Despite assurances from Ed and Keith, I must admit that I still don't get it. The NRC committee is looking at a number of issues, but the one that is most publicly noted is to determine whether, and with what confidence, we can say that recent temperatures have emerged from the band of natural variability over the last millennium or two. Millennial reconstructions with high time resolution are mostly tree-ring based, mostly northern hemisphere, and as I understand it, some are correlated to mean-annual temperatures and others to seasonal temperatures. The performance of the tree-ring paleothermometry is central. Taking the recent instrumental record and the tree-ring record and joining them yields a dramatic picture, with rather high confidence that recent times are anomalously warm. Taking strictly the tree-ring record and omitting the instrumental record yields a less-dramatic picture and a lower confidence that the recent temperatures are anomalous. When a big difference is evident between recent and a millennium ago, small errors don't matter; the more similar they are, the more important become possible small issues regarding CO2 fertilization, nitrogen fertilization (or ozone inhibition on the other side...).
Unless the "divergence problem" can be confidently ascribed to some cause that was not active a millennium ago, then the comparison between tree rings from a millennium ago and instrumental records from the last decades does not seem to be justified, and the confidence level in the anomalous nature of the recent warmth is lowered. This is further complicated by the possible small influence of CO2 fertilization.... I was just looking at some of the recent Mann et al. papers, and at the Osborn and Briffa paper from this year. In that one, as nearly as I can tell, there are 14 long records, of which 2 extend to 2000, 8 end in the early to mid 1990s, 1 in the early to mid 1980s, 2 in the early to mid 1970s, and one in the late 1940s. That looks to be a pretty small data set by the time you get into the strongest part of the instrumental warming. If some of the records, or some other records such as Rosanne's new ones, show "divergence", then I believe it casts doubt on the use of joined tree-ring/instrumental records, and I don't believe that I have yet heard why this interpretation is wrong.
I'm open to hearing what I have screwed up. Please note, I have no direct stake in this! I went to the meeting, I spoke, I'm done. But, I think you have a problem coming, that it involves the IPCC and particularly chapter 6 and paleo generally, that I really should let Susan know what is going on (if you've seen all the increasingly publicly disseminated emails, you know the story). I'd rather go back to teaching and research and raising money and advising students and all of that, but I'm trying to be helpful. Casting aspersions on Rosanne, on the NRC panel, or on me for that matter is not going to solve the underlying problem.
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