ArticlePDF Available

Calving rates at tidewater glaciers vary strongly with ocean temperature

Authors:

Abstract and Figures

Rates of ice mass loss at the calving margins of tidewater glaciers (frontal ablation rates) are a key uncertainty in sea level rise projections. Measurements are difficult because mass lost is replaced by ice flow at variable rates, and frontal ablation incorporates sub-aerial calving, and submarine melt and calving. Here we derive frontal ablation rates for three dynamically contrasting glaciers in Svalbard from an unusually dense series of satellite images. We combine ocean data, ice-front position and terminus velocity to investigate controls on frontal ablation. We find that frontal ablation is not dependent on ice dynamics, nor reduced by glacier surface freeze-up, but varies strongly with sub-surface water temperature. We conclude that calving proceeds by melt undercutting and ice-front collapse, a process that may dominate frontal ablation where submarine melt can outpace ice flow. Our findings illustrate the potential for deriving simple models of tidewater glacier response to oceanographic forcing.
Content may be subject to copyright.
ARTICLE
Received 27 Jan 2015 |Accepted 4 Sep 2015 |Published 9 Oct 2015
Calving rates at tidewater glaciers vary strongly
with ocean temperature
Adrian Luckman1,2, Douglas I. Benn3,4, Finlo Cottier5,6, Suzanne Bevan1, Frank Nilsen2,7 & Mark Inall5
Rates of ice mass loss at the calving margins of tidewater glaciers (frontal ablation rates) are
a key uncertainty in sea level rise projections. Measurements are difficult because mass lost
is replaced by ice flow at variable rates, and frontal ablation incorporates sub-aerial calving,
and submarine melt and calving. Here we derive frontal ablation rates for three dynamically
contrasting glaciers in Svalbard from an unusually dense series of satellite images. We
combine ocean data, ice-front position and terminus velocity to investigate controls on frontal
ablation. We find that frontal ablation is not dependent on ice dynamics, nor reduced
by glacier surface freeze-up, but varies strongly with sub-surface water temperature. We
conclude that calving proceeds by melt undercutting and ice-front collapse, a process that
may dominate frontal ablation where submarine melt can outpace ice flow. Our findings
illustrate the potential for deriving simple models of tidewater glacier response to
oceanographic forcing.
Q5
DOI: 10.1038/ncomms9566 OPEN
1Department of Geography, College of Science, Swansea University, SA2 8PP, UK. 2Department of Arctic Geophysics, University Centre in Svalbard, Po Box
156, 9171 Longyearbyen, Norway. 3Department of Arctic Geology, University Centre in Svalbard, Po Box 156, 9171 Longyearbyen, Norway. 4Department of
Geography, University of St Andrews, St Andrews, Fife, KY16 9AJ, UK. 5Scottish Association for Marine Science, Oban PA37 1QA, UK. 6Faculty of
Biosciences, Fisheries and Economics, UiT The Arctic University of Norway, Po Box 6050 Langnes 9037 Tromsø, Norway. 7The Geophysical Institute,
University of Postboks 7803 5020 Bergen, Norway. Correspondence and requests for materials should be addressed to A.L.
(email: A.Luckman@Swansea.ac.uk).
NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications 1
&2015 Macmillan Publishers Limited. All rights reserved.
Loss of mass at the termini of tidewater glaciers, or frontal
ablation, occurs by a combination of calving and submarine
melting1,2. In addition to its direct contribution, submarine
melting can amplify calving by undercutting and destabilizing the
glacier front3. Several recent studies have concluded that
submarine melting can dominate mass loss at tidewater termini,
particularly where glaciers are impacted by incursions of warm
water from the continental shelf or beyond2,4,5. However, there
remains a distinct lack of direct measurements of either calving or
subaqueous melt rates. Almost all estimates of the latter are based
on calculations of the frontal heat budget using oceanographic
data, for example, refs 1,6,7, and because these estimates rely on
extrapolations from spatially and temporally limited CTD and
current measurements, they tend to be subject to large
uncertainties. Some attempt has been made to directly measure
frontal ablation rates at tidewater glaciers8,9 but temporal and
spatial resolution is often poor, and winter months are
inadequately sampled. This scarcity of direct measurements of
frontal ablation rates and their relationship to conditions both
above and below the waterline places severe limits on our ability
to predict the response of tidewater glaciers to oceanographic
forcing10.
We address this problem by combining high temporal and
spatial resolution satellite radar data with oceanographic and
meteorological time series to investigate controls on frontal
ablation rates. We derive time series of frontal velocities, terminus
positions and frontal ablation rates for three Svalbard tidewater
glaciers using 11-day repeat, 2 m resolution, TerraSAR-X images
spanning more than a year. Analysis of these data allows us to
quantify evolution of frontal ablation rates in unprecedented
detail through a complete seasonal cycle, and to identify the key
environmental controls on calving behaviour. We find that rates
of frontal ablation are not dependent on glacier dynamics, nor
reduced by the onset of glacier surface freeze-up, but closely
follow the temperature of the fjord waters at depth. Our findings
imply that submarine melt undercut and collapse is the dominant
calving mechanism at these glaciers, and that frontal ablation is
therefore controlled primarily by ocean temperatures.
Results
Glaciers studied. We choose three glaciers for their contrasting
dynamic characteristics (Fig. 1): Kronebreen, a glacier with one of
the highest flux rates in Svalbard; Tunabreen, a quiescent-phase
surge-type glacier; and Aavatsmarkbreen, an active-phase surge-
type glacier that began a surge during our observation period. In
addition to their contrasting dynamical characteristics, these
glaciers also vary in their fjord settings (Fig. 1): Kronebreen
terminates in Kongsfjorden which has a deep, unrestricted
connection to the warm West-Spitsbergen Current (WSC), but
has a relatively shallow connection between the outer and inner
parts of the fjord at B60 m depth. Tunabreen terminates in
Tempelfjorden which is shallower and further removed from the
WSC than Kongsfjorden (Fig. 1). Aavatsmarkbreen terminates in
Forlandsundet, which is as close to the WSC as Kronebreen, but
lacks a direct, deep connection.
Calving rate measurement and potential controls. The rate of
ice loss from a tidewater glacier terminus is the sum of that due to
calving, and that due to submarine melting, and is commonly
referred to as the frontal ablation rate _
a(ref. 2). A change in
ice-front position over time must equal the rate at which ice is
delivered to the terminus minus _
a. Thus satellite data of sufficient
spatial resolution may be used to calculate this quantity through
regular measurements of ice-front position, from which dl/dtmay
be derived, and of terminus ice speed U
T
:
_
a¼UTdl=dtð1Þ
We use equation (1) to calculate the frontal ablation rate
between pairs of TerraSAR-X images from surface velocities
derived by feature tracking, and ice-front position change derived
by manual digitization and geographical information systems
(GIS) (Methods section). The images, in ‘Stripmap’ mode
(B2 m ground range pixel size), were acquired every orbital
cycle for 19 months during 2013 and early 2014, providing an
image pair every 11 days barring a few acquisition failures. In the
cases of Kronebreen and Aavatsmarkbreen, we incorporate
images from more than one satellite track, giving a greater
frequency of observations during some time periods.
To investigate the controls on measured frontal ablation rates
we explore a variety of environmental variables that have
previously been associated with calving behaviour. These include
local weather data (air temperature and precipitation), sea-surface
temperature (SST), fjord ice presence and water temperatures at
depth in the glacier fjords (henceforth ‘sub-surface temperatures’;
Methods section).
Glacier dynamic behaviour. Glacier terminus speeds (Fig. 2)
show significantly different behaviours between the three glaciers
during our observation period. Kronebreen has a winter speed of
1.5–2m per day, with summer peaks of 3–4 m per day associated
with positive air temperatures and periods of high rainfall. The
winter speed of Tunabreen is only 0.2 m per day, rising to a peak of
B1 m per day in October. Aavatsmarkbreen flowed at B1 m per
day before its surge but by November 2013 had attained B3 m per
day, a speed maintained without major fluctuations until May
2014 when a further summer speed-up occurred. These different
West
Spitsbergen
current
Billefjorden
A
K
T
Ny Ålesund
Kongsfjorden
Longyearbyen
km
0
–500 –400 –300 –200
Depth (m)
–100 0
100
Figure 1 | Svalbard location map. Arrows show positions of glaciers
(K ¼Kronebreen, T ¼Tunabreen, A ¼Aavatsmarkbreen), weather stations,
and moorings. Background bathymetry from IBCAO Version 3.0 (ref. 35)
illustrates the possible routes for warm water from the WSC via troughs
to fjords.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566
2NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications
&2015 Macmillan Publishers Limited. All rights reserved.
