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Mass balance investigation of Antarctica from budget methods
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During the last 20 years, West Antarctica has experienced enhanced ice discharge to the
ocean due to loss of buttressing from melting and collapsing ice shelves. On the other
hand, increases in precipitation have been reported in East Antarctica in line with an
expected wetter atmosphere in a warming climate. The big questions that still lie ahead
are therefore: (i) Will the enhanced precipitations in East Antarctica compensate the
dynamic mass losses observed in West Antarctica in the future? (ii) And what will be the
resulting contribution to sea level rise (SLR)? To answer those questions we need to have
a firm grip on the present day mass balance (MB) of Antarctica and on the mechanisms
that govern both the surface mass balance (SMB) and the ice discharge (D) into the
ocean. This thesis investigates the MB of Antarctica using the input-output method (10M)
allowing for a direct diagnoses of local, regional and global MB in Antarctica. It does this
for both the Antarctic Ice Sheet (AIS) and the ice shelves. Because the mass imbalance of
AIS is of the order of 5-10% of both accumulation and attrition terms of the mass budget
(~2000 Gt yr-1), all glaciers around Antarctica as well as each assumption made require
precise attention. This thesis starts with a chapter exploring the grounding zone (Chapter
2), and then goes on to the actual mass balance calculations of the AIS in Chapter 3 and
of the Antarctica ice shelves in Chapter 4.
The Grounding lines (GL) of Antarctica have been widely studied using various
techniques at a local and regional scale. In recent years GL datasets aiming for circumpolar
coverage have been published using different approaches. However these datasets
still bear unexplained discrepancies of up to tens of kilometres in numerous places around
Antarctica. In Chapter 2 four recent datasets are compared which track either the surface
break of slope (h) or the inward limit of tidal flexure (F) as proxies for the grounding
point (G). From visual examination and from a particle tracking scheme (PTS), it is found that all GL datasets agree within 1-2 km on slow moving ice and on the sides of fast
flowing features (FFFs). However it is confirmed that h, obtained from photogrametry or
photo clinometry, is not a reliable proxy in central parts of FFFs because of multiple
breaks-in-slope and artefacts. It is further confirmed that the most reliable methods to
map G in such places are those tracking F. In addition, a gravitational driving stress (td) is
computed from a 1 km Antarctic digital model elevation (DEM) and leads to the finding
that driving stress mapping (DSM) supports dynamic approaches in grounding line
location. This reconciles static and dynamic grounding line methods by showing that they
map the same features providing that altimetry is used rather than imagery for static
methods. Guided by these analyses a new, up-to-date, and complete grounding line of
Antarctica is compiled. The potential of DSM is successfully tested on a grounding line
migration case study in West Antarctica.
To investigate the grounding zone around Antarctica and its ice dynamics, DSM is
further used. DSM allows to map sharp transitions across G for slow moving ice, as well
as complicated transitions on fast flowing features (FFFs). Complicated transitions on
FFFs contradict the idea of there being an ideal transition occurring at G, whereby the ice
flow regime switches from basal drag-dominated to lateral drag-dominated. Rather, it is
found that acceleration occurs upstream of G and that deceleration occurs downstream of
G. This changes the understanding of the grounding zone ice dynamics, where ice was
believed to accelerate at G due to loss of basal drag. Using DSM in combination with ice
penetrating radar (lPR), reported and new ice plains (i .e. lightly grounded areas) are
detected and mapped. They extents cover ~55,000 km2 around the Ross, the Filchner-Ronne,
and the Larsen C ice shelves. These findings have implications for our
understanding of ice sheet stability since ice plains are particularly prone to grounding
line migration and can stretch up to ~300 km inland of G.
In Chapter 3 the MB of the AIS is assessed using the input-output method (lOM). The
grounding line fluxes (GLF) and 5MB are estimated for 110 drainage basins covering the
whole AIS. The GLF is computed using up to date grounding lines and additional radar
ice thicknesses data compared to previous 10M studies. 5MB values are re-evaluated in
light of new drainage basins defined from an ice velocity field rather than from
topography. 5MB is taken as the 30 years mean of three regional climate models. Due to a number of improvements in the GLF methodology, an unprecedented 94% of the ice
sheet area is surveyed, i.e. an increase of + 13% from the latest 10M study. Un-surveyed
areas are accounted for using mass trends (MT) from a Bayesian hierarchical modelling
solution from the RATES (Resolving Antarctic ice mass TrEndS) project. The integrated
AIS mass balance is -63 ± 83 Gt yr- I and divides into -22 ± 28, -62 ± 45, and 22 ± 64
Gt yr- I for the Antarctic Peninsula (AP), the West Antarctic Ice Sheet (WAIS), and the
East Antarctic Ice Sheet (EAIS), respectively. The integrated MB is therefore a lower
10M estimate compared to previous 10M studies and reconciles the 10M with the other
MB methods of satellite gravimetry and altimetry.
Because the stability of the AIS is intimately linked to the stability of ice shelves,
Chapter 4 finally focuses on the mass balance of ice shelves around Antarctica, giving the
partition between calving fluxes (CF) and basal mass balance (BMB), the main processes
by which Antarctic ice is lost. Before this study, iceberg calving had been assumed the
dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the
calving flux exceeding 2,000 Gt yr- I . More recently, the importance of melting by the
ocean had been demonstrated close to the grounding line and near the calving front. So
far, however, no study had reliably quantified the calving flux and the BMB (the balance
between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The
distribution of fresh water in the Southern Ocean and its partitioning between the liquid
and solid phases was therefore poorly constrained. Here, a first estimate of the mass
balance components for all ice shelves in Antarctica is produced using calving flux and
grounding-line flux from satellite and airborne observations, modelled ice-shelf snow
accumulation rates, and a regional scaling that accounts for un-surveyed areas. The total
CF is 1321 ± 144 Gt yr- I and the total BMB is -1454 ± 174 Gt yr-1 . These numbers mean
that about half of the ice-sheet surface mass gain is lost through oceanic erosion before
reaching the ice front, and that the calving flux is about 34% less than previous estimates
derived from iceberg tracking. In addition, the fraction of mass loss due to basal
processes varies from about 10 to 90 % between ice shelves. A significant positive
correlation between BMB and surface elevation change is found for ice shelves
experiencing surface lowering and enhanced discharge. It is therefore suggested that basal
mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.
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