Rapid retreat of Berry Glacier, West Antarctica, linked to seawater intrusions revealed by radar interferometry

Domain: earth_science
Published: 2025-10-21
Editorial outcome: major_revision

Abstract
We employ a time series of ERS-1/2, ALOS-1/2 PALSAR, Sentinel-1, COSMO-SkyMed, and RCM differential synthetic-aperture radar interferometry data from 1996 to 2023 to document the short and long-term migrations of the grounding line (GL) of Berry Glacier, West Antarctica, a tributary of Getz Ice Shelf that controls 10% of its ice discharge. In 2019–2021, we detected a short-term GL migration of 18.0 ± 0.9 km, which is exceptionally long and implies that the glacier bed is up to 1300 m deeper than previously known. On short timescales, the GL migrates between three states controlled by bed topography. The observed flexing of the glacier suggests that seawater is trapped in the newly formed ice shelf cavity in an irregular fashion during the tidal cycle. From 1996 to 2021, the most inland position of the GL retreated by 18.1 ± 0.4 km, or 0.7 km/year, the ice thickness decreased by 11 ± 1 m/year, the ice sped up by 64 ± 5%, and the glacier lost a total mass of 131 ± 23 Gt. We attribute the rapid retreat to an enhanced ocean heat flux from warm Circumpolar Deep Water (CDW) reaching the grounding line through favorable bathymetry channels, combined with km-sized seawater intrusions beneath the glacier that cause rapid melting of basal ice.

Introduction
The Antarctic Ice Sheet has been a major contributor to sea level rise in the last four decades1, but projections of its evolution in the coming century are affected by large uncertainties2. Central to these uncertainties is the modeling of the evolution of the grounding line (GL), i.e., the transition boundary between grounded and floating ice. The ice that crosses the GL floats in the ocean waters. Changes in GL position affect the basal friction regime of the glacier, which, in turn, controls its speed, ice discharge, and mass loss3. During a GL retreat, the basal resistance may drop and the glacier may accelerate. In contrast, during an advance of the GL, resistance to flow may increase, and the glacier may slow down. Most numerical ice sheet models do not account for any short-term variability in GL position (e.g., due to ocean tides) and should not apply basal melt at a fixed GL4. In this study, we revisit this assumption with novel interferometric synthetic aperture radar (InSAR) observations.

Global Positioning System (GPS), tiltmeters, satellite altimetry5, and InSAR data6 have been used to delineate GL positions. Altimetry detects vertical motion below a satellite track with a precision of 5–10 cm7. InSAR detects vertical motion on a 100 km wide strip with a precision of 2–3 mm8. InSAR GL mapping started in 1995-1996 with Earth Remote Sensing Satellites 1 and 2 (ERS-1/2) in a 1-day repeat cycle and resumed in 2014 with Sentinel-1a (S1a) in a 12-day repeat cycle9 and Sentinel-1b (S1b) in 2016, in a 6-day repeat cycle until the loss of S1b in December 2021. Here, we also include data from the ALOS-1/2 PALSAR mission for the years 2008–2009 and 2021, a time series from the COSMO-SkyMed (CSK) constellation at a 1-day repeat cycle for the years 2020–2021, and data from the RADARSAT Constellation Mission (RCM) in a 4-day repeat cycle for the year 2023. We characterize the short- and long-term variabilities in the GL position of Berry Glacier, which is a part of the Getz Ice Shelf, West Antarctica. Short-term variability is used to define an “Ice Grounding Zone” (IGZ), or a region of tidal-scale variations in the position of the GL.

Getz drains an area of 85,900 km2 that contains an ice volume equivalent to a 22-cm rise in sea level. Berry Glacier drains a sector 5593 km2 in size into the Getz Ice Shelf and controls 10% of the total ice flux (Fig. 1, Fig. 2). Berry flows at 760 m/year in the IGZ and holds an ice volume equivalent to a 1.5-cm rise in sea level. Near the GL of Getz, many glaciers are in contact with warm waters coming from Circumpolar Deep Water (CDW), which rapidly melt ice from below10–13. As a result, the Getz Ice Shelf is one of the largest producers of basal meltwater in Antarctica, which contributes to weakening ice shelf buttressing and enhancing dynamic mass loss14.

Results
Using a time series of InSAR data, we find that the length of the IGZ increased from 0.6 km in 1995/1996 to 18.0 ± 0.9 km in 2019–2021 (Fig. 1). Because we have only two interferograms in 1995/1996, the length of the IGZ is defined with less precision, but we find no residual signal upstream of state 1 in 1996 that would suggest that seawater could have intruded even farther upstream back in the 1990s (Supplementary Fig. 1a, b). In contrast, the most recent interferograms always display residual vertical motion between states 1 and 3 of the GL. We conclude that seawater intrusions were unlikely upstream of state 1 in 1996 (Fig. 3). In 2019–2021, the IGZ is considerably longer and the longest recorded in Antarctica15–17. Calculations of ice thickness assuming hydrostatic equilibrium18 using a WorldView (WV) DEM from year 2019, however, indicate that the IGZ should only be 300–600 m long (Fig. 3a). This discrepancy imposes a revision of the Berry bed topography in BedMachine v3.7 (BMv3.7), which is not derived from mass conservation in this area but from an interpolation between NASA Operation IceBridge (OIB) profiles.

In terms of long-term migration, we find that the upper limit of the IGZ retreated 18.1 ± 0.4 km from 1996 to 2021, i.e., the lower portion of the 2021 IGZ matches the upper limit of the 1996 IGZ, while the upper limit is 18 km further inland than in 1996. During the more recent time period 2009-2021 with denser observations, the GL retreated 7.0 ± 0.4 km, or 0.5 km/year (Fig. 1) but also started to migrate laterally by about 3 km to the west into an unnamed tributary glacier (Fig. 4). Another 1.5 km retreat of the upper limit of the IGZ occurred between 2021 and 2023 (Fig. 1).

We find excellent agreement between the vertical motion of the ice surface measured with the double difference interferograms and changes in sea surface height (SSH) calculated using the CATS2008 tidal model19 corrected for changes in atmospheric pressure, or the inverse barometer effect (IBE), calculated using the ERA5 pressure fields20 (Fig. 3c). We find no correlation between the position of the GL and SSH (Fig. 3d), even if we take into account the spring-neap cycles and ebb/flow (Supplementary Fig. 2), i.e., the GL does not migrate in phase with SSH as we would expect in a freely opening and closing cavity. Instead, the GL oscillates between three states: (1) state 1 near the 1996 IGZ, (2) state 2, about 6 km upstream, along a retrograde portion of the new bed that is discussed later, and (3) state 3 at the southern limit of the 2021 IGZ or 2 km further inland. The three states stand out in the density map of GL positions retrieved for 2019–2021 (Fig. 3a). We conclude that the GL is located in state 1 in 1996, reaches state 2 around the year 2008, and is located predominantly in state 3 in 2019–2023 with possibilities for the ice to touch the seabed at state 1 and state 2 on occasions during the tidal cycle.

We compare the GL retreat for 2006–2022 with changes in the elevation of the ice surface, h, the thickness of the ice, H, and the speed of the ice, V. Between states 1 and 2 along the centerline, h decreased by 24 m in 2006–2022, equivalent to a reduction in thickness H of 222 m, or a thickness thinning rate of 13 m/year. The lowering of the surface of the ice reaches 35 m along the west flank of Berry, where the IGZ migrated laterally, which is equivalent to a thickness reduction dH/dt of 19 m/year (Fig. 4a). During 2006–2022, the glacier speed increased by 65 m/year at the center between states 1 and 2, and 136 m/year along the west flank (Fig. 4b). Glacier speed-up is, however, the largest between state 2 and state 3 and extends 28 km beyond the 2021 IGZ (Fig. 4b). In that region stretching from state 2 to 28 km beyond state 3, the speed up averages 134 m/year, with a maximum of 224 m/year between 2006–2022.

From 1996 to 2022, along the G-H profile, the surface elevation decreased by 59 m and the speed increased by 48% from 448 m/year to 665 m/year (Fig. 4c, e). Along profile C-D, the acceleration is 52%, accompanied by rapid thinning, especially on the central and western sides (Fig. 4d, f). On average in the 2021 IGZ between 1996–2022, the ice thickness dropped at a rate of 11 ± 1 m (the surface elevation changed by 47 ± 3 m) and the increase in speed is 64 ± 5%.

We calculate ice discharge in state 1 in 1996, state 2 in 2006–2011, and state 3 in 2012–2022. The gaps in the time series of thickness and speed are filled with linear interpolation. We calculate anomalies in ice discharge, that is, excess ice discharge compared to average surface mass balance for 1979–2004 that would keep the glacier in a state of mass balance. The anomaly increases from 0.9 Gt/year in 1996 to 7.5 Gt/year in 2022 (Fig. 5a). The anomalies in the surface mass balance versus the 1979-2004 period remain at about -0.5 Gt/year between 1996 and 2022 (Fig. 5a). The anomalies in the total mass balance, M, therefore, increased by a factor of 6.6, from −1.3 Gt/year to -8.6 Gt/year, which is considerable compared to other glaciers in Antarctica1. In 25 years, the ice discharge of Berry Glacier has increased by 118 ± 8 Gt while the surface mass balance has decreased by 13 ± 15 Gt, that is, a cumulative mass loss of 131 ± 23 Gt (Fig. 5b), with 90% due to ice discharge (Fig. 5b). Despite its small size, Berry Glacier has therefore had an enormous influence on the mass loss of this region. Its mass loss is one third of the mass loss of the entire Getz Ice Shelf system when comparing our results with the estimates in 1.

We reconstruct the bed topography based on the minimum depth of ice that ensures flotation of ice over the zone of flexing during the tidal cycle, i.e., ice must be within a meter of flotation to allow ice flexing. The elevations of the ice draft deduced from the ICESat-2 and WV DEMs are consistent with each other (Fig. 4g). However, the calculated basal elevations are up to 800 m lower than those reconstructed in BMv3.7 along the profiles (Fig. 4g, h), with discrepancies reaching 1,300 m on the eastern side of the 2021 IGZ, where no radar profiles are available. The elevation of the BMv3.7 bed matches the thickness profiles of the MCoRDS only in the upstream part of the 2021 IGZ (Fig. 4g). We conclude that the thickness of the MCoRDS may have been misinterpreted in 2011. In fact, a close inspection of the radar echograms indicates weak bed echoes at the glacier center, i.e., a considerable amount of uncertainty in bed detection, i.e., the 2011 OIB Level 2 data are incorrect (Supplementary Fig. 3d). We infer a much deeper elevation of the bed, which is also consistent with the gravity gradients in that area21. Overall, we infer a much deeper bed than expected around state 3, narrow water columns at states 1, 2, and 3, a water column of several hundred meters between state 2 and state 3, and an unknown water column between state 1 and state 2. However, the thinning rate between states 1 and 2 suggests that the water column in that region should be on the order of several hundred meters.

Comparing the spatial distribution of the GL positions with the new bed topography along the G-H profile (Fig. 4g), we find that the GL migrated over a mostly retrograde bed during 1996–2021. A series of bed depressions between the three states may have created pockets capable of trapping seawater during the tidal cycle. We attribute the water trapping to the complex cavity shape, which involves the resistance to the outflow of the hydrological network and along-slope gravity gradients as modeled in ref. 22. In several InSAR data where the GL is detected in state 1 or 2, we observe fringes of opposite sense motion upstream, e.g., 11.5 cm (3.7 fringes of S1 DInSAR) in Fig. 3b, which suggests that seawater trapped in a prior tidal cycle is now able to leave the cavity. Similar fluctuations out of phase with the tide have been reported elsewhere and have been attributed to water trapping15,16,23.

We calculate the ice shelf basal melt rate, b_, on the west Getz Ice Shelf including the IGZ for 2019–2022, from mass conservation using the ice velocity of MEaSUREs, the ice thickness deduced from hydrostatic equilibrium, the thinning rate of ICESat-2, and the surface mass balance data of RACMO2.3p224. We use an Eulerian framework to apply mass conservation. To alleviate the deviations in the ice thickness from flotation, we apply a 4.5 km smoothing filter in the IGZ. We find the highest melt values at the entrance of the Berry IGZ and within the IGZ (Fig. 6). Within the Berry IGZ, which is 114 km2 in area, the basal melt rates range from -30 to 90 m/year and average 67 ± 26 m/year. High values are predominantly found along steep retrograde slopes. Low values are found in flatter parts of the bed and near bed ridges.

During the past 25 years, a subglacial cavity has developed beneath Berry Glacier, with direct observations along the 2012 OIB track near state 2 suggesting a local cavity depth of up to 300 m (Fig. 4h, Supplementary Fig. 4b–d). The depth of the cavity away from the OIB transects is uncertain. The DInSAR signal supports the presence of water intrusions, but does not constrain the cavity thickness. A time series of Landsat images confirms that bumps in surface height present in the earlier part of the record, which are indicative of ice grounded on a bumpy bedrock, disappeared during the retreat, to lead to smooth variations in ice elevation, which are consistent with flotation (Supplementary Video 1).

Conductivity temperature depth (CTD) data at the western Getz front show the presence of warm CDW near Berry Glacier as early as 1994, with an ocean temperature at 1,000 m depth around +0.2 °C (−0.1 °C in 2000), increasing to +0.4 °C by 2007 (Supplementary Fig. 5b)14,25, i.e., the ocean heat flux must have increased during that time period. The bathymetry on the continental shelf includes deep and wide channels that should facilitate the access of CDW to the IGZ of Berry (Supplementary Fig. 6). We delineate a thalweg along western Getz, from the ice front to the east of 134°W, which may provide access of CDW to Berry IGZ (Supplementary Fig. 5a)21,26. More recent CTD data indicate a continued warming trend of CDW along the western Getz Ice Shelf since 2007. Temperatures increased from +0.8 °C to +1.2 °C at 750 m depth in front of Dean Island between 2007 and 2018 (Supplementary Fig. 6). Although these stations are not located directly in front of Berry, they provide strong circumstantial evidence for widespread ocean warming throughout the western Getz. The warming trend has been attributed to improved Ekman pumping over the Siple Trough shelf break, which contributed to the presence of warmer CDW in front of the western Getz Ice Shelf in 2018 compared to 2000 and 2007, and also facilitated CDW inflow onto the continental shelf to increase the heat flux available at glacier grounding zones25. Together, these observations support the interpretation that CDW intrusions, modulated by shelf-break dynamics and channeled through topographic depressions, have played a central role in the retreat of Berry Glacier.

We compare the melt rates within 10 km downstream of the IGZ on the west Getz Ice Shelf for 2019–2022 (this study) with the inferred rates for 2003–200827 and 2010–201828 since we have data in this area for all three periods, but not inside the IGZ. Focusing on the area of coincident measurements, we find that the melt rate within 10 km of the Berry IGZ increased from 9.1 m/year in 2003-2008 to 14.9 m/year in 2019-2022. These values are higher than those for other glaciers, e.g., the De Vicq Glacier. The ocean temperature in front of De Vicq Glacier at 700 m depth increased from −1.2 °C in 2000 to +0.3 °C in 2007, more than Berry, but unlike Berry Glacier, we find no obvious deep channel that would allow 700 m deep CDW to reach the IGZ of De Vicq (Supplementary Fig. 5). The difference in melt between the two glaciers increased from a factor of 1.3× in 2003-2008 to a factor of 1.7× in 2019–2022. The melt rates downstream of the IGZ of Berry are also the largest on West Getz Ice Shelf (Supplementary Fig. 7). The melt rates for Berry in 2019–2022 are 110% higher than the steady-state values calculated for the year 2007–2008, that is, assuming no thinning (Supplementary Fig. 7d), indicating that the ice shelf in this sector is 110% out of balance.

