Introduction
Glacier mass loss constitutes the largest contributor to global mean
sea-level rise (SLR), followed by ocean thermal expansion
. Over the last two decades, both glaciers
and ice caps , as well as ice sheets, have lost mass
at accelerating rates . These ice mass
changes coincide with atmospheric warming that causes record summer
temperatures and glacier melt in the Arctic and
across the Greenland Ice Sheet . In addition to mass loss by
melting, glacier dynamics have the potential to significantly amplify
glacier response to climate change by altering the ice discharge to
the ocean. Acceleration of outlet glaciers of the
Greenland Ice Sheet are generally attributed to hydraulic lubrication and marine terminus destabilization by oceanic warming
and calving . West Antarctic ice-stream acceleration is
generally attributed to reduced buttressing by thinning or loss of ice
shelves . Recent observations of sustained ice flow and
mass loss from northeastern Greenland and West Antarctica are also
reported as evidence of a marine ice-sheet instability
. This phenomenon, also known as
tidewater-glacier instability, refers to glacier speed-up due to
terminus retreat into deeper water .
To date, glacier-dynamic feedback processes remain poorly constrained and are
therefore not yet incorporated in global projections of future glacier
mass loss . SLR projections released with the IPCC Fifth
Assessment Report range from 0.26 to 0.82 m over the 21st century
. The projections include ice discharge from the ice
sheets into the ocean, however, based on current rates and not
accounting for a future dynamic response to climate change. The IPCC
acknowledges but is not able to quantify the probability of significantly
higher SLR, such as that associated with disintegration of marine-based sectors of
the Antarctic Ice Sheet .
Variations in glacier flow, and hence ice discharge, encompass wide
ranges of timescales and magnitudes, from diurnal velocity
fluctuations to century-scale surge-type behaviour, and are mainly
attributed to changes in basal drag . Cyclic surge-type behaviour is thought to arise from internal instabilities , whereas seasonal
velocity variations are externally controlled by surface-melt-induced
acceleration through basal lubrication . Basal water pressure, and thus basal drag, is controlled by the
rate of water supply to the glacier bed and the capacity of the
subglacial drainage system to accommodate increased discharge . Increased meltwater supply may thus
accelerate but also slow down ice flow if it facilitates the establishment of
a hydraulically efficient drainage system . Several
recent studies therefore suggest that hydraulic lubrication alone has
a more limited effect on the future net mass balance of the Greenland
Ice Sheet than previously anticipated
. However, the two recent assessments do not account for
changes in the extent of the basal area subjected to hydraulic
lubrication or do not quantify changes in calving loss.
The effect of surface meltwater on the thermal
structure of glaciers has only recently been considered in modelling studies and
was termed cryo-hydrological warming CHW;.
CHW includes latent heat released during refreezing of meltwater, as well as direct
heat transfer between water and ice. CHW has the potential to change englacial and basal temperatures
within years, whereas changes in basal thermal regime by heat
conduction alone would require decades to centuries
. regard CHW as a phenomenon specific
to the ablation area and suggest that more areas of the Greenland Ice Sheet will be
subjected to CHW in the case of sustained climate warming and thus upward migration of
the equilibrium-line altitude (ELA). CHW has the potential to enhance ice deformation
through the temperature dependency of the ice viscosity , as well
as basal motion, if it were to increase the extent of the basal area at the pressure-melting
point . It has previously been suggested that present outlet glaciers
may accelerate significantly if meltwater drainage to the bed were to spread inland
. simulated ice dynamics within the
wet snow zone of western Greenland. They achieved a best fit with an observed increase in
wintertime velocities if both changes in englacial and basal temperatures, and hence ice deformation and basal motion, were considered.
Cyclic surge-type behaviour is an extreme example of variations in
glacier flow, and characterized by long quiescent phases (decades to
centuries) with slow ice flow followed by short-lived active
phases (months to years) with orders-of-magnitude increases in flow
velocities . Resistance to sliding during the quiescent
phase leads to build-up of an upper reservoir . During a surge, mass is transferred
from the reservoir area to a lower receiving area, often
associated with kilometre-scale advances of the terminus and greatly
enhanced calving flux in the case of tidewater glaciers . Glacier
surges are generally regarded as internal instabilities, and not thought to be driven by
external factors . Nevertheless, the climatic mass balance influences
the build-up of the reservoir area and thus surge occurrence and periodicity .
