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
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Stephan says:
May 8, 2010 at 4:32 pm
OT: One gets the distinct impression from recent posts here, at CA, and RC etc, that interest in the subject (AGW), is finally waning. Would be interesting to follow the sites statistics recently, to confirm/deny this. Basically the skeptics/deniers were right.. weather, aka climate is not changing due to AG. AGW news stories have certainly dipped. We can all go back and get a life instead of looking at graphs of temps, ice, polar bears, anxiety due to AGW, etc.. Of course a few diehards will keep banging away at it until they too, just give up, a good ol hahaha and LOL!
Staphan, in an ideal world you would be right. But, unfortunately, many governments have accepted the IPCC reports as gospel truth and have enacted legislation designed to reduce CO2 emissions. They will do nothing for the climate but WILL do great harm to the economy. It is as if we were on a hill overlooking a railway line on which a train is headed at high speed for a bridge that is no longer there. We know that a trainwreck is inevitable, but there is nothing that we can do to stop it.
IanM
Jeff Green
There is no question that CO2 absorbs infrared.
stevengoddard says:
May 8, 2010 at 4:48 pm
George Turner
Gas pressure P = nRT/V
There is no term for either mass or gravity.
Steven-
The term “R”, the gas constant, has units of 8.314472(15) J·K-1·mol-1
stevengoddard says:
May 8, 2010 at 4:48 pm
George Turner
Gas pressure P = nRT/V
There is no term for either mass or gravity.
Steven-
The term “R”, the gas constant, has units of 8.314472(15) J·K-1·mol-1
To me, “mol” implies “mass”, and in a gravitational field it will have weight. Doesn’t this contradict your statement?
IanM
Steven,
Gravity doesn’t appear in PV=nRT, but it does appear in F=ma. The mass is the mass of the atmosphere and the acceleration is due to gravity. That gives the downward force an atmosphere exerts on the surface (minus a small error because gravitational acceleration decreases with altitude).
Given a planet’s surface area S, the average surface pressure must be F/S = mg/S.
Venus atmophere’s mass is approximately 4.8×10^20 kg and its surface gravity is 8.87 m/sec^2, so the force it exerts on the surface is 4.26×10^21 Newtons. This is spread across a surface area of 4.6×10^8 square km, or 4.6×10^14 m^2, so the pressure is 4.26×10^21 N/4.6×10^14 m^2 = 9.26×10^6 N/m^2 (Pascals), which is 1343 psi or 91.4 atmospheres.
Even if the atmosphere condenses to a liquid or freezes to a solid, the pressure on top of the rocks is still going to be 91.4 atmospheres.
🙂
Ian L. McQueen
Joules is a unit of energy
Degrees Kelvin is a unit of temperature
moles is a non-dimensional number – 6.022 x 10^23 molecules
None of these are units of mass or gravity.
Steve,
I agree with your claim about the lapse rate explaining in part why the surface of Venus is so hot, but the lapse rate does not disprove the existence of a greenhouse effect, and I believe it is only part of the story about the high surface temperature.
First, a technicality – your dry lapse rate only applies for an adiabatic process where there is no condensation or heat transfer. It is an upper limit. The actual lapse rates on both Earth and Venus are lower than their dry lapse rates.
In your previous post “Hyperventilating on Venus”, you presented this plot (http://www.globalwarmingart.com/wiki/File:Atmospheric_Transmission_png) showing the transmissivity of various gases in Earth’s atmosphere. It shows that the 380-some ppm of CO2 in Earth’s atmosphere is enough to block all infrared radiation from 13 microns to 17 microns wavelength. Combined with the infrared opacity of Earth’s water vapor, that still leaves a broad window of atmospheric transparency to infrared between 8 microns and 13 microns, enough to keep the Earth at its current comfortable temperature range.
However, on Venus, the CO2 concentration is the equivalent of 89 million ppm, relative to Earth’s atmosphere, or 234,000 times the optical thickness of CO2 on Earth. As a result, the atmospheric transmissivity to infrared on Venus is quite different from the plot linked above.
I got into this Venus argument with a friend last year and to satisfy myself of the effect of CO2, I subscribed to Spectralcalc.com for a month to run some numbers. I was able to replicate the CO2 curve in the Atmospheric Transmission plot for Earth, and I found that for Venus, the absorptive range of the gas broadens so much to other wavelengths that the Venutian atmosphere is opaque to infrared from below 2 microns up to 28 microns except for three relatively narrow windows of transparency at 2.3, 3.4, and 5.7 microns, even for only 5 kilometers of atmospheric depth (near the surface). Also, my sources indicate there is enough water vapor in the Venutian atmosphere (somewhere above 50 km altitude) to effectively block infrared emissions above 28 microns.
