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IMPLICATIONS OF ICE LOSS
Global Sea Level
The most widespread and economically
important global impact of ice loss is sea-
level rise. As summarized by the IPCC
(2013), sea level has recently been rising at
just over 3 mm/year. A longer view shows
that this rise has accelerated and that fur-
ther acceleration is likely with continuing
warming. Recent contributions to ocean
volume have come from a combination of
the expansion of ocean water due to its
warming, retreat of mountain glaciers,
shrinkage of the Greenland and Antarctic
ice sheets through faster flow of land-orig-
inated ice into the ocean, and, primarily in
Greenland, increasing surface melting and
runoff. Relevant studies show accelerated
flow in coastal regions in response to
warming ocean waters that reduce the
buttressing of ice shelves (IPCC, 2013).
The Shepherd et al. (2012) synthesis
estimated that sea-level rise from the ice
sheets accelerated between 1992 and
2011, with an average over that interval of
0.59 ± 0.20 mm/yr. Complete loss of the
ice sheets would raise sea level ~60 m, so
at this average rate, more than 100,000
years would be required for complete
ice-sheet removal. As discussed below,
however, much shorter time scales may
be involved.
Some studies estimate the costs of sea-
level rise to be relatively small. These esti- larger long-term rise of sea-level (Pollard
mates are, in part, based upon using the
most-likely IPCC projections of a slow,
small, and well-anticipated rate, as well as
the assumption of an efficient response to
the rise (e.g., Darwin and Tol, 2001).
Growing knowledge about the ongoing
evolution and behavior of the primary out-
let glaciers in Greenland and Antarctica,
however, raises the possibility that future
increments of sea-level rise may not, in
fact, be slow, small, or well-anticipated
(e.g., Joughin et al., 2014; Pollard et al.,
2015; DeConto and Pollard, 2016).
As reviewed in Alley et al. (2015), the
distribution of possible rates of sea-level
rise includes values with slightly slower,
slightly faster, or a much faster rise than
the central IPCC projections. Of particular
concern is marine instability in West
Antarctica, especially the drainage through
Thwaites Glacier into the Amundsen Sea
(National Research Council, 2013).
Extensive retreat may already have been
triggered or may be imminent (Joughin et
8 GSA Today | August 2017
al., 2014; Parizek et al., 2013). In cold envi- Figure 3. Ice that enters the ocean typically
ronments, ice flowing into the ocean typi-
cally forms attached, floating ice shelves.
Friction between ice shelves and fjord
walls, or local sea-floor highs, slows ice-
shelf flow, in turn slowing the flow of non-
floating ice into ice shelves. Warming
ocean waters thin shelves, reducing this
ice-shelf buttressing, allowing faster flow
of non-floating ice into ice shelves. Beyond
some warming threshold, ice shelves typi-
cally break off completely, leaving tide­
water cliffs (reviewed in Alley et al., 2015).
Material strength limits the height of cliffs
(e.g., Fig. 3); ice cliffs much taller than
~100 m are likely to be unstable and break
rapidly (Hanson and Hooke, 2003; Bassis
and Walker, 2012). Retreat of Thwaites
Glacier, West Antarctica (Joughin et al.,
2014; DeConto and Pollard, 2016), could
generate a cliff much higher than this
limit, suggesting that very rapid retreat
could follow. Implementation of a param-
eterization for these processes in a well-
characterized ice-flow model produced
ice-sheet collapse more than one century
after initiation of rapid retreat, with >3 m
of sea-level rise from this one source alone
(Pollard et al., 2015; DeConto and Pollard,
2016). The full parameter space for such
cliff instability has not been extensively
explored, and faster collapse cannot be
eliminated (Alley et al., 2015). Even if such
rapid cliff collapse is not triggered, warm-
ing during the next one to a few centuries
could commit the world to a very much
et al., 2015), possibly including complete
loss of the ice sheets (Winkelmann et al.,
2015). Uncertainties remain great, with
potentially very large impacts upon human
society and economies. The importance of
this topic was highlighted by the recent
studies from the National Research
Council/National Academies (2013, 2015).
Water Supply
The loss of ice will have direct impact
on local populations through changes in
water availability, particularly during dry
periods. The glacial-fed streams in the
Andes, and elsewhere, are essential for
hydroelectric production, irrigation, and
municipal water supplies. Indeed, the gla-
ciers across the TP are sometimes referred
to as the “water towers” for southern Asia,
where >100,000 km2 of glaciers contain
one of the largest glacial stores of fresh
water outside of the Greenland and
forms cliffs that then fracture, as shown for this
iceberg with an approximate 60 m cliff face, in
Scoresby Sound, Greenland. Higher cliffs have
larger stress imbalances and so are more likely
to break rapidly. Ice cliffs of ~100 m height
appear to be near the highest that can support
themselves. Retreat in West Antarctica, partic-
ularly at Thwaites Glacier, could produce taller
cliffs that would fail rapidly, accelerating the
retreat and its contribution to sea-level rise.
Antarctic ice sheets (Yao et al., 2012;
Bolch et al., 2012). These glaciers dis-
charge meltwater into the largest rivers in
south Asia (Fig. 4), which are critical water
resources in the populous regions sur-
rounding the Himalayas. In 2009, the
Third Pole Environment (TPE) program
was launched in part to study the response
of this remote region to climate change
(http://www.tpe.ac.cn). The TPE program
includes a strong educational component,
as well as an integrated study of paleo-
records to develop the context essential to
assess and address the impact of anthro­
pogenic activities. Beside the larger-scale
impacts that are yet to unfold, people who
live in areas affected by glacial retreat are
already experiencing the consequences.
For example, in 2006, a lake that had grown
from the melting of Quelccaya’s Qori Kalis
outlet glacier (see Fig. 2J) breached its
moraine dam after an avalanche and
flooded the valley below, drowning herds
of grazing alpacas. Emblematic of these
concerns, the National Research Council
of the National Academies also conducted
a study to assess the role of Himalayan
glaciers within the context of climate