Australia: The Land Where Time Began |
||||||||||||||
Widespread increase in dynamic imbalance in the Getz region of
Antarctica from 1994 to 2018
Ice is being lost from the Getz region of West Antarctica at an
increasingly rapid rate, however, the mechanism of forcing has remained
unclear. In this paper satellite observations as well as an ice sheet
mode have been used to measure the change in ice speed and mass balance
of the drainage basin over the last 25 years. It is shown by their
results that between 1994 and 2018 there was a mean increase in speed of
23.8%, with 3 glaciers accelerating by more than 44%. Across the Getz
Basin the increase in speed has been linear, with increase in speed and
thinning being correlated directly, which confirms the presence of
dynamic imbalance. 315 Gt of ice has been lost since 1994 which
contributed 0.9 ± 0.6 mm global mean sea level, with increased loss
since 2010 which was caused by a reduction in snowfall. Overall, the
dynamic imbalance accounts for ⅔ of the mass loss from this region of
West Antarctica over the past 25 years, with a longer-term response to
ocean forcing the likely driving mechanism.
The Antarctic Ice Sheet has contributed 7.6 ± 3.9 mm to global sea level
over the last 25 years (Shepard et
al., 2018). It has been shown
by observations that ice loss from Antarctica is dominated by the
low-lying, marine-based sectors of West Antarctica (Shepherd et
al., 2018; Rignot et
al., 2008; Joughin Smith &
Holland, 2010; Shepherd, 2012), where the Amundsen Sea Sector glaciers
have thinned, accelerated, and retreat of grounding lines since the
1940s (Mouginot, Rignot & 2014; Rignot et
al., 2014; Smith et
al., 2017; Shepherd et
al., 2019). Elsewhere, on the
Antarctic Peninsula ice shelves have retreated (Cook & Vaughn, 2010) and
collapsed (Rott, Skvarca & Nagler, 1996; Rack & Rott, 2004). In West
Antarctica dynamic balance is driven by incursions of warm, modified
Circumpolar Deep Water (mCDW) melting the ice that is floating (Joughin,
Smith & Holland, 2010; Jenkins et
al., 2010), with the interannual and long-term variability of ocean
atmospheres linked to atmospheric forcing that is associated with the El
Nino-Southern Oscillation (ENSO) (Dutrieux et
al., 2014; Jenkins et
al., 2018) and anthropogenic
forcing (Holland et al.,
2019), respectively. The contribution of the ice sheet to the global sea
level budget remains the greatest uncertainty in projections of future
sea level rise (Church et al.,
2013), which is partially driven by positive feedbacks such as the
Marine Ice Sheet Instability (Favier et
al., 2014; Joughin, Smith &
Medley, 2014), and with scenarios that are the most extreme, more than 1
m by 2100 being possibly only through the onset of Marine Ice Cliff
Instability (MICI) (DeConto & Pollard, 2016; Edwards et
al., 2019). Antarctica has
been shown by satellite data that the dynamic ice loss (6.3 ± 1.9 mm sea
level equivalent (sle) is 86% greater than the modest reduction in
surface mass (0.9 ± 1.1 mm sle) since the 1990s (Slater, Hogg & Mottram,
2020). Long-term and emerging new dynamic signals must, however, both be
measured accurately in order to understand better how ice sheets will
behave in the future. 88% of the speedup of the ice has been observed
occurring on glaciers located in the Amundsen Sea, Getz and Marguerite
Bay sectors (Gardner et al.,
2018). The timing and pace of dynamic imbalance is characterised poorly
in regions that have been observed less frequently, and uncertainty has
remained about the physical mechanisms that are driving this change.
The Getz drainage basin, which covers 10.2% (177,625 km2) of
the West Antarctic Ice Sheet, lies in the coastal margin of Marie Byrd
Land (Shepherd et al., 2019).
Ice flows from the ice sheet into the Getz Ice Shelf through 14 distinct
glaciers that extend inland about 145 km, and flow at average speeds of
more than 500 m per year at the grounding line. The ice shelf, which is
650 km long by 110 km wide, which characterises the region, is the
largest in Antarctica providing buttressing support to the grounded ice.
There are 8 large islands and 23 pinning points which stabilise the ice
shelf calving front, which has resulted in relatively small area change
over the last 3 decades (Jacobs et
al., 2013; Swithinbank et
al., 2003), though retreat
has been observed since 2003 (Christie et
al., 2018). Strong thinning
of the ice shelf (-16.1 m/decade), in spite of the absence of
significant area change, has been observed since the 1990s (Paolo,
Fricker & Padman, 2015), producing one of the largest sources of
freshwater input to the Southern Ocean (Assmann et
al., 2019), more than double
the input of the ice shelves of the neighbouring Amundsen Sea (Nakayama
et al., 2014). Ocean currents
are channelled beneath the sub-ice shelf cavity by the complex network
of topographic rises at the ice front, which drives a highly localised
spatial pattern of ice thinning, which has the strongest rates that are
observed at the grounding line (Hogg et
al., 2020). It is expected
that there will be spatially variable ocean forcing along the Getz
coastline due to its zonal extent, about half the margin of the West
Antarctic Ice Sheet, and its location between the colder Ross and warmer
Amundsen Seas (Jacobs et al.,
2013).
