What makes a gas a greenhouse gas?

A water molecule

Something the course hasn't gone into yet is why certain gases behave as greenhouse gases (GHGs). So I thought I'd have a go at it myself :)

There are 2 requirements for a GHG:

1) The gas needs a frequency of vibration mode that matches the frequency of infra red light (IR). In other words it needs to be capable of vibrating in tune when IR hits it.

2) The vibration needs to perturb the gas molecule's electric field, creating a dipole. Dipoles are where a molecule has a positive and negative pole (like a battery).

When you accelerate and decelerate a dipole (i.e. vibrate it) electro-magnetic radiation - in this case IR - is created.

In summary, IR hits a GHG, the gas vibrates in tune with it and therefore fires off IR in all directions including back to the ground.

More here.