The Northern Hemisphere (NH) temperature reconstruction published by Esper, Cook, and Schweingruber (ECS) in 2002 is revisited in order to strengthen and clarify its interpretation. This reconstruction, based on tree-ring data from 14 temperature-sensitive sites, is best interpreted as a land-only, extra-tropical expression of NH temperature variability. Its strongly expressed multi-centennial variability is highly robust over the AD 1200-1950 interval, with strongly expressed periods of "Little Ice Age" cooling indicated prior to AD 1900. Persistently above-average temperatures in the AD 960-1050 interval also suggest the large-scale occurrence of a "Medieval Warm Period" in the NH extra-tropics. However, declining site availability and low within-chronology tree-ring replication prior to AD 1200 weakens this interpretation considerably.The temperature signal in the ECS reconstruction is shown to be restricted to periods longer than 20 years in duration. After recalibration to take this property into account, annual temperatures up to AD 2000 over extra-tropical NH land areas have probably exceeded by about 0.3degreesC the warmest previous interval over the past 1162 years. This estimate is based on comparing instrumental temperature data available up to AD 2000 with the reconstruction that ends in AD 1992 and does not take into account the mutual uncertainties in those data sets. (C) 2004 Published by Elsevier Ltd.
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