Geoscience Reference
In-Depth Information
Figure 2.5.
Concentration of CO
2
, in parts per million (ppm)
According to Arrhenius (1896), confirmed by Myrhe (1998), the CO
2
in
the atmosphere acts on the climate in relation to the logarithm of its
concentration. This is why a non-dimensional variable
u
1
is defined (indices
2 and 3 respectively refer to solar irradiance and volcanic activity):
CC
−
~
E
u
=
log(
C
/
C
)
1
E
E
Hence
u
1
= 0 for the pre-industrial value. At each doubling of CO
2
,
u
1
increases by
. After CO
2
has quadrupled,
.
log(
2
)
=
0.6931
u
=
1
.
3862
1
2.4. Solar activity
The fluctuations in solar activity constitute a second potential cause for
climatic variations. This has a number of different effects.
Total irradiance
A first manifestation of solar activity is the total energy flow intercepted,
at Earth's level, in
W
(TSI : Total Solar Irradiance). In fact, the solar
constant, of around 1366
Wm
-
2
, varies little (variations of around
o
/
oo
). The
first satellite measurements date from 1978. Since satellites have a limited
−
2
/
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