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Lessons from Venus

Filed under: — rasmus @ 11 April 2006

by Rasmus Benestad and Ray Pierrehumbert

Venus Express will make unprecedented studies of the largely unkown phenomena taking place in the Venusian atmosphere. Credits: ESA - AOES Medialab A special report in The Observer on Sunday (April 9) titled ‘Venus – The Hot Spot’, provides a well-written account on a mission called the Venus Express. The Venus express is an European Space Agency (ESA) mission to probe the the atmosphere of Venus and address questions regarding the differences between the climates on Venus and Earth. According to the plans, the probe will enter the final orbit around Venus in May 2006, i.e. within about a month.

What relevance does a mission to Venus have for a blog like RealClimate? Primarily, Venus offers scientists the chance to see how the same basic physics used to study Earth’s climate operates under a very different set of circumstances. In one sense, Venus is rather similar to Earth: it has nearly the same mass as Earth, and while its orbit is somewhat closer to the Sun, that effect is more than made up for by the sunlight reflected from Venus’ thick cloud cover. Because of the cloud cover, the surface temperature of Venus would be a chilly -42C if were not for the greenhouse effect of its atmosphere. In reality, the surface of Venus, at 740K (467C) is even hotter than the surface of Mercury, which is a (relatively!) pleasant 440K. Per unit of surface area, the atmosphere of Venus has as much mass as about 100 Earth atmospheres, and it is almost pure CO2. This accounts for its very strong greenhouse effect. In contrast, the CO2 in the Earth’s atmosphere accounts for a mere .00056 of the full mass of one Earth atmosphere.

Despite the fact that Venus has vastly more CO2 in its atmosphere than Earth, the same basic principles govern the operation of the greenhouse effect for both planets: the fact that air cools by expansion as it rises means that the upper parts of the atmosphere are colder than the surface, while the opacity of greenhouse gases to infrared means that infrared radiation can only escape from the upper portions of the atmosphere. Since the rate of radiation goes down with temperature, the net effect allows the planet to lose energy at a rate much lower than it would if the radiation from the surface escaped directly to space. Although most of the warm surface temperature of Venus is accounted for by its CO2 greenhouse effect, there are suggestions that it is warmer than it should be on the basis of CO2 alone. There are various theories that have been proposed for the source of the additional greenhouse effect, and sorting this out will be one of the major objectives of Venus Express.

Another interesting difference between Venus and Earth is that Venus has a very slow rotation rate, taking 243 Earth days to complete one rotation. This is actually somewhat longer than its year. Despite the low rotation rate of the surface, the upper atmosphere is whizzing along at a rate of about one rotation every four Earth days. Pinning down the mechanisms responsible for this super-rotation will teach us much about atmospheric dynamics in general. In particular, it would be interesting to know if there are circumstances in which the Earth’s atmosphere could kick over into super-rotation.

Although the atmospheres and climates of Venus and Earth differ very greatly today, it is generally believed that the two planets started out in a rather similar state, but subsequently evolved along divergent paths. Venus succumbed early to a "runaway water vapor greenhouse," in which the increased water vapor content arising from increased temperature reached an end state with much of the ocean evaporated into the atmosphere. Once this happens, it is easy for the water vapor to decompose in the upper atmosphere, whereafter the light hydrogen escapes and oxygen either escapes or reacts with rocks. One hypothesis is that the weak magnetic field at Venus, which otherwise would protect the planet from the solar wind, is one reason for why the oxygen and hydrogen escaped faster into space. Once water is lost, the reaction that turns carbon dioxide into limestone can no longer take place, so CO2 outgassing from volcanoes accumulates in the atmosphere instead of staying bound up in the rocks. The end state of this process is the current atmosphere of Venus, with essentially no water in the atmosphere and essentially the planet’s whole inventory of carbon in the form of atmospheric CO2. Earth, in contrast, kept its water, which allowed the planet to keep most of its carbon inventory safely bound up in the crust. The amount of CO2 in the atmosphere of Venus is approximately the same as the amount of CO2 bound up in the form of carbonate rocks on Earth today.

The runaway greenhouse that presumably led to the present Venus is an extreme form of the water vapor feedback that amplifies the effect of CO2 increases on Earth. Is there a risk that anthropogenic global warming could kick the Earth into a runaway greenhouse state? Almost certainly not. For an atmosphere saturated with water vapor, but with no CO2 in it, the threshold absorbed solar radiation for triggering a runaway greenhouse is about 350 Watts/m2 (see Kasting Icarus 74 (1988)). The addition of up to 8 times present CO2 might bring this threshold down to around 325 Watts/m2 , but the fact that the Earth’s atmosphere is substantially undersaturated with respect to water vapor probably brings the threshold back up to the neighborhood of 375 Watts/m2. Allowing for a 20% albedo (considerably less than the actual albedo of Earth), our present absorbed solar radiation is only about 275 Watts/m2, comfortably below the threshold. The Earth may well succumb to a runaway greenhouse as the Sun continues to brighten over the next billion years or so, but the amount of CO2 we could add to the atmosphere by burning all available fossil fuel reserves would not move us significantly closer to the runaway greenhouse threshold. There are plenty of nightmares lurking in anthropogenic global warming, but the runaway greenhouse is not among them.

