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The runaway greenhouse effect on Venus

What the science says...

Select a level... Basic Advanced

Venus very likely underwent a runaway or ‘moist’ greenhouse phase earlier in its history, and today is kept hot by a dense CO2 atmosphere.

Climate Myth...

Venus doesn't have a runaway greenhouse effect

"I bought off on the “runaway greenhouse” idea on Venus for several decades (without smoking pot) and only very recently have come to understand that the theory is beyond absurd." (Steve Goddard, WUWT)

At a glance

Earth: we take its existence for granted. But when one looks at its early evolution, from around 4.56 billion years ago, the fact that we are here at all starts to look miraculous.

Over billions of years, stars are born and then die. Our modern telescopes can observe such processes across the cosmos. So we have a reasonable idea of what happened when our own Solar System was young. It started out as a vast spinning disc of dust with the young Sun at its centre. What happened next?

Readers who look up a lot at night will be familiar with shooting stars. These are small remnants of the early Solar System, drawn towards Earth's surface by our planet's gravitational pull. Billions of years ago, the same thing happened but on an absolutely massive scale. Fledgeling protoplanets attracted more and more matter to themselves. Lots of them collided. Eventually out of all this violent chaos, a few winners emerged, making up the Solar System as we now know it.

The early Solar system was also extremely hot. Even more heat was generated during the near-constant collisions and through the super-abundance of fiercely radioactive isotopes. Protoplanets became so hot that they went through a completely molten stage, during which heavy elements such as iron sank down through gravity, towards the centre. That's how their cores formed. At the same time, the displaced lighter material rose, to form their silicate mantles. This dramatic process, that affected all juvenile rocky planets, is known as planetary differentiation.

Earth and Venus are the two largest rocky planets. But at some point after differentiation and solidification of their magma-oceans, their paths diverged. Earth ended up becoming habitable to life, but Venus turned into a hellscape. What happened?

There's a lot we don't know about Venus. But what we do know is that the surface temperature is hot enough to melt lead, at 477 °C (890 °F). Atmospheric pressure is akin to that found on Earth - but over a kilometre down in the oceans. The orbit of Venus may be closer to the Sun but a lot of the sunlight bathing the planet is reflected by the thick and permanent cloud cover. Several attempts to land probes on the surface have seen the craft expire during descent or only a short while (~2 hours max.) after landing.

Nevertheless, radar has been used to map the features of the planetary surface and analyses have been made of the Venusian atmosphere. The latter is almost all carbon dioxide, with a bit of nitrogen. Sulphuric acid droplets make up the clouds. Many hypotheses have been put forward for the evidently different evolution of Venus, but the critical bit - testing them - requires fieldwork under the most difficult conditions in the inner Solar System.

One leading hypothesis is that early on, Venus experienced a runaway water vapour-based greenhouse effect. Water vapour built up in the atmosphere and temperatures rose and rose until a point was reached where the oceans had evaporated. In the upper atmosphere, the water (H2O) molecules were split by exposure to high-energy ultraviolet light and the light hydrogen component escaped to space.

With that progressive loss of water, most processes that consume CO2 would eventually grind to a halt, unlike on Earth. Carbon dioxide released by volcanic activity would then simply accumulate in the Venusian atmosphere over billions of years, creating the stable but unfriendly conditions we see there today.

Earth instead managed to hang onto its water, to become the benign, life-supporting place where we live. We should be grateful!

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Venus may have experienced a runaway greenhouse effect in the geological past. To use the term 'runaway' is to refer to a highly specific process when discussed by planetary scientists. Simply having a very hot, high-CO2 atmosphere is not it. So let's start with a tutorial on Venus at the present day.

Venus’ orbit is much closer to the sun, which means it receives almost twice the solar radiation at the top of its atmosphere than Earth. Venus also has a very high albedo which ends up over-compensating for the closer distance to the sun. The result is that less than 10% of that incident solar radiation reaches the surface. High albedo can be attributed to sulphur-bearing compounds, along with minor water vapour (around 20 ppm). These substances form globally encircling sulphuric acid-dominated cloud decks (fig. 1). Venus’ atmosphere also has a surface pressure of around 92 bars (or if you like, 92,000 millibars), equivalent to what you’d feel on Earth beneath more than a kilometre of ocean.

Venus in its shroud of clouds.

