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 223

during the coldest parts of the last glacial period than during the Holocene. Dust
and sea salt also underwent abrupt threefold jumps during Dansgaard-Oeschger
cycles. Thus, it is clear that the atmosphere was far dustier and salt laden during
glacial times. The high sea salt content in glacial ice from both Antarctica and
Greenland points to storminess as the key cause. Broecker (2002) also emphasized
the suddenness of climate changes that may have taken place in less than a
decade. He eliminated changes in glacial extent, sea level, and atmospheric CO 2
content as candidates because they are incapable of such rapid climate changes.
While water vapor, snow cover, and cloudiness can change with sucient rapidity,
he argued that they do not seem to have the required property of being able to be
turned on and off. Only the ocean's thermohaline circulation appears to fulfill the
requirements. Broecker also emphasized that each of the major glacial periods
over the last million years ended abruptly. The climate typically switched from full
glacial to full interglacial in fewer than 5,000 years.
According to Adams et al. (1999), the circulation of the North Atlantic Ocean
probably plays a major role in either triggering or amplifying rapid climate
changes in the historical and recent geological record. The North Atlantic has a
peculiar circulation pattern: the northeast-trending Gulf Stream carries warm and
relatively salty surface water from the Gulf of Mexico up to the seas between
Greenland, Iceland, and Norway. Upon reaching these regions, the surface waters
cool and (with the combination of becoming cooler and relatively saltier) become
dense enough to sink into the deep ocean. The pull exerted by this dense sinking
water is thought to help maintain the strength of the warm Gulf Stream, ensuring
a current of warm tropical water into the North Atlantic that sends mild air
masses across to the European continent. If the sinking process in the North
Atlantic were to diminish or cease, sea ice would form more readily in the North
Atlantic. This would reinforce a much colder regional climate:

''The trigger for a sudden 'switching off' or a strong decrease in deep water
formation in the North Atlantic must be found in a decrease in density of surface
waters in the areas of sinking in the northern Atlantic Ocean. Such a decrease in
density would result from changes in salinity (addition of fresh water from rivers,
precipitation, or melt water), and/or increased temperatures

. During glacial
phases, the trigger for a shut-off or a decrease in deep water formation could be
the sudden emptying into the northern seas of a lake formed along the edge of a
large ice sheet on land (for instance, the very large ice-dammed lake that existed
in western Siberia), or a diversion of a melt-water stream from the North
American Laurentide ice sheet through the Gulf of St. Lawrence, as seems to
have occurred as part of the trigger for the Younger Dryas cold. A pulse of fresh
water would dilute the dense, salty Gulf Stream and float on top, forming a
temporary lid that stopped the sinking of water that helps drive the Gulf Stream.
The Gulf Stream could weaken and its northern end could switch off altogether,
breaking the 'conveyer belt' and allowing an extensive sea ice cap to form across
the North Atlantic, preventing the ocean current from starting up again at its
previous strength. Theoretically, the whole process could occur very rapidly, in

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