Overall Results - Climate Change: Identification and Projections - page 96

At this stage, the existence of a mechanism of solar activity with a
powerful influence over the climate seems irrefutable, unless we reject any
paleotemperature which is significantly different from the Jones and Mann
reconstruction.

8.3. Hypothesis test

Hypothesis of solar sensitivity restricted to the energy factor

We have seen (see section 5.5) that this hypothesis is expressed by the
constraint S 2 <1.62. Figure 8.1 shows (marked o ) , the couples S 1 , S 2
estimated under this constraint. The table of deviations between the
constrained models and the freely identified models is the following (rows:
temperature reconstructions, columns: irradiances):

21.7 26.7 13.5 16.3

⎡ ⎤

⎢ ⎥

38.3 20.6 16.1 14.8

⎢ ⎥

D = ⎢

⎥

439.8 32.5 344.7 35.1

⎢ ⎥

5.7 12.9 0.2 1.1

⎣ ⎦

The rejection threshold for a model at 6 degrees of freedom is D = 10.6.
This table therefore confirms the conclusions of the previous paragraph: the
hypothesis of a low sensitivity to solar activity must be rejected as being
incompatible with paleoclimatic reconstructions, apart from that of Jones
and Mann (in the final line).

Hypothesis of zero climate sensitivity

According to the IPCC, a climate sensitivity below 1 ° C is extremely
unlikely , and even more so, a sensitivity of zero. Nevertheless, we have
tested this hypothesis S clim = 0. We should recall that the climate sensitivity
identified is the sensitivity to the concentration of CO 2 , considered as a
global indicator of human activity. It combines the climate sensitivity in its
strictest sense (to CO 2 ), with sensitivities to other GHGs, industrial
aerosols and changes in land use. In the event where negative effects
are greater than positive forcings, the anthropogenic sensitivity could