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The runaway greenhouse effect on Venus

What the science says...

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

Venus is not hot because of a runaway greenhouse.

In keeping with my recent theme of discussing planetary climate, I am revisiting a claim last year made by Steven Goddard at WUWT (here and here, and echoed by him again recently) that “the [runaway greenhouse] theory is beyond absurd,” and that it is pressure, not the greenhouse effect that keeps Venus hot.  My focus in this post is not on his alternative theory (discussed here), but to discuss Venus and the runaway greenhouse in general, as a matter of interest and as an educational opportunity.  In keeping my skepticism fair, I’d also like to address claims (sometimes thrown out by Jim Hansen in passing by) that burning all the coal, tars, and oil could conceivably initiate a runaway on Earth.

It is worth noting that the term runaway greenhouse refers to a specific process when discussed by planetary scientists, and simply having a very hot, high-CO2 atmosphere is not it.  It is best thought of as a process that may have happened in Venus’ past (or a large number of exo-planets being discovered close enough to their host star) rather than a circumstance it is currently in.

A Tutorial of Present-Day Venus

Venus’ orbit is approximately 70% closer to the sun, which means it receives about 1/0.72 ~ 2 times more solar insolation at the top of the atmosphere than Earth.  Venus also has a very high albedo which ends up over-compensating for the distance to the sun, so the absorbed solar energy by Venus is less than that for Earth.  The high albedo can be attributed to a host of gaseous sulfur species, along with what water there is, which provide fodder for several globally encircling sulfuric acid (H2SO4) cloud decks.  SO2 and H2O are the gaseous precursor of the clouds particles; the lower clouds are formed by condensation of H2SO4 vapor, with SO2 created by photochemistry in the upper clouds. Venus’ atmosphere also has a pressure of ~92 bars, nearly equivalent to what you’d feel swimming under a kilometer of ocean.  The dense atmosphere could keep the albedo well above Earth’s even without clouds due to the high Rayleigh scattering (the effect of clouds on Venus and how they could change in time is discussed in Bullock and Grinspoon, 2001). Less than 10% of the incident solar radiation reaches the surface.

Observations of the vapor content in the Venusian atmosphere show an extremely high heavy to light isotopic ratio (D/H) and is best interpreted as a preferential light hydrogen escape to space, while deuterium escapes less rapidly.  A lower limit of at least 100 times its current water content in the past can be inferred (e.g. Selsis et al. 2007 and references therein).

The greenhouse effect on Venus is primarily caused by CO2, although water vapor and SO2 are extremely important as well.  This makes Venus very opaque throughout the spectrum (figure 1a), and 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 from IR imagery. In reality, Venus is even hotter than the dayside of Mercury, at an uncomfortable 735 K (or ~860 F). Like Earth, Venusian clouds also generate a greenhouse effect, although they are not as good infrared absorbers/emitters as water clouds.  However, the concentrated sulfuric acid droplets can scatter infrared back to the surface, generating an alternative form of the greenhouse effect that way.  In the dense Venusian CO2 atmosphere, pressure broadening from collisions and the presence of a large number of absorption features unimportant on modern Earth can come into play (figure 1b), which means quick and dirty attempts by Goddard to extrapolate the logarithmic dependence between CO2 and radiative forcing make little sense.  The typical Myhre et al (1998) equation which suggests every doubling of CO2 reduces the outgoing flux at the tropopause by ~4 W/m2, although even for CO2 concentration typical of post-snowball Earth states this can be substantially enhanced.  Figure 1b also shows that CO2 is not saturated, as some skeptics have claimed.

 

 Figure 1: a) Radiant spectra for the terrestrial planets.  Courtesy of David Grisp (Jet Propulsion Laboratory/CIT), from lecture "Understanding the Remote-Sensing Signatures of Life in Disk-averaged Planetary Spectra: 2" b) Absorption properties for CO2. The horizontal lines represent the absorption coefficient above which the atmosphere is strongly absorbing.  The green (orange) rectangle shows that portion of the spectrum where the atmosphere is optically thick for 300 (1200) ppm.  From Pierrehumbert (2011)

 How to get a Runaway?

To get a true runaway greenhouse, you need a conspiracy of solar radiation and the availability of some greenhouse gas in equilibrium with a surface reservoir (whose concentration increases with temperature by the Clausius-Clapeyron relation).  For Earth, or Venus in a runaway greenhouse phase, the condensable substance of interest is water— although one can generalize to other atmospheric agents as well.

The familiar water vapor feedback can be illustrated in Figure 2, whereby an increase in surface temperature increases the water vapor content, which in turn results in increased atmospheric opacity and greenhouse effect.  In a plot of outgoing radiation vs. temperature, this would result in less sensitive change in outgoing flux for a given temperature change (i.e., the outgoing radiation is more linear than one would expect from the σT4 blackbody-relation). 

 

Figure 2: Graph of the OLR vs. T for different values of the CO2 content and relative humidity.  For a fixed RH, the specific humidity increases with temperature. The horizontal lines are the absorbed shortwave radiation, which can be increased from 260-300 W m-2.  The water vapor feedback manifests itself as the temperature difference between b’-b and a’-a, since water vapor feedback linearizes the OLR curve.  Eventually the OLR asymptotes at the Komabayashi-Ingersoll limit.  Adopted from Pierrehumbert (2002)

 

One can imagine an extreme case in which the water vapor feedback becomes sufficiently effective, so that eventually the outgoing radiation is decoupled from surface temperature, and asymptotes into a horizontal line (sometimes called the “Komabayashi-Ingersoll” limit following the work of the authors in the 1960’s, although Nakajima et al (1992) expanded upon this limiting OLR in terms of tropospheric and stratospheric limitations).  In order to sustain the runaway, one requires a sufficient supply of absorbed solar radiation, as this prevents the system from reaching radiative equilibrium.  Once the absorbed radiation exceeds the limiting outgoing radiation, then a runaway greenhouse ensues and the radiation to space does not increase until the oceans are depleted, or perhaps the planet begins to get hot enough to radiate in near visible wavelengths.

