Comments on “A Surrogate Ensemble Study of Climate Reconstruction Methods: Stochasticity and Robustness”

Scott D. Rutherford Department of Environmental Science, Roger Williams University, Bristol, Rhode Island

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Michael E. Mann Department of Meteorology, and Earth and Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania

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Caspar M. Ammann Climate Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado

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Eugene R. Wahl National Climatic Data Center, National Oceanic and Atmospheric Administration, Boulder, Colorado

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Abstract

In a recent paper, Christiansen et al. compared climate reconstruction methods using surrogate ensembles from a coupled general circulation model and pseudoproxies. Their results using the regularized expectation maximization method with truncated total least squares (RegEM-TTLS) appear inconsistent with previous studies. Results presented here show that the poor performance of RegEM-TTLS in Christiansen et al. is due to 1) their use of the nonhybrid method compared to the hybrid method; 2) a stagnation tolerance that is too large and does not permit the solution to stabilize, which is compounded in another paper by Christiansen et al. by the introduction of an inappropriate measure of stagnation; and 3) their use of a truncation parameter that is too large. Thus, the poor performance of RegEM-TTLS in both Christiansen et al. papers is due to poor implementation of the method rather than to shortcomings inherent to the method.

Corresponding author address: Scott Rutherford, Department of Environmental Science, Roger Williams University, Bristol, RI 02809. Email: rutherford@fox.rwu.edu

Abstract

In a recent paper, Christiansen et al. compared climate reconstruction methods using surrogate ensembles from a coupled general circulation model and pseudoproxies. Their results using the regularized expectation maximization method with truncated total least squares (RegEM-TTLS) appear inconsistent with previous studies. Results presented here show that the poor performance of RegEM-TTLS in Christiansen et al. is due to 1) their use of the nonhybrid method compared to the hybrid method; 2) a stagnation tolerance that is too large and does not permit the solution to stabilize, which is compounded in another paper by Christiansen et al. by the introduction of an inappropriate measure of stagnation; and 3) their use of a truncation parameter that is too large. Thus, the poor performance of RegEM-TTLS in both Christiansen et al. papers is due to poor implementation of the method rather than to shortcomings inherent to the method.

Corresponding author address: Scott Rutherford, Department of Environmental Science, Roger Williams University, Bristol, RI 02809. Email: rutherford@fox.rwu.edu

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