Fig. 1: Venus in its shroud of clouds - a false colour composite created by combining images taken using orange and ultraviolet spectral filters on the Mariner 10 spacecraft's imaging camera.The images used to create this view were acquired in 1974; the RH one has been enhanced to bring out texture and colour. Image: NASA.

Observations of the water vapour content in the Venusian atmosphere show a high heavy to light hydrogen isotopic ratio (D/H). This is best interpreted as the product of a preferential light hydrogen escape to space: deuterium escapes less easily. Venus is considered to have had at least 100 times its current water content in the past (e.g. Selsis et al. 2007 and references therein).

The greenhouse effect on Venus today is primarily caused by CO2, although water vapour and SO2 are important as well. Since most of the radiation that makes its way out to space comes from only the very topmost parts of the atmosphere, it can look as cold as Mars in infra-red (IR) imagery. In reality, the surface of Venus (Fig. 2) is even hotter than the dayside of Mercury, at a deadly 477 °C (890 °F).

Like Earth, the Venusian clouds also generate a greenhouse effect. However, they are poor IR absorbers and emitters compared to water clouds. The sulfuric acid droplets forming the clouds can also scatter IR radiation back to the surface, producing another form of the greenhouse effect in that way. In the dense Venusian CO2-rich atmosphere, there are IR-handling processes at work that are unimportant on modern Earth.

The Soviet Union's Venera 14 probe.

Fig. 2: The Soviet Union's Venera 14 probe captured two colour panoramas of Venus's surface in 1982. This panorama came from the rear camera. Image: Russian Academy of Sciences. More images can be seen at:

How to get a Runaway?

To get a true runaway greenhouse effect on Venus, you need a combination of solar radiation and the presence of a greenhouse gas. That gas has two key requirements. It must be condensable and it needs to be in equilibrium with its surface reservoir. In addition, its concentration must increase with temperature, as explained by the Clausius-Clapeyron relation. For Venus to enter a runaway greenhouse state, the greenhouse gas of interest is water vapour, plus its liquid reservoir, the water making up the oceans.

The greenhouse effect on any planet involves impeding the flux of outwards longwave radiation to space. Water vapour is very good at this so can potentially lead to a positive feedback runaway scenario. That works as follows: higher temperatures cause ever more water to evaporate and then drive temperatures even higher and on and on it goes - while there is an available liquid water reservoir.

Through water vapour's effectiveness at blocking IR, the outward longwave radiation flux eventually flatlines. If the incoming Solar flux is constantly greater than that outgoing flatline value, the planet is tipped out of radiative equilibrium and we have that runaway. If you like, it has a fever. The reservoir for water vapour - the oceans - is vast. That means the system may only be able to return to radiative equilibrium once the runaway process has stopped. In the extreme runaway greenhouse effect, that cessation may only happen at the point when the whole ocean has evaporated.

On present-day Earth, there is a strong temperature inversion, called the tropopause. It is situated between the troposphere and stratosphere. You can see it on thundery days when the tops of storm-clouds spread out beneath it to form the familiar anvil-shapes. The tropopause thus forms an effective barrier to moisture getting into the stratosphere. At the height of the tropopause on Earth, in any case, it's already too cold for water to remain in the vapour phase. The wispy clouds making up thunderstorm anvils consist of ice crystals. This impediment to water vapour's ascent is often referred to as a 'cold trap'.

In a runaway scenario, such as that proposed for Venus, no such impediment exists. This means the upper atmosphere would have become moist too. On Venus, the troposphere extends to a much greater height than on Earth. There is no stratosphere - we're talking about a very different situation here. That is critical because water vapour, upon reaching such great heights, has energetic Solar ultraviolet (UV) radiation to contend with. The UV is effective at splitting the H2O molecule into its constituent elements. Once that has happened, the hydrogen in particular is easily lost to space. One can envisage that once a runaway greenhouse effect got going, Venus' water content got steadily depleted in this manner through time. If Venus ever had oceans, they must have evaporated into oblivion. Because of the 'cold trap', this form of water-depletion is of very little significance on Earth - thankfully.

Once that water was lost, the chemical processes that lock up carbon in rocks on Earth could not operate. All of them involve water somewhere. Thereafter, every addition of carbon to the atmosphere, large or small, stayed up there. Most CO2 was probably of volcanic origin. The result was the 96.5% CO2 atmosphere and hellish surface temperature of Venus today.

