By Steve Goddard
ESA’s Venus Express mission has been studying the planet and a basic atmospheric model is emerging.
Venus has long been the CO2 bogeyman of climate science. In my last piece about Venus I laid out arguments against the claim that it is a runaway greenhouse which makes Venus hot. This generated a lot of discussion. I’m not going to review that discussion, but instead will pose a few ideas which should make the concepts clear to almost everybody.
If there were no Sun (or other external energy source) atmospheric temperature would approach absolute zero. As a result there would be almost no atmospheric pressure on any planet -> PV = nRT.
Because we have a sun providing energy to the periphery of the atmospheric system, the atmosphere circulates vertically and horizontally to maintain equilibrium. Falling air moves to regions of higher pressure, compresses and warms. The greater the pressure, the greater the warming. Rising air moves to regions of lower pressure, expands, and cools. The amount of warming (or cooling) per unit distance is described as the “lapse rate.” On Earth the dry lapse rate is 9.760 K/km. On Venus, the dry lapse rate is similar at 10.468 K/km. This means that with each km of elevation you gain on either Earth or Venus, the temperature drops by about 10C.
It is very important to note that despite radically different compositions, both atmospheres have approximately the same dry lapse rate. This tells us that the primary factor affecting the temperature is the thickness of the atmosphere, not the composition. Because Venus has a much thicker atmosphere than Earth, the temperature is much higher.
dT = -10 * dh where T is temperature and h is height.
With a constant lapse rate, an atmosphere twice as thick would be twice as warm. Three times as thick would be three times as warm. etc. Now let’s do some experiments using this information.
Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more. Now suppose that we could instantly change the molecular composition of Venus atmosphere to match that of Earth. Because the lapse rate of Earth’s atmosphere is very similar to that of Venus, we would see little change in Venus temperature.
Experiment #2 – Now, lets keep the atmospheric composition of Venus constant, but instead remove almost 91/92 of it – to make the mass and thickness of Venus atmosphere similar to earth. Because lapse rates are similar between the two planets, temperatures would become similar to those on earth.
Experiment #3 – Let’s take Earth’s atmosphere and replace the composition with that of Venus. Because the lapse rates are similar, the temperature on Earth would not change very much.
Experiment #4 – Let’s keep the composition of Earth’s atmosphere fixed, but increase the amount of gas in the atmosphere by 92X. Because the lapse rates are similar, the temperature on Earth would become very hot, like Venus.
Now let’s look at measured data :
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-p.gif
Note that at one Earth atmospheric pressure on Venus (altitude 50km) temperatures are only about 50 degrees warmer than earth temperatures. This is another indication that atmospheric composition is less important than thickness.
Conclusions : It isn’t the large amount of CO2 which makes Venus hot, rather it is the thick atmosphere being continuously heated by external sources. It isn’t the lack of CO2 on Earth which keeps Earth relatively cool, rather it is the thin atmosphere. Mars is even colder than earth despite having a 95% CO2 atmosphere, because it’s atmosphere is very thin. If greenhouse gases were responsible for the high temperatures on Venus (rather than atmospheric thickness) we would mathematically have to see a much higher lapse rate than on Earth – but we don’t.
WUWT commentor Julian Braggins provided a very useful link which adds a lot of important information.
“The much ballyhooed greenhouse effect of Venus’s carbon dioxide atmosphere can account for only part of the heating and evidence for other heating mechanisms is now in a turmoil,” confirmed Richard Kerr in Science magazine in 1980.
The greenhouse theory does not explain the even surface temperatures from the equator to the poles: “atmospheric temperature and pressure in most of the atmosphere (99 percent of it) are almost identical everywhere on Venus – at the equator, at high latitudes, and in both the planet’s day and night hemispheres. This, in turn, means the Venus weather machine is very efficient in distributing heat evenly,” suggested NASA News in April 1979. Firsoff pointed out the fallacy of the last statement: “To say that the vigorous circulation (of the atmosphere) smooths out the temperature differences will not do, for, firstly, if these differences were smoothed out the flow would stop and, secondly, an effect cannot be its own cause. We are thus left with an unresolved contradiction.”
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An update for those interested in what Venus looks like at the surface.
On March 1, 1982, the Soviet Venera 13 lander survived for 127 minutes (the planned design life was 32 minutes) in an environment with a temperature of 457 °C (855 °F) and a pressure of 89 Earth atmospheres (9.0 MPa). The photo composite above shows the soil and rocks near the lander.
Here’s another Venera image that shows a hint of yellow atmosphere. – Anthony
To Nick Stokes:
OK, let’s say the surface emits 12,000 W/m2. Yet Venus only radiates 300-400 W/m2 from the top of the atmosphere, and the net radiation from the surface must be on the order of 70 W/m2. (That is 16% of the radiation received by Earth’s surface, which is what my source says reaches the surface of Venus.)
Doesn’t this prove how dramatically large the greenhouse effect is on Venus??!! The surface radiates 12,000 W/m2, but 11,930 W/m2 gets reflected back down to it by the atmosphere!!! That is a SUPER-GREENHOUSE.
