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

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.