From the modeled tidal height of ±1.2 m (Fig. 3d), we know that the average water in the IGZ cavity must be at least 0.6 m. If we assume that the cavity geometry tapers linearly from a maximum of 1.2 m at the entrance of the IGZ to zero at the inland limit, then given an IGZ area of 114 km2, a water flux of 0.7 × 108 m3 in 12 hours would translate into a mass flux of 1.6 × 106 kg3/s. Assuming an ocean temperature of +1.2 °C, inferred from 2018 CTD observations near Dean Island, the ocean thermal forcing, referring to the salinity-dependent and depth-dependent freezing point of seawater, is +3.6 °C at 1000 m depth. With a heat capacity for seawater of 3850 J/kg/K, this thermal forcing translates into a heat flux of 2.8 × 1010 W. If water intrusions flush the entire cavity during each tidal cycle and 30 ± 10% of the available heat is used to melt the ice with a latent heat of fusion of 3 × 105 J/kg, we calculate 0.7 ± 0.4 Gt/year of melt or an average melt of 6 ± 4 m/year, which is ten times lower than the remote sensing estimate for 2019–2022. The large difference between the back-of-the-envelope calculation and the remote sensing results is not due to an underestimation of ocean thermal forcing or the efficiency of ocean heat at melting ice; it reveals or confirms that the thickness of the water column coming in and out of the Berry IGZ cavity is much greater than 0.6 m, possibly by a factor of ten.

Methods
Double-difference interferogram
We used ERS-1/2 (Supplementary Fig. 1a, b), ALOS-1/2 PALSAR (Supplementary Fig. 1c–h), S1a/b, CSK and RCM (Supplementary Fig. 1i–l) InSAR data to delineate the GL of Berry Glacier from 1996 to 2023. ERS-1/2 operated on a 1 day cycle in 1995-1996 during the tandem mission and on a 35-day cycle otherwise, at the C-band frequency (wavelength 5.6 cm), with a spatial resolution of 13 × 4 m, respectively, in the sloping range (cross-track) and azimuth (long-track). ALOS-1 PALSAR operates on the L band (wavelength 24 cm) with a 41.5° look angle off the nadir, 7.5 m × 4.1 m resolution (slant range × azimuth), a width of 70 km, and a repeat cycle of 46 days. ALOS-2 PALSAR operates at L-band (wavelength of 24 cm), has a 14-day repeat cycle, and in Stripmap Fine mode provides 10 m resolution over 70 km swaths. The S1 data are acquired at the C-band frequency (wavelength 5.6 cm), in a 6-day repeat cycle, in Interferometric Wide Swath (IW) mode, with a spatial resolution of 2.4 m in the slant range versus 13.9 m in azimuth32,33. CSK is a constellation of 4 × low-Earth orbit satellites operating in the X-band (wavelength of 3.1 cm) at a slant-range spacing of 3 m with a 16-day repeat cycle. The SAR images are formed by concatenating 3 × CSK STRIPMAP-HIMAGE overlapping frames, each 40 km × 40 km, at 3 m post in both azimuth and range directions34. RCM is a three satellite constellation that operates at C-band (wavelength of 5.5 cm) and provides a 4-day repeat cycle (12 days per satellite). The mission offers a wide variety of imaging modes, we focus on the high resolution 5 m (Stripmap) mode data at supported incidence angles between roughly 20–45°. We differ 3 (or 4) radar images to form a differential interferogram that reveals the differential tidal motion of floating ice during 3 (to 4) epochs. This approach enables us to separate the short-term vertical motion of the ice caused by tidal flexing from the constant horizontal motion of the ice35. The GL is located at the upstream end of a dense set of interferometric fringes (Fig. 2a, c, e). We manually map the GL in 93 × high-quality double-difference interferograms. When the fringe boundary is unclear or the fringes merge or split, we estimate the positive and negative errors based on the fringe pattern to show the farthest and closest distances that the GL may reach. We define GL positions for years 1996, 2008, 2009 and 2023, and IGZs for 1996 and 2019–2021. The limited data availability for 1996 may lead to an underestimation of the IGZ length in 1996, but we detect no residual signal upstream of the 1996 IGZ, and the annual series of ∂h/∂t along the flow line indicates that it is unlikely for the GL to have reached state 2 before 2006. One fringe is a differential vertical displacement, Δd = λ / (2cosθ), where λ is the wavelength of the SAR and θ is the angle between the incident line of the radar and the vertical direction. Multiplying Δd by the number of fringes, we calculate ΔSSH from the InSAR data.

Tidal height calculation and correction
Floating ice flexes in response to variations in oceanic tides and atmospheric pressure. To quantify the tidal state, we use the CATS2008 model, which has a precision of 5 cm19 (Fig. 2b, d, f). We calculate the correction of the inverse barometer effect (IBE) at a rate of 1 cm/hPa36, using anomalies in mean sea level pressure from the fifth generation of atmospheric reanalysis of global climate (ERA5) by the ECMWF, with hourly outputs, at a horizontal resolution of 31 km20. We use the mean pressure for the period 2019–2021 as a reference pressure.

Grounding zone length from hydrostatic equilibrium
If the ice is in hydrostatic equilibrium, the length of the IGZ is ΔL HE = Δh [β + ρ i / ρ w (α - β)] -1, where Δh is the variation in the tidal height, the slopes of the surface α and the bed β are determined along the thickness gradients averaged over several ice thicknesses, and ρ i = 917 kg/m3 and ρ w = 1028 kg/m3 are the density of ice and seawater, respectively18. To calculate α and β, we use a WorldView DEM from September 6, 2019. The slopes are calculated along the direction of the migration of GL on a length scale of 3 × ice thickness.

Ice velocity and surface height data
The ice velocity is from MEaSUREs at a grid spacing of 450 m from 1996 to 202237,38. We used changes in surface elevation from ATL14 (land-ice DEM) timestamped to January 1, 2020 and ATL15 (land-ice height change) between 2019 and 2022, and MEaSUREs ITS_LIVE Antarctic Grounded Ice Sheet Elevation Change between 1996 and 2018. ICESat-2 produces a foundational DEM at a 100-m post and quarterly elevation differences at 1 km resolution that documents surface elevation changes with reference to the foundational DEM39,40. MEaSUREs products offer monthly ice sheet elevation changes derived from 5 × radar altimetry missions (Geosat, ERS-1 and -2, Envisat and CryoSat-2) and 2 × laser altimetry missions (ICESat and ICESat-2) at a resolution of 2 km41.

Ice mass balance calculation
Using ∂h/∂t along flow lines inferred from the elevation change products of ATL15 and MEaSUREs, we find that the IGZ reaches state 2 by 2006 and state 3 by 2012. We selected tracks from the OIB missions in 2016, 2012, and 2011 to serve as flux gates for 1996, 2006-2011, and 2012-2022, respectively, because the locations of the three OIB tracks overlap with the three states42. The OIB missions combine ATM altimetry and MCoRDS ice thickness. The ATM operates by emitting laser pulses toward the surface and measuring the two-way travel time, which is then converted into surface elevation data with an accuracy of a few centimeters. MCoRDS runs at low-frequency radar wavelengths (typically 150-600 MHz) to penetrate thick ice and detect internal layers and the ice-bed interface43–47. We calculated the ice thickness with the HE assumption at the designated flux gates for the period 1996-2022 by combining MCoRDS ice thickness with ∂h/∂t from MEaSUREs (1996-2018) and ICESat-2 (2019–2022). We apply the HE assumption only to regions where seawater intrusions are inferred at a specific time period. For the period 1996–2005, changes in ice thickness were equated directly with surface elevation changes, as the GL reached to state 2 around 2006. Between 2006 and 2011, the HE assumption was applied within the region from state 1 to state 2. From 2012 to 2022, HE was applied across the entire 2021 IGZ extent. By integrating these time-dependent thickness measurements with annual velocities in the direction perpendicular to the flux gates, we calculate annual ice discharge. We estimate the uncertainty in ice discharge by calculating its standard deviation from a central value in a region ±1 km from the original flux gate.

The annual SMB for the Berry Glacier basin is from the Regional Atmospheric Climate Model Version p2.3 (RACMOp2.3)48 with a resolution of 2 km with a 0.1 m/year error. The average SMB of the years 1979–2004, SMB 1979−2004, is used as a reference surface mass balance that would keep the glacier in a state of mass equilibrium, to calculate anomalies in ice discharge, δD, and SMB, δSMB. The total mass balance, δM is δSMB - δD, which is independent of the reference period.

Ice bottom and bed topography
We used the ICESat-2 DEM and the WorldView DEM to deduce the basal elevation of ice in the IGZ from flotation. The basal elevation of the ice h bottom = h - (ρ w / ρ i) h, where h is the elevation of the ice surface, and ρ i = 917 kg/m3 and ρ w = 1028 kg/m3 are the densities of ice and seawater, respectively. The WorldView DEM is generated from stereo imagery captured by Maxar’s WorldView satellite constellation with spatial resolutions of up to 0.5 m and vertical accuracies of 3 to 5 m49. Along profiles overlapping with OIB tracks, we utilize ice surface elevation from ATM minus MCoRDS thickness to calculate the elevation of the ice bottom. We also calculated the elevation of the bottom of the ice from the ATM. The elevation of the DEMs is corrected with the EIGEN-6C4 geoid of BMv3.7. Firn air content (FAC) is removed from all DEM using the values in BMv3.7, which uses a firn densification model50. Although we apply FAC from BMv3.7 to account for the density difference between snow and ice, we acknowledge that the FAC in IGZ is uncertain, as firn models may not capture the complex densification processes that occur in these transitional environments50.

Along profiles A-B, C-D, and C-F, the bed layers are questionable because the bed echoes are weak and incompatible with our calculations of flotation, which is equivalent to setting up a minimum ice draft (Supplementary Fig. 4a–c). Level 2 MCoRDS data for the year 2011 are not robust because the values read from the level 2 data do not match with the bed line picked in the radar map, which is why we find many 100 meters difference between our calculated bed and BMv3.7. The blue stripey areas in Fig. 4g mark the regions where vertical motion from seawater intrusion is detected, but the bed depth is unknown. The white stripey areas show regions outside the IGZ where flotation cannot be applied and the BMv3.7 bed is not reliable. In Fig. 4h, the brown stripey areas are regions where the bed inferred from MCoRDS is not compatible with flotation and, therefore, incorrect. The absence of clear bed echoes at that location confirms that the elevation of the bed was underestimated from the radar echograms.

