Disentangling the drivers behind the post-2000 retreat of Sermeq Kujalleq, Greenland (Jakobshavn Isbrae)

Abstract. Ocean temperatures have warmed in fjords surrounding the Greenland Ice Sheet, which is causing increased melt along their ice fronts, rapid glacier retreat, and contributes to rising global sea levels. However, there are many physical mechanisms which may mediate the glacier response to ocean warming and variability. Warm ocean waters can directly cause melt at horizontal and vertical ice interfaces or promote iceberg calving by weakening proglacial mélange or undercutting the glacier front. Sermeq Kujalleq (also known as Jakobshavn Isbræ) is the largest and fastest glacier in Greenland and has undergone substantial retreat starting in the late 1990s. In this study, we use a large ensemble modeling approach to disentangle the dominant mechanisms driving the retreat of Sermeq Kujalleq. Within this ensemble, we vary the sensitivity of three different glaciological parameters to ocean warming: frontal melt, subshelf melt and a calving stress threshold. Comparing results to the observed retreat behavior from 1985–2018, we select a best-fitting simulation which reproduces the observed retreat well. In this simulation, the arrival of warm water at the front of Sermeq Kujalleq in the late 1990s leads to enhanced rates of subshelf melt, leading to the disintegration of the floating ice tongue over a decade. Retreat into a substantially deeper bed trough around 2010 accelerates retreat, which continues nearly unabated despite local ocean cooling in 2016. An extended ensemble of simulations with varying calving threshold shows evidence of hysteresis in calving rate, which can only be inhibited by a substantial increase in calving stress threshold beyond values suggested for the historical period. Our findings indicate that accurate simulation of rapid calving-driven glacier retreats requires more sophisticated models of iceberg mélange and calving evolution coupled to ice flow models.