M7 Deglaciation

Topic

The process of deglaciation encompasses all the ways in which the cryosphere loses mass over time, as well as how the system itself responds to that sustained loss. In mountain glaciers, deglaciation manifests primarily as a retreat of the glacier front—reducing its total length—and as a thinning of the glacier's surface.

Glacier front retreat does not occur in direct proportion to the climate change driving it; the rate of retreat is modulated in a highly non-linear fashion by the glacier's specific geometry, the slope of the terrain it occupies, and the presence or absence of a proglacial lake at its terminus. A glacier terminating in a lake or directly in the sea retreats much faster than one ending on land, because iceberg calving and underwater melting are far more efficient at removing mass than simple surface melting.

Glacier thinning does not remain localized at the point of origin but propagates upstream via a mechanism involving kinematic waves: a change in the glacier's mass balance creates a thickness anomaly that travels downstream at a speed several times faster—typically three to five times faster—than the ice flow itself. This dynamic propagation means that a glacier's full response to climate change is both time-lagged and spatially uneven; the distance over which this thickness perturbation eventually dissipates and is fully dampened is known as the glacier's response length.

Post-glacial isostatic rebound is the response of the lithosphere and Earth's mantle to the removal of the ice load that previously weighed upon them. As the ice retreats, the Earth's crust first rises elastically—over the course of decades or centuries—and subsequently continues to rise viscoelastically for thousands of years more, as the Earth's mantle slowly flows back toward the area relieved of the ice's weight. In Scandinavia, the post-glacial isostatic rebound that began after the Last Glacial Maximum—some twenty-one thousand years ago—has not yet ended; the region's crust continues to rise today at a rate of five to ten millimeters per year.

This same post-glacial isostatic rebound affects the glaciers still present in these rising regions, as the uplift of the bedrock alters the position of the glaciers' grounding lines; it also affects sea-level measurements, since tide gauges record sea level relative to the land on which they are installed—a reading that already incorporates the movement of the land itself—thereby linking the cryosphere mass loss mentioned at the beginning of this description to the response the Earth continues to exhibit thousands of years later.

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