Non-linear retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

Abstract

Abstract. Rapid acceleration and retreat of Greenland's marine-terminating glaciers during the last two decades have initiated questions on the trigger and processes governing observed changes. Destabilization of these glaciers coincides with atmosphere and ocean warming, which broadly has been used to explain the rapid changes. To assess the relative role of external forcing versus fjord geometry, we investigate the retreat of Jakobshavn Isbrae in West Greenland, where margin positions exist since the Little Ice Age maximum in 1850. We use a one-dimensional ice flow model and isolate geometric effects on the retreat 5 using a linear increase in external forcing.We find that the observed retreat of 43 km from 1850 until 2014 can only be simulated when multiple forcing parameterssuch as hydrofracturing, submarine melt and frontal buttressing by sea ice-are changed simultaneously. Surface mass balance, in contrast, has a negligible effect. While changing external forcing initiates retreat, fjord geometry controls the retreat pattern.Basal and lateral topography govern shifts from temporary stabilization to rapid retreat, resulting in a highly non-linear glacier 10 response. For example, we simulate a disintegration of a 15 km long floating tongue within one model year, which dislodges the grounding line onto the next pinning point. The retreat pattern loses complexity and becomes linear when we artificially straighten the glacier walls and bed, confirming the topographic controls.For real complex fjord systems such as Jakobshavn Isbrae, geometric pinning points predetermine grounding line stabilization and may therefore be used as a proxy for moraine build-up. Also, we find that after decades of stability and with constant 15 external forcing, grounding lines may retreat rapidly without any trigger. This means that past changes may precondition marine-terminating glaciers to reach tipping-points, and that retreat can occur without additional climate warming. Present-day changes and future projections can therefore not be viewed in isolation of historic retreat.

Twenty-first century response of Petermann Glacier, northwest Greenland to ice shelf loss

“…In addition to the role of bed topography, channel width can also modulate grounding line retreat (Jamieson and others, 2012; Åkesson and others, 2018), and has been identified as a key control on the retreat of numerous glaciers using both modern (e.g. Carr and others, 2014; Steiger and others, 2018; Catania and others, 2018) and palaeo records (Jamieson and others, 2014; Stokes and others, 2014). PGIS is well-confined within its narrow fjord, and hence, its collapse leads to a loss of lateral resistive forces and buttressing.…”

Impact of Fjord Geometry on Grounding Line Stability

“…Studies of Antarctic ice streams (Jamieson et al, 2012(Jamieson et al, , 2014 as well as outlet glaciers in Greenland (Warren and Glasser, 1992;Carr et al, 2013bCarr et al, , 2014Steiger et al, 2017), Patagonia (Warren, 1993), and Alaska (O'Neel et al, 2005) have highlighted channel width as an important factor controlling retreat rates and stability. We build on these studies by showing the striking impact of fjord width on grounding line retreat in a systematic, synthetic framework, and on time scales longer than have been studied in the aforementioned studies.…”

“…Embayments are also common where a tributary glacier and the main trunk coalesce. For example, this situation is plausible for Jakobshavn Isbrae during the Little Ice Age, when a northern tributary may have become disconnected from the main channel, forming separate calving fronts and a wide open embayment (Steiger et al, 2017). A similar scenario holds true for Eqalorutsit Kangigdlit Sermia (EKS) in south Greenland, where the lower end of a formerly ice-filled valley now has become a wide embayment downstream of the present calving front (west-east oriented valley to the right in Figure 8b).…”

Basal drag of Fleming Glacier, Antarctica, Part B: implications of evolution from 2008 to 2015

“…due to detachment from a pinning point), as in the rapid retreat of Jakobshavn Isbrae in West Greenland (Steiger et al, 2017). The grounding line in 2008 may have experienced a retreat after moving across the geometric pinning band near the front, and then retreated further to the position in 2015 about 9 km upstream in the FGL downstream basin.…”