The applicability to Venus of concepts originating in the study of Earth climate is a testament to the beauty and generality of the physical underpinnings of climate science. In turn, testing the resilience of these ideas against radically different climate encountered on other planets and in the distant past of Earth serves a valuable role in helping to shake loose new ideas. We wish the Venus Express team the best of fortune for a successful mission.

35 Responses to “Lessons from Venus”

  1. 1
    Leonard Evens says:

    What effect does the extreme tilt of Venus’s axis, when coupled with the slow rotation, have on its climate?

  2. 2

    Re the article itself and “The atmosphere of Venus weighs as much as 92 Earth atmospheres, and it is almost pure CO2. This accounts for its very strong greenhouse effect. In contrast, the CO2 in the Earth’s atmosphere accounts for a mere .00037 of the full mass of one Earth atmosphere.”

    The 92 atmosphere figure is the pressure in bars, not the relative mass of the atmosphere. Venus has lower gravity than Earth. And 0.00038 is the volume fraction of CO2 in Earth’s atmosphere, not the mass fraction. For the mass fraction you want to multiply by the molecular weight of CO2 over the molecular weight of air in general.

    [Response: True enough. I bollixed that up when I changed the wording of my draft to deal with mass rather than pressure and molar mixing ratio. I also glossed over the fact that it’s the mass per unit surface area that counts, which I now took the trouble of making explicit, as well as referring to the mass of the atmosphere instead of the weight. Thanks for pointing it out. I’ve taken the liberty of correcting the numbers, but readers please note that Barton caught it. –raypierre]

  3. 3
    Duncan Young says:

    Venus’ axial tilt is only 2.6 degrees – however it spins retrograde which explains the very high number (177.4) which is published.

    The best explaination for Venus’ slow rotation is that friction from the thermal (not gravity) tides in the thick atmosphere slowed the planet down during the first billion years. Two sets of end states are predicted: prograde rotations of 77 days and retrograde solutions of 224 days. Venus picked that latter (Correia and Laskar, 2001)

  4. 4
    Steve Sadlov says:

    But what about lessons from Mars?

    [Response: We did that already: – gavin]

  5. 5
    Oliver Gause says:

    Regarding the temperature on Mercury it should be noted that the stated 440K is the mean temperature. Due to the fact that Mercury has no atmosphere and a very long day (rather similar to our own moon), the real temperatures are ranging from 100K on the night side to 700K on the day side. Not really that much more cozy than on Venus (apart from the crushing pressure there maybe). ;-)

    [Response: Very true. The 440K on Mercury is an essentially unoccupied mean (which we quoted only to make the point that it’s not the orbital distance of Venus that makes it so hot). Venus, owing to the great thermal inertia of its atmosphere, has a very steady temperature, rather like California, but hotter. –raypierre]

  6. 6
    Dano says:

    Re 5:

    And didn’t certain telescopes appear, a couple of years ago, to have found some sort of ice at Mercury’s south pole? All the more reason to reject Earth’s average temperature as a metric!!!!! ;o)



    [Response: For airless bodies like Mercury or the Moon, global mean radiation balance and global mean temperature are indeed not very informative statistics. Mars is more like Mercury than it is like the Earth in this regard. However, the heat transports and thermal inertia of the Earth’s atmosphere and ocean make the temperature uniform enough that one can do reasonably well, as a first approximation, to consider the planet as having uniform surface temperature. A 20K plus or minus variation about a 285K mean is under 10%, and as far as predicting mean surface temperatures go the error is even smaller than this would indicate since it’s only the second order nonlinear correction to the relation between Outgoing Longwave Radiation and surface temperature that throws off the calculation. Needless to say, modern calculations take into account the full geographical, diurnal and seasonal variations of Earth’s climate, but this remark shows that thinking in terms of global averages can get you pretty far. –raypierre]

  7. 7
    Ralph Hartley says:

    I’ve always wonderd why people say “the atmosphere of Venus has as much mass as about 100 Earth atmospheres, and it is almost pure CO2”.

    I would say that the atmosphere of Earth has much mass as two Venus atmospheres, and it is almost all liquid H2O, an outer layer of nitrogen and oxygen acounts for only 0.5% of its mass (or some numbers like that).

    Only by following the convention of not counting most of Earth’s atmosphere, can one say that Venus has more. Of course I follow that convention most of the time as well, but it is still only a convention.

    When the question is “why does Venus have so much more atmosphere?” it should be “why are the compositions so different?”

