Why CO2 is not the cause of “Hothouse Venus”

Update 30/11: I have now updated the model to include all CO2 absorption bands. The new profile of absorbed radiation is shown  in Figure 4 below. The conclusion remains the same.

We have been led to believe that a “run-away” CO2 greenhouse effect is the cause of  very high  surface temperatures found on Venus, and that unless we curb carbon emissions  we risk a similar fate on Earth.  I am now convinced  that this narrative is complete nonsense because  CO2 absorption of IR radiation from the surface of Venus turns out to be tiny, and the reason for this lies with Wien’s displacement law.  Only  2% of the 740K Planck spectrum emitted by the surface actually falls within the main CO2 absorption band. While this is  indeed  absorbed within the first few meters,  the rest of the heat radiation (>90% of it) passes straight through.  I was myself surprised to discover this after  running my simple greenhouse model for Venus (described here).  I expected to see huge upward radiative transfer and  “back radiation” fluxes. Instead I found very small CO2 induced radiative transfer and almost zero back radiation. Essentially all CO2 sourced IR radiation is absorbed within each layer in the atmosphere, because the optical depth is small for the Venusian troposphere.  Wien’s displacement law then governs how much IR is absorbed by the atmosphere at each successive height above the surface.   To understand  this – just look at Figures 1 and 2.

CO2 is NOT the cause of the “greenhouse effect” on Venus !   Despite the fact that it forms 95% of the atmosphere, CO2  really is  an insignificant greenhouse gas on Venus !

Fig 1: CO2 absorption for different temperatures. The blue shading represents the main CO2 absorption band. The percentage of total SB radiation absorbed within the CO2 band is shown next to each temperature.

The top graph in figure 2. below shows  the energy fraction absorbed by the atmosphere at different heights above the Venusian surface. Note how it peaks from 45-55 km and then rapidly falls to just 3% by 61 km. The temperature at 50 km  turns out to be about the same as the Earth’s surface (288K) !

Fig 2: Fraction of SB energy flux (sigmaT^4) absorbed by CO2 in Venusian atmosphere,

The CO2 greenhouse model is based on 100m thick layers, each in local thermal equilibrium at the measured lapse rate. The absorption and emittance of IR by CO2 molecules is calculated using derived values of partial pressure and measured Beer-Lambert absorption data. For Venus Ts= 740K, lapse rate = 10.45K/km, CO2=0.95, P0 = 93 atmospheres, g= 8.9 m/s2. The code can be found here and the result it produces are shown in Figure 3.

Fig 3: Black curve- Radiation flux within CO2 band 13-17 microns in the Venusian atmosphere composed of 95% CO2. The dashed curve shows the surface emittance in this band for a temperature of 740K (no CO2 present). The small magenta curve is the “back radiation” from levels above. It is zero until 60 km up in the atmosphere, as the optical length is << 100m below.


I am well aware that I have ignored pressure broadening of the CO2 absorption band, and also excluded other smaller narrow absorption lines. However, I think this cannot change the overall conclusion that the CO2 greenhouse effect on Venus is between 1 and 2 orders of magnitude too small to explain the high surface temperatures.  So if CO2 is not responsible for the scorching temperatures on Venus, what is ?

It must be the other 5% of the atmosphere, which contains  SO2 (150ppm), H2O (20 ppm) and  the thick H2So4  clouds covering the planet.  The high overall density means that even the partial pressure of H2O is not dissimilar to that on Earth. It is this greenhouse mixture  covering a much broader IR spectra which traps the bulk of the IR emitted from the surface. CO2 is insignificant in comparison.

Furthermore there are  violent convective circulations on Venus and strong polar vortex winds. Convection maintains the lapse rate moving heat up through the atmosphere far more effectively than by radiative transfer. When finally the air  sufficiently above about  60 km the heat can then radiate freely to space.

Update 30/11: The model has been updated to include all 4 absorption bands for CO2. The 4.1-4.5 micron band is important for the lower Venus atmosphere as the Planck spectrum shifts to lower wavelengths . The new profile with height is shown in figure 4 and the relative fraction for each band versus height shown in Figure 5.

Fig 4: Radiation profile versus height on Venus for all CO2 absorption bands. The profile just for the 13-17 micron band is shown in blue.

Fig 5: Relative fractions of full SB radiation falling in each of the 4 CO2 absorption bands versus height on Venus. The 4.1-4.5 micron band becomes dominant at low altitudes.