In addition, external factors may drive feedback processes
that lead to amplified dynamic response, pushing the glacier towards
an instability threshold.
On Svalbard, glacier surges are thought to be controlled by a thermally controlled
soft-bed surge mechanism ,
as previously proposed for surges of polythermal glaciers in subpolar environments
. This theory concerns the slow adjustment of
the basal thermal regime and driving stresses in response to geometric changes during
the quiescent phase, ultimately leading to a thermal switch from cold to temperate
basal conditions, permitting for fast basal motion. Basal melting sets in and then subglacial
till, if present, is thawed and its shear strength reduced by rising pore-water pressure,
destabilizing the glacier and initiating the surge . Dynamic
thinning and reduction in surface slope reduce the driving stress and sliding rates,
eventually bringing the surge to a halt .
Thermal models of glaciers generally account for heat conduction through the ice, heat
transfer by ice advection, and strain heating and frictional heating. The geothermal heat
flux and the annual air temperature (plus firn warming) form the lower and upper boundary
conditions, respectively. Thermal effects by water flow within glaciers have previously
been mentioned and considered important e.g., but only recently
has “cryo-hydrologic warming (CHW)” been put back into focus .
neglected CHW in thermal models of glacier surges by assuming that
water flow within cold ice is limited. However, recent studies suggest meltwater connections
from the surface to the bed through cold ice of substantial thickness e.g..
Surface and bedrock topography of
Austfonna/Basin-3. (a) Surface elevation contours at
50 m interval (solid black), overlain on a TerraSAR-X
backscatter image (30 April 2012). The insert provides the ice cap's
location within the Svalbard archipelago. Drainage basins are
outlined in solid grey; Basin-3 is outlined in solid green. The positions of five
GPS receivers and one stake on Basin-3, as well as the automatic
weather station (AWS), are marked. Repeat-GPR profiling revealed
crevasse formation in upper reaches of Basin-3 between 2004 and 2007
and between 2008 and 2012 (Appendix D). (b) Bedrock contours (black,
colour-filled) are at 25 m intervals, with the bedrock sea-level
contour highlighted in red and 50 m surface elevation contours
superimposed (white).
Direct observations of ice dynamics during glacier surges are scarce. The evolution of
several surges on Svalbard was reconstructed from archived satellite radar data
only after a surge was noticed. Snapshot velocities were derived preferably for the
winter season, i.e. when surface melt is absent, as liquid water deteriorates the
quality of the derived motion maps using satellite interferometric radar
. Changes in ice dynamics specifically during
the summer melt season have therefore not been captured. Based on the available motion
snapshots, proposed that Svalbard glacier surges are characterized
by gradual acceleration over several years, at a higher rate during the months towards
the peak of the surge, and followed by multiannual, gradual slow-down.
Here, we present a 5-year record of continuous GPS measurements (May 2008 to May 2013)
and satellite synthetic-aperture-radar-based velocity maps (since April 2012) from Basin-3,
a marine-terminating drainage basin of the Austfonna ice cap on Svalbard. Our observations
demonstrate strong links between surface-melt and ice-flow acceleration, culminating by
October 2012 in a surge and drastically enhanced ice discharge. To date (January 2015),
the surge continues. We propose a hydro-thermodynamic feedback mechanism to surface melt,
subsequently mobilizing the ice within the reservoir area and weakening cold-based marginal
ice that restricts inland ice from draining into the ocean. Finally, we discuss possible
implications of the proposed mechanism for the stability of marine ice-sheet regions
if exposed to significant surface melt in a warming climate.
Austfonna, Basin-3
At ∼ 7800 km2, Austfonna is the largest ice cap in the
Eurasian Arctic (Fig. 1a). The ice cap consists of a main dome with an
ice thickness of up to 600 m that feeds a number of drainage basins,
some of which are known to have surged in the past
. Austfonna has a polythermal structure. In
the ice cap's interior, basal temperatures are likely at the pressure-melting point , while the thinner ice-cap
margins are cold and frozen to the bed, except for distinct fast-flowing outlets that are dominated by fast basal motion and
imbedded within the generally slowly deforming ice cap
. The southeastern basins are to a large extent
grounded below sea level, and form a continuous, non-floating calving
front towards the Barents Sea . The regions
grounded below sea level are at least partially underlain by marine
sediments . Over 2002–2008, the climatic mass
balance of Austfonna was close to zero . Yet the
ice cap was losing mass due to calving and retreat of the marine ice
margin that accounted for 2.5 Gta-1 or 33±5 % of
the total ablation .