–500
–400
–300
–200
–100
0
100
200
300
400
500
–4
–2
0
2
4
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Jan
Feb
Mar
Apr
May
Jun
JulJul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Jan
Feb
Mar
Apr
May
Jun
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Kronebreen
Mean ice front position
Mean retreat rate
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0
1
2
3
4
5
6
7
8
Mean speed
Mean frontal ablation rate
–25
–20
–15
–10
–5
0
5
10
15
20
25
0
5
10
15
20
25
30
35
40
45
50
Precipitation (mm)
Mean daily temp
Daily precipitation
–2
0
2
4
6
8
0.0
0.4
0.8
1.2
1.6
2.0
Sea ice presence
or concentration (0–1)
2013 2014 2013 2014
Ocean surface temperature
20–60m ocean temperature
Sea ice concentration
–300
–200
–100
0
100
200
300
–3
–2
–1
0
1
2
3
Tunabreen
Mean ice front position
Mean retreat rate
0.0
0.2
0.5
0.8
1.0
1.2
1.5
1.8
2.0
0
1
2
3
4
5
6
7
8
Mean speed
Mean frontal ablation rate
–25
–20
–15
–10
–5
0
5
10
15
20
25
0
5
10
15
20
25
30
35
40
45
50
Mean daily temp
Daily precipitation
–2
0
2
4
6
8
0.0
0.4
0.8
1.2
1.6
2.0
Ocean surface temperature
20–60 m ocean temperature
Sea ice concentration
Fast ice presence
–600
–500
–400
–300
–200
–100
0
100
200
300
400
500
600
–6
–4
–2
0
2
4
6
Ice front position (m)
Retreat rate (m per day)
Aavatsmarkbreen
Mean ice front position
Mean retreat rate
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0
1
2
3
4
5
6
7
8
Speed (m per day)
Mean speed
Mean frontal ablation rate
–25
–20
–15
–10
–5
0
5
10
15
20
25
0
5
10
15
20
25
30
35
40
45
50
Temperature (°C) Temperature (°C)
Mean daily temp
Daily precipitation
–2
0
2
4
6
8
0.0
0.4
0.8
1.2
1.6
2.0
2013 2014
Ocean surface temperature
20–60 m ocean temperature
Sea ice concentration
Fast ice presence
Frontal ablation
rate (m per day)
Precipitation (mm)Sea ice presence or
concentration (0–1)
Ice front position (m)Speed (m per day)
Speed (m per day)
Temperature (°C)Temperature (°C)
Temperature (°C)Temperature (°C)
Retreat rate (m per day)
Ice front position (m)
Frontal ablation
rate (m per day)
Precipitation (mm)Sea ice presence
or concentration (0–1)
Retreat rate (m per day)Frontal ablation
rate (m per day)
Fast ice presence
Figure 2 | Glacier time series. Data derived from a series of 11-day repeat TerraSAR-X images from 2013 to 2014 for three contrasting Svalbard glaciers to
illustrate the frontal ablation rate and its two key components: the rate of change of ice-front position and the terminus speed. Also shown are: temperature
and precipitation from the nearest weather station (Ny Ålesund for (a) Kronebreen and (c) Aavatsmarkbreen; Longyearbyen for (b) Tunabreen);
OSTIA SST and sea ice concentration32 from close to the ice front; fast ice presence in each fjord immediately in front of the ice front; and mean weekly
water temperature between 20 and 60 m from the nearest ocean mooring. For (a,c) these data are from Kongsfjorden (Fig. 1); for (b) these data are from
Billefjorden. Dashed vertical lines indicate year transitions.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566 ARTICLE
NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications 3
&2015 Macmillan Publishers Limited. All rights reserved.
behaviours reflect contrasting controls on the terminus dynamics
of the three glaciers. The summer acceleration of
Kronebreen is consistent with enhanced basal lubrication from
surface melt water and precipitation11, and is in accord with
reports of seasonal dynamic variation at tidewater glaciers
elsewhere8,9,12,13. Detectable motion of the quiescent Tunabreen
is confined to the frontal zone, where high longitudinal stress
gradients induce outward stretching of the ice. Stretching rates are
proportional to ice cliff height, which exhibits systematic seasonal
variations in response to calving losses14. The sustained high
velocities of Aavatsmarkbreen between summer 2013 and the end
of our time series are characteristic of a surge, possibly modulated
by the influx of surface melt water during the summer months.
Mean ice-front positions at Kronebreen and Tunabreen reveal
comparable seasonal patterns, although of different magnitudes
(Fig. 2). Both glaciers advance modestly during winter and retreat
strongly during summer and autumn, a period associated with
greater seismic activity13, with net annual retreats of B350 and
B150 m, respectively. Similar patterns of seasonality in ice-front
position are seen in Alaska15 and Greenland16. Aavatsmarkbreen
followed the same pattern in spring 2013, but then advanced by
B500 m between the start of the surge in summer 2013 and May
2014, after which it began to retreat.
By combining ice-front positions with terminus velocities
(equation (1)) we quantify frontal ablation rates and their
seasonal variation at an unprecedented temporal resolution
(Fig. 2). Maximum frontal ablation rates for Kronebreen,
Tunabreen and Aavatsmarkbreen are B8, B3 and B5 m per
day, respectively. Despite their diverse dynamic behaviours and
fjord settings, the magnitudes of these frontal ablation rates are
notably comparable, and their seasonal patterns very similar.
Rates for all three glaciers peak in September and October and
continue at a high level well after air temperature has fallen
consistently below 0 °C. Frontal ablation lags air and SSTs by
1–2 months but is in synchrony with the peak in sub-surface
ocean temperature. We note that frontal ablation rates at a
sample of Alaskan Glaciers appear to peak earlier in the summer9,
but that a strongly similar seasonal pattern of calving rate to our
glaciers is seen at Columbia Glacier (also in Alaska)8. Of the three
glaciers studied, only Tunabreen experienced sea ice during our
study period. The loss of sea ice in spring heralds an increase in
calving, but frontal ablation drops to a minimum long before the
return of sea ice in January.
Controls on calving. To explore these interactions in more detail,
we further examine relationships between frontal ablation rate
and potential controls on calving at Kronebreen and Tunabreen,
the glaciers for which we have proximal oceanographic data.
Previous studies have investigated calving relationships with
water depth, ice thickness and ice speed8,17, or modelled calving
as a process driven by surface melt18. Since we need to explain the
seasonal variation in frontal ablation, we present relationships
with seasonally varying parameters: air temperature, ice speed
and sub-surface ocean temperature (Fig. 3). Simple linear
regressions between frontal ablation rate and each of the three
possible determinants are all statistically significant at the 99%
confidence level but demonstrate that sub-surface ocean
temperature is by far the strongest predictor of frontal ablation
rate at both Kronebreen (R2¼0.84) and Tunabreen (R2¼0.80).
Multiple linear regression reveals that including air temperature
as a second predictor accounts for only an additional 1.4%
of the variance for Kronebreen, and is not significant for
Tunabreen. Of the three variables, only sub-surface ocean
temperature can adequately explain the seasonal variation in
calving behaviour.
Discussion
Calving events at Kronebreen, Tunabreen and Aavatsmarkbreen,
in common with valley-confined tidewater glaciers elsewhere in
Svalbard and beyond, typically involve relatively small sections of
ice (o50 m across) detaching from the terminus19,20. Calving
events at these glaciers are therefore unlike the massive block
rotation events seen in the largest Greenland glaciers, for
example, refs 21,22 or tabular events occurring at ice shelf
margins, for example, ref. 23. Nevertheless, these Svalbard glaciers
may be typical of medium-sized glaciers terminating in fjords that
experience a seasonal incursion of warm water. The submarine
melt and calving processes investigated here may therefore be
representative of many high-latitude glaciers.
