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.”
======================================================
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
Mike M
As I have stated about 14 times now, the atmospheric pressure (in temperature ranges where it is a gas) is set by the weight of the atmosphere above it. P is fixed by the weight of the column of air above.
Venus has a large value of P because it has a large value of n. The compressibility of gases is not linear, and as a result, T is high on Venus.
PV = nRT
The main reason why earth is cooler than Venus is because we have oceans which remove CO2 (term n in the ideal gas law) from the atmosphere in the form of limestone. This causes the atmospheric pressure to be much lower and thus the temperature, as specified by the ideal gas law.
PV = nRT
OK, let me see if I can state what Steve says in a way that makes sense to us engineer types: We have three main processes producing a higher surface temperature on Venus than Earth. The first, of course is solar energy influx greater than that of Earth. The second is a blanketing layer of thick clouds that has a very high albedo. That cloud layer not only reflects most solar energy away from the planet but also reflects most energy from below them downward. That is the same mechanism we experience here on Earth that causes clear, starry nights to be colder than cloudy nights. The third is vertical motion of the atmosphere produced by some as yet poorly defined energy source that causes rising gases to cool and falling gasses to warm. The environmental lapse rate is primarily the result of this bi-directional vertical motion within Venus’s thick atmosphere.
I don’t know if I agree with the above description. I’ll have to dust of my calculator and see if the numbers work out.
DavidB
Is the purpose of a bicycle pump to increase the pressure or the temperature? You drive the pump in order to increase the pressure, and a side effect is that the temperature increases too because of the non-linear compressibility of gases.
My 7:41 post was not explained properly wrt how n relates to limestone removing CO2.
The existence of limestone reduces the mass of the atmosphere, which reduces P, which in turn reduces T.
A reply to Ralph’s question dated May 10, 2010 at 12:16 am:
> Just one question about the Venera pics – why did they never ‘normalise’ the
> colours? They have a colour chart there for comparison, but I have never seen an
> image of the surface as it would look under Earth-like light conditions.
Have a look at http://www.solarviews.com/raw/venus/v13corr.jpg – the surface looks black like lava.
Further discussion about the color of light on Venus is available at Don Mitchell’s excellent site: http://www.mentallandscape.com/C_CatalogVenus.htm
RE: Ralph: (May 9, 2010 at 11:59 pm) “You don’t need convection for an adiabatic lapse rate – it is purely gas density/pressure that makes the gas hot. I have adiabatic warming in my bicycle pump, but no convection.”
Apples and Oranges — an adiabatic lapse rate and adiabatic warming are two different things. At ground level we often have stable inversion conditions where a layer of cold air at the surface is under relatively warm air above. Also the stratosphere is characterized as a zone where temperature increases with altitude. Adiabatic cooling can only happen if a relatively warm parcel of air rises to a region of lower pressure, where it expands and cools as a result. That is convective cooling in a nutshell. If there are no rising or falling parcels of air then there can be no adiabatic cooling or warming.
I believe convection stops at the tropopause because that is the approximate altitude where the atmosphere finally becomes so thin that the greenhouse effect is unsupported and air that has risen to that level is finally able to release or relay its cargo of heat by direct radiation to space. This would suggest that the convection process might act as a safety valve to prevent ground temperatures rising above the lapse-rate span to the tropopause.
Nick Stokes, in re May 9, 7:49 PM
Yes I did mean to address you Nick. So thanks for setting me straight on that surface black body radiation thing (no, seriously). It’s the MechEs who get trained in that radiative heat transfer stuff (steam boilers and all that). Not my specialty.
Fascinating stuff, science. I could do this all day if someone PAID me for it.
GaryW
A much simpler empirical exercise would be to answer qualitatively this simple question.
What would happen to temperatures on Venus if you could strip away 99% of it’s atmosphere (making it like earth?)
