Climate reconstruction by regression – 32 variations on a theme

Regression-based methods fail to provide a sufficiently unique reconstruction of a given millennial history of Northern Hemisphere mean temperature. They instead offer a multitude of variants, depending on the specific data processing scheme. Using a simulated climate history with noise-disturbed pseudo-proxies, we systematically test a set of such configurations, each of which appears to be a priori reasonable, with existing applications elsewhere. This results in an entire spectrum between practically useless and almost perfect reconstructions. The reason lies in the fact that the training variations are not representative of the full millennium, and the regression equations have to be extrapolated. This creates an error that is proportional to both the model uncertainty and the proxy amplitudes. Estimation of that uncertainty is paramount for a useful millennial reconstruction, especially if it is of the parameter-loaded multiproxy type.