A2 Ice Shelf Collapse

Topic

Floating ice shelves are extensions of the continental ice sheet that float on the ocean and act as buttresses: they exert a back-stress on the tributary glaciers feeding each shelf, and this back-stress slows the flow of those glaciers toward the sea. When an ice shelf collapses—as happened with the Larsen B shelf in 2002—the tributary glaciers upstream accelerate dramatically upon losing the back-stress that previously held them back; this increases ice discharge into the ocean and raises the contribution of those glaciers to sea-level rise.

The first key physical mechanism behind ice shelf collapse is hydrofracture induced by surface meltwater ponds: during warm summers, melt ponds form on the shelf surface, and the water within them penetrates existing cracks in the ice. This water propagates the cracks vertically downward, driven by the hydrostatic pressure of the accumulated water—a pressure that overcomes the fracture resistance of the ice itself.

The second key physical mechanism is marine-based instability: the grounding line—the point where continental ice ceases to rest on the seabed and begins to float as an ice shelf—may lie on a retrograde-sloping seabed, one that deepens toward the continental interior rather than toward the open ocean. On a retrograde-sloping seabed, any retreat of the grounding line shifts it toward a deeper area of ​​the seabed; this increased depth at the new grounding line position results in greater ice thickness at that specific point. Greater ice thickness at the grounding line accelerates the flow of ice toward the ocean at that same location, and this faster ice flow drives the grounding line to retreat even further, creating a self-reinforcing instability on the retrograde-sloping seabed.

The expected behavior of a floating ice shelf collapse is nonlinear and irreversible: once the threshold for hydrofracturing or for retreat over a retrograde-sloping seabed is crossed, the collapse of the floating ice shelf can become self-sustaining, no longer dependent on the continued action of the climate forcing that initiated it. The loss of the buttressing effect previously provided by the floating ice shelf is rapidly transmitted upstream via pressure waves traveling through the ice itself, reaching the tributary glaciers with the same loss of resistive stress mentioned at the start of this description; this accelerates the tributary glaciers and increases the entire system's contribution to sea-level rise.

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