The persistence of frontal ablation at all three glaciers well after
winter freeze-up of the glacier surface implies that melt water in
crevasses is not a primary control on calving, in contrast to some
model findings, for example, ref. 18, and this is further supported
by the lack of substantial correlation between local air
temperature and frontal ablation. Despite significant contrasts
in dynamical behaviour, our studied glaciers have very similar
seasonal frontal ablation cycles, implying that dynamic regime
also has little influence on the calving process. This is further
confirmed by the weak correlation between terminus speed and
frontal ablation rate. Surprisingly perhaps, even the initiation of a
surge at Aavatsmarkbreen did not seem to influence its frontal
ablation rate. These observations imply that frontal ablation rate
controls terminus position and is not sensitive to terminus speed
(Equation (1)). Whilst a basic consideration of mass balance
implies that mean annual calving rates would be expected to
correlate well with mean annual ice speed, the weakness of
association between ice speed and calving rate on seasonal
timescales seen in our data is significant. At Columbia Glacier the
seasonal relation between calving rate and ice speed is also much
weaker than the annual one8. On the other hand, where
Kronebreen terminus speeds exceed 2.5 m per day, there is
some evidence that frontal ablation rate also varies with glacier
speed (Fig. 3b).
The strong correlation between frontal ablation rate and ocean
temperature at Kronebreen and Tunabreen is indicative of
melt-driven convection at the ice–ocean interface, which is highly
sensitive to ambient ocean temperature24,25. The less strong
relationship for Tunabreen may be because our oceanographic
data have weaker spatial and temporal relevance. The water
temperatures around the depths of the calving fronts of these
glaciers are comparable to those elsewhere that give rise to
submarine melt on the order of metres per day1,5,7. With the
possible exception of the fastest phases of summer flow, this rate
of melt appears sufficient to seasonally outpace the delivery of ice
to the terminus of our glaciers. Our findings are consistent
with melt undercut and collapse being the dominant calving
mechanism at these glaciers.
The seasonal cycles of observed ocean temperature in
Kongsfjorden and Billefjorden are similar, but the proximity of
Kongsfjorden to the WSC and the deep channel that supports a
greater exchange of Atlantic-origin waters during summer and
autumn26 compared with Billefjorden27 explain the difference in
magnitude of sub-surface temperature between Kongsfjorden and
Billefjorden in late September (Fig. 2). The greater frontal
ablation rate at Kronebreen, compared with Tunabreen and
Aavatsmarkbreen, may therefore be due to the more direct and
open connection between this glacier terminus and the source of
warm water. In addition, Kronebreen has a much larger ablation
area than Tunabreen, and will therefore experience higher
summer melt-water discharges. These factors will encourage
more efficient subaqueous melting and may help to explain the
higher calving rates at Kronebreen25,28.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566
4NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications
&2015 Macmillan Publishers Limited. All rights reserved.
On shorter timescales, frontal ablation may be influenced by
other factors. For instance, spikes in frontal ablation rates at
Kronebreen occasionally coincide with terminus velocity peaks
(Fig. 2). It is possible that on these occasions delivery of ice to the
glacier terminus outstrips the melt undercut process allowing
other calving mechanisms to contribute, and causing the frontal
ablation rate to respond more strongly to ice dynamics (Fig. 3b).
Alternatively, surface melt-water pulses, as well as promoting
faster glacier flow by lubrication, may lead to higher summer
melt-water discharge, enhancing convective circulation and melt
at the ice front28.
Since the relationships between frontal ablation and sub-
surface ocean temperature are strong and linear, the data from
Kronebreen and Tunabreen (Fig. 3) are in keeping with a simple
linear calving law:
_
a¼kTð2Þ
Where _
ais the frontal ablation rate (m per day), Tis the
temperature at a depth appropriate for each glacier (here
20–60 m) and the coefficient kcaptures the nature of the heat
transfer in this setting (1.015 for Kronebreen; 0.35 for
Tunabreen). General calving laws of this type, perhaps incorpor-
ating other variables such as melt-water discharge, might be
developed using data from further glaciers.
We propose that submarine melt is an integral part of a
mechanism that promotes calving by undercut and collapse,
rather than a process which renders calving potentially unim-
portant2. Where terminus ice velocity is matched or outpaced by
submarine melt and undercut calving, a glacier will be stable or
will retreat at a rate dependent on the availability of warm water
at depth and the circulation of that water through estuarine and
glacial discharge processes. Where the ice velocity is greater than
the rate at which ice can be removed by melting, the front may be
able to advance to a position where other calving processes come
–20
–15
–10
–5
0
5
10
15
Frontal ablation rate (m per day)
Kronebreen
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1.5
2.0
2.5
3.0
3.5
Mean ice-front speed (m per day) 20-60m ocean temperature (°C)
Frontal ablation rate (m per day)
Kronebreen
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.0
1.0
5.0
6.0
7.0
Frontal ablation rate (m/day)
Kronebreen
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
–20
–15
–10
–5
0
5
10
15
Frontal ablation rate (m per day)
R2 = 0.31 R2 = 0.36 R2 = 0.80
R2 = 0.84R2 = 0.30R2 = 0.43
Tunabreen
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.2
0.6
0.8
Frontal ablation rate (m per day)
Tunabreen
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
–2.0
–1.0
0.0
1.0
2.0
Frontal ablation rate (m per day)
Tunabreen
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Mean air temperature (°C)
4.0
4.0
3.0
2.0
20–60 m ocean temperature (°C)
Mean ice-front speed (m per day)
Mean air temperature (°C)
3.0
1.0
0.4
Figure 3 | Glacier scatterplots. Correlation data and coefficients of determination for Kronebreen and Tunabreen between frontal ablation rate and:
(a,d) mean air temperature (at Ny Ålesund for Kronebreen and at Longyearbyen for Tunabreen); (b,e) mean ice-front speed at the glacier terminus;
and (c,f) nearby sub-surface ocean temperature (Kongsfjorden for Kronebreen and Billefjorden for Tunabreen). Note that for Kronebreen in (b) frontal
ablation rate varies more strongly with ice-front speed when the ice is flowing faster than B2.5m per day.
30−12−2012
01−02−2013
06−03−2013
08−04−2013
11−05−2013
13−06−2013
16−07−2013
18−08−2013
20−09−2013
23−10−2013
25−11−2013
28−12−2013
30−01−2014
04−03−2014
06−04−2014
09−05−2014
Date
04−06−2014
0.0 0.5 1.0 1.5 2.0 2.5 3.0
km
01
Speed (m per day)
Figure 4 | Kronebreen velocity map. Example TerraSAR-X speed map of
Kronebreen from January 2013 illustrating glacier surface texture exploited
by feature tracking, typical quality of speed data acquired right up to the ice
front, and variation in ice-front position over the observation period (green
to magenta lines). The ice-front measurement flow-lines along which ice
velocity and retreat rate are measured are shown in white.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566 ARTICLE
NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications 5
&2015 Macmillan Publishers Limited. All rights reserved.
into play29. This view is consistent with the theoretical framework
proposed by Benn et al.30, in which ‘‘first-order’’ calving processes
(determined by the large-scale velocity structure of the glacier)
provide the ultimate limit on glacier extent, while ‘‘second-order’’
processes (including melt undercutting) may cause more rapid
calving and glacier retreat. The present study, and work
elsewhere, for example, ref. 2, indicates that second-order
processes currently dominate in some regions. First-order
calving may be the rate-limiting process at only the fastest
outlet glaciers, for example, ref. 22 or in the coldest ocean
settings, for example, ref. 23, these representing only a part of the
global ice–ocean interface. Knowledge of submarine melt rates, or
much better availability of high-resolution satellite data such as
those used in this study, are therefore required to properly
understand tidewater calving mechanisms and rates in full.
We have investigated in unprecedented detail the seasonal
variation in calving rates at three dynamically contrasting glaciers
in Svalbard and their relationships to environmental controls. We
find that rates of frontal ablation are not dependent on glacier
dynamics, nor reduced by the onset of glacier surface freeze-up,
but closely follow the temperature of the fjord waters at depth.
