I find myself scratching my head over the latest debate between the RealClimate crew and Roger Pielke Sr. regarding the definition and use of “chaos” (non-linear sensitive dependence on initial conditions) in weather and climate modelling.
As often is the case in debates like this, both sides are correct in the point they’re making, differing not on the technical issue but the choice of emphasis. (I can’t help but note the irony here that, in arguing over “chaos,” the disputants are engaging in exactly the sort of “proxy argument” that Roger Pielke The Younger complains about so articulately.)
William and James at RealClimate argue that weather is chaotic and climate (as defined as weather averaged over time) is not. Roger argues (in the RC comments and also his own blog) that such a definition of climate fails to capture an important characteristic: “Observations show chaotic behavior of the climate system on all time scales, including sudden regime transitions.”
I don’t disagree with either group here. Given their definition, which is useful for some purposes, James and William are correct. It just so happens that for me, as a consumer of climate science, Roger’s definition is more useful. I recognize the GCMs’ globally averaged long-term rising trends under greenhouse emissions as important, but the sort of abrupt transitions that the models don’t yet capture are far more important to me, because (see drought posts below), it’s variability that kills ya, not long term trends. From Roger in the comments at RC:
In terms of how we are altering the climate, it is these sudden transitions that we need to understand, rather than focus so much of our resources on assessments of the global averaged temperature trend.