FAQ #271: If Greenhouse Gases are such a Small Part of the Atmosphere, How Do They Change Its Temperature?

June 17th, 2010 by Roy W. Spencer, Ph. D.

NOTE posted 9:20 a.m. CDT 21 June, 2010: Upon reading the comments here, its obvious some have misinterpreted what I am discussing here. It’s NOT why greenhouse gases act to warm the lower atmosphere, it’s why a given parcel of air containing a very small fraction of greenhouse gases can be thoroughly warmed (or cooled, if in the upper atmosphere) by them.

Some of the questions I receive from the public tend to show up repeatedly. One of those more common questions I receive arrived once again yesterday, from a airplane pilot, who asked “If greenhouse gases are such a small proportion of the atmosphere,” (only 39 out of every 100,000 molecules are CO2), “how can they heat or cool all the rest of the air?”

The answer comes from the “kinetic theory of gases”. In effect, each CO2 molecule is a tiny heater (or air conditioner) depending on whether it is absorbing more infrared photons than it is emitting, or vice versa.

When the radiatively active molecules in the atmosphere — mainly water vapor, CO2, and methane — are heated by infrared radiation, even though they are a very small fraction of the total, they are moving very fast and do not have to travel very far before they collide with other molecules of air…that’s when they transfer part of their thermal energy to another molecule. That transfer is in the form of momentum from the molecule’s mass and its speed.

That molecule then bumps into others, those bump into still more, and on and on ad infinitum.

To give some idea of how fast all this happens, consider:

1) there are 26,900,000,000,000,000,000,000,000 molecules in 1 cubic meter of air at sea level.

2) at room temperature, each molecule is traveling at a very high speed, averaging 1,000 mph for heavier molecules like nitrogen, over 3,000 mph for the lightest molecule, hydrogen, etc.

3) the average distance a molecule travels before hitting another molecule (called the “mean free path”) is only 0.000067 of a millimeter

So, there are so many molecules traveling so fast, and so close to one another, that the radiatively active molecules almost instantly transfer any extra thermal energy (their velocity is proportional to the square root of their temperature) to other molecules. Or, if they happen to be cooling the air, the absorb extra momentum from the other air molecules.

From the above numbers we can compute that a single nitrogen molecule (air is mostly nitrogen) undergoes over 7 billion collisions every second.

All of this happens on extremely small scales, with gazillions of the radiatively active molecules scattered through a very small volume of air.

It is rather amazing that these relatively few “greenhouse” gases are largely responsible for the temperature structure of the atmosphere. Without them, the atmosphere would have no way of losing the heat energy that it gains from the Earth’s surface in response to solar heating.

Such an atmosphere would eventually become the same temperature throughout its depth, called an “isothermal” atmosphere. All vertical air motions would stop in such an atmosphere, which means there would be no weather either.

Now, I will have to endure the rash of e-mails I always get from those who do not believe that greenhouse gases do all of this. But that issue will have to be the subject of a later FAQ.

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39 Responses to “FAQ #271: If Greenhouse Gases are such a Small Part of the Atmosphere, How Do They Change Its Temperature?”

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  1. benpal says:

    “Without them, the atmosphere would have no way of losing the heat energy that it gains from the Earth’s surface in response to solar heating.”

    Care to explain this? Wouldn’t the atmosphere rather lose (i.e. not take up) heat energy if there were no greehouse gases?

  2. Massimo says:

    Dear Dr. Spencer,
    it’s always a pleasure to read your explanations about climate behaviours.
    I agree with your analysis, but I’m not sure about the static vertical atmosphere you suggest in case of missing GHGs. Without GHGs the Earth should still heat the atmosphere by conduction, and (I guess) convection because the gravitational field is still there. An hotter molecula rise over a colder one, but it still shares its energy by bouching against the neighbouring ones. When the molecula reaches the TOA it is still affected by the gravity which attracts back it to the planet. So, when the planet get colder (for example by night) the higher molecules still shares their energy by bouncing the underlyng ones which are colder, cooling themselves.
    What I’m supposing here, is that in case of a missing GHGs atmosphere, the convectional flux could increase during the night/day transitions because of the missing radiative “short-circuit” made-up by the GHGs.
    Maybe I’m wrong, but in few words what I’m guessing is that GHGs could just change the exit point of the energy, not the total amount of energy in atmosphere.


