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become negative. The table of deviations associated to S clim = 0 is the
following:
6.0 7.7 2.9 3.0
3.7 4.5 0.4 0.8
1.6 0.4 6.6 3.1
D
=
21.4 31.7 14.6 23.9
As could be expected, a zero anthropogenic sensitivity is compatible with
the first three reconstructions, and incompatible with that of Jones and Mann.
8.4. Comments
The fundamental characteristic of our approach of identification is that
once the input-output data is selected, no degree of freedom is left, meaning
there is no possibility of the conclusions being oriented in one direction or
the other. The constraints of forced identification are openly implemented, in
order to make the analysis process completely explicit. In this context, it is
impossible to reject a given reconstruction for the benefit of another. It is
also impossible to carry out average estimates to obtain the “ most likely
model. Moreover, it is impossible to propose ranges which would result from
the combination of regions which sometimes lack a common intersection.
We therefore concur with the IPCC's attitude whereby, in the absence of
consensus, no “ best ” climate sensitivity is assigned, and we remain with the
presentation of the raw results, leaving the reader the freedom to draw their
own conclusions. We will maintain this position in the following chapter on
climate projections. We can only showcase the overall results obtained,
allowing the reader to interpret them according to the confidence they have
in the various reconstructions and their own preconceived ideas, whether
they are skeptical or alarmist.
An initial conclusion is that the Jones and Mann reconstruction is the only
one which is coherent with the IPCC's assumed climatic parameters. If we
accept this as valid, the other three would have to be rejected, and vice versa.
In the event where the latter three are kept, we can reach two types of
conclusion. The first is positive: solar activity not only explains the previous
large climatic episodes, but also makes a strong contribution to the warming
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