Climate Change 2001:
Working Group I: The Scientific Basis
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Figure 12.2: Coloured lines: power spectra of global mean temperatures in the unforced control integrations that are used to provide estimates of internal climate variability in Figure 12.12. All series were linearly detrended prior to analysis, and spectra computed using a standard Tukey window with the window width (maximum lag used in the estimate) set to one-fifth of the series length, giving each spectral estimate the same uncertainty range, as shown (see, e.g., Priestley, 1981). The first 300 years were omitted from ECHAM3-LSG, CGCM1 and CGCM2 models as potentially trend-contaminated. Solid black line: spectrum of observed global mean temperatures (Jones et al., 2001) over the period 1861 to 1998 after removing a best-fit linear trend. This estimate is unreliable on inter-decadal time-scales because of the likely impact of external forcing on the observed series and the negative bias introduced by the detrending. Dotted black line: spectrum of observed global mean temperatures after removing an independent estimate of the externally forced response provided by the ensemble mean of a coupled model simulation (Stott et al., 2000b, and Figure 12.7c). This estimate will be contaminated by uncertainty in the model-simulated forced response, together with observation noise and sampling error. However, unlike the detrending procedure, all of these introduce a positive (upward) bias in the resulting estimate of the observed spectrum. The dotted line therefore provides a conservative (high) estimate of observed internal variability at all frequencies. Asterisks indicate models whose variability is significantly less than observed variability on 10 to 60 year time-scales after removing either a best-fit linear trend or an independent estimate of the forced response from the observed series. Significance is based on an F-test on the ratio observed/model mean power over this frequency interval and quoted at the 5% level. Power spectral density (PSD) is defined such that unit-variance uncorrelated noise would have an expected PSD of unity (see Allen et al., 2000a, for details). Note that different normalisation conventions can lead to different values, which appear as a constant offset up or down on the logarithmic vertical scale used here. Differences between the spectra shown here and the corresponding figure in Stouffer et al. (2000) shown in Chapter 8, Figure 8.18 are due to the use here of a longer (1861 to 2000) observational record, as opposed to 1881 to 1991 in Figure 8.18. That figure also shows 2.5 to 97.5% uncertainty ranges, while for consistency with other figures in this chapter, the 5 to 95% range is displayed here.

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