DOI: 10.5061/dryad.p2ngf1w4h
title: Thirty-three years of glacier grounding line retreat in Antarctica 1992-2025

description: The Grounding Line (GL) - the transition from ice grounded on the
 continent and ice afloat in the ocean - is a sensitive indicator of
 glacier stability and mass balance. Using differential SAR interferometry
 from ERS-1/2, Sentinel-1, RADARSAT-1/2, RCM, ALOS PALSAR-2, COSMO-SkyMed,
 and ICEYE, we assemble a continental scale record of grounding line
 migration from 1992 to 2025. Over 77±10% of Antarctic coastal length, we
 detect no GL migration. Stable areas include the vast Ross,
 Filchner-Ronne, Amery and West ice shelves, and broad sectors of Coats,
 Queen Maud, Enderby, and Princess Elizabeth Lands. Retreat is concentrated
 in (i) the Antarctic Peninsula - 2-18 km along Larsen A-B and 2-6 km along
 parts of GeorgeVI, and no change on Larsen C-D ice shelves; (ii) Wilkes
 and GeorgeV lands - 6-10 km on Denman, Totten, Moscow, Frost, Holmes,
 Mertz, Ninnis, and Cook, and 26 km on Vanderford; and (iii) West
 Antarctica - 5-7-km on Ferrigno, Fox, and Venable, with extreme retreat in
 the Amundsen and Getz sectors (Pine Island 33 km, Thwaites 26 km, Haynes
 20 km, Pope 23 km, Smith 42 km, Kohler 12 km, East Getz 9 km toward Berry
 18 km, Hull 14 km and Land 5 km). The ice sheet lost 12,820±1,873km2of
 grounded ice in 1996-2025, or 442±64 km2/year, with 62% from West
 Antarctica and 28% from East Antarctica. Retreat clusters in areas where
 bathymetry channelizes warm Circumpolar Deep Water toward deep grounding
 zones where beds are retrograde, except in the northeastern Antarctic
 Peninsula. The results provide a harmonized benchmark for ice grounding
 zone-based ice sheet models and identifies gateways where future retreat
 is likely to accelerate.
description: <strong>SAR Missions.</strong> We use radar
 interferometry (InSAR) observations from several satellite
 Synthetic-Aperture Radar (SAR) missions. The European Space Agency (ESA)
 Earth Remote Sensing ERS-1 collected repeat-pass interferometry at the
 C-band frequency (5.3 GHz) in 1992 and 1994 (3-day repeat). ESA collected
 data at a one-day repeat with ERS-1 and ERS-2 in 1995/1996. ERS-1 acquired
 3-day repeat data at the end of its mission in 2000. ERS-2 acquired 3-day
 repeat data at the end of its mission in 2011. ESA launched Sentinel-1a in
 2014, followed by Sentinel-1b in 2016, which acquired SAR data at the
 C-band frequency (5.4 GHz) with a 6 to 12 day repeat until Sentinel-1b
 ceased operations in 2021. Sentinel-1c was launched in December 2024 and
 collected data at a one-day repeat with S1a for one month over the
 Amundsen Sea. The Japanese Space Agency (JAXA) launched the ALOS PALSAR in
 2006 to acquire SAR data at the L-band frequency (1.27 GHz) at a 44-day
 repeat until 2011. In 2014, JAXA launched ALOS-2 PALSAR-2 (1.24 GHz) at a
 14-day repeat cycle. The Canadian Space Agency (CSA) launched RADARSAT-1
 in 1995 to collect data at a 24-day repeat cycle at the C-band frequency
 (5.4 GHz) until 2013; RADARSAT-2 in 2007 with the same cycle but left
 (i.e. south) looking capability; and the RADARSAT Constellation Mission
 (RCM) in 2019 started to acquire data in Antarctica in 2023 at a 4-day
 repeat cycle. The Agenzia Spaziale Italiana (ASI) launched the CSK2 and
 CSK3 satellite part of the CosmoSkyMed Constellation in 2007 and 2008,
 which started to acquire 1-day repeat track data at the X-band frequency
 (9.60 GHz) in 2015 until 2021 after which we no longer have access to
 data. Finally, ICEYE launched X6 and X7 in 2020 to operate at the X-band
 frequency (9.65 GHz) on a one-day ground track repeat (GTR) starting in
 2023. For each SAR, we form interferometric pairs with the natural ground
 repeat cycle and difference consecutive pairs to obtain a differential
 interferogram, or DInSAR, that measures the differential displacement
 between two consecutive SAR interferograms.
