

The reasons for Ice Shelf collapse continue to be identified, but one key thread emerges. The decade prior to collapse the Larsen-B Ice Shelf had thinned primarily by melting of the ice shelf bottom (by the ocean) by 18 m (Shepard and others, 2003). Thinning preconditions the ice shelf for failure by weakening its connection to pinning points at the grounding line as the shelf becomes more buoyant. Glasser and Scambos (2008) observed that prior to collapse that rifts and crevasses parallel to the ice front crosscut the meltwater channels and ponds, hence, post dated them. The number and length of the rifts increased markedly in the year before collapse. There was no evidence of relict rifts, illustrating that these rifts are a feature of the last 20 years. After ice shelf collapse the ice front receded to the pre-existing rifts, and the pre-existing rifts defined the area of collapse. In this case the thinning and resultant structural weaknesses preconditioned the ice to rapid breakup, which proceeded to lose only the preconditioned portion of the ice shelf.

The WIS is buttressed by Alexander, Latady, Charcot and Rothschild islands and by numerous small ice rises, indicating that they are touching the ocean floor. WIS was examined by Braun, Humbert and Moll (2008). They found that drainage of melt ponds into crevasses were of no relevance for the break-up at WIS. On WIS the evolution of failure zones is associated with ice rises. In 1993/94, rift formation started to expand at the northern ice front. Today, the central part of WIS is intersected by long rifts formed in and around ice rises. The rifts up to tens of kilometers long evolve and coalesce prior to break-up events. The conclusion for WIS is that preconditioning of the ice shelf by connection of the rifts in the failure zones near ice rises trigger break-up events. The thinning and rifting lead to a cascade of failure.

The Feb.-April break-up left a narrow 6 km wide fractured connection to Charcot Island. Existing rifts formed between already existing fractures, crossed almost the entire northern shelf. This fragile and vulnerable area was expected to collapse further the next austral summer. However, it instead has happened this austral winter with loss of an additional 160 km2 of ice. It is the first winter ice loss of an ice shelf ever observed, and so was surprising. However, looking at the image below, from the European Space Agency showing the extent of the rifts as winter began, makes this less surprising. The question is more what can possibly hold this together? The area of interconnected rifts seen is 2000 km2. If this is lost an additional 3000 km2 of the 13 000km2 of WIS, is at risk when this connection to Charcot Island is lost (Braun, Humbert and Moll, 2008).

It appears then that glacier or ice shelf thinning is the key preconditioning factor for collapse, retreat and acceleration, whether you are in Antarctica of Greenland. The mechanisms for ice shelf thinning include basal melting (from warming ocean waters), surface melting, reduction in glacier inflow and rift development. These are interrelated mechanisms that precondition the ice shelves to collapse. On marine terminating outlet glaciers the mechanisms to trigger thinning is surface ablation causing thinning, and potentially basal melting, though not yet observed (though see this recent paper by Holland et al, 2008). Once the process begins thinner less buttressed ice enables acceleration and more calving and more retreat. There is a potential difference between the two, in glacier such as most marine terminating outlet glaciers, where the glacier flow is rapid, acceleration results from retreat and thinning. In the case of ice shelves a glacier buttressed by them will accelerate after the loss, but the slow moving ice shelf may suffer from reduced inflow. Attention will continue to be focused on these rapid responders to climate change;marine terminating glaciers in Greenland and ice shelves in Antarctica. We can look forward to more details from the extensive 2008 summer field season in Greenland and the upcoming view of the Wilkins this fall.

Unlinked References:


Howat, I., I. Joughin, M. Fahnestock, B. Smith,T. Scambos 2008. Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000–06: ice dynamics and coupling to climate.Journal of Glaciology, 54(187).

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