Hint:
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
Thinking about the insulating property of the atmosphere as a function of thickness got me to thinking about insulating a house. Certainly the R-value goes up as the thickness of the fiberglass goes up. It also reminded me of the poor insulating properties of windows. Typically they put Argon in the higher grade double panes, but according to AGW theory wouldn’t you want to fill them with CO2? That way the windows could reradiate the heat back into the house. So if this doesn’t really work for double pane windows, should we expect a few ppm to work in the atmosphere?
Mike M
The ideal gas law is independent of gravity. That does not mean that any of the individual terms in the ideal gas law are independent of gravity. For instance, atmospheric pressure (P) is determined by the weight of the column of air above it.
P = cT/V where c is a constant
If P increases due to increased mass above you (i.e. you are descending through the atmosphere) then the ideal gas law tells us that the ratio of T/V must also increase. And because gas compressibility is not linear with pressure, we can then infer that T has to increase.
I have read ALL of your posts, Steve, which is what prompted me to try and explain the basic Physics of the situation, which I am eminently qualified to do. You keep bouncing around like a pinball, reversing cause and effect, using equations in ways that aren’t valid, and making one loose assertion after another, and then changing them to other things that are even looser, as has been pointed out by many posters before me. This discussion is no longer even remotely scientific. It’s more like the stupidity my kids used to get up to in the back seat of the car. “Do too!. Do not! …..” Bah……
It is time for me, at least, to go spend my time doing something more productive.
/dr.bill
Steve Goddard, in re May 9, 7:55Pm, “extremely unlikely that Venus was ever like Earth” “The atmosphere must have always been thick and hot”.
Nobody even knows what the primeval atmosphere of even Earth was like. There is no proxy measure for surface pressure. The latest theory I heard is that Earth originally had very little atmosphere after it accreted and that bombardment with comets is what provided the water, O2, N2, CH4, etc. In that case, Venus being closer to the sun would be even more dependent on comets as a source of gases. There is no way to know its atmospheric composition or pressure at the end of the Late Heavy Bombardment.
I’m just repeating the narrative of the “runaway GH theory” of Venus for those who haven’t heard it (apparently). “Once the hypothetical ocean started to boil, that increased the water vapor GH effect, leading to more boiling, more GH, more boiling, and the [so-called] runaway greenhouse”. Since nobody really knows what Venus’s primordial atmosphere was like, this is merely “an” hypothesis.
“Venus didn’t have limestone.” Initially Earth didn’t have limestone either, supposedly, but I have not heard any account from a single geologist or planetary scientist suggesting that Earth’s atmospheric pressure was every more than a few atmospheres, so what was the form of all the carbon that must have been here after the Late Heavy Bombardment if it wasn’t in the atmosphere?? Let’s just agree that nobody knows. (And by the way carbonate rock can also form from chemical weathering and precipitation, not just seashells.)
Steve Goddard, in re may 9, 10:08PM
“The idea that Earth could become like Venus was a central point of Sagan’s TV show – Cosmos. A whole generation was corrupted by this idea.”
Sorry, Steve, Earth COULD become like Venus. It’s not a corruption. (Sagan had a TV show???) As the sun converts hydrogen into helium, it’s core becomes denser and hotter thanks to the changing kinetics and thermodynamics of the fusion reaction, and as a result the sun is gradually becoming hotter over the gigayears. It is estimated to have been 30% cooler at the time of Earth’s formation (which presents all kinds of questions as to why the Earth was not therefore an ice-ball then, but let’s not get into that).
Over time, the sun will continue to get hotter, at a slowly accelerating pace, and it is estimated that in about a billion years it will be hot enough to raise the ocean temperature enough to create the runaway greenhouse effect of Venutian notoriety thanks to the increasing water vapor concentration leading to increasingly higher GH effect, more boiling, more GH, until the oceans are boiled dry. This would create a surface pressure of almost 250 atmospheres, and stop plate tectonics dead in its tracks.
