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Figure 2.9. Forcings due to various levels of CO
2
concentration.
other estimates. Another concern is that no consideration was taken of possible
changes in humidity or cloudiness.
If we modify Hansen and Sato's estimate by taking the forcing as 8.2W/m
2
(instead of 6.5W/m
2
), and if we choose
D
T
G
¼
4.5
C instead of 5.0
C, we obtain:
¼ D
T
55
C
m
2
Þ:
Hansen and Sato used their value for climate sensitivity (0.75
C/(W/m
2
)) in
conjunction with the forcing due to doubling of the CO
2
concentration (from 280
the CO
2
concentration. With the lower value of
=ð
Forcing
Þ¼
4
:
5
=
8
:
2
0
:
=ð
W
=
, one would obtain 2.2
C.
Crucifix (2006) provided alternate estimates of the forcings:
.
Change in sea level and vegetation changes (
4W/m
2
)
.
Reduction of greenhouse gas concentrations (
2.85W/m
2
)
.
Other forcings, dicult to quantify, such as increased dust concentration (
1W/
m
2
).
This sums to 7.85W/m
2
, but Crucifix added: ''There is also a small
contribution due to the surface being, on average, more elevated than today'',
which might bring the total close to the value 8.2W/m
2
previously estimated.
Crucifix felt that the value of
could not be pinned down well, primarily because
of uncertainty in
D
T
G
. He attempted to use climate models to bridge this gap, but
concluded:
the ratio between LGM and CO
2
feedback factors cannot be
accurately estimated from current state-of-the-art coupled models.''
Chylek and Lohmann (2008) carried out an independent estimate of climate
''
...
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