The Getz drainage basin has lost ~410 Gt of ice mass over the last 30
years (Shepherd et al.,
2019), and the rate has increased by more than
40% since 2010 (McMillan et
al., 2014). Regions of high
elevation bed topography at the ice divide has been provided by the
Flood, McCuddin & Executive Committee mountain ranges, which produces a
broadly prograde bed slope across the basin (Fretwell et
al., 2013). Inland
propagation of strong ice sheet thinning is prevented by this geometry
and makes the region less susceptible to onset of MISI, compared with
the retrograde sloping of the marine-based glaciers in the marine-based
glaciers in the neighbouring Amundsen Sea Embayment (Favier et
al., 2014; Joughin, Smith &
Medley, 2014). The snowfall pattern is heterogeneous across the basin,
with the highest rates being deposited on topography that is steeply
sloping which is aligned orthogonal (i.e. West to East) to the prevalent
direction of atmospheric moisture flux (Lenaerts et
al., 2018). Interannual
variability in Surface Mass Balance (SMB) is driven by the Amundsen Sea
Low, which accounts for 40% of the surface mass and 21% of the surface
melt variability, while summer melt is limited to the lower elevation
ice shelf (Lenaerts et al.,
2018). In Antarctica, the key to understanding the atmospheric and
oceanic forcing mechanisms that have been driving recent change is
partitioning of the influence of surface mass and ice dynamic signals
(Hogg et al., 2017). It has
been suggested by studies that thinning of glaciers that flow into the
Getz Ice Shelf are greater than the difference in ice discharge alone
that has been estimated, which indicates that surface processes are
responsible, at least in part, for the thinning that has been observed
(Shepherd et al., 2019;
Chuter et al., 2017). A
multi-decadal, continuous record of ice velocity is, however, required
to perform a detailed assessment of the change in ice flux. Satellite
data were used in this study to measure the change in the speed of the
ice of glaciers in the Getz drainage basin from 1994-2028, in order to
assess the localised pattern of dynamic imbalance in this large and
complex sector of West Antarctica. It was shown by the results of this
study that between 1994 and 2018, there was widespread speedup of the
majority of glaciers in the Getz drainage basin of West Antarctica. It
was observed that there was a mean increase of speed of 23.8%, with 3 of
the glaciers accelerating by more than 44%. 315 Gt of ice has been lost
since 1994, which contributed to 0.9 ± 0.6 mm to global sea levels.
Since 2010, increased loss was driven by a reduction in snowfall, with
dynamic imbalance likely to have been driven by a longer-term response
to ocean forcing.
The change in speed of glaciers in the Getz drainage basin was measured
in 2 previous studies, in which it was found that the largest ice flow
increase was observed behind Simple Island and at the far West of the
sector between 2007/2008 and 2014/2015 (Gardner et
al., 2018; Chuter et
al., 2017). According to
Selley et al. extend and fill
gaps in the spatial coverage, and show that the speedup of the ice flow
that is the largest and most extensive is concentrated on flow units 5
and 6 in the centre of the Getz drainage basin. These 2 glaciers
combined account for 41.1% of the total increase in the speed of the ice
across the Getz sector since 1994. The zone in which the highest speed
increase occurs coincides with the thickest and most rapidly thinning
part of the Getz Ice Shelf behind Siple and Carney Islands (Paolo,
Fricker & Padman; Gardner et al.,
2018). It was confirmed by this study that the 5 fastest flowing
glaciers in the in the far West of the Getz drainage basin (flow units
10 to 14) have also undergone a significant increase in the speed of ice
flow since 1994, ~25% on average, which accounts for 39.7% of the total
speedup in the region. It has been proposed that DeVicq Glacier (flow
unit 10) is a possible route through which future instability may
propagate in Marie Byrd Land, due to a deep bedrock trough that lies
more than 300 m below sea level at the grounding line and extends inland
for more than 200 km. It is indicated by model studies that in spite of
the geometry of the glacier it is not susceptible to imbalance even with
the presence of warmer ocean water in the Amundsen Sea (Holschuh et
al., 2014). It is shown, in
contrast, by the results of this study that this glacier has undergone
the 4th largest speedup in the Getz region over the last 25
years with ice speed increasing at a rate of 9.1 m/year2,
though it is limited to ~40 km inland of the grounding line.
