the excursions of the smoothed time series above or below selected thresholds (± 1 or 2 standard deviations). The authors showed that the excursions on the warm side were largest in the 20th century and that the deviation index produced graphs similar to those from other research (Figure 11-2). Osborn and Briffa’s method shifts the focus of the argument from the statistical methods used to reconstruct temperature time series to the selection of proxy and site. The degree of spatial coherence shown by Osborn
FIGURE 11-2 Upper panel: Fourteen temperature-related proxy records selected on the basis of their correlation with the instrumental record, as described by Osborn and Briffa (2006), filtered to remove variations on timescales less than 20 years and then normalized to have zero mean and unit standard deviation during the period A.D. 800–1995. Lower panel: Fraction of records available in each year that have normalized values greater than zero (red line), greater than 1 (light red shading), greater than 2 (dark red shading), less than zero (blue line), less than –1 (light blue shading), and less than –2 (dark blue shading), with the latter three series multiplied by –1 before plotting. SOURCE: Osborn and Briffa (2006).