Tornetrask Regressions

Briffa’s Tornetrask temperature reconstruction is done by regression analysis. Previously I reported that Briffa purported to justify his upward adjustment of 20th century MXD chronology (and downward adjustment of MWP reconstruction) by a very slight improvement of R2 (going from 0.503 to 0.553 – see Clim. Dyn 1992). I’ve attempted to replicate these regression calculations — I show an R2 using unadjusted data of 0.555 and with adjusted data of 0.558 (both using Briffa’s 1876-1975 reference period). With a reference period of 1851-1980, the corresponding unadjusted R2 is 0.571 (adjusted 0.572). As mentioned before, my RW chronology calculations are based on the 65-core dataset used in the MXD calculation.

As I noted a few posts ago, the discrepancy between 20th century RW and MXD series in the 20th century may very well derive from "Modern Sample Bias", where the minimum core diameter in modern trees eliminates slower growing trees from the distribution. This pretty much eliminates any quasi-justification for the adjustment. Even the very weak justification in terms of R2 cannot be justified. (BTW Briffa and co-authors are co-authors of the Rutherford et al 2005 diatribe against R2 statistics.)

Another curiosity of the Briffa regression is the retention of insignificant regressors. Briffa did an “inverse regression” of April-August temperature (AA) against 4 regressors: rw, mxd and next year’s rw and mxd (on the basis that the following year’s rw and mxd are influenced by prior year’s temperature).

fm.adjusted<-lm(AA~rw.RCS+lag.rw.RCS+mxd.RCS.corrected+lag.mxd.RCS.corrected, data=site[(1876-site[1,1]+1):(1975-site[1,1]+1),])

Value Error t value Pr > |t|
Intercept -0.232 0.074 -3.16 0.002 **
rw.RCS 0.002 0.084 0.018 0.986
lag.rw.RCS 0.089 0.092 0.976 0.332
mxd.RCS.corrected 0.643 0.077 8.303 6.9e-13 ***
mxd.RCS.corrected -0.057 0.077 0.737 0.463

Residual standard error: 0.6124 on 95 degrees of freedom
Multiple R-Squared: 0.558, Adjusted R-squared: 0.5394
F-statistic: 29.98 on 4 and 95 DF, p-value: 3.956e-16

Three of the 4 regressors — rw and the two lag variables — are insignificant. It is hard to understand why these insignificant regressors would be retained. The insignificance of the t-statistic for the rw regressor make the adjustment described previously even more bizarre. More Hockey Team oddities.

I’ve provided a textfile here tornetrask.dataframe.txt with my estimates of his underlying temperature and tree-chronology dataset hecking. The temperature data are July-August (JA) and April-August (AA) averages from gridcells described in Briffa et al [Nature 1990 page 435 col 1] using a contemporary gridcell dataset, which is as close to the one that he used as I can find (downloaded from http://cdiac.ornl.gov/ftp/ndp020/ndp020r1.txt.) Briffa has not archived any chronologies. I’ve included my emulations of his chronology versions, mxd nomenclature shown here, with corresponding rw versions (except for adjusted): mxd.spline – standardized by spline equal to 2/3 core length; mxd.RCS – standardized by linear age model; mxd.RCS.adjusted - post-1750 values “adjusted” as discussed before.


8 Comments

  1. Ed Snack
    Posted Apr 19, 2005 at 8:07 PM | Permalink | Reply

    It’s all gone so quiet ! Frankly this post doesn’t need much comment, I mean what else is there to say really. Tornetrask and the Bristlecones, the Tasmanian Huorn Pines, what is left of the dendrochronological record that one can rely on as used by the AGW crowd.

  2. Michael Ballantine
    Posted Apr 20, 2005 at 5:19 AM | Permalink | Reply

    Ed, I guess we can say that the only thing the trees can say unambiguously is whether conditions were favourable or unfavourable for growth.

  3. Chas
    Posted Apr 20, 2005 at 11:42 AM | Permalink | Reply

    Briffa’s “Trees tell of past climates: but are they spreaking less clearly today” contains a quite enjoyable discussion of ‘correction’ issues:
    http://www.journals.royalsoc.ac.uk/link.asp?id=43xa8lk6pcmvmh9h

    BTW would it be possible to have a ‘library page’ on the site?

  4. Chas
    Posted Apr 24, 2005 at 3:51 AM | Permalink | Reply

    Its intersting to see that in 1999 the reasons for the “reduced sensitivity” were “unknown”:

    http//:academic.engr.arizona.edu/HWR/Brooks/ GC572-2004/readings/vaganov-nature-siberia-tree-snow.pdf
    So the last 100 years precipitations are unique!
    It must frightening to notice that the trees are lining up with the meteosondes
    and the satellites against the contaminated thermometer record (LOTR?);-)

  5. Chas
    Posted Apr 25, 2005 at 12:41 PM | Permalink | Reply

    Woops lots of typos in #4
    The correct link is:
    http://academic.engr.arizona.edu:80/HWR/Brooks/GC572-2004/readings/vaganov-nature-siberia-tree-snow.pdf
    I hope!

  6. Chas
    Posted Apr 30, 2005 at 2:09 PM | Permalink | Reply

    By 2004 no justification for such corrections has materialised, as Briffa, Jones, Osborne and Schweingruber comment:
    “At present, no satisfactory explanation of the relative MXD decline has been identified”

    Yet they use it:

    ” we make the untested assumption that the decline is due to an anthropogenic factor that did not occur earlier in the reconstruction period.”

    But they concede
    ” further work must dictate whether this assumption will be supported or rejected.”

    ” uncertainty must surely be associated with the reconstructions because of this assumption, particularly for earlier warm periods.”

  7. TCO
    Posted Sep 18, 2005 at 4:57 PM | Permalink | Reply

    Trees don’t talk less clearly now than in the past…people were just reading too simple of a story into the record and are now starting to get whipped closer to truth.

  8. Mats Holmstrom
    Posted Sep 27, 2005 at 9:17 PM | Permalink | Reply

    It is interesting to note that the pine trees studied in the McCarroll paper is probably very similar to Briffa’s Tornetrask pines. These regions of northern Sweden and Finland are quite similar.

    The connection between tree growth and sunshine is interesting, especially since it seems that the solar radiation at Earth’s surface varies quite a lot (R.T. Pinker et al., Science, 308, p850, 2005). In that paper they state that At high latitudes, plant growth is light-limited, and a decrease in solar radiation can affect net primary productivity. Consistent with high latitude tree growth being a proxy for sunshine.

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