Our findings imply that submarine melt undercut and collapse is
the dominant calving mechanism at these glaciers, and that
frontal ablation is therefore controlled primarily by ocean
temperatures. The calving process of melt undercut and collapse
may dominate frontal ablation at any glacier where it can match
or outpace the delivery of ice to the glacier terminus. This opens
up the possibility of being able to predict the response of many
tidewater glaciers to oceanographic forcing. We have identified a
growing need to assess sub-surface temperatures at tidewater
glacier margins and to evaluate the relative prevalence of melt
undercut versus other calving processes.
Methods
Satellite image data.Feature tracking was applied to TerraSAR-X image pairs in
slant range using correlation windows of 200 200 pixels spaced every 20 pixels,
for example, ref. 31, and subsequently ortho-rectified to a pixel size of 40 m using a
Digital Elevation Model. Ice fronts were manually digitized from dB-scaled images
ortho-rectified to a pixel size of 2 m using the same Digital Elevation Model.
Ice-front position change was derived by locating the intersections between each
ice front and a set of flow-lines equally spaced at 100 m intervals across the active
glacier terminus width (Fig. 4), and finding the distance between these intersections
for each image pair. Surface velocities were extracted at the locations of these
ice-front intersections, and the frontal ablation rate at each intersection was
calculated using equation (1). These ‘‘specific frontal ablation rates’’ were averaged
to produce a mean frontal ablation rate for the entire glacier width for each image
pair. This is the first report of frontal ablation calculated using surface velocity
measured at the evolving ice front.
Uncertainties in surface velocity U
T
are estimated to be o0.4 m per day, and
comprise co-registration error (±0.2 pixels) and error arising from unavoidable
smoothing of the velocity field over the feature-tracking window size
(400 400 m). Uncertainties in the rate of ice-front position change dl/dtare
estimated to be o2.0 m per day and are dominated by digitization error (±4 pixels
for each of 2 images over 11 days). The combined estimated specific frontal
ablation rate error of up to 2.4 m per day is too large to confidently assess the
variation in specific frontal ablation rate between individual measurements across
the terminus. The RMS frontal ablation rate error for B30 flow-line crossing
points of B0.4 m per day, however, is sufficiently small to allow mean frontal
ablation rates to be meaningfully compared between glaciers.
Weather and sea-surface data.We used daily mean air temperatures and
precipitation totals from the Norwegian Meteorological Institute (www.yr.no),
measured at the closest available weather station. For Kronebreen and
Aavatsmarkbreen this was Ny Ålesund and for Tunabreen, Longyearbyen.
To characterize the fjord surface conditions, we used daily Operational
Sea-Surface Temperature and Sea Ice Analysis (OSTIA) data supplied by the Met
Office (Crown Copyright 2014). OSTIA SSTs are produced at B5 km resolution
using an optimal interpolation method to incorporate in situ and satellite
observations, and sea ice fractional coverages are based on data from the
EUMETSAT Ocean & Sea Ice Satellite Application Facility32.
We also present data on the presence of fast ice (sea ice frozen to the shoreline)
from the Norwegian Ice Service (wms.met.no/icechart). Ice charts are derived by
manual classification of satellite images33.
Ocean sub-surface temperature data.Sub-surface fjord water temperatures
were measured using moored instrumentation as described by Cottier et al.26
Temperature sensors, with a precision of better than 0.1 °C after calibration, were
positioned from B20 m below the surface to within 15 m of the seabed. The
moorings typically include 10 sensors over the full 200 m depth with 4–5 sensors at
10 m intervals over the 20–60 m depth range used in this study. To characterize
sub-surface ocean temperatures at Kronebreen and Aavatsmarkbreen, we used
mooring data from Kongsfjorden (78°580N11°480E; Fig. 1) available for the
entire 2013–2014 study period with the exception of September 2013. For
Tunabreen in Tempelfjorden, we used the closest available data from neighbouring
Billefjorden (78°390N16°410E; Fig. 1). These data may be considered a reliable
indicator of near-surface oceanographic conditions at Tunabreen because the two
fjords are similar in shape and connect adjacently to the same Isfjorden system.
The Billefjorden data were available only until September 2013 so our best
oceanographic comparison is to the mean value over the period 2008 and 2013.
The mean temperature in the 20–60 m depth range for the two moorings was
calculated at weekly intervals to provide an appropriate comparison with the
calving time series derived every 11 days or better. The choice of a 60 m maximum
for Kongsfjorden was dictated by the depth at which water in the inner fjord is
connected to the outer fjord34, and for Billefjorden by the presence of a sill at
B50 m (ref. 22). In add ition, both Kronebreen and Tunabreen are grounded in
B70 m of water.
References
1. Motyka, R. J., Hunter, L., Echelmeyer, K. A. & Connor, C. Submarine melting
at the terminus of a temperate tidewater glacier, LeConte glacier, Alaska, USA.
Ann. Glaciol. 36, 57–65 (2003).
2. Bartholomaus, T. C., Larsen, C. F. & O’Neel, S. Does calving matter? Evidence
for significant submarine melt. Earth Planet. Sci. Lett. 380, 21–30 (2013).
3. O’Leary, M. & Christoffersen, P. Calving on tidewater glaciers amplified by
submarine frontal melting. Cryosphere 7, 119–128 (2013).
4. Straneo, F. et al. Rapid circulation of warm subtropical waters in a major glacial
fjord in East Greenland. Nat. Geosci. 3, 182–186 (2010).
5. Inall, M. E. et al. Oceanic heat delivery via Kangerdlugssuaq fjord to the
south-east Greenland ice sheet. J. Geophys. Res. Oceans 119, 631–645 (2014).
6. Motyka, R. J. et al. Submarine melting of the 1985 Jakobshavn Isbræ floating
tongue and the triggering of the current retreat. J. Geophys. Res. 116, F01007
(2011).
7. Rignot, E., Koppes, M. & Velicogna, I. Rapid submarine melting of the calving
faces of West Greenland glaciers. Nat. Geosci. 3, 187–191 (2010).
8. van der Veen, C. J. Calving glaciers. Prog. Phys. Geogr. 26, 96–122 (2002).
9. McNabb, R. W., Hock, R. & Huss, M. Variations in Alaska tidewater glacier
frontal ablation, 1985–2013. J. Geophys. Res. Atmos. 120 doi: 10.1002/
2014JF003276 (2015).
10. Bassis, J. N. The statistical physics of iceberg calving and the emergence of
universal calving laws. J. Glaciol. 57, 3–16 (2011).
11. Schellenberger, T., Dunse, T., Ka
¨a
¨b, A., Kohler, J. & Reijmer, C. H. Surface
speed and frontal ablation of Kronebreen and Kongsbreen, NW-Svalbard, from
SAR offset tracking. Cryosphere Discuss. 8, 6193–6233 (2014).
12. Moon, T. et al. Distinct patterns of seasonal Greenland glacier velocity.
Geophys. Res. Lett. 41, 7209–7216 (2014).
13. Ko
¨hler, A., Chapuis, A., Nuth, C., Kohler, J. & Weidle, C. Autonomous
detection of calving-related seismicity at Kronebreen, Svalbard. Cryosphere 6,
393–406 (2012).
14. Flink, A. E. et al. The evolution of a submarine landform record following
recent and multiple surges of Tunabreen glacier, Svalbard. Quat. Sci. Rev. 108,
37–50 (2015).
15. Ritchie, J. B., Lingle, C. S., Motyka, R. J. & Truffer, M. Seasonal fluctuations in
the advance of a tidewater glacier and potential causes: Hubbard glacier, Alaska,
USA. J. Glaciol. 54, 401–411 (2008).
16. Bevan, S. L., Luckman, A. J. & Murray, T. Glacier dynamics over the last
quarter of a century at Helheim, Kangerdlugssuaq and 14 other major
Greenland outlet glaciers. Cryosphere 6, 923–937 (2012).
17. Brown, C. S., Meier, M. F. & Post, A. Calving speed of Alaska tidewater glaciers,
with applications to Columbia Glacier. US Geol. Surv. Prof. Pap. 1258-C,
C1–C13 (1982).