Ice basal melt rates
We apply an Eulerian approach to calculate the basal melt rates from mass conservation24 using ATL15 products39,40. The melt rate is b_ = a_ - ∂H/∂t - (H(∇⋅u) + u⋅(∇H)), where a_ is the surface mass balance, H is the thickness of the ice, u is the velocity, ∇⋅u is the velocity divergence and ∇H is the thickness gradient. Since ICESat-2 only provides elevation change relative to a single DEM, rather than absolute surface elevations at different time steps, as required for a full Lagrangian analysis, the Eulerian approach is the only option to calculate basal melt rates. This approach requires spatial filtering of the results to mitigate the effect of heterogeneities in ice thickness and bending stresses. We used a spatial filter of 4.5 km, which approximately corresponds to the size of the bending zone discussed by 51. Since the ice in the IGZ is not floating during the entire cycle, the melt rates calculated along the IGZ may be overestimated. Using DEMs separated by one year, we exclude points within a “limit of viability” zone where the ice was grounded at the beginning of the integration period, equivalent to half the annual displacement (Fig. 6b). To evaluate the uncertainty in melt rate, we assume that the errors in the components of the mass balance equation are independent, unbiased, and follow a Gaussian probability distribution. We calculate an uncertainty of 21%, similar to Rignot et al. (2013) using an error in the velocity of 1% or 5 m/year, 3% in thickness or 30 m, 7% in SMB, and 20% in thinning rate.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Summary: This study uses a wide range of satellite observations from 1996-2022 to document changes in grounding line (GL) position, tidal grounding zone (GZ) dynamics, ice velocity, discharge and mass balance of Berry Glacier, which feeds 10% of the Getz Ice Shelf in West Antarctica. The authors report substantial increase in the length of Berry's ice grounding zone (IGZ), from 0.6 km in 1995/96 to 18±0.9 km in 2020-21. Over the same period, they report changes in ice dynamics, including an estimated 17±1 m/yr of thinning, a 64±5% increase in ice flow speed, and a total mass loss of 131±23 Gt. The authors present novel observations of short-term GZ dynamics within the Berry IGZ show that the GL migrates between three states, independently of tidal forcing, suggesting complex interactions between tides and GL movement that are not adequately captured in current ice sheet models. These short-term dynamics are proposed to facilitate seawater intrusions, driving the increased basal melt, thinning, and ice discharge. Furthermore, the authors propose that the bed topography within the Berry IGZ is up to 1,300 m deeper than indicated by MCoRDS airborne radar.\n\nGeneral Comments: This work has impressive scope, integrating a wide range of satellite datasets over Berry Glacier – a region that has not previously been the subject of such a comprehensive study. I believe the results of the study will be of interest to the community, particularly regarding the novel observations of tidal seawater intrusions and their potential impact on basal melt rates within grounding zones. However, I have several significant concerns that should be addressed by the authors before publication. These concern the lack of robust uncertainty estimation or discussion of limitations, as well as certain aspects of the methodology and data interpretation that may affect the validity of the conclusions. It is crucial that the distinction between observations and interpretations is made clear throughout the manuscript. Additionally, the text and figures require refinement in many places to bring them up to publication standard. Below, I outline my major concerns below, followed by a list of specific suggestions (in attached file) for improving text clarity, figures, code and datasets.\n\nOriginality and Significance: The study provides new insights into the dynamics of the IGZ at Berry Glacier and the first published observations of changes in ice velocity, discharge and melt rates over the past 25 years. However, it should be noted that Berry Glacier is relatively small in the context of the Antarctic Ice Sheet. Thus, while the findings are significant for this setting and contribute to improved process-level understanding, their broader implications for ice sheet modeling and sea level rise projections are potentially more limited. I recommend enhancing the discussion of these broader implications, addressing—for example—how findings at Berry compare to those from other Antarctic IGZs and how they might contribute to modeling improvements on an ice-sheet-wide scale.\n\nTreatment of uncertainties: There is an insufficient and inconsistent treatment of errors and uncertainties throughout the analysis, including in the main text, methods and attached dataset. I strongly recommend that the authors include a more comprehensive discussion of the uncertainties and errors in each part of the analysis and how these propagate through the calculations. I recognize that quantifying some of these uncertainties may be challenging; nevertheless, an open discussion of these limitations is essential to avoid potentially misleading readers or overinterpreting the results.\n\nRobustness of Conclusions: I have some concerns about the validity of some of the data interpretation that form the basis of the paper’s main conclusions. I would like to see a response from the authors providing further clarification or justification of these.\n1. Long-term GL retreat 1995-2021\nThe authors report that the length of the Berry IGZ increased from 0.6 km in 1995/96 to 18±0.9 km in 2020-21. The 1995/96 IGZ length seems to be based on data from just two ERS1/2 interferograms, compared to >15 interferograms for 2020-21. I appreciate that there is limited data availability from 1995/96, but I wonder whether the reported shorter IGZ length in the 1990s may be influenced by the tidal sampling resolution. If the two ERS1/2 interferograms only captured the GL when it was in its Stage 1 position, it’s possible that any inland migration of the GL during other tidal phases was missed. This could lead to an underestimation of the IGZ length during this period, and therefore an overestimation of the long-term change by 2020-21. I recommend that the authors discuss this potential bias in the interpretation and clarify how tidal sampling might impact the results (1-2 sentences would suffice). Providing the modeled differential tide and maximum sampled tide for each of the ERS1/2 SAR scenes in both Supplementary Figure 6 and in the data spreadsheet (as for the Sentinel-1 and COSMO-SkyMed data) would provide valuable context.\n2. Calculations of ice thickness using HE\nThe authors compare the GL retreat at Berry to observed changes in velocity and ice elevation from 1996-2022 and estimate how this translates to changes in ice thickness, discharge and total mass balance. I am concerned about the validity of using hydrostatic equilibrium (HE) to convert ice elevation change observations to thickness changes. Additional justification of this approach is needed, along with a more open discussion of the associated limitations and uncertainties in the main text. While using HE for this calculating ice thickness is appropriate for fully floating ice, the InSAR data indicate that the ice in the Berry IGZ is grounded for a significant portion of the tidal cycle, with an average depth of seawater intrusions of ~0.6 m. This implies the ice within the IGZ cannot be in hydrostatic equilibrium, casting doubt on the validity of ice thickness values derived from this method, and, by extension, the estimates of thinning rates, ice discharge, basal melt rates and bathymetry. For example, the authors’ interpretation that a 24 m surface elevation decrease in the IGZ translates to 222 m of thinning (L101-102) is likely to be a significant overestimation.\nIt is well-documented that satellite-derived estimates of ice thickness and basal melt rate within GZs are subject to large uncertainties due to the breakdown of the hydrostatic assumption (e.g., Chuter & Bamber, 2015; Griggs & Bamber, 2011; Kim et al., 2024), which often leads to these regions being masked out (e.g., Griggs & Bamber, 2011; Paolo et al., 2015; Wilson et al., 2017) or interpolated across (e.g., Fretwell et al., 2013; Morlighem et al., 2020). While I note that some attempt has been made by the authors to address this (e.g. L150-152; 296-298), the discussion remains too vague and does not adequately address the impact on the key findings.\n3. Refinement of bed topography\nThe authors propose that the bed topography within the Berry IGZ is up to 1,300 m deeper than measurements from MCoRDS airborne radar and in BedMachine v3.7 (which interpolates between sparse radar measurements). However, I do not think that the evidence presented here is strong enough to support this conclusion; I suggest the authors either reconsider this interpretation or provide a more robust justification, clearly communicating any limitations. Specific concerns include:\na) Methods description: The description of methods used to reconstruct bed topography lacks sufficient detail, both in the main text (L125-127) and methods (L278-279). I recommend expanding the description to clarify the approach, including details on the geoid and specifying the areas or flowlines over which bed topography was calculated.\nb) Use of HE/flotation: If the new bathymetry estimates in the IGZ are based on ice thickness calculated using HE/flotation, they are subject to the same uncertainties outlined above. Prior studies have demonstrated that ice thicknesses calculated in this way close to the GL tend to overestimate values compared to radar measurements due to the breakdown of HE (e.g. Chartrand & Howat, 2023).\nc) Radargram interpretation: The authors propose that the bed depths from the MCoRDS radar in the Berry IGZ have been misinterpreted as too shallow (L132-134). However, upon reviewing Supp. Figure 2 (noting that I am not a radar specialist), I find this interpretation unconvincing. While weak bed echoes at the glacier center may well indicate higher uncertainty in bed depths, the picked bed depths (red dashed lines) in all four radargrams look reasonable and there is little indication of any deeper echoes at the depths suggested by the authors. Radar is generally considered most reliable way to measure ice thickness/bed depth in these regions, and it seems more plausible that the HE-based calculations in this study might overestimate thickness/depth compared to the radar. Labelling specific features in the radargrams that support the deeper bed interpretation would be helpful, as well as providing a comparison to data from the gravity inversion mentioned in L136.\nd) Clarification of the 1300m depth difference: The origin of the 1,300 m value is unclear. In Figure 3(g-h), the difference between the BedMachine bed elevation and the author’ new calculations appear to max out at around 1,000 m. Since this 1,300 m value is presented as a key result in the abstract, its basis should be clarified in the text.\n4. Long-term melt rates / basal melt regime\nThe text in this section (L146-192) needs refining; it is currently difficult to follow the specific areas and time periods for which melt rates have been calculated. Clear distinctions are needed between discussions of: (a) Melt rates within the Berry IGZ (2019-2022); (b) Melt rates within a zone 10 km seaward of the IGZ, compared to neighboring glaciers and previous measurements; and (c) Predicted melt rates within the Berry IGZ due to tidal seawater intrusions\nAgain, my concerns about the use of HE to calculate thinning rates within the IGZ alongside the lack of uncertainty quantification make me skeptical about the validity of the derived melt rates. The authors briefly mention using 4.5 km spatial smoothing to mitigate deviations from hydrostatic equilibrium (L150-152), but further detail is needed to explain what was done and how this reduces uncertainty. As I understand, the authors have interpreted the formation of a >100m thick cavity beneath the Berry IGZ over the past 25 years (L156), reaching depths up to 300 m near State 2. This implies the whole region is afloat, which starkly contradicts the InSAR GL results which show the whole IGZ is ephemerally grounded with depths of seawater intrusions averaging just 0.6 m. Clarification of this discrepancy is important.\n5. Tidal seawater intrusions\nThe estimates of melt rates due to tidal seawater intrusions (L181-192) appear to rely heavily on assumptions and values that are either poorly constrained or lack clarity on their uncertainties. This undermines confidence in the results and may mislead readers, especially if the values are subsequently used for in models. It is recommended that the authors clearly outline all assumptions, cite where the values are taken from (note specific comments below about the CTD data), and acknowledge the scale of the limitations.\nDiversity, Equity and Inclusion: I would like to draw attention to the lack of female representation among the nine co-authors, which might be something the authors or the journal could consider addressing in future to foster more diversity in scientific collaborations.\n(Remarks on code availability)\nThe Zenodo link points to code used to compute basal melt rates at Thwaites, originally from a different paper (Gadi, 2024). I am not sure of the specific journal guidelines on this, but I suggest this should either be appropriately cited alongside the dataset, or specific code used for this work at Berry should be provided.\nNo code is provided for the other calculations described in the methods, including ice bottom/bed topography, ice mass balance, GZ length or tides. This should be included to ensure full transparency.", "reviewer_2": "A note in advance of this review: I am focusing on the physical oceanography part of the manuscript “Rapid retreat of Berry Glacier, West Antarctica from satellite radar interferometry and other data” by Chen et al. have not commented on the remote sensing and glaciology aspects, since these are outside of my area of expertise as a physical oceanographer. I am referring quite heavily to my own work as a consequence of the scarcity of oceanographic observations in the area and have thus chosen to write this review non-anonymously.\nKaren Assmann\nOverall I think the manuscript contains exciting, new results, but the line of argument around the connection between the GL retreat of Berry and changes in CDW temperatures needs revision and refinement to make it convincing.\nGenerally, the manuscript needs some polishing. I suspect that shortening the text to make it fit within the Nature Communications format is the culprit here. This applies especially to the introduction that needs to be reworked – see my specific comments.\nThere are two main points in the manuscript. One is that you can identify tidal variability in the movement of the glacier and that this likely gives the warm CDW on the continental shelf access to the grounding line zone leading to high melt rates. The second one is that the grounding line of Berry has retreated rapidly over the past 25 years due to that CDW access.\nMy main problem here is that your line of argument focusses on the presence of CDW and not how it may have changed. There isn’t much CTD data available, but what there is could be used better:\nShow the temperature profiles that you are using and don’t just leave the reader to eye-ball them from Jacobs et al. 2013. Also give data sources for the CTD data, especially the mysterious 2018 CTD that you base your heat flux calculation on. Use these temperatures and the position of the thermocline vs the associated GL positions and cavity shapes to discuss the implications for basal melt.\nCDW access to the deep basin in front of Berry is most likely across the shelf break and not from the east on the shelf. The sill depth is shallower than for Siple Trough (see Jacobs et al. Oceanography 2012) and the off-shelf CDW core actually sits below the sill depth. This implies that it is likely even more episodic than we have shown for Siple Trough and even more dependent on winds along the shelf break and Ekman pumping (Assmann et al. 2019).\nThe implication of this is that you have a mechanism that explains why CDW access may have changed between 1994 and 2021: If you compare your mass balance time series in Fig. 4a with our Ekman pumping time series in Assmann et al. 2019 Fig 4f they agree rather well. You could calculate a similar time series for the shelf break in front of Berry, but it is unlikely that it would be much different because of the scales of atmospheric variability. Just like we argued that the enhanced Ekman pumping over the Siple Trough shelf break led to warmer CDW in front of the western Getz in 2016 & 2018 compared to 2000 and 2007 that argument could be used in this study as well.\nSpecific comments:\nLine 42: “affected” not “effected”.\nLine 45: You are implying that changes in GL position affect sea level, because of the addition of seawater by floating ice. Floating ice shelves are hydrostatically balanced, i.e., they don’t affect sea level. What does affect is the mass flux across the grounding line and the mass loss from the ice shelves by basal melting and calving that adds freshwater, i.e., volume to the ocean. Just remove this sentence.\nLine 50: This sentence is very out of context, move to last paragraph in the introduction where you introduce the location.\nLine 52-53: “due to ocean tides” not “changes in oceanic tides”. Tides imply variability.\nLine 56-67: For my taste this is too technical for an introduction. I got lost in the detail and wasn’t sure why you are using all of these satellite data sources.\nLine 66: You haven’t given any geographical context, i.e., the reader has no idea where or what Berry is. I assume this is result of shortening the text to fit within the Nature comms format.\nLine 71: Change “undiluted warm, salty CDW” to “warm CDW”. You don’t need salinity to make your point and I would remove it to simplify the story. CDW access on the western side of Getz is intermittent, so the CDW that makes it to the ice shelf front is anywhere from part of the thermocline to part of the CDW core (see Assmann et al. 2019).\nLine 147-152: Duplicates the methods section, so you could remove this.\nLine 161-162: Remove the reference to salinity. This is superfluous to your main storyline. Either write “mean bottom temperature of…” or change to something like “bottom temperature exceeding 1 C” to match the CTD sections in Jacobs et al. 2013 and sections and mooring records in Assmann et al. (2019).