Eli’s already covered this but I thought I would present a slightly different argument. The topic is a recent paper by Ned Nikolov and Karl Zeller called new insights on the physical nature of the atmospheric greenhouse effect deduced from an empirical planetary temperature model, an earlier version of which was retracted because they had published under pseudonyms (reversing the letters in their names).
The basic argument is that surface pressure sets surface temperatures. The idea being that for a planet like the Earth, we can get the pressure from the weight of the atmosphere and the surface area of the Earth, and that this then sets the surface temperature. This is patently nonsense, and a fairly simple way to see this is via the ideal gas law. The ideal gas law is that the pressure, , is given by
where is the volume being considered, is the number of molecules in that volume, is Boltzmann’s constant, and is the temperature of the gas. If the gas has mass and the mean mass per molecule is ( being some reference mass, and a constant) then and we can write
where is now the mass density.
As should be obvious from the above, the pressure alone cannot tell you what the temperature should be; it depends also on the density. For a given pressure, we could have a hot surface with a low density, or a cool surface with a higher density.
Update (09/08): As per Tom’s comment, their model does include a dependence on solar insolation and albedo and then indicates that the enhanced temperature depends on surface pressure. So, their model is not simply pressure, but it is still the case that surface pressure alone does not determine how the surface temperature is enhanced.
So, what actually sets the temperature?
Let’s imagine we have the Earth, but without an atmosphere (or with an atmosphere that is completely transparent). In such a scenario, the surface must radiate back into space – on average – as much energy as it recieves from the Sun. If it didn’t, it would either heat up, or cool down, until it did so.
If we assume this imaginary Earth has the same albedo as today’s Earth, and orbits today’s Sun, then it would reflect 30% of the incoming sunlight, and would absorb – on average – 240 W m-2. It would also, therefore, radiate 240 Wm-2 and would have an effective surface temperature of 255K. The exact distribution of temperatures on the surface, however, would depend on its rotation and the heat capacity of the surface (as discussed in this paper by Arthur Smith) but, in the absence of an atmospheric greenhouse effect, the surface has to have the same effective temperature as a blackbody that radiates – on average – 240 Wm-2.
Okay, so what about the actual Earth. Well, the surface of the Earth radiates almost 400 Wm-2. This is considerably more than the energy that we receive from the Sun. In the absence of an atmospheric greenhouse effect, the surface would be cooling rapidly, but it obviously does not.
How does this work? Well, there are many ways to explain this, but let’s go back to our imaginary Earth that does not have an atmospheric greenhouse effect. Now add an atmosphere with radiatively active gases. The surface would no longer be able to radiate directly to space. The atmosphere would act to block some of the outgoing longwavelength radiation coming from the surface. The atmosphere would then emit some of this energy back into space and some back down to the surface. However, initially, the amount escaping to space would be less than the amount being received from the Sun. The surface would then warm and emit more energy back into the atmosphere. The atmosphere would also warm, and emit more energy into space, and transfer more down to the surface. This would continue until the system (surface and atmosphere) had warmed until the amount of energy being radiated into space matched the amount of energy being received from the Sun.
A key point is that the amount of energy – on average – being radiated into space has to match the amount of energy being received from the Sun. In the absence of an atmospheric greenhouse effect, this comes directly from the surface. In the presence of an atmospheric greenhouse effect, this comes mostly from within the atmosphere and requires that the surface be warmer than it would be in the absence of the atmospheric greenhouse effect.
So, we can be very confident that Nikolov and Zeller’s argument that planetary surface temperature is set by pressure alone is wrong. Not only does pressure alone not define temperature, in the absence of a planetary greenhouse effect the surface should radiate as much energy as it receives from the Sun, which is clearly not the case for the Earth. The only way to explain why the surface radiates more energy than it gets from the Sun is because of the atmospheric greenhouse effect (to be clear, the surface radiates more than it gets from the Sun, but the planet radiates – into space – the same, on average, as it gets from the Sun). Also, if we add additional greenhouse gases to the atmosphere – as we are currently doing – then we’ll cause the surface to warm even more, as is currently happening.
Attribution – a post that tries to explain the energy balance.