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November 13th, 2014

Balanced on a knife point: Are glaciers self-organized critical systems?

Kronebreen, Svalbard in September 2014, above photographed by co-author D. Vallot, and below simulated by the authors’ computer model. Graphic by J. Hokkanen, CSC-IT.

A study involving INSTAAR researchers takes a new view of glaciers and ice sheets as self-organized critical systems, which may explain why a slight climate change can mean the difference between relatively stable ice and the complete collapse of entire ice shelves.

Driven by global warming over the next century, ice sheets and glaciers will contribute significantly to sea level rise through melting and “calving”—the breaking off of ice into the ocean. Observed rates of iceberg calving are not easy to reproduce in computer simulations, and their rapid, nonlinear fluctuations are especially difficult to predict under a changing climate. Better simulation of iceberg calving, and therefore more precise projections of sea level rise, remains a challenge in preparing for a warming world.

A new study published in Nature Geoscience this week found that calving is governed by the same power-law distributions that govern avalanching sand piles and epidemics, among other phenomena. That means that a glacier terminus can readily flip between a state of sub-critical advance and super-critical retreat in response to small changes in climate and geometric conditions. The researchers argue that their view of glaciers as self-organizing critical systems might explain why we are seeing glaciers retreat suddenly and ice shelves collapse in response to gradual warming in the environment—they may be systems fluctuating around their critical points.

The fracturing of a square piece of ice at the terminus of a larger glacier where it calves into in the sea, generated using the authors’ new computer simulation program of iceberg calving. The program reproduces the fracture dynamics of ice by modeling the interactions between millions of tiny ice blocks. Graphic by J. Hokkanen, CSC-IT.

To understand calving event rates and timing, INSTAAR PhD student Ethan Welty compiled a catalog of observations that spans 12 orders of magnitude, with data from Alaska, Svalbard, Greenland, and Antarctica. “It is actually the first time all this data has been compiled, some of it published for the first time,” says Welty. The researchers compared the calving data with simulations from a first-principles, particle-based numerical simulation that models calving events as the fracture of millions of individual ice blocks.

They found that calving events obey universal scaling laws. “This means that the probability of calving events obey a particular pattern no matter if they are small or large events—much like the Gutenberg–Richter law for earthquakes,” explains lead author Jan Åström of the CSC–IT Centre for Science, Finland.

Calving is thus extremely and unavoidably sensitive to its environment: a slight climate change can mean the difference between very little calving and the complete disintegration of entire ice shelves. This new view of iceberg calving explains the observed, sudden collapse of ice shelves and sudden retreat of tidewater glaciers as the polar regions gradually warm.

The authors, from Finland, Sweden, the United States, China, and Russia, propose that investigating ice sheets and glaciers as self-organizing critical systems can help better predict sea level rise. “More specifically, our results suggest that fluctuations—i.e., large, spontaneous events—dominate the sea level signal,” says Welty. “There is no such thing as an ‘average’ calving rate. Rather than trying to find better deterministic calving ‘laws,’ our results suggest that it might be more constructive to investigate how close particular glaciers are to their critical point and thus how sensitive they may be to near-future changes in climatic and geometric conditions.”