Over the past few days, we’ve discussed many peculiar aspects of Rahmstorf smoothing and centering: incorrect disclosure; seeming unawareness of what the smoothing did; unattractive properties of the triangular filter; the enhancement of “successful” prediction; opportunistic policy changes.
It’s not though IPCC hadn’t turned its mind to smoothing. IPCC AR4 enunciated a sensible policy on smoothing in AR4 chapter 3 : “Observations: Surface and Atmospheric Climate Change” Appendix A. In that chapter, they condemned Rahmstorf procedures and, unlike Rahmstorf, described their filter in unambigous terms – no cat-and-mouse. They stated:
In order to highlight decadal and longer time-scale variations and trends, it is often desirable to apply some kind of low-pass filter to the monthly, seasonal or annual data. In the literature cited for the many indices used in this chapter, a wide variety of schemes was employed. In this chapter, the same filter was used wherever it was reasonable to do so. The desirable characteristics of such filters are 1) they should be easily understood and transparent; 2) they should avoid introducing spurious effects such as ripples and ringing (Duchon, 1979); 3) they should remove the high frequencies; and 4) they should involve as few weighting coefficients as possible, in order to minimise end effects. The classic low-pass filters widely used have been the binomial set of coefficients that remove 2Δt fluctuations, where Δt is the sampling interval. However, combinations of binomial filters are usually more efficient, and those have been chosen for use here, for their simplicity and ease of use
These are sensible policies. “Easily understood and transparent” clearly excludes Rahmstorf’s Copenhagen description of a triangular filter of length 29 as – “smoothed over 15 years”. Criterion 2 -excluding ripples and ringing – excludes Rahmstorf’s triangular filter on other grounds. Criterion 4 – “as few weighting coefficients as possible” also precludes Rahmstorf’s filter.
IPCC went so far as to provide a standard filter to “remove fluctuations on less than decadal time scales” for chapter 3, described in unequivocal terms as follows:
The second filter used in conjunction with annual values (Δt =1) or for comparisons of multiple curves (e.g., Figure 3.8) is designed to remove fluctuations on less than decadal time scales. It has 13 weights 1/576 [1-6-19-42-71-96-106-96-71-42-19-6-1]. Its response function is 0.0 at 2, 3 and Δt, 0.06 at 6Δt, 0.24 at 8Δt, 0.41 at 10Δt, 0.54 at 12Δt, 0.71 at 16Δt, 0.81 at 20Δt, and 1 for zero frequency, so for yearly data the half-amplitude point is about a 12-year period, and the half-power point is 16 years. This filter has a very similar response function to the 21-term binomial filter used in the TAR.
Instead of simply complying with standard IPCC procedures, Rahmstorf used a filter procedure described only in the AGU newspaper – the triangular filter properties of which were not described in the original article and indeed the authors say that they unaware of this defect at the time.
As so often in climate science, Rahmstorf changed smoothing policy not just once, but twice. First, in Rahmstorf 2007, he abandoned IPCC policy in favor of an article in the AGU newspaper; then he changed accounting parameters in the Copenhagen Report – all without explicitly stating that he had changed policy from the IPCC report and accompanying the change notice with an explicit accounting of the impact of the change.
Here’s what happens with Rahmstorf’s results if IPCC filter procedures had been followed. Rahmstorf can no longer assert that observations are in the “upper” part of models, with the implication that things are “worse than we thought”. R07 is looking shakier and shakier.