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 224

the space of just a few decades or even several years. The result could be a very
sudden climate change to colder conditions, as has happened many times in the
area around the North Atlantic during the last 100,000 years.
The sudden switch could also occur in the opposite direction, for example if
warmer summers caused the sea ice to melt back to a critical point where the sea
ice lid vanished and the Gulf Stream was able to start up again. Indeed, following
an initial cooling event the evaporation of water vapor in the tropical Atlantic
could result in an 'oscillator' whereby the salinity of Atlantic Ocean surface water
(unable to sink into the north Atlantic because of the lid of sea ice) built up to a
point where strong sinking began to occur anyway at the edges of the sea ice
zone. The onset of sinking could result in a renewed northward flux of warm
water and air to the north Atlantic, giving a sudden switch to warmer climates, as
is observed many times within the record of the last 130,000 years or so.''
The process of switching off or greatly diminishing the flow of the Gulf
Stream would not affect Europe alone. Antarctica would be even colder than it is
now, because much of the heat that it receives now ultimately comes from Gulf
Stream water that sinks in the North Atlantic, travels down the western side of
the deep Atlantic Basin, and then partially resurfaces just off the bays of the
Antarctic coastline.
The role of ocean circulation and variations in salinity as a cause for the
sudden changes in glaciation has been discussed by many authors (e.g., Lehman
and Keigwin, 1992; Schmidt et al., 2006; Fleitmann et al., 2008).
Dokken and Jansen (1999) analyzed the sudden climate changes over the past
60,000 years based on ice core and ocean sediment data. They assumed that
Pacific sediment data reflected global mean glacial-interglacial transitions while
high-resolution sediment data from the Nordic seas represented regional variations
in that area superimposed on global trends. By subtracting the Pacific trends they
obtained residual differences between the deep Nordic seas and the deep global
ocean. These residual differences in d 18 O were so large that very large temperature
changes in the deep ocean would be needed to explain these isotope shifts if
temperature changes were the cause. They asserted that such large temperature
changes were impossible, so they concluded that the cause must have been the
vertical transport of 18 O-depleted waters from the surface to the deep. This ran
counter to the usual theory that convective vertical transport is impeded during
glacial periods due to the buoyancy of meltwater, and so another mechanism was
required. They suggested it was due to the higher density of brine as a result of
increased salinity as great masses of sea ice formed.
Thus, Dokken and Jansen (1999) concluded:
''Deep water was generated more or less continuously in the Nordic Seas
during the latter part of the last glacial period (60 to 10 thousand years ago), but
by two different mechanisms. The deep-water formation occurred by convection
in the open ocean during warmer periods (interstadials). But during colder phases
(stadials), a freshening of the surface ocean reduced or stopped open-ocean