Kitagawa, H. and Matsumoto, E. 1995. Climatic implications of δ13C variations in a Japanese cedar (Cryptomeria japonica) during the last two millennia. Geophysical Research Letters 22: 2155-2158.
The authors analyzed δ13C variations of Japanese cedars growing on Yakushima Island, southern Japan (30°20'N, 130°30'E), in an effort to reconstruct a high-resolution proxy temperature record over the past two thousand years. The Medieval Warm Period occurred between AD 800-1250 and from the authors' Figure 3, peak warmth during this time was about 1°C above that of the Current Warm Period.