    [Response: You seem to be counting the hydrosphere (oceans) as part of the mass of the “atmosphere”. If one is thinking of lateral heat transport there would be something to be said for this, but as far as figuring the greenhouse effect, it’s important to keep the two separate. Water in the ocean doesn’t contribute to the greenhouse effect. Water in the form of vapor in the atmosphere does. Maybe it’s best to think of the ocean as “potential atmosphere.” –raypierre]

  8. 8
    Lynn Vincentnathan says:

    The fact that earth cannot have a permanent runaway greenhouse condition like Venus (under present solar conditions), does not mean we can’t have a finite runaway (from human control) GH condition. Currently human emissions are increasing the GH effect, and reduction in those emissions should decrease that GH effect. However, we could (correct me if I’m wrong) reach a point of warming at which reductions in human GHG emission, even to zero, would not stop or reverse the warming process, wreaking great havoc for us & causing mass extinction — which would eventually stop & reverse from natural processes after a long time, as has happened several times in the past.

    In other words, it’s possible the warming that humans are currently causing could trigger nature to increase that warming in positive feedback fashion, by reduced albedo; releasing ice-stored GHGs; increasing wind-whipped, drought induced fires; killing plants that take in CO2, etc.

    [Response: I don’t mean to minimize the importance of the other positive feedbacks you mention, many of which could have effects that could take an exceedingly long time to reverse. A lot of what you are thinking of more properly goes under the name “hysteresis” rather than “runaway.” Some examples: If you increase CO2 to 8X present and (probably) melt Greenland, it’s not clear that Greenland comes back once you drop CO2 back to pre-industrial values. Certainly, if the polar bears go extinct in the wild (and the zoo population doesn’t keep them going), the polar bears aren’t going to suddently re-evolve once CO2 drops back. If the Amazon dies off at high CO2 (unknown, but possible), it’s not clear it would grow back either. Other things you mention are positive feedbacks, but shouldn’t really be called a “runaway.” The term “runaway greenhouse” is what lawyers would call a “term of art,” which has a rather specific meaning in planetary science. It refers to a state where a planet can’t get rid of the solar energy it’s absorbing until ALL of the ocean has evaporated into the atmosphere (yielding a surface temperature several thousands of degrees Kelvin, typically). Now, to confuse the issue further, Kasting has pointed out that it’s not absolutely clear that a true runaway could have actually occurred on Venus and the uncertainty is guess what– cloud effects again! He argues that maybe a true runaway isn’t necessary, since once you get the surface temperature up to 400K or 500K there’s enough water vapor in the upper atmosphere to break down and escape even if the whole ocean never winds up in the atmosphere all at once. The story goes on. It’s one of the most fascinating open areas of planetary science. –raypierre]

  9. 9
    Lynn Vincentnathan says:

    Thanks for the term, “hysteresis” (#8). I goggled it, and it is applied (for one situation among many) to abrupt CC, due to ocean conveyor halting (and you wouldn’t apply “runaway” to that, even figuratively).

    But I do prefer “runaway” modified by “finite” or “limited” as a lay term (if not a very scientifically acurate one) for the positive feedback GW situation I mentioned, because it seems more descriptive, than the more general “hysteresis.” Also “runaway” indicates the partial similarity between Venus & Earth — Venus having a runaway GH effect that keeps on going, and Earth, ones that eventually reverse.

    Anyway, it’s good there’s a scientifically acturate term for what I have been calling “runaway” or the “Venus effect” — aware of the vast, insurmountable differences between the situation on Venus & Earth, but stressing some similarity.

    Perhaps if people in public had been using “hysteresis” in relation to the positive feedback situation I mentioned, I would have gotten used to that term. But then we would need to distinguish between ocean conveyor halt (involving N. Atlantic cooling) & the positive feedback situation of increasing overall warming.

    Also I prefer “global warming” to “climate change,” which seems to imply that climate always changes one way or the other, no big deal (esp since most people confuse climate with weather). Even “global warming” sounds much more benign that it is in actuality.

    [Response: For the situations you have in mind where hysteresis doesn’t apply, what’s wrong with “positive feedback?” Perhaps that seems to imply some kind of value judgement, in which it is presumed that the feedback does something good. Perhaps “destabilizing feedback,” or “amplification of climate senstitivity?” –raypierre]

  10. 10
    Blair Dowden says:

    On Earth, the relationship between the concentration of carbon dioxide and its greenhouse effect is logartihmic – each doubling causes the same increase in radiative forcing. I understand that is not the case where the atmosphere is extremely dense, as on Venus, because carbon dioxide absorption bands expand with increased atmospheric pressure (collisional broadening) and higher temperatures (doppler broadening).

    My question is does this happen because the atmosphere is mostly carbon dioxide, or would a smaller amount of carbon dioxide have a larger effect than on Earth if the atmosphere on Venus had the same mass but was mostly (say) nitrogen? In other words is it the density of carbon dioxide, or simply atmospheric pressure?

