Suppressed basal melting in the eastern Thwaites Glacier grounding zone.

Thwaites Glacier is one of the fastest-changing ice-ocean systems in Antarctica1-3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat3,6, both of which are largely unknown. Here we show-using observations from a hot-water-drilled access hole-that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice-ocean boundary layer actively restrict the vertical mixing of heat towards the ice base7,8, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates.

Rapid iceberg calving following removal of tightly packed pro-glacial mélange.

Iceberg calving is a major contributor to Greenland's ice mass loss. Pro-glacial mélange (a mixture of sea ice, icebergs, and snow) may be tightly packed in the long, narrow fjords that front many marine-terminating glaciers and can reduce calving by buttressing. However, data limitations have hampered a quantitative understanding. We develop a new radar-based approach to estimate time-varying elevations near the mélange-glacier interface, generating a factor of three or more improvement in elevation precision. We apply the technique to Jakobshavn Isbræ, Greenland's major outlet glacier. Over a one-month period in early summer 2016, the glacier experienced essentially no calving, and was buttressed by an unusually thick mélange wedge that increased in thickness towards the glacier front. The extent and thickness of the wedge gradually decreased, with large-scale calving starting once the mélange mass within 7 km of the glacier front had decreased by >40%.

An Intensive Observation of Calving at Helheim Glacier, East Greenland.

Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the calving process at Helheim Glacier in East Greenland in an effort to better understand it. During an intensive one-week survey, we deployed a suite of instrumentation including a terrestrial radar interferometer, GPS receivers, seismometers, tsunameters, and an automated weather station. This effort captured a calving process and measured various glaciological, oceanographic, and atmospheric parameters before, during, and after the event. One outcome of our observations is evidence that the calving process actually consists of a number of discrete events, spread out over time, in this instance over at least two days. This time span has implications for models of the process. Realistic projections of future global sea level will depend on accurate parametrization of calving, which will require more sustained observations.