![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Antarctic Ice Sheet – Mass Balance from 1992 to 2017
According to the IMBIE Team the
Antarctic Ice Sheet is an important indicator of climate change and
a sea level rise driver. In this paper satellite observations of the
changing volume of the Antarctic Ice Sheet, flow and gravitational
attraction with modelling of its surface mass balance shows that it lost
2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which
corresponds to a mean sea level increase of 7.6 ± 3.9 mm (errors are 1
standard deviation). Rates of the loss of ice from the West Antarctic
Ice Sheet (WAIS) has increased from 53 ± 29 billion to 159 ± 26 billion
tonnes per year as a result of ocean-driven melting over this period and
ice shelf collapse has increased the rate of ice loss from the Antarctic
Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. It was
found by the IMBIE Team that large variations in and among the estimates
of models of surface mass balance and for East Antarctica, glacial
isostatic adjustments with its average rate of mass gain over the period
1992-2017 (5 ± 44 billion tonnes per year) being the least certain.
Enough water is held in the Antarctic ice sheets to raise global sea
level by 58 m (Fretwell et al., 2013). Ice is channeled to the oceans
through a network of glaciers and ice streams (Rignot, Mouginot &
Scheuchl, 2011), each of which has a substantial catchment (Zwally,
Giovinetto, Beckley and Saba, 2012). Fluctuations of the mass of
grounded ice sheets arise due to differences between the net
accumulation of snow at the surface, meltwater runoff and the discharge
of ice into the ocean. Reductions in the thickness (Shepherd, 2010) and
extent (Cook & Vaughn) of floating ice shelves have disturbed the flow
of inland ice flow, and this has triggered retreat (Rignot et al., 2014;
Konrad et al., 2018), acceleration (Joughin et al., 2002; Rignot et al.,
2004) and drawdown (Shepherd, Wingham & Mansley, 2002; Scambos et al.,
2002) of many ice streams that terminate at the ocean. There have been
various techniques that have been developed to measure ice sheet mass
changes, based on satellite observations of their speed (Rignot &
Thomas, 2003), volume (Wingham et al., 1998) and gravitational
attraction (Velicogna & Wahr, 2006) combines with modelled surface mass
balance (SMB) (van Wessem et al., 2018) and glacial isostatic adjustment
(GIA; the ongoing vertical movement of land associated with ice load
changes) (King et al., 2012). There have been more than 150 assessments
of the loss of ice from Antarctica based on these approaches (Briggs et
al., 016) since 1989. It was demonstrated by the inter-comparison of 12
such estimates (Shepherd et al., 2012) that the 3 principal satellite
techniques provide similar results at continental scale
and, lead, when combined, to an estimated loss of mass of 71± 53
billion tonnes of ice per year (Gt yr-1) averaged over the
period 1992-2011 (errors are 1 standard deviation unless otherwise
stated). In this paper the assessment has been extended to include twice
as many studies, which doubles the overlap period and extends the record
to 2017.
Satellite Observations
According to the IMBIE Team they collated 24 estimates that were derived
independently of ice sheet mass balance that had been determined in the
period 1992-2017 and were based on the techniques of satellite altimetry
(7 estimates), gravimetry (15 estimates) or the input-output method (2
estimates). The computation involved a total of 24, 24 and 23 estimates
of mass change within defined geographical limits (Zwally, Giovinetto,
Beckley & Saba, 2012; Rignot, Mouginot & Scheuchl, 2011) for the East
Antarctic Ice Sheet (EAIC), WAIS and Antarctic Peninsular Ice Sheet
(APIS), respectively. The rates of mass change of ice sheets were
compared over common intervals of time (Shepherd et al.,2012). The rates
of ice sheet mass balance was then averaged using the same class of
satellite observations to produce time series of mass change that were
independent of technique in each geographical region. The uncertainty in
the annual mass rate was computed within each class as the mean
uncertainty of the individual contributions. The final reconciled
estimated ice sheet mass change was computed for each region as the mean
of the values that are dependent on the technique that are available for
each epoch. The IMBIE Team assumed that the errors for each technique
are independent in computing the associated uncertainty. In order to
estimate the cumulative mass change and its uncertainty, estimates for
each ice sheet were integrated and the annual uncertainty was weighted
by 1/√n, where n is the number of years since the
start of each time series. Mass trends of the Antarctic Ice Sheet (AIS)
were computed as the linear sum of the regional trends and the
uncertainties in the mass trends as the root-sum-square of the regional
uncertainties.
