Effect of scaling and regression on reconstructed temperature amplitude for the past millennium

Examination of large‐scale millennial‐long temperature reconstructions reveals a wide range of datasets and methods used for calibration. Proxy time series are commonly calibrated against overlapping instrumental records, representing different seasons, Northern Hemisphere latitudinal bands, and including or excluding sea surface temperature data. Methodological differences include, using scaling or regression, the calibration time period, and smoothing data before calibration. We find that these various approaches alone can result in differences in the reconstructed temperature amplitude of about 0.5°C. This magnitude is equivalent to the mean annual temperature change for the Northern Hemisphere reported in the last IPCC report for the 1000–1998 period. A more precise assessment of absolute reconstructed temperature amplitudes is necessary to help quantify the relative influences of forcing mechanisms in climate models.