A Twin Otter airplane operated by the British Antarctic Survey leaves an inland camp to collect radar and gravity data around the South Pole in December 2015 as a part of the PolarGAP project. Credit: Kenichi Matsuoka
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By Kenichi Matsuoka, René Forsberg, Fausto Ferraccioli, Geir Moholdt and Mathieu Morlighem.
15 June 2022
An international initiative aims to collect a comprehensive airborne data set from the Antarctic Ice Sheet margin to better estimate ice discharge and sea level contributions today and in the future.
Most of Earth’s land surfaces have been mapped in great detail—the courses of rivers, outlines of ocean shores, mountain heights, and valley depths. Antarctica’s topography is a notable exception. Maps of the ice-covered land surface of this continent still show many areas that are defined vaguely or not at all.
Accurate projections of the future evolution of the Antarctic Ice Sheet are essential for mitigating potential risks to people and infrastructure along continental coastlines and on low-lying islands.
The Antarctic Ice Sheet (AIS) is the largest freshwater mass on Earth, holding a volume of water equivalent to 58 meters of global sea level rise (about the height of a 15-story building). A net loss of even just 1% of the AIS—not implausible by 2100—would raise sea levels by more than half a meter, enough to inundate vast stretches of land, especially when combined with the effects of melting elsewhere [DeConto et al., 2021]. Accurate projections of the future evolution of the ice sheet are essential for mitigating potential risks to people and infrastructure along continental coastlines and on low-lying islands.
In its most recent assessment report, the Intergovernmental Panel on Climate Change (IPCC) made unequivocal statements about the societal implications of recent and future net loss of Antarctic ice.
Another recent IPCC report identified major knowledge gaps that impede scientists’ ability to estimate Antarctic ice loss and contributions to sea level change, including the lack of knowledge about the shape of the subglacial land surface near the ice sheet edge, or margin. Filling this gap all around the 62,000-kilometer-long margin of the AIS (which is longer than Earth’s circumference) is too big a challenge for any one country alone. Driven by the need for an internationally collaborative approach for studying the AIS’s margins, the Scientific Committee on Antarctic Research (SCAR), a coordinating body and thematic organization of the International Science Council, established the RINGS Action Group, in 2021.
The immediate goal of this initiative is to develop plans for a comprehensive airborne geophysical survey encircling Antarctica in three concentric rings. These new measurements will improve our knowledge of the bed topography beneath the ice sheet close to the grounding line (where ice on land meets the ocean and begins to float; Figure 1), where it matters most for the stability of the ice sheet today and in the future. This information will help constrain ice discharge—the mass removed through the flow of glaciers to the ocean. Improved estimates of ice discharge (mass output), when combined with modeled rates of snow accumulation on the continent (mass input), will help better constrain the snow-ice mass balance (input minus output) and Antarctica’s net contribution to sea level rise.
By, National Snow and Ice Data Center, NASA.
The Landscape Below the Ice Sheet.
The interplay between deep (tectonic) and shallow (glacial and fluvial) Earth processes has created complex bed topography across Antarctica. Rivers and streams eroded Antarctic bedrock before the AIS formed 34 million years ago, after which glacial processes eroded it further. Between major glaciated periods, rivers and streams resumed their erosion [Paxman et al., 2019].
Today, this topography—buried under as much as 4 kilometers of ice in places—includes mountain ranges, flat-lying and sediment-filled regions, and deep fjords. Denman Glacier in East Antarctica, for example, flows through a tectonically controlled trough that has sidewall slopes exceeding 40° and that reaches a depth of 3.5 kilometers below sea level, the deepest ice-covered bed in Antarctica.
Thwaites Glacier in West Antarctica, in contrast, flows over a gently sloping (a few degrees) lowland basin mostly lying about 800 meters below sea level, although it begins to flow fast over the deep Byrd Subglacial Basin, another tectonic structure that lies farther inland.
The bed topography around Antarctica’s margins, which fundamentally affects the dynamics of ice flow and the vulnerability of glaciers and ice shelves, remains poorly known.
Both Denman and Thwaites Glaciers flow at speeds of about 2 kilometers per year or faster and discharge considerable amounts of ice into the ocean. The distinct bed topography under these glaciers—and, indeed, all around Antarctica’s margins—fundamentally affects both the dynamics of ice flow (i.e., speed, direction, and discharge rate) and the vulnerability of glaciers and ice shelves to ongoing atmospheric and oceanic changes.
