E3 Glacier Dynamics

Topic

Glacier dynamics arise from the balance between two opposing zones within the glacier itself: the accumulation zone, located above the equilibrium line, where the glacier gains mass as snow accumulates; and the ablation zone, located below the equilibrium line, where the glacier loses mass through melting, sublimation, and the calving of ice blocks into the sea or a lake. The net mass balance—calculated by subtracting the mass lost in the ablation zone from the mass gained in the accumulation zone—is the primary indicator of a glacier's health: a positive net mass balance causes the glacier to advance, while a negative net mass balance causes it to retreat.

The equilibrium line separating these two zones rises in altitude as the climate warms; this higher equilibrium line reduces the surface area of ​​the accumulation zone while expanding that of the ablation zone, driving the glacier's net mass balance toward increasingly negative values.

Glacier movement combines two distinct components: the internal deformation of the ice itself—following the same non-linear stress-strain relationship described for ice in general—and basal sliding, which is the movement of the entire glacier over the underlying bedrock. The speed at which a glacier's surface moves due to internal deformation increases with the fourth power of its thickness; consequently, a glacier that is only slightly thicker than another flows much, much faster than the thinner one. Basal sliding, unlike internal deformation, occurs only when the glacier's base reaches the melting point—that is, when the glacier is temperate, or at least partially temperate at its base. Meltwater accumulating at the base lubricates the contact between the ice and the bedrock, reducing friction and allowing the entire glacier to slide over the bed.

A glacier's continuity equation links changes in ice thickness at any given point to the surface mass balance at that location and the amount of ice entering or leaving via the glacier's flow. When glacier thickness remains constant over time, the downstream ice flux must exactly match the total net accumulation received upstream across the rest of the drainage basin. It is precisely these two elements—the continuity equation and the previously described equations governing ice motion—that an ice-flow model solves simultaneously to predict how the glacier will evolve under various climate forcing scenarios.

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