Figure 6 . Second principal stresses and compressive arch
locations for different front geometry conditions (non-rift scenario).
F1–F4 are the front locations in 1963, 2000, 2013, and 2017,
respectively. H1–H5 are the hypothetical front locations under future
retreat scenarios.
5.2 Dynamic responses to front retreat (non-rift scenario)
The retreat events during 1963–2020 neither exceeded nor modified the
compressive arch (Figures 6a–d). Under future retreat scenarios,
successive retreat would allow the compressive arch to migrate upstream
until H3 (Figures 6e, f). The front geometry change due to retreat can
result in flow accelerations and backstress loss (Figure 7), with the
magnitude and extent depending on the location of the retreat. The two
retreat cycles reduced the backstress at the areas adjacent to the ice
rises and the central downstream portion (Figure 7i–k). Flow
accelerations (greater than 20 m/year) caused by front geometry change
are limited to the central downstream portion and are less than 50
m/year for the majority of Larsen C (Figures 7a–c). Therefore, the
front geometry change due to the past retreat events is insufficient by
itself to explain the observed velocity increase. We compared the
derived flow velocities with the modeled flow velocities of
corresponding front geometry. Over the downstream portion of the ice
shelf, the averaged difference increases over time with the propogation
of R1 and R2 from 9.8±8.5% (2000–2002, F2 condition), to 16.3±8.6%
(2008–2010, F3 condition), to 17±12.9% (2013–2015, F3 condition), and
to 23.4±24.5% (2015–2016, F3 condition). This suggests that, to make a
realistic prediction of flow velocity change due to ice shelf retreat,
it is necessary to first consider rift development in response to front
retreat. Nevertheless, the modeling scenario considering only the front
geometry condition can be used with the observational data as a
diagnostic indicator. The increasing difference between observed and
modeled flow velocities over time may be an indicator for future calving
events.
However, even without considering rifts, once the ice front reaches the
current compressive arch, widespread flow accelerations
(~25%) would be triggered (Figure 7d), as a result of
the dramatic backstress loss (Figure 7l) from Bawden Ice Rise and Gipps
Ice Rise. This emphasizes the important role of ice rises in stabilizing
the ice shelf. Complete buttressing loss from ice rises would cause
ice-shelf-wide flow accelerations, and the backstress-reducing effect
would extend to the grounding line. After the compressive arch ceases to
exist, even small perturbations in the ice front would cause large-scale
backstress loss and flow accelerations (Figures 7g–h, o–p),
particularly over the C2 and C6 units. This effect would reach the
grounding line, and the upstream glaciers feeding the C2 and C6 units
would speed up in response to the buttressing loss caused by a small
retreat at the ice front. This scenario resembles the pre-collapsed
Larsen B, for which the compressive arch was removed and flow
accelerations occurred across the ice shelf. These modeling experiments
suggest that ice rises are crucial for determining the tipping point of
ice shelf stability. The compressive arch is the critical boundary,
beyond which the ice shelf flow velocities and stresses are highly
sensitive to front retreat. Once the compressive arch is absent, front
geometry change will become an important control on ice shelf flow
dynamics and stress conditions.