Trends in Antarctic ice sheet mass
The disagreement level between individual estimates of the mass balance
of ice sheets increases with the area of each region of the ice sheet,
the average standard deviations per epoch being
·
APIS 11 billion tonnes of ice
per year,
·
WAIS 21
·
EAIS 37
Gravimetric estimates are the most abundant among the techniques, and
are also the most closely aligned, though in East Antarctica their
spread increases, where the Glacial Isostatic Adjustment remains poorly
constrained (Bentley et al.,
2014) is at its least uncertain when integrated spatially (Peltier,
2004; Wahr & Zhong, 2013; Sasgen et al., 2013; Peltier, Argus &
Drummond, 2015; King, Whitehouse & van der Wal, 2016;
Whitehouse et al., 2012; Spada, Melini & Colleoni, 2015; Konrad,
Sasgen, Pollard & Klemann, 2015; Briggs, Pollard & Tarasov, 2014; Ivins
& James, 2005; Ivins et al., 2013; Nield et
al., 2014), because of the
vast extent of the region. Solutions that are based on satellite
altimetry, and the input-output method that is run for the full record
is roughly twice the duration of the gravimetry time series. Though most
(59%) estimates are within 1 standard deviation of the
technique-dependent mean, and a few (6%) depart by more than 3 standard
deviations.
For the Antarctic Peninsula, the 25-year average rate of mass balance is
-20 ± 15 billion tonnes per year, with an increase of about 15 billion
tonnes per year that has been lost since 2000. In West Antarctica the
strongest signal and trend has occurred, where mass loss rates increased
from 53 ± 29 to 159 ± 26 billion tonnes per year over the period between
the first and final 5 years of the survey by the IMBIE Team; the largest
of the increases occurred during the late 2000s when there was an
acceleration of the discharge of ice from the Amundsen Sea sector
(Mouginot, Rignot & Scheuchl, 2014). These regional losses are both
driven by reductions in the thickness and extent of floating ice
shelves, and this has triggered the retreat, acceleration and drawdown
of glaciers that terminate at the ocean (Shepherd, Fricker & Farrell,
2018). East Antarctica is where the least certain result was obtained,
where the average 25-year mass trend is 5 ± 46 billion tonnes per year.
The Antarctic Ice Sheet, overall, lost 2,720 ± 1,390 billion tonnes per
year between 1992 and 2017, which is an average rate of 109 ± 56 Billion
tonnes per year.
Surface mass balance
An essential component of the input-output method is knowledge of the
Surface Mass Balance of the icesheet, which subtracts the discharge of
solid ice from the net accumulation of snow, and aids in the
interpretation of mass trends that are derived from satellite altimetry
and Gravimetry. The main driver of temporal and spatial variability in
the Antarctic Ice Sheet surface mass change (Boening et al., 2012;
Medley et al., 2018) is snowfall. Though locally important, spatially
integrated sublimation and runoff of melt water are typically 1 and 2
orders of magnitude smaller, respectively. The Surface Mass Balance of
the Antarctic Ice Sheet is usually taken from the atmospheric models in
the absence of maps that are observationally based, evaluated with
observations that are remotely sensed and
in situ (van Wessem et al.,
2018; Favier et al., 2013; van de Berg & Medley, 2016; Palerme et al.,
2017; Van Wessem et al., 2014). The IMBIE Team compared 2 global
reanalysis products (JRA55 and ERA-Interim) and 2 regional climate
models (RACMO2 and MARv3.6) in order to assess the Antarctic Surface
Mass Balance. The best performing reanalysis product over Antarctica is
usually regarded to be ERA-Interim, albeit with a dry bias in the
interior and overestimated rain fraction (Palerme et al. 2017; Bromwich,
Nicolas & Monaghan, 2011; Behrangi et al., 2016). Accumulation rates
that are averaged spatially peak at the Antarctic Peninsula, and are
roughly 3 times lower in West Antarctica and roughly 7 times lower in
East Antarctica. Values are given by the regional climate models of 4.7%
higher and the reanalysis 7% lower. According to the INBIE Team these
differences can be attributed to the higher resolution of the regional
models, which resolve the steep coastal precipitation gradients in
greater detail and to their improved representation of the polar
processes. In all products the temporal variability is similar and they
all agree that there is an absence of a trend that is ice sheet wide in
the Surface Mass Balance over the period 1979-2017, which implies that
the recent loss of mass from the Antarctic Ice Sheet is dominated by
increased discharge of solid ice into the ocean.
