Heinrich Event

A Heinrich event is a period of extreme ice-rafted debris deposition in the North Atlantic, recorded in ocean sediment cores as layers of lithic fragments dropped from melting icebergs. These events occurred six times during the last glacial period (roughly 70,000 to 15,000 years ago), each lasting several centuries to a millennium, and each coinciding with abrupt cooling of the North Atlantic region.

The cause is thought to be the periodic collapse of the Laurentide Ice Sheet's Hudson Bay ice stream, which released massive armadas of icebergs into the North Atlantic. As the icebergs melted, they released fresh water and lithic debris, freshening the surface ocean and suppressing the Atlantic Meridional Overturning Circulation. The resulting reduction in northward heat transport produced regional cooling of 4–10°C, with effects propagating across the Northern Hemisphere through atmospheric and oceanic teleconnections.

Heinrich events are closely linked to the Dansgaard-Oeschger events — the rapid warming-cooling oscillations that characterised the last glacial climate — and to the Younger Dryas, the most recent and best-studied of the abrupt climate changes. Together, these events demonstrate that the thermohaline circulation is a tipping element capable of reorganising on centennial timescales under sufficient freshwater forcing.