  3. toby says:

    Dear Dr Spencer,

    Thank you for this explanation. I am trying to understand the dynamics of global warming, and this is very helpful.

    When a molecule of CO2 absorbs a photon of radiation, I presume the energy is transformed into vibrational and linear motion. Do we understand how much of each? Is vibrational energy alone enough to bring about the collisons you discuss?

  4. Massimo says:

    Uhmmm… I wrote a stupid thing in my previous message: is not the convectional flux that “could increase during the night/day transitions because of the missing radiative “short-circuit” made-up by the GHGs” (quoted myself) but the heat conduction.

  5. Baa Humbug says:

    The only way the planet loses it’s energy to space is via radiation provided by GHG’s.
    If there were no GHG’s, the rest of the atmosphere would still warm via conduction etc but couldn’t lose this gained warmth without radiation. Hence isothermal atmosphere.

    • Anonymous says:

      So the ground will never be heat by the hotter atmosphere by night?

    • Anonymous says:

      Hoops! I missed my name.
      What I mean is that without GHGs the “gained warmth” should still exit the atmosphere via the planet surface by conduction.

  6. Massimo says:

    Still missed my name (I put it into the “Name” field, but continued to mark “anonymous”).


  7. Baa Humbug says:

    Yes, without GHG’s the gained warmth cannot be radiated out to space. (remembering that some will always be radiated directly from the ground to space via the radiation window)
    If the gained warmth cannot radiate out to space, the remaining air will mix this warmth until there is a uniform warmth (isothermal atmosphere)

    But please, I’m a learner. That’s why I’m here. I’m sure Dr Spencer will set us straight.

    • Anonymous says:

      By night I can’t imagine the cold ground surface in contact with the hot atmosphere without any thermodynamic exchange. Can you imagine a system made of cold solid in contact with an hot gas which doesn’t reach a thermal equilibrium in the long run? I can’t. I’m a rookie here, but if you fill a cold bottle with oxygen (which is not a GHG) you finally get the oxygen and the bottle at the same temperature of the neighbouring environment. You can argue that the bottle constraints the volume of the gas and for this reason this one releases its energy to the container, but it’ my opininon that in the atmosphere/planet surface system the gravitational force does the same.

    • Anonymous says:

      You are missing one huge point, if the gases in the atmosphere cannot radiate at all (never really happens) then the radiation “window” is huge, in fact total. All radidation from the ground could then radiate directly to space without any retardation. Remember, an object or molecule that cannot radiate cannot absorb either. Your “window” would not exist for all radiation of any kind would pass directly to space always, 100%, see?

  8. Anonymous says:

    If there were no GHGs, there would be no weather. The temperature of the planet would depend on its albedo, with an oscillating regional temperature change as night turns into day. At equilibrium temperature, I doubt much convection would occur, as there would be no water vapour and not much wind.

    The planet would get very cold, if we assume that the oceans somehow remain without atmospheric turnover of H2O. Ice would accumulate at the surface, increasing albedo. With no atmosphere at all, the planet would be below freezing at night time, and a very hot, albedo-determined temperature at day time, like the moon.

    Or so it seems to this layman.

  9. Alan D McIntire says:

    Faster moving molecules would continue to move higher in the atmosphere than slower moving molecules, and gravity would continue to slow the molecules down. Molecules higher in the atmosphere moving fast enough would escape into space, again lowering the average speed of the molecules remaining in the higher atmosphere, so I think we would still have some kind of temperature lapse rate caused by gravity and by conduction from lower in the atmosphere. Of course, we’d have large temperature inversions every night as the earth radiated heat away faster than it could pick up heat by conduction from the atmosphere.

