 333 W/m^2 comes back down from the atmosphere as backradiation, along with 161 W/m^2 from the sun, but given that all incoming energy becomes surface temperature, you can't just difference the IR flows. Currently the difference between incoming and outgoing is something like +0.9 W/m^2, hence the observed global warming.
  17. Ned, thanks, that sounds better. It is not the terminology that confuses me but your use of it. As I understand it infra-red is rather large radiation spectrum that then had been subdivided for more precise meaning (near infra-red and thermal infra-red being two of them). Of cause you are right, I should have written "If incoming and outgoing radiation is (more or less) equally effected by the insulation it is quite hard to see how the result could be a warming of the earth." But unfortunately so far I have not found any comment about the green house effect on incoming radiation. Sorry, but you are wrong, take a closer look a the solar spectrum diagrams and you will see there is an effect on incoming radiation as well for H2O and CO2. More prominent with H2O but it is there.
  18. h-j-h, see this Global Heat Flows" diagram.
  19. Can someone please remove post 43 and change the posting software so that the comment field returns blank after a post is submitted. Thanks. KR, I just saw your post stating: Currently the difference between incoming and outgoing is something like +0.9 W/m^2, hence the observed global warming. I don't know what the correct numbers would be, but don't you think that we might need some of that energy to drive the climate system (winds, ocean currents, rainfall etc.). A lot of chemical processes need energy. Last, but not least is the biosphere of this planet depending on energy. All these energies won't show up at outgoing radiation. I'm not sure but there might be even more to be added to that difference due to the entropy implied in thermodynamics.
  20. h-j-m writes: Ned, thanks, that sounds better. Thanks! It is not the terminology that confuses me but your use of it. As I understand it infra-red is rather large radiation spectrum that then had been subdivided for more precise meaning (near infra-red and thermal infra-red being two of them). Sorry, I work with this stuff every day in my job, so I may be a bit casual in how I talk about it. The term "infrared" is ambiguous, because it is used to refer to a very broad range of the EM spectrum ... but there are hugely important differences in the origins and behavior of "infrared" radiation within the Earth's atmosphere. You asked a very natural question -- if greenhouse gases warm the Earth by blocking outgoing (emitted) radiation, shouldn't they also correspondingly cool the Earth by blocking incoming (solar) radiation? The answer to that question is one of the key principles of the greenhouse effect: given the current composition of the atmosphere, adding greenhouse gases has little direct effect on the wavelengths that comprise 99% of the downwelling solar radiation (visible, near-infrared, and shortwave infrared ... i.e., everything below 3 micrometers). However, it does have a significant direct effect on the wavelength range that comprises > 99% of the outgoing emitted radiation from the Earth (longwave infrared). So, to first order, adding CO2 to the existing atmosphere directly reduces outgoing radiation but doesn't directly reduce incoming radiation. That produces the warming effect. Notice all those "directs" and "directlys" in there? That's because the indirect effects of greenhouse gases include some feedbacks (involving water vapor and changes to cloud-albedo) that do influence incoming short-wavelength irradiance. This is the largest source of uncertainty in IPCC estimates of climate sensitivity. But these feedbacks are secondary effects and are almost certainly not large enough to counter the effects of CO2 warming. See here for a discussion of water vapor and here for a comparison of the magnitude of different forcings such as CO2 vs clouds.
  21. In another comment, h-j-m also writes: A lot of chemical processes need energy. Last, but not least is the biosphere of this planet depending on energy. All these energies won't show up at outgoing radiation. Well, all the processes that you mention were occurring in the past, too. Unless there's some change that's caused the biosphere or the oceans or whatever to suddenly start storing more energy than they were able to do so before, you wouldn't expect this to have any effect on the observed energy balance of the planet. In any case, though, this isn't really relevant to the question of whether the greenhouse effect is somehow a violation of the second law of thermodynamics (it isn't) or of whether greenhouse gases must have the same effect on incoming and outgoing radiation (they don't). If some mysterious chemical or physical process were discovered to have soaked up a lot of additional energy within the climate system, it would just imply a larger planetary radiative imbalance. The observed warming of the surface and atmosphere would still be a concern ... and in fact we'd have to worry about what would happen if your mysterious process X ever stopped absorbing excess solar radiation.
  22. Damorbel wrote: "Energy can only flow from a high temperature place to a lower temperature place." Interesting. How exactly do you explain sunlight traveling from space (very cold) to the Earth (much warmer) in your world? As explained before, the greenhouse effect acts like insulation. Think of a house in winter. If you've got a heating system (the Sun) but no insulation (greenhouse gases) then the heat escapes quickly and the house (planet Earth) stays cold. If you add insulation then the heat can't escape as fast and the maximum temperature which the heating system can maintain increases even though the amount of heat it puts out hasn't changed. No violations of the laws of thermodynamics... just an every day phenomenon that we have all experienced.
  23. Damorbel wrote: "Energy can only flow from a high temperature place to a lower temperature place." One of my favorites. Ice is invisible to Damorbel. Damorbel, your post #38 suggests that you think the atmosphere is fairly uniform in temperature from surface to top. After all, no energy can move from a colder place to a hotter place. Yet instrumental observations show a cooling stratosphere and a warming troposphere. How does your physics account for this?
  24. Ugh - when I say "uniform in temperature," I mean it uniformly decreases in temp from bottom to top.
  25. When I started writing on this thread it was caused by the repulsive argument in it's lead article. It should be obvious that the main reason for any insulation to raise temperatures is due to an energy source within the insulation. Therefore the analogy is outright wrong. Which of cause leaves the lead article without any argument. Nevertheless the subject is somewhat fascinating and I started thinking about it a lot. Finally I had to come to the conclusion that greenhouse theory indeed violates the second law of thermodynamics. Now, here is why. My argument has two parts. The first part deals with infra-red radiation, heat with respect to the second law of thermodynamics. The second part rests on the assumption that the digram about global energy flows by Trenberth, K. E. and Kiehl, J. T. (at it's latest version on american meteorological society March 2009 page 4) reflects the greenhouse theory. Part 1. The second law of thermodynamics states (repeating the quote from the lead article) "Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature". Now, it should be obvious that this says nothing about directions. In essence it says that heat hitting a body that is as warm or warmer than the heat's source it can not heat up that body further. Further more, if heat hits a body at a lower temperature than it's source it will not be able to heat this body up to the exact temperature of it's source as this would constitute a perpetual motion machine which the laws of thermodynamics don't allow for. As infra-red radiation constitutes a form of heat transport the same rules have to apply here. So it needs a closer look at infra-red radiation and how it transfers heat. Now that's simple, it is absorbed by matter transferring all it's energy to it. If that higher energy level renders the absorbing matter unstable it gives the excess energy up by again emitting radiation. Part 2: Unless I screwed something horribly up in part 1 the conclusion is as follows: Due to the second law of thermodynamics infra-red radiation is bound to hit matter it can not transfer it's energy to. As it obviously cannot be destroyed there is but one alternative, it needs to be reflected. Now let us look at the mentioned diagram and look for the reflection of infra-red radiation. Sorry, I can't see any, All infra-red radiation except for the part heading to space gets absorbed and in consequence transfers energy. As for me, that clearly violates the second law of thermodynamics.

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