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The Greenhouse Effect

Time for another physics blog.

For something that is supposed to be the most important issue facing mankind, it seems utterly amazing to me that so few people have any real understanding of how this thing called “the Greenhouse Effect” works. The explanation most people know, that is repeated endlessly in the media, is oversimplified to the point of being simply wrong.

That’s not a criticism of those people who have been deceived. That’s a criticism of those few who do know better, but who persist in using their reputations as experts to push this rubbish without explaining it. It does make it harder to challenge, but I don’t suppose that’s why they do it.

And before anybody jumps to conclusions about my ‘denier’ ravings, what I’m about to describe to you is the official, orthodox climate science version, as seen in peer-reviewed literature. (Behind a paywall, I’m afraid.) I will come on to its shortcomings on another day I’m sure, but it would be a good start if first we all understood the theory we’re supposed to be debating.

One thing we can get out of the way immediately is that it doesn’t work in the same way as a greenhouse. There used to be a theory, dating back to Joseph Fourier in 1824, that visible radiation could enter through the transparent glass, but because glass is opaque to infrared, when it is re-emitted it gets trapped. Fourier proposed that gases in the atmosphere could act the same way. This theory was proved wrong for actual greenhouses in 1909 by Professor Wood of John Hopkins University. An experiment comparing a pane of glass to a pane of crystallised rock salt (Sodium Chloride) which is totally transparent to infrared found no difference in temperature. In fact, greenhouses work by preventing convection, a mechanism that is of course impossible to freely floating CO2.

However, this idea that heat is “trapped” in the atmosphere by absorption by CO2 has persisted. While it is now more-or-less accepted that greenhouses don’t work this way, what isn’t so well known is that neither does the Greenhouse Effect.

We have a bit of a problem with terminology here. The Greenhouse Effect has nothing to do with with greenhouses, real or theorised. So what do we mean by the term, if not the mechanism of visible light in – outgoing infrared trapped? Here, I am using the term to describe a fact of which there is no doubt – the surface of the Earth is warmer than it should be on the grounds of energy balance alone. We know how much energy enters the Earth’s system from the sun. The Earth ought to settle at the temperature at which it radiates exactly the same amount to outer space. (Taking the average over time and latitude.) We can calculate this temperature fairly easily, and it turns out to be –24 C. But the average temperature of the Earth’s surface is about 14 C! This temperature difference and its dominant mechanism are what I’m choosing to call the Greenhouse Effect.

Right. Now here’s a puzzle for you. Everybody knows that hot air rises. So why are the tops of mountains so cold?

It’s one of those sort of questions that only geniuses and small children ask. We all know hot air rises. But if you measure the temperature of the atmosphere as you climb, it starts at about +14 C at the surface, and drops in a straight line to –54 C at about 10 km up. (I’m simplifying. It varies from about 8 km to 20 km, but 10 is the standard.) Then it levels out, and starts rising again with altitude.

We have another mystery, too. What keeps the layer at 10 km so cold? –54 C is far below the –24 C we expect on energy-balance grounds, so it can’t be by radiating to space. And the fact that there is a straight line all the way down to the ground suggests that whatever the mechanism is, it’s the same one that keeps the surface at +14 C. Straight lines don’t happen by accident.

I won’t keep you in suspense any longer. The answer is pressure. Because of the weight of air, the pressure at the surface is greater than it is higher up. This means that if air moves up and down, the pressure changes, and the air expands or is compressed. And when air is compressed its temperature increases.

Air is driven to circulate up and down by convection. As it rises, it expands and its temperature drops. As it descends, it is compressed and its temperature rises. This maintains a constant temperature gradient of about 6 C/km. (It would be bigger, but evaporation of water carries heat upwards too, which somewhat counteracts the effect.)

No heat passes in to or out of the air to effect this change. It is solely an effect of the changing pressure. (If you really want to know, the compression does ‘work’ on the gas, which increases its internal energy. It doesn’t come from any flow of heat or radiation.)

This temperature gradient is called the adiabatic lapse rate, and is an absolutely standard bit of physics.

Right, we’re half way there. There is a second piece to the puzzle, which is where those greenhouse gases come in. We’ve said there’s a slope, but we haven’t said what level the slope should be set at.

