Latitude, my understanding is that most of the tree ring data was calibrated on temperature data from 1881 to 1940 (Briffa, 1998) or 1881 to 1960 (Briffa, 2001). I’m not sure about the validation periods used, though, and I think this is a key point. Perhaps the only validation performed is on the error for the calibration period? But maybe someone with a better understanding of the methods could comment, since I’m not sure.
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