18. Cook, S. et al. Modelling environmental influences on calving at Helheim
Glacier in eastern Greenland. Cryosphere 8, 827–841 (2014).
19. Dowdeswell, J. A. On the nature of Svalbard icebergs. J. Glaciol. 35, 224–234 (1989).
20. Åstro
¨m, J. A. et al. Termini of calving glaciers as self-organized critical systems.
Nat. Geosci. 7, 874–878 (2014).
21. Amundson, J. M. et al. Ice me
´lange dynamics and implications for terminus
stability, Jakobshavn Isbræ, Greenland. J. Geophys. Res. 115, F01005 (2010).
22. James, T. D., Murray, T., Selmes, N., Scharrer, K. & O’Leary, M. Buoyant
flexure and basal crevassing in dynamic mass loss at Helheim glacier. Nat.
Geosci. 7, 593–596 (2014).
23. Alley, R. B. et al. A simple law for Ice-Shelf calving. Science 322, 1344 (2008).
24. Jenkins, A. A one-dimensional model of ice shelf-ocean interaction. J. Geophys.
Res. 96, 20671–20677 (1991).
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566
6NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications
&2015 Macmillan Publishers Limited. All rights reserved.
25. Jenkins, A. Convection-driven melting near the grounding lines of ice shelves
and tidewater glaciers. J. Phys. Oceanogr. 41, 2279–2294 (2011).
26. Cottier, F. et al. Water mass modification in an arctic fjord through cross-shelf
exchange: the seasonal hydrography of Kongsfjorden, Svalbard. J. Geophys. Res.
110, C12005 (2005).
27. Nilsen, F., Cottier, F., Skogseth, R. & Mattsson, S. Fjord–shelf exchanges
controlled by ice and brine production: the inter-annual variation of Atlantic
water in Isfjorden, Svalbard. Cont. Shelf Res. 28, 1838–1853 (2008).
28. Motyka, R. J., Dryer, W. P., Amundson, J., Truffer, M. & Fahnestock, M. Rapid
submarine melting driven by subglacial discharge, LeConte glacier, Alaska.
Geophys. Res. Lett. 40, 5153–5158 (2013).
29. Todd, J. & Christoffersen, P. Are seasonal calving dynamics forced by
buttressing from ice me
´lange or undercutting by melting? Outcomes from
full-stokes simulations of Store Glacier, West Greenland. Cryosphere 8,
2353–2365 (2014).
30. Benn, D. I., Warren, C. R. & Mottram, R. H. Calving processes and the
dynamics of calving glaciers. Earth-Sci. Rev. 82, 143–179 (2007).
31. Luckman, A., Murray, T., de Lange, R. & Hanna, E. Rapid and synchronous
ice-dynamic changes in East Greenland. Geophys. Res. Lett. 33, L03503 (2006).
32. Donlon, C. J. et al. The operational sea surface temperature and sea ice analysis
(OSTIA) system. Remote Sens. Environ. 116, 140–158 (2012).
33. Norwegian Meteorological Institute Ice Service, http://met.no/Hav_og_is/
English/Activities_and_tasks/Sea_ice/Ice_Service(2015).
34. MacLachlan, S. E., Cottier, F. R., Austin, W. E. N. & Howe, J. A. The salinity:
d18O water relationship in Kongsfjorden, western Spitsbergen. Polar Res. 26,
160–167 (2007).
35. Jakobsson, M. et al. The international bathymetric chart of the Arctic ocean
(IBCAO) version 3.0. Geophys. Res. Lett. 39, L12609 (2012).
Acknowledgements
TerraSAR-X data were provided by DLR (project OCE1503), and funded by the Conoco
Phillips-Lundin Northern Area Program through the CRIOS project (Calving Rates and
Impact on Sea level). A.L. and S.B. are affiliated to the Climate Change Consortium of
Wales (C3W). Mooring work is supported by the UK Natural Environment Research
Council (Oceans 2025 and Northern Sea Program) and the Research Council of Norway
(projects Cleopatra: 178766, Cleopatra II: 216537, and Circa: 214271/F20). Contribution
by F.C. was undertaken through the Scottish Alliance for Geoscience Environment and
Society (SAGES).
Author contributions
A.L. designed the study, analysed the satellite data and led the writing of the manuscript.
D.I.B. led the project that funded the satellite data acquisition and contributed
significantly to the writing of the manuscript. S.B. analysed the sea-surface data and
performed the statistical analyses. F.C. and M.I. provided ocean temperature data and
F.N. provided information on fjord bathymetry. All authors contributed to the writing of
the manuscript.
Additional information
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Luckman, A. et al. Calving rates at tidewater glaciers vary
strongly with ocean temperature. Nat. Commun. 6:8566 doi: 10.1038/ncomms9566
(2015).
This work is licensed under a Creative Commons Attribution 4.0
International License. The images or other third party material in this
article are included in the article’s Creative Commons license, unless indicated otherwise
in the credit line; if the material is not included under the Creative Commons license,
users will need to obtain permission from the license holder to reproduce the material.
To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9566 ARTICLE
NATURE COMMUNICATIONS | 6:8566 | DOI: 10.1038/ncomms9566 | www.nature.com/naturecommunications 7
&2015 Macmillan Publishers Limited. All rights reserved.
... Tunabreen is a 26 km long tidewater glacier in central Spitsbergen (Fig. 1). It is the most frequently surging glacier in Svalbard, with four documented surges during the past 100 years (Flink et al., 2015;Luckman et al., 2015). The surges occurred during 1924-1930 (advance 3 km), 1966-1971 (advance 2.1 km), 2002-2004 (advance 2 km), and 2016-2018 (advance 1.1 km). ...
... Apart from the length fluctuations related to the surges, over the past 100 years Tunabreen has become shorter by about 1.5 km. This is likely due to an increasing equilibrium line altitude (ELA) caused by rising air temperature (Førland et al., 2011), perhaps in combination with larger calving rates associated with higher ocean temperature (Luckman et al., 2015). ...
... Krug et al., 2014) but cannot simply be adopted in a more general calving law for use in large-scale models of tidewater glaciers. Recent field and remote sensing studies show that an important control on calving at many Svalbard tidewater glaciers is undercutting of the submerged ice front by melting (Petlicki et al., 2015;Luckman et al., 2015;How et al., 2019). This implies that the calving parameter depends on water temperature. ...
Article
Full-text available
The 26 km long tidewater glacier Tunabreen is the most frequently surging glacier in Svalbard, with four documented surges in the past 100 years. We model the evolution of this glacier with a minimal glacier model (MGM), in which ice mechanics, calving, and surging are parameterized. The model geometry consists of a flow band to which three tributaries supply mass. The calving rate is set to the mean observed value for the period 2012–2019 and kept constant. For the past 120 years, a smooth equilibrium line altitude (ELA) history is reconstructed by finding the best possible match between observed and simulated glacier length. There is a modest correlation between this reconstructed ELA history and an ELA history based on meteorological observations from Longyearbyen. Runs with and without surging show that the effect of surging on the long-term glacier evolution is limited. Due to the low surface slope and associated strong height–mass-balance feedback, Tunabreen is very sensitive to changes in the ELA. For a constant future ELA equal to the reconstructed value for 2020, the glacier front will retreat by 8 km during the coming 100 years. For an increase in the ELA of 2 m a−1, the retreat is projected to be 13 km, and Tunabreen becomes a land-terminating glacier around 2100. The calving parameter is an important quantity: increasing its value by 50 % has about the same effect as a 35 m increase in the ELA, with the corresponding equilibrium glacier length being 17.5 km (as compared to 25.8 km in the reference state). Response times vary from 150 to 400 years, depending on the forcing and on the state of the glacier (tidewater or land-terminating).
... For land-terminating glaciers, the major mechanism for melting is through surface meltwaters migrating to the ice-bedrock interface (e.g. Zwally et al., 2002;Tedstone et al., 2015), whereas ocean-terminating glaciers mainly lose mass through calving at the glacial front by warming seawater temperatures (e.g., Luckman et al., 2015). Whether through rising sea temperatures or changes in freshwater and sediment inputs induced by glacier melting, these climate-change induced phenomena are having, and will continue to have, several implications for Arctic marine ecosystems, with repercussions on the benthic and pelagic realms (Meredith et al., 2019). ...
