DON J. EASTERBROOK DISTINGUISHED SCIENTIST AWARD: THE ORIGIN OF HEINRICH EVENTS

Heinrich events (HEs) are widely believed to represent an ice-sheet instability that triggered abrupt climate change. Several mechanisms have been invoked to explain HEs, including binge-purge oscillation, which operates independently of climate, and ice-shelf collapse induced by surface forcing. However, existing stage 3 sea-level records dispute the binge-purge hypothesis by showing that sea-level rises (falls) during the predicted binge (purge) period, and HEs occurred during the coldest intervals in the North Atlantic, or the least likely time for surface forcing to cause an ice shelf to fragment. Fluckiger et al. (2006) implicate the ~1 m steric and dynamic sea-level rise that accompanies a collapse of the Atlantic Meridional Overturning Circulation (AMOC) as the primary trigger for HEs. Recent modeling, however, indicates that ice sheets are likely to be immune to such small sea-level forcing and significant sea-level rise occurred over a several-thousand year period prior to HEs during stage 3.

An additional mechanism by which climate change may trigger a Heinrich event by ice-shelf fragmentation involves a subsurface warming that develops at intermediate depths in the North Atlantic in response to a reduction or collapse of the AMOC. Model simulations indicate that, without an active AMOC and cooling of the ocean interior by convection, downward diffusion of heat at low latitudes warms subsurface waters to a depth of ~2500 m. Some of the heat accumulated in the subsurface is transported poleward causing a temperature inversion in the northern North Atlantic. Development of a subsurface warming following collapse of the AMOC is also evident in other models.

The plausible role of subsurface warming implies that HEs were thus responses to, rather than causes of, the shutdown of the AMOC. This conjecture is consistent with the observation that HEs occur at the end of a long-term cooling trend; such a cooling is likely caused by a reduction in the AMOC and expansion of sea ice. The synchronization of Antarctic and Greenland ice cores further places the HEs in the context of climate change occurring in response to a slowing AMOC. The bipolar seesaw pattern is most clearly expressed when Greenland is coldest and Antarctica is warmest, which is readily attributed to times of weakest AMOC; this is when Heinrich events occur.