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Table 5.11 Global mean RF for three types of anthropogenic aerosol
Type of aerosol
˖
=1
“
More realistic
” ˖
F (W/m
2
)
F (W/m
2
)
ʔ
ʔ
T
s
(
°
C)
ʔ
ʔ
T
s
(
°
C)
Sulphate
−
0.28
−
0.19
−
0.20
−
0.11
Organic
−
0.41
−
0.25
−
0.22
−
0.08
Dust
−
0.53
−
0.28
−
0.12
−
0.09
Total
−
1.22
−
0.72
−
0.54
−
0.28
Hansen et al. (1998) noted that most reliable value of total RF due to aerosol
constitutes
−
0.4
±
0.3Wm
−
2
instead of the value
−
0.54 given in Table
5.6
, though
in any case this estimate remains very uncertain due to unreliable input data on
aerosol properties.
Anthropogenic RF changes due to clouds are undouptedly more substantial than
those due to aerosol, but they are even more uncertain. Such changes (including the
impact of aircraft contrails) mainly result from the indirect impact of anthropogenic
aerosol, which causes (functioning as condensation nuclei) variations of cloud
droplets size distribution and optical properties. Rough estimates give the
“
cloud
”
RF
1.5 W m
−
2
, but it can change by an order of magnitude (depending on
prescribed input parameters). Conditional value
1
þ
0
:
5
from
−
1to
−
1
Wm
−
2
can be assumed. Some
increase of cloud amount observed in the 20th century may be attributed to indirect
impact of aerosol. Speci
cation of the obtained estimates requires an accomplishment
of complex observational programmes in different regions of the globe.
The contribution of land-use changes into RF variations is connected with
processes of deforestation, deserti
cation, and biomass burning, which affect the
surface albedo and roughness as well as evapotranspiration. It is also important that
the bare surface albedo changes more strongly being covered with snow than
vegetation. Approximate estimates of the ERB change due to land-use evolution
were
0.2 W m
−
2
.
The natural RF due to SC changes during the last century (including an indirect
impact on the ozone layer) can be assumed to equal 0.4
−
0.2
±
0.2 W m
−
2
. Since the total
RF constitutes only about 1 W m
−
2
, the contribution of extra-atmospheric insolation
variability could play a substantial role. Volcanic eruptions cause RF changes from
0.2 to
±
0.5 W m
−
2
(these estimates are, however, conditional). For analysis of
possible anthropogenic impact on global climate, extremely important are estimates
of sensitivity of the climate system to external forcings. Hansen et al. (1998)
assumed that the change of global mean SAT at a doubled CO
2
concentration
should constitute 3
−
C. Since the RF estimates are not reliable enough, it is
expedient to use different scenarios of RF change. One of the developments in this
sphere is a study of Tett et al. (1999) and IPCC (2007).
The simulations of the 19th and 20th centuries are based upon changes in long-
lived greenhouse gases (LLGHGs) that are reasonably constrained by the observa-
tional record. Therefore, the models have qualitatively similar temporal evolutions
of their radiative forcing time histories for LLGHGs (e.g., see Fig.
5.15
). However,
±
1
°
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