E4 Ice Sheets and Platforms

Topic

The continental ice sheets of Greenland and Antarctica together contain nearly ninety-nine percent of all terrestrial ice, making them the primary potential contributors to sea-level rise. The dynamics of these continental ice sheets are governed by flow moving from the central domes of each sheet—where the mass balance is positive due to snow accumulation exceeding loss—toward the margins, where the ice discharges into the ocean via calving icebergs or by forming floating ice shelves that extend over the sea.

A floating ice shelf is crucial to the stability of the continental ice sheet feeding it, as it exerts a buttressing effect on the tributary glaciers flowing into it: the resistive stress provided by the floating ice shelf slows the flow of land ice arriving from the continent's interior.

Marine-based instability is the most critical collapse mechanism a continental ice sheet can face: when the base of a marine-terminating glacier rests on a bed that deepens toward the continental interior rather than the open ocean, even a minor disturbance can trigger an irreversible retreat. This occurs because the grounding line—the boundary where the ice begins to float—recedes toward a section of the bed where the ice is thicker; this increased ice thickness at the new grounding line position boosts ice discharge into the ocean at that point, which in turn drives further retreat of the grounding line, creating a self-reinforcing cycle of instability. This same mechanism of marine-based instability could cause the collapse of the West Antarctic Ice Sheet, resulting in a global sea-level rise of several meters. The grounding line marks the precise boundary between ice resting on bedrock and ice floating on the ocean; the flow of ice across this line increases disproportionately relative to the ice thickness at that specific point, meaning a small increase in thickness results in a much larger increase in ice flux across the grounding line. This disproportionate relationship between thickness and flow is precisely what drives the marine ice-sheet instability described earlier.

The thinning of a floating ice shelf—caused by basal melting driven by warm ocean water known as Circumpolar Deep Water—weakens the buttressing effect the shelf exerts on the tributary glaciers feeding it. This weakening of the buttressing effect is currently the mechanism accelerating the discharge of ice from West Antarctica’s continental ice sheets into the ocean—the very process driving the sea-level rise mentioned at the beginning of this description.

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