Basin-3 (∼ 1200 km2 and a length of ∼ 60 km) is the largest drainage basin of
Austfonna and topographically constrained by a subglacial valley that
extends from south of the main dome eastwards towards the Barents Sea
(Fig. 1b). About one-third of the ice base is grounded below sea level
down to a maximum depth of ∼ 150 m within an overdeepening in the
central terminus region Fig. 1b;. A previous velocity map based on
satellite radar interferometry from data acquired in the mid-1990s revealed an ice
stream in the northern lower reaches of Basin-3 with flow velocities
≤ 200 ma-1 . The ice stream
was topographically constrained by a subglacial mountain to the north
(Isdomen) and near-stagnant ice, likely frozen to its bed, to the
south. The absence of surface lineations (e.g. crevasses) as late as
1991 identified the ice stream of Basin-3 as a recent feature
. During the 1990s, the front retreated on
average by 70±10 m a-1, accounting for two-thirds of
the calving flux of ∼ 0.4 Gta-1
. Basin-3 is known to have surged some time
prior to 1870 , and the advancing terminus
created a pronounced surge lobe nearly 25 km in width. The terminal
moraine associated with the last surge lies ∼ 8 km from
the present-day position of the calving front
. Based on lobe volume and accumulation rate, the
duration of the quiescent phase of Basin-3 was estimated to last
130–140 years , a very good estimate considering
the basin's renewed surge activity since autumn 2012, which is reported on here.
Results
Multiannual ice-flow acceleration
Winter velocities observed by GPS in May 2008
were significantly higher than in the mid-1990s (Fig. 2). The GPS time
series also reveal considerable overall acceleration, occurring in
pronounced steps, each of which coincides with the summer melt period,
as indicated by the temperature record of an automatic weather station
Fig. 2;. The 2008 summer speed-up is
followed by a gradual winter deceleration. In total, 60–95% of the velocity increase
during summer is reversible (Appendix D). However, during subsequent
years, an increasing fraction of the summer speed-up is of lasting nature,
i.e. increased summer velocities are sustained throughout the winter although surface melt had ceased.
Multiannual changes in ice dynamics are also evident from within the reservoir
area, higher up on Basin-3. The annual position change of a mass-balance stake close to the
ice divide (Fig. 1) revealed strong acceleration from 11 ma-1 between
2004 and 2007 to 28, 43 and 114 ma-1 for the period 2010–2013.
In addition, annually repeated GPR surveys reveal first and cumulative occurrence
of surface crevasses from ∼ 2004 onwards, between ∼ 2004 and 2007 along
the western profile, and between 2008 and 2012 along the eastern profile (Fig. 1a; Appendix C).
Mobilization of stagnant ice regions
The drastic acceleration during autumn 2012 coincided with the
expansion of the fast-flowing region across the entire basin, as
revealed by a time series of ice-surface velocity maps based on
intensity tracking of repeat-pass TerraSAR-X satellite radar
images (Fig. 3; Appendix B). Velocities in April 2012 were
up to 3 md-1, 1 order of magnitude larger than in the mid-1990s
(Fig. 3a). In April 2012, the fast-flow region extended farther
inland and had widened southward to a width of ∼ 6–8 km, as
compared to ∼ 5–6 km in the mid-1990s. As a result of southward expansion, the GPS
receivers were located ∼ 1 km north from the central
flowline and did not capture fastest flow velocities. In contrast to
the mid-1990s, the southeastern corner of Basin-3 also displayed fast
motion, at velocities of up to 1 md-1. These two distinct
fast-flow regions were completely separated by almost stagnant ice,
notably including the calving front. Low ice velocities,
< 0.1 md-1, indicate the absence of considerable basal
motion, and suggest frozen-bed conditions in this region. In August
2012, i.e. at the end of the summer melt season, velocities had
increased significantly, up to 6 md-1 for the northern and
4 md-1 for the southeastern fast-flow region, along with
further lateral expansion of the fast-flowing areas. Consequently, the
slow-moving ice region in between had decreased in size (Fig. 3b) and
disappeared by October 2012, when ice flow escalated into a surge
comprising the entire width of the basin, reaching velocities
>10 md-1 (Fig. 3c). Velocities increased further until
January 2013, reaching a maximum of 20 md-1 (Fig. 3d).
