Is the atmospheric pressure the cause of a planet's surface temperature or is it the temperature the cause of a planet's atmospheric pressure? I heard a climatologist on a talk show saying that one of the widely known arguments used by climate scientists to exemplify what a runaway greenhouse effect could cause to Earth's temperature and climate (a comparison with Venus' atmosphere) is basically a farce.
He argued that Venus' high surface temperature (which is over 400ºC) is caused mainly by its high surface pressure (which is over 90x greater than Earth's) and that the so-called "greenhouse effect contribution" to this temperature was "negligible" or simply "non-existent". He mentioned the ideal gas law in order to "prove" his argument, saying that temperature grows proportionally with pressure (which, of course, is true for an ideal gas). And this pretty much settled his train of thought.
This made me wonder and I'm trying to come up with a reasoning that debunks his argument. At first, I thought one could not simply apply the ideal gas law to Venus' atmosphere, since the molar density at the surface is too high and that one should use more terms of the virial expansion to account for the behavior of the pressure near the surface. However, even if that is true, his argument remains undisputed, since the leading term of the expansion is still the "ideal gas" term.
Then I realized his oversight may have been the cause-effect relationship he tried to establish between pressure and temperature. As a matter of fact, an equation of state doesn't tell us anything about causality: it only states the relationship between different state variables that describe a system. This takes us back to my question: Is it the really the high pressure of Venus' at surface the agent responsible for its high surface temperature, or is it the other way around or maybe even a mix of the two?
To better clarify my thinking, imagine this: if we take Venus and place it out of the Solar system where its exposure to solar radiation would be negligible, I think we can agree that its temperature would decrease drastically. Now, I know most of its atmosphere would solidify in such conditions, but the thing is: would the pressure on the same layer of matter where the surface of the planet was previously remain the same? No. So, does this tells us that it is the temperature of the planet that determines its pressure?
On the other hand, I am also aware that the pressure at the surface is simply a function of how much matter (gas) there is above it (no clear connection with temperature here). 
I've reached this crossroads and can't seem to establish any clear causality relationship between pressure and temperature. Am I going in the right direction or is it that I am just plain wrong and the climatologist correct when he made his assertion?
 A: Well, to clarify some things first
In atmospheric science, or more correct: If you do the math...
your only to free Variables are Density and Temperature. The equation of state which gives you the pressure, is a material property.
The equations for the atmospheric variables are interconnected at any moment, it is nonsense to say P causes T or T causes P. You could only make sense of this, if you would have a concrete change in T due to some radiative effect, or change P due to contraction.
The point here is, that your 'climatologist' doesn't recognize, is that the energy content of an atmospheric layer is always in flux. If the $F_{up} = F_{down}$ then this is $= \sigma A T^4$.
However to reach a high state of equilibrium flux (meaning: to reach a high T!) you have to have a mechanism to contain this flux somehow, and that is achieve by pulling a blanket over your atmosphere (a.k.a greenhouse effect). This makes some initial flux never be able to escape.
So to summarize: The Equation of state is true at any given moment, but doesn't explain how all the heat got there, or stayed there. You have to look at the thermal history of Venus or the body of interest, to understand how the heat got there. (and thermal history means to solve the above mentioned equations, involving radiative windows etc. that this guy ignore.)
Tell me, if you want me to clarify things up.
A: (I have only read the first paragraph of the question)
The pressure of Venus's atmosphere is about 90x greater than that on earth. It also happens to be about 90x more dense that that on earth. Coincidence? No. The density is the reason the pressure is so high.
If you were to descend 1 km into one of our oceans, the pressure would be comparable to that of the Venusian atmosphere. And yet the ocean isn't several hundreds of degrees is it? The reason for the high temperature on Venus is the greenhouse effect.
A: I look at the question this way, and it's very simple.
What happens when you pump up your bycycle tyre? ... it gets hot, because work is done on the air to compress it into the tyre. You leave it when finished. It cools down. The air is still at pressure (OK a little less on cooling). Why does it cool - because work is NO longer being done on the air. But it is still at high pressure.
The work done on the atmosphere by it's compression due to mass/gravity is a ONE time only event and NOT kept there by that process.
In the same way your bicycle tyre does not stay forever hot.
A: Your Brazilian Professor is partly right: the high temps at the surface of Venus are partly due to the high pressures at its surface.  But that's not sufficient to explain the total temp.  Other factors are that Venus is a lot closer to the sun than Earth is, and of course the greenhouse effect.  
As a thought experiment, if you took two planets that were identical in every way except that one had a thicker atmosphere than the other, the one with the thicker atmosphere would have higher surface temps than its twin, even if global warming were not a factor.  This is because higher pressures produce higher temps, and explains why it's hotter at the bottom of Mt. Everest than at the top.  
I think part of the problem is that most people don't understand how the greenhouse effect really works.  Here's a brief explanation: Gasses that are deemed "greenhouse gasses" are transparent at the peak wavelengths that the Sun emits, so that energy travels through the atmosphere relatively unscathed and heats up the surface of the Earth.  The Earth gets rid of that energy by re-radiating it back into space as black-body radiation.  But that re-radiation is at much longer wavelengths (well into the infrared), at which greenhouse gasses are not transparent.  (Non-greenhouse gasses are transparent at both the incoming and outgoing wavelengths).  This is not a matter of speculation: you can take a container full of methane or carbon-dioxide and measure its transparency as a function of wavelength in any well-equipped lab in the world.  We know exactly how much extra energy is being trapped inside the atmosphere due to these gasses.  The only dispute (among those versed in the science) is what the effects will be on climate, ocean currents, sea level, etc., and how fast those changes will occur.
