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
Just why are there clouds? Sunlight hits the top of cloud, some scattered, some absorbed, clouds evaporate.
Why does Venus have clouds during the day time? For clouds to form a liquid must have been heated, evaporated, rose up through the atmosphere and then cooled as the pressure drops.
So, why are there clouds?
I want to point out that the oceans of the Earth hold about 40 times as much Carbon compounds as the atmosphere, and that came from dissolved CO2. In addition, the carbonate compounds in the solid land surface and sea floor of the Earth contain about 40,000 times as much Carbon as the atmosphere, and that Carbon came from life on early Earth forming Carbonate shells and organic debris (and oil and coal) from early CO2, and that this debris was sequestered on the bottom of oceans and on land. The waste product of plants taking in CO2 is Oxygen. This implies that there was once mostly CO2 in early Earth atmosphere, and much was removed over time by living organisms. Additional atmospheric material was probably removed in the collision between Earth and another planet that resulted in the formation of the Moon. There is no big mystery why Earth has little CO2 and both Venus and Mars are mostly CO2. The differences in quantities depend mainly on the size of the planets and formation details.
An experiment:
We drop a spherical probe onto Venus, weighted on the bottom so it rolls right-side up. The probe is a hollow ball well insulated with asbestos (gotta use that stuff somewhere!) and with a large IR transparent window on top. The window is insulated against convection and conduction by having multiple panes, argon filled from a high-pressure canister and a pressure regulator.
The temperature on Venus’ surface is in balance, so the heat gained from downward radiation plus the heat gained from the atmosphere by convection equals the heat lost to upward radiation and the heat lost to convection.
The internal temperature of the well-insulated probe will stabilize to a value determined only by the radiant energy balance. If the probe is cooler than the outside atmosphere then the surface is losing more radiant energy than it’s gaining, which means Venus surface must gain heat from atmospheric convection. If the probe runs hotter than the outside atmosphere then the surface must be losing heat to the atmosphere by convection. The probe’s results should allow some numbers to be placed on the surface radiation balance.
Unfortunately, finding funds for a probe that might remove Venus’ status as a dire warning to Earthlings, and thus cause us to defund all future Venus missions, might be a bit difficult.
Darn it. That was in reply to Nick Stokes
Richard Sharpe
May 8, 2010 at 6:55 pm
I was under the impression that it is average kinetic energy within a volume.
DesertYote
Yes, you’re right – the dry lapse rate wouldn’t change, but would operate over a shorter distance as the gas cooled and contracted. But it doesn’t change my point that a N2 atmosphere would cool as the surface cooled in a way that just tracks that cooling. It doesn’t constrain the surface temperature.
Ian l. McQueen says:
May 8, 2010 at 2:40 pm
“……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.”
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At absolute zero…. No molecule is in a gasseous state…. It is solid and inert…. even with no pressure…. This is a theoretical limit of course…. for Absolute zero has never been observed….. So a planet with a 100 percent nitrogen atmosphere would have a surface of solid nitrogen at 0K. There would be no sublimation… Because no molecule has any energy left.
That’s my understanding anyway.
stevengoddard says:
Steve- check out a basic HVAC manual- there is a reason nobody designs a heat pump with a gas that doesn’t change phase.
Next, calculate the enthalpy of the constituents of air = be generous and assume 1% h20 and 500ppm co2 – do the math and see who carries the joules. The water does more than 50,000 times the work of the co2. Nevertheless, co2 does a tiny bit of work- and that only increases the efficiency of the heat flow from boiler to radiator.
Examine the profiles of earth – overlay the charts with precipitation and IR radiation.
If you don’t know the difference between the lapse rate of dry air and the actual measured lapse rate on earth which is much different, then look it up. Dithering on distractions doesn’t change these 2 fundamental facts:
Earth has a heat pump with water as the working fluid.
Any increase in efficiency of the working fluid in a heat pump (like adding some heat capacity the way CO2 does) only slows the flow for the same work. Measure and calculate the work.
At least understand that thermodynamics is about heat and that is not measured in degrees. Heat is NOT measured in degrees and there is no way to convert degrees to watts. NOT possible.
You want to study heat, you have to actually speak about heat. This has been hugely ignored for a reason.
Meanwhile- every time you suggest that the CRU and the rest are unscientific- let me clear this up once and for all:
They are brilliant psychologists and have the best and brightest marketing psychologists money can buy. You only imagine they aren’t scientific because the sad sorry truth is that you have been, are being and will keep on being duped because they are smarter than you where it matters in this game. You just haven’t really got a grip on what the game is. You’ve been outsmarted and never really stood a chance. You still argue about weather.
Strut and fret and in the end – you get shafted because you weren’t watching the pea, you were watching TV.
Troposphere, shmoposphere – clouds, rain, snow.
Once it was ‘don’t talk about religion or politics’ and it was safe to talk about the weather because nobody got offended and everybody knew something to say.
