Plain Language Summary
The Antarctic Ice Sheet is the largest source of uncertainty in projecting future sea-level rise. This is due to our limited understanding of drivers and mechanisms triggering tipping points in ice-sheet instability, and knowledge gaps regarding the retreat and disintegration of ice shelves. Understanding processes leading to ice shelf destabilization is critical to improving estimates of future Antarctic mass loss because of their important role in stabilizing ice flow. We studied Larsen C Ice Shelf changes using satellite data collected over 1963–2020, and conducted modeling experiments to elucidate the observed linkages between front retreat, flow acceleration, and rifts. We find the development of rifts near ice rises to be the primary control on Larsen C front calving and flow acceleration in the past six decades. Rifts can affect each other by causing structural weakening and modifying stress fields. To predict future dynamical changes, it is necessary to account for this feedback and capture how ice rigidity changes over time in response to rift growth. If Larsen C retreats such that the compressive arch ceases to exist, it will resemble the pre-collapsed Larsen B ice shelf, producing widespread flow accelerations in response to the backstress loss from ice rises.
1 Introduction
Ice shelves, as floating extensions of the grounded ice sheet, fringe ~75% of Antarctica’s coastline (Rignot et al., 2013). Ice shelves gain mass through ice flux from inland glaciers, local snow accumulation, and basal marine ice accretion, while they lose mass primarily via basal melting and iceberg calving (e.g., Depoorter et al., 2013; Liu et al., 2015), together with minor runoff of surface meltwater (Bell et al., 2017). Although the direct effect of ice shelf mass loss on sea-level rise is negligible, ice shelves provide an important buttressing force to stabilize the grounded ice sheet and regulate the contribution from inland glaciers to global sea level (Dupont & Alley, 2005; Gagliardini et al., 2010; Gudmundsson, 2013; Pritchard et al., 2012; Reese et al., 2017; Thomas, 1979). Understanding the processes driving ice shelf retreat is therefore essential to generating realistic projections of future Antarctic ice sheet mass loss in a warming climate (DeConto et al., 2021; Noble et al., 2020; Pattyn & Morlighem, 2020).
Satellite data have indicated increasing trends in thinning and retreat of Antarctic ice shelves (e.g., Depoorter et al., 2013; Paolo et al., 2015), revealing the vulnerability of ice shelves to atmospheric warming and oceanic forcing (Holland et al., 2015; Liu et al., 2015; Vaughan & Doake, 1996). As a region sensitive to climate warming, the Antarctic Peninsula has undergone pronounced regional warming (Vaughan et al., 2003), along with rapid retreat of ice shelves and glaciers on both the west and east coasts (Cook et al., 2005; Doake & Vaughan, 1991; Rott et al., 1996; Scambos et al., 2009; Scambos et al., 2004). Successive retreat has occurred on the Larsen Ice Shelf along the east coast. The Larsen Ice Shelf was formerly composed of Larsen A, B, C, and D, four sections from north to south (Figure 1). Larsen A disintegrated in 1995, with an areal loss of ~1600 km2 (Rott et al., 1996). Larsen B collapsed in February–March 2002, with an areal loss of ~3250 km2 (Scambos et al., 2004). The rapid disintegrations of Larsen A and Larsen B triggered immediate flow accelerations of upstream glaciers (Rignot et al., 2004; Scambos et al., 2004), which highlights the importance of studying how an ice shelf evolves to an unstable condition and how ice shelf retreat affects upstream flow dynamics. This has also led to an increasing concern as to whether Larsen C will disintegrate in the future (e.g., Borstad et al., 2017; Glasser et al., 2009; Hogg & Gudmundsson, 2017; Jansen et al., 2010, 2015; Kulessa et al., 2014), especially after a giant iceberg (A68, ~5800 km2) broke off from Larsen C in July 2017.
The disintegrations of Larsen A and Larsen B were preceded by warm air temperature and intense surface melt (Banwell et al., 2013; Scambos et al., 2003; Scambos et al., 2000). Meltwater-induced hydrofracturing has been postulated to be the dominant mechanism in the ice shelf collapse (van den Broeke, 2005; MacAyeal et al., 2003; Robel & Banwell, 2019), likely preconditioned by increased basal melting (Shepherd et al., 2003; Vieli et al., 2007). To predict the future stability of Larsen C, recent research has focused on surface melt and associated atmospheric drivers (Bevan et al., 2020; Luckman et al., 2014), and parallel efforts have focused on ocean-driven basal processes (Holland et al., 2015; Jansen et al., 2013; McGrath et al., 2014), such as basal melting and the stabilizing role of suture zones. Despite the importance of atmospheric and oceanic forcing, the stability of an ice shelf is also controlled by its stress field (Doake et al., 1998; Fürst et al., 2016; Kulessa et al., 2014; Lhermitte et al., 2020). The stress field, either compressive or tensile, is closely related to fracture opening. The spatial distribution and temporal evolution of fractures determine the ice shelf resilience, and enhanced fracturing along shear margins has been suggested to be critical for ice shelf stability and flow dynamics (Lhermitte et al., 2020; Joughin et al. 2008). Larsen A and Larsen B were already in an unstable stress state before their collapse as the front retreat had passed a “compressive arch” (Doake et al., 1998). Investigating the dynamic changes of ice shelves over the long term is important to understanding the destabilization processes. Multidecadal satellite observations on the retreat of Larsen C provide an excellent opportunity to study the long-term dynamic behavior of ice shelves and associated rifting and calving processes. Understanding these processes is also the key to predicting the future changes of other embayment-confined ice shelves, such as the Ross Ice Shelf and Filchner–Ronne Ice Shelf.
Here, we use multi-source satellite images spanning six decades and ice shelf modeling experiments to conduct a comprehensive analysis of the changes of ice shelf front, flow velocities, and stress fields over Larsen C. The earliest satellite imagery acquired in the 1960s allows us to extend the flow velocity data back to the 1960s–1980s. We analyze in detail the development of two rifts that led to the 2017 calving event, and reveal the close linkages between rift propagation, flow acceleration, and front retreat. We further carry out ice shelf modeling experiments to examine how ice shelf flow velocities and stress fields vary in response to the front geometry change due to ice shelf retreat and the mechanical weakening due to rift development.
2 Study area
Larsen C is confined by Jason Peninsula in the north and Kenyon Peninsula and Gipps Ice Rise in the south (Figure 1). Because of the barrier effect of the high mountain ridge on westerly winds, the east side of the Antarctic Peninsula is generally colder than the west side (King et al., 2017; Morris & Vaughan, 2003). However, strong downslope foehn winds occasionally bring warm and dry air masses over the northern Larsen C during the austral summer, causing intense surface melt in the northwest near the Cabinet Inlet (Elvidge et al., 2015; King et al., 2017). The structure of Larsen C (Figure 1) consists of fast flow units draining from the twelve inlets, and suture zones originating from the Churchill Peninsula, Cole Peninsula, Marmelon Point, Francis Island, Tonkin Island, and Joerg Peninsula (Borstad et al., 2017; Glasser et al., 2009). Suture zones connect those fast flow units and have been believed to play a significant role in stabilizing the ice shelf (Kulessa et al., 2014; McGrath et al., 2014). Two pinning points, the Bawden Ice Rise and Gipps Ice Rise, anchor the northern and southern ice front, respectively. The ice thickness varies from ~1000 m near the grounding line to ~200 m near the ice front.