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Why does CO2 cause the Greenhouse Effect?! | Climate Chemistry

Posted on 27 November 2018 by Guest Author

This is one of ClimateAdam's latest videos

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Comments 1 to 30:

  1. I hugely respect Climate Adam, and realise hes trying to make science fun, but this video is a trainwreck and just so childish and distracting. A few wiggles of CO2 molecules on a diagram would have been more informative, and taken a fraction of the time.  Give them smiley faces, if you want to make it fun.

    How do we get from measuring the level of energy absorption by the molecule to calculating a change in global temperature? It would be interesting to see the basic maths and physics but kept at a level the average person in the street can relate to.

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  2. I totally agree with nigelj here. I cannot picture kids being overwhelmed by this presentation. Frankly speaking, it's quite boring and not easy to grasp that way.

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  3. Much better link =>

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  4. If atmospheric heating is directly proportional to C02 consentration, how did the Earth go through at least three "Snow Ball Earth" periods when the atmosphere was essentially 100% C02?  Also, why is it so cold on Mars when the atmosphere may be thin, but it is 93% C02.  

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    Moderator Response:

    [DB] "how did the Earth go through at least three "Snow Ball Earth" periods when the atmosphere was essentially 100% C02"

    That's nonsense.  Atmospheric carbon dioxide has never been more than even 50% of the total:

    Atmospheric composition over geologic time

    "why is it so cold on Mars when the atmosphere may be thin, but it is 93% C02"

    Less solar insolation received at the surface due to the greater distance from the Sun.  

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive or off-topic posts or simply make things up, as you do. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site. 
    Finally, please understand that moderation policies are not open for discussion.  If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  5. Ken @4,

    A very quick reply, (but perhaps not as quick as the Moderator Response).

    (1) The GHG effect from CO2 is not "directly proportional" to the level of CO2. It is a lot more complicated than that. For small changes in the terrestrial atmosphere (200ppm to 1,200ppm) the relatinship is logarthmic. However your general assertion that more CO2 would result in more warming would be correct in most circumstances.

    (2) The Snowball Earth episodes were a long time ago so their circumstance are not very well nailed down. The favourite cause of Snowball Earth is that it begins with lowering levels of CO2 allowing global temperature to drop and this cooling is amplified by an increase in albedo - the extensive snow/ice fields increasing the amount of sunlight being reflected into space and so not warming the planet. Once a Snowball Earth has formed, it requires a very large increase in GHG (ie CO2) to warm up the planet enough to melt the snow/ice and release it from the Snowball. Note that through this time, the sun was much weaker. By the end of the Snowball Earth eposides, it would still have been 6% weaker than today, roughly equal to losing a third of today's GHG effect.

    (3) The Martian atmosphere is almost all CO2 and, although there is little gas in the Martian atmosphere, it still has more CO2 than Earth. Indeed Mars (95% x 6mb) has well over ten times the CO2 of Earth (0.06%[mass] x 1,000mb) in its atmosphere. The problem for a GHG effect on Mars is not the level of CO2 but the lack of any other GHGs to fill in the gaps of the electro-magnetic spectrum not being insulated by the CO2. On Earth, the CO2 warms the planet enough to boost H2O levels which then provides this extra insulation. [The Mars GHG is a bit like going out on a snowy day wearing just a hat and gloves. They don't keep you very warm. Even thick furry hat & gloves will not improve the situation.] And additionally, Mars has on 43% of the sunlight that Earth enjoys. A cold Mars with its CO2-laden atmosphere and a warm Earth with just a small portion of CO2 in a nitrogen atmosphere is what the theory expects and exactly what we see.

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  6. I am an old enough geezer & what catches young folks' attention is somehat outside my zone of understanding. So I think it might suit the generation it is aimed at, and kudos to Climate Adam for trying. I enjoyed it and watched to the end.

