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).
