mathman said:
That is how greenhouses work - light goes through glass, but heat does not.
Curiously, some people get really pedantic that the mechanism by which greenhouses (and blankets) work is mainly inhibiting convection (unlike the greenhouse effect of the atmosphere, noting that in general the atmosphere facilitates convection). But clearly what greenhouses (and blankets)
do is the same as what the atmosphere (its greenhouse effect) does: elevate the temperature passively by impeding net heat loss (at any given temperature).
A body reaches thermal equilibrium where it receives energy at the same rate that it gets rid of it. A planet gets rid of incident sunlight by a combination of reflection and thermal (Planck) radiation. Thus, by the "Stefan–Boltzmann law" we can figure out how hot it must be (to emit thermal radiation at the necessary rate). If the planet has no atmosphere (no greenhouse effect) then this will give us the temperature of the surface (otherwise, it will be telling us about the temperature of higher atmospheric layers). Considering how far the Earth is from the sun, this law suggests an average temperature that happens to be about 30 degrees cooler than what Earth's average surface temperature actually is. (We can verify our work by comparing with average temperatures of the atmosphereless moon.)
The atmosphere absorbs some IR, so it heats up until it is also emitting the same quantity of IR. Basically, half of this heat is being emitted back downward, and half is directed further upward (heating next higher layers of the atmosphere). If we increase the amount that the atmosphere interacts with IR, the very top layers will cool (since they become able to radiate their heat faster) but the surface would warm (since the lower layers are absorbing and emitting back more heat than before). (It might be roughly analogous to keeping the air composition the same yet making the atmosphere thicker/taller.) As it is, something like a quarter of the energy incident upon the Earth's surface arrives from the atmosphere (augmenting the energy received from the sun more directly). Since we have the spectral data of the various species of gas, it isn't hard to see how one would start to model exactly how various atmospheric compositions would produce effects of various surface temperatures.
PF now has a policy, by the way, that discussion of the physical mechanisms is acceptable (for example, it is ok to discuss how much the temperature of some planet would change if the composition of its atmosphere was different, hypothetically, like by adding whatever reactants would be obtained by oxidising a given amount of carbon like, say, the quantity of carbon fuel that on Earth we've recorded mining since the industrial revolution) but PF
does not allow discussion of global warming (so you would not be allowed to discuss whether or not mankind is causing any warming of Earth). Make sense?