It was my call not to "overplay" the importance of the divergence issue, knowing the subtlety of the issues, in the fortcoming IPCC Chapter 6 draft. We did always intend to have a brief section about the assumption of uniformitarianism in proxy interpretation , including mention of the possible direct carbon dioxide fertilization effect on tree growth (equally controversial), but it is likely to conclude that here as well , there is no strong evidence of any major real-world effect. This and the divergence problem are not well defined, sufficiently studied, or quantified to be worthy of too much concern at this point. The uncertainty estimates we calibrate when interpreting many tree-ring series will likely incorporate the possibility of some bias in our estimates of past warmth, but these are wide anyway. This does not mean that temperatures were necessarily at the upper extreme of the reconstruction uncertainty range 1000 years ago, any more than they may have been at the bottom. The real problem is a lack of widespread (and non-terrestrial) proxies for defining the level of early warmth, and the vital need to up-date and study the responses of proxies in very recent times.
Raymond Bradley and the Grand Old Duke of York
46 minutes ago