The atmosphere on Venus is fully opaque below ~40km for absorption of IR by CO2. There is no back radiation to the surface since all re-emitted photons are absorbed immediately. Heat is transported upwards by convection until a level where the atmosphere thins sufficiently to radiate to space.


About Clive Best

PhD High Energy Physics Worked at CERN, Rutherford Lab, JET, JRC, OSVision
This entry was posted in AGW, Climate Change, climate science, Physics, Science and tagged , . Bookmark the permalink.

29 Responses to Why CO2 is not the cause of “Hothouse Venus”

  1. I agree with your conclusion. But I think you have under-estimated the absorption by CO2. First of all, take a look at the absorption spectrum of pure CO2 at 100 kPa and 300 K here:


    The plot gives the absorption length versus wavelength, where absorption length is the distance over which 63% will be absorbed. At 5 um the absorption length is only a few meters. On the surface of Venus the pressure is around 9,300 kPa and 740 K. According to Wikipedia, the atmosphere of Venus is 96.5% CO2, so almost pure. At the surface the CO2 density will be around 40 times greater than that used to obtain the above plot. Thus in the lowest 100 m layer in your simulation, the CO2 will be opaque for all wavelengths except 8-11 um, and even in that band it might be opaque, it’s just that the data I plotted was unable to measure an absorption length longer than 30 m at 100 kPa.

    Furthermore, given that the CO2 emits only at wavelengths it absorbs, this blanket of CO2 will be effective at containing the heat it absorbs. You already know that, of course, because your simulation code implements radiative symmetry.

    We have already discussed the origin of the lapse rate on Venus. I claim convection alone is sufficient to set it up, and you suggest that the planet-wide circulations make it a certainty. Either way, the lapse rate dictates the temperature on the surface. Here is my calculation using the adiabatic expansion relation, if you’re interested.


    So, I claim that even if you were to assume CO2 is opaque at all wavelengths, you’d still get pretty much the same surface temperature, assuming you implemented the lapse rate with an adiabatic expansion relation. Which is why I agree with your conclusion.

    Also: liked your modified greenhouse simulation for Earth. It is heartening to see the 1.5 C effect being arrived at by independent investigators. Not that this makes it a fact, of course. We would have to experiment on a planet or two. But it’s still heartening.

  2. Don Simpson says:


    Sorry this is not on topic however I have read the following article (http://www.cbc.ca/news/yourcommunity/2012/12/un-climate-change-projections-made-in-1990-coming-true.html)

    Which essentially states that a study has been performed which proves that 1990 IPCC climate science predictions are accurate. I would be interested to read you opinion on this study in a future blog post. Thanks for you efforts.

  3. nuclear_is_good says:

    I think that (as usual in your quest for sensationalism and cheap celebrity) you are missing some of the basic facts – a CO2 greenhouse model based on 100m thick layers is a good approximation of reality on Earth since that is a decent estimate of the distance over which more than half of IR is absorbed under Earth-like condition; the equivalent distance on Venus I would guess is at most 5 meters. Maybe now that you keep making regular posts on that kind of stuff it’s time to take some organized courses on that subject.

    • Clive Best says:

      I am not really interested in sensationalism – only the truth.

      CO2 absorption spectrum covers just a small part of the BB spectrum. I am fairly sure that it is the thick H2SO4 thick clouds blanketing Venus that cause the GHE. The same would happen on Earth if cloud cover could ever became 100%. Luckily for us this could never happen as phase changes of H2O ensure temperatures remain between 0 and about 40C.

  4. You are ignoring the line wings, which spread out remarkably under Venus conditions. You can’t just ignore pressure broadening and assume it makes no difference. According to spectroscopic studies under simulated Venus conditions (see e.g. the work of David Crisp and others), all kinds of minor bands come out of the woodwork at high temperature and pressure, and you’ve essentially got continuum absorption to deal with through the whole thermal IR range. Modeling radiative transfer in the Venus atmosphere is a headache on many fronts, but you haven’t even begun to approach it correctly here.

    • Clive Best says:

      I agree that my analysis above was far too naive. Kevin showed this measured spectrum for absorption by pure CO2 at standard atmospheric pressure and temp=20C. “>
      In addition there will be further pressure broadening on Venus. However there still remains a strong CO2 window from 8-11 microns and other gaps. So I still think that the H2SO4 clouds and circulation must add a very strong ‘greenhouse’ effect much greater than CO2 alone.Why otherwise are polar temperatures so high?