Glacial isostatic adjustment
The method used to correct for Glacial Isostatic Adjustment (King et
al., 2012) strongly influences the gravimetric estimates (Peltier, 2004;
Peltier, Argus & Drummond, 2015; Whitehouse et al., 2012; Ivins & James,
2005; Ivins et al., 2013; Shepherd, Fricker & Farrell, 2018). There were
also 9 continent-wide forward-model simulations and 2 regional
simulations that were assessed in this study to understand better
uncertainties in the Glacial Isostatic Adjustment signal; the gravimetry
estimates of mass balance were reprocessed using W12a (White et al.,
2012) and IJO5_r2 (31) Glacial Isostatic Adjustment models for
comparison with earlier work (Shepherd et al., 2012). Across Antarctica
the net gravitational effect is positive, and the mean and standard
deviation of the continent-wide Glacial Isostatic Adjustment models (54
± 18 billion tonnes per year is very close to that of W12a (56 ± 27
billion tonnes per year) and IJO5_R2 (55± 13 billion tonnes per year)
models. The difficulty of quantifying the timing and extent of past ice
sheet change and the absence of lateral variations in Earth rheology
within some models (van der Wal, Whitehouse & Schrama, 2015), is
probably a reflection of the narrow spread. In areas where the Glacial
Isostatic Adjustment is a substantial component of the regional mass
change, such as the Amundsen, Ross and Filchner-Ronne sectors of West
Antarctica, though it is also the area of greatest variability (e.g.
standard deviation of more than 10 mm/yr in the Amundsen sector, the
models predict the greatest rates of solid Earth uplifts (on average,
5-7 mm/yr). There is a low variance among the models and broad agreement
with the GPS observations (Martín-Español et al., 2016), away from areas
of large Glacial Isostatic Adjustment signals. Most models that are
considered in this study, nevertheless, do not account for ice sheet
change over the past few thousand years, as it is not well known.
Gravimetric calculations (Caron et al., 2018) can also be biased by
inaccurate treatment of low degree harmonics that are associated with
the global Glacial Isostatic Adjustment signal. If a transient component
that is associated with recent ice sheet change is included in the
Glacial Isostatic Adjustment signal, it will bias the mass trend
estimates and should be accounted for in future work.
Outlook
It is still possible that assessments in ice sheet mass balance can be
achieved. A powerful tool for evaluating models of Surface Mass Balance
and compaction of firn over large spatial (thousands of kilometres) and
temporal (centennial) scales is airborne snow radar (Medley et al.,
2014; Lewis et al., 2017), as well as the ice cores that have been used
traditionally (Thomas et al., 2017). Glacial Isostatic Adjustment models
are enabled to be scrutinised and calibrated by geological constraints
of the ice sheet history (Bentley et al., 2014) and GPS measurement of
contemporary uplift (Martín-Español et al., 2016; Thomas et al., 2011).
The IMBIE Team suggest more of these types of datasets are needed,
especially in East Antarctica. The spread of models such as Glacial
Isostatic Adjustment and Surface Mass Balance should be evaluated in
concert with the satellite gravimetry, altimetry and velocity
measurements, given their apparent diversity. The imbalance that is
present in the current record would be addressed by a reassessment of
the satellite measurements that were acquired during the 1990s.
Alternative techniques (e.g. Wahr, Wingham & Bentley, 2000) for
combining satellite datasets should be explored, and satellite
measurements with common temporal sampling should be contrasted. The
mass balance of the ice sheet record should now be separated into
continuations due to short term fluctuations in in Surface Mass Balance
and to trends on longer terms in glacier ice. Continued satellite
observations are essential, as well as these improvements.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |