Figure
12.7: Global mean surface temperature anomalies relative to the 1880 to 1920
mean from the instrumental record compared with ensembles of four simulations
with a coupled ocean-atmosphere climate model (from Stott et al., 2000b; Tett et
al., 2000) forced (a) with solar and volcanic forcing only, (b) with
anthropogenic forcing including well mixed greenhouse gases, changes in
stratospheric and tropospheric ozone and the direct and indirect effects of
sulphate aerosols, and (c) with all forcings, both natural and anthropogenic.
The thick line shows the instrumental data while the thin lines show the
individual model simulations in the ensemble of four members. Note that the data
are annual mean values. The model data are only sampled at the locations where
there are observations. The changes in sulphate aerosol are calculated
interactively, and changes in tropospheric ozone were calculated offline using a
chemical transport model. Changes in cloud brightness (the first indirect effect
of sulphate aerosols) were calculated by an offline simulation (Jones et al.,
1999) and included in the model. The changes in stratospheric ozone were based
on observations. The volcanic forcing was based on the data of Sato et al.
(1993) and the solar forcing on Lean et al. (1995), updated to 1997. The net
anthropogenic forcing at 1990 was 1.0 Wm-2 including a net cooling of
1.0 Wm-2 due to sulphate aerosols. The net natural forcing for 1990
relative to 1860 was 0.5 Wm-2 , and for 1992 was a net cooling of 2.0
Wm-2 due to Mt. Pinatubo. Other models forced with anthropogenic
forcing give similar results to those shown in b (see Chapter 8, Section 8.6.1, Figure 8.15; Hasselmann
et al., 1995; Mitchell et al., 1995b; Haywood et al., 1997; Boer et al., 2000a;
Knutson et al., 2000).
