Northern Hemisphere temperature patterns in the last 12 centuries

According to Ljungqvist et al. (2012), “a number of Northern Hemispheric (NH) temperature reconstructions covering the last 1-2 millennia, using temperature-sensitive proxy data, have been made to place the observed 20th century warming into a long-term perspective.” And they say that “these studies generally agree on the occurrence of warmer conditions ca. 800-1300 AD and colder conditions ca. 1300-1900 AD, followed by a strong warming trend in the 20th century,” noting that “the earlier warm period is usually referred to as the Medieval Warm Period … whereas the later colder period is usually referred to as the Little Ice Age).” In addition, and “related to this issue,” as they continue, “is the question of whether or not the current warmth has exceeded the level and geographic extent of the warmth in the last millennium,” i.e., that of the Medieval Warm Period.
As Ljungqvist et al. describe it, they developed “a new reconstruction of the spatio-temporal patterns of centennial temperature variability over the NH land areas for the last twelve centuries based on 120 proxy records,” which were “retrieved from a wide range of archives including, but not limited to, ice-cores, pollen, marine sediments, lake sediments, tree-rings, speleothems and historical documentary data.” And with respect to how big an improvement their data base makes compared to prior studies of this type, in terms of the amount and distribution of data employed, they present a list of antecedent analyses where the number of proxy records used ranged from only 3 to 46 (compared to their 120), and where the number of records with annual resolution ranged from only 3 to 30, whereas their study included 49 such annual-resolution records.

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