Michael Mann made his mark in global warming alarmism with the use of dendrochronological data (a.k.a. tree-ring data) as a proxy for temperatures going back over 1000 years. He has clung to it despite its obvious ignorance of the Medieval Warm Period (to say nothing of how it deviated from recent history so much he had to truncate it from his analysis – hence the term “Mike’s Nature Trick”).
Well, as it turns out, the tree-ring data his a problematic bias (WUWT):
Basically, older trees grow slower, and that mimics the temperature signal paleo researchers like Mann look for. Unless you correct for this issue, you end up with a false temperature signal, like a hockey stick in modern times. Separating a valid temperature signal from the natural growth pattern of the tree becomes a larger challenge with this correction.
Trees that grow slower create lower temperature proxies, and thus can mask higher temperatures in periods like the MWP.
Yet another nail in alarmism’s coffin.
Cross-posted to the right-wing liberal