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  7. Ken, so just how much effort did you put into trying to answer your own question? Frankly looks like grasping at a straw because perhaps you cant imagine a solution to climate change that is compatiable with your political philosophy? Or if you are simply trolling, perhaps its time to find another site for your amusement.

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  8. MA Rodger at 02:58 AM on 29 November, 2018

    I appreciate your bringing up the log relationship for CO2 and GHG effictiveness.  That was going to be my next question, because no one seems to want to address it.  I believe what you meant by not "directly proportional" was "not linear", because it is directly proportional, but in a ln (natural log) curve, and that means it is a smooth, continuous function.  CO2 GHG effectiveness is a ln function of CO2 consentration.  This means that the slope of that ln curve is the effectiveness.  The problem is that the slope of the ln curve (or the first partial derivative) is equal to 1/(CO2 consentration).  For example, at 200 ppm the slope is 1/200  and at 400 ppm the slope is 1/400.  Eventually the ln curve approaches horizontal, which means the GHG effectiveness is reduced by 50% when the CO2 consentration doubles.  Any engineering of scientific system that follows the ln curve has the same result, the more you have of the independent variable (x-axis) the less effective the dependent variable is.  

    The best physical example of this is laying a chain on the ground, holding one end and running toward the other end.  You move the chain quickly at first, but pretty soon you can hardly move the chain and your speed, or effectiveness, goes to zero.

    I realize the interactions in the atmosphere are complex, but that complexity does not negate the properties of the natural log function.  Methane and other gases do not follow the ln curve, but CO2 and water vapor do follow it.

    If more CO2 means more temperature at some ratio, someone must have developed the first partial derivative of Temp with respect to CO2, otherwise how can we make estimates of containing Temp by reducing CO2 by a certain amount over a certain number of years.

    Just to clear the air here, I am very serious about wanting to know how the atmosphere really works.  I am a retired AF pilot, I have a BS and MS in aerospace and aeronautical engineering, a MS in software engineering/computer science.  I have worked in these fields and taught university courses for over 40 years.  I am a serious amateur astronmer and astro photographer, and design custom astronomy equipment.  

    I am sorry if scientific questions and comments upset people.  I was hoping to find people who want to find real answers and leave people and politics out of the scientific discussions.

    I am traveling right now, but will answer the other comments and show some data I have next week.

    Thank you for your time.

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    Moderator Response:

    [DB] Inflammatory rhetoric snipped.  Please read this venue's Comments Policy and stick to the science and all will be fine.  Thanks!

  9. So we can assume you are now satisfied on the questions of snowball earth and why Mars isnt hot? Given your stated background, it would seem that you could have easily consulted a text book.

    The amount of radiation acting on the earths surface is what is proportional to CO2. Double the CO2 concentration to double the forcing. The effect of change from 200ppm to 400ppm is same as change from 400ppm to 800ppm. The effect on surface radiation can be directly measured.

    The surface temperature that results from such a change in radiation is not so straightforward because of feedbacks. You cant change the earths temperature up or down without also affecting the amount of water in the atmosphere and the albedo. Hence the relationship of CO2 to temperature is much more complex.

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  10. Ken @8, I think I can see where you are going with this. Before you waste your time posting a whole lot of data, read the following:

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  11. Ken @8,

    Although it is not relevant to the substance of this interchange, you evidently have a different understanding of the term "directly proportional" than I do. And it seems my understanding is shared with Wikipedia.

    As for the log relationship resulting in there being a diminishing relationship between rising CO2 levels and extra direct climate forcing, there is also another non-linear relationship - the greater the forcing imposed on climate, the greater the harm to humanity from a unit rise in that forcing. Mankind really does not want to arrive at a point where CO2 emissions have a negligible direct effect on climate forcing. If we do arrive we will find that 'negligible' increases in CO2 forcing will nontheless bring with it non-negligable impacts, either in resulting feedbacks or in dramatic climatical effects.

    You say that "someone must have developed the first partial derivative of Temp with respect to CO2" but do not express this overtly as a question or why the inception of this work would be relevant to policy decisions today.