Earth and the Runaway: Past and Future

Currently, Earth is well under the absorbed solar radiation threshold for a runaway greenhouse effect to occur. Its water condenses and is recycled back to the surface as rain, rather than accumulating indefinitely throughout the atmosphere. The opposite is true for CO2, which builds up and up through our emissions, only checked by natural removal processes. Note here that the runaway greenhouse threshold is largely independent of CO2 since the IR opacity is swamped by the water vapour effect. This makes it difficult to justify concerns over a CO2-induced runaway on Earth.

However, this immunity to a runaway greenhouse effect will not last forever. The most realistic scenario for Earth entering a runaway occurs a few billion years in the future, when the sun's brightness has substantially increased. Earth will then receive more sunlight than the outgoing longwave radiation escaping to space. A true runaway greenhouse effect is then able to kick in. Caveats apply, though. For example, greater cloud cover could increase planetary albedo and delay this process.

Interestingly, some (e.g. Zahnle et al. 2007) have argued that Earth may have been in a transient runaway greenhouse phase within the first few million years of its existence. Geothermal heat and the heat flow from the moon-forming impact would have made up for the difference between the net solar insolation and the runaway greenhouse threshold. But if this happened it could only have lasted for a relatively short period of time - since we still have plenty of water on Earth.

For further reading, a recent review paper (Gillmann et al. 2022) explores the various hypotheses concerning the evolution of the Venusian atmosphere over geological time. There's also an excellent book chapter (Arney & Kane. 2020, currently available as a PDF at arXiv). As might be expected, difficulties in fieldwork are plural on Venus and designing a probe that survives touchdown and can go on to do the required data-collecting is still some time away. The key piece of evidence we need to confirm the existence of a runaway greenhouse effect in deep time would be for free water having once been present. But it is apparent that large parts of the surface were covered with lava flows from monster volcanoes at some point. Is that evidence nowhere to be seen, or is it just hiding? Time will tell.

Last updated on 21 January 2024 by John Mason. View Archives

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Comments 101 to 125 out of 268:

  1. Mike, Sorry, I didn't notice the second part of your question. You can't apply a simple climate sensitivity equation like that one to Venus because it's apples and oranges. That sensitivity includes things like albedo feedbacks (ice/desert/water), carbon feedbacks, water vapor feedbacks and more. It's specific to earth, and derived by a combination of observational, paleo and modeling studies. Really... you need a complex computer program to simulate the atmosphere of Venus. What exactly are you attempting to prove or investigate?
    Response: [Sph] Ignore this. It was wrong.
  2. Mike Myhre's formula is just a fit to the full radiative transfer code results. It has limited validity and it's not a functional form that can be applied to other situations. I'm not aware of any simplified form for Venus.
  3. Mike Mellor - Here's a direct link to Myhre et al 1998, which I believe is from Dr. Myhre himself. In short, they ran radiative modelling (akin to numerical integration, not global climate modelling, mind you) using HITRAN spectral data, over three different atmospheric profiles (tropical, northern, and southern hemisphere), and calculated the incremental forcings. They then arrived at F=5.35 ln(C/C0) W/m^2 as an appropriately scaled approximation of the forcing deltas. Note that this was an update from the value used in the 1990 IPCC reports, which was 6.3, not 5.35 - in other words the Myhre analysis has a slightly lower forcing for doubling CO2. --- Sphaerica - I don't believe you have that equation right. The forcing delta is in W/m^2, not Fahrenheit, and the resulting forcing change from a doubling of CO2 is 3.7 W/m^2. That leads to a temperature change (as per the SB equation, no feedbacks, assuming radiated power at TOA goes from 240 to 243.7) of ~1.1C, after which you have water vapor changes (another 1.1-1.3°C, total of ~2.4°C) and additional feedbacks - leading to a total climate sensitivity of ~3°C/doubling of CO2.
    Response: [Sph] Thanks, KR. My bad. I should have looked far more closely at the thread and question before responding. Requested edit applied.
  4. I'm not convinced of Venus having a runaway greenhouse effect. With 92X atmosphere than Earth - and if Venus has a radioactive heat source like Earth that the insolation effect, as well as some solor heat trapped in the atmosphere - would be keeping the planet from cooling off.

    Additionally - I object to comparing Venus to Earth in this regard because the fact is that you can't compare a planet with 92X atmosphere and being closer to the Sun and having different atmospheric chemestry as being similar.

