K.R. Briffa, P.D. Jones, F.H. Schweingruber and T.J. OsbornComplete Scientific Reference
Summary:A team of scientists has developed a circum-northern Hemisphere network of temperature sensitive tree-ring density data that was used to generate a 600 year record of Northern Hemisphere summer temperatures in this study. The calibration of tree-ring data to instrumental temperature records yielded a high-quality quantitative record of past hemispheric summer temperatures.
The data used in this study are distinct from the data used in other long temperature reconstructions, none of which have been based solely on tree-ring data. However, as with the other available state-of-the-art long temperature reconstructions, the results of Briffa et al. indicate that the 20th century is the warmest of the last 400-600 years. Many of the short (about 1-5 year) cold events during the last six centuries can be explained by the cooling effects of large volcanic eruptions.
The authors were able to compare (or calibrate) their density records directly against instrumental data; note that the tree-ring density records become de-coupled from temperature after 1950, possibly due to some large-scale human influence that caused wood densities to decline. Thus, the reconstructed temperature record after 1960 is considered unreliable.
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