Dansgaard–Oeschger event

[2] In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period.

[5] The impact of D-O events in Europe is also recorded by fluctuations in discharge and sedimentation patterns in fluvial systems like the Tisza River.

[8] This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic Ocean.

[8] D-O events are also believed to cause minor increases in atmospheric carbon dioxide concentrations on the order of around 5 ppm.

[citation needed] The events appear to reflect changes in the North Atlantic Ocean circulation, perhaps triggered by an influx of fresh water[8] or rain.

[12] The events may be caused by an amplification of solar forcings, or by a cause internal to the earth system – either a "binge-purge" cycle of ice sheets accumulating so much mass they become unstable, as postulated for Heinrich events, or an oscillation in deep ocean currents (Maslin et al.. 2001, p25).

[14] The former determines the strength of the Atlantic Ocean circulation via altering the northern hemisphere westerly winds, gulf stream, and sea-ice systems.

These studies corroborate the previously suggested existence of a "D-O window"[15] of AMOC bistability ('sweet spot' for abrupt climate changes) associated with ice volume and atmospheric CO2, accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene.

More specifically, D-O cold events, and their associated influx of meltwater, reduce the strength of the North Atlantic Deep Water current (NADW), weakening the northern hemisphere circulation and therefore resulting in an increased transfer of heat polewards in the southern hemisphere.

This allows the NADW to return to its previous strength, driving Northern Hemisphere melting – and another D-O cold event.

The theory may also explain Heinrich events' apparent connection to the D-O cycle; when the accumulation of meltwater in the oceans reaches a threshold, it may have raised sea level enough to undercut the Laurentide ice sheet – causing a Heinrich event and resetting the cycle.