Climate Change 2001:
Working Group I: The Scientific Basis
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Figure 12.10: Comparison between the amplitude of anthropogenic signals from observed and modelled JJA trend patterns using fingerprints from two different climate models (ECHAM3/LSG and HadCM2) and data from five climate models. (a) Comparison of the amplitude of a single greenhouse gas + sulphate aerosol (GS) signal (expressed as change in global mean temperature [°C] over 50 years). Results show that a significant GS signal can be detected in observed trend patterns 1949 to 1998 at a 5% significance level (one-sided test), independent of which pair of fingerprints was used. The observed signal amplitude is consistent with contemporaneous GS amplitudes for most modelsí GS simulations. 90% confidence intervals are shown by solid lines for estimates using ECHAM3/LSG fingerprints and by dashed lines for estimates based on HadCM2 fingerprints. Cases where a modelís and the observed amplitude disagree are marked by a cross on the axis. (b) and (c) show an estimate of the observed amplitude of a greenhouse gas signal (horizontal axis) and a sulphate aerosol signal (vertical axis) estimated simultaneously. Both signal amplitudes can be estimated as positive from observations based on ECHAM3/LSG fingerprints shown in (b) while only the greenhouse gas signal is detected based on HadCM2 fingerprints shown in panel (c). The amplitudes of both signals from the observations are compared with those from model simulations forced with various forcing histories and using different climate models (1: HadCM2; 2: ECHAM3/LSG; 3: GFDL; 4: ECHAM4/OPYC; 5: CCCma1; 6: CCCma2). Simulations with symbols shown in black are consistent with observations relative to the uncertainty in observations (grey ellipse) and that of the model simulations (not shown). Simulations which are inconsistent are shown in grey. Model simulations where only a single ensemble member is available are illustrated by thin symbols, those based on ensembles of simulations by fat symbols.
Results from consistency tests indicate that most greenhouse gas only simulations (G, shown by "x") are inconsistent with observations. Ten of the GS simulations in both panels are in agreement with observed trend patterns, discrepancies arise mostly from the magnitude of a sulphate signal (vertical axis). The failure to detect a sulphate signal as well as a greenhouse gas signal in panel (c) is due to the two signals being very highly correlated if only spatial patterns are used- this makes separation of the signals difficult. These results show that estimates of a sulphate aerosol signal from observations are model dependent and quite uncertain, while a single anthropogenic signal can be estimated with more confidence.
All units are in °C/50 year, values in the upper right quadrant refer to a physically meaningful greenhouse warming and sulphate aerosol cooling signal. The consistency test establishes whether the difference between a modelís and the observed amplitude estimate is significantly larger than the combined uncertainty in the observations (internal variability + observational uncertainty) and the model simulation (internal variability). The figure is derived by updating the data used by Barnett et al. (1999) (for details of the analysis see Hegerl et al., 2000) and then applying a simple linear transformation of the multi-regression results (Hegerl and Allen, 2000).
Results for 1946 to 1995 period used by Barnett et al. (1999) are similar, except fewer of the models in b and c agree with observations and the case of both signals being zero in c is not rejected. Simulations of natural forcing only ending before 1998 are also rejected in that case.

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