\nLine 162-165: see comment about Supp. Fig. 4a and general comment about CDW access from the shelf break.\nLine 165-167: This is the only mention you have in the entire article about changes in ocean temperatures and it is too hand-waving to support your main point. Show the actual temperature profiles in a figure, because it is hard to pin-point Berry in the Jacobs et al. (2013) figures.\nLine 181-185: I am not sure why you use 0.6 m as a thickness when you draw in 1.2 m worth of water volume over a tidal cycle.\nLine 186: The tide is diurnal at Berry (and generally in the Amundsen Sea), i.e., half a tidal cycle is 12 hours not 6 hours – see your Fig. 1.\nLine 187: Where is your 2018 CTD from? Add a source for all CTD data you use to your data statement. This is especially important for this 2018 CTD since you use the temperature for your heat flux calculations.\nLine 190: I like the Gadi et al. (2023) reference a lot and agree that not all heat in the inflowing seawater is used to melt ice, but I interpreted the 30% statement in their study that 30% of the ice flux across the grounding line was melted in the GZ and FZ.\nFig 1: Even with the improved resolution, panels in Fig 1 are still very small, this applies particularly to the top left insert in panel a.\nSupp. Fig. 4 a: Add the ice shelf front to panel and extend the figure to the continental shelf break to show the channels giving the CDW access. In my experience, the bathymetry on the continental shelf around the western Getz isn’t very detailed or reliable, because I don’t think there is more multibeam data than shown in Nitsche et al. (2007). I would qualify the statement you make about these channels in lines 162-165.\nSupp. Fig. 4 b & c: I do like the cavity sections, because they show nicely how the Berry GZ has moved to a retrograde slope, while the DeVicq GZ sits just at a small rise. I also suspect that bed shape is more responsible for the different reactions of the two glaciers rather than CDW temperature.\nThe temperature contours in panels b and c imply a thermocline at constant depth in front of Berry. I would have preferred a profile insert that shows that this is a point measurement. Showing would also give you the opportunity to show profiles for different years and to make your point about the CDW changes.\nSupp. Fig. 5: I do wonder if it would be worth to move some of the panels into Fig. 5, because they illustrate the exceptional reaction of Berry within the region and give context to your discussion of DeVicq glacier which feels somewhat stand-alone in the manuscript.\n(Remarks on code availability)", "reviewer_3": "Review for ‘Rapid retreat of Berry Glacier, West Antarctica, from satellite radar interferometry and other data’\nIn this study, Chen et al. used DInSAR interferograms generated from different satellite data to map both the short-term and long-term grounding line migrations in Berry Glacier located in the Getz Ice Shelf from 1996 to 2021. They discovered a short-term grounding line migration over 18 km and claimed that the glacier bed is up to 1300 m deeper than the BedMachine data. The methods used in this study are standard and not novel, particularly for research focused on grounding line mapping using DInSAR. The identification of a large ‘Ice Grounding Zone’ in Berry Glacier is also not new, as it was previously discovered by Mohajerani et al (2021). The paper does not offer significant advances in our understanding of either short-term grounding line migration or long-term changes related to ice dynamics, as these concepts have already been extensively explored and established in previous studies. Furthermore, the interpretation of bed topography and the calculation of short-term grounding line migrations require further consideration and improvements.\nMajor Comments:\nThe finding of the large tidally-influenced grounding line migration in Berry Glacier is not new, Mohajerani et al (2021) have already found grounding line changes in Berry Glacier to be about 15 km (https://www.nature.com/articles/s41598-021-84309-3). Given this, I feel this paper only provides a more detailed calculation building upon the work of Mohajerani et al (2021), and its main conclusion lacks novelty. Especially, Mohajerani et al (2021) have analyzed the entire Getz Ice Shelf. I am curious why the authors in this study chose to focus solely on a single glacier within the same region. Including all glaciers in Getz Ice Shelf would allow for a comparison of grounding line changes across glaciers that share similar oceanography settings, this would result in a more comprehensive study. Focusing on just one glacier significantly limits the scope of this research.\nI agree that short-term grounding line migrations have not been adequately incorporated into ice sheet modeling. However, this phenomenon has been well-documented in Antarctica in recent years using a similar DInSAR approach and datasets to those used in this study, with some examples listed below. While I acknowledge the efforts presented in this paper, I feel that its conclusions do not significantly advance our understanding of short-term grounding line migrations. Instead, the study only adds incremental evidence to the existing body of literature.\n- Milillo 2017: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074320\n- Milillo 2019: https://www.science.org/doi/10.1126/sciadv.aau3433\n- Brancato 2020: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL086291\n- Mohajerani 2021: https://www.nature.com/articles/s41598-021-84309-3\n- Milillo 2022: https://www.nature.com/articles/s41561-021-00877-z\n- Chen 2024: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL102430\nThe comparison between 1996 IGZ and 2020-2021 IGZ needs more care. In 1996 you only have two interferograms, it is very likely due to limited DInSAR data acquisition during that time, this may have resulted in incomplete capture of short-term grounding zone migrations. While in 2020-2021, there is a greater availability of data, enabling greater sampling and a more comprehensive imaging of the entire IGZ. Please address this in the analysis.\nFigure 2a shows the spatial density of different grounding line positions obtained in this study, capturing three different states. However, a limitation of this approach is the lack of consideration for temporal sampling. If the GL acquisitions are concentrated around same tidal phases, the resulting spatial distribution map may be biased. In addition, the gap between State 2 and 3 is too short (~1 km), making this classification unreliable.\nIt is interesting that the authors found the bed topography underneath Berry Glacier to be 1300 m deeper than the BedMachine. This is useful information that can be used to update the BedMachine dataset. However, I am not particularly surprised that the Bedmachine dataset is inaccurate in some of the glaciers due to a lack of observations. Additionally, it is unclear how this discovery of deeper bed topography changes our current understanding of the grounding line dynamics and regional mass imbalance.\nThe discussion about seawater intrusion is unclear. In Line 139: the authors claim that ‘A series of bed depressions between the three states must trap seawater…’ I feel this claim is not well supported and more evidence is needed to make sure this statement is scientifically correct. Although the authors claimed to detect the reverse fringes of opposite motion upstream to indicate the trapped seawater, there are not enough details on how the water extent is mapped in Figure 3 and how the presence of trapped seawater changes in sync with grounding line migrations.\nMismatching patterns in ice dynamics: The dhdt in the west flank is much lower than the main glacier trunk where there is an over-deepened retrograde bed slope, this does not match the basal melt rate patterns in Figure 5 and the ice velocity changes in Figure 3. Based on MISI theory, we would expect to see matching patterns of ice thinning, grounding line retreat, as well as ice speed up along the overdeepended bed. The authors, however, only provide one comment on this mismatching pattern in Line 114-115: ‘The pattern of thinning does not coincide with the pattern of speed up as the retreat expands laterally along a tributary’. This explanation is too simplistic and misleading. Please provide a more detailed explanation.\nSpecific Comments\nLine 51-55: the jump from basal resistance associated with the grounding line to the lack of consideration of short-term grounding line migration in glacier modeling feels out of place here. Please consider talking about the importance of short-term grounding line and its observations across Antarctica first.\nLine 57-59: the accuracy of GL mapping in altimetry and DInSAR needs references\nLine 97: ‘GL is predominantly in state 1 in 1996…’ also depends on the number of DInSAR interferograms and the tidal samplings.\nLine 129-130: ‘up to 1300 m lower than those reconstructed in BMv3.7’, could you explain more how this number is obtained in the figure or in the main text?\nFigure 1: please increase the size of the inset map of Getz ice shelf in subplot 1a, it is difficult to see the details in its current format\nFigure 3: please avoid using rainbow colormap in line plots 3c/d/e/f, it is hard to distinguish between different years, perhaps a sequential colormap is better. In 3g and h, please clarify the question mark annotated on the figure in the caption, this part is also unclear from the main text\nLine 151: why 4.5 km?\nLine 160: could you map or annotate the boundaries of this surface elevation bumps in the satellite images in Supplementary Video 1? It is difficult to distinguish this feature in the current video.\n(Remarks on code availability)\nI have not reviewed the code due to the above reasons."}, "author_response": "Dear Editor, Reviewers,\nWe are very thankful for the reviewers’ comments. In response to these, we have extensively\nrevised the manuscript. However, the impact of the revision on the results is minimal. We\nbelieve that the new version is significantly improved. We hope that the reviewers will find the\nrevised manuscript acceptance for publication.\nOn behalf of the co-authors – Hanning Chen.\nReviewer #1\n1. Originality and Significance: The study provides new insights into the dynamics of the IGZ\nat Berry Glacier and the first published observations of changes in ice velocity, discharge\nand melt rates over the past 25 years. However, it should be noted that Berry Glacier is\nrelatively small in the context of the Antarctic Ice Sheet. Thus, while the findings are\nsignificant for this setting and contribute to improved process-level understanding, their\nbroader implications for ice sheet modeling and sea level rise projections are potentially\nmore limited. I recommend enhancing the discussion of these broader implications,\naddressing—for example—how findings at Berry compare to those from other Antarctic\nIGZs and how they might contribute to modelling improvements on an ice-sheet-wide\nscale.\nResponse: Yes, Berry Glacier is a small glacier, with a low potential for sea level rise but\nits evolution illustrates the fundamental physical processes operating in the Antarctic\nenvironment that - in the presence case - led to the formation of a gigantic ice grounding\nzone and a rapid retreat along a deep trough. The magnitude of the retreat, the size of the\ncavity, the magnitude of the melt rates, and the bathymetry are relevant aspects to a broader\ntype of study of glacial retreat in a warmer ocean. Berry has also one of the longest IGZ in\nAntarctica, as first noted in Mohajerani et al. (2022). There are others: Barnett Glacier,\nVictoria Land; Vanderfjord Glacier, Wilkes Land, Lambert/Mellor/Fisher, Jutulstraumen,\netc. In the revision, we discussed the broader implications of this study at Lines 226-240.\n2. Treatment of uncertainties: There is an insufficient and inconsistent treatment of errors and\nuncertainties throughout the analysis, including in the main text, methods and attached\ndataset. I strongly recommend that the authors include a more comprehensive discussion\nof the uncertainties and errors in each part of the analysis and how these propagate through\nthe calculations. I recognize that quantifying some of these uncertainties may be\nchallenging; nevertheless, an open discussion of these limitations is essential to avoid\npotentially misleading readers or overinterpreting the results.\nResponse: We documented the uncertainty of the grounding line retreat, the time series of\nice velocity, surface height, and inferred bed topography, which led us to find that some\nMCoRDS data were not correctly interpreted. We double checked the discussion of the\nuncertainty of the melt rates.\n3. Robustness of Conclusions: I have some concerns about the validity of some of the data\ninterpretation that form the basis of the paper’s main conclusions. I would like to see a\nresponse from the authors providing further clarification or justification of these.\n1) Long-term GL retreat 1995-2021\nThe authors report that the length of the Berry IGZ increased from 0.6 km in 1995/96 to\n18±0.9 km in 2020-21. The 1995/96 IGZ length seems to be based on data from just two\nERS1/2 interferograms, compared to >15 interferograms for 2020-21. I appreciate that\nthere is limited data availability from 1995/96, but I wonder whether the reported shorter\nIGZ length in the 1990s may be influenced by the tidal sampling resolution. If the two\nERS1/2 interferograms only captured the GL when it was in its Stage 1 position, it’s\npossible that any inland migration of the GL during other tidal phases was missed. This\ncould lead to an underestimation of the IGZ length during this period, and therefore an\noverestimation of the long-term change by 2020-21. I recommend that the authors discuss\nthis potential bias in the interpretation and clarify how tidal sampling might impact the\nresults (1-2 sentences would suffice). Providing the modelled differential tide and\nmaximum sampled tide for each of the ERS1/2 SAR scenes in both Supplementary Figure\n6 and in the data spreadsheet (as for the Sentinel-1 and COSMO-SkyMed data) would\nprovide valuable context.\nResponse: Yes, because we have only two interferograms, the width of the IGZ in 1996 is\nnot as well determined as in later years. Yet, there is also several lines of evidence\nindicating that the GL was at state 1 in 1996. First, we see no residual signal upstream of\nthe state 1 IGZ in the two 1996 interferograms. In the more recent interferograms, there is\nalways a residual signal upstream, between state 1 and 3, that indicates that during some\npart of the tidal cycle, there is residual vertical motion in that region (Fig. 1). It is unlikely\nthat seawater intruded farther than state 1 in 1996. Furthermore, the glacier thinned\nconsiderably since 1996 (11±1 m/yr). Ice deviated from flotation by a few meters at state\n2, which the grounding line reached in 2006. See Lines 277-280. The 1996 IGZ could not\nhave extended to state 2. As requested, in the revision, we also added information about the\nmodeled differential tide for ERS1/2 SAR in Supplementary Fig. 7.\n2) Calculations of ice thickness using HE\nThe authors compare the GL retreat at Berry to observed changes in velocity and ice\nelevation from 1996-2022 and estimate how this translates to changes in ice thickness,\ndischarge and total mass balance. I am concerned about the validity of using hydrostatic\nequilibrium (HE) to convert ice elevation change observations to thickness changes.\nAdditional justification of this approach is needed, along with a more open discussion of\nthe associated limitations and uncertainties in the main text.\nWhile using HE for this calculating ice thickness is appropriate for fully floating ice, the\nInSAR data indicate that the ice in the Berry IGZ is grounded for a significant portion of\nthe tidal cycle, with an average depth of seawater intrusions of ~0.6 m. This implies the ice\nwithin the IGZ cannot be in hydrostatic equilibrium, casting doubt on the validity of ice\nthickness values derived from this method, and, by extension, the estimates of thinning\nrates, ice discharge, basal melt rates and bathymetry. For example, the authors’\ninterpretation that a 24 m surface elevation decrease in the IGZ translates to 222 m of\nthinning (L101-102) is likely to be a significant overestimation.\nIt is well-documented that satellite-derived estimates of ice thickness and basal melt rate\nwithin GZs are subject to large uncertainties due to the breakdown of the hydrostatic\nassumption (e.g., Chuter & Bamber, 2015; Griggs & Bamber, 2011; Kim et al., 2024),\nwhich often leads to these regions being masked out (e.g., Griggs & Bamber, 2011; Paolo\net al., 2015; Wilson et al., 2017) or interpolated across (e.g., Fretwell et al., 2013;\nMorlighem et al., 2020). While I note that some attempt has been made by the authors to\naddress this (e.g. L150-152; 296-298), the discussion remains too vague and does not\nadequately address the impact on the key findings.\nResponse: As ice is afloat during part of the tidal cycle and the tidal amplitude is only +/-\n0.6 m, ice must be within 1-2 meter of flotation, hence the error of using HE during the\ntidal cycle is less than 10-20 m. Following the 24 m surface lowering (Line 108), the\ngrounding line reached state 2 in 2006, so the assumption of HE is valid. Finally, in Figure\n3g, we match the bed depth from MCoRDS with the ice draft derived from the 2019 DEM,\nwhich gives another line of evidence that this calculation is reliable. This is now stated\nmore clearly in the manuscript.\n3) Refinement of bed topography\nThe authors propose that the bed topography within the Berry IGZ is up to 1,300 m deeper\nthan measurements from MCoRDS airborne radar and in BedMachine v3.7 (which\ninterpolates between sparse radar measurements). However, I do not think that the evidence\npresented here is strong enough to support this conclusion; I suggest the authors either\nreconsider this interpretation or provide a more robust justification, clearly communicating\nany limitations. Specific concerns include:\na) Methods description: The description of methods used to reconstruct bed topography\nlacks sufficient detail, both in the main text (L125-127) and methods (L278-279). I\nrecommend expanding the description to clarify the approach, including details on the\ngeoid and specifying the areas or flowlines over which bed topography was calculated.