Nonetheless, bed topography remains poorly known for coastal regions of the AIS because of limitations in the capacity and coverage of geophysical surveys. This knowledge gap must be filled urgently, considering predictions of future ice loss from Antarctica are largely linked to ocean-triggered retreat of the grounding line.
Geophysical Surveys, Then and Now.
In the early stages of Antarctic scientific expeditions, researchers measured ice thickness and bed topography primarily through active seismic surveys using dynamite. During the International Geophysical Year in 1957–1958, a less destructive approach emerged from radar surveys of the ionosphere over Antarctica.
Radar echoes were observed not only from the sky but also, unexpectedly, from the bottom of the ice [Turchetti et al., 2008], demonstrating the potential of radar to sound ice thickness and bed topography. Within a decade, international airborne radar surveys were routinely carried out to explore large parts of Antarctica. These data were also recently used as a reference to measure changes in ice thickness over 4 decades [Schroeder et al., 2019].
Photo by René Forsberg
We are not yet capable of measuring ice thickness and bed topography with satellite-based instruments, so large-scale bed topography under the ice is still measured by airborne radar.
Local features are also measured by ground-based radar surveys. However, radar does not always detect the bed clearly in challenging cases, such as when the ice surface is highly crevassed or when the ice lies in deep and steep valleys. In such cases, gravity data measured from airplanes can be inverted to estimate bed elevation and ice thickness. Gravity data are also a key method to infer bathymetry under floating ice shelves because radio waves cannot penetrate seawater. Inversion of airborne gravity data, however, provides only a low-resolution view (~5–10 kilometer) of bed topography and, given our lack of knowledge about Antarctic bedrock density, can be prone to sometimes large biases.
In 2001, the British Antarctic Survey compiled the first digital elevation model of the Antarctic bed, called BEDMAP, using radar, gravity, and seismic data. An update in 2013, BEDMAP2, refined estimates of the ice volume in Antarctica [Fretwell et al., 2013]. BEDMAP2 is a key data set for ice flow modeling and ice discharge calculations.
When BEDMAP2 was released, two regions of the continent were identified as “poles of ignorance,” where no radar data were available within 200 kilometers. These areas were recently surveyed during reconnaissance campaigns, but the data density from these surveys is limited [Cui et al., 2020; Jordan et al., 2018].
Combining bed elevation data with satellite-measured ice flow speeds, ice elevations from satellite altimetry, and estimates of surface mass balance allows researchers to infer bed elevations elsewhere.
The most recent map of bed topography under the AIS is based on an approach called BedMachine that applies the principle of conservation of mass to interpret available bed elevation data [Morlighem et al., 2020]. Combining bed elevation data with satellite-measured ice flow speeds, ice elevations from satellite altimetry, and estimates of surface mass balance (snowfall minus sublimation and melt) from regional climate models allows researchers to infer bed elevations elsewhere.
This approach revealed many topographic features that were poorly resolved or missing in the BEDMAP2 compilation, which applied geospatial interpolation algorithms to estimate topography between data points.
However, BedMachine is limited by uncertainties in satellite observations and climate models, and it does not work well in regions where ice flows slower than 20 meters per year because of inadequate accuracy in mass flux directions. These limitations mean that this scheme works well for only about 10% of the AIS. Also, surface mass balance is difficult to model accurately over rough terrain, particularly in coastal regions.
In addition to counting individual mass input and output terms to determine net Antarctic mass balance, this balance is also estimated using data from satellite gravity (e.g., Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO)) and altimetry (e.g., CryoSat-2 and Ice, Cloud and land Elevation Satellite 2 (ICESat-2)) missions. Results of these three methods have been reconciled for an ensemble estimate [Shepherd et al., 2018]. However, like their airborne counterparts, these satellite-derived methods have limitations.
For example, the gravity data have relatively low spatial resolution (200–300 kilometers), and a significant portion of the measured gravity signal is caused by past ice changes. After the ice sheet thins (or retreats entirely), the underlying lithosphere adjusts to the lighter load by rising, but typically at a rate of a few centimeters per year or less, creating a time lag and associated bias in the data. Altimetry satellites, on the other hand, measure ice surface elevation changes with much higher spatial resolutions, but converting altimetric height changes to ice mass changes requires knowledge of snow density near the ice sheet surface, which is highly variable and hard to model.
The shortcomings of all the available methods reinforce the need to better constrain individual mass input and output terms and thus the need for new high-resolution reference bed topography data.
