What is the Greenhouse Effect?

So what is the Greenhouse Effect anyway, and what does it have to do with carbon dioxide?

If you’ve ever walked inside a greenhouse on a sunny day you will notice that its warmer inside than outside.  The reasons for this may be seen if we look at the graphic below. The only difference between the waves that give us sound from our radio and the very high energy gamma rays that are emitted by radioactive matter is the wavelength.

Essentially, the shorter the wavelength the higher the energy (which is the same as saying the higher the frequency the higher the energy).

If you look in the middle part of the graphic below you will see a coloured section with black parts on either end.  This is called the visible spectrum as its the only part that we can see with our eyes.

Up one end of this section we see violet and at the other hand we see red.  We should recognise these colours as what we see in a rainbow.  Just beyond the violet (just outside our visible range) there is ultra violet (UV), and at the other end of the spectrum, just beyond the red, we see infrared (IR).

Now, a greenhouse is based upon the fact that glass is transparent to UV but not transparent to IR.

What happens is that the UV radiation shines through the glass and heats things inside the greenhouse, such as plants, or the ground.  Many organic molecules will absorb the UV light and then re-emit it as heat (infrared).

This is in fact how sunscreens work, but that’s another story.

So all this energy that came into the greenhouse as UV light, is now re-emitted as IR, except now it can’t get through the glass.

So that’s how the greenhouse heats up.

A greenhouse gas therefore is one that will allow UV light to pass through it but we’ll not allow IR to pass through it, but will trap it.

More tomorrow.