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Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia

  1. Michael E. Mann*,†,
  2. Zhihua Zhang*,
  3. Malcolm K. Hughes‡,
  4. Raymond S. Bradley§,
  5. Sonya K. Miller*,
  6. Scott Rutherford, and
  7. Fenbiao Ni‡
  1. *Department of Meteorology and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802;
  2. ‡Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721;
  3. §Department of Geosciences, University of Massachusetts, Amherst, MA 01003-9298; and
  4. Department of Environmental Science, Roger Williams University, Bristol, RI 02809

  1. Communicated by Lonnie G. Thompson, Ohio State University, Columbus, OH, June 26, 2008 (received for review November 20, 2007)

Abstract

Following the suggestions of a recent National Research Council report [NRC (National Research Council) (2006) Surface Temperature Reconstructions for the Last 2,000 Years (Natl Acad Press, Washington, DC).], we reconstruct surface temperature at hemispheric and global scale for much of the last 2,000 years using a greatly expanded set of proxy data for decadal-to-centennial climate changes, recently updated instrumental data, and complementary methods that have been thoroughly tested and validated with model simulation experiments. Our results extend previous conclusions that recent Northern Hemisphere surface temperature increases are likely anomalous in a long-term context. Recent warmth appears anomalous for at least the past 1,300 years whether or not tree-ring data are used. If tree-ring data are used, the conclusion can be extended to at least the past 1,700 years, but with additional strong caveats. The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels.

Footnotes

  • †To whom correspondence should be addressed. E-mail: mann{at}psu.edu

  • Author contributions: M.E.M., M.K.H., and R.S.B. designed research; Z.Z., S.K.M., S.R., and F.N. performed research; M.E.M. analyzed data; and M.E.M., M.K.H., and R.S.B. wrote the paper.

  • The authors declare no conflict of interest.

  • This article contains supporting information online at www.pnas.org/cgi/content/full/0805721105/DCSupplemental.

  • ↵‖ Here and elsewhere in the paper, “recent” is taken to correspond to the past decade unless noted otherwise.

  • ↵** For the purpose of this paper, we adopt the definition of “likely” used by the Intergovernmental Panel on Climate Change (IPCC): i.e., that the probability of the assertion being true is estimated as between 66% and 90%.

  • Freely available online through the PNAS open access option.

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