    I remember from my physics classes 40 years ago, that for
    ideal gases rising adiabetically, PV^ gamma is a constant.
    For a non-greenhouse atmosphere I’d think that gamma would be about 1.67 as it is with argon, or 1.4 as it is with Oxygen Or Nitrogen, rather than the 1.33 value for more complex gases. That would imply that the dropoff in pressure with altitude would be slightly greater in a non-greenhouse atmosphere.

    • Anonymous says:

      I agree with Alan D McIntire.
      I tried two times to reply to Baa Humbug, but I don’t know why my post has been lost.

  10. coturnix19 says:

    And yet, without greenhouse effect, earth would have been so cold. No GHG atmosphere would not only not radiate, but also not absorb – essentially, we’d have surface warm up as if there weren’t any atmosphere, and atmosphere would follow-up after it because of conduction. But would there still be meridional circulation? I think – yes, as air would warm from surface at equator and cool at poles – again, through conduction. Am i wrong?

    • Anonymous says:

      I’m not a scientist but your argument sounds right to me. What I would like to know is if anybody has estimated whether or not the thermal conduction of non GHGs is reduced when they are mixed to GHGs as in our current atmosphere.

    • Anonymous says:

      I think the answer is that dry air is an extremely poor conductor of heat.

  11. Allan Kiik says:

    “It is rather amazing that these relatively few “greenhouse” gases are largely responsible for the temperature structure of the atmosphere. Without them, the atmosphere would have no way of losing the heat energy that it gains from the Earth’s surface in response to solar heating.”

    This is amazing only when we are considering ideal gas, which can’t lose energy in collisions, because no accelerating charges = no photons. But real gases have electrons and they can be ionized. So, if atmosphere is made of non-GHG gases which are not ideal, there must still be some radiation from it and most probably it looks very similar to flat Planck curve according to gas mixture temperature at radiation height. When there are GHG-s in the mixture, their characteristic peaks will be on top of it.

    Please take a look at this image:

    The best one is “b” – thermal emission spectrum from mediterranean area, recorded by Nimbus 4 spacecraft. We know how good absorber is CO2 at 15um, total extinction height is about few hundred meters from surface, after that there’s no radiation from ground left. But there still is some radiation visible in this band, and it follows nicely ~220K Planck curve. CO2 can’t do this, actually we can see here CO2-s characteristic peak too, that’s what it does.

    I guess, this is one good piece of evidence against ideal gas approximation – it does not work this way in real Earth atmosphere.

    • Anonymous says:

      I did some games with the on-line Dr.Archer’s MODTRAN application some times ago, and the simulator shows how that peak rise just above the end of tropopause. I guessed that under that point the CO2 absorbed energy was shared with the WV which spreaded it all along its continuum (in that case the WV/CO2 ratio should define how much of it was re-emitted at 15um), while above that point the residual energy had just the CO2 as emission gate to the open space, for this reasone the peak grows up there. If that hypothesis was right, maybe that the spectrometer amplitude resolution is not sufficient to measure the change of few W/m2 due to the CO2 doubling if it is diluted on the VW spectrum in the troposphere.
      Mybe it was a silly conjecture, but that time I didn’t find any alternative explanation.

  12. […] FAQ #271: If Greenhouse Gases are such a Small Part of the Atmosphere, How Do They Change Its Temper… […]

  13. Bomber_the_Cat says:

    Haven’t you got this one wrong Roy?
    What you are describing here is a thermal interaction. This is not what greenhouse gas theory is about. It is not a question of heating the atmosphere, it is a question of directly heating the surface of the Earth by ‘back radiation. CO2 molecules absorb photons leaving the Earth’s surface (within narrow infra-red bands) and then subsequently re-emit the photons in those same bands, some of which will go back to Earth and re-heat it. It is true that a small fraction of CO2 molecules will collide with other gas molecules before they re-emit and consequently pass on some energy; such molecules are said to be ‘thermalised’ and they no longer have enough energy to re-emit the photon that they absorbed. However, the thermalised molecules cannot keep passing their energy on to every other molecule that they collide with, in some sort of chain reaction as you seem to describe. The principles of Conservation of Energy and Momentum limit any increase to the energy of the original photon.

