Tuesday, August 23, 2011

Chaotic systems, rivers and global warming.


Someone asked, “If global warming is happening how come each year is not warmer than the previous?”
And also, “What’s the difference between weather and climate?”
The simple answer that climate is weather averaged over time is a bit too simple.
The weather is a chaotic dynamic system.  Forecasting tomorrow’s weather is fairly easy, forecasting next week’s is much harder, next month’s impossible.  But generalised trends, July will be warmer than January, are easy.  The timescale of the view is relevant.
Climate is, as we’ve seen, weather averaged over time, and has its roots in a chaotic dynamic system.  Should it not display chaotic attributes, wobbling too and fro, back and forth, while maintaining a general direction of travel?

An analogy.

A river flows from the mountains to the sea.  Downhill.  Gravity determines the direction of flow.
But let’s take a closer look, get down on our knees by the riverbank and watch a leaf caught up in an eddy.  We are dealing with fluid dynamics, a chaotic system, with mini hurricane-like whirlpools in which our leaf may, for a few seconds, actually move the ‘wrong’ way, away from the sea, before resuming its calm progress.
Then step back, back to a vantage point overlooking the broad flood-plain.  The meanders need to be viewed from a distance but also over a lengthy period of time.  Some of those meanders are so contorted they send the river flow towards the mountains, for a short distance, before resuming the seaward journey.  Just as the little eddies come and go, so too the meanders change their position over the years so that at any one point the river may be seen to flow through all points of the compass if one waits long enough.
Thus we have, at two very different scales of both time and distance, opportunities for the water to defy the general trend, not flowing directly down to the sea but eddying and meandering about.

Back to global warming.

Just as we are confident that gravity will pull the mountain waters down to the sea, however circuitously, we are confident that the anthropogenic greenhouse gases will shift global temperatures higher.  But climate is a chaotic system.  Expect eddies, expect meanders.  Don’t assume that tomorrow will be warmer than today; that next year will be warmer than this.  But do be assured that this century will be warmer than last.  The influence of the greenhouse gasses on global warming is certain as gravity on a river.

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