There is excellent agreement between the tree ring proxies and direct temperature measurements up to the 1960s, and a divergence thereafter. Now, we are pretty sure that the thermometers are not wrong; but we do know that the 1960s onwards was a time of industrialization in the Soviet union and Eastern Europe, with all the medium range pollution – acid rain,etc – that this implies. So there is a perfectly plausible explanation for why the series should diverge.
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