... In summer, AW reached the inner part of Kongsfjorden only in the last decade (Holmes et al., 2019). This recent hydrological change strongly enhanced submarine melt and calving (i.e., frontal ablation) of tidewater glaciers (Luckman et al., 2015;Holmes et al., 2019), thus increasing meltwater discharges and associated sediment load at the fjord head. As a result, the higher water turbidity in the vicinity of the marine termini of Kronebreen and Kongsvegen glaciers reduces local euphotic depth, disrupting phytoplankton distribution and biomass at the head of the fjord (Payne and Roesler, 2019). ...
... During summer, those tidewater glaciers highly contribute to the fjord dynamics with important freshwater and sediment inputs. Kronebreen, a fast-flowing tidewater glacier, has one of the highest flux rates in Svalbard (winter speed of 1.5-2 m d − 1 , with 3-4 m d − 1 peaks in summer; Luckman et al., 2015). ...
Article
Kongsfjorden (Svalbard archipelago) is subjected to strong environmental gradients creating high physical and geochemical stress on benthic faunas. The present study aims at understanding the environmental drivers governing benthic foraminifera in the innermost part of the fjord. Surface sediments from 9 stations were sampled during August 2018 along a transect starting at ca. 2 km from the tidewater glacier Kronebreen and ending 12 km seaward. Three biozones were identified in response to disturbances linked to the proximity of the Kronebreen front (i.e., high water turbidity, freshwater, and sediment inputs, reduced organic fluxes). Close to the terminus (proximal biozone), few stress-tolerant and glacier proximal species were present (i.e., Capsammina bowmanni and Cassidulina reniforme). At about 6–8 km from the front (medial biozone), reduced turbidity, and increased organic fluxes, resulted in a higher diversity, and a high abundance of the phytodetritus-indicator Nonionellina labradorica. Relatively high diversity persisted until 12 km from the front due to higher organic inputs and reduced stressful conditions. The distal biozone was dominated by the Atlantic Water (AW) indicator Adercotryma glomeratum, in coherence with the presence of warm and salty AW detected far inside the fjord. Physical stress related to the glacier dynamics appears to favour the establishment of opportunistic species close to the terminus, whereas reduced disturbance away from the glacier induces the establishment of diverse assemblages. Our results show that benthic foraminifera may be effective bioindicators to monitor the long-term retreat of tidewater glaciers induced by climate change in Kongsfjorden.
... Methods that take into account the CFL require a time series of satellite imagery that is of appropriate temporal and spatial resolution. Very dense time series of satellite imagery have successfully captured the variation in calving-front locations for frequently-calving tidewater glaciers (e.g., Luckman et al., 2015;Mottram et al., 2019). However, Antarctic ice shelves tend to produce large tabular icebergs after extended periods of calving quiescence, which can last from months to decades Giles, 2017). ...
... This sensitivity suggests that the temporal resolution of the calving-front time series has to be high enough to avoid temporal aliasing, which occurs when the satellite observation frequency is insufficient to capture the cyclic nature of the calving-front position (Liu et al., 2015). Further to having frequent CFL observations within the time series, the imagery itself needs to be of a sufficient spatial resolution to capture any calving-front geometry changes (Luckman et al., 2015;Qi et al., 2020) or to clearly show any surface features (if they are present) for feature-tracking approaches (Liu et al., 2015). ...
Article
Full-text available
In order to determine whether the calving flux of an ice shelf is changing, the long-term calving flux needs to be established. Methods used to estimate the calving flux either take into account non-steady-state behaviour by capturing movement of the calving-front location (e.g., using satellite observations), or they assume the calving front is stationary and that the ice is in steady state (e.g., flux-gate methods). Non-steady-state methods are hampered by the issue of temporal aliasing, i.e., when the satellite observation frequency is insufficient to capture the cyclic nature of the calving-front position. Methods that assume a steady state to estimate the calving flux accrue uncertainties if the ice shelf changes its physical state. In order to overcome these limitations we propose and implement a new observation-based approach that combines a time series of calving-front locations with a flux-gate method. The approach involves the creation of a unique semi-temporal domain as a mechanism to overcome the issue of temporal aliasing, and only requires easily accessible ice thickness and surface velocity estimates of the ice shelf. This approach allows for complex calving-front geometries and captures calving events of all sizes that are visible within the satellite imagery. Application of the approach allows the long-term average calving flux to be estimated (provided sufficient temporal coverage by satellite imagery), as well as identification of the minimum temporal baseline needed to produce a representative estimate of the long-term average calving flux, for any ice shelf. Implementation of the approach to multiple ice shelves would enable comparisons to be made regarding the spatial variability in the long-term calving flux of Antarctica's ice shelves, thereby highlighting calving regime change around the continent.
... Calving from tidewater glaciers is a consequence of stress induced by different mechanisms, including (1) longitudinal stretching, (2) melt undercutting at or below the sea surface, (3) changes in terminus height and position (bay and glacier geometry) and (4) buoyancy forces (van der Veen, 2002;Benn and others, 2007). Submarine melting is a major trigger of calving in a warming climate, as demonstrated by model simulations (O'Leary and Christoffersen, 2013;Benn and others, 2017) and observational data (Bartholomaus and others, 2013;Luckman and others, 2015). Different driving mechanisms lead to different calving modes (see, e.g. ...
Article
Full-text available
Iceberg calving is one of the major mechanisms of ice loss from tidewater glaciers and ice sheets, but obtaining accurate estimates of ice discharge that are both continuous and accurate is a challenging task. Recent results have demonstrated the effective application of passive cryoacoustics – the use of naturally generated sounds to study the cryosphere – to quantify subaerial calving fluxes. However, little is known about the acoustic signatures of submarine calving. This study investigates the underwater noise from 656 subaerial and 162 submarine calving events observed at Hansbreen, Svalbard in the summers of 2016 and 2017. Statistical analysis of the acoustic signal shows that the normalized power of the calving noise is log-normally distributed regardless of the calving mode. However, submarine events can be distinguished from subaerial events by using the shape parameter of the log-normal distribution paired with the calving signal duration. The newly developed classification model may potentially be used for two purposes: (1) to study potential causal relationships between these two calving modes and (2) to separate calving fluxes into subaerial and submarine components. The latter will also require knowledge of the relationship between ice mass and sound spectral level for submarine calving events.
... Our target in this study is Kronebreen, a grounded, fast-flowing tidewater glacier in the Northwest of Svalbard about 15 km East of Ny-Ålesund (Fig. 1), which is a research station hosting the three-component seismic broadband station KBS (STS-2 seismometer). Mass loss at Kronebreen is dominated by frontal ablation, i.e., dynamic ice loss through calving and frontal melting (Nuth et al. 2012;Luckman et al. 2015). In recent years the glacier has experienced a rapid retreat (Schellenberger et al. 2015;Köhler et al. 2019a;Vallot et al. 2018;Deschamps-Berger et al. 2019). ...
Article
Full-text available
Seismic signals generated by iceberg calving can be used to monitor ice loss at tidewater glaciers with high temporal resolution and independent of visibility. We combine the Empirical Matched Field (EMF) method and machine learning using Convolutional Neural Networks (CNNs) for calving event detection at the SPITS seismic array and the single broadband station KBS on the Arctic Archipelago of Svalbard. EMF detection with seismic arrays seeks to identify all signals generated by events in a confined target region similar to single P and/or S phase templates by assessing the beam power obtained using empirical phase delays between the array stations. The false detection rate depends on threshold settings and therefore needs appropriate tuning or, alternatively, post-processing. We combine the EMF detector at the SPITS array, as well as an STA/LTA detector at the KBS station, with a post-detection classification step using CNNs. The CNN classifier uses waveforms of the three-component record at KBS as input. We apply the methodology to detect and classify calving events at tidewater glaciers close to the KBS station in the Kongsfjord region in Northwestern Svalbard. In a previous study, a simpler method was implemented to find these calving events in KBS data, and we use it as the baseline in our attempt to improve the detection and classification performance. The CNN classifier is trained using classes of confirmed calving signals from four different glaciers in the Kongsfjord region, seismic noise examples, and regional tectonic seismic events. Subsequently, we process continuous data of 6 months in 2016. We test different CNN architectures and data augmentations to deal with the limited training data set available. Targeting Kronebreen, one of the most active glaciers in the Kongsfjord region, we show that the best performing models significantly improve the baseline classifier. This result is achieved for both the STA/LTA detection at KBS followed by CNN classification, as well as EMF detection at SPITS combined with a CNN classifier at KBS, despite of SPITS being located at 100 km distance from the target glacier in contrast to KBS at 15 km distance. Our results will further increase confidence in estimates of ice loss at Kronebreen derived from seismic observations which in turn can help to better understand the impact of climate change in Svalbard.
