M4 Centennial to Millennial Variability

Topic

Centennial-to-millennial-scale variability in the climate system during the last glacial period is dominated by two types of abrupt events. The first consists of Dansgaard-Oeschger cycles—temperature oscillations in the Arctic and Greenland ranging from eight to sixteen degrees, with a periodicity of approximately 1,500 years. The second comprises Heinrich events—massive discharges of icebergs into the North Atlantic originating from the North American Laurentide Ice Sheet. Both types of abrupt events are mediated by changes in the Atlantic Meridional Overturning Circulation: Heinrich events coincide with collapses of this circulation, whereas Dansgaard-Oeschger cycles correspond to "stadials" (periods when the circulation is active) and "interstadials" (periods when it is weakened).

The physical mechanism proposed for Dansgaard-Oeschger cycles involves a bifurcation of the Atlantic Meridional Overturning Circulation between two stable states: an active state, characterized by deep convection in North Atlantic deep-water formation regions and northward heat transport; and a weakened state, lacking deep convection in those same regions. Freshwater input from melting ice acts as the trigger for this bifurcation by reducing water salinity in the North Atlantic deep-water formation regions, thereby causing the Atlantic Meridional Overturning Circulation to cross the threshold between the active and weakened states.

Heinrich events, by contrast, are triggered by an inherent instability in the Hudson Bay ice stream—part of the same North American Laurentide Ice Sheet. Ice within this stream begins to slide much faster than usual over its underlying bed, and this accelerated sliding discharges the massive volume of icebergs into the North Atlantic that characterizes a Heinrich event. This instability in the Hudson Bay ice stream can evolve autonomously—without any external trigger—or it can be modulated by the temperature of the ocean water washing against the base of the ice stream itself.

Intermediate-complexity models and paleoclimatic general circulation models reproduce both Dansgaard-Oeschger cycles and Heinrich events by coupling the previously described bifurcation of the Atlantic Meridional Overturning Circulation with ice-sheet dynamics. The roughly 1,500-year timescale of Dansgaard-Oeschger cycles has been linked to three distinct explanations: an inherent solar-forcing periodicity, an internal oscillator within the Atlantic Meridional Overturning Circulation, and the time required for the Laurentide Ice Sheet to rebuild its ice mass between successive Heinrich events.

Key observational sources for testing these models of centennial-to-millennial variability include Greenland ice cores—which directly record Arctic temperatures during each Dansgaard-Oeschger cycle—and marine sediment cores, which capture the rock fragments dropped onto the North Atlantic seafloor as icebergs from Heinrich events melted. By combining data from Greenland ice cores and marine sediment cores, it is possible to reconstruct the full sequence of Dansgaard-Oeschger cycles and Heinrich events driven by the Atlantic Meridional Overturning Circulation throughout the last glacial period—the very same centennial-to-millennial variability with which this description began.

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