A little while ago, I mentioned here the curious and very large adjustment to 19th century sea surface temperatures based on changing hypotheses about the relative use of wood and canvas buckets. It’s always worth checking whether there’s a hidden agenda for seemingly innocent adjustments. Sometimes my instincts are pretty good. Here’s Figure 3.20 from IPCC SAR  together with the original caption:
Original Caption: Figure 3.20. Decadal summer temperature index for the Northern Hemisphere, from Bradley and Jones . up to 1970-1979. The record is based on the average of 16 proxy summer temperature records from North America, Europe and east Asia. The smooth line was created using an approximately 50-year Gaussian filter. Recent instrumental data for Northern Hemisphere summer temperature anomalies (over land and ocean) are also plotted (thick line). The instrumental record is probably biased high in the mid-19th century, because of exposures differing from current techniques (e.g. Parker, 1994b). [my bold]
Here the instrumental record in the 19th century fits sufficiently poorly with the proxy reconstruction that it requires explanation. One wonders whether this might have contributed even a little to the re-tuning of 19th century SST results. Especially when the bucket adjuster (Folland) is closely affiliated with the proxy proponent (Jones, who is also the primary author of the land temperature data set.) The bucket adjuster is then the lead IPCC chapter author (and is still lead author in IPCC TAR), so it’s all pretty incestuous. The "validation" for the various multiproxy studies is claimed because they supposedly track last half 19th century results a little bit. But doesn’t this show at least enough prior tuning to affect cross-validation statistics? The nuance is important: I’m not arguing the adjustments per se (at least for now), but whether the tuning process needs to be considered in defining statistical significance benchmarks.