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
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
What disturbs me is the following;
Personally I havent given Venus much thought before reading Steven Goddards posts here at WUWT. Why should I have. I have enough other thinks to worry about on a daily basis.
But now, reading about Venus, thinking about it, the grey cells working…over at Stephen Wildes post…it stands clear to me, at least for now….that, hey, it makes sense!
And then the disturbing thought;
Surely J. Hansen must know this too? Surely? I mean, if its your job to use the grey cells on such matters?
Surely?
Haha, love the title! Now I’ll go back and read the article.
When you compress or decompress a gas, it either warms or cools. Due to the temperature differential to the surroundings, after some time it will cool or warm by exchanging heat with the surroundings until the temperature differential is approaching zero and hence the temperature of the gas approaches that of the surroundings.
Why should Venus behave any different?
Steve,
You are correct on all the cases you quoted, but the basic reason Venus is hot is in fact due to the presence of greenhouse gasses combined with the high pressure. It is not how much of the gas is greenhouse gasses, but the fact that there is at least some present (even <1% CO2 or water vapor and 99% N2 would do about the same). If there were no greenhouse gas at all, but the gas pressure were still high, a lapse rate would still be present (but modified due to real gas effects and greatly different temperatures), but the surface of Venus would then be the location where all of the radiation to space would balance incoming absorbed energy. This would make the surface close to Earth's temperature, and the lapse rate would result in a very cold upper atmosphere. The presence of even a small amount of greenhouse gasses moves the location of the radiation that goes to space out to the edge of the atmosphere, and the lapse rate heats the surface. The main difference is where the radiation to space occurs.
Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more.
I am always puzzled by the statement that the atmospheric pressure on the surface of Venus is a large multiple of that on earth.
The MW of earth’s atmosphere is about 29, and that of Venus around 44. But gravity at the surface of Venus is only 0.904 that of earth.
Atmospheric pressure is the result of the weight of a column of gas above the surface pressing down. I won’t embarrass myself by trying to calculate the effect of higher MW and lower force due gravity on Venus, and compressibility of the gas(es), but I would expect the atmosphere of Venus to extend out a large multiple of the distance that it does on earth to produce an atmospheric pressure 92X that on earth. But I have not heard that the Venusian atmosphere extends out so far…..
So, the question remains why the atmosphere is so “thick”. It is not constrained to a fixed volume, but can expand as the mass of gas is increased.
A further niggle comes from the statement: “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.”
At 0K all gases would have condensed to liquid or solid, but the mass of the gas would still be present as a layer on the surface. I would think that the liquid/solid would be as heavy (=attraction due to gravity) as the atmosphere from which it was produced, and possibly a tiny bit higher since the molecules would be concentrated at the surface instead of extending a distance above the planet and thus minutely farther from the center of attraction of the mass of the planet.
It’s always been clear to me from the timeline of the discoveries on Venus and the genesis of the global warming panic that a strong argument can be made that the scare aspects of global warming (its original and most honest name) are an example of a massive case of medical students’ disease. James Hansen got his nose so deep into Venus he couldn’t see his way out. Just because it’s not real, doesn’t mean it doesn’t seem real. The mind is a powerful thing and can overwhelm logic when placed under enough stress. Ask any medical student if this has ever happened to them.
Leonard Weinstein
You are correct.
Earth already has plenty of greenhouse gases, so my point is that even if earth went to 100% CO2, the maximum temperature increase would be less than 36C – nothing like Venus. (In reality, it would get cooler because of the loss of water vapour.)
Ian l. McQueen
You can infer how thick the atmosphere of Venus is from this diagram:
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
Ian l. McQueen
Liquids are not very compressible and don’t heat up much under pressure. Consider the bottom of a swimming pool.
On Earth we have water that Venus does not have. We have a wet lapse rate that is less than a dry lapse rate because of the processes of evaporation and condensation. Condensed moisture also controls the atmospheric concentration of CO2. If we had no atmospheric moisture, CO2 concentrations could be higher and the atmosphere could be heavier. If Venus ever had water like Earth, it must of boiled off because it was too close to the Sun and not in the sweet zone that sustains life here.
John caten
Title is Anthony’s idea. He also added the ESA and Venera content.
One of the great things that NASA did was the planetary explorer missions. Without comparison to other planets, there would be little hope of understanding Earth. It is unfortunate that some have hypochondriated that splendid knowledge into doomsday scenarios. Earth, like the other planets, has been as it is for at least the last billion years. The foundation for what separates Earth from the other planets is Carbon-based life that transformed the place. Else, who knows what it might have become, or not become. Better start thinking about taking care of Earth’s precious Carbon, as geologic forces are relentlessly conspiring to sequester it.