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  12. Ken,

    I am astonished that someone who claims to be interested in the science behind AGW is so ignorant of the basic data.  All gases follow the log relationship.  Please provide a citation to support your wild claim that methane and other gases do not. 

    You are about 100 years behind in your understanding of the science.  Claiming you want a scientific discussion while criticizing those who know the science is insulting.  Follow your own advice instead of making political, insulting statements.    You appear to have informed yourself with a brief review of nonsense (non-science) sites on the internet. 

    Your wild claims contradict your claims that you want a non-poloitical discussion.  You are the only one who is making wild, incorrect claims.

    People here are willing to help you obtain an understanding of the science and will provide you references to the science.  If you want to make false assertions and insult those who reply to you they will quickly lose interest. Try to up your game.

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  13. Pretty sure this is on topic. I’m struggling with the details of the physics. BTW, I thought the video was good but it didn’t go far enough. My question is whether the absorption of infrared radiation (IR) by greenhouse gas molecules directly warms the air. I have seen an explanation regarding the hot atmosphere giving off IR. But, since 1859, we know that the majority of the atmosphere doesn’t absorb IR.So I'm assuming it can't emit IR. The atmosphere isn't a black body (maybe a gray body).

    GHGs absorb IR because they have at least three atoms and two bonds allowing more than one quantum vibrational state. A GHG molecule is boosted to a higher state only by a photon with the right energy. The energy must match the energy gap between two allowed quantum states of the molecule.

    Similarly, it seems the molecule could only emit a photon of the same energy and return to its base state. Some of the emitted photons will be reabsorbed by the Earth causing further warming.

    But gas molecules are also in kinetic motion---whizzing about. The faster the molecules move---the more kinetic energy they have---the warmer the air is. Can the higher quantum state of an excited GHG molecule be transferred to another molecule through a collision? I know that the kinetic energy isn’t quantized so it seems that in order for that to happen the excited molecule would have to emit a lower energy photon, to return to its base state, as part of the energy transferring collision---the transferred kinetic energy plus the energy of the photon would have to equal the energy gap between the two allowed quantum states---kind of the reverse of collisional broadening of the absorption bands.

    I hope I’ve made my question clear and I’d really like to know the answer.

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  14. dkeiereber @13,
    Gas molecules do increasingly whiz about with increasing temperature. If the gas contains polyatomic molecules (like CO2) it will have a higher Specific Heat Capacity as there will also be energy being absorbed into extra rotation and vibration as temperature increases. The vibrational energy can be transferred to other molecules/atoms as kinetic, rotational or vibrational energy but can also result in the emission of a photon. So the answer to your question "Can the higher quantum state of an excited GHG molecule be transferred to another molecule through a collision?" is 'Yes.'
    And the opposite can occur; a collision or a photon of the correct wavelength can set a polyatomic molecule vibrating in a particular mode.
    Quantum rules apply to vibration (and rotation) so the photon is always of the same wavelength, abet the rotational energy and kinetic energy do create small variations (broadening) in wavelength for emitted/absorbed photons.
    Relative to the energy transferred between molecules/atoms in a gas, the emitting/absorption of photons are a rare events and most absorbed photons will become added to the energy in the gas rather than being immediately emitted as another photon.

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  15. @14, MA Rodger

    Can you provide references for your response?

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  16. dkeierleber @15,

    Can I provide references? Not easily.

    On line, perhaps Siegel & Howell (1971) may be of use to you. Alternatively, you may wish to narrow your enquiry within what is a piece of physics that doesn't get described within published literature in the manner that is understandable to the general reader. Or more correctly, I have never found anything on-line more complete than the likes of this overly-simplistic UCAR page.

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  17. dkeierleber @15, I came across this when reading up on the greenhouse effect some time back, appears to be written by a chemistry teacher.