This shows the Venutian atmosphere is way more opaque to infrared than Earth’s atmosphere and does indeed reveal a strong greenhouse effect on Venus. Even the lower atmosphere, while efficiently passing infrared from 28 microns and higher (because there is virtually no water vapor below the clouds) is still almost opaque from 2 to 28 microns and presents a substantial impediment to radiative cooling of the surface.
As I said in the beginning, I agree with your claim that the lapse rate helps to explain partly why the Venutian surface is so very hot, but I think that lapse rate is not the whole story. With the surface receiving only 1/6th the sunlight that the Earth’s surface receives, something has to act as a barrier to slow the rate of radiation from the surface to the much cooler haze layer at 30-50km altitude, and I think that something is the CO2 infrared opacity. I’m no atmospheric scientist or physicist and I lack the tools to model this, but I suspect that if CO2 were transparent to infrared the Venutian surface would be a lot cooler than it is, but still substantially hotter than the upper atmosphere thanks to the lapse rate.
Heat from below ground cannot possibly explain the high surface temperature as some have suggested. On Earth, the heat radiating from the ground is only 1/10th of a watt per square meter, and even if it is ten or 100 times that on Venus, this is still dramatically less than the visible light reaching the surface.
[Interesting factoid: On Earth, refraction or bending of sunlight makes the horizon light during dusk and dawn even though the sun is not up. Well according to one source, on Venus the refractive effect from the thick atmosphere is so much stronger than on Earth that even the darkest region of the dark side still gets some glow from the sunlit side.]
I disagree on the PV=nRT analysis Steve. Assuming the gas remains a gas, the pressure remains constant at the surface because we must obey conservation of mass. Given a control column of a given mass of atmosphere, open at the top to vacuum space, the pressure at the surface will not change** with temperature, (** ignoring, as another poster above rightly pointed out, differences in gravity WRT altitude), because it’s weight remains the same. So it’s ‘V’ that changes not ‘P’. As temperature decreases, the height of the column will decrease thus decreasing the volume.
Speaking of radiation… How much of Venus’s heat is coming from it’s own core? Given its formation closer to the Sun, I would surmise that Venus might have a greater proportion of heavier/more unstable elements at its core than Earth. ?
I need to amend my previous comment.
If the CO2 on Venus froze it would form dry ice about 2000 feet deep, on average, and would of course form glaciers, thus increasing the pressure in the lowlands and leaving the mountain tops in a vacuum.
As a side note, the black body spectrum at 735 Kelvin starts emitting at around 700 nm, in the visible spectrum, peaks at about 3.9 microns, then tapers off towards 15 microns.
blackbody calculator
Since very little energy makes it down to the surface, successfully maintaining Venus’ temperature purely due to IR blocking would require trapping 99% of the energy emitted between 1 to 15 microns. I don’t think CO2 alone is going to accomplish it.
The whole idea of a runaway greenhouse effect causing the dense atmosphere of Venus is so silly that it could only have been as created in the minds of Marxist brainwashed pseudo-scientists with a political agenda. The Universities have been pushing the nonsense for so long that even some real scientists have been hood winked by it. It is not that Venus has an unusually high atmospheric density for its size and location, its that the Earth has an unusually low density. That density can be quite adequately be attributed by that absurdly big ball we call the Moon.
Why are so many so confused by the relationship of temperature and density. Gas is compressible so its density is highly related to pressure. Energy will distribute evenly through the mass. The more mass per volume, the more kinetic energy per volume, the higher the temperature. It is not pressure causing heating. Sheesh! Liquid is not a compressible fluid so its density is not very related to pressure. The behaviour of the Earth’s oceans tells us nothing about the behaviour of an atmosphere. So bringing up the thermal characteristics of the Ocean is meaningless.
Jeff Green, please educate yoursel on how little of the infrared band is active with CO2 even at the higher pressure and temp of the lower Venusian atmosphere.
IF all of the IR was concentrated in the tiny active bands of CO2 (which it isn’t, see black body and grey body emisssions), then CO2 could have a higher influence on temperature in the system. As it is the MASS of CO2 has a much larger effect than its GHG properties.
This is, without a doubt, the most authoritative description of the planet Venus and its environment: http://www.feedbooks.com/book/2482.pdf
With regards to the atmospheric pressure comment, it appears as though he is talking about the pressure by the gaseous envelope surrounding the planet, the atmosphere. If the temperature were dropped to 0K, the gases would solidify, leaving the planet without an atmosphere. Hence, no atmospheric pressure.
There’s a basic fallacy here which Leonard Weinstein has covered fairly well. Yes, the adiabatic lapse rate is as described. With a gas in motion and under gravity, there will be a temperature difference across any layer approx as determined by the ~10 K/km figure.
But that’s a difference. It doesn’t tell you how hot it will be at top or bottom. The analogy is a battery. There’s a 1.5V difference end to end, but the actual voltage depends on how it’s connected. Where it is earthed.
