Good question LH. And timely. there was a paer last month in Science magazine on exactly this (Mann et al., 2009).
Turns out the "Medieval Warm Period" or MCA, was a highly locallized phenomenon. It affected the North Atlantic, but other regions of the globe were actually cooler. Thus, there was no "Global Climate Change" at that time. The Earth was close to the baseline 1961-1990 global average temperature.
In contrast, today, all regions of the Earth are warmer than the 1961-1990 baseline. It is truly a global phenomenon.
Compare the temperature anomlies for the MCA on this map:
With modern temperature anomalies on this one:
Big difference between then and now.