Overview of the various models for ice ages in the recent past (3 MYBP to present) - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 220

Figure 8.3. Near-surface waters (red lines) flow towards four main deepwater formation
regions (yellow ovals)—in the northern North Atlantic, the Ross Sea, and the Weddell
Sea—and recirculate at depth (deep currents shown in blue, bottom currents in purple)
(#Rahmstorf, 2002#). 4
origin fill the deep Atlantic basin. While these groupings are somewhat arbitrary
and not necessarily discontinuous, they describe the essential range of variability
of NADW. There is also firm evidence for links between these changes in ocean
circulation and changes in surface climate.
Rahmstorf (2002) was concerned with the onset of the most recent ice age,
which began about 115,000 years ago based on Greenland data (see Figure 4.7 ),
about 135,000 years ago based on Antarctica data (see Figure 4.9 ), and at varying
times according to ocean sediment data (see Figures 5.4 and 5.5 ) . As Figure 9.8
shows, there was a steep minimum in solar input around 115,000 ybp , although it
quickly increased after that. Rahmstorf quoted Paillard's model as evidence.
This model is described in Section 9.6.1, but it appears to contain several flaws.
Rahmstorf then raised the question: ''Does a weakening in Atlantic Ocean circula-
tion have a role in glacial inception?'' He partly answered his own question by
saying: ''there are no palaeoclimatic data showing that NADW formation slowed
at this time.'' He claimed that model simulations achieve glacial inception with
only minor changes in ocean circulation. However, the validity of such models is
dicult to confirm. Rahmstorf also criticized the reverse theory—that a warm
North Atlantic could have induced ice sheet growth by enhanced moisture
supply—saying that this ''goes against our knowledge of glacier mass balance:
glaciers grow when climate is cold, not warm and moist.'' Actually, that is not
4 According to Carl Wunsch, Figure 8.2 is a ''memorable graphic''. However, he argues that it
provides some misleading inferences, but it is so visually appealing that his students tend to
remember the graphic—not his objections to it (pers. commun. from C. Wunsch, December
2008).