Change in Antarctic Ice Shelf Area from 2009 to 2019

https://doi.org/10.5194/tc-17-2059-2023

© Author(s) 2023. This work is distributed under the Creative Commons Attribution 4.0 License.

Julia R. Andreasen, Anna E. Hogg, and Heather L. Selley

Abstract

Antarctic ice shelves provide buttressing support to the ice sheet, stabilising the flow of grounded ice and its contribution to global sea levels. Over the past 50 years, satellite observations have shown ice shelves collapse, thin, and retreat; however, there are few measurements of the Antarctic-wide change in ice shelf area. Here, we use MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data to measure the change in ice shelf calving front position and area on 34 ice shelves in Antarctica from 2009 to 2019. Over the last decade, a reduction in the area on the Antarctic Peninsula (6693 km2) and West Antarctica (5563 km2) has been outweighed by area growth in East Antarctica (3532 km2) and the large Ross and Ronne–Filchner ice shelves (14 028 km2). The largest retreat was observed on the Larsen C Ice Shelf, where 5917 km2 of ice was lost during an individual calving event in 2017, and the largest area increase was observed on Ronne Ice Shelf in East Antarctica, where a gradual advance over the past decade (535 km2 yr−1) led to a 5889 km2 area gain from 2009 to 2019. Overall, the Antarctic ice shelf area has grown by 5305 km2 since 2009, with 18 ice shelves retreating and 16 larger shelves growing in area. Our observations show that Antarctic ice shelves gained 661 Gt of ice mass over the past decade, whereas the steady-state approach would estimate substantial ice loss over the same period, demonstrating the importance of using time-variable calving flux observations to measure change.

How to cite.  Andreasen, J. R., Hogg, A. E., and Selley, H. L.: Change in Antarctic ice shelf area from 2009 to 2019, The Cryosphere, 17, 2059–2072, https://doi.org/10.5194/tc-17-2059-2023, 2023.

Received: 12 Oct 2022– Discussion started: 01 Nov 2022– Revised: 10 Mar 2023– Accepted: 08 Apr 2023– Published: 16 May 2023

1 Introduction

Ice shelves fringe three-quarters of the Antarctic coastline, providing buttressing support to the grounded ice and linking the ice sheet with the Southern Ocean. The calving front represents the seaward limit of the ice shelf edge and is the boundary of the Antarctic coastal margin. The calving front location (CFL) can change gradually through sustained growth or retreat (Cook and Vaughan, 2010) or more suddenly due to large events such as iceberg calving (Hogg and Gudmundsson, 2017) and ice shelf collapse (Rott et al., 1996; Rack and Rott, 2004; Padman et al., 2012). Mapping the time-variable calving front location on Antarctic ice shelves is important (i) for estimating the total ice shelf freshwater budget, (ii) as a precursor for dynamic instability and therefore ice sheet sea level contribution, (iii) as an indicator of changing ice shelf structural conditions, and (iv) as a proxy for changing ocean and atmospheric forcing. Satellite observations have shown that a reduction in ice shelf area can cause upstream glaciers to thin (Scambos et al., 2004) and accelerate by up to 8 times their previous speed (Rignot et al., 2004), increasing the ice dynamic sea level contribution from the affected region. Some zones of floating ice provide significantly more structural stability to the ice sheet, with ice inland of the compressive arch or in contact with a pinning point triggering instability if lost (Holland et al., 2015). The effect of change in ice shelf area is not always local, with studies showing that ice shelves provide far-reaching buttressing support to grounded ice hundreds of kilometres away (Fürst et al., 2016). However, many iceberg calving events form part of the natural cycle of ice shelf evolution, with the steady regrowth and advance of the calving front typically seen after a calving event (Hogg and Gudmundsson, 2017).

Over the past 30 years, ice shelves across Antarctica have been observed to advance steadily, retreat after iceberg calving events, and collapse catastrophically, as seen in the case of the Larsen A (Rott et al., 1996), Larsen B (Rack and Rott, 2004), and Wilkins ice shelves (Padman et al., 2012) on the Antarctic Peninsula. Tracking the change in the calving front location is a vital input parameter for ice flow models, as it is used to inform studies of calving processes and their driving forces (Trevers et al., 2019) and is required to compute ice shelf mass change from calving, a component part of the total budget along with basal melt and surface mass input (Rignot et al., 2013). Measurements of the ice shelf calving front location have been made using a range of methods, including historical ship-based observations dating from 1842 on the Ross Ice Shelf (Jacobs et al., 1986; Keys et al., 1998), manual delineation of images acquired by aerial photography (Cook et al., 2005), and optical and synthetic aperture radar (SAR) satellites (Cook and Vaughan, 2010; MacGregor et al., 2012), automated ice front detection (Baumhoer et al., 2019), and by applying edge detection techniques to satellite radar altimetry elevation data (Wuite et al., 2019). The spatial resolution, accuracy, and frequency of these complementary techniques vary, with the temporal and spatial extent of calving front measurements largely dependent on the repeat period and coverage of data acquired and the manual intensity of the processing technique used. While data prior to the satellite era (pre-1960s) are extremely limited, historical records are an important reference dataset for understanding long-term change in the ice front position and its response to environmental forcing. Due to the importance of this glaciological parameter, there are several recent publications that measure change in the Antarctic ice shelf calving front locations, from regional assessments to full continent-wide evaluations (MacGregor et al., 2012; Lilien et al., 2018; Wuite et al., 2019; Baumhoer et al., 2018, 2019, 2021; Greene et al., 2022; Christie et al., 2022). In this study, we expand on this previous work and provide a circum-Antarctic survey by mapping the annual calving front location on 34 ice shelves around Antarctica from 2009 to 2019, using MODIS (Moderate Resolution Imaging Spectroradiometer) satellite imagery (Scambos et al., 1996). The results provide a comprehensive assessment of ice front migration across Antarctica over the last decade, expanding on historic patterns of ice movement and enabling areas of growth and retreat to be accurately quantified (Fig. 1).