    [Response: These are very interesting questions. Actually, the logarithmic behavior of CO2 in Earth’s atmosphere only applies over a limited (but rather extensive) range of concentrations. At very low concentrations (say, around 1 ppm) bands are unsaturated and OLR becomes more sensitive to CO2 than in the logarithmic range. At sufficiently high concentrations (say, when you start to get around 10% or 20% of CO2 in the atmosphere) the absorption starts to be dominated by weak bands that have a different probability distribution than the bands that dominate in the present climate; this again starts to lead to an increase in sensitivity. Radiative transfer is complicated because of the complex line structure of greenhouse gases, but for a long time I have been looking for a simple, accessible explanation of the typical logarithmic behavior. I’m writing that section of my climate book now (check Chapter 4 of The Climate Book in a few months). As far as I can tell, the simplest way to put it is like this: CO2 opacity for the present Earth is dominated by the 15 micron band group. The envelope of the absorption strength in this group tails off roughly exponentially from the center of the group, once the lines are broad enough to overlap significantly within each sub-band of the interval, and the resulting probability distribution of absorption can be shown to give rise to the logarithmic behavior. However, the exponential envelope is only approximate, and only extends a certain distance out from 15 microns, so once you put in enough CO2 you get out of the logarithmic range. Hence the answer to your question, roughly, is that it depends both on pressure/temperature broadening and on CO2 concentration. To get a logarithmic behavior, you need enough pressure or temperature to make the lines broad enough to start overlapping, but if you put in too much CO2, you make the overall width of the principal absorption region (that’s not the line width!) wide enough that you get out into a different shape of envelope, and lose the logarithmic behavior. Play around with Dave Archer’s online Modtran model to get a feel for this. On Venus, the main issue is that, with so much CO2 around in the atmosphere, a lot of weak bands that are inconsequential on Earth become dominant in determining the changes in OLR. I haven’t done the calculation, but I’m guessing that a 90 bar N2 atmosphere with a millibar or so of CO2 in it would still be in the logarithmic range, since the pressure broadening would just put each of the sub-bands of the 15 micron group into the strongly overlapping regime; I don’t see that this would too much affect the overall envelope of the group, or the importance of the far outlier bands. –raypierre]

  11. 11
    Eachran says:

    I would be very interested to know what areas of climate science are in need of research funds. Please dont reply : “all”.

    I have seen references to cloud effects (post 8) before by you all and recall someone commenting that it would be useful to have a few satellites up to measure whatever it is that needs measuring – in this case I guess ; creation, movements, disappearance, density, altitude and reflectivity.

    Obviously without measuring something you are stuck, and you need the right tools to measure accurately.

    This doesnt just apply to clouds, but also surface and water temperatures, ocean circulation and the rest. One of my sons in law works on undersea surveying for bigoil and the telecomm companies : he has given me tuition on ocean temperature measurement so I know a little of what one needs to get “good” readings and it doesnt come “cheap”.

    My guess is that the general public trust climate scientists and accept that there is something “strange” going on with the climate that needs researching and fixing. (I am not of the “do nothing” school. Nor do I agree with some of your posters who believe humankind is effectively dead.)

    So just approximately, give or take a few hundred millions of Euros (or $ or £ if you prefer), indulge me a little please and tell me as a best ballpark guess (you understand the qualifications I am sure) what you most need to measure, how you need to do it and why?

    Perhaps a general post on this very subject for all would be terrific. In my view it is the right time to discuss it.

    My guess is that the brainpower for analysing the data and thinking about it, are on the net and dont require university chairs (I may be wrong).

    PS The AGW argument is now effectively won but as I said on another post : “Never let a challenge go unanswered”. We need to move on to more positive areas like understanding better and fixing it if we can. And I am sure we can fix it.

    [Response: My pro domo favorite is to understand “tipping points” (to use the buzz word): where are points in the climate system where things could go really and perhaps irreversibly wrong? Ice sheets, ocean circulation, Amazon forest, … But overall, the highest priority in my personal opinion is to spend money on fixing the problem rather than studying it – energy efficiency, renewables, etc. -stefan]

    [Response: My favored priorities are: (1) Reliable long-term monitoring of cloud properties, so that at least over the next 20 years we can get a better fix on which of the various cloud parameterizations is most accurate; (2) Intensive research on the impacts of climate change, especially regarding natural ecosystems, agriculture, and diseases — this is by far the biggest and most consequential uncertainty in predicting the future; (3) Intensified research on theory, simulation and reconstruction of paleoclimates, particularly the warm Cretaceous and Eocene climates (4) More flexible climate modelling software, better ‘intermediate models,’ and more mid-sized computers in more places so that more people can test innovative ideas; (5) Ice sheet and sea ice dynamics; (6) Research on carbon sequestration, improved IGCC coal technology and coal-to-hydrogen technology, technology for direct removall of CO2 from air, and nuclear waste disposal and proliferation-resistant nuclear fuel cycles. –raypierre]

    [Response:I would add the need for ‘seasonal forecasting’, i.e. being able to make good/better probabilistic forecasts for El Ninos occurring, for droughts in given locations (eg Africa), or for tropical cyclones. This would improve our ability to tacticly deal with problems. -rasmus]

  12. 12

    Re #3 and “The best explaination for Venus’ slow rotation is that friction from the thermal (not gravity) tides in the thick atmosphere slowed the planet down during the first billion years. Two sets of end states are predicted: prograde rotations of 77 days and retrograde solutions of 224 days. Venus picked that latter (Correia and Laskar, 2001)”

    I haven’t seen that paper, but I’d like to. The previous major paper I remember on the Venus rotation was Dones and Tremaine (1993), which proposed that a large, late planetesimal hit Venus going the wrong way. I don’t know offhand what empirical test you could make to distinguish between the two.