Admit that it was a brilliant maneuver to suck common man into war over the most innocuous topic. Look at the ROI. Look how well it worked and continues to work.
No, sir, they are extremely scientific and the science is valid – the results prove it.
How long ago was venus a molten ball, man? And how many trillions will it cost to play make-believe?
You don’t need a weatherman to know which way the wind blows – Dylan.
Sorry for the long winded rant. It’s been building up. Nothing personal – I’m looking at where the train is about to go dig a dirt road and the passengers chat and gossip and that bothers me because I did my Dostoevsky period long ago and didn’t want to revisit it in america, in real time.
Hugo Chavez is a piker compared to the creeps stealing america. But hey- some evolution is about to happen and guess who will get the darwin awards- it won’t be Hansen, Schmidt and Jones. Who’s the unscientific one? Well, who has the whose money?
I think I can rest my case.
Steve Goddard says:
May 8, 2010 at 7:27 pm
Mole is a non-dimensional number. A mole of H2 weighs much less than a mole of UF6
The mole is the SI unit of the amount of substance, it is not non-dimensional, a mole of a substance has a mass equal to the substance’s molecular weight.
Steve Goddard
I usually agree with your postings. This one I am having trouble with. Adiabatic Lapse Rate is about temperature changes as air masses move up and down in altitude. Environmental Lapse Rate is what I think you are trying to describe. That is the temperature lapse rate with altitude. That is what you get when the air is not moving. Yes, I realize the atmosphere of Venus is probably moving up and down. However, that movement balances out producing a net neutral effect with respect to pressure change.
Consider a tall column of perfectly insulated gas. Given sufficient time for temperatures to stabilize in the column, there will be no difference in temperature top to bottom. Though at a higher pressure, the bottom of the column will not be warmer than the top. Physics show us that the gas at the bottom is at a higher energy level because of the greater potential energy associated with the higher pressure. That does not mean, however, that the gas molecules at higher pressure exhibit greater kinetic energy than the those at lower pressure.
For planetary atmospheres, the Environmental Lapse Rate is a function of the energy lost versus gained from radiation and conduction at a given altitude. Pressure figures in only in the density of the molecules available to absorb and release energy and whatever phase or chemical changes may occur.
We should actually be calling what is happening on Venus a “blanket” effect, not a “greenhouse” effect.
Dave McK
You can stop hyperventilating now. The average measured lapse rate on Earth is a little lower than the dry lapse rate. About 6.5. On Venus the average measured lapse rate is also a little lower than the dry lapse rate at 8.5. Not much difference.
Your argument doesn’t provide much useful information about why Earth is 20C and Venus is is 485C.
Jacob says:
May 8, 2010 at 4:14 pm
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?
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You are confusing gasses and liquids Jacob… Seems a few others are doing the same. Gasses are compressible, liquids are not….. Compress a gas enough, and it becomes a liquid…..
Compressing a solid or liquid results in a crystalline structure I believe, but only under immense pressure…..(?)
Gary W,
If the atmosphere on Venus stopped circulating, then I agree that the compression effects would dissipate. But the atmosphere there is constantly rotating in two giant Hadley cells. The air rises near the equator, moves to the poles, descends, and moves to the equator again, so it is always undergoing cycles of compression and expansion. This means it is also undergoing cycles of adiabatic heating and cooling.
“You are ignoring the fact that Venus has almost 100% CO2 and it’s lapse rate is the same as Earth’s up to 60 km. The empirical evidence does not agree with your theory.”
I am not saying that the temperature contribution of GHGs comes from changing the lapse _rate_. I am pointing out that to use the lapse rate to determine the surface temperature, you have to know both the temperature at the tropopause and the altitude of the tropopause. GHGs (and the resulting optical thickness/depth/opacity) change the altitude at which the atmosphere emits to space. Once you know that altitude, and the temperature at that altitude, _then_ you can use the lapse rate to calculate the surface temperature.
A totally transparent atmosphere will result in a planet that emits to space from the surface, and therefore the temperature of the surface will be in radiative balance with the incoming radiation from the sun. As you add GHGs, the height at which the atmosphere emits to space will increase, and the temperature of the surface increases. There is not some magic concentration of GHGs at which Venus suddenly jumps from -46 degrees C (the temperature of the surface given the existing albedo and no greenhouse effect at all: yes, this requires assuming clouds that exist at that temperature and reflect incoming light but don’t trap outgoing light, but this is a thought experiment) to 450 degrees C.
Replacing 90% of your CO2 with Folgers Crystals (I mean, N2) would change the optical thickness, and therefore the height at which you emit to space, and therefore the surface temperature EVEN IF THE LAPSE RATE REMAINS CONSTANT.
“Mars is even colder than earth despite having a 95% CO2 atmosphere, because it’s atmosphere is very thin.”
No, Mars is even colder than earth despite having more CO2 than earth because it is a lot farther from the Sun, and the additional radiative forcing from the CO2 is not enough to make up for the lower solar radiation.