    "Certain gases in the atmosphere have the property of absorbing infrared radiation. Oxygen and nitrogen the major gases in the atmosphere do not have this property. The infrared radiation strikes a molecule such as carbon dioxide and causes the bonds to bend and vibrate - this is called the absorption of IR energy. The molecule gains kinetic energy by this absorption of IR radiation. This extra kinetic energy may then be transmitted to other molecules such as oxygen and nitrogen  (as they bump together) and causes a general heating of the atmosphere. Analogy: Think of a partially stretched "toy slinky" - if you bump the slinky, the energy of the bump is absorbed by the vibrations in the slinky."

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  18. I’m dense and not quite there yet. From Pierrehumbert Principles of Planetary Climate (p 221), “Since energy is conserved, the absorption or emission of a photon must be accompanied by a change in the internal energy state of the molecule. It is a consequence of quantum mechanics that the internal energy of a molecule can only take on values drawn from a finite set of possible energy states, the distribution of which is determined by the structure of the molecule.”

    So molecules can only take on specific vibrational states, constrained by quantum mechanics. A molecule with 2 atoms has only 1 allowed vibrational state (3 x 2 – 5). GHG molecules have more allowed energy (vibrational) states (3 x the # of atoms -5 for linear molecules, -6 for nonlinear ones). But the energy states are still confined to whole multiples of the quanta that excites them. Just like a photon energy must match the energy gap between electron shells in order to be absorbed and boost the electron to a higher shell. The photon could not boost 2 electrons half way to the next shell. Quanta can’t be divided.

    It is this quantum constraint that makes the GH work. If GHGs were excited by any LWR the atmosphere would go crazy. GHG molecules can only absorb energy in narrow bands---they are little receptors tuned to specific frequencies. So loosing part of their constrained vibrational energy via collisional transfer would leave them in a non-allowed quantum state.

    Also from Pierrehumbert (p 227) collisional broadening of the absorption bands occurs because the kinetic energy of the collision is not quantized. So a molecule can borrow some of that energy to be boosted to the next allowed vibrational state by photons whose frequency (and thus energy) would ordinarily be outside their absorption band.

    That is the missing part of MA Rodgers response on why Mars is so cold. The lack of water vapor is a big factor but so is the thin atmosphere---too thin to allow for collisional broadening. On Venus, collisional broadening extends high into the atmosphere. This is explained on the American Chemical Society’s post on Multi-Layer Model.

    Digressing for a bit, in a sufficiently thick atmosphere, non GHG molecules can absorb photons during a collision. Nitrogen in Titan’s cold dense atmosphere, eg. That’s a different process called Continuum Absorption and also happens on Venus.

    So, again, I can see the vibrational energy state of molecules being affected by a collision only if the process involved the emission of a lower energy photon to return the molecule to a lower vibrational state. Of course the problem may be my failure to understand the process and learning difficulties associated with my aging brain. Also, Pierrehumbert likely explains the process in the next 2 chapters but I struggle a bit with a graduate level climate science book. I was hoping for a short cut to understanding this

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  19. PS I will read the 1971 NASA publication today to see how they explain it.

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  20. dkierleber,

    You have asked good questions that are on topic.  I think you have a small misunderstanding that will soon be corrected.  Keep up the discussion till you feel you are informed.  It helps other readers to hear your questions. Everyone looks harder at their understanding.

    I think you are just a little off.  As I understand the physics, it is easy for carbon dioxide molecules to lose the extra vibrational energy they get from IR photons through collisions with other molecules.  Due to the long time before a new photon is released, there are usually thousands or millions of collisions before a photon is emitted.  It is not necessary for a photon to be emitted at the same time as the collision.  The extra vibrational energy ends up as kinetic energy and heats up the atmosphere.

    Because other molecules of CO2 can be put in the excited state by collisions there is always a population of molecules that can emit photons.  Even though most of the molecules do not emit the energy as photons, enough do to keep the system in equilibrium.  At higher temperatures more molecules are in the excited state and more photons are emitted.

    I think line broadening is only a small contribution to the greenhouse effect.  The primary issue is that when CO2 is at low concentrations emitted photons escape to outer space.  At high concentrations of CO2 the photons are absorbed by other CO2 molecules before they can escape. 