When we look at the Earth in infrared wavelengths, we see it merrily glowing away, like a coal ember, radiating all the heat it has absorbed from the sun. But unlike the view in visible light, where we can clearly see the surface, in infrared the atmosphere is fuzzy and opaque. It is full of water vapour, and a few other trace gases, that fog our view of the surface. And so when we ask what temperature the surface of the Earth should radiate at, the surface we see isn’t solid ground, but this fuzzy layer high up in the air. And therefore, it is this surface that settles down to –24 C, to radiate exactly the right amount of heat away.

It is about 4 km up, and held at –24 C by the heat rising from below balancing radiation directly to space. Below it, compression increases the temperature. Above it, decompression lowers it. The actual mechanism and explanation for the Greenhouse Effect is in fact pressure. To be specific, it is the pressure difference between the surface and the average altitude from which heat radiates to outer space. Moreover, it is the exact same mechanism by which the upper atmosphere is cooled to –54 C, and there is no way you can explain a massive cooling by heat being in any sense “trapped”.

Heat is not trapped by absorption by CO2. That is Wrong, Wrong, Wrong! Such trapping does go on, but it has no long-term effect on the temperature because the adiabatic lapse rate has overriding control. You can even theoretically get a greenhouse effect with no greenhouse gases at all! All you need is some high altitude cloud to radiate heat to space.

Take Venus, for example. Venus has a layer of thick cloud about 50-80 km above the surface, and a surface pressure of 92 (Earth) atmospheres. Can you tell me now, why do you think its surface is so hot? The cloud tops are at a roughly Earth-like temperature. (And an Earth-like pressure. A point that has some interesting possibilities for colonisation. Balloons, anyone?) It’s 50 km down to the surface at about 8 C/km. And there’s plenty of convection to keep things mixed up. It’s simple physics. No need for a ‘runaway’ anything.

Now supposedly, according to rather more complicated calculations, doubling CO2 levels in Earth’s atmosphere will raise the average altitude of emission about 150 m, which will therefore raise the pressure difference and hence the surface temperature about 1.1 C. If we raise CO2 by only 40%, surface temperature will go up about half that. So we had half a degree last century (an amount too small to reliably measure). We’ll have half a degree next century. And that’s all the standard Greenhouse Effect can give you.

You have to invent a bunch of other effects on top to turn it into a disaster, and that’s where it gets controversial. But I’m sure we’ll talk about that some other day.


  1. Pa Annoyed says:


    I’ll have a go at answering your questions, but please bear in mind that some of them are more complicated to answer than I can easily deal with in a blog comment.

    “Here, for instance, you talk qualitatively about rising and falling columns of warm and cold air respectively. These motions surely cancel out in the mean, don’t they?”

    It depends what you mean by “cancel out”. It is true that as much air goes up as comes down – but as it goes up in a different place to coming down, there is no cancellation in the sense of the motion stopping. A lot of it rises in the tropics and descends at greater latitudes – look up “Hadley cells” for a picture.

    “Some molecules absorb radiation and later they emit it again. Fair enough.”

    They do, but bear in mind that the molecules don’t necessarily get the energy from radiation. They can also get it from collisions with other molecules. Thus, Nitrogen is heated at the surface, rises by convection, bumps into a water molecule, which promptly radiates the energy into outer space.

    “My physics is admittedly very shaky, but is it not so that all the radiation does is to change the temperature of the molecule, if the temperature can be viewed as a measure of stored energy?”


    “If I take a horizontal slice through the atmosphere at any altitude, how is the radiation flux affecting it, quantitatively?”

    The radiation flux is irrelevant. The temperature of air in the troposphere is controlled by the adiabatic lapse rate, not radiation. The only component of the radiation flux that has any effect is the last leg, when it is radiated into outer space.

    “Years ago I did an engineering course which included a discussion of the international standard atmosphere. [...] Given the temperature as a function of altitude it is simple to compute how other properties vary.”

    I’m basically using the ISA here. But bear in mind that it is only approximately true. The height of the tropopause varies from location to location – it varies massively with latitude, and it also changes with air pressure and the passage of weather systems. (e.g.

    “But the existence of a temperature gradient implies a classical heat flux analogous to diffusion.”

    Actually, no it doesn’t. You see, the temperature can change without any heat flowing at all, if you compress a gas or allow it to expand. The word ‘adiabatic’ means without transfer of heat. (look it up.) If air is moved up and then down, carried along without undergoing any heat transfer, it’s temperature will fall and then rise. And yet the heat flux is zero. It’s physically the same as the way a refrigerator works, or the way a bicycle pump gets hot.

