The majority of the Antarctic coast is fringed by ice shelves that are formed from the floating extensions of outlet glaciers and ice streams mixed with marine accreted ice, snow and firn. The calving front location (CFL) marks the seaward limit of the shelf, where the ice front meets the ocean. This edge is usually characterized by a steep ice cliff, often tens of meters high above the ocean surface, with the remaining ~90% of the ice shelf thickness concealed below. Its position is linked to climate and ocean forcing as well as internal ice dynamics and is subject to large fluctuations as the ice flows seaward, advancing the front, or as the ice front retreats following an iceberg breaking off. Iceberg calving from ice shelves, ice tongues and outlet glaciers is estimated to account for nearly half of the total volume of freshwater released from Antarctica [1
], yet is difficult to quantify exactly due to incomplete or a lack of observations.
In recent years and decades, large calving events have reshaped the Antarctic coastline considerably. These include the calving of iceberg B-15 [3
], the largest ever recorded, the calving of iceberg A-68 from Larsen-C in 2017 [4
] and the disintegration of several ice shelves on the Antarctic Peninsula, such as Larsen-A and Larsen-B [5
]. Calving, collapse and ungrounding of ice shelves from pinning points, i.e., rocky outcrops underneath the ice, can initiate a dynamic response that leads to increased ice export and sea level rise [8
]. While large episodic calving events draw much scientific and public interest, smaller events often go unnoticed despite being an important component of the ice shelf cycle. From the ice velocity (IV), ice thickness (H) at the calving front and a time sequence of CFLs, the iceberg calving rate can be computed. This can be used to estimate the background calving rate and total export of ice mass to the ocean. Therefore, monitoring the temporal evolution of the CFL is important, as a fundamental climate data record, indicator of changing boundary conditions or a precursor of dynamic imbalance. The CFL also serves as a practical boundary of the Antarctic coastal margin and is required as an input parameter for ice flow modelling, studies of calving processes and their driving forces, computing mass fluxes at the calving gate and ice shelf mass balance, as well as mapping glacier and ice shelf area change.
Baumhoer et al. [14
] provides an overview of satellite sensors and existing methods used to detect Antarctic glacier and ice shelf front dynamics. Calving fronts of glaciers and ice shelves are in general mapped using Synthetic Aperture Radar (SAR) and optical satellite sensors [15
]. These imaging instruments provide excellent spatial resolution for measuring the location of the ice edge, as the CFL can often clearly be discriminated due to marked differences in reflectance, spectral properties or texture of shelf ice and open water or sea ice. The use of active SAR is particularly valuable because it enables images to be acquired in all weather conditions and during the austral winter, when optical images would be obscured by clouds or have insufficient illumination. Furthermore, there is a stronger contrast between different types of ice (e.g., sea ice, fast ice and shelf ice) in SAR imagery, which are difficult to distinguish in optical imagery and allow ice shelf edges to be tracked more easily. To date, both manual and (semi-) automated techniques are applied to map CFLs in geocoded SAR and optical imagery, with manual delineation being the primary method. The automated approaches usually make use of some form of image segmentation followed by edge detection. More recently, advanced computer techniques have also been employed, including a method based on convolutional neural networks trained with manually determined calving fronts [17
Several studies have extracted Antarctic-wide calving fronts and coastlines which are available through online data portals. The Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database (ADD) provides an Antarctic coastline, based on a variety of datasets, with sections occasionally updated. Liu and Jezek [18
] and Jezek [20
] used the 1997 and 2000 RADARSAT-1 mosaics to extract the entire coastline of Antarctica through a sequence of automated image processing techniques. Scambos et al. [21
] also derived complete coastlines from MODIS-based mosaics of Antarctica (MOA) acquired in 2004, 2009 and 2014. The NASA Making Earth System Data Records for Use in Research (MEaSUREs) project also provided a coastline product for the entire Antarctic margin, including all ice shelves, derived from a variety of satellite radar interferometry data acquired during the International Polar Year (IPY 2007–2008) [2
]. Other studies have provided coastlines for selected smaller study regions (for example, [22
]). Baumhoer et al. [14
] consolidated these existing CFL products to map and assess patterns of glacier and ice shelf front changes around Antarctica between 1972/1975 and 2009/2015. They reported difficulties caused by data gaps in time and space, differences in temporal sampling and the variety of different methods necessitating undesirable assumptions. The authors expressed the need for more homogenized data sets, with regular and more frequent mapping intervals, that would permit the use of dynamic calving fronts instead of the often-used steady-state ice fronts for mass balance estimates. They also noted the added value height information would provide for extracting ice front positions.