Between January and May 2013 the maximum velocities decreased
to 15.2 md-1, while the upglacier regions continued to
accelerate (Fig. A1b). By the end of 2013, fast flow of Basin-3
continued (Appendix A).
Surface velocity fields of Basin-3, Austfonna, derived from
TerraSAR-X feature tracking: (a) April/May 2012,
(b) August, (c) October, and (d)
January/February 2013. Red circles represent mean position of GPS
receivers over the particular repeat-pass period; fill colour
according to colour coding of receivers in Fig. 2. The red arrows
indicate associated GPS velocity vectors. Glacier elevation contours
plotted in grey at 100 m intervals; front position at time of
repeat pass in orange.
Calving flux
The TSX data allowed for calculation of the calving flux components,
i.e. (i) the ice flux through a fixed fluxgate near the calving
front, and (ii) the mass change of the terminus downglacier of that
fluxgate, accounting for front position changes (Fig. 4; Appendix B). The observed ice flux peaked at a rate of
13.0±4.2 Gta-1 in December 2012/January 2013, after which it decreased slightly (Fig. 4a). Prior to October 2012, the position
of the entire calving front of Basin-3 was remarkably stable (slight
retreat; Fig. 4b), indicating that the entire ice flux was balanced by
iceberg calving. After November 2012, the southern and central parts
of the front advanced by >1 km, reducing ice mass loss
through calving by 61 %, as compared to a stable front position.
Direct conversion of calving mass loss to SLR contribution is only
meaningful for a static calving front. Glacier surges are typically
accompanied by significant terminus advances. An advancing terminus
reduces the mass loss from the glacier; however, the submerged part of
the terminus replaces sea water instantaneously, causing an
instantaneous sea-level rise. We therefore distinguish between
a glacier mass balance and a sea-level perspective on the calving flux
(Fig. 4c; Table 1). From 19 April 2012 to 9 May 2013, calving mass loss
[yearly rate] from Basin-3 accounted for
4.4± 1.6 Gt [4.2± 1.6 Gta-1], an
increase by an order of magnitude compared to 1991–2008
, nearly tripling the calving loss from the
entire Austfonna ice cap. The related sea-level rise contribution of
7.6± 2.7 Gt [7.2± 2.6 Gta-1] is as
large as the total glacier mass change from the entire Svalbard
archipelago for the period 2003 to 2008, estimated to be
-6.6±2.6 Gta-1 . Rates of sea-level
rise contribution are expected to decline once the surge of Basin-3 has
terminated, i.e. once the ice flux has diminished and the terminus advance has come to a halt.
Nevertheless, iceberg calving and hence ice mass loss will be maintained,
depending on future rates of marine-terminus retreat.
Calving flux from Basin-3, Austfonna, April 2012 to May
2013. Calving components are expressed in terms of the instantaneous (a) and cumulative mass change (b) and allocated
to the effect on glacier mass balance and sea level (c) (see
Sect. 4.3). Whiskers in (a, b) indicate uncertainty bounds
calculated using propagation-of-uncertainty analysis, and shaded
areas in (c) indicate upper and lower bounds given maximum or minimum
ice thickness.
Discussion
The dynamic changes that have been observed at Basin-3 over the last
two decades can be separated into three phases: (1) activation of
a spatially confined fast-flow region in the early 1990s
, (2) multiannual acceleration and expansion of the fast-flow region from < 2008 to
2012, and (3) active surge phase following the destabilization of the entire terminus in autumn
2012.
Phase 1: Initiation of spatially confined fast flow
Spatially confined fast flow was initiated in the northern part of lower Basin-3
in the early 1990s, and interpreted as a temporary flow instability .
Activation of fast flow within this region could be explained by internal mechanisms,
i.e. by gradual changes in basal thermal regime and driving stress, associated with
long-term geometric changes during the quiescent phase e.g..
Numerical simulations of Austfonna support the concept of a thermally controlled
soft-bed surge mechanism , as proposed earlier for Svalbard
glacier surges . Spatially
confined fast flow since the early 1990s entailed an ice flux in excess of the
balance flux and thus a drawdown of the reservoir area.
Extensional flow within the reservoir area is expected to form crevasses ,
as indeed observed by GPR since ∼ 2004 (Fig. 1a; Appendix C).
Estimate of calving flux components and total calving flux over the TSX observation period from 19 April 2012 to 9 May 2013.
