The Medieval Warm Period is known and here's some help debunking misconceptions stemming from it. A tree ring can only record the data for the location of the tree, and not all locations experience the same climate or change in climate. Even then, it may be possible to extrapolate some human causes even back then. Human deforestation may have had a notable effect in some regions. The lack of trees may have allowed warming due to a different albedo. The burning of trees without new tree plantings would have created a net increase in CO2. Furthermore, burning wood would have resulted in soot and recent studies have shown that soot contributes to warming over time due to albedo changes in the atmosphere, which also reduce precipitation in the area.
Et tu BAUT? Quantum mutatus ab illo.