 

Figure 3: Qualitative schematic of how the ocean reservoir is depleted in a runaway.  From Ch. 4 of R.T. Pierrehumbert’s Principles of Planetary Climate

 

On present-day Earth, a “cold trap” limits significant amounts of water vapor from reaching the high atmosphere, so its fate is ultimately to condense and precipitate out.  In a runaway scenario, this “cold trap” is broken and the atmosphere is moist even into the stratosphere.  This allows energetic UV radiation to break up H2O and allow for significant hydrogen loss to space, which explains the loss of water over time on Venus.  An intermediate case is the “moist greenhouse” (Kasting 1988) in which liquid water can remain on the surface, but the stratosphere is still wet so one can lose large quantities of water that way (note Venus may never actually encountered a true runaway, there is still debate over this).  Kasting (1988) explored the nature of the runaway /moist greenhouse, and later in 1993 applied this to understanding habitable zones around main-sequence stars.  He found that a planet with a vapor atmosphere can lose no more than ~310 W/m2, which corresponds to 140% of the modern solar constant (note the albedo of a dense H2O atmosphere is higher than the modern), or about 110% of the modern value for the moist greenhouse.

 

Earth and the Runaway: Past and Future

 

Because Earth is well under the absorbed solar radiation threshold for a runaway, water is in a regime where it condenses rather than accumulating indefinitely in the atmosphere.  The opposite is true for CO2, which builds up indefinitely unless checked by silicate weathering or ocean/biosphere removal processes.  In fact, a generalization to the runaway threshold thinking is when the solar radiation is so low, so that CO2 condenses out rather than building up in the atmosphere, as would be the case for very cold Mars-like planets.  Note the traditional runaway greenhouse threshold is largely independent of CO2 (figure 2 & 4; also see Kasting 1988), since the IR opacity is swamped by the water vapor effect.  This makes it very difficult to justify concerns over an anthropogenic-induced runaway.

 


 

Figure 4: The H2O–CO2 greenhouse. The plot shows the surface temperature as a function of radiated heat for different amounts of atmospheric CO2 (after Abe 1993). The albedo is the fraction of sunlight that is not absorbed (the appropriate albedo to use is the Bond albedo, which refers to all sunlight visible and invisible). Modern Earth has an albedo of 30%. Net insolations for Earth and Venus ca. 4.5 Ga (after the Sun reached the main sequence) are shown at 30% and 40% albedo. Earth entered the runaway greenhouse state only ephemerally after big impacts that generated big pulses of geothermal heat. For example, after the Moon-forming impact the atmosphere would have been in a runaway greenhouse state for ∼2 million years, during which the heat flow would have made up the difference between net insolation and the runaway greenhouse limit. A plausible trajectory takes Earth from ∼100 bars of CO2 and 40% albedo down to 0.1–1 bar and 30% albedo, at which point the oceans ice over and albedo jumps. Note that CO2 does not by itself cause a runaway. Also note that Venus would enter the runaway state when its albedo dropped below 35%.  Se e Zahnle et al 2007

 

This immunity to a runaway will not be the case in the long-term.  In about a billion years, the sun will brighten enough to push us into a state where hydrogen is lost much more rapidly, and a true runaway greenhouse occurs in several billion years from now, with the large caveat that clouds could increase the albedo and delay this process.

Interesting, 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, with geothermal heat and the heat flow from the moon-forming impact making up for the difference between the net solar insolation and the runaway greenhouse threshold, although this would last for only a brief period of time.  Because the runaway threshold also represents a maximum heat loss term, it means the planet would take many millions of years to cool off following such magma ocean & steam atmosphere events of the early Hadean, much slower than a no-atmosphere case (figure 5).

 

Figure 5: Radiative cooling rates from a steam atmosphere over a magma ocean. The radiated heat is equal to the sum of absorbed sunlight (net insolation) and geothermal heat flow. The plot shows the surface temperature as a function of radiated heat for different amounts of atmospheric H2O (adapted from Abe et al. 2000). The radiated heat is the sum of absorbed sunlight (net insolation) and geothermal heat flow. The different curves are labeled by the amount of H2O in the atmosphere (in bars). The runaway greenhouse threshold is indicated. This is the maximum rate that a steam atmosphere can radiate if condensed water is present. If at least 30 bars of water are present (a tenth of an ocean), the runaway greenhouse threshold applies even over a magma ocean. Note that the radiative cooling rate is always much smaller than the σT4 of a planet without an atmosphere

Conclusions

Venus likely underwent a runaway or “moist greenhouse” phase associated with rapid water loss and very high temperatures.  Once water is gone, silicate weathering reactions that draw down CO2 from the atmosphere are insignificant, and CO2 can then build up to very high values.  Today, a dense CO2 atmosphere keeps Venus extremely hot.

Last updated on 11 April 2011 by Chris Colose. View Archives

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Comments 101 to 125 out of 267:

  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.

    Response:

    [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. 

       

    Response:

    [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:

    http://hockeyschtick.blogspot.com.au/2016/06/new-paper-demonstrates-gravito-thermal.html

    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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