During the last glacial period, the Laurentide Ice Sheet sporadically discharged huge numbers of icebergs through the Hudson Strait into the North Atlantic Ocean, leaving behind distinct layers of ice-rafted debris in the ocean sediments1,2,3. Perplexingly, these massive discharge events—Heinrich events—occurred during the cold portion of millennial-scale climate oscillations called Dansgaard–Oeschger cycles2,4. This is in contrast to the expectation that ice sheets expand in colder climates and shrink in warmer climates. Here we use an ice sheet model to show that the magnitude and timing of Heinrich events can be explained by the same processes that drive the retreat of modern marine-terminating glaciers. In our model, subsurface ocean warming associated with variations in the overturning circulation increases underwater melt along the calving face, triggering rapid margin retreat and increased iceberg discharge. On millennial timescales, isostatic adjustment causes the bed to uplift, isolating the terminus from subsurface warming and allowing the ice sheet to advance again until, at its most advanced position, it is poised for another Heinrich event. This mechanism not only explains the timing and magnitude of observed Heinrich events, but also suggests that ice sheets in contact with warming oceans may be vulnerable to catastrophic collapse even with little atmospheric warming.
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We were supported by grant NSF-ANT 114085, Polar Programs grant PLR-1341568, NOAA Award NA13OAR4310096, grant NSF-OCE-PRF 1420902 and the Michigan Society of Fellows.
The authors declare no competing financial interests.
Reviewer Information Nature thanks A. Vieli and the other anonymous reviewer(s) for their contribution to the peer review of this work.
This file contains Supplementary Figures 1-13, Supplementary Table 1, Supplementary Methods, a Supplementary Discussion and Supplementary References. (PDF 4492 kb)
Animation showing simulated Heinrich Events using the flowline model over the 120 ka simulation period.
Snapshots from the animation are shown in Figure 1 in main text. (MOV 3904 kb)
Animation showing 15 ka simulation of Heinrich Events using a regional model with 180 km channel width.
Behaviour is similar to the flowline model with ocean warming triggering a rapid retreat and acceleration. Retreat continues until the deepest portion of the sill isolates the terminus from the warm subsurface water or the pulse arrests at which point the ice stream advances again. (MOV 1099 kb)
Animation showing 15 ka simulation of Heinrich Events using a regional model with 100 km channel width.
Behaviour is similar to the flowline model with ocean warming triggering a rapid retreat and acceleration. Retreat continues until the deepest portion of the sill isolates the terminus from the warm subsurface water or the pulse arrests at which point the ice stream advances again. (MOV 1088 kb)
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Bassis, J., Petersen, S. & Mac Cathles, L. Heinrich events triggered by ocean forcing and modulated by isostatic adjustment. Nature 542, 332–334 (2017). https://doi.org/10.1038/nature21069
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