Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 40

Their estimates for greenhouse gas forcings were 2.25W/m 2 for CO 2
(185 ! 275 ppm), 0.43W/m 2 for CH 4 (350 ! 675 ppb) and 0.32W/m 2 for N 2 O
(200 ! 270 ppb) for a total greenhouse gas forcing of 3.0W/m 2 . They also esti-
mated the forcing due to surface albedo changes to be 3.5W/m 2 , but this estimate
appears to be rather approximate. Nevertheless, they argued that a total negative
forcing of 6.5W/m 2 would bring about the LGM-pre-industrial transition. They
assumed that the D T G associated with this transition was 5 C. In that case, the
Earth's scientific climate sensitivity would be:

75 C m 2

¼ D T G =ð Forcing Þ¼ 5 : = :

5 0

: =ð W = Þ:

0 6

There are several problems with this calculation.
One problem is that the estimate of the forcing due to greenhouse gases
appears to be a bit low. According to Hansen's papers, the forcings due to
various levels of CO 2 (without feedbacks) are as shown in Figure 2.9 . In Figure
2.9 , vertical lines represent:

A ¼ typical CO 2 at glacial maximum in an ice age
B ¼ typical CO 2 during an interglacial period between ice ages
C ¼ current CO 2 level due to human impact on environment
D ¼ CO 2 level after it doubles compared with pre-industrial levels

:

The estimated forcings are shown as vertical double arrows:

F1 ¼ forcing in the transition from a glacial maximum to an interglacial
period ( 3W/m 2 )

F2 ¼ forcing due to CO 2 rise from before the industrial period to the present
( 2W/m 2 ). (Note that according to these calculations, the rise in CO 2
from pre-industrial times to the present has already produced about
half the forcing that will result from doubling CO 2 from pre-industrial
times.)

F3 ¼ forcing due to change from pre-industrial levels to doubled CO 2

( 3.7W/m 2 ).
Thus the forcing due to CO 2 is not 2.25W/m 2 (as claimed), but 3W/m 2 , and
the total forcing due to all greenhouse gases is not 3.0W/m 2 (as claimed), but

3.7W/m 2 .
Another problem is that Hansen and Sato used D T G ¼ 5.0 C, whereas 4.5 C
appears to be a better choice.
In addition, Hansen and Sato did not appear to adequately consider the
forcing due to high dust levels in the atmosphere during the LGM. One estimate
is that dust would produce a forcing of about 1W/m 2 (Crucifix, 2006). It is also
noteworthy that Bielefeld (1997) estimated that at the height of the last ice age
(18,000 ybp ) global radiation absorption was lower by 7-10% than it is today.
That would indicate a negative forcing of 24 to 34W/m 2 , which is far greater than