Venus Envy

I am finding this thread as baffling as the previous one about Venus. Why are the following points not obvious?
(1) The surface must support the weight of the atmosphere, regardless of its temperature. Gases have weight! Thus, the surface pressure is determined by the mass of the atmosphere and the gravity of the planet, independent of temperature.
(1′) There is a very small effect of temperature, since it does affect the depth of the atmosphere, and gravity diminishes slightly with height. This is a small effect.
(2) An atmosphere should approximately follow an adiabat — that is, pressure and temperature varying at the adiabatic lapse rate — provided the conditions are suitable for convection. This is true in the lower atmosphere of both Earth and Venus. But convection will only occur if the fluid is heated from beneath. In other words, the net heat flow in the fluid in the convective zone must be upward.
(3) If the atmosphere of Venus were heated from the top — which I gather is Steve’s theory — then there should be no upward heat flow in the lower atmosphere. Ergo, no convection. The temperature profile would be much flatter than the adiabatic lapse rate. Indeed, in the long run it should be quite flat, an isothermal atmosphere.
(4) This is not what is happening. Thus, the surface is heating the atmosphere at the bottom. Internal energy sources, volcanism and such, are orders of magnitude too small. The surface must be heated by the short-wave radiation that reaches it. That is, it must be heated by sunlight (however attenuated). This net heat input to the surface accounts for the net upward heat flow in the atmosphere.
(5) The surface is really hot, and so it radiates strongly. (Most natural surfaces are pretty good blackbodies at IR wavelengths.) In fact, the surface radiates dozens of times more IR radiation than actually emerges into space at the top of the atmosphere. So almost all of this must be absorbed by the atmosphere, and reradiated (both up and down, of course, in each layer).
So we have an atmosphere heated from below, by a surface that is getting its energy input from short-wave radiation. The thermal radiation from the surface is strongly absorbed by the atmosphere. If we turned off this absorption and made the atmosphere transparent to IR, then the planet would quickly cool off to a much lower temperature, until radiative equilibrium would be re-established. Doesn’t that pretty much define the greenhouse effect?
I keep expecting this thread to be about WHY Venus has developed such a strong greenhouse effect, and whether the term “runaway greenhouse” appropriately describes this process. Fascinating question, and present theories involve a good deal of speculation. I would probably count as a moderate AGW skeptic — a “luke-warmer” in common parlance — but the discussion here does not inspire me with confidence that this community has a firm grasp on the basic physics. Sorry to be so blunt, but the thing is pretty distressing to someone who reads WUWT regularly and with pleasure.
I’d like to see Steve respond to the points about hydrostatic equilibrium and the upward heat flow associated with the adiabatic lapse rate.

Here’s another clear explanation of the role of the adiabatic lapse rate in ‘greenhouse’ warming.

Steven
Over at “The Reference Frame” Lubos Motl has a detailed dicussion of your WUWT posts on Venus temperature.
You will be interested to find that in his conclusion he is largely in agreement with you. i.e. that the greenhouse effect on Venus amounts to a few dozens of degrees C and that the very high temp at the surface relates primarily to the environmental lapse rate.

It’s been confusing on both articles how people are talking about pressure.
Pressure is force exerted on a surface, can be expressed as pounds per square inch or even newtons per square meter. Atmospheric pressure is being discussed, which basically is the average of the forces involved from gas molecules hitting a surface.
So if the temperature drops to where all the gas molecules precipitate out, what atmospheric pressure is acting on a vertical surface? None. What if I set up a pressure gauge with a horizontal surface pointing upwards, a meter above the ground, what would it read? Nothing, as all the formerly gaseous molecules are already in a layer on the ground below the gauge. Same thing if the horizontal surface is pointing downwards, rotated to vertical, or in any position.
Yes, you can talk of pressure from a substance laying on a surface that’s not completely vertical. But that would be pressure based on gravitational force, while here atmospheric pressure (all-surrounding) is being discussed. So why all the nit-picking?

Didn’t mean to be condecending, but using the closed volume ideal gas laws to describe atmospheric pressure, equating temperature with atmospheric pressure, and implying that gravity disappears at the lowest temperatures are your ascertions. When lucid comments are made to correct these misconceptions there is no acknowlegement nor attempt at retraction. It would therefore appear that your intent is somewhat other than achieving an understanding of the question at hand.

If you changed the composition of Venus atmosphere to 100% nitrogen, the planet would be much colder due to the lack of absorption of IR. The atmosphere would also necessarily be much thinner (lower volume.) P = nRT/V
However, if you changed the composition of Venus atmosphere to 95% nitrogen and 5% CO2, the temperature and height of the atmosphere would not be hugely different than it is at present. Why? Because the greenhouse effect is logarithmic. After the first few percent, additional CO2 makes much less difference to the temperature.
Consider earth, where a doubling of CO2 only increases temperatures by <1 (Lindzen) to 3 (IPCC) degrees C.

