Environmental Engineering Reference
In-Depth Information
Source: Boulton et al. (1977).
THE STABILITY OF ICE SHELVES
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Ice shelves are pinned to their bed landward of the grounding line (see p. 314) and where
they cross offshore rises or islands, but otherwise move unimpeded and may reach
velocities of 1-2 km a −1 . They avoid outstripping their ice supply by thinning seawards as
they move 10 1-3 km offshore. Shelf stability requires additional ice to the flow extruded
from their landward margin, to offset high calving losses at their seaward margin. This is
achieved by snowfall at the shelf surface and bottom freezing from seawater. Freezing-on
rates of 10-35 mm yr −1 and 300-600 mm yr −1 occur below the Ross and Amery ice
shelves respectively in Antarctica. Oceanic and atmospheric heat fluxes and continuous
flexing by tides and currents eventually break up the outer shelf, often calving bergs from
ice cliffs 100-200 m high.
Icebergs more than 2,000 km 2 have recently calved from the Ronne-Filchner and
Larsen ice shelves, which occupy protected embayments east of the Antarctic Peninsula,
raising the spectre of widespread destabilization by global warming. For any given shelf
mass balance, the extent and nature of the pinning points strongly influence stability.
There are four types of dynamic ice shelf boundary - where the shelf ice stream exits its
feeder glaciers; at the shear zone where fast- and slowmoving ice meet at lateral margins
in confining embayments; where ice meets bedrock lubricated by sediment and water;
and where grounded and floating ice meet (i.e. the grounding line).
Earth's potentially most serious glacier hazard may be the West Antarctic Ice Sheet
(see Figure 15.1a and Colour Plate 13 between pp. 272 and 273) which holds a water
equivalent of 7 m of global sea-level rise. The Ronne-Filchner and Ross ice shelves,
which it shares with outlet glaciers breaching the Transantarctic Mountains to the east,
comprise most of its eastern sector. Much of the remainder is grounded on bedrock below
sea level or close to the pressure-melting point. It is therefore metastable , with an
unpredictable response to global warming and sea-level rise. The ice shelves will retreat
if rising air and ocean temperatures reduce ice fluxes. Antarctic warming by 0·5° C in the
last fifty years, rising to 2° C in the Antarctic Peninsula, has pushed the climatic
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