\nb) Use of HE/flotation: If the new bathymetry estimates in the IGZ are based on ice\nthickness calculated using HE/flotation, they are subject to the same uncertainties outlined\nabove. Prior studies have demonstrated that ice thicknesses calculated in this way close to\nthe GL tend to overestimate values compared to radar measurements due to the breakdown\nof HE (e.g. Chartrand & Howat, 2023).\nc) Radargram interpretation: The authors propose that the bed depths from the MCoRDS\nradar in the Berry IGZ have been misinterpreted as too shallow (L132-134). However, upon\nreviewing Supp. Figure 2 (noting that I am not a radar specialist), I find this interpretation\nunconvincing. While weak bed echoes at the glacier center may well indicate higher\nuncertainty in bed depths, the picked bed depths (red dashed lines) in all four radargrams\nlook reasonable and there is little indication of any deeper echoes at the depths suggested by\nthe authors. Radar is generally considered most reliable way to measure ice thickness/bed\ndepth in these regions, and it seems more plausible that the HE-based calculations in this\nstudy might overestimate thickness/depth compared to the radar. Labelling specific features\nin the radargrams that support the deeper bed interpretation would be helpful, as well as\nproviding a comparison to data from the gravity inversion mentioned in L136.\nd) Clarification of the 1300m depth difference: The origin of the 1,300 m value is unclear. In\nFigure 3(g-h), the difference between the BedMachine bed elevation and the author’ new\ncalculations appear to max out at around 1,000 m. Since this 1,300 m value is presented as\na key result in the abstract, its basis should be clarified in the text.\nResponse: 1. As stated in Lines 348-349, the EIGEN-6C4 geoid model from BedMachine\nv3.7 is used to geoid-correct the DEMs. The spatial extent over which the bed topography\nis reconstructed is clarified in Lines 137-139 and 340-341; 2. As noted earlier, the\nagreement between the ice draft inferred from HE and the basal interface detected by\nMCoRDS radar supports HE in this region; 3. The MCoRDS data document the ice–bed\ninterface; therefore, in the IGZ, the bed elevation inferred from MCoRDS cannot be below\nthe HE-inferred ice draft. Along profile A–B, the radar signal over the central IGZ region\n(above the red dashed line) suggests that the bed was misidentified, prompting us to shade\nthe area between the MCoRDS-derived bed and the ice draft to highlight this uncertainty.\nAlong profiles C–D and C–F, radar wave reflections from the steep valley sidewalls (off-\nnadir returns) obscure the basal interface in the valley center. For profile E–F, discrepancies\nare observed between the 2011 OIB Level 2 dataset and the corresponding radargrams:\nwhile the radargrams have the correct bed picks, the Level 2 data contained erroneous\nvalues, leading to the substantial mismatch between our reconstructed bed topography and\nBedMachine. Unfortunately, users have used the Level 2 values instead of the Level 1\nvalues. We have updated Supplementary Fig. 2d to show the erroneous data for clarity; 4.\nIn the revision, we note that the maximum depth discrepancy of 1300 m is located on the\neastern side of the IGZ, but it does not overlap with any radar profile examined in this study\n(See Lines 140-143), so it is a maximum offset with BedMachine which is interpolated data\nat this location.\n4) Long-term melt rates / basal melt regime\nThe text in this section (L146-192) needs refining; it is currently difficult to follow the\nspecific areas and time periods for which melt rates have been calculated. Clear distinctions\nare needed between discussions of: (a) Melt rates within the Berry IGZ (2019-2022); (b)\nMelt rates within a zone 10 km seaward of the IGZ, compared to neighboring glaciers and\nprevious measurements; and (c) Predicted melt rates within the Berry IGZ due to tidal\nseawater intrusions Again, my concerns about the use of HE to calculate thinning rates\nwithin the IGZ alongside the lack of uncertainty quantification make me skeptical about\nthe validity of the derived melt rates. The authors briefly mention using 4.5 km spatial\nsmoothing to mitigate deviations from hydrostatic equilibrium (L150-152), but further\ndetail is needed to explain what was done and how this reduces uncertainty. As I\nunderstand, the authors have interpreted the formation of a >100m thick cavity beneath the\nBerry IGZ over the past 25 years (L156), reaching depths up to 300 m near State 2. This\nimplies the whole region is afloat, which starkly contradicts the InSAR GL results which\nshow the whole IGZ is ephemerally grounded with depths of seawater intrusions averaging\njust 0.6 m. Clarification of this discrepancy is important.\nResponse: 1. The distinction of areas and periods has been updated at Lines 162-163 and\n190-193; 2. We used different window sizes for the filtering until the melt rates do not\nchange at a detectable level and heterogeneities are eliminated. The window size is 4.5 km,\nwhich roughly corresponds to the size of the bending zone discussed by Chartran et al.\n2024; 3. We attribute the formation of the subglacial cavity at state 2 to basal melting.\nThe\nDInSAR fringe patterns observed in this region reflect the vertical displacement of the ice\nsurface driven by subglacial water motion, rather than the total water thickness. In other\nwords, the cavity has areas of partial grounding, and the water column must be thicker in\nbetween these pinning points, perhaps 10’s m, but we do not have data to quantify it.\n5) Tidal seawater intrusions\nThe estimates of melt rates due to tidal seawater intrusions (L181-192) appear to rely\nheavily on assumptions and values that are either poorly constrained or lack clarity on their\nuncertainties. This undermines confidence in the results and may mislead readers,\nespecially if the values are subsequently used for in models. It is recommended that the authors clearly outline all assumptions, cite where the values are taken from (note specific comments below about the CTD data), and acknowledge the scale of the limitations.\nResponse: Yes, the melt rates quoted in this section are order-of-magnitude melt numbers.\nIn the revision, we made this clearer (Lines 205-225).\n4. Diversity, Equity and Inclusion: I would like to draw attention to the lack of female\nrepresentation among the nine co-authors, which might be something the authors or the\njournal could consider addressing in future to foster more diversity in scientific\ncollaborations.\nResponse: This comment is not appropriate in a scientific review.\n5. Specific Comments\n1) Title – The title doesn’t really reflect the key findings of the paper; I suggest this could\nbe updated to include something on seawater intrusions which I think are arguably the\nmost interesting result. Also ‘and other data’ is a bit weak.\nResponse: The title has been changed to “Rapid retreat of Berry Glacier, West Antarctica,\nlinked to seawater intrusions revealed by satellite radar interferometry data”.\n2) Abstract – should be updated to reflect any changes made to the manuscript during\nrevision.\nResponse: The changes in the manuscript are updated in the abstract.\n3) L42 – typo; ‘affected’\nResponse: Fixed at Line 43.\n4) L47-49 – These sentences are quite vague. Are you referring to the effect of short-term\nGL retreat and advance periods (i.e. with the tides), or longer-term retreat/advance\nphases? Please clarify and consider adding references for support.\nResponse: If we refer to \"GL retreat reduces bottom friction and speeds up the glacier, GL\nadvance increases drag and slows down the glacier\", then it is the same effect on short and\nlong time scales.\n5) L51 – This paper doesn’t directly discuss CDW; you might consider citing more\nrelevant oceanography papers, such as Dinniman et al. (2012)\nhttps://doi.org/10.1175/JCLI-D-11-00307.1, Wåhlin et al. (2010)\nhttps://doi.org/10.1175/2010JPO4431.1; Schmidt et al. (2023)\nhttps://doi.org/10.1038/s41586-022-05691-0; Thoma et al. (2008)\nhttps://doi.org/10.1029/2008GL034939.\nResponse: We added these references at Line 72: Thoma et al. 2008, Dinniman et al. 2012,\nand Schmidt et al. 2023.\n6) L66 – I recommend using the full name ‘Berry Glacier’ here, since this is its first\nmention in the main text.\nResponse: Added at Line 64.\n7) L67 – ‘locus’ suggests a single point; consider using ‘area/region’.\nResponse: “locus” changed to “region” at Line 65.\n8) L75 – The dash between 0.6 and km can be removed here, and similarly in other places\nwhere it is not used as an adjective.\nResponse: The dash has been removed at Line 75, and other places.\n9) L79 – It is not clear where this observation is shown on Figure 2; I suggest adding\nlines/labels\nResponse: The GL from HE has been added to Figure 2a.\n10) L87-98 – This analysis would be strengthened by discussing potential reasons for the\nlack of correlation between tidal forcing and short-term GL migration, as well as\nconsidering other controls that may drive this short-term variability. Comparing to\nstudies that have documented different patterns of short-term GL migration elsewhere\nin Antarctica (e.g. Rignot et al., 2024; Chen et al., 2023; Freer et al., 2023) would be\nvaluable. For example, could this interpretation be affected by calculating modelled\ntides 50-100 km seaward of the IGZ, where tidal forcing may be out of phase with the\nresulting tidal flexure within the IGZ itself? Exploring whether there any correlations\nwith the longer-period spring-neap tide cycle, phase lags between rising and falling\ntides, or interactions with upstream subglacial hydrology—as observed in the\nThwaites IGZ (e.g. Chartrand et al., 2024; Rignot et al., 2024)—would enhance the\nanalysis. I note that Figure 5b shows several subglacial channels upstream of the IGZ,\nbut this has not been addressed in the text.\nResponse: The spreadsheet in the Dryad archive includes tidal height computed at two\noffshore locations, 50 km and 100 km seaward of the IGZ. The record shows negligible\ndifference in tidal amplitude and phase between the two sites. To further investigate the\ntidal influence on GL migration, we analyzed the spring–neap tidal cycle as well as the ebb\nand flow phases for each GL state, as now shown in Supplementary Fig. 1. Our results\nshow no discernible correlation between grounding line retreat and sea surface height, even\nwhen accounting for variations caused by the spring–neap cycle and tidal phase.\n11) L87-90 – It would also be interesting to see a comparison of the modelled Δtide (at\nthis same position 50-100 km seaward of the GL) compared to the measured InSAR\nΔSSH within the IGZ. This may provide some insight into the non-linear relationship\nbetween tidal forcing further from the ice front and the ice shelf flexural response/ GL\nmigration within the IGZ.\nResponse: This was shown in Figure 2c, which indicates that tide model and DInSAR agree.\n12) L97 – I suggest it is challenging to confidently state that the GL was \"predominantly\"\nin State 1 in 1996 based on only two observations. Perhaps rephrasing to reflect this\nuncertainty would be provide more context.\nResponse: Yes, we added a note of caution and indicated no evidence for signal upstream\nof that position in 1996, in contrast with recent interferograms (See Lines 76-80).\n13) L101-115 – This section is somewhat challenging to follow as currently structured.\nConsider reorganizing and re-wording to improve clarity. It appears to mix different\ntime periods (2006-2022 and 1996-2022), various regions (e.g., \"between states 1 and\n2,\" \"west flank,\" \"between the 1996 and 2008 GLs,\" \"from the 2008 GL to 28 km\nbeyond the 2021 IGZ upper limit,\" \"along profiles G-H and C-D,\" \"glacier on\naverage\"), and different values (ice elevation, thickness, thinning, speed, and speed-up).\nThis makes the text difficult to follow. Additionally, when Δh, ΔH and ΔV are\naveraged over specific areas, these areas should be clearly defined or located on a map.\nResponse: Agreed. We revised the paragraph for improved clarity and logical flow (Lines\n106-123). We clarified our description of the western flank (Lines 110-111), now explicitly\ndefining it as the region where the IGZ expanded laterally. Figure 3b highlights the area of\nice flow acceleration, extending up to 28 km beyond the 2021 inland limit of the IGZ.\n14) L104 – Clarification is needed regarding how glacier speed changes were calculated;\nnot enough detail is given here or in the methods section. Are these glacier speed\nchanges calculated as an average along flowlines, at specific points, or averaged across\nspecific areas? Including this information in the text or figure would provide useful\ncontext.\nResponse: The reported speed changes represent average values calculate over specified\nregions or along specific profiles. We clarified each case in the revised text.\n15) L105 – The sentence on glacier speed feels out of place between other sentences\ndiscussing glacier speed-up. Consider moving it to a more logical position or removing\naltogether. It is important here to be specific about the time stamp of the velocity value.\nResponse: The purpose of referencing the maximum speed here, along with the subsequent\nstatement, was to highlight that the location of peak ice speed does not coincide with the\nregion exhibiting the largest speed change. The sentence has been removed for clarity.\n16) L106 – The 1996 and 2008 GLs are not currently shown on Supplementary Figure 1,\nbut should be added.\nResponse: Based on the comment above, we removed the sentence related to the maximum\nspeed, the only place where Supplementary Fig. 1 was cited, therefore we removed the\nSupplementary Fig. 1 as well.\n17) L107-109 – It is important to clearly define the region over which the average speed-\nup of 134 m/yr and a 59 m elevation decrease is calculated.\nResponse: Agreed. As noted at Line 115-117, the region refers to the area extending from\nthe state 2 to inland by 28 km beyond its upper limit of 2021 IGZ.\n18) L110-111 – The results in L100-109 discussed change in V and h over the period 2006-\n2022, but it now seems for the flowline results you switch here a longer period 1996-\n2022. Please explain this discrepancy in time periods, and ensure the text explicitly\nmentions these timeframes to avoid confusion.\nResponse: Agreed. In the revision, we separate the discussion of the change in 2006-2022,\nfor which we have complete spatial maps for the basin, from the discussion for 1996-2022\nfor which the 1996 have data gaps, therefore we only analyze profiles.\n19) L110 – Consider rephrasing for clarity: ‘… the surface elevation decreased by up to\n59 m and ice sped up by 48% from 448 m/yr in [YEAR] to 665 m/yr in [YEAR]’.\nResponse: The sentence has been updated at Lines 118-119.\n20) L113 – is ‘on average’ referring to the entire glacier basin?\nResponse: The statement refers to the 2021 IGZ. See update at Line 121.\n21) L113 – As explained in the general comments above, I suspect these thinning rates are\nsignificantly overestimated given the region is not in hydrostatic equilibrium\n(especially if these values are referring to the entire glacier basin, the majority of which\nis fully grounded).\nResponse: We apply the hydrostatic equilibrium (HE) assumption only to regions where\nseawater intrusions are inferred at a specific time period. For the period 1996–2005,\nchanges in ice thickness were equated directly with surface elevation changes, as the GL\nreached to state 2 around 2006. Between 2006 and 2011, the HE assumption was applied\nwithin the region from state 1 to state 2. From 2012 to 2022, HE was applied across the\nentire 2021 IGZ extent.\n22) L114-15 – Is this referring to the spatial pattern of thinning across the whole basin?\nI suggest you are more explicit about what is being cited here from Selley et al. (2021);\nis this an observation they also made?\nResponse: This was a mistake. Text has been corrected.\n23) L116-123 – I recommend the authors clarify the time period and locations for ice\ndischarge calculations, and how this relates to the two time periods of observed h and\nV change discussed above (2006-2022 vs 1996-2022). It is currently difficult to\nascertain how this was done.\nResponse: The time period and locations of flux gates for ice discharge are indicated in Fig.\n1a and mentioned more clearly at Lines 316-320. The relationship between ice discharge\nand changes of h/V is discussed at Lines 327-329. We initiate the ice discharge calculation\nin 1996. For the time periods 1996-2005, 2007, and 2013, a linear interpolation is used to\nfill gaps in ice discharge as mentioned in the caption of Fig. 4. This is now clarified at Lines\n124-126 and 316-331. The time period and locations for the ice discharge calculations are\nrelated to the timing when the GL reaches each state.\n24) L116 – Against which baseline are these discharge anomalies measured? Is it the same\n1979-2004 reference period cited for the SMB anomalies in the following sentence?\nIf so, please clarify.\nResponse: The reference SMB is 1979-2004 is used as the balance discharge. See Lines\n126-130 and 334-337.\n25) L119 – Consider rewording for clarity: ‘Between 1996 and 2022, annual anomalies in\nmass balance, M, increased by a factor of 6.6, from X Gt/yr to Y Gt/yr’.\nResponse: The sentence has been re-written at Lines 131-133.\n26) L122 – It would be more logical to introduce the total increase in ice discharge at the\nstart of this paragraph, and then transition from discharge > SMB > mass balance.\nResponse: The sentence has been rewritten at Lines 133-135.\n27) L127 – ‘jack up’ is a somewhat colloquial phrase which can have negative\nconnotations; a different term like ‘lift up’ would be more suitable here.\nResponse: Agreed. The sentence has been rewritten at Lines 137-139.\n28) L131 – The interchangeable use of \"OIB\" and \"MCoRDS\" in the text can be confusing.\nConsider specifying \"MCoRDS\" when referring to radar-derived thickness data and\n\"ATM\" for altimetry data from OIB and updating this consistently throughout.\nResponse: Agreed. “OIB” has been replaced by “MCoRDS” when referring to radar-\nderived thickness.\n29) L141 – It would help to clarify here that you are comparing results to a modeling study,\nnot direct observations. Consider rewording to something like: \"... gravity gradients,\nas modeled in [20].\".\nResponse: The sentence has been rewritten at Line 156.\n30) L144 – A number of previous studies have also observed this; you may wish to cite\nworks such as Chen et al. (2023), Freer et al (2023), Rignot et al. (2024).\nResponse: These citations have been added at Line 160.\n31) L146 – Please specify the time period over which basal melt rates were calculated—\nwas this solely for 2019-2022, as indicated in Figure 5?\nResponse: The time information has been added at Line 163.\n32) L154 – It would be helpful to clarify which area is being referred to here and to mark\nthis region on Figure 5.\nResponse: This has been clarified at Line 168. The region is the IGZ.\n33) L156 – you should use “meters” instead of “m’s”.\nResponse: “m’s” has been changed to “meters” at Line 172.\n34) L171-173 - There appears to be a contradiction between these sentences. The text\ninitially states that the Berry IGZ was unknown during previous studies, but then\nreferences comparisons of melt rates \"in the IGZ of Berry\" between 2003-2008 and\n2019-2022. Please clarify this.\nResponse: Yes, we needed to clarify that we compare the melt rates within 10 km of the\nlower limit of the IGZ as stated at Line 190 where we have coincident measurements.\n35) L172 - The “area of coincident measurements\" for comparing the three time periods\nof basal melt rates is unclear. I suggest the authors label this area in Supplementary\nFigure 5. This applies to the areas used for the melt rate calculations at the other\nglaciers, such as De Vicq, as well.\nResponse: We added white dashed polygons in Fig. 5c–d and Supplementary Fig. 5 to\ndelineate the region of coincident measurements.\n36) L176-177 – The statement \"the melt rates in the IGZ of Berry are also the largest for\nthe Getz Ice Shelf\" appears in a section discussing melt rates in the 10 km downstream\nof the IGZ, rather than the IGZ itself. Consider moving this sentence to the previous\nsection or clarifying that it refers to the downstream zone.\nResponse: “the downstream” has been added at Line 201.\n37) L177-180 – I suggest adding a reference to Supp. Fig 5d here, as that is the only place\nthis result is shown.\nResponse: The reference has been added at Line 204.\n38) L182 – There should be a unit given in ‘… from 0 m at state 3 to…’.\nResponse: The sentence has been re-phased at Lines 205-206.\n39) L185 – Does the 0.6 m value represent a calculated average of the observed InSAR\ndisplacements? Or is this a prediction? More clarity (+associated uncertainties) is\nneeded.\nResponse: “0.6” is half of the maximum vertical amplitude at state 1, 1.2 m, which comes\nfrom the tidal model. “Observed vertical displacements at the entry of the IGZ of 1, 4 and\n4.7 CSK fringes translate into a minimum change in water column of 1.4, 5.5 and 6.5 cm,\nrespectively” has been deleted to avoid confusion.\n40) L186 – the stated water flux of 6.9 x 107 m3 in 6 hours would assume that the GL\nmigrates all the way to state 3 each tide cycle. However, your results indicate there is\nno observed correlation between tide forcing and GL migration distance (Fig 2) – so\ndo you think this is still a reliable estimate of water flux? Please clarify and/or be more\nopen about the associated limitations of this assumption.\nResponse: This calculation is a maximum melt rate if water intrusions flush the entire cavity\nand all available heat is used to melt the ice.\n41) L186 – You use an area of 114 km2 for the IGZ here, but earlier a 100 km2 IGZ area\nwas used to calculate melt rates (L154)? Clarifying why these values differ would be\nhelpful.\nResponse: Agreed. “100 km2” has been changed to “114 km2” at Line 168.\n42) L187 – This is the first and only time you mention a 2018 CTD, but there is not\nassociated citation. Earlier, you referenced a 0.7C ocean temperature at 1,000m depth\nin front of Berry measured from a 2007 CTD (Jacobs et al. 2013). The ‘2018 CTD’\ndata gives ocean thermal forcing at 1,000m depth of +3.6C. Is this a mistake in the text\nwith the date, or are you referencing two different datasets here? If this is the case,\nplease clarify the date and location, alongside the appropriate citation.\nResponse: We originally cited Selley et al. 2021. Upon careful examination of the CTD\ndataset from KOPRI—used in the 2021 study—we found that the measurements extend\nonly as far west as Dean Island but do not reach Berry Glacier. Consequently, to estimate\npost-2007 ocean temperature changes at the ice front of Berry Glacier, we analyzed CTD\nprofiles collected at distant locations in west Getz Ice Shelf after 2007 instead\n(Supplementary Fig. 6). The CTD records indicate a warming trend since 2000, which\npersisted beyond 2007. If we assume that the ocean in front of Berry Glacier experienced\na comparable degree of warming, we find a thermal forcing of +3.6°C at 1,000 m depth in\n2018. This discussion has been added at Lines 208-216.\n43) L188-189 – I suggest more detail is needed regarding the calculation of ocean heat and\nfreshwater fluxes, including any associated uncertainties or assumptions.\nResponse: The primary sources of uncertainty in the ocean heat estimate stem from two\ncomponents: the thermal forcing and the volume of intruding seawater. We revised the\ncalculation using the CTD from 2018 (Supplementary Fig. 6). For the intrusion water\nvolume, we assume an average cavity thickness of 0.6 m. We estimate a low efficient of\nthe melt process at 30% and obtain an average melt rate of 6 m/year which indicates that\nwater thickness in the IGZ must be higher than 0.6 m as noted at Lines 221-225.\n44) L190 – the statement that ‘only 30% of the heat will be used to melt ice’ is based on\nmodelling at Petermann Glacier in Greenland. How confident are you that this can also\nbe applied in this setting? Without providing uncertainty estimates or being open about\nthese limitations, this quantification could be misleading.\nResponse: The citation of the 30 ± 10 % is from a simulation of ocean circulation using\nthe MITgcm ocean model in a grounding zone. There is no fundamental reason for the\nphysics to be different in Greenland vs Antarctica. We revised the calculation, fixing the\nerror in water height, and used a more recent value of ocean temperature from 2018 at Lines\n205-221.\n45) L191 – Please clarify the basis for the statement that the seawater intrusions may be\n\"thicker by a factor of two\"; has this been quantified or just speculative?\nResponse: Sentence has been removed.\n46) L206 – In general, the methods lack sufficient detail to reproduce the analysis,\nalthough perhaps this has been limited by the word count of the article. Citations\nshould be provided for all datasets.\nResponse: Reference has been added.\n47) L218 – I suggest this should cite a paper describing the Sentinel-1 instrument, rather\nthan an ice sheet study that has also used Sentinel-1.\nResponse: The citation of the Sentinel-1 instrument has been added at Line 264.\n48) L222-228 – This section is missing a description of how the differential SSH was\ncalculated from the interferograms, and whether the GLs were delineated manually or\nusing an automated approach. Also, how do you define a ‘high quality’ interferogram?\nResponse: The descriptions of how the differential SSH is calculated and the method for\ndelineating the GLs have been added at Lines 283-283 and 273, respectively. The “high\nquality” means we have a high-precision, low-noise determination of the GL position.\n49) L226 – should be ‘dense’ instead of ‘densely’.\nResponse: Changed at Line 272.\n50) L226 - The attached data spreadsheet includes a column for GL migration distance\n('GL migration_Berry') as well as columns of 'positive error' and 'negative error',\nhowever there is no description of these errors in the text. I suggest the authors include\na robust description of these error terms and the calculations used to derive them here.\nResponse: When the fringes are noisy or merge or split, we estimate the positive and\nnegative errors based on the fringe pattern to show the potential minimum and maximum\nposition of the GL. See Line 274-276.\n51) L249-258 – The temporal coverage of each of the elevation change products used here\nshould be provided:\n• ICESat-2 ATL14 DEM is timestamped to Jan 1 2020\n• ICESat-2 ATL15 elevation change products – between 2019 and 2024\n• Add date ranges for other satellite missions used in the MEaSUREs elevation change\nproducts.\nPlease clarify whether you use the ATL15 ICESat-2 elevation products (1km) as well\nas the MEaSUREs product using ICESat-2 (2 km)? Or just the former? And this\nsection is missing information and citation for the WorldView-derived DEM and ATM\ndata (e.g. used in Fig. 3, and in calculation of ice bottom and bed topography below).\nResponse: The information of the products above has been added in Lines 304-313. We\nonly use the MEaSUREs product for the period of 1996-2018. We do not use ICESat-2 at\n2 km resolution. The citation and information for WV and ATM have been added in Lines\n343-345 and 321-325.\n52) L262-3 – this seems to be a new result that hasn’t been presented in the main text?\nPlease clarify.\nResponse: Agreed. The elevation change (∂h/∂t) along flowlines, derived from ICESat-2\nATL15 and MEaSUREs products, was not intended to be a new, independent result. Rather,\nit serves as supporting evidence to corroborate the GL migration established based on\nDInSAR-derived GL positions. These indicators help understand the GL migration and the\nplacement of flux gates at different times.\n53) L278 – The current description lacks sufficient detail. In the main text it says the bed\ntopography is reconstructed ‘based on the minimum depth of ice that ensures flotation\nof ice over the zone of flexing, i.e. where a meter of change in SSH is sufficient to\njack up the ice surface’. Please elaborate here on this calculation, specifying e.g. the\ngeoid used and whether the reconstruction was performed over the whole IGZ region,\nor along individual flowlines?\nResponse: Agreed. We elaborated on the bed reconstruction method in the revised text.\nSpecifically, the reconstruction was performed across the entire IGZ, see Lines 340-341.\nThe method involves calculating the minimum ice draft required to ensure flotation based\non the surface elevation data (using the BedMachine v3.7 geoid; Lines 348-349) and firn\nair content. After corrections, we calculate the minimum depth of the ice bottom (Lines\n340-353).\n54) L281 - Given that FAC values are known to be uncertain over GZs, it would be\nimportant to acknowledge this uncertainty here (even if it cannot be precisely\nquantified). Additionally, consider citing the firn model used in BM3.7 to provide\nmore context.\nResponse: Agreed. The citation about the firn model have been added at Line 349-350.\nAlthough the uncertainty in firn air content (FAC) over grounding zones is not quantified\nin BedMachine v3.7, Ligtenberg et al. (2011)—the basis of the firn densification model—\nhighlight that firn properties exhibit substantial spatial variability across Antarctica, and\nthat firn correction values may vary up to 7–9% in coastal and shelf regions. The authors\nacknowledge that their model is semi-empirical and does not fully capture the physical\ncomplexity of firn processes in dynamically evolving zones such as the IGZ where complex\nprocesses such as surface melt, tidal flexure, and dynamic thinning may deviate from the\nmodeled steady-state assumptions. We added a statement to the revised text at Line 350-\n353.\n55) L283-286 – see previous comment on this interpretation; I would suggest it’s more a\ncase that your calculations are inconsistent with the radar measurements (rather than\nthe other way round). A stronger justification is required, beyond simply that than the\nL2 data is ‘not as robust’.\nResponse: The GL positions derived from DInSAR suggest that portions of profiles A–B,\nC–D, and C–F are partially afloat. Under these conditions, the bed elevation inferred from\nMCoRDS radar sounding cannot be deeper than the ice draft derived from HE using ATM\nsurface elevation data. In our previous response, we discussed in detail the limitations of\nthe MCoRDS-derived bed along these profiles, including misidentification of the basal\nreflector due to off-nadir returns or weak radar backscatter in the valley center. These issues\nare well documented in the radargrams and explain why the radar-derived bed is\ninconsistent with flotation-based estimates. It is not the first time erroneous bed picks have\nbeen reported from MCoRDS.\n56) L289 – It would be useful to see a justification of applying a Eulerian vs Lagrangrian\napproach here, or acknowledgement of the associated uncertainties when applied to\nfloating ice.\nResponse: Agreed. We clarified in the revised text that the use of a Eulerian approach is\nrequired for ICESat-2 ATL15 data. Specifically, ATL15 provides surface elevation\nchanges relative to a fixed DEM, rather than absolute surface elevations at discrete time\nsteps referenced to moving ice parcels. We acknowledge that applying the Eulerian method\nto floating ice introduces additional uncertainties, particularly in regions experiencing\nadvection or basal geometry changes. To mitigate these effects, we have applied a spatial\nfilter based on the length of the bending stresses and the size of heterogeneities. We\nincluded this discussion in the revised Methods section (Lines 372-378).\n57) L293-4 – Please provide more detail on this, as per previous comment.\nResponse: The information regarding uncertainties has been provided at Lines 383-385.\n58) L295-299 - It is good that the potential overestimation of melt rates is acknowledged\nhere. However, this point should be emphasized more strongly within the main text,\naccompanied by a more detailed explanation of the uncertainty estimation process.\nSpecifically, it is unclear how you arrived at the stated uncertainty of \"about 36%\". It\nwould be helpful to clarify this methodology, and any reported uncertainty values\nshould be presented alongside the corresponding results in the main text for\ntransparency and clarity.\nResponse: We evaluate the uncertainty at 21%, see methods section, where we explain the\ndetails.\n6. Figures\n1) In general, the figures and captions (in main text and supplementary) deserve some\nattention to bring up to publication standard. Most text is too small to read\ncomfortably, there are a mix of font types and sizes, and overlapping data and sub-\npanels obscures some important information. I recommend using a consistent structure\nfor figure captions throughout, i.e. Fig X - main title. (a) subpanel caption; (b) subpanel\ncaption; (c) etc. I have made some suggestions here for a number of (relatively minor)\nedits that could be made to the main text figures that I believe would significantly\nimprove their quality. The same principles should be applied to the supplementary\nfigures.\nResponse: Thank you. We increased the size of letters and made the styles of the figures\nmore consistent.\n2) Figure 1: This figure contains a lot of valuable and important data, but its impact is\ncurrently limited by the way it is presented. I suggest that the authors should consider\nre-formatting to a portrait layout, which would allow it to be displayed as a full-page\nfigure. Alternatively, consider separating panel (a) as its own figure, and organizing\npanels (b-g) into a separate portrait figure (e.g. a 2x3 grid). Some suggested\nimprovements include:\n• Panel (a) –\no Avoid use of < > symbols in the velocity legend for clarity.\no The colors of the flux gates get a bit lost; perhaps keeping these in bold black\nwhile reducing the opacity of the rest of the OIB lines would improve clarity.\no Add a box to the map to show the area in panels (b-d).\no The elevation contour values could be removed; they are somewhat redundant\nalongside the blue elevation color scale. This would simplify things and avoid\ncontour values overlapping the main data you are presenting (i.e. the GL positions).\n• Panels (b-d)\no These panels are well-presented, but the caption should provide more\ninformation (i.e. explain these are interferograms composed from images sampled\nfrom the 3-4 tidal states shown in panels (e-g); and describe what the color bar\nrepresents).\no Adding labels for the maximum tide height and/or differential SSH of each\ninterferogram could assist readers in understanding the data presented.\no Clarify whether the inclusion of three interferograms comprised of scenes all\nsampled at low tides is a deliberate choice; if so, consider briefly explaining this in the\ncaption as it is interesting for the interpretation!\n• Panels (e-g)\no Consider reducing the number of values on each axis label (e.g. intervals of 12h\non x-axis) to allow for an increase in text size.\no The grey lines pointing to sections of the longer tide time series clutter the\nfigure. Consider removing or ensuring they do not cross over one other.\no Consider repositioning the inset panels to prevent them from covering\nimportant data on the main panel, as this is an issue across all three panels (e-g) where\nsome of the inset panels cover the red dots.\nResponse: Figure 1 has been updated based on the comments, along with the caption. The\ninterferograms are not chosen deliberately for the low tides. We choose them to show the\nbest example of each state.\n3) Figure 2:\n• Panel (a)\no This density plot is a very effective way to illustrate the three GL states.\no Some clarification is needed for the values presented in the DInSAR/HE box.\nConsider adding details to the caption, including an explanation of HE as hydrostatic\nequilibrium, and incorporating lines on the map to show the locations of the HE GLs\nfor better reference.\n• Panel (b)\no Is there a particular reason for using a different color map and phase difference\nscale compared to Figure 1(b-d)? I’d say the color scale in Figure 1(b-d) is clearer.\no From your caption, I would expect the downstream seawater intrusions to be\nmarked by a region of uplift, and upstream extrusion of seawater trapped in a prior\ncycle marked by a region of subsidence – but it appears that these labels are reversed\non the map? Or is the region of extrusion further upstream (between states 2 and 3)?\nThis may be my own misinterpretation, but it would be good to clarify this. Adding\nlabels to show the regions of intrusion vs extrusion would be beneficial.\no Including arrows above/below the legend to illustrate the meaning of the\ndiverging fringe colors could greatly enhance comprehension of this figure for those\nless familiar with DInSAR maps (i.e. pink>yellow>blue = uplift; and\nblue>yellow>pink = subsidence).