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A review of the various global (or hemispheric) millennial temperature reconstructions was carried out. Unlike previous reviews, technical analyses presented via internet blogs were considered as well as the conventional peer-reviewed literature.
There was a remarkable consistency between all of the reconstructions in identifying three climatically distinct periods. These consisted of two relatively warm periods – the “Medieval Warm Period” (c. 800-1200 AD) and the “Current Warm Period” (c. 1900 AD on) – and a relatively cool period – the “Little Ice Age” (c. 1500-1850 AD). Disagreement seems to centre over how the two warm periods compare to each other, and exactly how cold, and continuous the cool period was.
However, many of the assumptions behind the reconstructions have still not been adequately justified. In addition, there are substantial inconsistencies in the data on which they are based, and between proxy-based and thermometer-based estimates.
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Supplementary Information is available through the FigShare website at http://dx.doi.org/10.6084/m9.figshare.977937
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