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

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.

Fig. 1.

Spatial distribution of proxy database (see SI Text for distribution of screened network and other details). Nine different proxy types are denoted with different symbols as shown in the map. Beginning dates of proxy records are represented by color scale.

Fig. 2.

Comparison of CPS: hemisphere screened (A); “no dendro” (B); and EIV full global (C) and “no dendro” (D). NH reconstructions (colored curves; 95% confidence intervals shown by lightly shaded regions of similar color) with decadally smoothed CRU NH land mean series (thick black curve). Reconstructions are shown over late (A.D. 1950–1995) and early (A.D. 1850–1895) validation intervals corresponding to early (A.D. 1850–1949) and late (1896–1995) calibration experiments respectively. RE validation scores are shown (Insets) as a function of the starting date of the network along with 90% and 95% significance levels based on Monte Carlo experiments. Only reconstructions that passed validation are shown. Color bar indicates network starting dates.

Fig. 3.

Composite CPS and EIV NH land and land plus ocean temperature reconstructions and estimated 95% confidence intervals. Shown for comparison are published NH reconstructions, centered to have the same mean as the overlapping segment of the CRU instrumental NH land surface temperature record 1850–2006 that, with the exception of the borehole-based reconstructions, have been scaled to have the same decadal variance as the CRU series during the overlap interval (alternative scaling approaches for attempting to match the amplitude of signal in the reconstructed and instrumental series are examined in SI Text). All series have been smoothed with a 40-year low-pass filter as in ref 33. Confidence intervals have been reduced to account for smoothing.