We've all seen how well temperature proxies and CO2 concentrations are correlated in the Antarctic ice cores — this has been known since the early 1990's and has featured in many high - profile discussions of climate change. (realclimate.org)
He could have concluded that tree rings are not sufficiently good temperature proxies and therefore thrown out the tree ring study on that basis, and never had the MBH98 paper published. (notrickszone.com)
(It's the data he used to prove that MBH's method wildly underestimates past climate variability, even when fed with good temperature proxies.) (climateaudit.org)