I’m thinking of a Carbon Appreciation Day.
In addition to pressure, Venus also has high concentrations of other greenhouse gasses such as carbon monoxide. If one of these gasses absorbs at the frequency corresponding to earths atmospheric window then it could have a significant effect on temp. I also wonder if atmospheric pressure could explain the faint sun warm earth paradox.
Leonard Weinstein says:
May 8, 2010 at 2:38 pm
… The presence of even a small amount of greenhouse gasses moves the location of the radiation that goes to space out to the edge of the atmosphere, and the lapse rate heats the surface. The main difference is where the radiation to space occurs.
That’s interesting Leonard. It would seem to a layman like me that a small amount of green house gas would radiate a small amount of energy back into space at the effective altitude of the gas. Are you saying that just a small amount of GHG at some altitude implies a large amount of re radiated energy is leaving the planet at that same altitude? I’m having trouble understanding that non-linear relationship between gas density and radiated energy.
Willis- the lapse rate of earth’s atmosphere is NOT the lapse rate of a dry atmosphere.
In fact, the profile of earth’s atmosphere is that of a refrigerant system because of the working fluid water.
Venus has no such phase change gases in its atmosphere so it has the dry lapse rate.
The lapse rate of earth’s atmosphere is definitely not that of a dry atmosphere.
The are completely different- not similar at all.
The modeling of Venus and earth atmospheres was reported (1,2): The authors agree adiabatic lapse rate is the crucial physical process. They point out three things
1) the adiabatic exponent for a hypothetical CO2 atmosphere replacing a nitrogen oxygen one of the same mass would result in a cooling of 6.4C.
2) Adding Carbon to oxygen in the atmosphere makes it slightly heavier, in principle resulting in tiny warming
3)Ultimately the CO2 dissolves in seas forming carbonate rocks, oxygen is thus lost from the atmosphere and atmospheric mass drops causing (tiny)cooling.
They conclude:
“Accumulation of large amounts of carbon dioxide in the atmosphere leads to the cooling, and not to warming of climate…. This conclusion has a simple physical
explanation: when the infrared radiation is absorbed by the molecules of greenhouse
gases, its energy is transformed into thermal expansion of air, which causes convective
fluxes of air masses restoring the adiabatic distribution of temperature in the troposphere.”
FWIW I am not a qualified climate scientist, take my reading with a pinch of salt.
1)
Environ Geol (2008) 54:1567–1572
Response to W. Aeschbach-Hertig rebuttal of ‘‘On global forces
of nature driving the Earth’s climate. Are humans involved?’’
by L. F. Khilyuk and G. V. Chilingar
2)
Energy Sources, Part A, 30:1–9, 2008
Cooling of Atmosphere Due to CO2 Emission
G. V. Chilingar a; L. F. Khilyuk a;O. G. Sorokhtin b
a Rudolf W. Gunnerman Energy and Environment Laboratory, University of Southern California, Los
Angeles, California, USA b Institute of Oceanology of Russian Academy of Sciences, Moscow, Russia
Leonard Weinstein says:
. . . but the basic reason Venus is hot is in fact due to the presence of greenhouse gasses combined with the high pressure. It is not how much of the gas is greenhouse gasses, but the fact that there is at least some present . . .
The basic reason Venus is so darn hot is that it is so darn close to the sun. The high albedo moderates the temperature. The difference in composition affects the adiabatic lapse rate, but swapping Venus’ atmosphere with the same weight of air would give you no clouds and a much hotter planet.
The difference in lapse rates boils down to the gas molecules. Air is mainly diatomic, while Venus is triatomic CO2. The specific heat ratio Cp/Cv for air is ~1.4, CO2 is ~1.3, and that affects the temperature change as you move a parcel up and down, how much energy goes to ‘work’ as opposed to what’s left for temperature. Triatomic water vapor, our main ghg, has a Cp/Cv of ~1.3 like CO2. Methane, the next ghg suspect, also has a Cp/Cv of ~1.3. Sounds like a conspiracy.
Bottom line is that things on Venus are quite in accord with what you’d expect, no “runaway greenhouse” effect. I must acknowledge, though, that with no weather, Venus has solved the problem of climate change.
Mike McMillan
There isn’t a large difference in lapse rates, and the albedo of Venus is very high, so it’s distance from the Sun has little effect on temperature.
