I was thinking about all those proxies indicating medieval warmth that were reported in the NIPCC report. I found myself worrying that they might suffer from the same problem as the tree rings - namely that their proxy nature might be justified post-hoc, by showing that they correlate to temperature in the instrumental period. This of course leaves you with the possibility that the correlation is spurious.
Presumably to do this properly, you have to
One would hope then that the proxy papers cited by NIPCC would justify their somethings as proxies in this way (or by citing other papers that show this). I wonder if they do.
I've emailed Bob Carter, but in the meantime look at de Menoccal et al 2000 (this was picked at random, although I had several false starts as I tried to find an open access paper), a paper that reconstructs sea surface temperatures from sediments from a hole drilled off the coast of Africa:
Warm and cold season SST estimates were calculated from the [foraminiferal] census count data using the F13' transfer function derived from faunal analysis of 191 Atlantic core tops (15,16).
In terms of justifying these sediments as proxies, that appears to be it. One hopes that the justification can be found in references 15 and 16, but unfortunately I can't lay my hands on these. Perhaps readers can help. The references are:
15. N. G. Kipp, in Investigation of Late Quaternary Paleoceanography and Paleoclimatology, R. M. Cline and J. D. Hays, Eds. (Geological Society of America, Boulder, CO, 1976), pp. 3–42.
16. W. F. Ruddiman and L. K. Glover, Quat. Res. 5, 361 (1975).