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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
to 560 ppm): 3.7W/m 2 (see F igure 2.8 ) to obtain D T G 3.0 C for a doubling of
the CO 2 concentration. With the lower value of
Forcing Þ¼ 4
:
5
=
8
:
2 0
:
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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