Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 36

Figure 2.6. Variation of D T G with latitude (adapted from Shakun and Carlson, 2010).
surface air temperature decreased by 7 to 11 C at the LGM compared with
pre-industrial times.
In addition, Schneider von Deimling et al. (2006) estimated a global cooling of
5.9 C during the LGM and a total of all forcings in the range 8.0 to 8.5W/m 2 .
Shakun and Carlson (2010) carried out an extensive review of the LGM-
interglacial transition. They found, as would be expected, that the D T in this
transition varied with latitude as shown in Figure 2.6. Their estimate of D T G (the
temperature at the LGM minus the pre-industrial temperature) was 4.5 C. If
we couple the temperature during the LGM (14.3 C 4.5 C ¼ 9.8 C) with an
estimated CO 2 concentration of 170-180 ppm, we can plot a point representing
the LGM, as shown in Figure 2.7 .
Hansen and Sato (2011) estimated D T G as follows. Estimates exist for deep-
sea temperature over the past 70 million years, as shown in Figure 2.7 . While
oxygen isotope data clearly indicate a cooling trend, converting oxygen isotope
data to temperature is a tricky (and uncertain) matter. Hansen and Sato (2011)
relied heavily on the paper by Zachos et al. (2001), but Zachos et al. mainly pre-
sented oxygen isotope data, and only obliquely and briefly tacked on temperature
to their graph. Hansen and Sato (2011) adopted their temperature scale, as shown
in Figure 2.7 , but no indication was provided by either Zachos et al. or Hansen
and Sato as to how these temperatures were derived, or what the uncertainty is in
the estimates. Basically, the plot in Figure 2.7 represents the isotope ratio. Zachos
et al. interpreted the first part of the curve (prior to about 34 million years ago) as
defining an equivalent deep-sea temperature. After that date, variable amounts of
buildup of ice, initially in Antarctica, and most recently in Greenland, distorted
the isotope ratios, and only part of the observed isotope ratio can be attributed to
temperature change. Hansen and Sato pointed out that when there is heavy glacia-