I’ve been handed an AP Environmental Studies class to teach, and a kid asked about this:
If greenhouse gases are acting as a partial mirror to the infrared that the earth is emitting, wouldn’t it also act as a partial mirror to the light coming in from the sun?
I pointed out that the sun is broadcasting much more than infrared, and I assume that the effect of those gases is much greater in trapping heat, because all of the escaping heat from the ground and oceans is in the form of infrared, which is partially absorbed and re-reflected by the greenhouse-type gases.
Who out there knows if I’m right? What else is there to point out about this?
“Mirror” is a poor term to use here, the gases do not reflect IR. They absorb and emit IR.
But otherwise, it sounds like your understanding is correct. Greenhouse gases absorb very little visible light, but a lot of IR. Most of the energy from the sun is in the form of visible light, so very little of it is absorbed or attenuated by greenhouse gases. The energy loss by the earth, on the other hand, is mostly through IR, so it is blocked by greenhouse gases (i.e. absorbed by it, and some of the energy emitted back towards the ground).
Yes, I didn’t actually say to them that it reflected IR, but that it absorbed it and re-emitted it in all directions, including down, which then could be said to be a bit of a “reflection”.
As for which frequencies give us the most energy, I did find this:
The visible spectrum, however, accounts for just under half of the Sun’s total energy. Much of the Sun’s energy is made up of ultraviolet (UV) radiation, which has shorter wavelengths (higher energy levels) than visible light and extends off of the purple end of the visible spectrum. An even larger amount of this invisible energy can be found in the longer infrared wavelengths (lower energy levels) of light that extend off the opposite end of the visible spectrum.
When the earth’s surface absorbs the sunlight, it re-emits the energy at a different wavelength, in the infrared range. It’s that re-emitted energy that is held in by the greenhouse gases. Wikipedia has a much more detailed discussion of how this works.