  5. The clouds do have an effect, and so do minor absorbers such as H2O, SO2, HCl, and HF. In addition, although you can ignore scattering in the thermal IR on Earth or Mars, you can’t do so for Venus–there is serious multiple scattering at IR wavelengths through the Venus atmosphere. But I understand most of the studies that have been done still attribute most of the warming (not all of it) to CO2.

  6. ConcernedCitizen says:

    There isn’t any GHE on venus if you understand GHE to be ‘visible light heats the surface, frequency change, emitted IR backradiated’ definition because:
    1) There is no diurnal or polar temperature variation on venus, ie, the sun doesn’t heat it.
    2) The surface is already far hotter then the sun could possibly heat it to.

    So the solar energy is absorbed in the atmosphere many tens of kilometres above the surface, and in my opinion the lapse rate cranks this up to produce the surface temp.

    • Gerry Morrow says:

      Velikovsky forecast the surface of Venus would be very hot in 1950. He claimed it was a new planet spewed out of Jupiter.

  7. ConcernedCitizen says:

    Yep, its incredible how many people miss this. Not only that the fact the surface isn’t actually warmed by the sun (it has no diurnal temperature variation) means the GH effect, which is posited on a surface warmed by the sun, is impossible.

  8. Glenn Tamblyn says:

    Clive, as an update to your post, you might like to read this chapter by Crisp & Titov on the development of the understanding of absorption in the Venusian atmosphere and its thermal structure. The book dates from 1997 so lots of understanding will have been added since then.


    In contrast with your analysis there are a lot of other factors that need to be considered.

    – Obviously pressure and temperature broadening of the existing absorption lines/bands which expands the range of wavelengths enormously..

    – Also Collision Induced Absorption. This is where molecules that may not otherwise be absorbers, or wavelengths where absorption may not normally occur at all become able to absorb during the transient time when a collision is occurring. For example, Nitrogen collisions (N2/Ns) makes N2 the major GH gas on Titan due to CIA and Nitrogen/Hydrogen (N2/H2) collisions may have been a contributor to the GH Effect in the Early Earth atmosphere.

    – Scattering, which is negligible in the IR in Earth’s atmosphere needs to be included when dealing with the much denser Venusian atmosphere.

    – Continuum Absorption is another mode of absorption that can occurr where a molecule absorbs over a continuus spectrum. H2O continuum absorption needs to be included when considering H2O vapour on Earth. CO2 also exhibits continuum absorption on Venus. At the time of writing of this book, the understanding of continuum absorption was still developing. It is better understood today.

    – Spectral data from the HiTran spectroscopic database doesn’t apply for Venus, what is used is the HiTemp database of data for higher temperatures.

    Consider, gases absorb in lines/bands but solids and liquids absorb/emit with continuous spectra. Venus has an atmospheric density nearly 100 times that of Earth. That means it has a density nearly 10% of liquid water. Is the lower Venusian atmosphere a thick gas or a thin liquid? At what density does the transition from discrete band/line absorption to continuous whole-of-spectra absorption occur?

    The conclusion from Crisp & Titov, citing a large body of work, is that CO2 is the most important GH gas on Venus.Other gases play a part and obviously the continuous cloud cover also but CO2 is the largest part.

    • Clive Best says:

      Thanks Glenn,

      Pressure broadening and ‘Collision’ induced absorption must be due to the huge CO2 atmospheric pressure being nearly 100 times larger than earth.

      I don’t understand how CO2 can develop continuum absorption like H2O.

      I also don’t understand why the massive Venus CO2 atmosphere remains gravitationally contained, when Earth’s gravity is higher.

      Otherwise what you write seems spot on.

      • Earlierapex says:

        Very thoughtful analysis. It’s so nice to come across one of the rare souls trying hard to find the objective truth rather than demand that others align with their political views!

        would the molecular weight of CO2 (about 5-6x O2, N2 and H2O) explain the gravity-based density of Venus’ atmosphere?

        How would you assess the role of convection in the atmospheric heat transfer? Perhaps it doesn’t matter how the heat got there if CO2 emission spectra is narrow, temperature dependent and can be measured in total re-radiation to space?

        Is it possible there is a net exothermic heat transfer from high volcanic activity?

