Figure 12.12: (a) Estimates
of the �scaling factors� by which we have to multiply the amplitude
of several model-simulated signals to reproduce the corresponding changes in the
observed record. The vertical bars indicate the 5 to 95% uncertainty range due
to internal variability. A range encompassing unity implies that this combination
of forcing amplitude and model-simulated response is consistent with the corresponding
observed change, while a range encompassing zero implies that this model-simulated
signal is not detectable (Allen and Stott, 2000; Stott et al., 2000a). Signals
are defined as the ensemble mean response to external forcing expressed in large-scale
(>5000 km) near-surface temperatures over the 1946 to 1996 period relative
to the 1896 to 1996 mean. The first entry (G) shows the scaling factor and 5 to
95% confidence interval obtained if we assume the observations consist only of
a response to greenhouse gases plus internal variability. The range is significantly
less than one (consistent with results from other models), meaning that models
forced with greenhouse gases alone significantly overpredict the observed warming
signal. The next eight entries show scaling factors for model-simulated responses
to greenhouse and sulphate forcing (GS), with two cases including indirect sulphate
and tropospheric ozone forcing, one of these also including stratospheric ozone
depletion (GSI and GSIO respectively). All but one (CGCM1) of these ranges is
consistent with unity. Hence there is little evidence that models are systematically
over- or under-predicting the amplitude of the observed response under the assumption
that model-simulated GS signals and internal variability are an adequate representation
(i.e. that natural forcing has had little net impact on this diagnostic). Observed
residual variability is consistent with this assumption in all but one case (ECHAM3,
indicated by the asterisk). We are obliged to make this assumption to include
models for which only a simulation of the anthropogenic response is available,
but uncertainty estimates in these single-signal cases are incomplete since they
do not account for uncertainty in the naturally forced response. These ranges
indicate, however, the high level of confidence with which we can reject internal
variability as simulated by these various models as an explanation of recent near-surface
temperature change.
A more complete uncertainty analysis is provided by the next three entries, which
show corresponding scaling factors on individual greenhouse (G), sulphate (S),
solar-plus-volcanic (N), solar-only (So) and volcanic-only (V) signals for those
cases in which the relevant simulations have been performed. In these cases, we
estimate multiple factors simultaneously to account for uncertainty in the amplitude
of the naturally forced response. The uncertainties increase but the greenhouse
signal remains consistently detectable. In one case (ECHAM3) the model appears
to be overestimating the greenhouse response (scaling range in the G signal inconsistent
with unity), but this result is sensitive to which component of the control is
used to define the detection space. It is also not known how it would respond
to the inclusion of a volcanic signal. In cases where both solar and volcanic
forcing is included (HadCM2 and HadCM3), G and S signals remain detectable and
consistent with unity independent of whether natural signals are estimated jointly
or separately (allowing for different errors in S and V responses). (b) Estimated
contributions to global mean warming over the 20th century, based on the results
shown in (a), with 5 to 95% confidence intervals. Although the estimates vary
depending on which model�s signal and what forcing is assumed, and are less
certain if more than one signal is estimated, all show a significant contribution
from anthropogenic climate change to 20th century warming (from Allen et al.,
2000a).