After that, it may take a very very long time, much longer than it “MAY HAVE” on Venus thanks to Earth’s protective magnetic field, for solar radiation to dissociate all the water vapor and strip away all the hydrogen from the atmosphere, but eventually it will happen. And all the free oxygen that is liberated by this will have to oxidize something. First all the (dead) biomass will turn to CO2 (adding still more to the weight and pressure of the atmosphere) long before all the water vapor has been stripped of hydrogen, and eventually maybe it will get hot enough to bake all the CO2 out of the carbonate rock, aka limestone. Plate tectonics having been stopped, anything that comes out of any remaining volcanos will stay in the atmosphere. We’ll probably even wind up with sulfuric acid clouds.
Fortunately, AGW is nowhere NEAR strong enough to make this happen.
Steven, again thank you for taking your time doing this post. It has really started the gray cells working.
I have read the post many times now, and its starting to sink in. Most people here in Norway remembers PV=NRT .
The problem I think is this lapse rate stuff in combination with the ideal gas law.
Its the notion that we all know that if you compress a cylinder with a piston, we know that temerature must go up.
Because PV=NRT.
But after this compression ….thats the point I got problems with your post.
At first.
Then its the lapse rate. I understand now what you are saying.
I think.
You say that hot molecules migrate upwards, and radiates heat to space.
And sinks.
So the sun (with gravity as helping hand) is acting almost like a continous piston….
Jbar
Regardless of the early history of either planet, it is unlikely that Venus was ever like current day earth because there is no limestone on Venus.
dr.bill
Your appeals to your own authority and non-specific complaints don’t impress me.
kwik
You are correct that there has to be a a continuous external heat source to keep the gradient from equalizing. I’m not attempting to explain cause in this article, just pointing out that there is a vertical temperature gradient through Venus atmosphere which corresponds with pressure.
Jbar
Yes, when the sun becomes a red giant the polar ice caps are going to melt, but that has nothing to do with what we are discussing.
The greenhouse effect can not turn earth into Venus because of all the CO2 trapped in limestone.
Count me as also unimpressed with dr. bill. He sounds like a perfumed academic coasting on tenure when he says he’s “eminently qualified.”
dr. bill doesn’t understand that science is done by batting around ideas until only the conclusions that remain standing are accepted as valid. That’s what we’re doing here. But dr. bill can’t stand the heat.
Anthony is pretty good about publishing articles. Since dr. bill is so eminently qualified, maybe he’d like to write up a rebuttal, and see what it’s like to be on the receiving end of the pot shots.
dr.bill
If you disagree with me, then please use your eminent qualifications to demonstrate that Venus high temperature is unrelated to it’s atmospheric pressure.
David L.
The thickness of CO2 you can put between two window panes would not absorb enough IR to make any difference. There’s not enough mass. Besides, if I’m not mistaken glass reflects IR. [Which is supposedly why greenhouses get hot, but actually it’s because the hot air is physically sealed in by the glass and not allowed to convect with the rest of the atmosphere!!! So “greenhouse effect” is actually a gross misnomer!!! What happens in a greenhouse is COMPLETELY different from what happens in the atmosphere due to greenhouse gases!]
They put argon in windows “because it reduces convection currents”, but I suspect the effect is pretty small. Air Products and Air Liquide must be sitting on piles of argon because it’s left over from making liquid nitrogen and liquid oxygen (it’s 1% of our atmosphere).
Smokey
Good points. I have yet to see anyone argue against the central thesis that the key difference between the temperatures of Earth and Venus is the atmospheric pressure. Some people are getting bogged down in specific details that have way too many degrees of freedom to define in a simple model.
Steve Goddard:
“all the Co2 is trapped in limestone”
So how do you know that when the water vapor/ biosphere oxidation greenhouse raises Earth’s surface temperature to 523.15C that it won’t cause massive amounts of buried limestone to rise to the decomposition temperature of 825C, thereby releasing CO2 into the rock and atmosphere??
Where are your computer models?
Jbar ,
The IPCC estimates that each doubling of CO2 raises earth’s temperature by 3C. That means a maximum possible increase of about 36C.