We’re all taught that carbon dioxide is a greenhouse gas and therefore the cause of our climate change woes, but the physics behind this rarely gets mentioned, which is a shame considering its remarkable simplicity. Simple enough, even, to explain in 500 words*…
First, we need to introduce dipolar molecules. Molecules are atoms of certain elements connected by bonds that result from the different elements sharing electrons (this bit you will just have to trust me on). However, many molecules contain elements that are heavier than the others in the molecule. This means they attract the electrons more strongly towards them, as electrons are negatively charged and the protons that make up most of the weight of an atom are positively charged. This gives the individual atoms a net charge (normally an atom is neutral – the electrons and protons cancel each other out); the heavier atoms will be slightly negatively charged and the lighter ones positively charged, forming what’s known as a dipole, like the one in Figure 1. This is important!
Next we need to consider electromagnetic (EM) waves. These are a form of energy and are like waves on the surface of the ocean, except instead of being oscillations in sea-surface height they are oscillations of charge. If you were the size of a molecule in the path of an EM wave, as each peak and trough of the EM wave passed by, you would experience a change in charge from positive to negative and vice versa.
Perhaps you can see where this is going. We have two charges – that of the molecule of gas and that of the wave, which oscillates as the wave passes. The two charges will interact, by attraction and repulsion, exerting a force on the molecule and causing it to move in various different ways. When this happens the energy associated with the wave is transferred to the molecule – which has absorbed the radiation.
However, it does get a little (only a little, I promise) more complicated – molecules will only absorb specific wavelengths of light in this way because they themselves oscillate at certain frequencies (frequency simply being another measure of wavelength). This is akin to pushing someone on a swing – if you push them when they are closest to you at the top of their swing they will rapidly gain height and speed after a few pushes, whereas if you push them at the bottom of the swing a) this will hurt your arms and b) they won’t get nearly as high. Here, the person on the swing is the molecule and you pushing them is the force exerted by the EM wave. The EM wave has to be the specific wavelength (frequency) so that it exerts the right force at the right time to cause the molecule to oscillate and absorb the energy.
Right, now, just like any other object above absolute zero, Earth emits these EM waves, as does the sun. It just so happens that we can see those emitted by the sun because our eyes contain molecules that oscillate at the frequency of the EM waves emitted by the sun (and hence absorb their energy), but not those emitted by the Earth. However, the EM waves from the sun do not cause the gases in our atmosphere to oscillate, so the light from the sun is not absorbed, but those emitted from the Earth do – and it is this that gives rise to the greenhouse effect. A greenhouse gas is simply one that absorbs the EM waves that are emitted by Earth. Crucially, once these gases have absorbed this radiation they re-emit it both upwards and downwards back towards Earth, meaning that the Earth receives extra energy at the surface than it otherwise would and warms up – the greenhouse effect that we all know and love.
And finally, Earth emits EM waves at a range of wavelengths, some more than others, and it just so happens that carbon dioxide is able to absorb the wavelengths that Earth emits the most of, and thus absorbs and re-emits a large fraction of the radiation emitted by Earth. It’s this effect that makes it such a potent greenhouse gas.
For more info on these concepts, visit the links in the text. For a slightly more detailed explanation of molecular absorption, see the second answer to this question.
*Okay I got that wrong. More like 650.
“In a nutshell my research interests include atmospheric/climate dynamics (i.e. how the atmosphere and climate system work), African rainfall and drought prediction. I am also known to dabble in a bit of storm-chasing over in the US. Oh and I like rollercoasters. A lot.”