Monday, 27 April 2009

Clarifying chaos

While out at dinner the other night someone raised the overly-familiar "well, if you can't predict weather, how can you predict climate?" objection to climate change. At the time I dispensed it with the usual, and likely unsatisfying, "climate isn't weather" riposte, but thinking about it again this morning, a helpful analogy occurred to me ("helpful", that is, to me, so it might do nothing for anyone else).

Consider the lottery. Specifically, consider the sort of lottery that uses one of those ball-tumbling machines that "randomise" the generation of the winning numbers by bouncing a large number of numbered balls within the enclosed space of the tumbler. If we think of a case in which only a single ball is used, we can view the exact position of the ball inside the machine as being akin to the weather. If we observe the ball at one time, we can probably reasonably estimate where it'll be for the next few microseconds, but beyond that chaos prevents a reliable prediction. Similarly with weather, where we can only go a relatively short distance into the future before the so-called butterfly effect overwhelms predictability.

To continue the analogy, climate is not like the ball, climate is like the tumbler itself. This contains the ball, and gives a shape to the possible positions in space that the ball can find itself in. So climate describes the range of possible states that weather can find itself in. Rather than initial conditions, climate is dependent on larger scale phenomena like latitudinal solar heating, the position of oceans and continents and on certain factors like air density and composition. As such, it becomes much more predictable - much like the shape of the tumbler itself. Of course, if some of these phenomena change, as they are currently doing under anthropogenic guidance and as they have done in the glacial-interglacial cycles of the past, the shape of the tumbler changes and different weather can result.

Anyway, when I next get the chance, I'll test out this analogy, and see if it's any more satisfying. If nothing else, it may make me feel better. As it stands, I always come away from discussing climate change with non-scientists with the feeling that I've either patronised them and/or have a pulled a fast-one by relying on science-platitudes like "climate-is-not-weather". While analogies have their limits, and can themselves be used to convey misleading information, I tend to think of them as being a useful short-hand to getting some scientific idea across.

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