Calving flux components
(Gt)
(Gta-1)
Ice flux, Qfg
8.3±2.8
7.8±2.7
Terminus change, Qt
3.8±1.2
3.6±1.1
Terminus-seawater displacement, Qtsd
3.2±1.1
3.0±1.0
Total calving flux
Mb perspective, Qmb=Qfg-Qt
4.4± 1.6
4.2± 1.5
SLR perspective, Qsl=Qmb+Qtsd
7.6± 3.9
7.2± 2.6
Phase 2: Mobilization of the reservoir – multiannual, stepwise acceleration
The multiannual acceleration of Basin-3, observed by GPS since 2008, occurred
in discrete steps, each of which coincident with consecutive summer melt periods
(Fig. 2). The high sensitivity and short response
time (days) of glacier dynamics to melt events clearly suggest
surface-melt-triggered acceleration. Short-lived acceleration during the melt
season is consistent with current understanding of hydraulic lubrication
. In contrast, the multiannual acceleration of background
velocities (Appendix D) cannot be explained by this mechanism
and suggests a fundamental change in dynamics.
Enhanced post-summer velocities sustained throughout the winter can be explained by
successive activation of previously stagnant ice regions during the
previous summer melt period. Mobilization of increased ice volumes
within the reservoir area leads to increased velocities and
discharge. A widening of the fast-flow region itself allows for higher
centreline velocities due to the increased distance from the lateral
shear margin, analogous to the behaviour of Antarctic ice streams .
Mobilization of the reservoir area is evident from the
surface-crevasse formation within the upper accumulation area of
Basin-3 since ∼ 2004 (Fig. 1a; Appendix C). Crevasses are
a manifestation of longitudinal extension and
evidence of upglacier migration of the fast-flowing region. This is in
line with the upglacier expansion of the ice stream revealed by TSX
and the observed strong acceleration of the mass-balance stake within the reservoir area (Fig. 1). The first occurrence
of crevasses signifies the development of potential meltwater routes to
the glacier bed, subjecting an increasing region to CHW and basal hydraulic
lubrication. Similarly, meltwater reaching
the bed beneath the heavily crevassed shear margin of the fast-flow region efficiently weakens
the basal drag that balances the lateral drag, which in turn regulates
ice velocities . CHW and
increased frictional heating both act to enhance this mechanism,
weakening the flow resistance exerted by the lateral shear
margins and possibly cold-based ice patches acting as “sticky
spots” . The interplay between CHW and the
emergence of basal hydraulic lubrication over an expanding area of the
ice base constitutes a hydro-thermodynamic feedback. Unconsolidated subglacial
sediments underneath the marine-based ice of Basin-3 act to further enhance this
feedback. Input of surface meltwater during consecutive summers progressively
raises the pore-water pressure within the sediment, reducing its shear strength and favouring rapid
destabilization .
Phase 3: Destabilization of the marginal ice plug – basin-wide surge
A surge comprising the full width of Basin-3 and the subsequent
advance of the terminus followed the mobilization of the remaining
stagnant ice regions, notably including the calving front. Water flow may
have accessed the cold-ice base through partially unfrozen subglacial sediments,
as suggested in the case of Trapridge Glacier, Yukon, Canada .
Field measurements of thermal structure and ice flow during a surge of Trapridge
Glacier do not indicate that the cold ice below the surge front acts as a thermal
barrier to water flow. observed very high temperature
gradients in the cold basal ice below a surge front, about 10 times larger
than expected from geothermal heat flux, and attributed this to water flow through
an unfrozen substrate beneath the cold ice. The abrupt onset of fast flow in
previously stagnant regions of Basin-3 suggests sheer tearing of the glacier from
its bed. This activation was possibly related to a combination of warming of the
glacier bed through CHW and mechanical stress transfer from active ice further upstream.
During the fieldwork in spring 2012, large ice blocks were observed to be pushed upward
above the glacier surface near the lower shear margin, an indication of strong longitudinal stresses.
While fast flow continued by the end of 2013, it is expected to slow down or
come to a halt within a few years. Massive ice redistribution from the
reservoir into the receiving area and towards the calving front
efficiently lowers the driving stress. Eventually, temperate basal
conditions underneath the dynamically thinned and decelerating
terminus are expected to be no longer maintained, at which time the
base of the ice will start to refreeze to its bed .