    And - since almost no light makes it to the surface of Venus all solar energy is absorbed in the atmosphere. Therefore if a solar heating event is happening then the atmosphere would have to be hotter than the surface and heating the whole planet by convection and conduction. But the atmosphere is not hotter than the surface so that tends to disprove the runaway greenhouse idea.

    The simplier and more plausable explanation is that like Earthg Venus was molten and because of 92X atmosphere it never cooled off, as opposed to the idea that it used to be cool and was heated up.

    I'm not buying it.

  5. What's interesting in reading through the comments section here about possible greenhouse warming in Venus' history is the degree to which several factors can contribute to what I think is really an unhelpful paradigm.

    You have first of all a lack of experimental rigor. Data we can measure is incomplete, and we have no current ability to conduct on a planetary scale, any sort of experiment which could yield telling conclusions.

    Secondly, to incomplete data, you have an excess of theoretical mathematics. Interpretation of current data, divorced from adequate experiemental results, VERY quickly becomes a creative endeavor. It's extrmely easy to twist aspects of the data to fit a predetermined philosophical stance.

    Thirdly, you have an issue when you bring in a philosophical stance to an issue that should ideally be bereft of one. For example, anyone who uses the term "denier"  is really bringing in an unethical a priori point of view to something that should be science based.

    Lastly, my overarching point is that we have a lack of ability to carry out valid experiment, and that we shouldn't be overly eager to marry a particular philosophical point of view with inadequate data.

  6. protagorias @105, your comments are so lacking in specifics as to be void of content.  Where they rise to any level of specificity, they amount merely to an ad hominen, accusing people of basing their theory on a "philosophical point of view" rather than science, again without specifics so as to avoid detailed refutation.  In all, your post is a classic example of sloganeering, which is banned by the comments policy.  Ergo, it is not worthy of further comment.

  7. tom curtis @106,

    On the contrary I think my points are inherently self-evident to anyone who knows how to conduct a scientific experiment and properly interpret data. Quite frankly it's sheer folly and arrogance to take any particular data point, which may be transient in nature, and lacking any ability to test for such transience, ascribe undue meaning to it. I have no interest in playing games of whose creative interpretation of incomplete data is better.

  8. Protagorias, perhaps you could provide a specific example of where you think this terrible error in judgment has occurred.

  9. protagorias, your definition of "science" requiring experimentation on a whole-planet scale is incorrect.  Indeed, "science" in general does not require experimentation.  Unfortunately, your definition of "science" is what typically is taught in grade school.

  10. Tom Dayton, thank you for pointing that out. Begrudgingly I have to admit that I stand corrected. We are, I think, a civilization going through a modern and enlightening period in history.

  11. This is rediculous. We have an absolutely mass of data on Venus. Where is there any sign of an observation that disagrees with known physics? By your definition, geology and maybe biology are not sciences because we cant rerun a planet. As a reminder, the core of science is about testing of ideas, usually expressed as models, against what is possible to be observed. Experiments are a way to generate observations but no mean the only way. Sending a probe to the surface of Venus and measuring all the way down is another perfectly valid one.

    The idea for instance that the Radiative Transfer Equations (which is how you calculate the GHE for Venus) are derived from "a single point that may be transient in nature" is absurd.

  12. scaddenp @111, it is more than (or should that be worse than) ridiculous.  Logically, the instruments used to obtain empirical emissions data from Venus were not experimentally tested on Venus.  Therefore it is possible that Venusian conditions result in a change in physical laws such that the data is misinterpreted if we use the theory based on experiments on Earth to interpret the data.  If, therefore, we apply protagorias' restrictions on the use of theory, we can not use any data from Venus we have obtained.

    If, on the other hand, we assume that physical laws tested in experiments on Earth, that work well in Earth's atmosphere and appear to work well in Venus (and Mars') atmosphere also work on Venus, then we obtain the results protagorias excoriates as too theoretical.  His objection, therefore amounts to no more than pseudo-philosophical cant,  which disguises that fact by not applying it explicitly to any particular observations or theories (where such application would show immediately he is resorting to unjustified obscurantism).

    Given his chosen internet name, this should not surprise us.  He has chosen the name of a philosopher who argued that theoretical maths (specifically, Euclidean geometry) was not applicable in the real world, and excessively theoretical; and that truth was relative.  (Note, Protagoras lived before Euclid, so the geometry he objected too had not yet been axiomatized, but was Euclidean in the sense that it treated parallel lines as never meeting.)  Given that his namesake would not even accept that a line could be tangent to a curve, why would we expect him  to accept the maths behind the Kombayashi-Ingersol limit?  Or consider it worth discussing with him, as he wants to imunize his views from debate by avoiding specifics.

