There has been some discussion recently about the formation of a large rift on Larsen C, the largest section of the Larsen Ice Shelf on the Antarctic Peninsula. The matter is interesting because two northern portions of this same ice shelf, called Larsen A and B, collapsed in 1995 and 2002, respectively, as a consequence of the climate warming in that region. As the climate continues to warm, Larsen C may be next in turn and therefore everything that happens in there is interesting. Let’s have a look at what is happening:
The rift is a large crack that cuts the whole thickness of the ice shelf. The one in question here developed from an already existing chasm in the southernmost part of the shelf:
Note the eastward displacement of the surface feature marked in a black square. This is the result of the normal movement of the ice shelf. The displacement is about 8800 m, which gives a (very) crude estimation of the ice velocity of about ~ 680 m per year. The rift developed from the chasm marked in a red square. If things proceed as usual this rift will continue to open northwards until the external portion of the ice shelf will separate and become an iceberg. We call this process iceberg calving.
Ice shelves are floating parts of ice sheets, attached either to land or to ice sheets themselves. Ice sheets resting on land, as those in Antarctica and Greenland, flow towards the ocean by a combination of internal (shear) deformation of the ice, basal sliding, and basal sediment deformation. As an ice sheet reaches the ocean the ice may start to float. As it losses contact with the base, the basal shear becomes almost zero and shear extension, that is stretching, becomes the dominant flow mechanism.
Given the right conditions, such as the presence of an embayment that provides lateral constraint and protects the ice shelves from the destructive forces of waves and tides, ice shelves can grow large and become stable features. Ice shelves spread then by extension, flowing towards the open, unconstrained side facing the ocean, becoming thinner and mechanically weaker towards the front. As they slowly extend beyond their protective bays, ice shelves may from time to time release an iceberg off their fronts. This process is natural and can have recurrence times of the order of decades. In a “normal” world (one without anthropogenic climate warming at the scale and rate we are witnessing now) this phenomenon is spectacular but signals no impending danger for an ice shelf.
Ice shelves are polar features located in cold climates with little surface melting. They can grow large and exist during long periods of time because the thinning of the ice body by the extensive flow is largely compensated by accumulation of ice at the surface due to snowfalls and in part some accretion of marine ice (frozen sea) from below:
Enter climate change. Ice shelves are extensive and relatively flat surfaces lying roughly at sea level. Accordingly, changes in climate affect them almost in its entirety and almost instantaneously. As the Earth’s polar climates warm, existing ice shelves are exposed to conditions under which these cannot survive. An ice shelf under a warming climate might suddenly find itself in the ablation zone, the area where the surface mass balance of a glacier is negative. This is bad news for ice shelves because in such conditions they will not be able to compensate the thickness reduction due to its extensive flow. This makes the ice shelf thinner and mechanically more fragile with time.
In addition, since ice shelves are essentially flat and have no developed drainage system, excess melt water from the increasingly warmer summers can accumulate in the surface and penetrate to the cold interior, where it will freeze and expand, further debilitating the ice shelf. When this process happens almost every summer during several years, a former healthy and mighty ice shelf will become irreversibly unstable. At this point, any external perturbation, such a Föhn storm, that in another circumstances would not have influenced the shelf much, will now provide the final trigger for the collapse. This is exactly what happened in Larsen A and B.
The collapse of an ice shelf does not in principle affect sea level because ice shelves are already floating, and they have already displaced the volume of water their submerged parts occupy. There is however a secondary effect of the removal of ice shelves that actually impacts in sea level, namely the enhanced flow of its tributary glaciers. In the case of Larsen C this secondary effect will likely not impact so much (some decimeters at most), but it would be huge in case of the collapse of any of the two giant ice shelves Ross and Filchner/Ronne ice shelves.
Those were already many words inspired by a rift on an ice shelf! Let’s keep an eye on Larsen C, it might give us a surprise in the near future.
De Angelis, H. and Skvarca, P., 2003. Glacier surge after ice shelf collapse. Science, 299(5612), pp.1560-1562.
Jansen, D. , Helm, V. , Neckel, N. , Luckman, A. J. and Bevan, S. (2016): Observed rift propagation in the Larsen C Ice Shelf from Sentinel 1-A radar data, EGU General Assembly 2016, Vienna, Austria, 17 April 2016 – 22 April 2016 .
Skvarca, P. and De Angelis, H., 2003. Impact assessment of regional climatic warming on glaciers and ice shelves of the northeastern Antarctic Peninsula. Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives, pp.69-78.