Currently the majority of the Antarctic coast is continuously covered by the Copernicus Sentinel-1 satellite mission at 6 to 12 day repeat intervals. Nevertheless, gaps in current and historical Earth observation coverage remain, CFLs are only available at relative moderate resolution or require a combination of different data sets making subsequent glaciological interpretation challenging. Moreover, SAR and optical images are subject to changes in radar backscatter or spectral reflectance, caused by for example ice melt or changing solar illumination, complicating automated approaches.
Here, we present a novel approach for CFL detection based on swath mode processed satellite radar altimetry data acquired by CryoSat-2 [26
], between 2011 and 2018, which can fill existing temporal and spatial gaps, overcome some of the critical issues and meet some of the recommendations mentioned above. The method is also a new application for the CryoSat-2 mission that has not been previously exploited. The CFL detection is based on the premise that the ice edge is usually characterized by a steep cliff, with a steep drop of tens of meters towards the ocean surface or sea ice cover and which is clearly resolved in the elevation data. Our method, referred to here as the elevation-edge method, applies computational theory known as Canny edge detection and vectorization of the sharp ice edge in gridded elevation data for generating shapefiles of the CFL. Our main objective is to demonstrate the feasibility and added value of the approach. For this, we derived a contemporary update on calving front positions on the Filchner-Ronne Ice Shelf in East Antarctica allowing to measure change in ice shelf area and, in combination with ice shelf thickness measured from freeboard and ice velocity from Sentinel-1, iceberg calving rate between 2011 and 2018. The quality of the derived CFLs is assessed through intercomparison with manually derived CFLs from contemporaneously acquired SAR satellite data.
The Antarctic Ice Sheet has been portrayed as a barometer of climate change and is currently losing mass at an accelerating rate, and is expected to do so in the future, with significant implications for global sea-level rise [46
]. In order to accurately determine the future response of the Antarctic ice sheet to ongoing climate warming it is essential to quantify the major components of current mass loss and understand their principal drivers. Antarctic ice is primarily drained through the peripheral ice shelves surrounding most of the continent. Although these floating ice shelves do not contribute directly to sea-level rise, they play a critical role in providing resistance to flow from upstream grounded ice [8
]. It is therefore essential to accurately determine their mass budget. The iceberg calving rate forms a key element of the ice shelf mass budget, which is traditionally determined from estimates of the surface mass balance (net snow accumulation), basal melting and the mass flux divergence. The latter being determined from the ice flux at the grounding line (total solid ice inflow) and at the ice front (total solid ice outflow), based on measurements of ice thickness and ice velocity. In absence of detailed knowledge on the exact position of the calving front previous studies have relied on the steady-state calving approach as a proxy for iceberg calving, assuming a fixed position of the ice front [1
The combined steady-state calving flux calculated for RON and FIS between 2011 and 2018 is 222 ± 11 Gt a−1
, which compares well with previous estimates of 212 ± 17 Gt a−1
by Liu et al. [41
] for the period 2005–2011 and 221 ± 26 Gt a−1
by Rignot et al. [2
] for the period 2007–2008. Previous estimates of freshwater flux from FRIS have assumed these steady-state calving rates alone because time-dependant calving data were unavailable, thereby ignoring any changes in areal extent of the ice shelf. We show that such approach for 2011–2018 would overestimate the freshwater budget from FRIS by 206 Gt of ice or 224 km3
of freshwater every year. This example serves to illustrate that the steady front flux can be very different than the actual iceberg calving flux, particularly on sub-decadal time scales, and that CFLs should start to be more routinely monitored.
The observed CFLs and subsequent analysis of terminus position changes, and iceberg calving rates on FRIS, presented in this study, demonstrate the usability of swath elevation data for effectively, and continuously mapping the complete ice front position on one of the largest ice shelves in Antarctica. Compared with conventional CFL mapping techniques, such as by manual delineation using SAR or optical imagery, the elevation-edge method has several advantages and drawbacks to be noted.