Finding and Filling the Data Gaps.
When ice is discharged across the grounding line to the ice shelf, it contributes directly to sea level rise. Quantifying this ice flux requires accurate measurements of ice thickness and flow speed at the grounding line.
Nearly three quarters of the AIS’s margin comprises floating ice shelves (Figure 1). When ice is discharged across the grounding line to the ice shelf, it contributes directly to sea level rise. Quantifying the ice flux from the ice sheet to the ocean therefore requires accurate measurements of ice thickness and flow speed at the grounding line.
Whereas ice thickness can change relatively rapidly, the elevation of the bed does not change significantly over decadal timescales. So once the bed topography is measured with high precision using ice-penetrating radar, future thickness changes of the grounded ice can be monitored with ice sheet surface elevations measured by satellite altimetry.
In an analysis in support of the RINGS initiative, we surveyed the availability of radar data in the vicinity of the grounding line using the data submitted to the BedMachine compilation. Historical data collected prior to the GPS era have relatively low positioning confidence, so we considered only modern data collected after 2007, which constitute 67 million data points spread across the AIS (Figure 2).
Our analysis revealed no proper data coverage in coastal regions. Only 12% of the grounding line is within 1 kilometer of a radar data point; nearly 50% of the grounding line is not within 6 kilometers of a data point, and about 28% of the grounding line is not within 20 kilometers of a data point. Data coverage is better for fast-flowing glaciers than for slow-moving ice in many regions, but even for glaciers that are well studied, data are not always available continuously along the margin. The reason is that radar data are often collected along ice flow lines, rather than across the glacier, for ice flow modeling purposes.
The rapidly changing Amundsen Embayment in West Antarctica has the highest data availability, yet still nearly 23% of the grounding line in the region near the embayment is not within 5 kilometers of a data point. We thus did not identify any regions where adequate data exist for accurate mass discharge calculations. To enable robust estimates of current and future ice discharge flux using satellite data, comprehensive reference bed elevation data are needed along the entire margin of the ice sheet.
The RINGS initiative will facilitate collection of these data from the coastal regions all around Antarctica.
A Three-Ring Survey.
RINGS aims to organize individual efforts to carry out three vital rings of pan-Antarctic surveying. The primary survey will follow the grounding line around the perimeter of Antarctica, and the other two will cover the seaward and landward sides of the grounding line.
Bed topography landward of the grounding line determines subglacial hydrology, which can in turn affect glacier dynamics and the location of meltwater runoff outlets. Nearly half of the base of the AIS melts because of geothermal and frictional heating. Although melting rates at the base of the AIS are low (a few centimeters per year or less), the total volume of subglacial meltwater is substantial. This meltwater lubricates the base of the ice sheet—affecting the dynamics of fast-flowing glaciers that largely control regional mass balance—before draining into the ocean.
Detailed topography upstream of the grounding line is also needed to predict future grounding line retreat [Pattyn and Morlighem, 2020]. Such retreat could happen rapidly—and could represent major glacial tipping points in coastal Antarctica—during so-called marine ice sheet instabilities in which the bed is below sea level and ice recedes down a retrograde slope (i.e., the ground level is lower inland than it is nearer to shore; Figure 1). On such slopes, however, small topographic bumps can act as docking, or anchor, points that slow glacier retreat, so it’s important to know whether and where such bumps exist.
The mass balance of the ice shelves is a key indicator of potential near-future grounding line retreat.
Seaward of the grounding line, floating ice shelves around the ice sheet buttress and slow ice flow and, therefore, ice discharge into the ocean. As relatively warm ocean water underneath these ice shelves melts and thins them (Figure 1), their impeding effects on ice sheet flow decrease, which may accelerate ice discharge and cause ice sheet retreat. The mass balance of the ice shelves is thus a key indicator of potential near-future grounding line retreat. Revealing oceanic melt processes requires detailed knowledge of seabed bathymetry under the ice shelves and heavily packed sea ice, areas where research vessels equipped to map bathymetry from the sea surface have limited access.
Many studies have approximated ice thickness at the grounding line to quantify ice discharge fluxes there on the basis of ice shelf thicknesses immediately seaward of the grounding line, which can be estimated using satellite altimetry data and assuming hydrostatic equilibrium (i.e., the ice shelves float freely on the ocean).