    On Mars the CO2 is in a low concentration (measured as grams per liter) even at the surface so the energy escapes to space.  On Earth the escape altitude was about 10.00 kilometers in 1850.  Since then the CO2 concentration has increased about 130 ppm.  That increase caused the escape altitude to increase to about 10.16 km (only about 160 meters).  The lapse rate (the rate of decrease of temperature due to increasing altitude) is about 6C per kilometer.  That means it is about 1C cooler at the new escape altitude.  Less energy is emitted when it is cooler.  In order to preserve conservation of energy, it has to heat up at the new escape altitude so that the same amount of energy is emitted.  This increase then propagates down to the surface. 

    At 10km altitude there is little water vapor in the atmosphere since it is so cold.  CO2 is the main greenhouse gas at that altitude.  Since CO2 is the main gas at the escape altitude it exerts the most control over warming.

    I have digressed a lot from your questions about vibrational energy.  Does this information help?  Unfortunately, I do not know of a good reference. For what it is worth, I teach introductory College Chemistry.  We touch on energy exchange.  I did not take quantum chemistry.  I learned the most about the greenhouse effect reading Skeptical Science for the past 10 years.

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  21. michael sweet @20,

    To add a bit more to your comment.

    On the number of collisions etc - The collisions of a molecule in air are measured in microseconds while the the relaxation from vibration of CO2 is in tenths of seconds. So the ratio is in the millions.

    On the pressure broadening - It won't reduce the atmospheric warming as the IR photons are absorbed over a broader spectrum but with less probability at any specific wavelength. But it does leave more gaps, a larger protion of the spectrum which can leak energy to space (if there is no other GHG operating at those broadened wavelengths). And @5, my comment about the "the lack of any other GHGs to fill in the gaps of the electro-magnetic spectrum" didn't consider the relatively minor effect of pressure broadening but was addressing the large parts of the specrum which would have no insulating GHG operating on Mars as there is only CO2 as a GHG in the martian atmosphere.

    On the temperature of Mars - Mars does actually have a greater thickness of CO2 in its atmosphere than Earth, over ten times more. But on its own CO2 would not even compensate for the effects of the diurnal and zonal temperature variations which reduce the average temperature below the simplistic S-B average value.

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  22. I think spectral broadening increases the warming. I mentioned it because focusing only on the lack of water vapor on Mars begs the question of Venus which also lacks appreciable water vapor.

    Thanks to everybody for the kind feedback. I think I owe a response. I’ve been working on an online course for engineers on the science of global warming. I think the A/E community is already being impacted by changes and, being a sometimes pretty conservative group, engineers could use some exposure to this material. I want to be explicitly correct in everything I write. So if it seems like I’m a troll running a gotcha game, nothing could be further from the truth. I’m trying to construct a narrative that informs an audience that may be more knowledgeable than I am in thermodynamics, energy transfer, and quantum mechanics. I’m a retired structural engineer and my one online course is Wood Connections for heavy Snow Loads so I’m well outside my career field.

    In my course material, I wrote, “When an emitted photon of long wave radiation (LWR) is intercepted by a molecule of a greenhouse gas (GHG) the molecule will absorb the photon and be raised to a more energetic state. The excited molecule may then bump into other molecules, thereby raising the temperature of the air, or it may re-emit a photon of LWR to return to its base state.” But, after reading the Pierrehumbert explanation of frequency bands and the quantum nature of excitation states in GHG molecules, I began to wonder if what I wrote was correct and how well I really understood the physics. By now I’m questioning the wisdom of venturing into the submicroscopic realm but, at least for my own edification, I’d like to understand the details of what really happens. Thus far I’m frustrated by easily understandable descriptions of what happens in the quantum mechanics constrained absorption of a photon while similarly detailed explanations of the other end of the process seem elusive. At the risk of sounding impertinent, any information given without a reference is taken with a grain of salt.