Firstly, this new approach applies a single technique using data from only one sensor, with minimal manual intervention, which ensures a consistent data product that is not biased by subjective judgements. This facilitates the assessment of ice front migration by avoiding the need to merge different data sets derived by different techniques with different respective accuracies, therefore avoiding systematic offsets and image artefacts. As the ice shelf cliff height during the year or between different years is, arguably, less variable than radar backscatter and solar reflectance the method requires less manual intervention or post-processing making it more robust and easier to automate. Secondly the CryoSat-2 altimetry record runs, at time of writing, into its 10th year of mission operations and thus provides an extensive relatively long-term systematic record with short and regular mapping intervals, that permits study of sub-annual and longer term (inter-)annual variation for the first time. Thirdly, the high latitudinal limit of the CryoSat-2 data coverage is Antarctic-wide, with complete coverage over all known ice shelf calving fronts across the continent, providing an opportunity to derive regular calving front locations for ice shelves around Antarctica. Finally, the use of elevation data allows to simultaneously derive ice cliff position, height, thickness and calving volume, which is advantageous for estimating the total freshwater flux.
The principal weakness of the altimetry derived elevation-edge technique is its lower spatial resolution source data (DEMs) in comparison with higher resolution optical or SAR images (5 to 10 m) used for manual CFL mapping. In this study, we used a grid spacing of 200 m which is near the limit for the biannual DEMs based on the data density of CryoSat-2 swath data. Nevertheless, this is of a similar magnitude as MODIS data used for continent-wide coastline mapping [21
] and justified as the ice shelf is mostly relatively simple and flat terrain. The lower temporal resolution causes the ice edge to become blurred as changes occurring in the 6-month epochs are averaged. The use of a Lagrangian framework, as done in elevation change studies to remove this effect (for example, [35
]), would change the calving front position which is not desirable. The lower spatial resolution of our new technique might limit the applicability of it on smaller outlet glaciers and ice shelves, such as those on the Antarctic Peninsula. These glaciers are often situated in narrow fjords, which can be complex terrain for satellite altimetry [48
]. As is the case when using imagery, complex calving fronts with many fractures and/or icebergs/ice-melange require additional manual post-processing. Overall, the dramatically reduced time required to calculate the calving front for nearly a decade on the FRIS, vastly outweighs the stated limitations, making it possible for us to evaluate the validity of the steady-state calving approach, which has been the default ‘modelled’ method for all historical studies.
In this study, we have demonstrated the capability of CryoSat-2 radar altimetry data to map CFL and calving front migration of ice shelves, with a height of a several tens of meters, using time series of gridded elevation data from swath processing. We demonstrated the feasibility of the approach by deriving a unique data set of ice front positions for the Filchner-Ronne Ice Shelf between 2012 and 2018. Our results compare to within two to three pixels compared to manually derived ice front positions from Sentinel-1 SAR imagery and reveal an overall gradual advance of the entire ice front, interrupted only by a calving event on FIL in 2012/13 and several smaller-scale events. By combining our CFLs with ice velocity measurements from Sentinel-1 and ice thickness, we used this new data set to calculate area change and iceberg calving rates. We found that the Filchner-Ronne Ice Shelf is currently growing at a rate exceeding 800 km2 per year. Along flowlines, we measure an advance rate between 0.4 and 1.7 km a−1 and calculated calving rates of 7 ± 1 Gt a−1 and 9 ± 1 Gt a−1 for RON and FIL, respectively. This is of an order of magnitude less than their steady-state calving fluxes.
The novel elevation-edge approach outlined in this study is complementary to standard CFL detection methods. Although with its limitations regarding spatial resolution and applicability in ‘difficult’ terrain it has the potential to be applied for derivation of a homogeneous and regular sub-annual circum-Antarctic record based on one single sensor and uninterrupted for the CryoSat-2 mission lifespan. To our knowledge there is no other sensor or approach that is capable of this. Moreover, it enables existing gaps to be filled in regions and during periods with no or few observations from other sensors. The ability to derive of both CFL and ice thickness simultaneously from a single platform is another clear benefit facilitating applications such as iceberg calving rate and ice shelf mass balance calculations.
This new application of CryoSat-2, outlined in this article, will greatly benefit the investigations of environmental forcing on ice flow and terminus dynamics by providing accurate calving front locations and improved constraints for calving models. With the extension of the CryoSat-2 mission, this provides excellent opportunity for satellite radar altimetry to derive valuable new CFL data sets for monitoring climate change impacts in Antarctica.