This approach is problematic for three main reasons. First, ice shelves are often thickest near the grounding line, and ocean-induced melt is greatest there, so there are typically large gradients in ice thickness immediately seaward of the grounding line. Second, the assumption of hydrostatic equilibrium is not strictly valid in many locations because of incomplete ice flexure near the grounding line. Third, limited knowledge of the thickness and density of partially compacted snow (firn) complicates estimations of ice thickness from ice surface elevation data.
The net effect of these three factors has barely been evaluated, creating a knowledge gap that hinders accurate estimation of ice discharge and ocean-induced melting, which could, in turn, affect assessments of the potential risks of future rapid grounding line retreat. Improved data on grounding line topography collected by RINGS will help fill this gap.
An International, Interdisciplinary Effort
To maximize the efficiency of and information learned from costly airborne operations, it will be essential to collect complete geophysical measurements when RINGS surveys are carried out. Such measurements include those from both deep-sounding ice-penetrating radar and shallow-sounding firn radar to map recent surface mass balance history, as well as gravity and magnetic measurements. Gravity measurements are needed to infer seabed bathymetry via data inversion. Gravity and magnetic measurements together will help constrain subglacial geology (e.g., lithology) and geothermal flux heterogeneity. In coastal regions, these data will also help tie outcrop-scale geological studies together into a larger regional picture.
Complementary offshore surveys by research vessels and uncrewed underwater vehicles are also required to ground truth seabed bathymetry inversions from airborne gravity data and to gain seamless, high-resolution seabed bathymetry data needed to model ocean currents and melting under ice shelves.
Completing the RINGS surveys will require an international, pan-Antarctic collaboration that integrates existing and novel technical and logistical capabilities of numerous countries.
Completing the RINGS surveys will require an international, pan-Antarctic collaboration that integrates existing and novel technical and logistical capabilities of numerous countries. Recent regional geophysical surveys, such as PolarGAP around the South Pole [Jordan et al., 2018], ROSETTA over the Ross Ice Shelf [Tinto et al., 2019], and ICECAP (International Collaborative Exploration of Central East Antarctica through Airborne geophysical Profiling) in Princess Elisabeth Land [Cui et al., 2020], have demonstrated the efficiency and advantages of small, international, and interdisciplinary teams. Such regional surveys, designed to meet common protocols established under the RINGS initiative, are best for characterizing individual study regions and will be key in meeting RINGS’s objective. However, the conventional use of small airplanes like Twin Otters and Baslers, which have relatively limited flight ranges from existing research stations, will need to be supplemented by long-range airplanes that will cover data gaps farther from research stations and help close the rings.
There is much planning still to do before these new surveys begin. In late June, the RINGS Action Group will lead a first workshop to discuss science priorities, survey requirements, data policy, and future survey plans. Meanwhile, some international projects are already being developed to carry out the first surveys during the 2023–2024 Antarctic field season and to provide early opportunities to refine RINGS guidelines and requirements with real-world lessons.
When completed, the RINGS surveys will provide a key missing reference data set for the international research community, giving us a clear picture of what coastal Antarctica looks like below its icy blanket. Combined with satellite monitoring of ice flow and numerical models, RINGS will also underpin more accurate longer-term predictions of ice sheet behavior and stability and of how Antarctica will contribute to global sea level rise and the displacement of coastal communities in the decades to come.
Acknowledgments.
We acknowledge David Bromwich, the chief officer of SCAR’s Physical Science Group, for presenting the RINGS initiative and the SCAR national delegates who approved it as a new Action Group in 2021 under exceptional conditions caused by the COVID-19 pandemic. SCAR’s Action Group RINGS welcomes new members to join at any time. Participant registration for virtual attendance of the first RINGS workshop is open until 25 June 2022.
Author information.
Kenichi Matsuoka (kenichi.matsuoka@npolar.no), Norwegian Polar Institute, Tromsø; René Forsberg, National Space Institute, Technical University of Denmark, Lyngby; Fausto Ferraccioli, National Institute of Oceanography and Applied Geophysics, Trieste, Italy; also at British Antarctic Survey, Cambridge, U.K.; Geir Moholdt, Norwegian Polar Institute, Tromsø; and Mathieu Morlighem, Dartmouth College, Hanover, N.H.
Citation: Matsuoka, K., R. Forsberg, F. Ferraccioli, G. Moholdt, and M. Morlighem (2022), Circling Antarctica to unveil the bed below its icy edge, Eos, Published on 15 June 2022.
