Figure 15: 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 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 off line simulation and included in the model. The changes in stratospheric
ozone were based on observations. The volcanic and solar forcing were based on
published combinations of measured and proxy data. 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 Mount Pinatubo. Other models forced with anthropogenic forcing give similar
results to those shown in (b). [Based on Figure 12.7]