 <strong>Differential Interferometry.</strong> One SAR
 interferogram measures the line of sight displacement of the ice surface
 over one repeat cycle, combined with topography which we remove
 automatically using a digital elevation model of Antarctica. In a
 differential interferogram, or DInSAR, the steady component of the glacier
 motion cancels out, leaving only the motion associated with tidal flexure
 of the ice on floating ice or blurbs of subsidence/uplift caused by
 subglacial water migration on grounded ice, or noise. When the glacier
 accelerates, the assumption of steady state breaks down, but the
 corresponding pattern of motion will resemble a pattern of horizontal
 motion rather than a pattern of tidal flexure. To mitigate the effect of
 speedup, we tend to use nearest neighbor pairs of interferograms and do
 not combine interferograms acquired months apart.
 <strong>GL detection.</strong> We do not unwrap the
 interferometric phase to quantify vertical displacements but use the
 pattern of fringes (or 360 degrees in phase) instead to detect the GL, as
 in Rignot et al. (2011), i.e., we detect the first interferometric fringe
 in the seaward direction, or initial vertical motion indicating that the
 ice lifts off its bed at high sea level height. For Sentinel-1a/b/c, we
 use a Machine Learning algorithm to map a large number of GL (Mohajerani
 et al., 2021) in 2018, 2019 and 2020, but these products still require a
 final stage of manual filtering. At the C-band
 frequency, we have one fringe of signal for every 28 mm of line of sight
 deformation. This ratio decreases by a factor 4 at the L-band frequency
 and increases by a factor 2 at the X-band, i.e. for the same tidal signal
 we get 4 times less fringes at the L-band and 2 times more at the X-band.
 The operating frequency of the radar, however, does not matter for
 grounding line mapping, which is based on a differential displacement of
 the ice surface caused by an increase in basal water pressure measured
 independent of the radar frequency. Frequency selection affects the noise
 level of the signal. <strong>Retreat
 estimates.</strong> Because the glaciers are broad, 120 km wide for
 Thwaites, it is difficult to characterize the retreat with a single
 number. We measure the retreat along a flow line of the fastest-moving
 portion of the glacier, or central flow line, between the most retreated
 position of the GL in the 1990s and the most retreated position of the GL
 in the latest data (2021-2025). The quotation is therefore approximate. We
 complement it with an estimation of the total area of retreat, which is
 more rigorous and well defined. Our estimates of grounded ice loss exclude
 glaciers terminating in a calving cliffs in the Peninsula, but the
 difference is small and probably at the 100 km<sup>2</sup>
 level. <strong>Errors in GL detection and areas
 of ungrounding</strong> assume a precision of ±500 m in GL mapping,
 which is conservative given that at a given epoch using one DInSAR, we
 pick the GL position within 100 m. The ±500 m accounts for the fact that
 we do not fully sample the tidal-induced, short-term migration of the GL,
 especially where we have few tracks. We measure the area of ungrounding in
 square kilometer. We transform the polygon data from EPSG 4326 (latitude,
 longitude, WGS84) into EPSG 6932 (x, y, WGS 84, named NSIDC EASE-Grid 2.0
 South, which is a Lambert Azimuthal Equal Area centered on the South
 Pole). We delineate polygons between the most retreated position of the GL
 in the 1990s and the most retreated position of the GL in the most recent
 data (2021-2025). All GL positions include the time of acquisition of the
 data, which makes it possible to correct for changes in tides and
 atmospheric pressure. We do not attempt to do this correction herein
 because the relationship is not linear (seawater intrusions can be trapped
 in between tides (Chen et al., 2023) and the correction is non critical
 with multiple observations (Rignot et al., 2024).