  13. 13
    Timothy says:

    Re: #11 – My understanding is that the principal requirements for [new] funding would be:

    * Global observing system suitable for climate trends. Much of weather observations are made primarily from the viewpoint of feeding into NWP models for short-range weather forecasts, leaving climate scientists with a fiddly job of trying to construct trends. It can sometimes be hard for systems to have long-term funding: e.g. the UK contribution to the ARGO system of ocean buoys is currently funded as part of a temporary “demonstration project”, but no-one seems willing to committ to funding it for the long term [~decades].

    * Digitisation. There are a lot of old records [eg Navy ship log books] that would be extremely valuable were they digitised. At the moment this is done in a very ad-hoc manner, with the scientists involved roping in their wives, etc, on a voluntary basis, to help with the process. This would be beneficial for looking at decadal variability of climate, which would help with climate model validation.

    * Although there are many other uncertainties that need addressing in climate science, such as cloud feedback or aerosol forcing, it is much less obvious that large increases in funding will increase the rate of progress of research in these areas.

    * I imagine that bigger and faster supercomputers will always come in handy, though.

    [Response: These are all reasonable issues, but to my mind there is a more fundamental gap in current funding. It turns out that it is quite easy to get funding within specific disciplines – like aerosol microphysics, or watershed-level hydrology. And it is also quite easy to get funding for global scale modelling (such as NCAR, GISS or the Hadley Centre). However, there is far more known about small-scale interactions that is (or even could be) incorporated into the ‘big picture’, or that feeds into the interpretation of paleo-climate data for instance. This is because it is very difficult to find people who are good ‘aggregators’, or bridge-builders between the specialists and the generalists. Very few people want to take the complexity that they have spent their careers appreciating and boil it down to a usable package for the wider community- this is as true for sea-ice specialists, cloud modellers, ecosystem researchers or atmospheric chemists. Yet the ‘big picture’ people need this information, and so often find that they need to do the simplifications themselves – and since they are not specialists, they don’t always do a good job. So if I was to suggest one focus for increased funding – it would be for translators of the specific to the general. – gavin]

  14. 14
    Duncan Young says:

    Re: #12
    The Correia and Laskar paper is in Nature with fuller descriptions in Icarus. The problem with the giant oblique impactor is that you need to drive off most of the radiogenic argon while leaving the primordial argon and CO2, and somehow avoid creating moons, and have it hit such that you are left with a very small (absolute) obliquity.
    Re: #7 An interesting wrinkle with the definition of Venus’ atmosphere is that in the lower few kilometers CO2 is no longer a true gas but a supercritical fluid – an issue, that, so far as I can tell, has not been fully explored.

  15. 15
    ocean says:

    What is a suprcritical liquid?

  16. 16
    Duncan Young says:

    Re: 15.
    Beyond what is known as the critical point on a pressure-temperature graph, there is no clear distinction betwen gas and liquid – for example, a supercritical fluid can act as a solvent, yet lacks surface tension. This makes supercritical CO2 ideal for, example, decaffeinating coffee. It is also something that needs to be dealt with in CO2 geological sequesteriation schemes.

    CO2 goes supercritical at 73 bar and 31 degrees C.

    [Response: Yes, beyond the critical point there is no longer any gas-liquid phase transition. The density just increases smoothly with pressure, and decreases smoothly with temperature. That means that it would be just as valid to think of the lower reaches of the atmosphere of Venus as a CO2 ocean as part of a CO2 atmosphere. In my mind, it’s nonetheless more like an atmosphere than an ocean, since it is still quite compressible, which has a lot of important climatic effects. The deep atmosphere of Jupiter is supercritical with respect to H2, and so one could think of that, too, as an “ocean,” as was exploited in Ben Bova’s science fiction novel Jupiter. By the way, the supercriticality of CO2 on Venus is taken into account in the nonideal equation of state used for computing things like the adiabatic temperature profile; nonideal gas effects are very important on Venus. I wonder whether it’s an accident that the surface pressure on Venus is not too far above the critical point pressure. Is there some physical or chemical process that keeps the atmosphere from being too far supercritical? Given that surface pressure is fixed by the mass of the atmosphere, any such limiting mechanism would have to involve processes in the planetary formation stage that determined Venus’ carbon inventory, or would have to involve CO2 interchange between the atmosphere and the crust and interior of the planet. –raypierre]

  17. 17
    ocean says:

    Thanks Duncan Young and Raypierre.

  18. 18
    Lynn Vincentnathan says:

    I’m wondering if Venus might have had life on it at one time, or is the rotation too slow (those nights must be killers, as well as those days). Could any of the CO2 in its atmosphere have been from organic sources?

