Read here and here. The stalagmites from an Alps cave in Austria provides evidence that Medieval Warming (MWP) temperatures were significantly higher than current temperatures during times of low CO2 levels. The Medieval temperatures were high enough to cause the AGW hypothetical "positive climate feedback" that the IPCC predicts would "naturally" happen in the climate, and is the basis for the obsessive-compulsive fear over temperatures.
If negative feedbacks did not rule the climate, then the high temperatures of the MWP would have triggered the hypothetical postive feedbacks.