A 9.2 MPa atmosphere of nitrogen will have a big temperature difference, top to bottom, but it is not “earthed” at the top. No heat is exchanged there. The temperature at the bottom is entirely determined by the radiative balance there. That’s why the bottom, in Venus conditions, couldn’t be at 700K. The 12000 W/m2 IR emitted at 700K would escape unhindered. The surface would have to cool until balance with avaible sunlight was restored. The whole column of N2 would cool with it, maintaining the lapse rate. The top would cool at the same rate as the bottom.
This is what the greenhouse effect does. It “earths” the system at TOA. CO2 emits to space at the top, not at the bottom. The radiative balance sets the temperature there. About 164 W/m2 (less than Earth) has to be emitted there to balance net sunlight absorbed (after albedo) and than determined the temperature there. The lapse rate then determines the temperature at the surface, and makes it hot. Just as on Earth, except that with our smaller GHG component, Earth also has significant emission from the surface at atmospheric window frequencies.
Well to those who replied above all I can say is Australia is out.. looks like the USA is gonna be out too.. I don’t think the French or Germans really care anymore. The British government will be much less interested that the Laborites.. and so on.. In fact it looks like Bolivia and the UN is now taking over the AGW cause. In fact I think the next Winter in both SH and NH (because that is the only thing the ordinary fellow/fellaw in the street really understands), is going to be so so cold…Good luck to ol Evo Morales Hahaha. BTW I think it is the actual weather that determines whether the ordinary person on the street will decide whether AGW is happening or not and most of the shift is due to that (ie very cold winters is Europe and USA this year). They are having doubts….
Nick Stokes,
“CO2 emits to space at the top, not at the bottom.”
Which side of the molecule is the TOP??
HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA
In case any one listens to you, CO2 emits to space from tropopause out to the edge of the atmosphere here on earth and a similar depth on Venus. You are being simpleton again and more confusing than informative.
I’m not sure where anyone got the idea that I am suggesting that there isn’t a greenhouse effect on Venus. It certainly wasn’t from anything I have written. This is a discussion of relative magnitude.
DesertYote at May 8, 2010 at 6:11 pm said:
Wikipedia defines temperature as the average kinetic energy of particles in matter. You seem to be talking about energy density. While Wikipedia often contains things I disagree with, there seems to be a disconnect here.
George Turner
Your explanation is correct in that gravity is the cause of the pressure, which drives the temperature.
Well Steve, I just thought it was assumed. Since Venus is totally devoid of vegetation, the idea of a greenhouse on it is rather silly. ^_^
Anyway, given the little F=ma thing, imagine if we reduced Venus’ surface gravity by a factor of 90, to about 0.01G or so (Vulcan terraformers probably do this all the time). Then we have a planet with the same amount of CO2, the same clouds, the same basic atmospheric structure (just stretched way up), but a surface pressure that’s Earth normal. If GHG completely drives Venus’ temperature then the dramatic change in surface pressure wouldn’t matter in the long term, and it would still be 700K+. If adiabatic heating is largely responsible then the change should produce a much cooler surface.
Nick Stokes
May 8, 2010 at 6:32 pm
“The top would cool at the same rate as the bottom.”
No it would not unless the density was constant which it would not be.
Steve Goddard
“I’m not sure where anyone got the idea that I am suggesting that there isn’t a greenhouse effect on Venus. It certainly wasn’t from anything I have written. This is a discussion of relative magnitude.”
Well, Steve, what then is your point? Relative magnitude of what? The fact that the lapse rate creates a temperature difference between where GHG’s emit (high) and the surface has always been an essential part of the greenhouse effect.
Mole is a non-dimensional number. A mole of H2 weighs much less than a mole of UF6
The heat MUST be created by planetary forces, see http://www.esa.int/esaSC/SEMUKVZNK7G_index_0.html .
It has long been recognised that there are simply not enough craters on Venus. Something is wiping the planet’s surface clean. That something is thought to be volcanic activity…….. and “Now we have strong evidence right at the surface for recent eruptions,” says Sue Smrekar, a scientist at NASA’s Jet Propulsion Laboratory in California. They do say They estimate that the flows are possibly as geologically recent as 2 500 000 years – and likely much less, possibly even currently active. In other words they don’t know, but something must be bringing heat from the interior of the planet to lava flow the surface. This must be the major phase change going on in ‘Her’. Recent laval surface remodelling seems to me to be the cause of the night/day hot surface temperatures, the sun’s role on the surface heat seems minor. The (relatively) waterless atmosphere without significant convective
forces seems to be keeping the heat in.
This enormous heat must be coming from the planet itself, the atmosphere is merely a blanket, the adiabatic lapse rate is too limp to explain such persistent high night /day heat.