    [Response: The normal composition of rocky planets has plenty of minerals that can decompose to put CO2 into the atmosphere inorganically. There’s never any need to invoke biology for that, and anyway biology doesn’t put CO2 into an atmosphere. It takes CO2 out of the atmosphere in the course of photosynthesis, gets eaten by heterotrophs (like us), who respirate it and put it back. Biology can greatly increase the methane content of an atmosphere, but for CO2 biology should be seen as a means of recycling CO2 back to the atmosphere, with perhaps some leakage into sequestered organic carbon (like fossil fuels). As for possibilities of early life on Venus, the time window for habilitability for Venus would have been very narrow. My own estimates are that even with no CO2 in the atmosphere, if Venus had an ocean and just a pure 1 bar N2 atmosphere, it still would have been quite close to the runaway greenhouse threshold even 4 billion years ago when the Sun was a lot fainter — close, but perhaps not so close as to close the window entirely. Without any CO2 in the atmosphere, the water vapor greenhouse would give Venus a global mean temperature of 300K. The slow rotation isn’t too much of a problem, since a global ocean would even out the day/night cycle. A half billion to a billion years later, though, Venus would have hit the runaway greenhouse threshold. The more CO2 there is in the atmosphere, the smaller the window of habitability. As I mentioned earlier, nobody is too sure yet what clouds do to the the runaway greenhouse threshold, and that would affect the habitability window as well. –raypierre]

  19. 19
    Ian K says:

    Forgive me playing devil�s advocate here but I would think that a denialist/skeptic would have grounds for complaint about your discussion of the dynamics of Venus vis-à-vis life on Planet Earth.

    You say: *the upper atmosphere is whizzing along at a rate of about one rotation every four Earth days. Pinning down the mechanisms responsible for this super-rotation will teach us much about atmospheric dynamics in general.* Surely a skeptic as to the worth of current climate models would counter: *here is a simple system where they don�t have to worry about the problems of water vapour, patchy cloud cover,oceans, methane, aerosols, etc. If their models are so good, surely they should be able to plug Venutian values into their models and at least demonstrate rotation, if not a correctly quantified super-rotation*.

    Sorry if such an argument is off the planet but I am still a bit mystified as to exactly how climate models work despite having an initial read of prior postings on models.

    [Response: A fair enough question, and not at all “off the planet.” In fact, Tony Del Gennio did achieve super-rotation in a Terrestrial general circulation model by reducing the planet’s rotation rate and increasing the upper air cloud thickness (which stabilizes the atmosphere and somewhat isolates it from drag against the surface). The basic mechanisms do seem to operate in a conventional GCM, but they depend on the way eddies generated in the model accelerate the flow. It’s therefore a long way from reproducing super-rotation in an idealized model to saying that the same mechanism is really happening in Venus (we also have Titan as an example of super-rotation). Nobody has yet achieved a full general circulation model of Venus which could be used to test such hypotheses, because doing the radiative transfer needed for the thick CO2 atmosphere is very computationally demanding and by conventional means can’t be done fast enough to interact with the requirements of doing the fluid dynamics. One also needs to learn a lot about how to model the chemistry and optical properties of the Venusian cloud deck. New ideas are coming out, and people are hard at work on this problem. –raypierre]

  20. 20
    M. Simon says:

    Re: Greenland,

    Some of the old Viking colonies are under 6 ft of ice. The polar bears will just have to adjust if temperatures there return to “normal”.

    BTW there has been evidence of global warming on Mars thought to be due to increased solar output. Yet during the same time frame (the last 6 to 10 years) the Earth temperatures have either not changed or have gone down slightly. So we have increased solar output, increase CO2 and yet temperatures have stalled. No model has predicted this.

    Me? I worry about the coming ice age. Plants do not grow well under ice.

    I propose mirrors in space.

  21. 21

    Does anyone have an equation of state for supercritical CO2? I’d love to put it into my RCM and see if it makes a difference. Still trying to get that surface temperature up…

  22. 22
    Hank Roberts says:

    M. Simon — what’s your source for your belief in ‘global warming on Mars’? Did you trust it, or did you check the footnotes and find out what the basis was for the claim? If it lacked footnotes, try here:

  23. 23
    Blair Dowden says:

    Re #10 – Thank you, RayPierre. You answered my question at just the right level of detail. I am not ready yet for the differential equations of your chapter four, but maybe someday.

    [Response: And maybe someday I’ll actually finish Chapter 4. –raypierre]

  24. 24
    Coby says:

    Some of the old Viking colonies are under 6 ft of ice. The polar bears will just have to adjust if temperatures there return to “normal”.

    …a quick reality check shows that Greenland’s ice cap is hundreds of thousands of years old and covers 95% of that island, so just how different could it have been only 1000 years ago?

    Greenland was called Greenland by Erik the Red (was he red??) who was in exile and wanted to attract people to a new colony. He believed that you should give a land a good name so that people want to go there! It very likely was a bit warmer when he landed for the first time than it was when the last settlers starved due to a number of factors, climate change a likely major one….

    BTW there has been evidence of global warming on Mars thought to be due to increased solar output.