  9. Glenn Tamblyn says:


    “There isn’t any GHE on venus if you understand GHE to be ‘visible light heats the surface, frequency change, emitted IR backradiated’ definition”

    But that isn’t the definition of the GHE!

    – Light from the Sun is absorbed by a planet, whether by its surface or its atmosphere/clouds or some mix of that.

    – Heat enters the atmosphere from the surface vis radiation absorbed, convection, and evapotranspiration on planets such as the Earth where a substance such as water can undergo a phase change.Heat is also returned to the surface from back radiation.

    – Atmospheric circulation distributes this heat vertically across the air column according to the Lapse Rate.

    – GH gas concentrations and cloud amounts determine the average altitude at which the planet radiates IR to space.

    – Radiative balance then sets the temperature of that emission layer..

    – And atmospheric mixing ensures that layers above/below this have temperatures relative to this layer according to the Lapse Rate.

    Venus does have a GHE, a powerful one.

  10. Erik Anderson says:

    Hi Clive,

    At the end of the updated section you write “Heat is transported upwards by convection until a level where the atmosphere thins sufficiently to radiate to space.”

    Isn’t this also what happens in Earth’s atmosphere? When I look at infrared satellite images all I see are swirling clouds giving off thermal radiation. And if this is true, how do we know CO2 warms our atmosphere a tiny bit instead of cooling it off? I mean if CO2 is heated near Earth’s surface and then convects upward until it can radiate more effectively to space.



  11. Ray says:

    This is outside the box but here goes:

    If you take the wikipedia profile for Venus atmosphere and the Barometric equation and slice it by increments of altitude, the average pressure of a molecule in Venus’ atmosphere is 4639523 pa. The average temp is 628 K. The heat capacity ratio for a mix of gases like Venus is about 1.293. Using that as a starting pressure and temperature condition, if we raise the surface pressure to 92.1 Earth atmospheres, the result is a surface temperature of 735.6 K.

    In other words, the surface temperature can be explained by heat capacity of the gases and surface pressure alone in the absence of a cooling mechanism like the water cycle and the general lack of surface convection.

    Sun supplies heat, gases soak it up and share it via conduction with the surface, convection where it exists, and absorbing radiation. Gases and the surface then emit radiation to space, but the atmosphere retains total heat based on heat capacity and pressure. It is not always distributed by pressure for a variety of reasons (clouds, convection, ozone layers etc), but the starting point is heat capacity, pressure, and distance from the sun to provide an input of energy vs the output on the dark side.

    This has been ignored on Earth. The sun heats the ground, and the ground heats the air by contact. Even oxygen and nitrogen retain heat and have a heat capacity. In the absence of greenhouse gases, the Earth’s temp would still be above its point by point average blackbody because the ground would heat the air and the air would store some heat, returning it to the ground at night to be radiated into space.

    This can be proven on the Moon. Calculate the average blackbody temperature point by point on a sphere (not using average radiation, but point by point or angle by angle). The whole dark side has a blackbody temp of zero, don’t forget to include that. The Moon’s average surface temp is higher than blackbody because the soil stores heat by day and radiates it slowly by night. The Earth with no greenhouse gases could store heat in both the soil and in the oxygen/nitrogen gases by day and then release it slowly by night.

    I applaud your honesty in pointing out what you have found, that Venus is not a runaway greenhouse at all. I would suggest exploring the pressure and heat capacity angle further. If you apply it to other solar system bodies with an atmosphere you have to include cooling of the surface by water (or methane on Titan) and the interception of radiation by ozone (or exotic molecules on Titan) and convection on those bodies as well as Mars. The method fits all these places and explains the surface temperatures.

    • Clive Best says:

      Brilliant comment – thanks !

    • Erik Anderson says:

      “Even oxygen and nitrogen retain heat and have a heat capacity. In the absence of greenhouse gases, the Earth’s temp would still be above its point by point average blackbody because the ground would heat the air and the air would store some heat”

      I’ve thought about this exact point a lot and agree, however on Earth most of the heat is in the ocean, so I think the ocean would determine the temp much more than the atmosphere when greenhouse gases are completely absent. But I do think of Venus’ atmosphere more like our ocean because it contains so much more mass than our atmosphere.

  12. Kim Libera says:

    Venus is an interesting case in that its carbon sources are fewer than earth. Its cloud layers keep lots of heat in. Prof. Richard Lindzen notates that the high pressure of Venus constrains the co2 to bottom layers.

