In a twitter exchange among Jean S, Ronan Connolley and Tim Osborn, Ronan drew attention to an early spaghetti graph in a comment on MBH98 published by Phil Jones in Science on the day after (Apr 24, 1998) publication of Mann et al 1998. The Briffa reconstruction is in purple below. Like IPCC 2001, it hides the decline in the Briffa reconstruction (here a 1998 version) by deleting late 20th century values – here after 1950.
Jones stated that all three reconstructions “clearly show” that the 20th century is the warmest in the 600-year period, with the most “dramatic feature” being the 20th century rise:
Despite the different methods of reconstruction and the different series used, or alternatively, because a few good ones are common to all three series, there is some similarity between the series. All clearly show the 20th century warmer than all others since 1400. The dramatic feature of all three records is the rise during the 20th century.
Mann et al published a Reply to Jones’ comment in June 1998 (with Jones as coauthor of the Reply). They agreed that Jones’ spaghetti diagram “demonstrate[d] the the robustness of the conclusion that the 20th-century warming is unusual in the context of the past several centuries”L
The comparison shown by Jones between Mann et al.’s Northern Hemisphere temperature reconstruction (1) and two other recent estimates is useful in several ways. For example, it demonstrates the robustness of the conclusion that the 20th-century warming is unusual in the context of the past several centuries, on the basis of largely independent estimates.
However, these claims are based on hide-the-decline: Jones deleted post-1940 values of the Briffa reconstruction, slightly enhancing the effect of the deletion by smoothing with post-deletion values only. Jones noted this truncation in the caption to the figure, where he stated that “tree-ring density data show a decline since the 1940s unrelated to temperature [see (9 – Briffa et al, in press; 10 – Briffa et al 1998 (Phil Trans London)] for more details], and the curve from (9) ends at 1940”, a precaution not taken by Mann in IPCC AR3. In the next graphic, I’ve done a blow-up of the 1900-2000 portion of the graphic to demonstrate this. I’ve also shown the deleted values of the Briffa reconstruction (using the nhlmt version from Briffa et al 1998). This hide-the-decline incident is a year earlier than hide-the-decline in Briffa and Osborn 1999 and Jones et al 1999.
Figure 2. Blow-up of Jones 1998 comment on MBH98. Briffa version is nhlmt from Briffa et al 1998.
In the next graphic, I’ve plotted the complete series – no hide-the-decline. When the decline is shown, one feels that even a reviewer for Science or Nature would cavil at the assertion that the “dramatic feature of all three records is the rise during the 20th century” or that the conclusion that “20th-century warming is unusual in the context of the past several centuries” is “robust”:
Figure 3. Jones 1998 diagram, showing decline in Briffa et al reconstruction (nhd2 version rescaled on same ratio as nhlmt to nhd1 and re-centering to match visually.
Jones did make some sensible comments in his Comment that have not previously drawn attention. Jones observed that one should be able to easily extract relative importance of the various proxies, speculating that “much of their success, in a statistical sense” must come from tree rings:
The mathematical technique used by Mann et al. (3) to produce the reconstructions could easily be adapted to show which proxy series are the most important. Although Mann et al. (3) do not explicitly rank the various proxies, much of their success, in a statistical sense, must come from the large number of tree-ring width series used.
Jones’s observation was correct, but no one seems to have attempted extracting the contribution of different proxies types until I did so in 2004-2005. My analysis showed that the contribution of all proxies except bristlecones was little different than white-to-low-order-red noise and that the HS came from a very small subset of all proxies. The climate community has chosen to ignore this point.
Jones also made the sensible observation that each new class of proxy had to prove itself – a precaution immediately abandoned in favor of Mannian armwaving.
Each paleoclimatic discipline has to come to terms with its own limitations (6, 7) and must unreservedly admit to problems, warts and all. A particular issue for all ice core and coral series and some new tree-ring work is what exactly an isotope series (be it O, H, or C) tells us about past temperature. Sensitivity to temperature cannot be assumed; it must be proved with instrumental data on both interannual, and, where possible, on longer (more than 20 years) time scales (7).
I must say that I was a little intrigued to find an example of hide-the-decline a full year before Briffa and Osborn 1999 or Jones et al 1999, previously the earliest hide-the-decline example. In previous analyses of hide-the-decline, I’ve repeatedly emphasized that the technique of deleting data to hide inconvenient results originated with CRU (I’ve previously termed it “Keith’s Science Trick”). Mann knew of CRU’s deletion of the decline, but, as Lead Author of IPCC AR3, Mann willingly and enthusiastically participated in the hide-the-decline scheme, because he didn’t want to “give fodder to the skeptics” or “dilute the message”. But the technique originated with CRU and the “exoneration” of Jones, Briffa and Osborn on hide-the-decline by Muir Russell and Oxburgh was totally undeserved. (Nor does assignment of blame to CRU excuse Mann’s participation in hide-the-decline as Lead Author of AR3, where Mann and CRU both were culpable.)
Postscript: the twitter exchange also discusses the provenance of the Briffa reconstruction version in IPCC AR3 Figure 2-21. IPCC cited Briffa 2000 (QSR), while the actual version used in AR3 comes from Briffa et al 2001. Tim Osborn stated that the IPCC version matched the green LFD curve in Briffa (2000) Figure 5. This seems to be only partly true: the LFD curve has the same shape as the AR3 curve, but appears to be scaled differently.