# What happens to a greenhouse gas after it absorbs infrared radiation?

We say that a greenhouse gas increases the temperature of the surface of the earth but how does it do that ? Only by re emitting the absorbed IR towards the earth ?

Actually a greenhouse gas absorbs light in the infrared wavelengths of light. One of two things will happen after the gas absorbs the IR radiation: 1) the molecules to move around and vibrate faster, which is really the definition of temperature, so the air gets warmer, and 2) the gas molecules can re-emit the IR radiation. In the second case, the radiation is emitted isotropically, so that some of it just goes out into space as it would have had the gas not been there in the first place, but some of it will be directed back towards the surface of the earth, warming the surface by absorbtion of the IR (heat).

Note that most of the heat being absorbed by greenhouse gases is coming from the ground after the ground absorbs sunlight and warms up, not from the sun shining through the air and being absorbed by the greenhouse gases directly.

• @RodrigodeAzevedo, definitetly. Thanks for the input. – airhuff Apr 13 '19 at 22:48

In addition to the point made by airhuff, which is that greenhouse gases essentially prevent the earth from emitting infrared radiation into space, this slight effect shifts the equilibria of greenhouse gases which actually amplifies the effect.

That is to say, when people speak of $\ce{CO2}$ being a big problem, this mostly because of an indirect effect. $\ce{CO2}$ slightly warms the earth which increases the amount of water vapor present in the atmosphere (in an average sort of way because this fluctuates quite a bit). This is a problem because water is a much more potent greenhouse gas than $\ce{CO2}$ and it absorbs at different wavelengths in the IR than $\ce{CO2}$ does. This matters because the amount of IR which $\ce{CO2}$ absorbs is fairly close to saturation. That is, doubling the amount of $\ce{CO2}$ does not mean doubling the amount of IR radiation absorbed and redirected back towards earth. The relationship is much closer to logarithmic than linear. But, this is only true for the wavelengths at which $\ce{CO2}$ absorbs, so introducing more water vapor allows a wavelength of IR to be absorbed which may not be saturated.

This is only one mechanism by which greenhouse gases warm the earth. The atmosphere is an extremely complicated system. None of this mentions how well certain gases mix in the atmosphere and how long they stay there and anything about methane for that matter. All of these things have been written about in scholarly articles and books, so those are gonna be the places you find the nitty-gritty details and data.

Also a note on what I mean when I say carbon dioxide is roughly saturated in terms of absorbance. It is true that carbon dioxide quite effectively absorbs IR light, but it can never truly be saturated. This is because the relationship is logarithmic, so it always increases, but just more slowly as you add more $\ce{CO2}$. Second, it has been said that convection can play an important role here because one shouldn't really think of bulk concentration of carbon dioxide but really the concentration in different layers of the atmosphere. This complication actually allows $\ce{CO2}$ to absorb more radiation than you might otherwise expect.

The 'greenhouse' gases such as $\ce{CO2, H2O, CH4}$ absorb ir radiation and become vibrationally and rotationally excited. When excited their temperature is effectively greater than that of their surroundings.

Two things can now occur; the vibrationally excited molecules may radiate, some of this energy will be lost to space, and some absorbed by the earth and can therefore be re-radiated. Secondly a collision with other air molecules will compete with radiating and so transfer some or all of the extra vibrational/rotational energy to the collision partner until after several collisions all gases are at the new equilibrium temperature. This increase in temperature can only occur because the majority gases in the atmosphere, nitrogen, oxygen cannot radiate ir radiation as they have no electric dipole. Thus the energy is trapped as vibration and rotational energy in the ground state of these molecules and so heats up the atmosphere.