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Occasionally it's worth going back to basics on climate change and having a look at why we're concerned about the things we're concerned about. For example, why does warm up the atmosphere?

I initially thought this was too basic to blog, but a highly unscientific straw poll of friends and acquaintances revealed that only one of those surveyed actually knew the answer. (He also knew why a slice of toast is more calorific per 100g than the bread it was made from, but that's a different story.)

So here goes. The reason is down to the molecular structure of CO₂. It doesn't absorb the very , but it does absorb the longer wavelength (heat, basically) released by the surface of the Earth.

Once it has absorbed this thermal infrared radiation, the carbon dioxide molecule becomes agitated and unstable. It can only achieve a stable state again by emitting this stored radiation, again as heat. Some of this heads outwards to space, but some stays here. So the more CO₂ there is in the atmosphere, the greater the warming effect. (Needless to say, this is a massively simplified description that skates over all the tricky stuff like and . Apologies to the purists.)

But why focus on carbon dioxide, a trace gas in Earth's atmosphere, rather than, say, , which has a much more potent greenhouse effect? (Between 17 and 25 times depending on your source.) Well, basically because there's much less methane and it has a shorter life in the atmosphere - roughly 8.5 years to CO₂'s roughly 100 years. Carbon dioxide has a much smaller effect, but for a much longer time.

And the biggest of all, accounting for as much as 85% of the total ? Water vapour, if you count clouds. But before we all stop boiling kettles and breathing out, it's worth bearing in mind that without this effect, Earth would be just another frozen rock floating in space.

As with the climate change debate itself, it's all about balance.

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