Originally Posted by National Academy of Sciences

Even less confidence can be placed in the original conclusions by Mann et al. (1999) that "the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium" because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods [...]

Prior to about 1600, ...periods of medieval warmth are seen in a number of diverse records, including historical information from Europe and Asia; cave deposits; marine and lake sediments; and ice cores from Greenland, Ellesmere Island, Tibet, and the equatorial Andes. [...]

Using proxies sensitive to hydrologic variables (including moisture-sensitive trees...) to take advantage of observed correlations with surface temperature could lead to problems [...]

For tree ring chronologies, the process of removing biological trends from ringwidth data potentially obscures information on long-term changes in climate. [...]

Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the “Medieval Warm Period”) and a relatively cold period (or “Little Ice Age”) centered around 1700. The existence and extent of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse ...set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia