M2 Ablation

Topic

Ablation encompasses all processes that cause the cryosphere to lose mass: melting at the ice surface and base, direct sublimation from the surface, evaporation of meltwater, and iceberg calving at the marine or lacustrine fronts of glaciers. Melting is the dominant ablation mechanism for both mountain glaciers and the margins of polar ice sheets; the rate of surface melt is governed by the surface energy balance—specifically, the sum of all radiative and turbulent fluxes reaching the ice surface must be positive for melting to occur.

The positive degree-day model is the most widely used tool in applied glaciology due to its simplicity and effectiveness. In this model, cumulative seasonal melt is proportional to the magnitude and duration of air temperatures remaining above freezing. The degree-day factor—which converts accumulated temperature into actual melt—is higher for ice than for snow, precisely because ice has a lower albedo and therefore absorbs more solar radiation per degree of air temperature.

Although the positive degree-day model does not explicitly calculate the ice surface energy balance and thus lacks a rigorous physical basis, its parameters can be easily calibrated using field data; consequently, the model achieves excellent predictive performance as long as climate changes remain within moderate ranges.

Of all ablation processes, iceberg calving is the most difficult to model, precisely because it occurs in a discontinuous and highly non-linear manner. An empirical calving relationship links the rate at which a glacier sheds icebergs to the speed at which the ice flows at its terminus; thus, a faster-flowing glacier front also tends to calve icebergs more frequently.

More sophisticated calving models further distinguish between "dry," purely mechanical calving and calving driven by meltwater accumulating in surface crevasses: when meltwater fills such a crevasse, it can propagate the crack downward to the glacier base—provided the accumulated water column exceeds a critical fraction of the total ice thickness—thereby completing the set of ablation mechanisms with which this description began.

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