Sudden Transitions

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.


  1. One of the comments on RC said it better than me: that the megadroughts (hey! wanna make your drought/wetness/temperature sound more interesting? Then prefix it with “mega”!) aren’t really an example of chaos. They are just a local transition. If they were chaos, the entire climate system would subsequently be totally different (a-la Lorentz). But no-one seems to think thats true.

  2. We can all agree that models that accurately predict regional and/or non-linear aspects of climate shifts would be a great improvement. Unfortunately, *they don’t yet exist*. RP Sr. seems to be advocating the abandonment (for public communication purposes) of current science/models that, however imperfect, have been shown to be useful in communicating the very real climate problems with which we are faced.

    The farthest I’ve been able to nail him down about what he thinks ought to replace surface temp is to elicit his view that we ought to focus on ocean heat content changes and avoid mentioning surface temp. Please do try writing such a story; my bet is that the first thing your editor will do is ask about what this means for surface temp. Anyway, unless you think you can write that story now, your defense of RP Sr. isn’t quite justified.

    Also, I don’t think it is an accurate statement to say that a lot of *resources* are being focused on the surface temp statistics. Attention yes, resources no. As with his misrepresentation of the conclusions of the NRC report (which did not, not, not suggest downplaying surface temp), a lot of what RP Sr. writes includes a little bit too much of this kind of conflation.

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