    I’m still not buying the transfer of energy residing in the vibration of chemical bonds to the kinetic energy associated with molecular collisions. From the 1971 NASA paper on thermal radiation heat transfer, Chapter 4, “Microscopic Basis for Gas Properties” we find, “Neglecting scattering, the gains or losses are due to spontaneous emission, induced emission, and absorption.” And, “…the discrete transitions result in absorption of photons of only very definite frequencies… Hence this process is termed line absorption. Because both the initial and final states of the atom or molecule are discrete bound states, these energy changes between states are called bound-bound transitions.”

    I’m also unsure about the statement that photon emission is a rare event. It accounts for 333 W/sq meter of energy absorbed by the surface of the Earth compared to 160 W/sq meter absorbed by the surface from incident solar radiation.

    At the macroscopic level, I recently found, “Overall, considering both solar and infrared radiation, the atmosphere is radiatively cooled. The radiative cooling is balanced by the latent heat released when the water evaporated from the ocean recondenses to form clouds.” And, “Atmospheric processes convert a small portion of the thermodynamic energy into the kinetic energy of atmospheric motion.” David Randall, Princeton Primers in Climate, Atmosphere, Clouds, and Climate, p 25.

    From the same source, pp 15-16, “Roughly speaking, the atmosphere is ‘heated’ by contact with the boundary; the heat enters directly into the base of the troposphere. In response, the troposphere churns like a pot of water on a stove, as buoyant chunks of air break away from the lower boundary and float upward, carrying energy (and other things) with them. The upper-level air is cooled by emitting infrared radiation to space.”

    I’m waiting for delivery of another volume in the Princeton Primer Series but, so far in my mind, the picture that’s emerging is that the greenhouse effect is due to the back radiation of energy that raises the temperature of the Earth’s surface. The air is warmed by physical processes (convection and evapotranspiration). Global warming increases the back radiation from higher concentrations of GHGs.

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  23. dkeierleber @22,
    (Sorry that this is a bit backwards in replying to your comment from the bottom paragraph up.)
    I would agree with your final paragraph except to add that it concerns "the" greenhouse effect, thus it is talking about the Earth. Thus the major "physical processes" warming "the air" are Surface Radiation (396), Latent heat from Evapo-Transpiration (80), Incoming Solar Radiation (78) and Thermals ie heated by conduction fron the surface rather than "convection" which is how it is transmitted through the air (17).

    So in this context, the quote in the second-last paragraph from pp15-16 of Randall (2012) is saying that the vast majority (86%) of the heating experienced by the atmosphere originates from the surface. Most of this (70%) is due to radiation absorbed by GHGs.

    The p25 quote in your 6th paragraph is only making explicit that the largest non-radiative part of atmospheric warming/cooling is evaporation which thus provides a balance (14%) to the net radiative flux. (The presence of the Trenberth Global Energy Flows diagram on p24 makes plain this meaning.)
    trenberth energy balance

    The second quote from p25 is trying to say that there will be small amounts of this energy-transferred-that-result-in-a-local-heating-of-th-air which is then transfered away as warmth within air movements, its movement thus cooling some locations (that lose warmed air) and heat other locations (that recieve warmed air), "like the heating, ventilating and air conditioning system in a building." The point made is that such processes are small relative to the radiative fluxes.

    Moving up to your 5th pargraph, the numbers you present are seen in the Trenberth diagram. But when @14 I called the absorption/emission of photons "rare," this was in slightly different terms, not energy as in Trenberth but as number-of-events. Consider that the energy held by a photon is inversely proportional to its wavelength. Thus a photon from the sun will be 30-time more energetic than a photon from the atmosphere. So the ratio of photons will not be 333/160 = 2 but 333*30/160 = 60.
    And it was this sort of number I am describing when I say "the emitting/absorption of photons are a rare events and most absorbed photons will become added to the energy in the gas rather than being immediately emitted as another photon." This is because the average time taken to re-emit a photon is very long relative to the time a CO2 molecule has before it collides with another air molecule. We are talking parts of a seconds to emit a photon from a vibrating CO2 molecule at 15 microns (see Blauer et al (1973) p48) relative to parts of a microsecond between collsions (which can be more readily calculated).

