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On the long-term context for late twentieth century warming


Fig. 5. Comparison of STD and RCS NH reconstructions with mean annual land (20N-90N) temperatures.
Fig. 5. Comparison of STD and RCS NH reconstructions with mean annual land (20°N-90°N) temperatures. Top: Annual time series from 1200 AD. Bottom: 20-year smoothed time series from 1200 AD. Click image for full figure, time series from 713AD.

On the long-term context for late twentieth century warming

Journal of Geophysical Research - Atmospheres
Vol. 111, No. D3, D03103, doi:10.1029/2005JD006352, 07 February 2006.

Rosanne D'Arrigo1, Rob Wilson 2, Gordon Jacoby1

1 Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, New York, USA
2 School of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh, UK
ABSTRACT:
Previous tree-ring-based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the "Medieval Warm Period"(MWP), "Little Ice Age" (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional (STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations (e.g., fifteenth to sixteenth century). Results indicate clear MWP (warm), LIA (cool), and recent (warm) episodes. Direct interpretation of the RCS reconstruction suggests that MWP temperatures were nearly 0.7C cooler than in the late twentieth century, with an amplitude difference of 1.14C from the coldest (1600-1609) to warmest (1937-1946) decades. However, we advise caution with this analysis. Although we conclude, as found elsewhere, that recent warming has been substantial relative to natural fluctuations of the past millennium, we also note that owing to the spatially heterogeneous nature of the MWP, and its different timing within different regions, present palaeoclimatic methodologies will likely "flatten out" estimates for this period relative to twentieth century warming, which expresses a more homogenous global "fingerprint." Therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences, at hemispheric scales, about MWP warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local/regional scale.
Download data from the WDC Paleo archive:
Northern Hemisphere Tree-Ring-Based STD and RCS Temperature Reconstructions, Text or Excel format.
Files include Gulf of Alaska Regional RCS Temperature Reconstruction.

To read or view the full study, please visit the AGU website.
It was published in Journal of Geophysical Research - Atmospheres, Vol. 111, No. D3, D03103, doi:10.1029/2005JD006352, 07 February 2006.
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