A Note on Possible Non-Climatic Tree-Ring Trend Bias

As noted by MBH98 (and references therein) non-climatic influences related to intrinsic tree growth trends, difficulty in their removal, and the combining of different tree segments, make the lowest frequency (ie, century and longer timescale) variations potentially somewhat suspect in studies that rely heavily on dendroclimatic indicators.

In selecting data from the International Tree-Ring Data Bank (ITRDB), MBH98 set criteria designed to minimize these problems. Starting with the full data bank available in 1997, they identified 1589 site chronologies, each representing a unique combination of species, variable (e.g. ring width or maximum latewood density) and location. Only the 251 chronologies that met the following criteria were retained:

  • Reliable information on the methods used to remove biological trend was available;
  • The median length of the individual segments used to build the chronology was greater than 150 years;
  • The mean correlation of these individual segments with the site chronology was greater than 0.5;
  • The first year of the chronology was before AD 1626, and it contained at least 8 segments by 1680;
  • The last year was after 1970, and there were still 8 segments after 1960.

Of the 251 ITRDB chronologies that met these criteria, 229 were ring-width chronologies, and 22 maximum latewood density.

MBH98 argued, furthermore, that biases unique to a particular type of proxy indicator (e.g., tree-ring widths) are less problematic for "multiproxy"-based reconstructions that make use of the complementary information in a diverse proxy network. MBH98 found through statistical proxy network sensitivity estimates that skillful NH reconstructions were possible without using any dendroclimatic data, with results that were quite similar to those shown by MBH98 based on the full multiproxy network (with dendroclimatic indicators) if no dendroclimatic indicators were used at all. We show this below for annual-mean reconstructions of Northern Hemisphere mean temperatures.

Full Multiproxy Network Data
No Tree Ring Data
Tree Ring Only Data

NH temperature reconstructions based on all records, and on subsets excluding or comprising exclusively of, tree-ring data.

Note that the NH reconstruction based on the sparse "non-dendro" multiproxy network (19 non-dendro indicators available back to 1760) is remarkably similar to that based on the full (more than 100 indicators) multiproxy network of MBH98. Because the sampling of the "no-dendro" dataset is much sparser, we expect that it will be more influenced by regional variations, and less representative of the true NH mean temperature. Accordingly, it calibrates significantly less (47% vs. 74%) of the instrumental NH variance, and the variability and uncertainty in the reconstruction is larger. Nonetheless, the overall variation--and the long term trends in particularly--are remarkably similar. Also shown is the reconstruction based ONLY on dendroclimatic indicators (ie, no coral, ice core, or historical or instrumental indicators). Again, the primary features of the reconstruction are very similar. Whether we use all data, exclude tree rings, or base a reconstruction only on tree rings, has no significant effect on the form of the reconstruction for the period in question. This is most probably a result of the combination of our unique reconstruction strategy with the careful selection of the natural archives according to clear a priori criteria. Furthermore, we note that Jones et al. (1998), get similar results for the recent changes using an almost completely different tree-ring network based on wood density from high latitude trees. These comparisons show no evidence that the possible biases inherent to tree-ring (alone) based studies impair in any significant way the multiproxy-based temperature pattern reconstructions discussed here.

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