Wilson, A.T., Hendy, C.H. and Reynolds, C.P. 1979. Short-term climate change and New Zealand temperatures during the last millennium. Nature 279: 315-317.
Description
Temperatures derived from an 18O/16O profile through a stalagmite found in a New Zealand cave (40.67S, 172.43E) revealed the Medieval Warm Period to have occurred between AD 1050 and 1400 and to have been 0.75C warmer than the Current Warm Period.

18O/16O are the isotopes of oxygen. The MWP was global and was warmer than today.