\n• Panel (d)\no This is an effective presentation of these results. A few minor clarifications\nwould further aid interpretation:\nIndicate what the error bars represent (both for modelled max tide and GL\n■\nmigration distance).\nConfirm that h refers to the maximum modelled tide and clarify in the\n■ max\ncaption.\nResponse: Figure 2 has been updated based on the comments. The caption related to\nintrusion/extrusion is corrected. 1. The different color map and phase difference scale for\nthe interferograms are because the CSK and Sentinel-1 are generated with different\nscaling; 2. For panel (d), the horizontal error comes from tides model and mean sea level\npressure, and the vertical error shows the range that the farthest and closest distance that\nthe GL may reach because when the fringe boundary is unclear or fringes merge or split.\n4) Figure 3:\n• The caption currently lacks clarity, specifically,\no L2 – Clarify that (a) and (b) represent change in h / change in V\no L3-4 - Remove ‘Year 2006 is the reference for (a-b)’; this info is already\nconveyed in the preceding text and figure.\no L4-5 – Maintain consistent phrasing for panels (c,d) and (e,f), i.e. ‘V(/h) along\nprofiles G-H and C-D for each year 1996-2022’\no L6 – specify that the elevation data is from the WorldView (WV) DEM (i.e.\nnot just from WV imagery)\no L6-12 – separate captions for panels (g) and (h); currently confusing as different\ndatasets are used in each for the same thing.\no L6-12 – Clarify descriptions of the different blue lines (‘blue line’/’blue dash\nline’/’blue solid line’/’dark blue line’). It is confusing where the same line style is used\nin panels (g) and (h) to show different data (e.g. in (g) the solid light blue line is from\nthe 2019 WV DEM, but in (g) the same line style shows the ‘2012 DEM’). Consider\nchanging the line styles or providing a more explicit legend.\no L7 – please clarify which DEM is used for ice draft in (h)? It is unclear what\ninstrument is used for the ‘2012 DEM’ – it says ICESat-2 in the caption, but this cannot\nbe correct as ICESat-2 only launched in 2018.\no L8 – please clarify if by ‘flotation from ICESat-2’ you mean: flotation depth\nderived from the 2020 ICESat-2 DEM?\no L10-12 – ensure consistency in the description of the OIB data points.\n• GL positions/IGZ zones\no On panels c, e and g, it is confusing having both the horizontal and vertical bars\nshowing GL locations, positioned both above and below the main data. I suggest\nkeeping just the horizontal bars for 2019-21 to show the short-term range of GL\npositions in each of the 'states’ and labelling the three states in each panel (as has been\ndone in g). Then you can use a vertical bar/dot for 1996 and 2008/9 GL positions where\nyou don’t have sampling of the IGZ length.\no For panels d, f and h, it is fine to use vertical lines, as this is showing the total\nwidth of the glacier/shelf.\no I suggest positioning all GL positions at the bottom of each panel (including in\ng) for consistency, rather than having some above and below the main data.\no Reflect any changes made in the caption.\n• Panels g-h – These are currently quite confusing to interpret.\no Consider moving both the x-axis and IGZ labels to the bottom of each figure\n(rather X than in the middle of the glacier).\no Clarify if elevation measurements are relative to mean sea level\no Legend and caption currently missing info on the ?s, striped areas and blue\nshaded areas.\no It is currently unclear what the areas labelled as ice/bed/water are using data\nfrom? BedMachine? Or the bed you’ve deduced from flotation? In (h) looks like the\n‘bed’ is from the OIB data, but in (g) it’s from a mixture of BedMachine and your\nWV-DEMderived bed.\no In panel (g), the blue dots representing the OIB data are not easily visible. The\nlegend entry for OIB reads like a date (i.e. 2011/12/16 could be interpreted as 16\nDecember 2011). I suggest since there are only four OIB points on this panel, each\ncould be individually labelled with their measurement year.\no There doesn’t seem to be a consistency in the results at the expected crossover\npoint between profiles G-H in panel (g) and C-D in panel (h); perhaps this is the result\nof using the same colors in both panels for different data?\nResponse: Figure 3 and its caption have been revised in response to the reviewer’s\ncomments. 1. Elevation measurements are referenced to mean sea level using the EIGEN-\n6C4 geoid model from BedMachine v3.7 for geoid correction; 2. Descriptions of the\nshaded areas have been added to the figure caption and the main text (Lines 359-365);\n3. We clarified the priority order for bed topography data sources: where available, we adopt\nthe bed from MCoRDS; within the IGZ, we use flotation depths derived from geoid-\ncorrected surface DEMs when MCoRDS is not available, as BedMachine performs poorly\nover Berry Glacier; outside the IGZ, BedMachine is used; 4. The crossover point between\nprofiles G–H and C–D in panel (h) does not correspond to the deepest part of the bed.\nWe have added the crossover locations to the X-axes in the relevant panels for clarity.\n5) Figure 5:\n• The sizing of this figure is much better\n• On panel (b) red GLs get lost in the red melt areas; consider using a different color\nscale?\nResponse: A thin white outline has been added to the red GLs to enhance their visual\ncontrast against the background in Figure 5.\n7. Datasets\nThe authors have provided easily accessible datasets of derived GL positions, flux gates\netc. that were straightforward to open in GIS software. I recommend that all processed\ninterferograms should also be included in the dataset. The excel spreadsheet ‘GL analysis,\ntidal heights, speed & surface height.xlsx’ contains a lot of useful data, but some\nclarification and reorganization is required to improve its usability. Notably:\n• Columns F and G (‘positive error’ and ‘negative error’) appear to indicate the number\nof days between the first and last SAR image date and the midpoint between these dates.\nPlease clarify how this constitutes an error, and—if so—how this error is dealt with in the\nsubsequent processing.\n• Columns P and Q are also labelled ‘positive error’ and ‘negative error’, but these\nappear to indicate some error in the derived GL position. Please provide an explanation or\nthis error and its calculation, assumptions etc.\n• This dataset contains columns for modelled tides/IBE at 50 km and 100 km. Please\nclarify what these distances refer to – i.e. is it the distance between the downstream IGZ\nlimit and the position used to calculate the tide? If so, which of these locations (50 or 100\nkm) did you use? If only one location is used for the tide calculation, only these values\nshould be included in the dataset to avoid confusion.\n• It is confusing that the SAR image acquisition dates are repeated three times in the\nsame Excel sheet: in columns A-D, S-V and AP-AS. There is also repeated GL migration\ndata in columns O (‘GL migration_Berry’) and AM/AN (‘GL migration_Sentinel-1’/’GL\nmigration_COSMO-SkyMed’). Consider consolidating these columns to remove\nunnecessary duplication.\nResponse: 1. The interferograms generated from Sentinel-1 and CSK cannot be uploaded\ndue to copyright; 2. The purpose of the \"positive/negative error\" of the dates is to show the\ntemporal range of the interferograms. We removed it; 3. The positive/negative error of the\nGL position is explained above; 4. The value is the distance between the downstream IGZ\nlimit and the position used to calculate the tide. The location at 100 km is used. The reason\nof including the SSH at 50 km is to show the SSH does not significantly change from 100\nkm to 50 km; 5. The duplication has been removed. We have revised the figure to improve\nclarity.\n8. Code availability\n• The Zenodo link points to code used to compute basal melt rates at Thwaites,\noriginally from a different paper (Gadi, 2024). I am not sure of the specific journal\nguidelines on this, but I suggest this should either be appropriately cited alongside the\ndataset, or specific code used for this work at Berry should be provided.\n• No code is provided for the other calculations described in the methods, including ice\nbottom/bed topography, ice mass balance, GZ length or tides. This should be included to\nensure full transparency.\nResponse: 1. Ratnakar Gadi is a co-author of this manuscript. The code is identical; 2. The\ncalculations are done using the built-in tools and the Python console in QGIS.\nReviewer #2\nOverall I think the manuscript contains exciting, new results, but the line of argument around\nthe connection between the GL retreat of Berry and changes in CDW temperatures needs\nrevision and refinement to make it convincing.\nGenerally, the manuscript needs some polishing. I suspect that shortening the text to make\nit fit within the Nature Communications format is the culprit here. This applies especially to\nthe introduction that needs to be reworked – see my specific comments.\nThere are two main points in the manuscript. One is that you can identify tidal variability\nin the movement of the glacier and that this likely gives the warm CDW on the continental shelf\naccess to the grounding line zone leading to high melt rates. The second one is that the\ngrounding line of Berry has retreated rapidly over the past 25 years due to that CDW access.\nMy main problem here is that your line of argument focusses on the presence of CDW and\nnot how it may have changed. There isn’t much CTD data available, but what there is could be\nused better:\nShow the temperature profiles that you are using and don’t just leave the reader to eye-ball\nthem from Jacobs et al. 2013. Also give data sources for the CTD data, especially the mysterious\nof the thermocline vs the associated GL positions and cavity shapes to discuss the implications\nfor basal melt.\nCDW access to the deep basin in front of Berry is most likely across the shelf break and\nnot from the east on the shelf. The sill depth is shallower than for Siple Trough (see Jacobs et\nal. Oceanography 2012) and the off-shelf CDW core actually sits below the sill depth. This\nimplies that it is likely even more episodic than we have shown for Siple Trough and even more\ndependent on winds along the shelf break and Ekman pumping (Assmann et al. 2019).\nThe implication of this is that you have a mechanism that explains why CDW access may\nhave changed between 1994 and 2021: If you compare your mass balance time series in Fig. 4a\nwith our Ekman pumping time series in Assmann et al. 2019 Fig 4f they agree rather well. You\ncould calculate a similar time series for the shelf break in front of Berry, but it is unlikely that\nit would be much different because of the scales of atmospheric variability. Just like we argued\nthat the enhanced Ekman pumping over the Siple Trough shelf break led to warmer CDW in\nfront of the western Getz in 2016 & 2018 compared to 2000 and 2007 that argument could be\nused in this study as well.\nResponse: We thank the reviewer for the thoughtful and detailed feedback. We appreciate the\nrecognition of the novelty of our results and agree that the argument linking GL retreat of Berry\nGlacier to changes in CDW temperature and access requires further refinement. We have made\nthe following revisions in response to these suggestions:\n1. We now explicitly show the CTD temperature profiles used in our analysis, rather than\nreferring readers to previous figures (e.g., Jacobs et al., 2013). These profiles are presented in\nSupplementary Fig. 4b and 6b, with labeled data sources including a 2018 profile, which is\nfrom Korean Polar Center.\n2. We revised the text to emphasize that CDW likely enters the deep basin in front of Berry\nGlacier from the continental shelf break, rather than from the east (Lines 181-183). We note\nthat the CDW core offshore sits below the sill depth—implying episodic access, likely regulated\nby wind-driven Ekman pumping, as the reviewer rightfully pointed out (Lines 186-189).\n3. We have incorporated the reviewer’s suggestion to better connect CDW variability with\nchanges in Ekman pumping. Specifically, we have added additional CTD profiles from 2014\nand 2018 in the western Getz region, which exhibit clear signals of ocean warming (Lines 209-\n214, Supplementary Fig. 6b). These observations support the interpretation that enhanced\nintrusions of CDW—potentially driven by intensified Ekman pumping during periods such as\n2016–2018—may have contributed to increased basal melt and the observed IGZ retreat.\nThank you very much.\nSpecific comments:\n1) Line 42: “affected” not “effected”.\nResponse: Fixed at Line 43.\n2) Line 45: You are implying that changes in GL position affect sea level, because of the\naddition of seawater by floating ice. Floating ice shelves are hydrostatically balanced,\ni.e., they don’t affect sea level. What does affect is the mass flux across the grounding\nline and the mass loss from the ice shelves by basal melting and calving that adds\nfreshwater, i.e., volume to the ocean. Just remove this sentence.\nResponse: Removed.\n3) Line 50: This sentence is very out of context, move to last paragraph in the introduction\nwhere you introduce the location.\nResponse: Moved to Lines 70-72.\n4) Line 52-53: “due to ocean tides” not “changes in oceanic tides”. Tides imply\nvariability.\nResponse: Changed to “due to ocean tides” in Line 51.\n5) Line 56-67: For my taste this is too technical for an introduction. I got lost in the detail\nand wasn’t sure why you are using all of these satellite data sources.\nResponse: The reason we use all satellite data sources is because ERS and ALOS-2\nPALSAR allow us to look back in time, while Sentinel-1 and CSK provide high frequency\nrepeat data to show the short-term GL dynamics. We noted this at Lines 57-63.\n6) Line 66: You haven’t given any geographical context, i.e., the reader has no idea where\nor what Berry is. I assume this is result of shortening the text to fit within the Nature\ncomms format.\nResponse: A short geographical context has been added at Line 64.\n7) Line 71: Change “undiluted warm, salty CDW” to “warm CDW”. You don’t need\nsalinity to make your point and I would remove it to simplify the story. CDW access\non the western side of Getz is intermittent, so the CDW that makes it to the ice shelf\nfront is anywhere from part of the thermocline to part of the CDW core (see Assmann\net al. 2019).\nResponse: The sentence has been simplified in Lines 70-72.\n8) Line 147-152: Duplicates the methods section, so you could remove this.\nResponse: Statement removed.\n9) Line 161-162: Remove the reference to salinity. This is superfluous to your main\nstoryline. Either write “mean bottom temperature of…” or change to something like\n“bottom temperature exceeding 1 C” to match the CTD sections in Jacobs et al. 2013\nand sections and mooring records in Assmann et al. (2019).\nResponse: The sentence has been modified based on the comments at Lines 178-179.\n10) Line 162-165: see comment about Supp. Fig. 4a and general comment about CDW\naccess from the shelf break.\nResponse: Supplementary Fig. 4a is updated based on the comments to show the ice shelf\nfront and the continental shelf break.\n11) Line 165-167: This is the only mention you have in the entire article about changes in\nocean temperatures and it is too hand-waving to support your main point. Show the\nactual temperature profiles in a figure, because it is hard to pin-point Berry in the\nJacobs et al. (2013) figures.\nResponse: The numbers have been corrected in Line 185. The temperature profiles have\nbeen added to Supplementary Fig. 4b.\n12) Line 181-185: I am not sure why you use 0.6 m as a thickness when you draw in 1.2\nm worth of water volume over a tidal cycle.\nResponse: We assume the water thickness linearly decreases from +/-0.6 m at downstream\n(state 1) to 0 m at upstream (state 3), like a right triangle, therefore the water thickness of\nthe entire IGZ is 0.6 m.\n13) Line 186: The tide is diurnal at Berry (and generally in the Amundsen Sea), i.e., half\na tidal cycle is 12 hours not 6 hours – see your Fig. 1.\nResponse: The value has been corrected to 12 hours at Line 207 in the revised text.\n14) Line 187: Where is your 2018 CTD from? Add a source for all CTD data you use to\nyour data statement. This is especially important for this 2018 CTD since you use the\ntemperature for your heat flux calculations.\nResponse: We originally cited the number from Selley et al. 2021. But upon careful\nexamination of the 2018 CTD dataset from the Korean Polar Center—used in the 2021\nstudy—we found that the measurements extend only as far west as Dean Island and do not\nreach the vicinity of Berry Glacier. Consequently, to estimate post-2007 ocean temperature\nchanges at the ice front of Berry Glacier, we analyzed CTD profiles collected from other\nlocations in the west Getz Ice Shelf region with data available after 2007 (Supplementary\nFig. 6). These CTD records indicate a general warming trend in the west Getz since 2000,\nwhich persisted beyond 2007, as evidenced by observations near Dean Island and east of\nSiple Island, which is consistent with your argument above that enhanced Ekman pumping\nover the Siple Trough shelf break contributed to the presence of warmer CDW in front of\nthe western Getz Ice Shelf in 2016 and 2018 compared to 2000 and 2007. If the ocean in\nfront of Berry Glacier experienced a comparable degree of warming, a thermal forcing of\n+3.6°C at 1,000 m depth represents a reasonable estimate. These data are now included in\nthe discussion.\n15) Line 190: I like the Gadi et al. (2023) reference a lot and agree that not all heat in the\ninflowing seawater is used to melt ice, but I interpreted the 30% statement in their\nstudy that 30% of the ice flux across the grounding line was melted in the GZ and FZ.\nResponse: Your interpretation is correct—the 30% figure refers to the fraction of ice flux\nmelted in the grounding and flexure zones in the 2023 Gadi et al. paper.\n16) Fig 1: Even with the improved resolution, panels in Fig 1 are still very small, this\napplies particularly to the top left insert in panel a.\nResponse: Figure 1 has been updated based on the comment.\n17) Supp. Fig. 4 a: Add the ice shelf front to panel and extend the figure to the continental\nshelf break to show the channels giving the CDW access. In my experience, the\nbathymetry on the continental shelf around the western Getz isn’t very detailed or\nreliable, because I don’t think there is more multibeam data than shown in Nitsche et\nal. (2007). I would qualify the statement you make about these channels in lines 162-\n165.