Hydro-thermodynamic feedback mechanism
We cannot explain our observations using standard theory of glacier surges alone
e.g.. The multiannual acceleration did not occur uniformly
or gradually, as previously thought characteristic of Svalbard glacier surges
. Incorporating the effect of CHW over an expanding
area of the glacier bed provides, we believe, a good explanation of our observations
(Fig. 5). The hydro-thermodynamic feedback proposed here is not limited to
CHW; it also includes the additional hydraulic lubrication effects. Ice regions undergoing
a switch from cold to temperate basal conditions were previously characterized by
frozen conditions, i.e. absence of water. In the initial stage of basal–hydraulic
drainage system development, water input is likely to raise water pressure and enhance
basal motion, in line with the established theory on basal lubrication
e.g.. In addition, thawing of underlying frozen till
and rising pore-water pressure lead to enhanced sediment deformation. These
processes all result in an enhancement of ice flow. Longitudinal extension
opens new surface crevasses that provide englacial pathways for meltwater in
subsequent summers, subjecting a larger region of the bed to surface melt.
Eventually, the entire drainage basin is sufficiently destabilized, initiating
the surge. Once the entire basin is moving by fast basal motion, i.e. the entire
base is at pressure melting, and driving stresses are decreasing, the hydro-thermodynamic
feedback no longer operates.
Schematic illustration of the proposed hydro-thermodynamic feedback to summer melt,
imbedded within the surge cycle of Basin-3, Austfonna. The approximate start of each phase is
indicated at the bottom. Phase 1 follows from long-term changes in glacier geometry, i.e.
build-up of a reservoir, and associated changes in driving stress and basal thermal regime.
The hydro-thermodynamic feedback loop operates over several years during phase 2 and 3, each
loop coinciding with consecutive summer melt periods. Successive mobilization and destabilization
initiates the surge. Dynamic thinning, reduction in driving stress and basal heat dissipation eventually terminate the surge.
We would like to stress that the hydro-thermodynamic feedback proposed here does not oppose,
but can be understood as an integral part of the theory of thermally controlled soft-bed
glacier surges, as illustrated in Fig. 5. An increasing fraction of the glacier
bed is subjected to surface melt, and basal processes conductive for surge
behaviour are amplified. Basal processes include, in particular, (1) CHW that
leads to a switch from cold to temperate basal conditions, permitting for fast
basal motion; (2) expansion of the area subjected to basal lubrication; and (3) rising
pore-water pressure and sediment deformation. The feedback requires an initiation
zone in which surface meltwater can access the bed, such as the spatially confined
fast-flow region described in phase 1. Alternatively, supra-glacial lake drainage
may establish full ice-thickness meltwater pathways . Our observations
of Basin-3 suggest, on the one hand, a significant contribution of the hydro-thermodynamic feedback on
the mobilization of the reservoir area and, on the other hand, weakening of the flow
resistance within the receiving area.
Implications for the future stability of ice sheets
Surface melt has likely been a widespread phenomenon on Svalbard throughout
most of the Holocene. The current surge of Basin-3 should therefore not be mistaken
as a response to recent Arctic warming. The hydro-thermal feedback could also have
played a role in previous Svalbard glacier surges. Given continued global warming,
characterized by more widespread and intense occurrence of surface melt, we hypothesize
that the hydro-thermodynamic feedback may gain importance in other glaciated regions, including the ice sheets.
Rapid marine ice-sheet
disintegration is evident from geological records both in the Northern and
Southern Hemisphere and typically associated with air temperatures similar to or
warmer than those predicted for the end of the 21st century
. Ice-rafted debris distributed across the
North Atlantic ocean floor provides evidence of substantial calving associated
with the rapid disintegration of the Laurentide Ice Sheet, so-called Heinrich
events . Palaeoclimatic records suggest rates of sea-level
change much greater than currently observed or projected for the 21st century,
e.g. 3.5 to 5 m SLR per century (35–50 mm a-1) at the
end of the Last Glacial Maximum, ∼ 14.5 ka ago .
Analogies between glacier surges and partial ice-sheet collapses are widespread
throughout the glaciological literature e.g..
