Evidence from a variety of sources, such as tree rings, ice cores, historical documents, glaciers, geology, and borehole temperatures, suggests that there may have been a period of climatic warming between the ninth and fourteenth centuries, when temperatures were higher than at the start of the 20th century, at least in some parts of the globe. These include Scandinavia, China, the Sierra Nevada in California, the Canadian Rockies and Tasmania. Evidence from other regions, such as the south eastern United States, Mediterranean Europe, and parts of South America, shows that during the same period temperatures were little different from later times. This postulated period has been called the Medieval Warm Period, and is widely believed to have been followed by the Little Ice Age.
Climate reconstructions from tree rings show little evidence for a sustained warm period during the last 1000 years, except in the Sierra Nevada, the Great Basin, and the Polar Urals. However, consistent features are a cooling tendency in the early and mid- 17th century, and a warm period between the mid-12th and early 14th centuries. Ice core evidence is being treated with caution, although it suggests warming during the early centuries of the second millennium. Historical evidence is largely based on records of crop planting in more northern latitudes than the present, and on archaeological evidence from settlements in Iceland and Greenland. Glacial evidence appears stronger, with glacial retreats in Europe and elsewhere before AD 900 and after 1250, but not between these dates. Borehole temperature records are too coarse at present to give accurate indications, but it appears that sometime between late in the first millennium and the middle of the second, there was a period of undetermined length with above average temperatures.
The evidence is not conclusive. Warmer conditions may have prevailed in some seasons in some regions during the period of interest. Much of the data that had been formerly used to present a more conclusive picture of the Medieval Warm Period, and of the Little Ice Age, have now been superseded or discarded. What the current data does show is that variability in climate on a decadal scale is apparent over the last 1000 years, but because the data is so incomplete, it is very difficult to use it to predict future climate trends. (M.K. Hughes and H.F. Diaz, "Was there a 'Medieval Warm Period?", Climatic Change, v.26, p.109-142, March 1994).
GREENPEACE Climate Impacts Database