Geoscience Reference
In-Depth Information
Ice sheet and ice shelf stability
NEW DEVELOPMENTS
Ice shelves form at the marine margin of ice sheets, pinned to their bed landward of the grounding line(see p. 366)
and where they cross offshore submarine rises or islands, but moving unimpeded seaward of that line at velocities
of 1-2 km yr
-1
. They avoid outstripping ice supply by thinning as they move, 10
1-3
km offshore. Shelf stability requires
additional ice to that 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 sea water. Freezing-on rates of 300-600 mm
yr
-1
occur below the Amery ice shelves in east Antarctica. Oceanic and atmospheric heat fluxes, and continuous
flexing by tides and currents, eventually break up the outer shelf, 'calving' bergs from ice cliffs 100-200 m high.
Ice shelves are fed by usually fast-moving outlet glaciers, marking a major transition in ice sheet dynamics where
slowly accumulating and slow-moving ice accelerates into the ablation zone. Shelf dynamics are therefore a useful
indicator of overall ice sheet 'health' in advancing or equilibrium states but can, in other situations, destabilize the
ice sheet itself. For any given shelf mass balance, the extent and nature of pinning points strongly influence stability.
There are four dynamic ice shelf boundary zones - where the shelf ice stream exits its feeder glaciers; at the shear
zone where fast- and slow-moving ice meet at lateral margins in confining embayments; where ice meets bedrock
lubricated by sediment and water; and where grounded ice and floating ice meet (i.e. the grounding line).
Global climate change inevitably has potentially major implications for ice shelves which are inherently metastable,
responding unpredictably to atmospheric and ocean warming and sea-level rise. General Antarctic warming by 0·5C
in the past fifty years, and
C in the Antarctic peninsula, has pushed climatic limits of the ice shelves polewards.
This has probably led to the progressive collapse of small ice shelves in the Antarctic peninsula since 1994, culminating
with all 3.3k km
-2
of Larsen B inside six weeks in 2002. Earth's potentially most serious glacier hazard is therefore
the West Antarctic Ice Sheet (se
e
Figure 15.11a,
b
and
Plate 15.2
)
and its water-equivalence of 6-7 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. Sensitivity to sea-level rise is the most alarming scenario, since it
moves grounding lines inland and undermines their pinning effect. Catastrophic shelf collapse could - literally - put
the skids under the ice sheet, leading to rapid draw-down as its feeder glaciers surge and rapidly disintegrate. It is
thought that a smaller-scale ice shelf collapse may have taken place in the northern Irish Sea basin c. 16·7 ka
BP
,
triggering rapid final deglaciation of the British ice sheet
(
Figure 15.11c
)
.
Although not feeding an ice shelf, west Greenland's Jacokbshavn Isbrae currently provides the clearest demonstration
of accelerating and collapsing iceflow. It retreated 40 km from 1851 to 2001 but a further 20 km from 2001 to
2007, whilst its flow velocity almost doubled in the same time to 14 km yr
-1
- it's rushing to its doom! This underlines
how non-linear and positive feedbacks can rapidly destabilize individual glaciers or whole ice sheets. Rapid terminations
at the end of Quaternary cold stages (see, p 577) reveal that ice sheets collapse much faster than they form - carrying
clear warnings for our future. Shelf and glacier collapse draws down ice from accumulation zones faster than it forms,
thinning the glacier and reducing the accumulation area. With sufficient access and circulation beneath floating ice
shelves, a 1
2
10 m yr
-1
basal melt in ice shelves. Atmospheric warming brings subsurface
ice to melting point extremely slowly, taking 10
2-4
yr to penetrate the base - whilst surface meltwater can reach it
in hours via crevasse and englacial stream systems. Water-based glaciers slide more rapidly, generating friction,
increasing basal melting and sliding velocities further.
What is the current 'pulse' of the Antarctic and Greenland Ice Sheets telling us? The Greenland Ice Sheet core has
thickened slightly but shrunk at its margins (
Plate 15.12
),
steepening its average slope - which may lead to further
acceleration and draw-down. IPCC assesses its net mass change from 1961 to 2003 as between - 60/+ 25 Gt yr
-1
,
compared with - 200/+ 100 Gt yr
-1
for the Antarctic Ice Sheet in the same period. The good news, for now, is that
rapid disintegration of the Greenland Ice Sheet or collapse of the West Antarctic Ice Sheet is 'not likely' during the
twenty-first century. The bad news is that either event becomes 'more likely' with increasing climate disturbance.
Total West Antarctic Ice Shelf loss, with sea-level rise potential of 7 m, stands at 95 ± 11 Gt yr
-1
and there is satellite
evidence of rapid reactions amongst its feeder glaciers. For Greenland, IPCC considers that the threshold temperature
( 3C) beyond which the ice sheet enters irreversible decline and eventual disappearance may be crossed this century
- and may have already been reached.
C SST increase can induce
