Geoscience Reference
In-Depth Information
Finally, in order to define the input signal
u
of the climatic process to
identify, TSI anomalies are considered in relation to the respective average
values
U
E
of each of them. Furthermore, a factor of 1/4 is introduced, in
relation to the ratio of the area of the cross-section to that of the sphere. Net
irradiance can then be obtained by taking into account the nominal terrestrial
albedo (0.3) with the co-factor 0.7:
1
(in
Wm
-
2
)
uUU
=−××
(
)
0.7
2
E
4
The anomalies for net irradiance are of the order of a tenth of a
W/m
2
.
This will again appear extremely minor compared to the other energy flows.
Clearly, this does not exclude the possibility that solar activity variations, for
which we used TSI as an indicator, may have an effect on the climate by
other mechanisms.
2.5. Volcanic activity
Volcanic activity affects the climate through the production of aerosols
(ash, sulphates). The sun's rays are obscured by these aerosols and partially
reflected back into space, having a negative effect on temperatures. Aerosols
also have an indirect effect by contributing to the formation of clouds in a
complex way depending on the chemical composition and the dimensions of
the particles. The archives of the Antarctic or Greenland ICE can be used to
calculate the optical depth
8
of aerosols caused by eruptions (AOD:Aerosol
Optical Depth), year after year, as well as the dimension of the volcanic
particles. In an identification context, where the aim is not to reconstitute the
corresponding radiative forcing by physical rules, it is good enough for us to
use an indicator, and the optical depth is perfectly suitable. NOAA released a
reconstruction of optical depth over the period 800-2000, designed by T.J.
Crowley and M.B. Unterman [CRO 13].
For input
u
3
of the model, we will take the difference of the AOD with
respect to the average value over the period 800-2000, with a changed sign
so as to create positive sensitivity. Volcanic activity which is temporarily
zero is therefore negative in relation to the average which is thus represented
8 Optical depth depends on wavelength (here, 550 nm). For weak values, optical depth equals
a fraction of the rays obscured.
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