In view of a future discussion on the next future of the PIIS I would like to recall an analysis of the calving of PIG made in the article : S.Jeong et al. 2016 (references below)
“Recent observations of continued acceleration, retreat, and thinning of Pine Island Glacier affirm its dynamic instability, suggesting that irreversible retreat has already begun. Observational analysis and ice flow models suggest that current degenerative change of Pine Island Glacier will persist for a century or more. The triggers for the ongoing changes remain poorly understood. Recent studies, however, have increasingly pointed toward ice-ocean interaction as the dominant driver. In addition to increased thinning and grounding line retreat, over the past several decades, the Pine Island Glacier has undergone increased rifting and expansion of the lateral shear zones flanking the fast-flowing ice shelf. Since the late 1990s, both the southern and northern ice shelf shear margins have become increasingly fractured and the northern margin has progressively opened, reducing the area of contact between the ice shelf and the shear margins. Because shearing along the ice shelf edges generates a stress that resists flow while ice thinning reduces the amount of stress that can be generated, fracturing and disintegration of the shear margins can cause ice flow acceleration, creating a potential positive feedback between shear margin rifting and acceleration leading to unstable disintegration. Additionally, three times since 2000, marginal rifts have propagated from the northern shear zone through the width of the ice shelf, resulting in the calving of large tabular icebergs. Similar to rifting and calving events observed on other ice shelves, these transverse rifts initiate where the shelf flank loses contact with the margin and thus may be related to the resulting loss in lateral shear stresses that resists outward flow of the shelf. The reason(s) that a particular rift propagates laterally across the entire shelf is unknown, although structural heterogeneities, such as basal crevasses or suture zones between merging ice streams, remain a possibility. While the most recent (2011) rifting and calving event initiated further inland than the two prior events, the net change in ice front position was small, with little resulting change to the ice shelf’s structure.”
“However, two anomalous rifts appeared in late 2014 and early 2015 that, in contrast to previous events, initiated in the center of the ice shelf and propagated toward the margins.”
“The style of ice shelf rifting currently underway at the Pine Island Glacier is fundamentally different from previous episodes of rifting and calving in the last decade, in which preexisting, marginal rifts propagated from the seaward end of the northern shear margin across the tongue, perpendicular to the mean flow direction. The initiation of multiple rifts in the center of a fast-flowing (faster than 1 km/yr) ice shelf is unusual. Basal crevassing near the grounding line has been observed at the Pine Island Glacier and rifts transverse to flow are observed at the grounding line of neighboring Thwaites Glacier, which is undergoing similar acceleration and thinning. The series of basal crevasses observed upstream of rifts R1 and R2 suggest that, in contrast to previous rifting events, basal crevasses have initiated at the grounding line and have widened and deepened as they advect downstream. Recent studies show that the presence of wide channels at the bottom of the ice can be sufficient to trigger full thickness ice fracture. Moreover, melting within basal crevasses can widen them, enabling penetration through an increasingly large fraction of the ice thickness as they propagate downstream. At the Pine Island Glacier, basal crevasses may have advected with the ice shelf and then propagated through the full ice thickness due to a combination of increased bottom melting, ice thinning and increased deviatoric stresses. The cause of the initial formation of the basal crevasses are uncertain but may be the result of periods of enhanced basal melt due to episodic intrusions of warm deep water and/or subglacial meltwater discharge associated with lake drainage.
Rather than being transverse to flow, these rifts are oriented oblique to the average flow direction and approximately perpendicular to the strike of marginal rifts on the northern side. Thus, their formation was not likely caused by increasing along-flow deviatoric stresses as the ice reaches the front, as is typical. Instead, we suggest that the evolution of rifts is accelerated by the same overall stress regime that is causing both northward rotation of the terminus and northern migration of the southern shear margin. An explanation for this northward migration is the retreat of the highly rifted portion of the northern shear margin. The removal of this section of the ice shelf has now completely decoupled the ice shelf from the northern coast, resulting in a nearly unconfined ice tongue. While this mélange-filled and highly rifted shear margin was likely weak, providing small resistance along flow, it may have still acted to confine the shelf on the northern flank and provide resistive stresses transverse to flow similar to the way that mélange-filled fjords in Greenland have been found to provide a backstress large enough to inhibit calving of fast flowing outlet glaciers.”
To which I add the subsequent changes:
NSM : end of the zone of ice mélange and flowing on a new pinning point (hope that it will hold for a few years)
SSM : end of pinning point created by the joint with the SWT and zone of ice mélange, (no pinning point before 40km)
Reference :
S. Jeong, I. M. Howat, and J. N. Bassis(2016),
Accelerated ice shelf rifting and retreat at Pine Island Glacier, West Antarctica,
Geophys. Res. Lett.,43,11,720–11,725, doi:10.1002/2016GL071360.