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I’ve been reading in PhysicsWorld about some grand ideas for controlling the earth’s climate by engineering on a global scale. Some sound pretty fanciful, though some might be just plausible. But before I get there (which will probably be another entry) I think it’s worthwhile reminding you what the greenhouse effect actually is. As in, why is it a greenhouse gets hot, and what has this got to do with the atmosphere?

Let’s take the greenhouse first of all. Just a few pieces of glass enclosing your tomato plants. Why is it so hot in there? The sun emits a lot of radiation. Because it’s so hot (around 5500 degrees celcius or so) a substantial fraction of its energy it emits at wavelengths in the visible spectrum. When this light arrives at the surface of the earth (let’s ignore the atmosphere for now) it travels through the glass of our greenhouse and hits the ground / tomatos or whatever is in there, where some of it is absorbed. This generates heat, which heats up the material inside.

However, as we know, hot things radiate the heat again. But, unlike the sun, which radiates hugely in the visible spectrum, things at say 20 or 30 Celcius radiate mostly at infra-red wavelengths. And at infra-red, unlike visible, glass is opaque. So the radiated heat can’t escape. (It can enter, but not leave, because it tries to leave at a different wavelength from what it arrives). Contrast this with the case of no greenhouse – the light from the sun falls on the ground – the ground heats up, but this heat can now escape back upwards as it is radiated. It doesn’t get so hot.

Now let’s go to the atmosphere. Water vapour and carbon dioxide molecules (amongst others) in the atmosphere do a similar thing to glass. They let visible light through, but absorb infra-red light. The sunlight makes it to the surface, but the heat radiated from the surface gets trapped. Same mechanism as the greenhouse, hence the name greenhouse effect.

NB The different properties of materials at infra-red and visible wavelengths are often overlooked. Paint is a good example – I often hear people remark that radiators should be painted black to maximise the amount they radiate (black bodies radiate well.) The argument doesn’t hold because pretty well any colour paint you buy is close to ‘black’ if you look at it in infra-red – i.e. it absorbs (and hence radiates) infra-red radiation (heat) very well. There is no need to repaint your radiators.