  14. Paolo M. says:

    Bomber, you had it half right.
    GHGs absorb and re-emit radiation (H20 mainly), but the only way to heat the ocean (i.e. the Earth surface) is by means of the short wave radiation.
    Putting aside some losses for a moment (conduction and evaporation), the GHGs only reduce the speed at which energy goes back to space via IR radiation, so that Sun energy can accumulate.
    The Sun is the only heater, GHGs influence the restitution rate.

  15. Leonard Weinstein says:

    You are so right on many subjects, but I think you oversimplified it here. The Earth’s atmosphere and energy balance are particularly complex, but the main transfer of heat from the Sun warmed surface is water evaporation and thermally driven convection up through the lower troposphere (and condensation of the water vapor at modest altitudes followed by rain), with some smaller amount of direct longwave radiation also passing straight through. In the upper atmosphere your argument is more valid. It is true that the radiation absorption and re-radiation, and thermal transfer of energy from heated molecules to surroundings enter in the mix, but they are not dominate sources of the heat transfer near the surface. It is the moving of the level of re-radiation to space from a high level in the atmosphere, rather than all re-radiation from the surface, along with the convective driven lapse rate that cause the greenhouse effect.

    • the “convectively driven” temperature lapse rate of the troposphere would not exist without the “greenhouse effect”. The radiatively active gases DE-stabilize the atmosphere (all GHGs tend to warm the lower part of the atmosphere, cool the upper part, thus a destabilization). Convection kicks in when the lapse rate (combined with the vertical humidity distribution) are sufficient to force convective ascent. Again, convection would not occur without a greenhouse effect. The atmosphere would become isothermal, and thus very convectively stable.

  16. wayne says:

    Paola M., just want to reaffirm and agree with your statement, “the GHGs only reduce the speed at which energy goes back to space via IR radiation.” That is exactly correct.

    And Roy, does not O2 and N2 radiate in some of the far infrared and microwave bands, albeit weakly? I’m surprised you didn’t mention that factor, to be exact and complete, instead of a blanket statement that the atmosphere without GHGs had NO way to cool.

  17. John Shotsky says:

    Interesting variety of opinions, if not all according to the thermal laws and gas laws.

    Here is the simplest explanation:
    There is a continuous input of radiated energy to earth by the sun. There is a continuous exit of energy from earth by surface and atmospheric radiation. These two flows are equal, or else the earth would warm or cool.

    If we remove greenhouse gases (including water vapor) from the atmosphere, all outgoing radiation would be from the surface. (but see note below) The thermal laws state that the amount of radiation from any matter is proportional to its absolute temperature.

    If there were no greenhouse gases, the surface would have to radiate more. In order for that to happen, it would have to warm. Earth’s surface and atmoshere would be warmer than they currently are. At some point, earth would warm enough to balance the incoming energy at night. That would become the new global average temperature.

    I’ve seen calculations that attribute about 27C to this phenomena. Imagine if the earth’s average temperature were 27C (about 50 degrees F) warmer.

    Note, all gases radiate. However, passive gases radiate at discrete frequencies rather than broadband, so the amount of radiation is miniscule, and can be safely ignored. If you’ve ever seen an IR photo of a lightning strike just AFTER the visible light disappears, you wlll see that ALL the gases are radiating. That is due to superheating them. As stated earlier, everything radiates at a rate proportional to its absolute temperature – even inert gases.


  18. 1DandyTroll says:

    Even if we disregard that most GHGs are pressed down to ground level thereby reducing the amount of heat they could shovel along out to space by themselves, removing the water from the atmosphere would essentially kill the planet anyway since it’d mean all the water molecules on the planet would have to go. So essentially all the oxygen gone as well, and we’d end up with pretty much just nitrogen, since the last dying phytoplankton would’ve used up the last CO2.

    But of course being the optimist that I am, I, of course should point out that nitrogen seem like a likely candidate for keeping the climate real, what with that what it lacks at ground level it makes up for higher up in the atmosphere, well, apparently anyway. Would that mean nitrogen would finally become one of them harmful GHG?

  19. Kevin says:

    Dear Dr. Spencer,

    WOW, boy did you oversimplify this topic. With all due respect I have a few clarifications to add. These are from the perspective of an engineer with many decades of “heat transfer” experience.