... Some studies cover the entire ice sheet for over a decade but map termini only decadally or in non-consecutive years (Howat and Eddy, 2011;Moon and Joughin, 2008). Other studies map termini more frequently but only for a small sector of the ice sheet (Bjørk et al., 2012;McFadden et al., 2011;Moon et al., 2015) or for a few specific glaciers (Holland et al., 2016;Joughin et al., 2008b;Larsen et al., 2016;Motyka et al., 2017;Schild and Hamilton, 2013). Murray et al. (2015) mapped terminus positions at high spatial and temporal resolutions but only for a single decade. ...
Article
Full-text available
The retreat and acceleration of marine-terminating outlet glaciers in Greenland over the past 2 decades have been widely attributed to climate change. Here we present a comprehensive annual record of glacier terminus positions in northwest and central-west Greenland and compare it against local and regional climatology to assess the regional sensitivity of glacier termini to different climatic factors. This record is derived from optical and radar satellite imagery and spans 87 marine-terminating outlet glaciers from 1972 through 2021. We find that in this region, most glaciers have retreated over the observation period and widespread regional retreat accelerated from around 1996. The acceleration of glacier retreat coincides with the timing of sharp shifts in ocean surface temperatures, the duration of the sea-ice season, ice-sheet surface mass balance, and meltwater and runoff production. Regression analysis indicates that terminus retreat is most sensitive to increases in runoff and ocean temperatures, while the effect of offshore sea ice is weak. Because runoff and ocean temperatures can influence terminus positions through several mechanisms, our findings suggest that a variety of processes – such as ocean-interface melting, mélange presence and rigidity, and hydrofracture-induced calving – may contribute to, but do not conclusively dominate, the observed regional retreat.
Article
Ice shelves play a key role in the dynamics of marine ice sheets by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice-sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last 5 years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and a release of tabular bergs on both the eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear, upstream of the main pinning point, developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedback between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished over time as the pinning point shrunk, accumulation of damage has ensured that the ice in the shear zone remained the weakest link in the system. Experiments with the BISICLES ice-sheet model indicate that additional damage to or unpinning of TEIS is unlikely to trigger significantly increased ice loss from WAIS, but the calving response to the loss of TEIS remains highly uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice-shelf margins or grounding points. Our results indicate a third mechanism, backstress triggered failure, that can occur if and when an ice shelf is no longer able to withstand stress imposed by pinning points. In most circumstances, pinning points are essential for ice-shelf stability, but as ice shelves thin and weaken, the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.
Article
Full-text available
With increase in the Arctic warming, fjords in the region are undergoing significant decline in sea-ice, increase in glacier retreat, and changes in primary productivity. Warm and saline Atlantic Water (AW) advected from the open ocean is attributed as one of the primary reasons for these changes. Vertical mixing of the subsurface AW with the cold surface Arctic water positively feeds many of these changes. One of the major contributors of vertical mixing in the water column is the near-inertial waves (NIW). In this study, we report prominent NIW activity in Kongsfjorden, an Arctic fjord in west Svalbard, using hydrography observations and numerical simulations. Shallow summer mixed layer depth facilitates generation of strong near-inertial currents in the mixed layer (ML), even with relatively weak storms. During storm events in summer, the near-inertial currents induce large shear at the base of the ML and downward propagating NIW induce shear in the fjord interior leading to an enhanced vertical mixing. These two processes during summertime storms lead to redistribution of the subsurface AW thereby warming the ML. Numerical simulations from the Regional Ocean Modelling System (ROMS) are used to understand the generation, dissipation and energetics of the NIW and their impact on vertical mixing in the fjord. Near-inertial energy budget estimations show that ∼45% of near-inertial energy input in the fjord and continental shelf-slope region dissipates within the upper 150 m of the water column. Numerical experiments are performed to confirm the role of storms in inducing vertical mixing of the AW. When storms are removed from the forcing field, eddy diffusivity reduces and fails to reproduce the upper ocean warming that was present in the simulation with storms. Even though stability in the fjord upper layer is enhanced with an increased glacier discharge, strong vertical mixing is still profound during storms. Consistent with this, in the experiment with double the glacier discharge, near-inertial energy flux increases by nearly 10% whereas the viscous dissipation of the NIW increases by about 13%. The findings of the study underscore the potential significance of the NIW dynamics and their impacts in the Arctic fjords. This is further relevant in understanding the vertical mixing in shallow regions such as shelves, slopes and fjords in the future Arctic with more storms and reduced sea-ice cover as projected by climate models.
Article
Full-text available
Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near‐glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill‐driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non‐glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016–2017 in LeConte Bay, Alaska, with a three‐dimensional numerical model of the fjord to quantify sill‐driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume‐driven buoyancy forcing, sill‐generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%–72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re‐entrained into the subglacial discharge plume at the fjord head. As a result, near‐terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.
Article
Iceberg calving, the process where icebergs detach from glaciers, remains poorly understood. Moreover, few parameterizations of the calving process can easily be integrated into numerical models to accurately capture observations, resulting in large uncertainties in projected sea level rise. Recent efforts have focused on estimating crevasse depths assuming tensile failure occurs when crevasses fully penetrate the glacier thickness. However, these approaches often ignore the role of advecting crevasses on calculations of crevasse depth. Here, we examine a more general crevasse depth calving model that includes crevasse advection. We apply this model to idealized prograde and retrograde bed geometries as well as a prograde geometry with a sill. Neglecting crevasse advection results in steady glacier advance and ice tongue formation for all ice temperatures, sliding law coefficients and constant slope bed geometries considered. In contrast, crevasse advection suppresses ice tongue formation and increases calving rates, leading to glacier retreat. Furthermore, crevasse advection allows a grounded calving front to stabilize on top of sills. These results suggest that crevasse advection can radically alter calving rates and hint that future parameterizations of fracture and failure need to more carefully consider the lifecycle of crevasses and the role this plays in the calving process.
Article
Full-text available
While it has been shown repeatedly that ocean conditions exhibit an important control on the behaviour of grounded tidewater glaciers, modelling studies have focused largely on the effects of basal and surface melting. Here, a finite-element model of stresses near the front of a tidewater glacier is used to investigate the effects of frontal melting on calving, independently of the calving criterion used. Applications of the stress model to idealized scenarios reveal that undercutting of the ice front due to frontal melting can drive calving at up to ten times the mean melt rate. Factors which cause increased frontal melt-driven calving include a strong thermal gradient in the ice, and a concentration of frontal melt at the base of the glacier. These properties are typical of both Arctic and Antarctic tidewater glaciers. The finding that frontal melt near the base is a strong driver of calving leads to the conclusion that water temperatures near the bed of the glacier are critically important to the glacier front, and thus the flow of the glacier. These conclusions are robust against changes in the basal boundary condition and the choice of calving criterion, as well as variations in the glacier size or level of crevassing.
Article
Full-text available
Calving is an important mass-loss process for many glaciers worldwide, and has been assumed to respond to a variety of environmental influences. We present a grounded, flowline tidewater glacier model using a physically-based calving mechanism, applied to Helheim Glacier, eastern Greenland. By qualitatively examining both modelled size and frequency of calving events, and the subsequent dynamic response, the model is found to realistically reproduce key aspects of observed calving behaviour. Experiments explore four environmental variables which have been suggested to affect calving rates: water depth in crevasses, basal water pressure, undercutting of the calving face by submarine melt and backstress from ice mélange. Of the four variables, only crevasse water depth and basal water pressure were found to have a significant effect on terminus behaviour when applied at a realistic magnitude. These results are in contrast to previous modelling studies, which have suggested that ocean temperatures could strongly influence the calving front. The results raise the possibility that Greenland outlet glaciers could respond to the recent trend of increased surface melt observed in Greenland more strongly than previously thought, as surface ablation can strongly affect water depth in crevasses and water pressure at the glacier bed.
