New clue to aid the understanding of the breakdown and melting of glaciers

Glaciers image from destination360.com

Glaciers and ice caps play an important role on earth’s climate; they reflect 80-90% of incoming solar radiation; and they’re a major carbon sink, especially in the Arctic, which accounts for up to 15% of the earth’s carbon sink. As I reported in my previous blog, scientists alerted that the contribution of glacial snow melt to the Himalayan river basins remains uncertain, due to a lack of reliable and consistent data. Today, I came across a new study by researchers from the Massachusetts Institute of Technology, just published in the Journal of Physics D: Applied Physics, which provides new clue to help us understand the reason for the fast rate of glaciers melting witnessed in the past decades, if not centuries.

The article by Zhao Qin and Markus Buehler entitled ‘Carbon dioxide enhances fragility of ice crystals’ used molecular dynamics simulations to investigate the role of carbon dioxide (CO₂) in ice fracture. The authors believe the fracture mechanism of ice is of paramount importance for the understanding of glacial dynamics, which in turn plays an important role in controlling the global climate.

Their work involved the reconstruction of the free energy landscape in varied chemical microenvironments, using enhanced molecular sampling with metadynamics. Their results showed that increasing concentrations of CO₂ molecules significantly decrease the fracture toughness of the ice crystal, making it more brittle and therefore susceptible to fracturing. The fracture toughness of pure ice is 19.4 kPam1/2 without adding CO₂ (C=0), while it decreases to 12.0 kPam1/2 with C = 2% CO₂, which equates to a 38% reduction in fracture toughness.

The diagram below explains how CO₂ molecules reduce ice crystals toughness.  The authors explained that CO₂ molecules affect the bonds between water molecules at crack tip and decrease their strength by altering the dissociation energy of hydrogen bonds. The CO₂ molecules first move closer to the crack tip in a flipping motion and then attempt to form hydrogen bonds by attacking water molecules under stress. This process repeats itself many times, leaving broken hydrogen bonds behind and thereby effectively increasing the brittleness of the ice.



The authors of the new study believe that, in the context of glacier dynamics, their findings may provide a novel viewpoint that could aid the understanding of the breakdown and melting of glaciers. This suggests that the chemical composition of the atmosphere can be critical to mediate the large-scale motion of large volumes of ice.

It appears CO₂ in the atmosphere not only causes glaciers to melt by inducing the greenhouse effect and global warming, but may also affect ice dynamics directly.

Reference

Zhao, Q.; Buehler, M.J. (2012). Carbon dioxide enhances fragility of ice crystals. J. Phys. D: Appl. Phys. 45 (2012) 445302. (http://iopscience.iop.org/0022-3727/45/44/445302)