Troels,
The atmosphere is not static and conductive heat transfer to surroundings is small. Here’s a description:
“An adiabatic temperature change occurs in a vertically displaced parcel of air due to the change in pressure and volume (refer to the gas equation in section 1.2) occurring during a short time period, with little or no heat exchange with the environment. Upward displacement and consequent expansion causes cooling; downward displacement and subsequent compression causes warming. In the troposphere, the change in temperature associated with the vertical displacement of a parcel of dry (i.e. not saturated) air is very close to 3 °C per 1000 feet, or 9.8 °C / km, of vertical motion; this is known as the dry adiabatic lapse rate [DALR]. As ascending moist air expands and cools in the adiabatic process, the excess water vapour condenses after reaching dewpoint and the latent heat of condensation is released into the parcel of air as sensible heat, thus slowing the pressure-induced cooling process.”
Ian l. McQueen
Yep. I think the pressure going to zero argument is an oopsie. The pressure is determined by mass and gravity, and so is fixed (except for some small dynamic effects due to weather). It’s the volume and temperature of the atmosphere that vary.
Another great post. I have been trying to go through similar ideas in the Climate Sceptics group for a while myself.
May I suggest a couple of interesting links:
(1) UTexas teaching notes about why there is an adiabatic lapse rate at all (hint: there is no mention of any GH effect)
(2) From the New Mexico State University a table about lapse rates in various atmospheres in the Solar System (and showing that composition might matter, alongside gravity)
In particular, the lapse rate is roughly g/Cp, where g is the acceleration due to gravity and Cp is the atmosphere’s specific heat at constant pressure (cp) divided by the molecular weight. It would be interesting to find out why exactly Cp for Venus would only be 85% of Earth’s.
Forgot to close the hyperlink in previous comment. Apologies.
Why is the atmosphere on Venus 92 times more dense than on Earth? That is the big question, that I don’t understand.
Now, if the atmosphere is denser – it means there are 92 times more molecules of gas in the same volume (or layer) of the atmosphere, therefore they would absorb 92 time more outgoing radiation (much more anyway). So, the heat on Venus’ surface is caused by the greenhouse effect after all, by the fact that the greenhouse effect is much stronger there, due to the density of the atmosphere, and not necessarily due to it’s composition (Co2).
If pressure alone could cause heating – why isn’t the bottom of the oceans very hot?
Steve,
You have almost got it now — here is the relevant question:
For Earth and Venus (as you note) the adiabatic lapse rate is similar; For Earth and Venus, the planetary equilibrium temperature is similar (Venus is closer to the sun but has a higher albedo). So now the question: To what altitude does the atmosphere mix vertically from the surface close to the adiabatic lapse rate (note on Earth that the true lapse rate is ~6.5o/km, e.g. the atmosphere is slightly stable)? You assume in your posts that it must be to the same pressure — but where does this assumption come from? On Earth, the temperate at 50 km is ~250K, if your 10K/km held to this altitude, the surface of Earth would be 750K — clearly something amiss. The answer is that at the tropopause OLR balances the incoming solar: outgoing energy (emission) from the atmosphere by cooler greenhouse gases and from the warmer surface (in the IR windows) balances the solar headed down.
So the important question for Venus is why is the tropopause at 60Km? You make the assumption that it is because this is (approximately) the same pressure as the tropopause on Earth. But there is no physical reason why the tropopause must be at the same pressure on other bodies. As on Earth, the tropopause is where the atmosphere must mix vertically to achieve thermal balance (e.g. read very nice discussion of radiative-convective models by Manabe). If instead of being made of IR absorbing constituents, the atmosphere was made only of N2 (even if it was as massive), the surface temperature would be much colder (and there would be a large day-night difference).
There is not much to this (at zeroth order) beyond the first law of thermodynamics in a compressible (nearly-ideal) gas.
“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.”
The accepted view is that present day Venus is a result of a runaway greenhouse, she got too warm, her oceans boiled, the resultant water vapour created a super GH effect, which melted the planets crust, the oxygen released from ionize H2O combined with crustal carbon to create CO2. The planets surface is hot because of its present atmosphere, its atmosphere is a product of a runaway greenhouse.
“we have a sun providing energy to the periphery of the atmospheric system”
Not sure what you mean by this, the sun’s EM radiation is absorbed by atmospheric constituents throughout the depth of the Venusian atmosphere, not at the “periphery”.
“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”
The surface pressure is almost entirely independent of the temperature, assuming that all the atmospheric constituents remain suspended.