  13. Gerry Morrow says:

    It’s a new planet, that’s why it’s hot. Suggest you read Immanuel Velikovsky’s “World’s in Collision”, The bits about Biblical plagues are unprovable, but Venus not being in the night sky until around 1500BC in any culture worldwide is compelling evidence. Velikowsky forecast the Venus would be very hot, that it’s atmosphere would not contain water, but would have CO2 and hydrocarbons, Tests have shown that the planet is emitting more heat than it gets. Many more of his forecasts turned out to be true. Mate of Einsteins’t too.

  14. Jon F Council says:

    Well, I guess people are at least willing to question the narrative about greenhouse gases and their effect on the planet. And I’m glad that Venus is the subject of comparison and debate. However, it has repeatedly surprised me that in a search for an answer for Venus, several got really close to solving what really is a simple dilemma that I’m sure these climatologists trying to scare everyone already know.

    Someone mentioned that Venus can’t possibly have a GHE going on because not enough of the Sun’s rays even makes it to the surface to be absorbed and reradiated over and over again. This is true.

    Looking at a chart of how much sunlight reaches each planet, you will see that via this formula:

    Hsun is the power density at the sun’s surface (in W/m2) as determined by Stefan-Boltzmann’s blackbody equation;
    Rsun is the radius of the sun in meters as shown in the figure below; and
    D is the distance from the sun in meters

    The actual energy reaching any point in space does not rise or fall directly 1 to 1 in line with increasing or decreasing your distance. Thus, Venus may be 1/3 closer to the sun than Earth, and about half the distance compared to Mars, but it receives way more energy than both. As measured in Watts per square meter, this is what the chart says:

    Planet Distance (x 109 m) Mean Solar Irradiance (W/m2)
    Mercury 57 9116.4
    Venus 108 2611.0
    Earth 150 1366.1
    Mars 227 588.6
    Jupiter 778 50.5
    Saturn 1426 15.04
    Uranus 2868 3.72
    Neptune 4497 1.51
    Pluto 5806 0.878

    Now keep in mind this is at the planet’s position in space BEFORE taking into account it’s atmosphere (if any), which will begin the process of Enthalpy on that energy as it gets absorbed and broken down to lower energies. How much depends on the atmosphere, it’s density, makeup, etc…
    This is what some are trying to say. Venus, by way of having such a think dense atmosphere, doesn’t get 2611 watts peer square meter at it’s surface. It suffers enthalpy fighting it’s way through all the layer, giving up IR radiation at different levels. Until it gets to the surface, where NASA itself says only 10% of the original amount in space ever reaches. That’s 260 watts/sq meter…less than HALF of what Mars gets twice as far away! And we all know Mars is colder than hell even though wait… it has an atmosphere of 98% Carbon Dioxide too!!!! Where’s it’s runaway greenhouse effect? Doesn’t seem to be working too good.

    I know someone will say… but Mars’ atmosphere is only 1/100th that of Earth! It can’t hold much heat it’s too thin. Sure, but it should be holding something as I bet there’s more CO2 on Mars than all of Earth. CO2 is a trace gas, yet it’s doing all this catastrophic damage to our climate. Mars has nearly all CO2, little clouds to block the sun, yet it’s cold enough to freeze it’s own CO2 at the poles. So what gives?

    The Gas Giants would give you the answer…

    Pressure guys. Ideal Gas Law…

    Earth atmospheric pressure as everyone knows 14.7 psi at sea level

    My 1992 Mercedes 300D turbodiesel engine cylinder compression: 320 psi

    Venus atmospheric pressure on the surface : 1350 psi

    My engine is designed to compress Earth’s atmosphere hot enough to ignite diesel, which by nature you can’t ignite with a lit match.

    Venus’ natural atmospheric pressure exceeds my diesel by a factor of more than 4X

    As Jupiter would tell you if it could talk, that’s where your heat comes from, as it has temps well past Venus; near it’s “surface”.

    Nasa is not stupid. If I could figure this out, they sure as hell know…

  15. Bob Peckham says:

    Sorry, but I can’t see the chart or the formula so am having trouble understanding it.
    The (x 109 m) bit seems somewhat arbitrary, maybe some characters or formulae are not getting through to the post ?
    Apart from that I do think it would be a good thing if more astronomers who are expert/competent in planetary atmospheres would get involved…

  16. cockneygit says:

    Interesting new video on YouTube…

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