5 Conclusions
Despite Crane Glacier being invoked in many conversations of unstable
ice cliff failure, the limited data collected during the LBIS collapse
never directly measured a cliff failure event. Previous analysis of
Crane Glacier’s behavior from 2002-2004 is predicated on an inaccurate
2002 grounding line position. Our analysis shows that retreat in 2002
and 2003 occurred in a remnant, floating ice shelf, but in the two years
following LBIS collapse, calving rates accelerated, consistent with a
positive feedback and geometric instability. This includes the final
phase of retreat, which may have occurred by the failure of a grounded
ice cliff with an initial height of 111 m, with a minimum calving rate
of ~8.50 km/a.
The glacier terminus retreated to a configuration where calving rates
slowed, and the system maintained a constant terminus position for 7
years with no further retreat. Meanwhile, the mass dis-equilibrium
induced by terminus changes drove widespread thinning in the glacier
interior. Finally, as sea-ice filled the fjord in 2011, Crane Glacier’s
terminus advanced and an ice shelf was re-established, supporting a
thickening and slowing (but overall thinner than pre-LBIS collapse)
glacier profile.
Given a maximum terminal cliff height of only 111 m, available process
models indicate that terminus stresses would not exceed the failure
threshold of undamaged ice. However, using model values consistent with
damaged ice, cliff failure at Crane Glacier becomes consistent with the
process models discussed here. This highlights the importance of better
understanding ice shelf damage in projecting future ice sheet behavior
– if high damage is plausible for Crane Glacier, it means that cliff
failure likely did govern the final stage of glacier retreat, and future
ice sheet margins may be more susceptible to fracture than anticipated.
The rapid nature of ice shelf break-up makes observing ice cliff failure
an inherently challenging problem. Brittle processes are fast and
therefore require high temporal sampling to observe, best accomplished
in situ. Measuring cliff failure processes will require having field
sensors in the right place at the right time, something not easily done
at scale. This means that observational evaluation of ice cliff models
must, in part, rely on the temporally limited (and therefore at times,
ambiguous) remote sensing record. While studies like this one cannot end
the debate over model realism and the role of cliff failure in the
future evolution of the Antarctic Ice Sheet, they contribute to the
larger corpus of evidence required to justify any novel treatment of
cliffs in continental scale models of Antarctica and Greenland.