Article
Full-text available
We use a full-Stokes 2-D model (Elmer/Ice) to investigate the flow and calving dynamics of Store Glacier, a fast-flowing outlet glacier in West Greenland. Based on a new, subgrid-scale implementation of the crevasse depth calving criterion, we perform two sets of simulations: one to identify the primary forcing mechanisms and another to constrain future stability. We find that the mixture of icebergs and sea ice, known as ice mélange or sikussak, is principally responsible for the observed seasonal advance of the ice front. On the other hand, the effect of submarine melting on the calving rate of Store Glacier appears to be limited. Sensitivity analysis demonstrates that the glacier's calving dynamics are sensitive to seasonal perturbation, but are stable on interannual timescales due to the strong topographic control on the flow regime. Our results shed light on the dynamics of calving glaciers and may help explain why neighbouring glaciers do not necessarily respond synchronously to changes in atmospheric and oceanic forcing.
Article
Full-text available
Kronebreen and Kongsbreen are among the fastest flowing glaciers on Svalbard, and therefore important contributors to glacier mass loss from the archipelago through frontal ablation. Here, we present a time series of area-wide surface velocity fields from April 2012 to December 2013 based on offset tracking on repeat high-resolution Radarsat-2 Ultrafine data. Surface speeds reached up to 3.2 m d-1 near the calving front of Kronebreen in summer 2013 and 2.7 m d-1 at Kongsbreen in late autumn 2012. Additional velocity fields from Radarsat-1, Radarsat-2 and TerraSAR-X data since December 2007 together with continuous GPS measurements on Kronebreen since September 2008 revealed complex patterns in seasonal and interannual speed evolution. Part of the ice-flow variations seem closely linked to the amount and timing of surface melt water production and rainfall, both of which are known to have a strong influence on the basal water pressure and lubrication. In addition, terminus retreat and the associated reduction in backstress appear to have influenced the speed close to the calving front, especially at Kongsbreen in 2012 and 2013. Since 2007, Kongsbreen retreated up to 1800 m, corresponding to a total area loss of 2.5 km2. In 2011 the retreat of Kronebreen of up to 850 m, responsible for a total area loss of 2.8 km2, was triggered after a phase of stable terminus position since ~1990. The retreat is an important component of the mass balance of both glaciers, in which frontal ablation is the largest component. Total frontal ablation between April 2012 and December 2013 was estimated to 0.21-0.25 Gt a-1 for Kronebreen and 0.14-0.16 Gt a-1 for Kongsbreen.
Article
Full-text available
Over the next century, one of the largest contributions to sea level rise will come from ice sheets and glaciers calving ice into the ocean. Factors controlling the rapid and nonlinear variations in calving fluxes are poorly understood, and therefore difficult to include in prognostic climate-forced land-ice models. Here we analyse globally distributed calving data sets from Svalbard, Alaska (USA), Greenland and Antarctica in combination with simulations from a first-principles, particle-based numerical calving model to investigate the size and inter-event time of calving events. We find that calving events triggered by the brittle fracture of glacier ice are governed by the same power-law distributions as avalanches in the canonical Abelian sandpile model. This similarity suggests that calving termini behave as self-organized critical systems that readily flip between states of sub-critical advance and super-critical retreat in response to changes in climate and geometric conditions. Observations of sudden ice-shelf collapse and tidewater glacier retreat in response to gradual warming of their environment are consistent with a system fluctuating around its critical point in response to changing external forcing. We propose that self-organized criticality provides a yet unexplored framework for investigations into calving and projections of sea level rise.
Article
Full-text available
Predicting Greenland Ice Sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms. We present a 5 year record of seasonal velocity measurements for 55 marine-terminating glaciers distributed around the ice sheet margin, along with ice-front position and runoff data sets for each glacier. Among glaciers with substantial speed variations, we find three distinct seasonal velocity patterns. One pattern indicates relatively high glacier sensitivity to ice-front position. The other two patterns are more prevalent and appear to be meltwater controlled. These patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. The difference may be determined by meltwater availability, which in some regions may be influenced by perennial firn aquifers. Our results highlight the need to understand subglacial meltwater availability on an ice sheet-wide scale to predict future dynamic changes. First multi-region seasonal velocity measurements show regional differencesSeasonal velocity fluctuations on most glaciers appear meltwater controlledSeasonal development of efficient subglacial drainage geographically divided.
Article
This study focuses on the glacial landform record associated with recent surge events of Tunabreen - a calving tidewater glacier in Tempelfjorden, Spitsbergen. Submarine geomorphology and recent terminal fluctuations of Tunabreen's glacier front were studied using high-resolution multibeam-bathymetric data and a range of published and remote-sensing sources, including topographic maps, satellite images and aerial photographs. The retreat moraines in the inner part of Tempelfjorden have been correlated with glacier terminus positions during retreat from the 2004 surge maximum. Glacier surface velocity and ice-front positions derived from high-resolution TerraSAR-X satellite data show ice movements at the glacier front during minor advances of the front in winter when calving is suppressed. This suggests that the moraines have formed annually during quiescent phase winter advances. Tunabreen has experienced three surges since the Little Ice Age (LIA). This is in contrast with most Svalbard surging glaciers which have long quiescent phases and have typically only undergone one or two surges during this time. The landform record in Tempelfjorden is distinguished from previously studied glacier-surge landsystems by four, well-preserved sets of landform assemblages generated by the LIA advance and three subsequent surges, all of which partly modify earlier landform records. Based on the unique landform record in Tempelfjorden, a new conceptual landsystem model for frequently surging glaciers has been put forward improving our understanding of the dynamics of the surging glaciers and, most importantly, how they can be distinguished from the climatically-controlled glaciers in the geological record.
Article
Our incomplete knowledge of the proportion of mass loss due to frontal ablation (the sum of ice loss through calving and submarine melt) from tidewater glaciers outside of the Greenland and Antarctic ice sheets has been cited as a major hindrance to accurate predictions of global sea level rise. We present a 28 year record (1985–2013) of frontal ablation for 27 Alaska tidewater glaciers (representing 96% of the total tidewater glacier area in the region), calculated from satellite-derived ice velocities and modeled estimates of glacier ice thickness. We account for cross-sectional ice thickness variation, long-term thickness changes, mass lost between an upstream flux gate and the terminus, and mass change due to changes in terminus position. The total mean rate of frontal ablation for these 27 glaciers over the period 1985–2013 is 15.11 ± 3.63 Gt a−1. Two glaciers, Hubbard and Columbia, account for approximately 50% of these losses. The regional total ablation has decreased at a rate of 0.14 Gt a−1 over this time period, likely due to the slowing and thinning of many of the glaciers in the study area. Frontal ablation constitutes only ~ 4% of the total annual regional ablation, but roughly 20% of net mass loss. Comparing several commonly-used approximations in the calculation of frontal ablation we find that neglecting cross-sectional thickness variations severely underestimates frontal ablation.
Article
Iceberg calving accounts for a significant proportion of annual mass loss from marine-terminating glaciers(1,2) and may have been a factor in the rapid demise of ancient ice sheets(3). The largest contributions from the main outlet glaciers of the Greenland ice sheet to sea-level rise over the next two centuries have been projected to be dynamic in origin, that is, driven by glacier flow and calving(4). However, present physical models remain a coarse approximation of real calving mechanisms because models are poorly constrained by sparse glacier geometry observations(5). Here we present a record of daily digital elevation models from the calving margin of Greenland's Helheim Glacier at a high spatial resolution. Our digital elevation models are derived from stereo terrestrial photography taken over the summers of 2010 and 2011. We find that during these two summers dynamic mass loss at Helheim Glacier was dominated by calving events exceeding 1 km(3) that were the result of buoyant flexure and the propagation of basal crevasses. We suggest that this buoyancy-driven mechanism for calving may be common elsewhere in Greenland and could be a first-order control on the ice sheet's future contribution to sea-level rise.