Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 43

Table 2.1. Parameters used by C&L.

CO 2 CH 4 D T Forcing via GHG

( C) (W/m 2 )

(ppm) (ppb)

42,000 years ago 209 548

LGM 182 340

Pre-industrial 285 667

42,000 ) LGM 2.16 0.23 0.93

LGM ) pre-industrial 4.6 0.5 2.67

they were not able to attribute forcings to these changes a priori. They assumed
that the forcing due to aerosols were 58X and 53X for the LGM ) pre-industrial,
and 42,000 years ago ) LGM transitions, respectively, but X could not be speci-
fied a priori. In order to estimate the forcing due to aerosols, C&L carried out a
comparison of the two transitions, assuming that the relation between D T G and
total forcing was the same for both transitions. Thus, they put

D T 1
F GHG1 þ F Alb1 þ F Aer1 ¼

D T 2
F GHG2 þ F Alb2 þ F Aer2

in which transition (1) refers to LGM ) pre-industrial, and transition (2) refers to
42,000 years ago ) LGM. Their estimates for D T 1 and D T 2 and F GHG1 and F GHG2
are given in Table 2.1 . Their estimate for F Alb1 was 3.5W/m 2 , but they did not
seem to specify what they used for F Alb2 .IfF Alb2 is known and setting
F Aer1 ¼ 58X and F Aer2 ¼ 53X, the above equation provides a means to estimate
X. C&L reported that their best estimate for X was 0.056W/m 2 . Working back-
wards, we may surmise that they must have used F Alb2 ¼ 1.58W/m 2 . Using the
above value for X, they estimated the total forcing for the LGM ) pre-industrial
to be 2

056 ¼ 9.4W/m 2 and, with D T 1 ¼ 4.6 C, climate sensitiv-

:

67 þ 3

:

5 þ 58 0

:

¼ 4.6/9.4 0.5 C/(W/m 2 ). This implies that when CO 2 goes from 280 to
560 ppm, the expected temperature rise is 0.5 3.7 1.8 C. C&L also examined
prior glacial to interglacial transitions and, from this, estimated slightly higher
values for

ity is

. However, they pointed out:

''At this time it is not clear whether these higher sensitivities, compared to the
climate sensitivity deduced from the LGM to Holocene transition, really reflect
higher climate sensitivity at the time of the considered climate transitions or
whether they are artifacts due to imperfect ice core data and uncertainties in
the used approximations.''

The main difference between the calculations of C&L and Hansen and Sato is