    Warming on another planet would be an interesting coincidence but it does not necessarily have to have the same cause. The only relevant factor the Earth and Mars share is the sun, so if the warming were real and related it would have to be due to the sun. The sun is being watched and measured very carefully back here on earth and it is not the primary cause of the current climate change.

    As for this alledged finding, there is very little evidence to go on when it comes to discerning a global climate change on Mars. The only evidence out there that I am aware of is a series of photographs of a single icey region in the southern hemisphere…

    Yet during the same time frame (the last 6 to 10 years) the Earth temperatures have either not changed or have gone down slightly.

    …This argument is clearly lacking in substance. 1998 was a record high year, and according to NASA GISS, it was elevated .2oC above the existing trend line by the strongest El Nino of the century. Choosing that year as a starting point is a classic cherry pick and demonstrates why it is necessary to remove the very chaotic inter-annual variability that exists in the annual mean…

    So we have increased solar output, increase CO2 and yet temperatures have stalled. No model has predicted this.

    No Yes No. Yes

  25. 25
    Joel Shore says:

    Re #20 (M. Simon)– Response #24 by coby covered this well. I’d only add a few things:

    First, as an addendum to the issue of solar mechanisms, since direct solar forcing doesn’t work, some scientists have tried to propose more exotic solar mechanisms such as the sun’s affect on intergalactic cosmic rays and their affect on clouds. However, besides the fact that the trend in the cosmic rays doesn’t seem to fit the observed warming, the problem with such mechanisms is they rely on specific aspect of the earth’s atmosphere that seem like they would be unlikely to hold in Mars’s much different and thinner atmosphere. Thus, an observed correlation between warming on Earth and Mars probably could not be explained by them.

    Second, in claiming that warming has not been observed recently, I assume that you are probably referring to Bob Carter’s recent piece ( ) that has been wildly touted in certain circles. However, Carter’s claim is laughable once you look at the actual graph that he makes the claim from: It is clear that the general trend is still up…and while it is true that the warmest year is still 1998 according to this CRU / UK MET data (although NASA data shows 2005 just barely beating out 1998 for the record), it is one clear outlier and the last 4 years occupy the 2nd – 5th positions amongst the hottest years in the instrumental record. His claim is basically akin to me denying the existence of seasonal cycles because it was colder here in Rochester last Saturday than it was in most of January!

  26. 26
    Lynn Vincentnathan says:

    I think I’m going to go with “Venus effect” for the positive feedback GW situation that may happen here on Earth under present trends (& has happened in the past during some of our mass extinctions). It’s short & catchy, and gives some information (assuming people know Venus is a planet, and that it’s closer to the sun, etc.), or at least a basis for curiosity, discussion, and argument about in what ways Earth’s situation is like that on Venus, and in what ways is it different. And I do think we need a quantum leap in arguments here from whether or not GW is happening, to whether or not a Venus effect will be starting (or perhaps has started).

    And I figure the term is no worse a metaphor than “greenhouse effect” – people don’t really think there is a glass enclosure surrounding Earth or Venus. (Greenhouses are also called “hothouses,” and perhaps we should have used the term “hothouse effect” since it is more acturate and “green” has lovely connotations of plants & grass.) And people are smart enough to understand that Earth is not Venus, only a bit like it.

    Then maybe we can use “hysteresis” for the reversal part of the Venus effect (getting back to habitability), as in “Now [it would be thousands of years from now if we are indeed headed toward a Venus effect] we’re in the hysteresis phase of the Venus effect” (assuming there are people with culture still persisting).

    So there’s the runaway phase and hysteresis phase of the Venus effect (I hope I’m not totally killing off the scientists here with fits of incredulous laughter.

    BTW, RayPierre, you’re right about the words “positive feedback.” Most would think it is something good — I only learned about it in grad school when we were studying a Mexican village where a rich guy replaced a negative feedback rule that kept people more or less equal, with a positive feedback one that made the rich richer & poor poorer. And I remember my college roommate getting her cancer test result back marked “negative,” & she went into “hysteresis.”

  27. 27
    C. W. Magee says:

    Dear Ray:
    A couple of questions: First, would a runaway H2O/CO2 greenhouse on Venus heat the atmosphere enough for H2O to escape as a molecule? If so, that ought to give you a maximum possible temperature.

    Secondly, to what extent does water dissociation play a part in the oxidation state of the Earth’s surface? I know that I’ve been told that photosynthetic critters are responsible for the early Proterozoic oxidation, but it seems to me that you need to sequester a tonne of organic carbon for every 12.5 tonnes of ferrous iron that goes ferric. There’s a whole lot of early Proterozoic ferric iron lying around- shales, BIF’s, and terriginous sediments in addition to the economic deposits- but I’ve never heard of anyone finding contemporaneous carbon deposits. And that doesn’t even start to take into account the sulfates, manganese, and free O2 in the atmosphere today. Got any good references on this topic?