    You wrote:

    “The answer comes from the “kinetic theory of gases”. In effect, each CO2 molecule is a tiny heater (or air conditioner) depending on whether it is absorbing more infrared photons than it is emitting, or vice versa.”

    The classical engineering definition of a “heater” is a device that converts non-chemical energy (i.e. electric current, steam, hot water, etc.) to other forms of thermal energy (usually hot air, or a hot liquid). The generic term “heater” represents an item THAT DOES NOT CREATE “EXTRA” ENERGY. A “furnace” or “burner” (sometimes a portion of a boiler) does CREATE THERMAL ENERGY by breaking the chemical bonds stored in fuels. But a gas that absorbs infrared radiation and converts it to thermal energy, while possibly called a “heater” DOES NOT CREATE “EXTRA” ENERGY.

    Sorry, but the “air conditioner” analogy does not work. An air conditioner transfers hot air from one volume (usually an interior space) to another volume (usually an exterior space), usually by consuming quite a bit of electrical energy. In NO WAY DOES AN AIR CONDITIONER ABSORB “EXTRA” ENERGY, IT SIMPLY MOVES IT TO A LOCATION THAT IS LESS ANNOYING (i.e. the HOT condenser outside your residence).

    Also, if a molecule is absorbing more photons than it is emitting, it will warm, but this is only a statement of conditions at ONE INSTANT in time, it does not represent a permanent gain in thermal energy. BTW this also holds for molecules heated by conduction or convection although the term ”emitted” is replaced by the more generic term “transferred”.

    You wrote:

    “2) at room temperature, each molecule is traveling at a very high speed, averaging 1,000 mph for heavier molecules like nitrogen, over 3,000 mph for the lightest molecule, hydrogen, etc.”

    Respectfully, I believe a more accurate statement might be:

    “2) at room temperature, each molecule is vibrating in place at a very high speed, averaging 1,000 mph for heavier molecules like nitrogen, over 3,000 mph for the lightest molecule, hydrogen, etc.”

    There is no linear motion imparted to any molecule when heated, for a simple example; does your teakettle zoom off of your stove at 1,000 mph when you make your tea in the morning ?

    You wrote:

    “It is rather amazing that these relatively few “greenhouse” gases are largely responsible for the temperature structure of the atmosphere. Without them, the atmosphere would have no way of losing the heat energy that it gains from the Earth’s surface in response to solar heating.”

    With all due respect this is patently absurd; you seem to be arguing that GHGs are the only way that the atmosphere of the Earth can cool and that GHGs are the only way the Earth can warm. This flies in the face of centuries of thermodynamic knowledge (theoretical and empirical) that clearly shows that thermal energy ALWAYS finds a way to move from warmer locations to colder locations via conduction, convection and radiation in various proportions as determined by the physical behavior of real materials.

    Without quoting all of the other comments I will just state this:

    YOU CANNOT CREATE “EXTRA” ENERGY BY REDIRECTING THE FLOW OF ENERGY VIA THE ABSORPTION AND REMISSION OF ELECTROMAGNETIC ENERGY (a.k.a. “The Greenhouse Effect”). The behavior of GHGs only serves to SLOW the flow of energy (thermal or electromagnetic). The only meaningful question is if this slowing of energy is significant with respect to the daily cycle of sunrises and sunsets. Note that the time for a photon of infrared radiation to transit from the surface of the Earth to the upper atmosphere is about 100 microseconds. Ask yourself how many microseconds are in the typical day (24 hours) ?

    Just as an aside, for several decades the “consensus” was that stomach ulcers where caused by “stress” and “spicy foods”. The person that postulated that a virus (or perhaps a bacterium, I’m an engineer remember) was RIDICULED for his belief. Turns out he was correct!

    Dr. Spencer, I do appreciate the fact that you have resisted the “herd” mentality and perhaps prevented all of us from running off a cliff. But I do believe that climate science still has some maturing to do. Thanks for your efforts to increase our knowledge of the very complex system known as the climate of the Earth.