A region of orographically driven snowfall of more than 3,000 mm w.e./year,
is resolved by a high spatial resolution (5.5 km) regional climate
models, corresponds to the Berry, Venzke and Land Glaciers (Lenaerts et
al., 2016). The highest
surface mass variability in Antarctica outside of the peninsula in
located in the Getz drainage basin (Shepherd et
al., 2019), and it is shown
by the cumulative anomaly that snowfall into the basin since ~2008 has
been significantly lower than the long term mean. It is shown by the
multidecadal time-series of speed change that the increase in ice flow
that has been observed on all glaciers in the Getz study region, has
been relatively linear over the past 25 years (Supplementary Fig. 1d).
The accelerated mass loss that has been observed by this study and
others (Shepherd et al.,
2019; Rignot et al., 2019)
from the Getz sector since ~2010, has therefore been driven by the
long-term gradual increase in dynamic imbalance, as well as combined
with the effect of a short-term surface-mass deficit. It is shown by
this that extreme snowfall years have a significant influence on mass
balance of the entire drainage basin, which suggests that localised
regions of high snowfall that are resolved by high-resolution models may
impact the balance of individual glaciers.
Located in the transition zone between the cooler Ross Sea and the
warmer Amundsen Sea, with potential warmer temperatures that are present
in the West of the Getz study region, is the Getz drainage basin. It is
suggested by observations over the last 3 decades in West Antarctica
that temperatures have warmed offshore (Rignot et
al., 2004), and there have
been periodic incursions of warm mCDW onto the continental shelf which
drives shorter-term sub-decadal to decadal variability (Dutrieux, 2014;
Jenkins et al., 2018; Jacobs
et al., 2013). Annual mean
temperature depth profiles in the Getz region, collected from ship-based
CTD sampling between 1994 and 2018 have shown annual variation in the
depth of the thermocline which divides a cold and fresh upper layer from
the mCDW. The thermocline was ~2,000 m deeper in 1994, 2000, 2012, and
2014, which led to cooler integrated heat content of the ocean, which is
in contrast to the warmer years in 2007, 2009, 2016 and 2018 when the
thermocline was shallower. Selley et
al. suggest these
observations are in line with previous ocean temperature studies, and
have been linked to the ocean response to interannual variability of the
atmosphere (Jacobs et al.,
2013; Assmann et al., 2019).
Changes in the depth of the thermocline are observed as a band of
variability of temperature between ~500 and 800 m below sea level, and
in the west of the Getz sector it is particularly strong. The ocean heat
content is made particularly sensitive to atmospheric forcing at the
continental break, even more than in the Eastern Amundsen Sea (Orsi,
Whitworth & Nowlin, 1995; Dotto et
al., 2019), by the relatively
short distance between the continental shelf break and the calving front
of the Getz Glacier, which is consistent with the large ocean heat
content viability.
Warmer ocean water must be transported from the open ocean under the ice
shelf at depth (below ~400 m), where it can come into contact with and
melt deeply grounded ice, in order for the spatial and temporal
variability to affect the rate of melting of the ice and the dynamic
imbalance of the Getz drainage basin. The Coriolis force, which causes
warm ocean water to enter ice cavities on the Eastern side, before being
guided by the sea bed and the geometry of the base of the ice and
exiting on the Western side, directs ocean circulation beneath the ice
shelves of Antarctica. Coriolis driven circulation of the Getz Ice
Shelf, brings warm ocean water from the Amundsen Sea between Wright and
Duncan Islands and to a lesser extent between Dean and Siple Islands.
Warmer water moves along the grounding line towards the West, in the
process melting the ice and gaining buoyancy and upwelling with
potential freshwater, as well as buoyancy and circulation input from
subglacial meltwater channels. Under the Getz Ice Shelf the pattern of
ocean circulation is visible through high meltwater fraction that exits
the cavity at ~150-200 m ice draft on the eastern side of islands, most
clearly Wright, Duncan and Grant. Selley et
al. attributed the ice
speedup that was observed on flow units 4-6 partially to the impact of
warm ocean water reaching the grounding line in these locations, which
drives high rates of ice speedup and melting of the ice shelf (Paolo,
Fricker & Padman, 2015; Hogg et
al., 2020). It appears that relatively warmer water reaches the
Western Getz (Nakayama et al.,
2014). Within the Getz cavities where tides seem to be relatively weak
and melting seems to be relatively large, however, most of the ocean
circulation is thought to be driven by the melt-induced upwelling and
conducted by the evolving geometry, much as is the case in Pine Island
and Thwaites cavities (Nakayama, 2019). In the case of the Getz, its
many ice stream tributaries, and islands which act as pinning points for
the ice flow make for a circulation pattern that is complex and
expectedly sinuous.
