Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - RESPONDER (Resolving subglacial properties, hydrological networks and dynamic evolution of ice flow on the Greenland Ice Sheet)

Teaser

The Greenland Ice Sheet is losing mass at a growing rate and is today contributing to global sea level rise at a rate of 1 mm/year. The most severe changes occur in the drainage basins of marine-terminating glaciers, which flow rapidly and drain 88% of the ice sheet. The...

Summary

The Greenland Ice Sheet is losing mass at a growing rate and is today contributing to global sea level rise at a rate of 1 mm/year. The most severe changes occur in the drainage basins of marine-terminating glaciers, which flow rapidly and drain 88% of the ice sheet. The latest report by the Intergovernmental Panel on Climate Change concluded that the widespread acceleration of these glaciers in recent years was a response to interaction with the ocean; yet, observations have since shown that many of these glaciers respond to the growing volume of surface meltwater, which reaches the bed through fractures and conduits. The mechanism whereby these hydrological connections form is unknown, and there is a lack of observational constraints to assess how the basal drainage system responds when surface water is injected at the bed. This lack of knowledge is a major source of uncertainty in the current generation of ice sheet models used to predict global sea level rise.

The fundamental goal of RESPONDER is to understand how hydrological networks at the base of the Greenland Ice Sheet evolve over seasons and over multiple years, and how this evolution impacts on ice flow in the interior and at the coast. To address this goal, RESPONDER is monitoring ice flow while observing the hydrological networks that form in summer when the surface melts and are expanding due to climate change. The work in RESPONDER takes place on Store Glacier, which flows rapidly and discharges 30-40 million cubic metres of ice into the ocean every day. Specifically, the project is exploring englacial and basal conditions, using a pressurised hot-water drill to gain access to the bed and install instruments in boreholes.

The project has the following aims:

AIM 1 is to identify glaciological ‘hotspots’ and sites for subglacial access drilling and borehole exploration by tracking hydrological pathways beneath Store Glacier, a large marine-terminating glacier in Uummannaq Fjord, using novel geophysical imaging techniques and unmanned aerial vehicles (UAVs)

AIM 2 is to observe and quantify the hydrological networks of Store Glacier while measuring basal slip and strain within ice with probes and sensors installed in boreholes drilled at ‘coastal’ and ‘interior’ targets.

AIM 3 is to predict the co-evolution of ice flow and hydrological networks in the Store Glacier drainage basin, and assess the vulnerability of the Greenland Ice Sheet as a whole, by integrating field observations in state-of-the-art ice sheet models.

The work in RESPONDER is designed with three work packages and objectives formulated to address each aim.

Work performed

Work performed in the first half of the project includes the development of a 3D full-Stokes model of Store Glacier. The model is the first to incorporate the iceberg calving mechanism. Progress in numerical modelling also includes the incorporation of a new and more advanced scheme for basal hydrology and ice-ocean interactions.

Work has also included the discovery of cascading supraglacial lakes drainage, a powerful new mechanism which shows that lakes on the surface of the Greenland Ice Sheet can drain simultaneously in domino-type effect. Using a numerical model forced with a record of observed lake drainages, the team has showed that basal lubrication from the delivery of water from surface to bed when a lake drains can open fractures, which cause more lakes to drain and transfer yet more water to the bed of the ice sheet. The RESPONDER team has been able to confirm this mechanism by directly observing rapid lake drainages in Greenland.

The first polar expedition in RESPONDER was a 4-week field campaign in June and July 2017. The team installed instruments at two specific target sites in order to measure elevation change, rates of ice deformation and speed up in ice flow. The work also included novel UAV surveys on an ice sheet where there is no ground control. The second expedition took place in April and May 2018 when the subglacial conditions around each target site were surveyed using radio-echo sounding and seismic reflection techniques. The team also installed a network of continuously recording geophones in order to record and locate ice-quakes occurring at the glacier bed.

The third expedition took place in June and July 2018, when the team drilled multiple boreholes through 1-km-thick ice in order to make observations at the glacier bed. Sensor strings to measure englacial tilt and temperature were installed in two boreholes, together with basal probes to measure subglacial water pressure. Two other boreholes were drilled in order to sample subglacial material and to record ice stratigraphy with an optical televiewer. During this expedition, the team members also recorded how opening of fractures at the surface can cause rapid drainage of supraglacial lakes. The observational record from this unique event, captured by multiple UAV surveys, has produced new observational understanding of the lake drainage mechanism.

Final results

Progress includes:
1. A custom-designed Unmanned Aerial Vehicle (UAV) capable of surveying autonomously along pre-programmed flight paths for up to 1 hour and covering distances of 80-90 km. Progress beyond the state-of-the-art was achieved by fitting the UAV with a carrier-phase GNSS receiver, resulting in extremely accurate surveying. The impact from developing this UAV is a new technical ability to capture transient ice sheet dynamics from detection of localised surface uplift and ice flow acceleration occurring over timescales of hours to days.

2. The ability to drill multiple boreholes of length >1 km at fast-flowing glaciers. Over the past three years we have adapted our hot-water drilling equipment and techniques to enable rapid drilling to depth in demanding glaciological conditions. The impact of this ability is that we can now access and install experiments routinely in the englacial and basal environment of fast-flowing glaciers.

3. The ability to install digital borehole probes along stretchable cable. The impact of this advance is to enable longer-term records to be obtained from boreholes that are subject to substantial ice deformation, which breaks non-stretchable cables.

4. The ability to image ice sheet\'s internal geometry using autonomous phase-sensitive radio-echo sounding (ApRES) systems set up with antennae in multiple arrays. The impact from this novel technical advance is that ice deformation can be remotely sensed in 3D with unsurpassed temporal resolution (hours) over a whole year.

5. Observing and recording vertical ice deformation and basal melt rates using ApRES systems set up with a single antenna pair. The impact from this technical advance is that vertical ice deformation can be observed from the displacement of internal layers detected with extremely high spatial solution (millimetres) on timescales of months, years and longer.

6. A 3D finite element model of Store Glacier solving the full Stokes equation. This model is the first to feature a freely evolving 3D geometry in which icebergs break off the terminus when fractures extend downwards from the surface or upwards from the base. The model has been used to address the impacts from climate change. With a novel new scheme for basal hydrology and ice sheet interactions with the ocean, the model continues to offer technical advances beyond the state of the art.

Website & more info

More info: http://www.erc-responder.eu.