    Cheers, Kevin.

    • Kevin:
      1) IN EFFECT, each CO2 molecule is a tiny heater (or air conditioner)…
      2) you obviously have not read much of what I have written…(previously)

      • Anonymous says:

        Dr. Spencer, with Respect,

        I have read most but not all of your previous writings on this subject. I appreciate your efforts and contributions in the field of climate science. I mean no disrespect, but you really need to unambiguously define what you mean by a “heater” and a “air conditioner” before you postulate the possible effects of these elements on the climate.

        I’m sorry, but a “heater” (as understood by the most generally accepted terms) does not create “extra” heat/energy, and an “air conditioner” (as understood by the most generally accepted terms) does not make “extra” heat/energy disappear.

        I’m sorry, but “IN EFFECT, each CO2 molecule is a tiny heater (or air conditioner)…” needs additional definition of terms before it is deserving of further consideration.

        Note, I do not find it necessary to mention my education and practical engineering history to buttress my argument. No disrespect to your credentials, a PhD clearly demonstrates significant accomplishments. However, the truth is the truth, and a title does not override this, and no, I am not suggesting in any way that you are distorting the truth, but perhaps you have been captured by a “paradigm” inherent to the climate sciences. Note, I do dislike the word “paradigm”, my best quote for this is “I burned out my clutch once on a paradigm shift”, probably lost on the younger generation that does not known what a clutch is.

        For reference, you started this discussion by stating that
        effectively “greenhouse gases are the only way that the Earth “warms” and also “cools”. I still find this an amazing conclusion which runs contrary to all of my empirical observations about what “heat” actually does in the real world.

        I’ll refrain from future comments……..

        Cheers, Kevin

  20. Deuce says:

    CO2 has a greater heat capacity than the O, N and H that make up most of our atmosphere. This means it takes a greater gain or loss of heat to change its temperature by one degree. This means the more CO2 you put in the air, the more heat is stored in it at any temperature than can be stored by just a mixture of N, O and H. However the flip side of this is that the more CO2 is in the air, the more heat it takes to raise the temperature. This is where the alarmists are disingenuous. The same property of CO2 that causes warming, can also create cooling with exactly equal efficiency.

    Your demonstrated lack of understanding of the difference between heat and temperature is common. It doesn’t make you ignorant or anything, its just that you, like most, are not a physicist. However those pushing the agenda know this and thus have latched onto temperature as their weapon of mass confusion. Most people understand temperature, but not everything that’s behind it. They say CO2 causes more warming then show you some graphs of increasing temperatures to “prove” it. And they ALWAYS leave out the fact that CO2 causes equally more cooling. A fact proven by you having to ask this question. Temperatures fluctuate by natural variations of many independent variables in a very very complex system. The amount of CO2 in the air can only sharpen the slope of the temperature changes, that is make both warming and cooling happen faster or slower. But given that heat capacity functions in two directions, increased CO2 ultimately can have no long term net effect.

    That said, I’m not sure what Dr. Roy is going for here. All he describes is the mechanism by which ALL molecules transfer heat. Yes it is part of how our atmosphere stores heat, but it also means that ALL gases are greenhouse gases. They all store heat. Invoking term greenhouse gases doesn’t imply any new mechanism at work, just different heat capacities, a trait common to all chemicals and compounds anyway.

  21. Cold Lynx says:

    The GHG theory is that reduced radiative heat transfer from earth to space in the atmosphere will increase average temperature in the atmosphere.

    The problem with this theory is that in reality will any increase of a well mixed GHG broaden the spectral band. The result is precisely as Dr Spencer say increase outgoing radiation.
    This because the atmosphere is heated by convection, shortwave radiation, latent heat, conduction and, but with very small amount, “captured” outgoing infrared radiation. But cooled nearly completely by infrared radiation.
    With a cooling/heating factor in the range of about 10 are the atmosphere a more efficient radiant cooler than heater.
    The AGW promoters tend to forget that the atmosphere is not so much heated by IR radiation as it is cooled by IR.
    Radiative balance between slabs of the atmosphere does not exist since heat is lifted by mainly convection and latent heat.
    The needed temperature difference for driving radiative heat transfer in any spectral band for a well mixed gas is also small inside the atmosphere due to the lapse rate but large towards cold space.
    Water vapor and especially clouds is not well mixed in the atmosphere and have seen from space as well as from earth a surface with a wide spectral band. That cause green house effects or cooling due to boundary conditions both for incoming as well as for outgoing radiation.
    But that is a completely different thing than for a non condensation well mixed gas. As CO2.