Look: there are now two models of the Earth + Venus that we’re discussing: the “Standard Model” which depends on GHGs, and the “Goddard Model” which apparently requires some non-zero amount of GHGs but mostly depends depends on pressure. Why do I prefer the Standard Model, if both models apparently get the surface temperature right? I prefer the Standard Model because using the same set of physical laws it can also explain the mesospheric, stratospheric, and thermospheric temperature profiles. It can explain why the tropopause changes altitude and temperature depending on latitude. Effectively, it is a sophisticated model that has been used by atmospheric and planetary scientists for decades and survived all their tests. Is it perfect? Well, it doesn’t, for example, tell us what Climate Sensitivity should be, but for problems like explaining Venusian, Terran, and Martian temperature profiles given existing GHG concentrations, clouds, solar radiation, etc. it does really well – whereas the Goddard Model can only explain temperatures from the surface to the tropopause, and only if you tell it where the tropopause is and what temperature the tropopause is.
George Turner says:
May 8, 2010 at 7:54 pm
George, I think you are on the right track, all you need is a downward sensor facing the crust in the shadow of the craft and an upward sensor sensing radiative energy from above and compare the two.
Willis, in the earlier thread, suggested that the surface pressure was such that a super critical fluid would occur, the images above would suggest otherwise.
What about the role of upper atmosphere sulfuric acid clouds? They are dense and heavy, right? Can they act like a glass of plastic film in a real greenhouse? http://astronomyonline.org/SolarSystem/VenusIntroduction.asp says “Extremely high atmospheric winds – 360 km/h – yet mild breeze near the surface”. This implies there is little heat exchange via convection near the surface. So, it’s like a car with rolled up windows in a parking lot under direct sunlight–it gets unbearably hot inside because hot air cannot escape. High atmospheric pressure and denser atmosphere is just the result of this.
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.
No it doesn’t, all it tells you is that the temperature gradient through the atmosphere is approximately the same. The actual temperature reached depends on the boundary condition, it doesn’t follow that the thicker atmosphere will be warmer. The composition does have a controlling role, a nitrogen atmosphere of similar pressure and thickness on Venus would result in a cold planet (surface ~230K).
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.
No, this does not follow!
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.
No, the temperature difference from surface to tropopause would be the same, not the actual temperatures.
This and the associated thread show quite clearly that an IR-opacity of the atmosphere is the cause of atmospheric heating, i.e. ‘the greenhouse effect’.
Titan has a thicker atmosphere (mostly N2) than Earth yet is much colder, surface T 95K.
To get the atmosphere on Earth as dense as Venus, I think we would ‘need’ something like a nearby supernova to flash-boil the ocean. If most of the energy received went into boiling the water, then the total mass of the planet should remain about as it was. I do not know if the atmosphere would cool and the oceans would reform after this event or if the new condition would remain stable. It would be a moot point.
Ugh, stop quoting Wiki as a source guys.
Nick Stokes
May 8, 2010 at 8:40 pm
Got it. Sorry for not seeing the point you were trying to make to begin with.
Phil.
You must have forgotten your high school chemistry.
A mole is most definitely a non-dimensional number. It is 6.023 * 10^23 molecules.
One of the interesting properties of gases (Avogadro’s law) is that they all occupy about the same volume at the same pressure and temperature, regardless of molecular weight. That is why the ideal gas law does not have a term for mass.
http://en.wikipedia.org/wiki/Avogadro's_Law
Avogadro’s law (sometimes referred to as Avogadro’s hypothesis or Avogadro’s principle) is a gas law named after Amedeo Avogadro who, in 1811,[1] hypothesized that “Equal volumes of ideal or perfect gases, at the same temperature and pressure, contain the same number of molecules.” Thus, the number of molecules in a specific volume of gas is independent of the size or mass of the gas molecules.
Hansen presents a dust insulation model as the cause of Venus’ high temperature in his 1967 article:
http://pubs.giss.nasa.gov/docs/1967/1967_Hansen_Matsushima.pdf
stevengoddard says:
George Turner –
Gas pressure P = nRT/V . There is no term for either mass or gravity.
stevengoddard says:
MaxL –
Gas pressure is created by the motion of molecules. As you approach absolute zero, molecules cease to move. So there is no pressure. Volume has to remain above zero, because the molecules have a finite size. So yes, it is that simple.
Like special relativity, the gas pressure equation works well in the absence of gravity. Given the temperature and pressure at the surface of the earth, we should be able to calculate the volume of the atmosphere. V = nRT/P
I must be doing it wrong, because I get different volumes for toasty sea level Houston and cool mountainous Salt Lake City. I do get the same surface pressure, though.
Mike McMillan
The ideal gas law works just fine, and gravity has nothing, nada, zippo to do with the accuracy of the equation.
You are comparing different temperatures and pressures in Houston, so you simply plug in different numbers into the equation. The ideal gas law works just as well in Timbuktu as it does in Salt Lake City.