Friday, October 12, 2018

"...because the stakes are so low"

In an earlier blog I alluded to the fact that someone had written a paper on ensemble means. I didn't think it was particularly interesting or useful since we had written a more correct and complete analysis some years previously (as blogged variously here here here). The particular point that the author Bo Christensen got most obviously wrong was his claim that the CMIP ensemble behaved like it was sampled from a high-dimensional space. In fact, as we showed via a variety of analyses, it behaves much more like it was sampled from an intermediate-dimensional space. Specifically, we found an effective dimensionality of about 3-11 depending on the details of the analysis. On top of getting that wrong, it didn't seem like his analysis actually added anything of significance to our results. Furthermore, Bo's analysis relies on the ensemble being perfect in the sense that the verifying observational data set (ie reality) is drawn from the same distribution as the ensemble members. We already demonstrated that the good performance of the ensemble mean does not depend on this very strong (and generally false) condition.

Amusingly, when I commented critically on his analysis after his EGU talk, he assumed I was Jonty Rougier, who seems to have been a dissatisfied reviewer of the paper (Jonty had also extended and improved on our work in some ways back in 2016).

So far, so boring, although it's a bit frustrating to see our own work so comprehensively misunderstood and denigrated. Bo did cite our paper, and Jonty's too, so was not unaware of them, but claimed (without much justification) that their explanations were not compelling.

What prompted me to bother writing it all down now is that a comment/reply pair has just appeared in Journal of Climate (full versions on sci-hub for anyone who cares). The comment is from Jonty Rougier of course. It's good to see that academic politics continues in our absence...

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