    [Response: I’ll have to check Kasting’s paper and the follow-ups to see what the possibilities are for H2O to escape as a molecule. It’s not something I’ve thought much about yet. The Rayleigh-Jeans escape calculation is easy to do, but the real trick is to compute the homopause temperature and to figure out what the solar wind will do to your outer atmosphere. There have even been some recent disputes about the H2 escape rates for Early Earth (see the paper by Tian et al in Science a year or so ago) so there’s a lot of room for surprises and basically only a small handful of people working on a problem that has enough open ends to occupy several universities. As for photodissociation and oxidation of Earth, at present photodissociation is unimportant compared to photosynthesis, but for the Early Earth you have to have some oxygen around before there’s any incentive for respirators to evolve. Current thinking seems to be that this happened by photodissociation. Check Liang et al in a few months, in PNAS, for an interesting story about how PaleoProterozoic Snowball episodes may have aided oxygenation. As for the carbon deposits, remember there were no land plants at the time, so the buried carbon would have been in the sea floor, which has almost all been subducted since. There’s a paleoproterozoic session at Spring AGU, and I hope to learn more about current research there. –raypierre]

  28. 28
    David B. Benson says:

    “Venus is dead” By this a geologist meant that the radar mapping of the surface of Venus shows no signs of plate tectonics. Persumably a radar mapping of Terra and Mars at the same resolution would.

    I have some difficulty with the thought that there are no convention cells in the interior of Venus. So is it the surface conditions and atmosphere that somehow mask the surficial results of magma circulation? Is the actual composition of Venus that much different from Terra?

    [Response: You raise several very interesting points here. First, would one be able to detect plate tectonics by radar mapping of Earth? If one had a map of the ocean bathymetry, one could certainly infer plate tectonics from the mid-ocean ridge and so forth. Radar doesn’t give you that, but the shape of the continents, and the distribution of topography (together with some estimate of weathering rates) would give you some information about tectonic processes. Now, on Mars, the cratering pattern, which gives the age of the various terraines. tells you with fair certainty that the crust hasn’t been moving around for a very long time. Some intriguing streak-like patterns in the residual magnetic field of the crust led some people to conjecture that there was plate motion in the distanct past, but the evidence is hard to interpret, and is in dispute. Now, as to your final remark, about Venus — Mars is “dead” tectonically, but Venus is anything but. One must distinguish “plate tectonics,” which refers to a certain kind of crustal motion, from mantle convection. Venus has very vigorous mantle convection, which can be seen in Hawaii-like surface features arising from mantle plumes, and a kind of orange-like wrinkling and rifting of the crust. Some planetary geologists even say the crust is young, and was absorbed by the mantle and regenerated within the last half billion years. There has been a lot of good thinking about why Venus has no plate tectonics while Earth does, but the matter is unresolved — one of the great challenges of comparative planetology. Many people think it has something with liquid water oceans, which change the mineralogy and may play some role in lubricating the plates and keeping a thin oceanic crust. Google Mark Jellinek and read almost anything by him to get a view from a real expert. I’m just a dabbler. –raypierre]

  29. 29
    David B. Benson says:

    Raypierre, thanks for taking the time with your modestly labeled ‘dabbler’ reply. Clearly the geologist’s remarks I read were mistaken. For the record, I should have said that “Mars was, in the past, tectonically active.” There are big extinct volcanos, for example.

  30. 30
    Jeff DeLaune says:

    Your website is extremely informative and useful in helping us understand the direction and implications of increasing levels of CO2 in the atmosphere. It may be helpful, at least as a sidebar, to understand part of the picture of what it means, from a technology perspective, to actually limit CO2 emissions for the electric industry. For that, I direct you to a recent presentation from Steve Specker, President of the U.S. based Electric Power Research Institute (EPRI). His presentation describes the technology options, the cost implications, and the technology advances necessary to limit CO2 levels to 550 parts per million.

  31. 31
    Nomad says:

    What’s to be gained from the study of the Venusian climate, surely its difference from Earth lies in the slow rotation speed. A comparison of the two is like apples and bananas?

    [Response:Even apples and bananas are subject to the same laws of physics/nature. The fact that they are different gives us a wider case-range for testing our knowledge. -rasmus]

  32. 32
    Jim Baerg says:

    RE: comments 20 & 24 & Greenland.

    I just read Jared Diamond’s book _Collapse_ which devotes a chapter to the Norse Greenland colony. According to Jared, the Norse settled in what were then & are now 2 of the greenest & most pleasant parts of the island. The settlements didn’t get overrun by glaciers, but the norse agriculture was marginal enough there that a slight temperature decrease was enough that a several cooler summers in a row were enough to cause starvation, especially when combined with more sea ice preventing trade with Europe.

  33. 33

    […] the normally Gore supporting says, “Is there a risk that anthropogenic global warming could kick the Earth into a runaway […]

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    […] is quite possible that genuine runaway climate change is not possible on Earth – that the existing feedbacks are of the bank lending rather than the nuclear blast variety. That […]

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    […] global warming could kick the Earth into a runaway greenhouse state? Almost certainly not. RealClimate And if you disagree with the above, consider that the earth’s CO2 levels millions of years ago […]