Hughes, M. K., and H. F. Diaz, 1994: Was there a "Medieval Warm Period", and if so, where and when? Climate Change, 26, 109-142. Reprinted in The Medieval Warm Period, M. K. Hughes and H. F. Diaz (Eds.), Kluwer Academic Publishers, 109-142.
It has frequently been suggested that the period encompassing the ninth to the fourteenth centuries A.D. experienced a climate warmer than that prevailing around the turn of the twentieth century. This epoch has become known as the Medieval Warm Period, since it coincides with the Middle Ages in Europe. In this review a number of lines of evidence are considered, (including climate-sensitive tree rings, documentary sources, and montane glaciers) in order to evaluate whether it is reasonable to conclude that climate in medieval times was, indeed, warmer than the climate of more recent times. Our review indicates that for some areas of the globe (for example, Scandinavia, China, the Sierra Nevada in California, the Canadian Rockies and Tasmania), temperatures, particularly in summer, appear to have been higher during some parts of this period than those that were to prevail until the most recent decades of the twentieth century. These warmer regional episodes were not strongly synchronous. Evidence from other regions (for example, the Southeast United States, southern Europe along the Mediterranean, and parts of South America) indicates that the climate during that time was little different to that of later times, or that warming, if it occurred, was recorded at a later time than has been asssumed. Taken together, the available evidence does not support a global Medieval Warm Period, although more support for such a phenomenon could be drawn from high-elevation recores than from low-elevation records.
The available data exhibit significant decadal to century scale variability throughout the last millennium. A comparison of 30-yr averages for various climate indices places recent decades in a longer term perspective.