Steve:
“However, if you changed the composition of Venus atmosphere to 95% nitrogen and 5% CO2, the temperature and height of the atmosphere would not be hugely different than it is at present. Why? Because the greenhouse effect is logarithmic. After the first few percent, additional CO2 makes much less difference to the temperature.”
I have difficulty seeing this. I can understand the reason for the thick CO2 blanket since there are no water/carbonates to remove the volcanically generated CO2. However, this wouldn’t apply to nitrogen and there should still be less of it relative to Venusian gravity using earth as an example. Couldn’t the major source of heat trapping on Venus simply be because of the thick atmosphere and lack of water vapor, making vertical convective transfer to space difficult (unlike Earth)? It has already been mentioned that the ground level density of the CO2 may be in the supercritical or near liquid state, which would make vertical thermal transfer very difficult relative to that of a gas.
As covered in other comments regarding CO2 depth pressure and temperature, if one could enclose a vertical section of Venusian atmosphere in a sealed insulated tube (closed system), after a period of time the temperature would completely equalize throughout the volume through convective transfer even though the gravity pressure at the tube bottom would still be 93 atmospheres. If this insulated tube of CO2 where originally in space and dropped into Venus’ gravity field, there would be immediate heating at the ‘bottom’ from the pressure increase but this would be cause by mass and potential/kinetic energy transfer to that section. Again when this closed system is left alone in the constant gravity field, the temperature would eventually equalize through convective transfer throughout the gas (at a higher temperature because of the gravity potential energy added) even though the pressure would be higher at the ‘bottom’ of the tube.
If this were not the case, all one would have to do for free energy is to install a heat engine between the top and bottom of the tube since there would be a constant temperature difference even in the closed system.

RealClimate [yeah, yeah, I know] provides some insight into why Venus’s surface temperature is so uniform even though there is very little surface wind to redistribute energy, and minimal light on the dark side.
The Venutian atmosphere is incredibly massive, roughly 40% of the weight of Earth’s oceans. Consequently it would take an incredibly long time for the entire atmosphere to heat up or cool down. (In contrast, much of Earth’s thin atmosphere easily heats and cools in a day.) So expecting the surface temperature of Venus to vary would be like expecting the temperature at the bottom of Earth’s ocean to vary. This massiveness means that the lower atmosphere of Venus is able to equilibrate in temperature all the way around the planet with very little convection (i.e. very low wind). A simple solution when you think about it.
Venus Express finds no significant “daily” variation in Venus’s atmospheric temperature below an altitude of 45km. That altitude is equivalent to a depth (or rather, pressure) of two Earth atmospheres. All the action takes place in the upper atmosphere. The high speed winds in the upper atmosphere transfer energy from the day side to the night side and by the time you get down to 45km altitude, the temperature there remains constant. (This is a few km below the lowest cloud layer.)

stevengoddard says:
May 9, 2010 at 12:36 pm
Venus has no oceans, so it has no limestone. No limestone means a thick atmosphere of CO2. A thick atmosphere means high temperatures.
Bless the clams for saving us from venereal discomfort, for while Venus was being repaved with lava, they were constructing the white cliffs of Dover and starting to crawl from the sea.
Did I get the physics of this right, finally, Steve? It’s all about the fizz in the beer?
I think Dr. Pangloss was right and it’s time to get back to the farm where they don’t believe the rooster actually makes the sun come up.

First let’s clear up some very basics physics.
The ideal gas law is appropriate when talking about gases in confined volumes. When talking about atmospheric pressure, you use the barometric formula.
http://en.wikipedia.org/wiki/Barometric_formula
There are several constants used that are specific to the atmosphere you are trying to use it on.
Atmospheric pressure is dependent on mass and gravity, among other things, and is derived from the ideal gas law.
To wit, as you approach zero kelvin your atmospheric pressure approaches infinity, not zero. (NOTE: Of course, under that scenario you can’t use basic formulas anyway.)
But back to the main discussion. Mr. Goddard is proposing that pressure is somehow driving the temperature on Venus. However, this makes no sense. Titan is a solid counter-example. It has a dense N2 based atmosphere and yet it is still bone chilling cold. There is simply nothing there that can hold onto the meager amounts of heat it receives from the sun (or Saturn).
Venus is another matter entirely. You don’t need a complex experiment to verify that the atmosphere is preventing almost all heat from escaping. We have a good idea of the surface temperature. We also can figure out how much combined IR radiation is escaping the planet (both reflected and emitted through day and night side IR measurements). If what is radiating from the planet is much less than what it should be given the other constraints, then SOMETHING between the surface and the TOA is preventing heat from escaping.
If it were merely density of the gas and NOT composition, then we can verify this quite simply. Take a few different gases (GHG and non-GHG) and place them in IR transparent containers at various pressure levels. Fire an IR laser through each sample at each level and measure. If pressure is indeed the key player, then we should see similar results across all the gas samples. However, if composition is the key player, then we should see disparate results across the gas samples. GHG gasses at higher pressures would absorb more IR while non-GHG gasses would show little to no differences regardless of pressure level.
Fortunately, these have already been done many times over in establishing thermal properties for various gases. It should come as no surprise that atmospheric composition is what matters. Density matters as well. After all, if you don’t have much of an atmosphere you’re not going to be absorbing much of anything no matter the composition. But you can have an N2 atmosphere at 1000 atmospheres and it still won’t create a greenhouse effect. Saying otherwise is the equivalent of saying you can can make salt taste like sugar by adding more salt.
Can what happened to Venus happen to Earth? Not very likely. We’d have to try really hard to do so, and it would take a long time of trying hard (centuries). I’d like to think that we would be smart enough to notice palm trees growing in Antarctica and know that something might be amiss.
At any rate, composition AND density determine the IR profile of an atmosphere. Basic high school/college level physics that has been validated many thousands of times in labs across the planet.
~X~