  22. Kevin says:

    With respect, Dr. Spencer,

    I did not mean to imply that the cooling mechanism you identified, specifically the transfer of heat from warmer “non-GHGs” to colder “GHGs” followed by the radiation of infrared radiation to space does not exist. It clearly does exist. It may in fact be the primary cooling mechanism for gases in the atmosphere. It may in fact be the most efficient (i.e. the fastest) mechanism. However, I dispute the proposal that it is the ONLY cooling mechanism. As one additional cooling mechanism I might suggest that falling sleet in the upper atmosphere absorbs heat from all the gases (thereby cooling the gases) and thereafter warms, melts and falls as rain. I’m sure there are other examples as well.

    Regarding the thermal capacity of a material, this is only one determining factor in the rate (i.e. speed) of heat flow. Let’s examine a hypothetical example of two materials in contact at the exact same temperature, Unobtainium and Administratium. Unobtainium has a thermal capacity near infinity; Administratium has a thermal capacity near zero. In the proposed example, no heat flows between the two since there is no temperature difference. Now, some believe that as soon as the most infinitesimal temperature difference (Administratium warmer than Unobtainium in this example) occurs the material with the higher thermal capacity (Unobtainium in our example) will immediately absorb all of the heat from the Administratium, thereby cooling it to the new lower temperature. Well, at first blush this is sort of correct, however, there is another very important factor involved; the thermal conductivity of the materials. A material with near infinite thermal capacity and near zero thermal conductivity is not very effective at cooling or heating another material. Likewise, a material with very high thermal conductivity and a very low thermal capacity is not very effective at cooling or heating another material.

    This is of course described by a very important characteristic of real materials; the thermal diffusivity. This considers both the thermal capacity and the thermal conductivity of a material. The thermal diffusivity of a material is literally the “Speed of Heat” in a material and has units of area/time (i.e. m^2 /sec). I postulate that the increase of “GHGs” in the atmosphere actually increases the “Speed of Heat” in the atmosphere (since more heat/energy is transferred at the “Speed of Light” rather than at the “Speed of Heat”). Therefore, a permanent warming of the Earth due to “GHGs” is impossible.

    Note that when Engineers want to move heat quickly (i.e. cool a microprocessor or laser diode) the materials of choice are Diamond (costly) and Copper (more practical) precisely because they exhibit some of the highest “Speed of Heat” characteristics available. Aluminum is an economical compromise used in most devices. Aluminum heat sinks and fans in computers act together to; quickly move the thermal energy to a large surface area so that the heat transfer via convection from the moving air is more effective. In fact many plastics have thermal capacities close to copper, but the thermal conductivity is “awful” so their usefulness in transferring heat is limited.

    So if you are a Heretic in the climate science world, I must be an anti-Christ. (No offense intended against anyone’s religious beliefs).

    Cheers, Kevin.

  23. Kevin says:

    Just one more observation, Diamond (synthetic) has in fact been used as a “heat sink” in a few limited applications for laser diodes. Synthetic Diamond has many useful applications and is not an unobtainable material. (Disclosure, I do not work for or own stock in any synthetic diamond companies)

    It turns out that the fastest heat transfer happens when all those atoms are dancing together to exactly the same tune (kind of like a little molecular conga line). This is what happens in a diamond, and to a lesser extent in a metal like copper.

    So, historically speaking I am on the side of the person that postulated that stomach ulcers where not caused by “stress” and “spicy foods”, but believed that the “consensus” was wrong. I believe that increases of “greenhouse gases” actually increase the “Speed of Heat” in the atmosphere of the Earth and probably have such a small effect that we could not possibly spend enough money to actually measure it with any believable accuracy.