  13. I see, but i think I make a valid point when I say we don't have enough data. We don't have accurate enough measurements regarding the composition of gases on Venus today to warrant speculation about what happened on the planet millions of years ago.


    [JH] You've made your point. Please move on to a different topic.

    Excessive repititon is prohibited by the SkS Comments Policy. Any future posts by you on this topic will be summarily deleted.

    Sloganeering is also prohibited by the SkS Comments Policy. Any future posts by you that lack credible documentation to support your position will be summarily deleted.

  14. Thank you for clarifying. I wanted to address, as a last point, specifics regarding the degree of confidence in the accuracy of the recorded data by the instrumentation from the Venus missions. Is the data coming back from the cameras on the Venus climate orbiter adequate and accurate?

  15. Sorry for joining this discussion so late. 

    My understanding is that Chris Colose's piece (the origin of this debate) said is not totally inconsistent with what Rosco was saying (Rosco was, unfortunately, mobbed out of this thread; shame on us, scientists, for being unable to conduct a civilized discussion with a well-meaning outsider without patronizing, antagonizing, provoking, name calling, etc.).

    So, Chris essentially said (again, this is my understanding) that Venus surface is far too hot for the current state of the affairs (insolation, albedo, atmosphere composition, etc.), so there MUST HAVE BEEN a runaway greenhouse effect in some (uspecified) past that heated it up, and the current state of the affairs does not let it cool too quickly.

    What Rosco was saying in the beginning is that Venus surface is far too hot for the current state of the affairs. I don't understand why he had to be chased out of this thread for this, even though here Rosco and Chris seem to agree.

    Where I disagree with Chris is when he says that "Less than 10% of the incident solar radiation reaches the surface." There is no evidence that even 1% of solar radiation reaches Venus surface, so dense is the Venusian atmosphere (if we live under an equivalent of 10 m of water - our atmosphere compressed, the  on Venus the equivalent depth is >900 m, and this is without taking into account the dense clouds).  The light observed by the Russian station was likely due to the  lightening that is constantly illuminates the Venusian atmosphere. 



    [PS] Rosco came to debate with a history of trying the moderator's patience and a strong dislike for either reading or comprehending information that contradicted his/her preconceptions. Please note that moderation complaints are always offtopic.

  16. RomanEmpire @115,

    1)  Apart from the obvious point that 1% <10% so that there is no contradiction between Colose's claim and yours, we have the fact that Svedhem et al (2007) says:

    " Less than 10% of the incoming solar radiation penetrates through the atmosphere and heats the surface."

    Apparently a similar claim is made in Titov et al (2006).  Finally we have Tomasko et al (1980) that concludes from the comparison of measurements with models that:

    "Averaged over the planet, about 17 W/m² are absorbed at the ground (some 2.5% of the total solar energy incident on the planet)."

    This is definitely inconsistent with the claim that there "... is no evidence that even 1% of solar radiation reaches Venus surface", which is revealed as hyperbole at best.

    2)  Chris Colose writes above:

    "note Venus may never actually encountered a true runaway, there is still debate over this"

    It follows that when you write "Chris essentially said ... that Venus surface is far too hot for the current state of the affairs ..., so there MUST HAVE BEEN a runaway greenhouse effect..." you are clearly misrepresenting his argument.  His argument is that while the TOA insolation on Venus is sufficient to drive a runaway greenhouse effect, it is not sufficient on Earth.  Venus may have reached its current conditions by either never having cooled down sufficiently from its initial heat of formation (due to a strong greenhouse effect) or to having experienced a runaway greenhouse after cooling down as the Earth did.

    3)  Roscoe espoused absurd theories (on the level of geocentrism).  He refused to either be convinced by clear argument or evidence provided.  Scientists, no matter how respectful, cannot be expected to persuade those who come into the discussion with a closed mind as Roscoe clearly did.  Nor, if you abuse their patience by continuing to espouse nonsense rather than learn something new, can you expect the patience of scientists to persist.

  17. Tom Curtis @116.

    I think it is wrong to characterise the initial position of Rosco up-thread as being "on a level of geocentrism". RomanEmpire @115 is pursuaded that there was something in Rosco's initial position and thus it would be wrong to dismiss it entirely off-handedly.

