Presumed Parameters - Climate Change: Identification and Projections - page 61

of the SPM: in the exhaustive list of radiative forcing (pages 11 and 12), no
reference is made to any solar forcing other than energy flow itself.
Everywhere else, a high level of confidence is reaffirmed regarding the fact
that variations in solar irradiance could only have a minor influence on
climate change.

To our knowledge, none of the IPCC models (GCM or EBM) include an
“amplifying mechanism”, nor any other mechanism, whether physically
based or empirical. In conclusion, the sensitivity to solar activity, for the
IPCC, is therefore reduced to only the sensitivity to energy flow. The upper
limit defined above (

1. 6 2 C / Wm −

2

°

) thus quantifies the IPCC's official

position.

On the contrary, since our goal is to compare these assumptions to
observations, we will not impose an upper limit for sensitivity to irradiance
in our algorithms, even if the result may be identified values which
significantly exceed this assumed limit.

5.6. Sensitivity to volcanic activity S 3

Nowhere in the IPCC reports is the coefficient of radiative forcing
quantified, nor is the sensitivity to volcanic activity (measured by optical
depth). However, a number of authors, including Hansen (2002), have
looked into the issue. Wigley, et al ., (2005), cited by Gao (2008), give the
successive values 30 Wm -2 , 27 Wm -2 , 21 Wm -2 , before settling on 20 Wm -2
(per unit of optical depth) as a “somewhat arbitrary” estimate for the
conversion factor α 3 of the optical depth in radiative forcing.

Multiplying the bounds by 20 0.27 ° CWm /

− 2

< S < 1.62 ° CWm /

− 2

G

Wm -2 gives the range for S 3 :

5 . 4 ° C < S < 32 . 4 ° C

3

Identification will confirm this order of magnitude.

5.7. Climate or anthropogenic sensitivity

The concentration of CO 2 is the only input in our model which is related
to human activity. We know that this also entails the emission of other