Jbar,
The radiation between two surfaces (or a surface and a gas layer) at different temperatures will have both an outgoing radiation from the surface and an incoming absorbed radiation from the gas layer. The NET flux is the difference, but the fact of the outgoing radiation is valid. The surface of Venus does radiate about 12,000 W/msq (assuming near unity emissivity), but absorbed back radiation is also very close to the same level due to the absorbing greenhouse gasses back radiating. The process is actually much more complicated, both due to the specific active wavelengths in the gas and due to it being an extended thickness effect. The emissivity and absorptivity of the ground may not be real close to 1 but probably is not too far off at the ground temperature and long wave radiation wave lengths. I think your disagreement is more one of semantics, not truth vs false.
Marc77:
Another thing about supercritical fluids – the supercritical point is the point at which the transition between liquid and gas disappears. There is no longer liquid and gas – there is only this supercritical fluid thing.
Something weird must go on at the altitude in Venus’s atmosphere where the pressure makes the CO2 supercritical.
I appear to have mis-linked a citation in the above post:
Deconflicting the conflation …
Tectonic Effects of Climate Change on Venus
F. S. Anderson and S. E. Smrekar, 1999
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/18365/1/99-1842.pdf
Title:The Thermal Balance of the Venus Atmosphere
Authors: Crisp, D. & Titov, Dmitri
Issue Date: 1995
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/32268/1/95-1621.pdf
Unfortunately, this scan is unreadable.
Bjorn & others: You haven’t heard the story of the “runaway greenhouse”?
Supposedly a long time ago Venus may have been like Earth, but being so close to the sun the oceans got hotter until the ocean started to boil. Water is a good greenhouse gas, so once the water started to boil, it made the greenhouse effect stronger, boiling more water, which made the GH effect stronger still, etc. and the greenhouse “ran away” as the oceans completely boiled away.
After that, there might have been 100 atm of water vapor in the atmosphere for some time. With water vapor reaching all the way to the top of the atmosphere, solar radiation could knock the hydrogens loose from water. Venus’s gravity is strong enough to hold onto oxygen but not hydrogen, so the hydrogen gets kicked out into space. Eventually almost all the hydrogen from the water got stripped away from Venus.
Meanwhile all the free oxygen (created by the stripping away of hydrogen from water) reacted with anything it could near the hot surface, especially carbon. Eventually this turned all the carbon on Venus into CO2. 90 atmospheres of CO2 makes a strong enough greenhouse to heat the surface to 470deg C.
Some evidence – what little water vapor there is left in the upper atmosphere is rich in deuterium. Deuterium is twice as heavy as hydrogen and so it’s harder for solar radiation to knock into space, so it gets more concentrated as the hydrogen goes away faster.
OkieSkeptic:
Venus has about 3% nitrogen, equivalent to roughly 3 Earth atmospheres of partial pressure. Earth in contrast has 0.8 Earth atmospheres partial pressure of nitrogen.
More like radiative transfer as you are assuming(I think) no convection in your column.
You are right on the last point, a point I feel is the Achilles Heel of Steves conjecture. If Steve is correct, then he has invented the Venusian equivalent of perpetual motion.
Hi Again Steve,
As I said a little earlier, I don’t know the answer myself, but I would give serious thought to the suggestion made by:
Jbar: May 9, 2010 at 5:26 pm
It makes more sense than anything I have previously seen.
/dr.bill
Xyrus
You wrote ” as you approach zero kelvin your atmospheric pressure approaches infinity, not zero. “
PV = nRT
Pressure is proportional to temperature, not inversely proportional as you claim. Now start over.
Xyrus
If you think pressure is inversely proportional to temperature, try sticking an aerosol can in the oven. Actually, don’t try that. You will probably get hurt.
Nick Stokes
Your heat transfer analogy doesn’t work, because the amount of required work also increases by 90X.
The key issue is wind velocity, because that limits how far the differentially heated air can move in a given amount of time.
MaxL-
The gas law applies to gases. As the temperature drops you get a change in phase and the law no longer applies. If Venus were moved out beyond Pluto, where there would be minimal heat from the sun or any other source, the temperature would start to drop. The volume of the atmosphere would decrease. When the temperature reached ~200 degK the CO2 would change to a liquid or solid, depending on the pressure, and eventually all turn into solid CO2 as the temperature fell. The other trace gases would condense out at their characteristic temperatures too, eventually turning almost all the atmosphere into solid.
The ESA work shows that Venus has a very strange atmosphere and climate compared to earth, primarily due to the lack of water. Venus lies just sunward of the habitable zone and apparently lost all of its water, probably even as it formed.
Bob_FJ and Steve, if pressure could create heat it would mean gravity is not a conservative force. Energy could be extracted from gravity and converted to heat via the “Goddard PV = nRT” relation.
If Steve was right you would have to take an oxygen tank with you to the Arctic because low temp would mean low pressure. Low pressure systems would have lower temperature than high ones. Etc.
If it’s not greenhouse I think the answer lies more along the lines of: 1) There’s a lot more internal planet heat than given credit for. 2) There’s heat generated somehow from the solar wind and Venus gets twice as much of that as Earth.
