Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 44

the much higher values of aerosol forcing used by C&L. As in the case of Hansen
and Sato, C&L did not consider changes in humidity or cloudiness.
Hargreaves and Annan (2008) (H&A) wrote a commentary on the paper by
C&L. They pointed out (properly) that the data in Figure 2.10 are vacillating and,
depending on exactly how one chooses the data points, one can derive different
results. They provided two examples. In their first example, they chose to read the
temperature curves such that D T 2 0.8 C instead of the value 2.16 C used by
C&L. 3 In this case, however, the dust forcing turns out to be negative—the
implication is that it was less dusty at the LGM. Hargreaves and Annan (2008)
seem to imply that this is an equally good interpretation of the Vostok data.
However, there are two things wrong with this. One is that the choice of tempera-
tures by H&A does not fit the data well in Figure 2.10 . But, more importantly, the
end result of a negative dust forcing at the LGM is contrary to our physical
understanding and suggests that the figures chosen by H&A cannot be correct.
In their second example, H&A claimed that they arrived at a dust forcing of
0.9 1.2W/m 2 , as compared with the estimate by C&L of 58 0.056 ¼ 3.25W/m 2 .
This led to an estimate of D T G 2

5 0.7 C for a doubling of CO 2 from 280 to
560 ppm. However, H&A did not specify which temperatures they used in this
calculation, so it is impossible to reproduce what they did. H&A then extrapolated
beyond science by asserting that an estimate D T G 2

:

5 0.7 C for a doubling
of CO 2 from 280 to 560 ppm ''does not pose any significant challenge to the
widely-held view that climate sensitivity is likely to lie in the range 2-4.5 C
[3.25 C 1.25 C].'' H&A evidently desired to derive as high a climate sensitivity
as they could from glacial-interglacial transitions, and the best they could do was
2.5 0.7 C—which they say does not pose a challenge to 3.25 1.25 C. If we con-
sider that C&L (known skeptics) derived a value of D T G ¼ 1.8 C for a doubling of
CO 2 from 280 to 560 ppm, and H&A (defenders of orthodoxy 4 ) derived 2.5 C, it
seems possible that perhaps the most credible value from this type of analysis lies
between these values.
None of these calculations take into account potential changes in humidity
and cloudiness during these transitions, which are likely to be as large or larger
than the forcings that were included.
If one were to simplistically take the result of Kohler et al. (2009) that a
forcing of 12.43W/m 2 produces a D T G of 5.8 C, one might conclude that their
estimate of climate sensitivity is

:

¼ 5.8/12.43 ¼ 0.47 C/(W/m 2 ), which agrees with
the result of C&L, although the data are different in both cases. It appears likely
that Kohler et al. made the most detailed analysis of the forcings, and it is poss-
ible that their estimate of the total forcing (12.4W/m 2 ) is likely to be the most
reliable. However, great uncertainty remains regarding D T G .If D T G is as small as

3 They did not actually provide the number 0.8 C but they did provide a graph and that was the
value I read from their graph.
4 One can discern the attitude of these authors toward orthodoxy regarding D T G for doubling
of CO 2 from their other publications. Furthermore, in the cited reference, H&A emphasize that
estimates by the IPCC are inviolable.