Hegerl et al [2003]: a re-posting

Raymond Pierrehumbert at realclimate has recently posted on climate sensitivity, citing Hegerl et al. [2003] approvingly. As less weight is being put on Hockey Team arguments, more weight is being put on these detection and attribution arguments. Maybe I’ll put a few inquiries out for data and methodology on these studies. For now, I’ll point out again how large the residuals are for Hegerl et al [2003]. There is no digital data available, but visually it seems impossible that there is anything like 57% explained variance. I’m recycling here part of a post made last February on this study, when climateaudit wasn’t as active as now (although it was already starting to get a lot of hits.)

Posted in February
In some recent commentary trying to backpedal from the hockey stick, "detection and attribution" studies have been cited as alternative validations and Hegerl et al [2003] is cited as a key example. I have not looked in detail at these studies, but some features of Hegerl at al. [2003] struck me as so obvious that they are worth pointing out.

The most striking aspect to me is how little "explanation" of the proxy reconstruction CLH seems to be accomplished by Hegerl et al. [2003]. Look at the top panel and then look at the residuals in the bottom panel. For most of the period, the residual is about the same size as the original series and it looks to me like virtually nothing is "explained" in statistical terms. Table 1 of Hegerl et al [2003] states that 57% of the variance is explained by the forcing model. It sure doesn’t look like it. It would be nice to see the calculations.


Original Caption: Figure 1. Detection results for the updated Crowley and Lowery [2000] reconstruction of decadal Northern Hemispheric mean temperature (north of 30N, calendar year average). Upper panel: Paleo reconstruction (black) compared to the instrumental data (grey) and the best estimate of the combined forced response (red), middle panel: response attributed to individual forcings (thick lines) and their 5–95% uncertainty range (thin lines), lower panel: residual variability attributed to internal climate variability and errors in reconstruction and forced response. An asterisk “‹Å“”‹Å“*” denotes a response that is detected at the 5% significance level.

The CLH reconstruction shown in the top panel is a new version of Crowley and Lowery [2000]. It is described in the text as follows:

a modified version [T. J. Crowley et al., in preparation, “‹Å“”‹Å“CLH”] of the Crowley and Lowery [2000, hereinafter referred to as CL00] reconstruction (correlation with CLH 0.94). The latter is a weighted average of 9 long decadal or decadally averaged records over the Northern Hemisphere mid-to-high latitudes (30–90 N, the records sample both the warm and cold season, with a likely bias towards the summer half year). The weights are determined from the regression coefficients of individual records with the 30–90 N annual mean instrumental record during the period of overlap [Jones et al., 1999]. The resulting paleo time series was scaled so that the regression fit with the instrumental data from 1880–1960 had a slope of 1.0 [decadal correlations of 0.81 (with trend) and 0.66 (detrended)]. For consistency, the scaling of E02 is based on the same period and also decadally filtered data.


  1. TCO
    Posted Dec 17, 2005 at 3:43 PM | Permalink

    They’re all in bed with each other.

  2. John A
    Posted Dec 17, 2005 at 4:28 PM | Permalink

    There could be a “chinese wall” that prevents them from discussing each other’s work.

    As Steve has already demonstrated, you can get better statistical significance from red noise – which means, of course, no significance at all.

    Is significance testing (like Monte Carlo simulations for example) not taught in statistics courses any more?

  3. Steve McIntyre
    Posted Dec 17, 2005 at 10:55 PM | Permalink

    A point to remember here is that the Crowley reconstruction used here (CLH) and in Crolwey [2000] has been fiddled to enhance 20th century relative to the MWP. The target changes pretty dramatically if the bristlecones are excluded (see my AGU PPT). The “explanation” of the MWP with the forcing parameters here is virtually non-existent – visually compare top and bottom panels above. It would be worse with a target series not relying on bristlecones.

    The issue is not simply “variability” as it is presently being cast – but the relative MWP-modern levels. It is not a gimme that MWP is lower than modern levels – see my post on Naurzbaev.

  4. JerryB
    Posted Dec 18, 2005 at 6:41 AM | Permalink

    Without editorial comment, let me mention that at http://www.nicholas.duke.edu/people/faculty/hegerl.html Hegerl is said to have “expertise” in “statistical techniques for climate research” in addition to her other “exprtise”.

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