Nature Journal Club

Ian Howat

Ohio State University, Columbus

A glaciologist ponders iceberg calving from a safe distance.

If the sea level rises catastrophically, it will be due to rapid retreat of Earth’s ice sheets. At the perimeter of these sheets, where warm, saline waters meet flowing ice, complex processes occur, including the fracturing of ice to form icebergs — a process known as calving. Calving is poorly understood owing to a lack of detailed observations: researchers willing to install instruments in frigid water beneath a continually collapsing wall of ice that is prone to frequent floods of meltwater have been scarce.

To better understand calving, Jason Amundsen at the University of Alaska Fairbanks and his colleagues took a clever, and much safer, approach. They deployed an impressive array of instruments several kilometres from the calving front on and near Jakobshavn Isbræ, one of Greenland’s largest glaciers. They then ‘listened’ to the sounds of calving using sophisticated audio equipment, ‘watched’ the motion of the ice with time-lapse photography and ‘felt’ the rumble of icebergs using seismometers and tide gauges.

By combining these remote observations with straightforward theory, they found that the ice front behaves similarly to road traffic, with dense packs of icebergs and sea ice forming a jam. Once this icy mélange weakens, large bergs capsize, pushing others out of the way, and the calving wall retreats. Calving continues until the front migrates far enough inland that the ice is too thick to fracture all the way through, putting on the brakes (J. M. Amundson et al. J. Geophys. Res. doi:10.1029/2009JF001405; 2010).

The results are encouraging to those interested in modelling ice-sheet behaviour because they provide a mechanism to explain relationships between ice thickness, fracturing and retreat. They also provide a great example of how a diverse arsenal of observational tools can solve the most formidable problems in Earth science.

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