Buoyancy gains at the grounding zone provided by subglacial drainage
outflows can also enhance ocean melt. It is indicated by the results of
this study that a number of glaciers in the Gets study region that have
increased in speed coincide with regions of high subglacial water flux
beneath the ice sheet, including flow units 4-6 and De Vicq Glacier
(flow unit 10). It is known that ice melt that is driven by subglacial
runoff or, in the case of the Amundsen sector, ocean heat, decreases
with distance from the grounding line (Dutrieux et
al., 2014; Le Brocq et
al., 2013; Wei et
al., 2020), as the
entrainment of warm water at the ice-ocean interface increases with
buoyancy and velocity, though it diminishes downstream. Contrasting with
this, beneath the ice shelf meltwater channels can extend to the ice
front, depending on whether the supply of warm ocean water is sustained
(Gourmelen et al., 2017), or
if the features are simply advected downstream. It is known that plumes
of buoyant freshwater drive high melt rates on Getz Ice Shelf
hydrological pathways that route subglacial freshwater have a
significant role in determining the location of and rate of basal melt
beneath the ice shelf (Wei et al.,
2020). It is shown by the change in the speed of the ice that increases
in the Getz study region are strongest locally near the grounding zone
and do not extend to the ice front, which indicates that subglacial
hydrology could also be responsible for some of the speedup and thinning
that has been observed. If the volume of subglacial water flux changes
over time, as is the case of glaciers in Greenland that are due to
seasonal surface melt (Sundal et
al., 2011; Nienow, Sole, Slater & Cowton, 2017), this may be a
factor that has not been accounted for that drives changes in ice flow
and thinning in the regions of Antarctica with high subglacial water
flux. The presence of subglacial water has not been accounted for in
most ice flow models and to the knowledge of Selley et
al. about any change in
subglacial flux over the last 25 years is limited regions that are not
characterised by active subglacial lakes. According to Selley et
al. the Getz drainage basin
may therefore be a valuable test region of any studies that investigate
the coupling between subglacial hydrology and the ocean, and the impact
this may have on the localised ice sheet dynamics pastern in Antarctica.
It is shown by the results of Selley et
al. that ice speeds have
increased on the majority of glaciers in the Getz drainage basin, ice
speeds have increased at a rate that is broadly linear. Given that the
Getz basin, more so than its unstable neighbours in the Eastern Amundsen
Sea, is thought to be relatively immune to positive feedback processes
like MISI and MICI due to the prograde topography its bed, it is
difficult to explain this trend as a runaway response to a step or
oscillatory ocean forcing, at least during the past 25 years (Jenkins et
al., 2018). It is suggested
by this that the dynamic imbalance that was observed in the Getz may be
primarily a response to ocean forcing on the longer-term which is
suggested by Selley et al. to
possibly be of anthropogenic origin (Holland et
al., 2019).
Research programs in the future that deliver continuous annual
monitoring of the velocity of the ice and ocean temperatures across the
study region at present locations will be critically important,
preventing gaps in the record and enabling an assessment of the link
between the localised pattern and short-term variability of the dynamics
of the ice and the complex transport of ocean temperature variability
that is more direct.
The record of the ice speed in this study over 25 years shows for the
first time that widespread linear speedup has occurred on the majority
of glaciers in the Getz drainage basin of west Antarctica since 1994. It
is shown by the changes in ice flow that are concentrated zones of very
high speed up (>44%) at the grounding line of 3 glaciers (flow units 6,
5 and Venzke Glacier (flow unit 12), and high speedup of more than >20%
on an additional glaciers (flow units 3, 4 DeVicq Glacier (flow unit
10)) since 1994. 46.8% of this acceleration is accounted for by central
regions of the Getz drainage basin, and contains the most spatially
extensive areas of change. A localised response on individual glaciers
is indicated by this pattern of ice speedup, which demonstrates the
value of high resolution observations that resolve the detailed pattern
of dynamic imbalance across the Getz drainage basin. The speed increase
coincides with regions of high surface lowering on all glaciers, with a
approximately 50% of speed up which corresponds to a reduction of ~5% of
the thickness of the ice. It is shown that 315 Gt of ice has been lost
from the Getz drainage basin since 1994, by the results of the optimised
model of Selley et al., which
contributed 0.9 to 0.6 mm to global se levels, and increased the rate of
ice loss by 4 times in the 2010s compared to the 1990s. The topography
of the prograde bed makes the Getz region inherently less susceptible to
unstable geometry that drives feedbacks such as MISI and MICI compared
with its neighbours in the Amundsen Sea, and indicates that in the
long-term warming of the ocean may be driven by the dynamic imbalance in
this region of Antarctica. Understanding of the dynamic imbalance of
remote areas of Antarctica in the future will be helped by consistent
and temporally extensive sampling of both ocean temperatures and ice
speed.
Selley, H. L., et al. (2021). "Widespread increase in dynamic imbalance
in the Getz region of Antarctica from 1994 to 2018." Nature
Communications 12(1): 1133.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |