Antarctica’s ice shelves help to control the flow of glacial ice as it drains into the ocean, meaning that the rate of global sea-level rise is subject to the structural integrity of these fragile, floating extensions of the ice sheet1,2,3. Until now, data limitations have made it difficult to monitor the growth and retreat cycles of ice shelves on a large scale, and the full impact of recent calving-front changes on ice-shelf buttressing has not been understood. Here, by combining data from multiple optical and radar satellite sensors, we generate pan-Antarctic, spatially continuous coastlines at roughly annual resolution since 1997. We show that from 1997 to 2021, Antarctica experienced a net loss of 36,701 ± 1,465 square kilometres (1.9 per cent) of ice-shelf area that cannot be fully regained before the next series of major calving events, which are likely to occur in the next decade. Mass loss associated with ice-front retreat (5,874 ± 396 gigatonnes) has been approximately equal to mass change owing to ice-shelf thinning over the past quarter of a century (6,113 ± 452 gigatonnes), meaning that the total mass loss is nearly double that which could be measured by altimetry-based surveys alone. We model the impacts of Antarctica’s recent coastline evolution in the absence of additional feedbacks, and find that calving and thinning have produced equivalent reductions in ice-shelf buttressing since 2007, and that further retreat could produce increasingly significant sea-level rise in the future.
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We use velocity data from ITS_LIVE51, which is available at https://its-live.jpl.nasa.gov/ and MEaSUREs InSAR-Based Antarctica Ice Velocity Map, Version 252, which is available at https://doi.org/10.5067/D7GK8F5J8M8R. Static ice-thickness data are primarily from BedMachine Antarctica Version 253, which is available at https://doi.org/10.5067/E1QL9HFQ7A8M. We also use surface elevation data from REMA54 (https://www.pgc.umn.edu/data/rema/), Bedmap255 (https://www.bas.ac.uk/project/bedmap-2/), Antarctic 1-km digital elevation model from Combined ERS-1 Radar and ICESat Laser Satellite Altimetry, Version 156 (https://doi.org/10.5067/H0FQ1KL9NEKM) and Radarsat Antarctic Mapping Project Digital Elevation Model, Version 257 (https://doi.org/10.5067/8JKNEW6BFRVD). Ice-shelf masks are adapted from MEaSUREs Antarctic Boundaries for IPY 2007–2009 from Satellite Radar, Version 258 (https://doi.org/10.5067/AXE4121732AD). Ice-shelf-thickness time series36 are available at http://its-live-data.s3.amazonaws.com/height_change/Antarctica/Floating/ANT_G1920V01_IceShelfMelt.nc. Radarsat-derived coastlines59 are available at https://asf.alaska.edu/data-sets/derived-data-sets/ramp/ramp-get-ramp-data/, coastlines from MODIS Mosaic of Antarctica60 are available at https://doi.org/10.5067/68TBT0CGJSOJ and coastlines from the fast-ice dataset61 are available at https://doi.org/10.26179/5d267d1ceb60c. Sentinel 1b-based mosaics used in this work are available at http://seaice.dk/. Our final, processed ice-shelf-thickness and -mass time series data are presented in Supplementary Table 1 and are also available in .mat format at https://github.com/chadagreene/ice-shelf-geometry. The coastline data presented in this work are available as plain-text files of ice-sheet outlines and also as binary grids at 240-m resolution in .mat format at https://doi.org/10.5281/zenodo.5903643.
Analysis and figure generation were performed in MATLAB, with functions from Antarctic Mapping Tools for MATLAB73 and the Climate Data Toolbox for MATLAB74. All code that was developed for this work to generate coastlines, analyse the data and produce the figures is available at https://github.com/chadagreene/ice-shelf-geometry. Ice-shelf surface elevation time series were processed with the Cryosphere Altimetry Processing Toolkit, which is available at https://doi.org/10.5281/zenodo.3665785. The ISSM and GEMB models used in this work are open-source and publicly available at https://issm.jpl.nasa.gov.
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We thank ESA/Copernicus and R. Saldo of DTU Space/Technical University of Denmark for making Sentinel 1 mosaics available via http://seaice.dk. This research was supported by the NASA Postdoctoral Program, along with the NASA Cryospheric Science, Sea Level Change Team, and Modeling, Analysis and Prediction (MAP) Programs, and was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration ©2022 All rights reserved. This project received grant funding from the Australian Government as part of the Antarctic Science Collaboration Initiative program. We gratefully acknowledge computational resources and support from the NASA Advanced Supercomputing Division.
The authors declare no competing interests.
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Extended data figures and tables
The coastline evolution over the past quarter of a century, shown for selected regions that do not appear in Fig. 1.
The mass imbalance of ice shelves in the blue triangles is dominated by coastal processes, while the balance of only a few ice shelves, mostly located in the Amundsen Sea Embayment, are dominated by thinning. Marker colour corresponds to the central longitude of each ice shelf, and marker area scales with ice-shelf area.
To quantify changes at the ice-sheet margins, where publicly available velocity and thickness datasets may not reach the maximum observed extents of the ice sheet, we combine all available data, then extrapolate thickness, velocity, and a mask of ice-shelf names in the direction of ice flow. Although the resulting grids fill the entire map, we only use thickness and velocity data close to the Antarctic periphery, wherever ice has been observed in at least one of the 24 mappings since 1997.
Weeks after our March 2021 annual pan-Antarctic coastline mapping, the Ronne Ice Shelf calved the 4320 km2 iceberg A76. By considering changes in area and mass that have occurred between each of our 24 consecutive mappings, we estimate that iceberg A76 took 1,002 Gt of ice from Ronne Ice Shelf.
To obtain steady-state calving flux, we calculate pixel centre locations (x1,y1) of a 240-m resolution grid after one year of ice displacement. We use (x1,y1) as query points to linearly interpolate a binary ice mask, which gives the fraction of each grid cell that is still within the initial mask after a year of displacement. Calving flux is calculated by multiplying the fraction of ice that has calved after a year of displacement by the area, thickness, and density of ice in each grid cell.
All sensitivity measurements described in this manuscript present grounding-line-flux measurements relative to the control values shown here. Marker area scales with ice-shelf area and marker corresponds to the central longitude of each ice shelf.
Time series of observed ice-shelf area and grounding-line-flux responses to calving.
Ice-shelf-mass and -area time series.
This zip file contains animations of observed calving front change, and ice-sheet response to hypothetical calving.
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Greene, C.A., Gardner, A.S., Schlegel, NJ. et al. Antarctic calving loss rivals ice-shelf thinning. Nature 609, 948–953 (2022). https://doi.org/10.1038/s41586-022-05037-w