Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations

Michael E. Mann and Raymond S. Bradley
Department of Geosciences, University of Massachusetts, Amherst, Massachusetts

Malcom K. Hughes
Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona


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Published: © 1999, American Geophysical Union
Geophysical Research Letters, Vol. 26, No. 6, p.759

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Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20thcentury is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.


Figure 3:
Millennial temperature reconstruction. (top) NH reconstruction (solid) and raw data (dotted) from AD 1000 1998. Smoother version of NH series (thick solid), linear trend from AD 1000-1850 (dot dashed) and two standard error limits (shaded) are also shown.

(bottom) Power spectrum of the NH series based on full (AD 1000-1980) and pre-calibration (AD 1000-1901) intervals. Robustly estimated median and 90%, 95%, and 99% significance levels relative to red noise are shown [see Mann and Lees, 1996].

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17 March 1999