From Wikipedia

Twenty-year smoothed plots of averaged ring-width (dashed) and tree-ring density (thick line), averaged across all sites, and shown as standardized anomalies from a common base (1881–1940), and compared with equivalent-area averages of mean April–September temperature anomalies (thin solid line). From Briffa et al. 1998.^[1]‘

So we have a problem using tree rings to measure temperature. The problem is that as temperatures have risen in recent decades, the same tree rings that purport to show that the Medieval Warm Period (MWP) was not as warm as today, sort of peter out, you know, “decline.”  This is known as the Divergence Problem.  How does Wikipedia describe the possible causes of this issue?

The explanation for the divergence problem is still unclear, but is likely to represent the impact of some other climatic variable that is important to modern northern hemisphere forests but not significant before the 1950s. Rosanne D’Arrigo, senior research scientist at the Tree Ring Lab at Columbia University‘s Lamont-Doherty Earth Observatory, hypothesises that “beyond a certain threshold level of temperature the trees may become more stressed physiologically, especially if moisture availability does not increase at the same time.” Signs suggestive of such stress are visible from space, where satellite pictures show “evidence of browning in some northern vegetation despite recent warming.”

“[S]ome other climactic variable … not significant before the 1950s” Well that is convenient, isn’t it? You can explain anything by positing “some other variable.”

““beyond a certain threshold level of temperature the trees may become more stressed physiologically,”

Do you think it is possible that during the MWP it was warm enough to stress the same trees as well? It’s a rhetorical question. Of course if it were very warm during the MWP, the same factors would likely apply.

In other words, Mann’s claim of today’s warming being “unprecedented in the past 1500 years” is not supported by the tree ring methodology that he uses.  The idea that some new unknown factor has come into play is just a fig leaf to avoid drawing the obvious conclusion.