Xyrus says:
May 9, 2010 at 6:11 pm
Only if the gov’t is trying to bail out the atmosphere (it’s too big to fail!) by adding more air. Otherwise, the pressure at ground level doesn’t change, as the weight of the air above doesn’t change. The height of the atmosphere shrinks until it turns to liquid or ice, then it still shrinks, but a lot less quickly.
Jbar says: May 9, 2010 at 6:16 pm To Nick Stokes:
Did you mean to address that to me? What you say has been my argument from way back.
Paul Clark
Try again. Low temperature means low volume in Antarctica. The atmosphere is thinner at the poles.
PV = nRT
I really suggest that everyone pass a high school chemistry class before trying to tackle this problem.
Jbar
It is extremely unlikely (impossible) that Venus was ever like Earth, because there are no limestones on Venus. The atmosphere must have always been very thick and hot.
Steve Goddard
“If there were no greenhouse gases or clouds (as Wikipedia implied) temperatures would be very cold.”
Well, what they said was that they would be Earth-like. Maybe that’s high-ball. But the estimate you put up against the Wiki statement was 400C. And you said, with emphasis:
“The high temperatures there can be almost completely explained by atmospheric pressure – not composition.”
And here
“Your heat transfer analogy doesn’t work”
I wasn’t making an analogy. I was just pointing out that at 90 atm, even a 10 km/h wind can transfer a lot of heat. Heat transfer goes as mass flux, not velocity.
The ideal gas law and Avagadro’s Law have been around a lot longer than I have.
The ideal gas law has four degrees of freedom (pressure, volume, number of molecules, temperature.) You can’t infer what one variable will do in response to a change in another, without also knowing what the other two are doing. For the sake of these discussions we can assume that n is constant. So the three degrees of freedom are P, V and T which are related by P = T*c/V where c is a constant.
If we assume in a given atmosphere that vapour pressure P is fixed through a reasonable range of temperature – then as T increases V also has to increase, and as T decreases V also has to decrease.
ALL: Concerning the assertions above of high emissions from the surface, per Stefan-Boltzmann, I suggest that the calculation is incorrect, as discussed below:
a} Imagine a block of dry ice. It will emit a low level of EMR as a consequence of its T. Now let surface melting occur; what happens? Does it emit from the newly constituted solid surface, straight through the liquid, or does the thin liquid become a new effective surface, or what?
(Incidentally, if the wet surface behaves like a body, and the solid beneath continues to emit in the same way, then a wet body would emit more than a dry one…. Without considering evaporative cooling, if any).
b} If a dry body is immersed in say an Earthly thin gas, having a lower temperature, there will not only be EMR emission resulting in net HEAT loss, but also HEAT loss via conduction and convection. Where does this “extra energy” come from without upsetting the balance of the EMR? Or, to put it another way, the surface molecules are busy losing energy as EMR, (photons), and as HEAT, (KE). Can they do that without affecting each other?
c} Perhaps b} can be ignored in a thin gas, but what if the gas is at high pressure, and approaching a liquid state? (either absorbent or transparent)
d} What if we substitute a liquid in c}? We are back to a}, except the liquid is thicker/deeper.
As an engineer, I’ve puzzled over this stuff for many years but have not found it described anywhere. (with modest looking, admittedly).
Here is one typical assertion from above:
“…it is still true that the surface emits (about) 12,000 W/m2. The S-B law applies at that level. Of course, many wavelengths are then quickly absorbed in the real atmosphere. But they were emitted.”
I suggest that this is incorrect. Also, I believe that conductive/ convective/ advective surface heat loss must be very high on Venus for Steve‘s hypothesis to work. Can anyone show that these suggestions are wrong? (without simply appealing to authority)
Nick Stokes
If you have 90X as much thermal flux and 90X as much mass, they cancel each other out. Wind can move heat no faster than it’s velocity.
As far as atmospheric composition goes, it makes little difference on Venus if it has 5% CO2 or 95% CO2. It is only the first few tenths of a percent that are really important to the greenhouse effect (relative to the importance of pressure.).
Steve Goddard
“The atmosphere is thinner at the poles.”
This is getting surreal. The air pressure at the N pole surface is basically the same as elsewhere, and the density is higher (colder air). The S Pole has lower pressure, but nothing to do with the ideal gas law – just altitude.
You’re getting tied in knots by talking about volumes in a continuum situation. You’d do better to use the ideal gas law in the form:
P=(R/M) ρT
where ρ = density and M=molecular weight. (R/M) is constant.
Wouldn’t be because of the rotation of the earth would it?
I’m surprised that people are still arguing about this. You can see very clearly that temperature increases steadily downwards through the Venusian atmosphere.
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif‘
If the atmosphere were thinner (like earth) the temperature would be much lower. Does anyone actually doubt this? This isn’t theory. It is observational data.
Nick Stokes,
I said “can be almost completely explained by atmospheric pressure – not composition.”
Given the presence of a minimal amount of greenhouse gas, the pressure becomes dominant. The whole point of these articles is to demonstrate that Earth could not become like Venus, unless all the limestones dissociated. That would require a catastrophic event which would kill all life on earth anyway.