Modeling the response of northwest Greenland to enhanced ocean thermal forcing and subglacial discharge

Abstract

Abstract. Calving-front dynamics is an important control on Greenland's ice mass balance. Ice front retreat of marine-terminating glaciers may, for example, lead to a loss in resistive stress, which ultimately results in glacier acceleration and thinning. Over the past decade, it has been suggested that such retreats may be triggered by warm and salty Atlantic Water, which is typically found at a depth below 200–300 m. An increase in subglacial water discharge at glacier ice fronts due to enhanced surface runoff may also be responsible for an intensification of undercutting and calving. An increase in ocean thermal forcing or subglacial discharge therefore has the potential to destabilize marine-terminating glaciers along the coast of Greenland. It remains unclear which glaciers are currently stable but may retreat in the future and how far inland and how fast they will retreat. Here, we quantify the sensitivity and vulnerability of marine-terminating glaciers along the northwest coast of Greenland (from 72.5 to 76∘ N) to ocean forcing and subglacial discharge using the Ice Sheet System Model (ISSM). We rely on a parameterization of undercutting based on ocean thermal forcing and subglacial discharge and use ocean temperature and salinity from high-resolution ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II) simulations at the fjord mouth to constrain the ocean thermal forcing. The ice flow model includes a calving law based on a tensile von Mises criterion. We find that some glaciers, such as Dietrichson Gletscher or Alison Glacier, are sensitive to small increases in ocean thermal forcing, while others, such as Illullip Sermia or Cornell Gletscher, are remarkably stable, even in a +3 ∘C ocean warming scenario. Under the most intense experiment, we find that Hayes Gletscher retreats by more than 50 km inland by 2100 into a deep trough, and its velocity increases by a factor of 3 over only 23 years. The model confirms that ice–ocean interactions can trigger extensive and rapid glacier retreat, but the bed controls the rate and magnitude of the retreat. Under current oceanic and atmospheric conditions, we find that this sector of the Greenland ice sheet alone will contribute more than 1 cm to sea level rise and up to 3 cm by 2100 under the most extreme scenario.

Twenty-first century ocean forcing of the Greenland ice sheet for modelling of sea level contribution

“…Fig. 3c) into fjords and below the present-day ice sheet by taking into account ocean bathymetry and subglacial topography in the same manner as Morlighem et al (2019), based on the BedMachine v3 topography (Morlighem et al, 2017). Specifically, for each location in a fjord and beneath the present-day ice sheet, the deepest point that is openly connected to the wider ocean is determined; this depth is hereafter termed the effective depth.…”

“…The submerged area will change over the course of the ice sheet model simulations as the termini retreat through fjords of various depths and widths. Since dynamically calculating the submerged area is difficult within an ice sheet model, we assume that the submerged area of each terminus remains constant at presentday values (see Morlighem et al, 2019) but highlight this as an area for improvement in future efforts. The presentday submerged surface area is calculated based on presentday calving front position and bed topography as defined by BedMachine v3 (Morlighem et al, 2017).…”

“…This implementation has the advantage that the important interactions between submarine melting, calving, ice dynamics and bed topography can be resolved by the model (e.g. Aschwanden et al, 2019;Morlighem et al, 2019). The disadvantages are that there is large uncertainty in the submarine melt rates obtained from the parameterisation and we still lack a good understanding of and parameterisation for calving.…”

“…In contrast, the NW region has the smallest projected retreat but has a large number of tidewater glaciers that currently account for ∼ 20 % of Greenland's ice flux (Table S1), and this region is much more likely to be a significant dynamic contributor to sea level. Within the submarine melt implementation there is also the possibility for non-linear or threshold response of glaciers to submarine melting, where small changes in forcing may result in large excursions in terminus position and mass loss (Morlighem et al, 2019).…”

“…Given the described process uncertainty, the small scale of key processes and the current lack of parameterisations for these processes, projecting ocean-induced ice mass loss from the Greenland ice sheet is very challenging. To date, attempts to project future ice discharge from tidewater glaciers have often relied on extrapolation from a few glaciers to the whole ice sheet Nick et al, 2013;Peano et al, 2017;Beckmann et al, 2019;Morlighem et al, 2019) or have employed ad hoc methods to mimic the impact of ocean forcing that are not easily relatable to climate warming scenarios (Price et al, 2011;Bindschadler et al, 2013;Fürst et al, 2015). In a single ice sheet model, a significant advance was recently made by Aschwanden et al (2019), who ran full ice sheet projections that resolve tidewater glaciers and were forced by estimated submarine melt rates, but many of the ice sheet models taking part in ISMIP6 do not currently have the resolution or technical capability for this approach (Goelzer et al, 2018).…”