I’m working away at Jones et al . Here’s an interesting diagram from Jones et al [Science, 2001] , which purports to provide confidence intervals for the J98 reconstruction (blue). There’s (at least) one really strange feature in this diagram. See if you can pick it out.
Original Caption: Fig. 2. (A) Northern Hemisphere surface temperature anomalies (deg C) relative to the 1961-1990 mean (dotted line). Annual mean land and marine temperature from instrumental observations (black, 1856-1999) (5) and estimated by Mann et al. (red, 1000 to 1980) (6, 10) and Crowley and Lowery (orange, 1000-1987) (7). April to September mean temperature from land north of 20N estimated by Briffa et al. (green, 1402-1960) (8) and estimated by re-calibrating (blue, 1000 to 1991) the Jones et al. Northern Hemisphere summer temperature estimate (9, 16). All series have been smoothed with a 30-year Gaussian-weighted filter. (B) Standard errors (SE, deg C) of the temperature reconstructions as in (A), calculated for 30-year smoothed data. The proxy average series (6-10) do not extend to the present because many of the constituent series were sampled as long ago as the early 1980s.
In most multiproxy studies, the number of series gets fewer in the earlier portions. In the Jones 1998 reconstruction, there are only 3 series in the first part of the 11th century (Polar Urals, Tornetrask and Greenland dO18); a fourth enters in the mid-11th century (Jasper, Alberta tree rings) and the roster is only 4 until 1400. Eventually, there are 10 "proxies", including the CEngland instrumental record and the CEurope documentary record. The J98 standard errors (second panel – blue) using 10 proxies are only a little lower than the standard errors using 3-4 MWP period proxies. That’s curious enough. But look at what happens at AD1400. One proxy series is added – Svalbard ice melt percentage. For some reason, the standard error nearly doubles. I wonder what’s going on.
There are some other frustrating aspects to this diagram. It turns out that the Jones et al  version used in the compilation in Jones et al  is not the same as the archived version for Jones et al , but has been "re-calibrated". I think that I’ve figured out the "re-calibration", but it leads into more murky by-ways of the multiproxy underworld.