"Density of the atmosphere has more effect on the temperature than any of you people realize."

It may be true that many don't appreciate this fact, though at same time many people are aware that if Venus had significantly less atmosphere, it would have a lower average temperature and they are aware that if Mars had significantly more atmosphere than it has, then it would have a higher average temperature.

Let's look at the greenhouse effect:
"The Earth's average surface temperature of 15 °C (59 °F) is about 33 °C (59 °F) warmer than it would be without the greenhouse effect."
http://en.wikipedia.org/wiki/Greenhouse_effect

Earth is said to be 33 C warmer than it should if one regarded Earth as a black body. It should be noted that Earth isn't a "good example" of a black body, but more important than this "problem" is the assumption of what is Earth's average temperature. What we consider Earth's average temperature is largely based upon a particular *custom* we have adopted- namely reading temperature in a little box painted white which 5 feet about the ground.

If we were sentient whales, your temperature might based on temperature reading of say 1000' under the surface of the Ocean. If we were sentient badgers, it might be the temperature reading 3' under ground.
In other words the temperature reading from a shaded white box 5' above the ground has little to do with the taking atmosphere's temperature or the ground temperature [the temperature of sand, dirt, rocks, or concrete].
For instance, why not measure the atmosphere's temperature at the mid-point of the atmosphere?

Now, this 5' in a box does has a relationship to temperatures at higher elevation, but the question why is it THE temperature of the atmosphere that SHOULD therefore be related to the Black body temperature. 5' in a box is a human arbitrary which is related to human needs.

Earth's air density and pressure vary depending upon elevation, temperature, and weather conditions in general.
One can pick normal condition and get average values- for example, being in the middle of Hurricane is not a normal condition on this planet or for humans, they generally avoid/dislike winds exceeding hundred mph.

So, for normalized conditions we have-
Air Density Calculator:
http://www.denysschen.com/catalogue/density.asp

Let's put in 5' and 59 F =
Air Density (lb/ft3): 0.0761

Half of that density would be .03805 lb/ft3

If elevation is 20,000' and 32 F =
Air Density (lb/ft3) is about 0.0369 which about 1/2 of sea level density.
[Actually, if it's 59 F at sea level then with 3 F decrease in temperature per 1000' at 20,000' it would actually be around -1 F [60F cooler & 0.0395 lb/ft3]

So, if all humans lived at 16,000 to 20,000' [perhaps because they are terrorized about rising sea level and maybe they really liked having a view]. And we measured our temperature in the little white boxes 5' above the ground, then we might then wonder why our average global temperature was *below* the black body temperature that it *should* be.
Now the conclusion they reach might be that Earth lacks enough land which is at 16000' or higher. And maybe the Left might have a global campaign to build more mountains.