    But, hey, I’m just a crazy engineer like Marconi, the Wrights, Edison, and others, but still hoping to someday make an impact anywhere near the impact they made.

    Dr. Spencer, I do appreciate your work and respect your candor and attention to detail. I do hope that you are able to make additional contributions to our understanding of the still complex system known as the Climate of the Earth.

    Cheers, Kevin.

  24. Kevin says:

    Ok, one more observation about the “Speed of Heat” in real materials, it’s ironic, but it turns out that the material with the highest “Speed of Heat” does not control the final outcome! When you examine the final outcome of a heat transfer problem the material with the slowest “Speed of Heat” actually determines the outcome! In the engineering field this is known as the “Rate Limiting Step”.

    If we assemble a hypothetical system comprising a “heater” (electrical in this case) followed by a slab of Copper, followed by a slab of PVC, followed by a slab of Copper, it turns out that the SLOWEST heat transfer mechanism determines the final outcome! If you replace the Copper with Diamond in this system, the final outcome is STILL determined by the PVC in the system! And this when we know that the PVC has a relatively high ”Thermal Capacity”.

    I hate to say this but there has been way too much concentration on the effects caused by the materials with the highest alleged capacity to affect the outcome (i.e. “greenhouse gases”) it is also necessary to carefully observe the behavior of the “less active” materials.

    It’s ironic but when it comes to romantic relationships the person with the least interest in the relationship actually has the most power. Funny how human relationships mimic physical relationships…

    Dr. Spencer, I do think you might benefit from a short internship at some place that deals with heat on a daily basis, perhaps a power plant. Even though I have a pretty good educational background, I did in fact learn quite a bit of useful things on the “factory floor”. I think this could add valuable insight to your already impressive understanding of what is going on above our heads in the atmosphere.

    Cheers, Kevin.

  25. Nerdlas the Poet says:

    I have one (probably very stupid) question. I was kind of puzzled by the sentence “In effect, each CO2 molecule is a tiny heater (or air conditioner) depending on whether it is absorbing more infrared photons than it is emitting, or vice versa.”
    Whenever you read elementary stuff about the greenhouse effect, it says that a CO2 molecule can absorb a photon and then, at some quantumillionth of a second later, emit it. Or somehthing like that. That is, a single CO2 molecule can absorb a single IR photon and then shoot it back out again in a random direction, thus slowing the flow of IR photons out into space.
    But according to the qoute above, it would seem that a CO2 molecule can absorb many photons and later “choose” to emit only some of them, while keeping the others in their quantum backpack. Have I understood this correctly?

  26. […] Otherwise the earth would be 22C cooler on average: down around -8C or less, inimical to life. FAQ #271: If Greenhouse Gases are such a Small Part of the Atmosphere, How Do They Change Its Temper… Only deniers are obsessed with the statistics – if they want to disprove global warming, they need […]

  27. Jim Macdonald says:

    I believe that temperatues decrease with height because atmospheric pressure decreases with height. Gravity compressing the gas near the surface. I can’t conceive of temperatures being uniform throughout the depth of the atmosphere.
    I also believe that black body radiation would excape to space regardless of the presence of greenhouse gases.
    As Paolo wrote, greenhouse gases only slow down the radiation to space.

  28. Frank says:

    The atmosphere so intangible that it is difficult to explain why 0.04% carbon dioxide does anything important. I like to make the atmosphere more tangible by imagining it as a liquid. It is simple to calculate that the 14.7 psi of air most people accept turns into a layer of liquid 10 meters deep (density 1 g/cc). If we separate the various gases into layers, the CO2 layer is 0.4 cm thick. Automobile glass is only slightly thicker than this, and everyone has experienced what happens to a closed car left in the sun. People protect themselves from sunburn using a far thinner layer of PABA.

    It is also interest to imagine our liquid atmosphere evaporating. Low MW liquids expand about a thousand fold upon evaporating. So 10 m of liquid becomes 10 km of atmosphere. About 5 km in altitude the pressure is halved and the volume doubles. Go another 5 km and the same thing happens.