Comparison of Large-Scale Proxy-Based Temperature Reconstructions over the Past Few Centuries M.E. Mann, Department of Environmental Sciences University of Virginia, USA S. Rutherford (U. Virginia, USA), T.J. Osborn (CRU, UK) EGS XXVI General Assembly Nice, France March 29, 2001

OVERVIEW Review of Existing Proxy-Based Northern Hemisphere Temperature Reconstructions Sensitivity of Proxy-Based Estimates of Northern Hemisphere Temperature to: Conclusions Non-stationarity Differing proxies, distribution, Seasonality, Differing Statistical Methodology Regularized Expectation Maximization (“RegEM”) method

Science

MANN ET AL MULTIPROXY NETWORK Symbols: Tree – tree-ring; plus – PCs of dense tree-ring networks, star - ice core/ice melt, ‘C’ – coral, early instrumental- square – temperature – diamond – precipitation. From Mann et al., 1998.

BRIFFA ET AL MXD DENDRO NETWORK

MULTIPROXY MXD DATA

Alternative Statistical Approach: (T. Schneider, Analysis of Incomplete Climate Data: Estimation of Mean Values and Covariance Matrices and Imputation of Missing Values, J. Climate, 14, , 2001) Regularized Expectation Maximization (“RegEM”) Algorithm

Regularized Expectation Maximization Algorithm iterate Estimate Covariance Matrix Regularize with ridge regression Adjust Imputed Values

SENSITIVITY TO NON- STATIONARY?

submitted

Training Interval Reconstruction Interval

NINO3 INDEX

SENSITIVITY TO TYPE OF PROXY DATA USED?

Mann et al, Earth Interactions, 2000

NORTHERN HEMISPHERE (0-90N) (ANNUAL MEAN)

NORTHERN HEMISPHERE (0-90N) (ANNUAL MEAN)

ANNUAL MEAN RECONSTRUCTIONS

SENSITIVITY TO STATISTICAL METHODOLOGY?

ANNUAL MEAN COMPARISONS

NOT MASKED MASKED WARM SEASON COMPARISONS (HEMISPHERIC)

WARM SEASON COMPARISONS (EXTRATROPICAL)

PATTERN CORRELATION

CONCLUSIONS Main hemispheric trends in proxy-based climate reconstructions are robust with respect to the precise methodology, type of proxy indicators used, seasonality of reconstructions. Non-stationary during the calibration period does not seem to be important. The principle source of differences in hemispheric reconstructions is associated with spatial sampling variations. The next most significant source of sensitivity of hemispheric reconstructions is the precise nature of the statistical reconstruction methodology