Melt rates in the kilometer-size grounding zone of Petermann Glacier, Greenland, before and during a retreat

Abstract

Warming of the ocean waters surrounding Greenland plays a major role in driving glacier retreat and the contribution of glaciers to sea level rise. The melt rate at the junction of the ocean with grounded ice—or grounding line—is, however, not well known. Here, we employ a time series of satellite radar interferometry data from the German TanDEM-X mission, the Italian COSMO-SkyMed constellation, and the Finnish ICEYE constellation to document the grounding line migration and basal melt rates of Petermann Glacier, a major marine-based glacier of Northwest Greenland. We find that the grounding line migrates at tidal frequencies over a kilometer-wide (2 to 6 km) grounding zone, which is one order of magnitude larger than expected for grounding lines on a rigid bed. The highest ice shelf melt rates are recorded within the grounding zone with values from 60 ± 13 to 80 ± 15 m/y along laterally confined channels. As the grounding line retreated by 3.8 km in 2016 to 2022, it carved a cavity about 204 m in height where melt rates increased from 40 ± 11 m/y in 2016 to 2019 to 60 ± 15 m/y in 2020 to 2021. In 2022, the cavity remained open during the entire tidal cycle. Such high melt rates concentrated in kilometer-wide grounding zones contrast with the traditional plume model of grounding line melt which predicts zero melt. High rates of simulated basal melting in grounded glacier ice in numerical models will increase the glacier sensitivity to ocean warming and potentially double projections of sea level rise.

Modeling Ice Melt Rates From Seawater Intrusions in the Grounding Zone of Petermann Gletscher, Greenland

“…We measure tidal motion with Interferometric Synthetic Aperture Radar (InSAR) data from the Earth Remote Sensing satellite ‐1 (ERS‐1), Sentinel‐1, CosmoSkyMed, and ICEYE (Ciraci et al., 2023; Millan et al., 2022). We distinguish three regions of vertical ice motion (Figure 1b): (a) The freely floating ice shelf, which experiences a vertical motion nearly in phase with the oceanic tide (Reeh et al., 2003) and of the same exact amplitude; (b) a FZ, which experiences a vertical tidal motion that decreases linearly with distance from the freely floating ice shelf and reaches zero at the grounding line; and (c) a zone of migration of the grounding line during the tidal cycle, or GZ.…”

“…If the grounding line is fixed in time, the GZ is less than one model element. Here, the GZ length varies from 1 to 6 km (Ciraci et al., 2023). Within the GZ, the vertical motion of the ice measured with radar interferometry is similar to that recorded in a FZ, that is, less than the tidal amplitude and typically a few centimeters to a few tens of centimeters.…”

“…We select a two‐dimensional (2D) section along the center line of Petermann Glacier (Figure 1c). Bathymetry is from a three‐dimensional (3D) inversion of high‐resolution gravity data with a nominal accuracy of ±60 m (An et al., 2019; Ciraci et al., 2023). Ice thickness is derived from a TANDEM‐X digital elevation model of the ice surface from year 2022 assuming hydrostatic equilibrium of the ice, with an accuracy of about 13 m (Ciraci et al., 2023).…”

“…Bathymetry is from a three‐dimensional (3D) inversion of high‐resolution gravity data with a nominal accuracy of ±60 m (An et al., 2019; Ciraci et al., 2023). Ice thickness is derived from a TANDEM‐X digital elevation model of the ice surface from year 2022 assuming hydrostatic equilibrium of the ice, with an accuracy of about 13 m (Ciraci et al., 2023). The GZ cavity is a rectangular cavity of one vertical ocean element, that is, 1 m here.…”

“…The GZ delineates the region of migration of the grounding line itself, which is equivalent to the migration of the inland limit of flexure. Until recently (Ciraci et al., 2023; Horgan et al., 2013; Parizek et al., 2013), it was thought that the GZ was narrow and therefore equivalent to a line. We have, however, now evidence for kilometer‐size grounding zones.…”