【気候科学】温暖な海洋亜表層水を原因とする氷山の大群の流出

An explanation for why an ice sheet that once covered much of North America ‘belched’ vast iceberg armadas during some of the coldest periods of the last ice age is revealed in a paper published in this week’s Nature. The study also provides insights into how small increases in ocean temperature can trigger disintegration of vulnerable portions of today’s ice sheets, even with little atmospheric warming.

The Laurentide Ice Sheet covered much of Canada and part of the northern United States during the last glacial period. Unexpectedly, it periodically discharged vast armadas of icebergs through the Hudson Strait and into the North Atlantic Ocean during some of the coldest portions of the period. This is in conflict with the expectation that ice sheets expand during colder periods and contract during warmer periods, and the underlying mechanism that triggered these ‘Heinrich Events’ remains intensely debated.

Jeremy Bassis and colleagues present modelling evidence that shows the Heinrich Events were produced by subsurface ocean warming that destabilized the ice sheet where it terminates in the ocean and where icebergs break off (or calve). The authors found that the warm subsurface water melts the underwater portion of the calving face, triggering rapid retreat of the ice sheet and abrupt iceberg release. They conclude that, once the ice sheet has retreated, the ocean bed that once supported the ice sheet rises up over millennial timescales, cutting the calving face off from the warm subsurface waters, and allowing the ice sheet to re-advance until the next Heinrich Event.

The mechanism proposed by the authors suggests that Heinrich Events are driven by the same processes that are driving retreat of modern marine-terminating glaciers. Hence, portions of West Antarctica already at risk of collapse - where wide ice streams are grounded deep beneath sea level - could be vulnerable to this process, even in the absence of atmospheric warming.