    The basic idea that seemed to confound Rosco was that he held that the sun (less albedo) should warm the insulating outer atmosphere of Venus to some 250 K and then he was perplexed that the surface temperature of Venus is some 750 K. How could this be? Addressing this point was not helped up-thread as Rosco arrived with a heavy load of misconceptions but let us ignore them. What Rosco simply failed to grasp was that when a planet gains a powerful greenhouse atmosphere, it takes very little energy to raise the temperature at its surface. So the vulcanism within Venus, if it had a similar heat output as Earth (which is likely) would require only a few million years (a mere blink of an eye in the evolution of a planet's climate) to warm its thick lower atmosphere from 250 K to today's 750 K.

  18. MA Rodger @117, this is Rosco's original post upthread (@3):

    "Venus is nothing like the earth - it is (-snip-) to claim it is. I have seen claims that the "greenhouse" effect on Venus is responsible for heating the planet by ~500 k. This is clearly impossible given the albedo of Venus reflects most incoming solar radiation.

    If such an effect were possible it could easily solve Earth's energy problems - simply collect all the hot exhaust gases from a coal fired power station and force it into a chamber under 92 bar pressure, add sunlight and the runaway greenhouse would raise the temperature to over 700 K - and we could use this heat to drive turbines and eventually shut down the coal fired power station.

    Yeah right - the whole idea is "beyond absurd"."

    In successive paragraphs he shows repeatedly that he does not think there is any such thing as a greenhouse effect, that he does not know how it works, and (at the end of the second paragraph) that he does not understand the laws of thermodynamics.

    The greenhouse effect is sufficiently well understood, and sufficiently well evidenced that the probability of it not existing is not meaningfully distinguishable from the probability that geocentrism is true.  Ergo, his initial contribution is very much on a level with geocentrism.

    There are things that are reasonably disputable, and even controversial in climate science.  That the origin of the 20th century increase in CO2 is anthropogenic, and that an greenhouse effect and an enhanced greenhouse effect exist are not among them.  Any denial of those facts merely shows an abysmal scientific ignorance.  The lack of a civilized discussion that RomanEmpire points to is a direct consequence of that fact.  It is not possible to have civilized discussion defending the thesis that black is white (or geocentrism; or rejecting the existence of a greenhouse effect) because one participant must lack an essential of civilized discussion in any such case - the desire or ability to be rational.  Those who sheet home the failure of civilized discussion to the rational side of the discourse need to be reminded of this fact in no uncertain terms.

  19. Tom Curtis @118.

    Sorry. When I said "initial", and RomanEmpire @115 said "beginning", this was not the very beginning @3 which is pretty incoherent stuff, but the Rosco comment @7. This fits with the description @115 "What Rosco was saying in the beginning is that Venus surface is far too hot for the current state of the affairs."  I must admit that re-reading #7 I did manage to mis-interprete the comment, somehow reading into it the idea that the sun only heats the outer atmosphere (thus the 250 K limit), an idea which is actually absent. But in my defence, the actual argument/question presented @7 is entirely self-defeating - there is no greenhouse effect (thus temperature is as a black body) so how can there be a greenhouse effect?

  20. MA Rodger @118 even considering Roscoe @7, his primary argument consists of the claim that there is an energy imbalance between incoming solar energy at the surface of Venus and outgoing thermal radiation at the surface of Venus with the later being 122 greater than the former, a gap that "no physical process" could explain in his view.  He maintains this view by, first, not understanding the greenhouse effect, and second, not considering all energy flows at the surface.

    The later point is best illustrated with the Earth's energy flows which are better known:

    The equivalent numbers for the Earth are 161 W/m^2 for solar energy at the surface, and 397 W/m^2 outgoing thermal radiation - a significant energy imbalance that creates exactly the same theoretical problems as the imbalance at Venus.  However it is obvious from the full chart once all energy flows are considered, the surface energy balance is close to 0.  From the numbers given it is out by 1 W/m^2 downward, an obvious rounding error from the 0.6 W/m^2, ie, the imbalance at the TOA due to the temperature response to current forcing not yet reaching equilibrium values.