    In your 4th paragraph, your reasons for "not buying" do not appear sensible. The first quote you make from p125 Siegel & Howell (1971) concerns photon absorption/emission and so do not concern the transfer of vibrational energy to/from other molecules. The three modes of gain/loss listed are 'spontaneous' (a photon emitted from an excited state) 'induced' (ditto but caused by a passing photon) and 'absorbed' (the capture of a photon causing an excited state or as on p128, "The absorption of a photon can cause a transition of some state of the atom or molecule to a state of higher energy.") The second quote in paragraph 4 is stating that only descrete wavelengths can excite a molecule, "bound-bound" meaning that the energy involved is not enough to rip electrons from the molecule.

    In paragraphs 2 & 3, I see descriptions of doubt rather than reasons for doubt.

    The first paragraph considers the GH-effect of Venus and is probaly best answered by this SKS post.

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  24. Missing link within #23.

    (which can be more readily calculated)

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  25. How is it that CO2 has such a dominant effect athough its concentration in the atmosphere is so small? Why doesn't the low-energy radiation from the earth's surface simply escape through the atmosphere (N2, O2) into space?

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    Moderator Response:

    [PS] You might also like to look at the "CO2 is just a trace gas" myth for more understanding.

  26. greyowl @25,

    To answer your second question first, while the atmosphere consists of gases that are transparent for the IR radiation emitted by the earth (the dry atmosphere comprises 78.08% N2, 20.95% O2 & 0.93% Ar), the important consideration is how far an IR photon can travel before encountering a GHG that will absorb it. Bascally, air has masses of molecules and even at 400ppm (or 0.04%), a photon in the 15 micron waveband will encounter a CO2 and be absorbed in a matter of a few metres. It thus has 12,000m of atmospheric depth to negotiate in some very short steps.

    And the first of your questions, the importance of CO2 is two-fold. Firstly it is a very long lived GHG and will persist in the atmosphere for tens of thousands of years. CO2 differs from H2O in this respect which is quickly lost from the atmosphere if the temperature drops. Secondly, CO2 is by far the most abundant long-lived GHG. It provides roughly 20% of our planet's GH effect. Much of the rest of the GH effect results from H2O in its various atmospheric forms but the level of H2O is dependent on that 20% CO2 GH effect. With no CO2 there would be trivial levels of H2O.

    Perahps I should add that these are direct answers to your two questions and may not hit the nail-you-were-seeking entirely squarely on the head.

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  27. Thanks very much, Rodger, your explanation exactly answers myquestions.

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  28. Sorry if this question is too simple.

    Am I correct in thinking that the primary mechanism of CO2 heating the atmosphere is

    - infrared photon strikes a CO2 molecule

    - CO2 molecule moves faster as a result, transferring energy to other molecules in the atmosphere

    and the secondary mechanism is

    - infrared photon strikes a CO2 molecule

    - at some later time, CO2 emits an infrared photon

    - if that photon happens to go downwards, it will either strike another CO2 molecule, or strike the earth's surface


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  29. nigelk @28,

    Your description seems pretty close to complete.

    The UCAR Center for Science Education explanation of "Carbon Dioxide Absorbs and Re-emits Infrared Radiation" is more complete, including the mention of the "Vibation" effect Climate Adam included in his video.

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  30. nigelk @28,

    In addition to a photon from a higher energy CO2 molecule going in any direction, reducing how much energy 'Goes up and out', the increased movement of CO2 molecules transfers movement energy to other molecules in all directions, so that is also energy that does not continue to 'Go Up and Out'.

    Less energy 'going directly up and out' means it will be warmer inside of the stuff that intercepts the energy on its way 'up and out' if there is more of that stuff.

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