The Evolution of Climate Over the Last Millennium

S. Hemisphere Temp. The Evolution of Climate Over the Last Millennium
Science, v.292(5517), pp.662, April 27, 2001

P. D. Jones*, T. J. Osborn, K. R. Briffa

Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK.

*To whom correspondence should be addressed.
E-mail: p.jones@uea.ac.uk


ABSTRACT:
Knowledge of past climate variability is crucial for understanding and modeling current and future climate trends. This article reviews present knowledge of changes in temperatures and two major circulation features--El Nio-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO)--over much of the last 1000 years, mainly on the basis of high-resolution paleoclimate records. Average temperatures during the last three decades were likely the warmest of the last millennium, about 0.2C warmer than during warm periods in the 11th and 12th centuries. The 20th century experienced the strongest warming trend of the millennium (about 0.6C per century). Some recent changes in ENSO may have been unique since 1800, whereas the recent trend to more positive NAO values may have occurred several times since 1500. Uncertainties will only be reduced through more extensive spatial sampling of diverse proxy climatic records.


Fig.1 Proxy Source Map Fig.1. Schematic map indicating the principal regions from which well-dated pre-1750 temperature information could potentially be obtained with approximately annual resolution. Additional proxies providing such information in more limited regions have not been included; neither have the many proxies providing precipitation or drought information, nor those providing temperature information with only decadal-to-century time scale resolution.

Fig. 2 N. Hemisphere Temp. Fig. 2. Northern Hemisphere surface temperature anomalies (C) relative to the 1961-1990 mean (dotted line). Annual mean land and marine temperature from instrumental observations (black, 1856-1999) and estimated by Mann et al. (red, 1000 to 1980) and Crowley and Lowery (orange, 1000-1987). April to September mean temperature from land north of 20N estimated by Briffa et al. (green, 1402-1960) and estimated by re-calibrating (blue, 1000 to 1991) the Jones et al. Northern Hemisphere summer temperature estimate. All series have been smoothed with a 30-year Gaussian-weighted filter. (B) Standard errors (SE, C) of the temperature reconstructions as in (A), calculated for 30-year smoothed data. The proxy average series do not extend to the present because many of the constituent series were sampled as long ago as the early 1980s.

Fig.3. S. Hemisphere Temp.

Fig. 3. Southern Hemisphere surface temperature anomalies (C) relative to the 1961-1990 mean (dotted line). Summer (December to February) land and marine temperature from instrumental observations (black, 1856-1999) and from estimates by Jones et al. (blue, 1600-1991). Annual mean (January to December) land and marine temperature estimated by Mann et al. (red, 1700-1980). All series have been smoothed with a 30-year Gaussian-weighted filter.


Fig. 4 Boreal Winter SOI

Fig. 4. Boreal winter Southern Oscillation Index (SOI, standardized anomalies relative to 1961-1990). Observed October to March mean SOI (black, 1867-2000), the reconstruction of Stahle et al., rescaled to match the observed mean and variance over 1867-1977 (blue, 1706-1977), and the Nio 3 sea surface temperature reconstruction of Mann et al., inverted and then rescaled to match the observed SOI mean and variance over 1867-1977 (red, 1650-1980). All series have been smoothed with a 30-year Gaussian-weighted filter.

(B) Warm and cold (inverted) ENSO event counts within a sliding 50-year window, defined as those events exceeding 1 standard deviation from the mean (mean and standard deviation are from the 1867-1977 common overlap period), from the observed SOI (black), Stahle et al. (blue), and Mann et al. (red). For warm events only, values are also given from the Quinn and Neal historical reconstruction (green, 1526-1988), after choosing an appropriate threshold to match the observed frequency of events over 1867-1977. Sequential years exceeding 1 standard deviation are counted as multiple rather than single events.


Fig. 5 Boreal Winter NAO

Fig. 5. Boreal winter NAO index (standardized anomalies relative to 1961-90). Observed December to March mean NAO (black, 1824-2000), the reconstruction of Cook et al. 2002 (blue, 1400-1979), and the reconstruction of Luterbacher et al. (red, 1500-1990). Both reconstructions have been shifted to match the observed mean over 1901-1974, and all series have been smoothed with a 30-year Gaussian-weighted filter.


To read or view the full study, please visit the Science website. It was published in Science Vol. 292, Issue 5517, pp. 662, April 27, 2001.

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