Analogous to the ice sheets, ice caps like Austfonna consist of slow-moving inland
ice interspersed with faster-flowing outlet glaciers and ice streams
that deliver inland ice towards the calving front. Drainage basins of Austfonna in
their quiescent phase are characterized by margins frozen to their bed
. Similarly, the coastal margins of the
Antarctic Ice Sheet contain large regions of cold-based ice
that may currently prohibit efficient drainage of
warm-based interior ice towards the ocean – e.g. in the Wilkes Basin,
East Antarctica, which is known to have been dynamically active during
the Pliocene warmth . A very recent model study confirms the
potential dynamic instability of the Wilkes Basin, following the removal of a
cold-ice plug . In that study, a retreat of the grounding
line is forced by oceanic warming, thereby eliminating the cold-ice plug.
Our proposed mobilization of the reservoir area of Basin-3 is in line with
, pointing at the potential of CHW in mobilizing more inland
ice regions of the Greenland Ice Sheet, thereby increasing ice discharge
through existing outlet glaciers. An obvious feature that distinguishes Basin-3
from Greenland outlet glaciers is the presence of a cold-based ice plug that,
until autumn 2012, restricted drainage from the reservoir area. In the case of
fast-flowing Greenland outlet glaciers, the thermo-dynamic feedback would
therefore lead to gradual acceleration, as long as the ELA continues migrating
upglacier, rather than leading to surge-type behaviour. In southwestern Greenland, further
acceleration of Jacobshavn Isbræ followed the 2012 record summer
melt season . Although the authors attribute the
acceleration to terminus retreat into a bedrock depression, the
occurrence of pronounced summer speed-up at higher elevations may also
reflect inland migration of surface melt and the effect of associated
hydraulic feedbacks on glacier dynamics . Similarly, the
hydro-thermodynamic feedback may play a role in the sustained mass
loss recently reported from northeastern Greenland and mainly attributed
to sea-ice decline due to regional warming .
Summary and conclusion
For the first time, to our best knowledge, we have obtained continuous
in situ velocity observations during glacier-surge initiation. Our observations
from Basin-3 of Austfonna reveal details of a surge that have never been recorded before.
Unlike proposed earlier for Svalbard glacier surges ,
multiannual acceleration during surge initiation of Basin-3 was not gradual but
occurred in discrete steps, coincident with successive summers. We propose a
hydro-thermodynamic feedback mechanism triggered by surface melt reaching a
growing fraction of the glacier bed. Intrusion
of surface melt to the glacier bed provides an efficient heat source
through CHW, facilitating a thermal switch from cold to temperate basal conditions,
permitting for basal motion. Initiation of hydraulic lubrication, along with rising
pore-water pressure within subglacial sediments, further enhances basal motion,
eventually destabilizing the overlying ice. These processes have earlier been
summarized as thermally controlled soft-bed surge mechanism e.g..
However, the active role of surface melt and the associated hydro-thermodynamic
feedback mechanism contrast the previous understanding of glacier surges as
purely internal instabilities.
The recent calving flux of Basin-3 has strong
implications for the mass balance of the ice cap and its contribution
to sea-level rise. From 19 April 2012 to 9 May 2013, the calving flux
[yearly rate] of Basin-3 amounted to 4.4±1.6 Gt
[4.2±1.6 Gta-1], an increase by an order of magnitude
compared to 1991–2008 . With the
terminus advance accounted for, the related sea-level rise contribution of
7.6±2.7 Gt [7.2±2.6 Gta-1] equals
the total annual glacier mass loss from the entire Svalbard
archipelago for the period 2003 to 2008, estimated to be
6.6±2.6 Gta-1 .
Given continued climatic warming and increasing surface melt, we hypothesize that the
hydro-thermodynamic feedback may gain significance in other glaciated areas, including the ice
sheets. In light of recent record melt and rising ELA of the Greenland
Ice Sheet, the proposed mechanism has the potential to lead to
a long-term enhancement of outlet-glacier discharge and calving
loss, as earlier proposed by . Our expectation contrasts with recent studies that indicate limited
effects of surface-melt-induced acceleration on the future net mass
balance of the Greenland Ice Sheet
. Surface melt in Antarctica is presently
mainly constrained to the ice shelves . Given strong
continued warming, surface melt will increasingly occur over coastal
areas of Antarctica, making the grounded ice-sheet margins vulnerable
to the hydro-thermodynamic feedback.
Our study of the Austfonna ice cap highlights the importance of dynamic ice-mass loss for glacier mass
balance. Current model projections of future SLR
still do not account for the dynamic response of
glaciers to continued global warming and might need to be revised
upward after incorporating mechanisms such as CHW and the hydro-thermodynamic
feedback.