Introduction and Background

IPCC’s recent Special Report on Ocean and Cryosphere in a Changing Climate (SROCC) addresses rapidly increasing sea-level contribution from the Antarctic Ice Sheet. The lack of ice thickness data at the margin of the ice sheet (grounding zone) is pointed out as one of the main sources of uncertainty for accurate estimation of Antarctic ice discharge. It is also the location where the bed topography matters the most as it controls the stability of the grounding zone. There is therefore an urgent need to carry out airborne surveys around the entire Antarctic Ice Sheet margin.

The ice discharge of the Antarctic Ice Sheet to the ocean can be calculated by a combination of ice thickness data and satellite-measured ice flow speed near the margin. While satellites such as Sentinel-1 and Landsat-8 can routinely measure ice-flow speed, limited knowledge of ice thickness leads to large uncertainties in ice discharge and eventually in overall assessments of Antarctic mass balance, estimated as the difference between ice discharge and mass input (snowfall) to the entire ice sheet. Mass balance estimated in this way currently has large discrepancies from other estimates based on gravity changes detected by GRACE/GRACE-FO satellites or ice-elevation changes detected by the CryoSat-2 and ICESat-2 satellites.

Ice thickness changes with time. However, once bed topography is measured using ice-penetrating radar with high precision and positioning control, ice thickness in the future can be monitored using ice sheet surface elevations measured with satellite altimetry missions such as CryoSat-2 and ICESat-2. According to existing BEDMAP and BedMachine compilations, ice thickness for more than half of the Antarctic margin remains inadequately known for estimating ice discharge. Also, availability of bed topography data is not uniform around Antarctica. Even for glaciers that are well studied, data are not always available continuously along the margin. This is because radar data are often collected along the ice flowline, rather than across the glacier. Compiling individual datasets collected for different purposes with different standards is a pragmatic, but not an ideal solution. Systematic collection of new radar data in the vicinity of the margin specifically aiming for ice-discharge estimates is a crucial step to monitor the current status, and predict the future, of the Antarctic Ice Sheet.

Aims, Goals and Objectives

To fill this major knowledge gap across all the margins of Antarctica for the first time, the Antarctic RINGS Action Group will be an ambitious and challenging initiative in a truly international cooperation. The primary goal is to provide more accurate and complete reference bed topography data for robust assessments of ice discharge from all around Antarctica. This dataset will also tremendously improve the accuracy of ice-sheet models by providing a better mapping of the grounding zone. When this dataset is combined with satellite altimetry data over the entire ice sheet, it can also be used to constrain mass input from snowfall to Antarctica, which is currently estimated with regional climate models but hardly validated over a large region. The secondary goals are (1) to better constrain the likelihood and rates of predicted future retreat of the ice-sheet margin by determining basal boundary conditions in adjacent inland areas, (2) to better quantify ice-ocean interactions by providing novel knowledge of the bathymetry of the cavity beneath adjacent ice shelves, and (3) to perceive subglacial hydrology for constraining basal mass balance of the ice sheet and subglacial geology particularly relevant to sediments and heat flow. To reach these objectives, it is necessary to complete not only the primary ring at the margin, but also a seaward ice-shelf ring and a landward ring, using radar, gravity, magnetics and lidar instruments altogether.

Proposed Milestone Activities with Timeline

The RINGS group will first analyze location data of BedMachine’s input radar data and soon-to-be-available BEDMAP3 open-data depositary. This will lead to the development of a set of protocols followed by future surveys. Available logistical resources and relevant constraints will be considered to examine first-order survey options. This process is interactive and will start with an international workshop, followed up with online meetings of smaller groups. This process also serves as a discussion forum to develop survey plans to balance the primary and secondary objectives of RINGS in the most efficient manner.

There are four milestones M1-M4:

Terms of Reference

The RINGS Action Group will work for two years to facilitate community efforts to fill critical knowledge gaps in the Antarctic Ice Sheet margins to monitor and predict the future of the Antarctic Ice Sheet. As a SCAR Action Group, RINGS aims to:

• Define knowledge gaps of geophysical datasets in the ice-sheet margins.
• Assess impacts of efforts to fill these knowledge gaps at different levels on estimates of ice discharge from Antarctica and on predicting the future of the ice sheet.
• Develop feasible plans to fill these knowledge gaps by generating interdisciplinary, and international synergies.