2 Data and methods

We measured the annual calving front position on 34 ice shelves, encompassing 80 % of the Antarctic coastline, over the 11 years from 2009 to 2019 (Fig. 1). We used over 350 multispectral optical images, acquired by the MODIS instrument on board the NASA Terra and Aqua satellites (Scambos et al., 1996; Table S1 in the Supplement). Images acquired during the austral summer, from mid-January to the end of February, were selected throughout the decade to ensure consistent sampling and to avoid aliasing seasonal variation in the calving front position. Cloud-free satellite images, with open ocean at the calving front, were preferentially selected whenever possible, as the presence of sea ice and iceberg melange can reduce the accuracy with which the calving front location can be visually identified. Images acquired around midday were also prioritised, as the illumination at this time provides better contrast, enabling clearer identification of the ice shelf edge. The study period started in 2009 on 30 ice shelves; however, on the Wordie, Baudouin, Nansen, and Drygalski ice shelves, suitable images were not acquired until 2011. Therefore, this year was used as the earliest start date in these three regions throughout this study (Table 1). We produced an annual measurement of the ice front position on 34 ice shelves around Antarctica by manually delineating the calving front location at the point where the ice shelf surface visibly transitioned to open ocean or sea ice in each satellite image (Cook et al., 2005; Cook and Vaughan, 2010).

Figure 1 Antarctic map of ice shelf area change from 2009 to 2019, with ice shelf names overlaid on a Bedmap2 surface of Antarctica. The circle areas denote the total amount of ice shelf area (in km2) lost (red) or gained (blue). The bold black line represents the Antarctic coastline, combining 2015 and 2019 data.

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Equidistant points were plotted every ∼1000 m along the ice front, using a Polar Stereographic projection to standardise the distance scale, point density, and accuracy of the calving front boundary on every ice shelf. The ice shelf calving front location is constantly evolving, with the measured location representative of the time stamp of the satellite image used; however, for the purposes of this study, we assume that this reflects the annual location. The accuracy of the calving front position is limited by the georeferencing precision of the image and the digitisation of the boundary. We assessed the uncertainty in the measurement technique by delineating the calving front boundary five times, using the 2017 image on the Dotson Ice Shelf, and then measuring the variance from the mean position. The results show that the standard deviation of the calving front measurement is 254 m, which we assume to be our measurement uncertainty. This reflects the spatial resolution of the MODIS imagery, which has a pixel size of 250×250 m. The contemporary calving front positions from this study were combined with historical measurements on the Antarctic Peninsula to extend the record of change back to 1947, including the Larsen A to C, George VI, Wilkins, Wordie, Bach, and Stange ice shelves (Cook and Vaughan, 2010). Overall, this study has produced 366 calving front measurements between 2009 and 2019 and utilised 53 historical measurements on the Antarctic Peninsula to provide the most temporally and spatially extensive assessment of change in ice front position across Antarctica.

The annual area of each ice shelf was measured from 2009 to 2019 by combining the digitised calving front locations with a reference grounding line position, Making Earth Science Data Records for Use in Research Environments (MEaSUREs) Antarctic Grounding Line from Differential Satellite Radar Interferometry, Version 2 (Rignot et al., 2016), which marks the inland limit of the ice shelf boundary (Thomas et al., 1979). The grounding line and calving front positions were polygonised and then intersected for each Antarctic drainage basin fringed by an ice shelf, with isolated islands and nunataks subtracted from the area, creating a bounded area for each ice shelf for each year of the study. The total area change over the decade-long study period was calculated by differencing the most recent ice shelf area observation (2019) from the oldest (2009 or 2011; Table 1). We computed the mean annual rate of calving by dividing the total area change by the number of years observed and calculated the percentage area change by dividing the total area change by the 2009 area (Table 1). To assess the volume and ice mass change caused by calving front evolution, we extracted ice thickness from Bedmap2 (Fretwell et al., 2013) across the most inland measured calving front position, which ranged from 2009 to 2019, depending on the ice shelf (Table 1). We then calculated a mean thickness at the calving front for each shelf. We calculated the annual mass change in each ice shelf due to calving processes by computing the volume change in the calved ice and multiplying each annual area by the mean ice thickness and ice density (0.9166 Gt km−3). The mean rate of volume change was computed by dividing the annually varying ice shelf volume change by the study period. As the accuracy of ice shelf area measurements depends on both variations in the width and the length of the coastline, we rounded this to 1 km2 precision, which is in line with the methodology of previous studies (Cook and Vaughan, 2010), and to account for errors within the calving front delineation (254 m). The same method of calculating the area, volume, and calving mass change was applied to the historical calving front positions on the Antarctic Peninsula.