 <strong>References</strong> H.
 Chen, E Rignot, B Scheuchl, S Ehrenfeucht, Grounding Zone of Amery Ice
 Shelf, Antarctica, From Differential Synthetic-Aperture Radar
 Interferometry. <em>Geophys. Res. Lett.</em> 50(6), 613
 e2022GL102430 (2023). Y Mohajerani, et al., Automatic
 delineation of glacier grounding lines in differential interferometric
 synthetic-aperture radar data using deep learning. <em>Nat. Sci.
 Rep.</em> 11, 4992 (2021). E Rignot, J Mouginot,
 B Scheuchl, Antarctic grounding line mapping from differential satellite
 radar interferometry. <em>Geophys. Res. Lett.</em> 38, 1–6
 (2011). E Rignot, et al., Widespread seawater
 intrusions beneath the grounded ice of Thwaites Glacier, 597 West
 Antarctica. <em>Proc. Nat. Acad. Sci.</em> 121(22),
 e2404766121 (2024).
description: # Thirty-three years of glacier grounding line retreat in Antarctica
 1992-2025 Dataset DOI: [10.5061/dryad.p2ngf1w4h](10.5061/dryad.p2ngf1w4h)
 ## Description of the data and file structure ***The grounding line
 delineations***, **InSAR_GL_Antarctica_v1-1992-2025.gpkg** and **.shp**
 are distributed as a geopackage file (gpkg) and ESRI shape files which
 include the following fields Each double difference is made from 3 to 4
 images which correspond to 3 to 4 orbits and 3 to 4 dates. When the set is
 limited to that, it simply means that the middle image is used twice, once
 to make a pair forward in time, once to make a pair backward.  **Sensor:**
 **String**. Name of the satellite constellation used to collect the data.
 Values are: CSK (CosmoSkyMed), Sentinel-1, RSAT-1 (RADARSAT-1), RCM
 (RADARSAT Constellation Mission), ALOS (ALOS PALSAR-2 and 3 missions), ERS
 (Earth Remote Sensing Satellite 1 and 2), ICEYE (ICEYE X6 and X7
 satellites, with a few data acquired with X38).  **Orbit_1: Integer*32**.
 Absolute orbit number of the satellite in reference to launch date. For
 ICEYE, as the satellites do not have a fixed repeat pass, the number is
 99999.  **Date_1: Date** for the first orbit in calendar days. 
 **Orbit_2**: **Integer*32.** Same for second orbit
 **Date_2**: **Date** for the second orbit **Orbit_3**: **Integer*32.**
 Same for third orbit (if needed) **Date_3**: **Date** for the third orbit
 (if needed)  **Orbit_4**: **Integer*32.** Same for fourth orbit (if
 needed) **Date_4**: **Date** for the fourth orbit (if needed)
 **Time_s:** **Integer*32.** Reference time in seconds since Jan. 1 of the
 year for the first orbit. This time reference can be used to evaluate
 tidal amplitude and atmospheric pressure (to calculate the Inverse
 Barometer Effect - IBE) at the time of passage of the satellite. Because
 of the repeat cycle, only one time is mentioned. The times of passage of
 Orbit_2,_3, and \_4 only differs from the first Orbit by seconds. 
 **Track: Integer*32.** Absolute track number based on the repeat cycle.
 See note below on calculations of Orbits and Tracks for the different
 sensors.  **Glac_name: String.** Glacier or ice shelf or region name.
 **Filename: String.** Comment field listing the filename of the DInSAR
 being digitized. For ERS, the names only include the orbit numbers. For
 RCM, RADARSAT-2, ICEYE, the filenames include more information.  **Year:
 Integer*32.** Year of data collection.  ***Area of retreat:*** We supply
 an ESRI Shape file of the area of grounding line retreat between 1992 and
 2025, named **GL_Area_Retreat_1992-2025.shp** and a geopackaged version
 **GL_Area_Retreat_1992-2025.gpkg** which includes:  **Length:**
 **Integer*32.** Length of the GL over which we document the area of
 grounded ice retreat in kilometers.  **Name: String.** Name of the glacier
 or ice shelf.  **Epoch: String.** Year Start and Year End of the
 observations of GL positions.  **Area:** **Integer*32.** Area of retreat
 for the time period quoted in "Epoch" in square kilometers. 
