Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 91

salinity contrast between the Atlantic and Pacific had already started to develop
by 4.2 mybp . Thus, Haug raised the question: ''If the salinity had already changed
by 4.2 million year ago, why didn't glaciation start until 2.7 million years ago? On
the contrary, the Earth experienced a warm spell between 4.5 million and 2.7
million years ago.'' And in a similar way, we could ask why did glaciation begin
about 2.7 mybp ? Haug (2004) provided a possible answer but it seems rather
contrived.
A more recent study (Bartoli et al. 2005) emphasized that the Pleistocene
climate has been characterized by a persistent succession of glacial-interglacial
cycles that they believe were driven by orbital forcing. However, these Northern
Hemisphere glaciations (NHGs) and large-scale Arctic sea ice did not begin until
about 3 mybp , which they argued was an epoch when a major reorganization of
the ocean-climate system took place. This event appears to be connected to the
final closure of the CAS, ending the introduction of Pacific surface water into the
Atlantic Ocean. They raised two questions in regard to this event:
(1) What was the precise timing of the final closure of the CAS?
(2) Was there a causal relationship between the CAS closure and the onset of
NHGs?
Bartoli et al. (2005) studied high-resolution data from planktic and benthic
foraminifers in the northern North Atlantic, a region crucial for the understanding
of NHGs. They examined evidence of continental-scale ice sheets, especially on
Greenland, recorded as ice-rafted detritus released from drifting icebergs into
sediments of the mid- and high-latitude ocean. After a transient precursor event at
3.2 mybp , signals of large-scale glaciations suddenly started in the North Atlantic
in two steps, at 2.92-2.82 mybp and 2.74-2.64 mybp . They also noted that the time
period around 2.7 mybp was the onset of ice-rafted detritus in the Pacific. Thus
they described an irreversible ''climate crash'' that began from 2.74-2.64 mybp .
This climate crash is reflected in the sudden change in benthic and planktic species
that took place over a small time interval. They also found evidence for a ''major
expansion of global ice volume'' and the consequent drop in sea level from 2.92 to
2.64 mybp was estimated at 45m.
Bartoli et al. (2005) established the time scale for the onset of glaciation in
the NH. However, it is not clear that they established the connection between
closure of the CAS and the onset of glaciation except to point out that
glaciation followed soon after the closure. The two questions raised by them
remain answered only in vague terms. The cause-effect relationship between
closing of the CAS and the onset of glaciation in the NH remains dicult to
prove.
Several studies have employed climate models to investigate the matter.
Murdock et al. (1997) employed a coupled ocean-atmosphere climate model to
study two cases: the CAS fully open and the CAS fully closed. They found that