Figure 16:(a) Estimates
of the “scaling factors” by which the amplitude of several model-simulated
signals must be multiplied 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. Signals are defined as the ensemble mean response to external
forcing expressed in large-scale (>5,000 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 with the assumption
that 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
over predict 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). One is 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 internal variability, as simulated by
these various models, can be rejected 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, multiple factors are estimated 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. [Based
on Figure 12.12]