 **Subregion: String.** Name of subregion.  **Region: String.** WAIS = West
 Antarctica; EAIS = East Antarctica; AP = Antarctic Peninsula. 
 **Error:** **Integer*32.** Error in the area of retreat in square
 kilometer, calculated as Length x 0.5 km.  **Area Adj.:** **Integer*32.**
 Area of retreat in square kilometer for the reference period 1992-2025 ###
 Files and variables #### Files: ####
 InSAR_GL_Antarctica_v01-1992-2025.gpkg ####
 **InSAR_GL_Antarctica_v1-1992-2025.shp** ####
 **InSAR_GL_Antarctica_v1-1992-2025.cpg** ####
 **InSAR_GL_Antarctica_v1-1992-2025.dbf** ####
 **InSAR_GL_Antarctica_v1-1992-2025.prj** ####
 **InSAR_GL_Antarctica_v1-1992-2025.shx** **Description:** Geopackage
 (.gpkg) and ESRI shape files (all others) of GL positions \**Filea: **
 **GL_Area_Retreat_1992-2025.gpkg** **GL_Area_Retreat_1992-2025.shp**
 **GL_Area_Retreat_1992-2025.cpg** **GL_Area_Retreat_1992-2025.dbf**
 **GL_Area_Retreat_1992-2025.prj** **GL_Area_Retreat_1992-2025.shx**
 **Description:** ESRI shape files of GL retreat for 108 glaciers, 31
 subregions, 3 regions.  ## Note on the calculation or reporting of orbit
 numbers and track number For **ERS**-1/2, the orbit numbers and track
 number are provided by ESA in the leader file of the raw data. For the
 tandem mission data (1995/1996), the first orbit is ERS-1, the second
 orbit is ERS-2, the third orbit is ERS-1, and the fourth orbit is ERS-2.
 For the 3-day repeat missions in 1992, 1994, 1999, and 2011, the orbits
 are ERS-1, ERS-1, ERS-1 and ERS-2, respectively.  For ALOS-2/PALSAR-2
 (**ALOS-2)**, ALOS-1/PALSAR-1 (**PALSAR**), we use the Orbit numbers and
 Track number provided by the Japanese Space Agency, JAXA. For RADARSAT-1
 (**RSAT**), RADARSAT-2 (**R2**), and the RADARSAT Constellation Mission
 (**RCM**), we use the Orbit numbers and Track number supplied by the
 Canadian Space Agency, CSA. For **ICEYE**, there is no strict book keeping
 of orbit numbers and track numbers, so we adhere to a practical scheme
 approved by the company and that is defined as follows.  Launch_date of 
 X6 and X7 is 28 Sept. 2020 at 11hr 20min UTC. The data are either X6 or
 X7.  Orbit  = (Date1 - Launch_date )/15 , where Date_1 and Launch_date are
 expressed in fraction of Julian Days. Similar expressions for Orbit_2, 3
 and 4. \ \ Track = (Orbit_1 modulo (15)) + 1 For **CSK**, the orbit
 numbers and track number follow a similar definition.  Launch_dates of
 CSKS1 is June 8, 2007 at 2hr 34 min UTC. CSK2 is December 9, 2007 at 2 hr
 31 min UTC. CSK3 is October 25, 2008 at 2 hr 28 min UTC. CSK4 is November
 6, 2010 at 2 hr 20 min UTC. Orbit_1  = (Date1 - Launch_date )/237, where
 Date1 and Launch_date are expressed in fraction of Julian Days. Similar
 expressions for Orbit_2, 3 and 4. \ \ Track = (Orbit_1 modulo (237)) + 1
 ## Code/software Data can be viewed freely on open source qGIS and any GIS
 of your choice.  ## Access information Other publicly accessible locations
 of the data: * NSIDC MEaSUREs Product 0498 Version 3 Data was derived from
 the following sources: * A suite of satellites operated by international
 space agencies and commercial vendors.