    “it would be very helpful to me, for my understanding of the greenhouse effect, if I could start with the radiation phenomena and obtain the temperature as part of the computation, but I don’t know how to do this.”

    Me neither. It’s actually a tremendously complicated calculation involving lots of quantum mechanics and databases of tens of thousands of spectral lines. It’s one of the great deceptions of climate physics – pretending this stuff is simple or obvious. But you can look up a program called HITRAN if you like.

    The explanation I’ve given here is about as simple as you can get and still be at least conceptually correct. To do it properly, you would have to model the atmosphere, hydrosphere, and radiation fields at most on the order of 10 metre resolutions. It’s mindbogglingly complex. Nobody can do that – not even climate scientists. But if you do want to follow up what they do actually do, go buy a textbook on atmospheric science (e.g. Wallace and Hobbs) and follow the references. It’s a very good idea, if you’ve got a physics background (and it sounds as if you have). Most are quite hot on the GHE these days.



    You say “Departing from the physically incorrect assumption of radiative balance”, but as I explain above, the GHE does not operate by means of a radiative balance, and competent climatologists know it. So there’s no point in discussing what happens if you depart from it.

    Like I said, G&T debunk a version of the GHE that has been pushed on the public, but is actually on the same scientific level as you would give to primary school children. It’s like trying to discredit the theory of gravity by pointing out that if everything falls downwards, then people in Australia would fall off the Earth into outer space! (Although if you’re Australian, it would presumably be the Europeans falling off.) You can calculate a mathematically correct trajectory for said Aussie in great detail, if you like, but it really doesn’t help.

  2. Bea Wildred says:

    To Pa Annoyed (on January 14, 2010 at 9:07 am)

    Many thanks for your words of wisdom. I see I will have to get hold of that book.

  3. Hi Pa Annoyed !

    A German engineer called Heinz Thieme has about the same ideas as you. Here is the address of his website:
    What I really don’t understand and that it is so much talking about an effect, which possibly doesn’t exist at all and which will ruin our industry if we shall pay carbon taxes to avoid an nonexistent problem. Have you seen any websites, which have the same opinion as you, that means that the GHE doen’t exist.
    Regards Martin
    P.S.: I am a German, so my English is not so good.

  4. [...] May 10: WUWT has a new post on Venus. Among the comments, a link to another blog making a similar point (Oct 7, 2009) and to a brief communication by Carl Sagan in the pages of the Astrophysical Journal (1967) [...]

  5. Paul Arveson says:

    The Internet has given a mouthpiece to apparently millions of people with time on their hands, mostly men, who are angry, skeptical of everything, believing nothing, trusting no one, affirming nothing, but reacting with vulgarity and vitriol to the news of the day, discoveries of science, and all the religious beliefs, cherished traditions, arts and cultures of humanity. All in the name of critical thinking, skepticism, and freedom.

    These angry men are not creative enough to find a solution to world problems. They are not thoughtful enough to reconcile conflicting data. They are satisfied simply to find fault, and leave it at that, satisfied that they are smarter than anyone else, and happy to let you know that via the safety and cheapness of the Internet.

    Unfortunately this trend is leading to a continued breakdown of trust and more polarization in politics. As pointed out in a note on AMC’s recent showing of “Conan the Barbarian”, modern people don’t have to be as polite as barbarians; they can say something insulting without getting their skulls bashed in.

    Sometimes I wish we could return to those barbaric days.

  6. Mark N says:

    @Paul Arveson I feel your pain. It’s hard to understand why people are so skeptical, too, since no politician has ever lied to us.

    Forget that Al Gore, Nobel Peace Prize recipient, in his famous movie stated ‘dumbest thing I ever heard’, when referring to people who did not believe there was a correlation between average global temperature and CO2. Funny tho, that he didn’t mention nor did anyone check the science when it came out that the CO2 change FOLLOWED the temperature change. An oversight? Sloppy science that missed the scrutiny of the Norwegian Nobel Committee? Doubtful. Duplicity? I’m more inclined to think so.

    But I agree, we should be more trusting, and because you identified this problem, we should probably start with you. Please send me a lot of YOUR personal money to help solve this problem.

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