    Given the full equivalent values for Venus, the surface energy balance would sum to zero with the downward thermal radiation consisting of a very large component.  That the energy balance would sum to zero is confirmed by the comparison of the temperature profile of Venus with altitude versus two one dimensional radiative convective models (Tomasko et al 1977):

    The solid lines are what is expected from the operation of the greenhouse effect together with convection given two slightly different assumptions about the composition of Venus' atmosphere, and closely match the observed profile.  One dimensional models such as those shown work by maintain energy balance throughout the vertical profile.  It should be noted that the slight difference in the observed lapse rate will be due to a slight difference in from the expected heat capacity of the atmosphere, ie, an error in understanding the variation in the precise chemical composition of the atmosphere with altitude, not with the radiative model.

    Another way of noting the incoherence of Roscoe's argument (ie, the argument from the lack of energy balance when he ignores all components) is to note that by ignoring back radiation, he assumes that there is no greenhouse effect, as you note @119.  That would again make his theory absurd "on the level of geocentrism".  Perhaps more important to this discussion is that none of the above has any bearing on the runaway greenhouse effect, which is a theory about how Venus developed from a (putative) Earth like state to its current greenhouse dominated state with no surface water.

    Roscoe may have a secondary "argument" that does specify the runaway greenhouse effect.  Specifically, he writes:

    "Where does this come from when a University Professor tells me the sutface of Venus receives only 132 W/sq m ?

    I think this is a fair question. If it is from the greenhouse effect how did this develop initially ? 132 W/sq m couldn't possibly do it."

    IMO the most sensible way to interpret this comment is that Roscoe did not distinguish between the runaway greenhouse effect (a process that would have taken tens or even hundreds of thousands of years) and the current Venusian greenhouse effect (an almost static state in quasi equilibrium and with no significant variation on decadal scales).  However, assuming he was correctly referring to the runaway greenhouse effect it should be noted that, first, his argument consists of an argument from personal incredulity; and second, that he falsely assumes the albedo of Venus in the initial, earthlike, state of the runaway greenhouse effect must equal its current albedo.  Without the later assumption he cannot assume that the surface insolation on Venus equaled the current value.

    As a final note, 132 W/m^2 is not the insolation at the surface, contrary to Roscoe's repeated assertions.  Rather it is the insolation at the TOA after adjustment for albedo and averaging over globe.  A flat surface perpendicular to incoming sunlight and at the orbit of Venus would recieve 2625 W/m^2.  Given the bond albedo of 0.9, that means total TOA insolation averages as 66 W/m^2, with only less than 6.6 W/m^2 reaching the surface. 

  21. The greenhouse effect doesn't explain why the dark and sunlit sides of Venus are the same temperature, and why the poles are as hot as the equator. This does:

    Venus is not like earth, in that its atmosphere directly absorbs sunlight on the way in, via the H2SO4 clouds. That heat absorced by H2SO4 is transferred to the surface via the gravity pump described here.

    The reason the temperature everywhere on Venus is the same is, gravity is the same all around Venus.

  22. Mike Hillis @121.

    You are incorrect in presenting that link to a denialist webpage while proclaiming "The greenhouse effect doesn't explain why .... . This does:" The link you provide offers no explanation but rather presents unsubstantiated assertion spruced up with a couple of sweeps of Ockham's broom. The nonsense is explained however (as much as such things can be explained) by Nikolov and Zeller's 2011 conference poster.

  23. That Nikolov and Zeller poster, not to mention the related Hockeyschtick blog post, could best be summarized as:

    "We didn't like GHG physics, so we made up our own."

    In short, utter nonsense. If you want actual physics, I would suggest reading the quite approachable Pierrehumbert 2001 article, "Infrared radiation and planetary temperature"

  24. Gee, I am surprised this rubbish keeps coming up, when it was laughed at even in "skeptic" circles. You might like to read what Roy Spencer has to say on the subject as I suspect you would trust that source rather than "warmistas" here or say physics textbooks. This has come up here before and even our friend Camburn wouldnt buy it. If you think this is plausible, then I think he has a bridge he would like to sell you.

  25. KR @123.

    The Nikolov and Zeller poster is not entirely make believe. Their initial modelling of a planet with zero GH-effect is correct but is so silly with its assumptions I wasn't bothered to look further into their arguments. So I cannot speak for how bad the rest of it is.

    That initial model in 2.1A is for a planet with its day-side locked to always face the sun. The average temperature will thus be a little over half the black body temperature. And this should not be a great surprise; with its dark side permanently unheated, half the planet has a temperature effectively at absolute zero. Smith (2008) helpfully have done these sums & show the effects of rotation & thermal inertia.

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