\nResponse: Supplementary Fig. 4 has been updated based on the comments. Cochran et al.\nuse a geologically consistent inversion of NASA OIB airborne gravity data for bathymetry\nbeneath the GIS, which extends for ~650 km along the Amundsen Sea coast of Antarctica,\nshowing the existence of the thalweg.\n18) Supp. Fig. 4 b & c: I do like the cavity sections, because they show nicely how the\nBerry GZ has moved to a retrograde slope, while the DeVicq GZ sits just at a small\nrise. I also suspect that bed shape is more responsible for the different reactions of the\ntwo glaciers rather than CDW temperature.\nThe temperature contours in panels b and c imply a thermocline at constant depth in\nfront of Berry. I would have preferred a profile insert that shows that this is a point\nmeasurement. Showing would also give you the opportunity to show profiles for\ndifferent years and to make your point about the CDW changes.\nResponse: Supplementary Fig. 4 has been updated based on the comments. We add the\ndiscussions related to the CDW and CTD from different years to Lines 196-199.\n19) Supp. Fig. 5: I do wonder if it would be worth to move some of the panels into Fig. 5,\nbecause they illustrate the exceptional reaction of Berry within the region and give\ncontext to your discussion of DeVicq glacier which feels somewhat stand-alone in the\nmanuscript.\nResponse: Supplementary Fig. 5 a&c has been added to Figure 5.\nReviewer #3\nIn this study, Chen et al. used DInSAR interferograms generated from different satellite data to\nmap both the short-term and long-term grounding line migrations in Berry Glacier located in\nthe Getz Ice Shelf from 1996 to 2021. They discovered a short-term grounding line migration\nover 18 km and claimed that the glacier bed is up to 1300 m deeper than the BedMachine data.\nThe methods used in this study are standard and not novel, particularly for research focused on\ngrounding line mapping using DInSAR. The identification of a large ‘Ice Grounding Zone’ in\nBerry Glacier is also not new, as it was previously discovered by Mohajerani et al (2021).\nThe paper does not offer significant advances in our understanding of either short-term grounding\nline migration or long-term changes related to ice dynamics, as these concepts have already\nbeen extensively explored and established in previous studies. Furthermore, the interpretation\nof bed topography and the calculation of short-term grounding line migrations require further\nconsideration and improvements.\nMajor Comments:\n1) The finding of the large tidally-influenced grounding line migration in Berry Glacier\nis not new, Mohajerani et al (2021) have already found grounding line changes in\nBerry Glacier to be about 15 km (https://www.nature.com/articles/s41598-021-84309-\n3). Given this, I feel this paper only provides a more detailed calculation building upon\nthe work of Mohajerani et al (2021), and its main conclusion lacks novelty. Especially,\nMohajerani et al (2021) have analyzed the entire Getz Ice Shelf. I am curious why the\nauthors in this study chose to focus solely on a single glacier within the same region.\nIncluding all glaciers in Getz Ice Shelf would allow for a comparison of grounding\nline changes across glaciers that share similar oceanography settings, this would result\nin a more comprehensive study. Focusing on just one glacier significantly limits the\nscope of this research.\nI agree that short-term grounding line migrations have not been adequately\nincorporated into ice sheet modeling. However, this phenomenon has been well-\ndocumented in Antarctica in recent years using a similar DInSAR approach and\ndatasets to those used in this study, with some examples listed below. While I\nacknowledge the efforts presented in this paper, I feel that its conclusions do not\nsignificantly advance our understanding of short-term grounding line migrations.\nInstead, the study only adds incremental evidence to the existing body of literature.\n- Milillo 2017:\nhttps://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074320\n- Milillo 2019: https://www.science.org/doi/10.1126/sciadv.aau3433\n- Brancato 2020:\nhttps://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL086291\n- Mohajerani 2021: https://www.nature.com/articles/s41598-021-84309-3\n- Milillo 2022:\nhttps://www.nature.com/articles/s41561-021-00877-z\n- Chen 2024:\nhttps://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL102430\nResponse: We appreciate the reviewer’s thoughtful comment and acknowledge that\nMohajerani et al. (2021) was the first to document the presence of a large tidally-influenced\nIGZ in Berry Glacier using Sentinel-1 DInSAR. Our study builds upon their important\nfinding by providing higher-quality interferometric observations and a more detailed\nspatiotemporal analysis of the evolution and mechanisms behind the observed GL behavior.\nSpecifically, the shorter wavelength and repeat cycle of Sentinel-1 limits coherence in\ninterferograms over fast-flowing ice near GLs. To overcome this, we incorporated CSK\ndata, which offers high temporal resolution and superior coherence in the IGZ. This enabled\nus to more precisely resolve GL migration patterns, tidal flexure, and transitions between\ndifferent grounding states. CSK coverage is limited spatially, and unfortunately, we do not\nhave comprehensive CSK data across the entire Getz Ice Shelf.\nWe chose to focus on Berry Glacier because it exhibits unique dynamic behavior not seen\nin other Getz tributaries. Unlike nearby glaciers, Berry hosts a deep, wide IGZ with distinct\ntidal signatures, and a complex bed geometry that allows—but also limits—warm CDW\npenetration. Through analysis of elevation time series, interferometric phase patterns, and\nreconstructed basal topography, we identify the processes by which CDW accesses the IGZ\nvia a thalweg and how melt efficiency is modulated by subglacial bed conditions. These\nfindings, including the role of trapped seawater and temporally evolving tidal states, are\nnot presented in previous studies.\nTherefore, while our study builds on earlier DInSAR-based GL mapping, it contributes new\ninsight into the ocean-ice-bed interaction at fine spatial and temporal scales, and highlights\nprocesses critical to improving ice sheet model representation of short-term grounding line\nvariability.\n2) The comparison between 1996 IGZ and 2020-2021 IGZ needs more care. In 1996 you\nonly have two interferograms, it is very likely due to limited DInSAR data acquisition\nduring that time, this may have resulted in incomplete capture of short-term grounding\nzone migrations. While in 2020-2021, there is a greater availability of data, enabling\ngreater sampling and a more comprehensive imaging of the entire IGZ. Please address\nthis in the analysis.\nResponse: Yes, in the revision we note the following. In the two 1996 interferograms, we\nsee no residual signal upstream of the IGZ. In more recent interferograms, there is always\na residual upstream, indicative that during some part of the tidal cycle, seawater intrudes\nfarther up. It is therefore unlikely that seawater intruded much farther in 1996. Secondly,\nas shown in the study, the glacier thinned considerably since 1996 (11±1 m/yr). In\nparticular, ice deviated from flotation by 1 of meters at the location of state 2, which the\ngrounding line reached around 2006. See Lines 277-280. The IGZ in 1996 could not have\nextended to state 2.\n3) Figure 2a shows the spatial density of different grounding line positions obtained in\nthis study, capturing three different states. However, a limitation of this approach is\nthe lack of consideration for temporal sampling. If the GL acquisitions are\nconcentrated around same tidal phases, the resulting spatial distribution map may be\nbiased. In addition, the gap between State 2 and 3 is too short (~1 km), making this\nclassification unreliable.\nResponse: We appreciate the reviewer’s comment and agree that temporal sampling and\ntidal phase distribution are important considerations when interpreting GL positions. To\naddress this, we note that Fig. 2d demonstrates the presence of all three GL states under\nboth positive and negative tidal phases, suggesting that the classification is not an artifact\nof biased tidal sampling.\nRegarding the distance between state 2 and state 3, the spatial separation is approximately\n2 km, rather than ~1 km. While the gap is relatively narrow, we argue that the two states\nare physically distinct. Specifically, the presence of reversed motion between states 2 and\n3 implies the existence of subglacial space available to accommodate trapped seawater\nduring tidal cycles. This behavior supports our interpretation that states 2 and 3 represent\ndifferent stages in the evolution of grounding line migration, rather than minor fluctuations\nwithin a single transitional state.\nWe have also considered the influence of spring–neap tidal cycles and the distinction\nbetween ebb and flood tides (Supplementary Fig. 1); however, no discernible correlation\nbetween tidal phase and GL migration is detected. This is now included in the paper.\n4) It is interesting that the authors found the bed topography underneath Berry Glacier to\nbe 1300 m deeper than the BedMachine. This is useful information that can be used to\nupdate the BedMachine dataset. However, I am not particularly surprised that the\nBedmachine dataset is inaccurate in some of the glaciers due to a lack of observations.\nAdditionally, it is unclear how this discovery of deeper bed topography changes our\ncurrent understanding of the grounding line dynamics and regional mass imbalance.\nResponse: There are numerous examples of poor delineation of bed depth in the OIB data\nbecause the radar echoes from the bed are sometimes difficult to pick. This study will\nindeed have an impact on BedMachine in this region.\n5) The discussion about seawater intrusion is unclear. In Line 139: the authors claim that\n‘A series of bed depressions between the three states must trap seawater…’ I feel this\nclaim is not well supported and more evidence is needed to make sure this statement\nis scientifically correct. Although the authors claimed to detect the reverse fringes of\nopposite motion upstream to indicate the trapped seawater, there are not enough details\non how the water extent is mapped in Figure 3 and how the presence of trapped\nseawater changes in sync with grounding line migrations.\nResponse: The extents of the three GL states were determined based on DInSAR-derived\nGL positions. We infer the presence of seawater in the gaps between these states—\nspecifically between state 1 and state 2, and between state 2 and state 3—based on two lines\nof evidence: 1. Fringe patterns of reversed motion in Figure 2b: The locations of reversed\ninterferometric fringes, which indicate vertical deformation, spatially coincide with the\nregions between GL states. This consistency supports the interpretation that these zones are\nperiodically flooded with seawater during high tides and partially drained during low tides,\nconsistent with the flexural response of floating ice over subglacial water; 2. Elevation\ndifference between ice draft and radar-derived bed: Along profile C–D, the 2012 MCoRDS-\nderived bed is significantly deeper than the 2019/2020 flotation-derived ice draft, indicating\nthe presence of sub-ice water. However, we acknowledge that this evidence is limited to\nspecific locations, and that we cannot constrain the full spatial extent of the deeper bed or\nthe thickness of the trapped water in other regions.\n6) Mismatching patterns in ice dynamics: The dhdt in the west flank is much lower than\nthe main glacier trunk where there is an over-deepened retrograde bed slope, this does\nnot match the basal melt rate patterns in Figure 5 and the ice velocity changes in Figure\n3. Based on MISI theory, we would expect to see matching patterns of ice thinning,\ngrounding line retreat, as well as ice speed up along the overdeepended bed. The\nauthors, however, only provide one comment on this mismatching pattern in Line 114-\n115: ‘The pattern of thinning does not coincide with the pattern of speed up as the\nretreat expands laterally along a tributary’. This explanation is too simplistic and\nmisleading. Please provide a more detailed explanation.\nResponse: The apparent mismatch noted in the manuscript refers to the western region\noutside the IGZ, where we observe substantial surface lowering without a marked increase\nin ice velocity. Within the IGZ, we have consistent patterns of GL retreat, ice thinning, and\nspeedup, in line with the MISI theory. On the west of the IGZ, the discrepancy may be due\nto a combination of factors"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I commend the authors for their thorough revision of this manuscript, and I agree that it is now significantly improved! The title is much improved and the majority of my concerns have been addressed, regarding the text, methods descriptions, and the treatment of uncertainties. The figures are in much better shape too. I have one major remaining concern regarding the discussion about the cavity formation (detailed below), and a few additional minor comments / suggestions that the authors may like to consider before final submission. Once addressed, I would recommend this manuscript is acceptable for publication.\n\nI have attached my comments in a separate PDF file.\n(Remarks on code availability)\nI have not re-reviewed the code but the authors have made no changes.", "reviewer_2": "In general, the whole manuscript reads more balanced and is greatly improved after the authors incorporated mine and my fellow reviewer’s comments.\n\nThe discussion of the influence of the CDW and its recent changes is now much better. Especially Supp Figs. 4 and 6 reflect the hydrography and data base much better.\n\nAs a minor revision before final acceptance of the manuscript I would like to see some re-organisation of the discussion of the influence of CDW in the main manuscript.\n\nGeneral comment:\nLine 178-189 and 208-214 which contain the main discussion on CDW are somewhat repetitive and would benefit from merging and re-ordering. I suggest doing this following your very nice figures Supp. Fig. 4 and Supp. Fig. 6.\n\nSupp. Fig. 4 shows that there are direct observations of CDW at the ice shelf front of both Berry and duVicq with some warming between 1994 and 2007. Use these to discuss access of the CDW to the GL and the importance cavity shape.\nThen use Supp. Fig. 6 to take in the more recent observations and the CDW warming to present day. Since these are not located directly at Berry, this is the place to discuss CDW pathways on the continental shelf and the enhanced Ekman pumping at the shelf break as the mechanism that controls CDW access to the continental shelf and thus the warming. It is worth mentioning that the CDW at Berry may not have reached 1.2 C in 2018, since CDW there appears to have been cooler that at Dean in the earlier years.\n\nSpecific comments:\nl.181: remove “because they are deep enough” Also since this refers to the bathymetry on the continental shelf outside of the ice shelf cavities that figure reference should be Supp. Fig. 6.\nl. 208-209: “The ocean temperature on the western Getz Ice Shelf” should be “The ocean temperature along the western Getz Ice Shelf”\nl. 236-238: This sentence is unclear. Re-write.\nSupp Fig 6: Remove the CTD stations east of Siple. CDW there doesn’t have direct access to your region of interest and this would make the figure easier to read.\n(Remarks on code availability)", "reviewer_3": "Thank you for addressing my comments and revising the paper. I only have some minor suggestions below.\nLine 71: Please consider rephrasing the sentence into ‘are in contact with warm waters coming from Circumpolar Deep Water that …’\nLine 72: The mention of ice shelf meltwater feels out of place here. I suggest adding a statement on the implications of ice shelf basal melting.\nFigure 1a: It’s difficult to discern the black arrow associated with Berry Glacier from the background ice velocity map. I suggest changing to a different color. The same issue applies to the star marker at the tide location. Also, the star marker is very small.\nLine 78: ‘but there is no residual signal upstream of state 1 in 1996’, please add a reference to Figure 7.\nLine 83-86: Please specify which bed topography product you are referring to.\nLine 172: Please rephrase ‘several 100 meters’\nLine 194 - 201: I can’t tell changes in basal melt rates in different time periods or different locations mentioned in this paragraph directly from Figure 5, as the current colorbar of basal melt rate has a limit of [-6,6] m/yr. Please revise the figure to improve the illustration of basal melt rate variations.\nLine 204: Consider removing the repetitive ‘that is’ in this sentence\n(Remarks on code availability)"}, "author_response": "Dear Editor, Reviewers,\nWe are very thankful for the reviewers’ comments. In response to these, we have extensively\nrevised the manuscript. However, the impact of the revision on the results is minimal. We\nbelieve that the new version is significantly improved. We hope that the reviewers will find the\nrevised manuscript acceptance for publication.\nOn behalf of the co-authors – Hanning Chen.\nReviewer #1\n1. Originality and Significance: The study provides new insights into the dynamics of the IGZ\nat Berry Glacier and the first published observations of changes in ice velocity, discharge\nand melt rates over the past 25 years. However, it should be noted that Berry Glacier is\nrelatively small in the context of the Antarctic Ice Sheet. Thus, while the findings are\nsignificant for this setting and contribute to improved process-level understanding, their\nbroader implications for ice sheet modeling and sea level rise projections are potentially\nmore limited. I recommend enhancing the discussion of these broader implications,\naddressing—for example—how findings at Berry compare to those from other Antarctic\nIGZs and how they might contribute to modelling improvements on an ice-sheet-wide\nscale.\nResponse: Yes, Berry Glacier is a small glacier, with a low potential for sea level rise but\nits evolution illustrates the fundamental physical processes operating in the Antarctic\nenvironment that - in the presence case - led to the formation of a gigantic ice grounding\nzone and a rapid retreat along a deep trough. The magnitude of the retreat, the size of the\ncavity, the magnitude of the melt rates, and the bathymetry are relevant aspects to a broader\ntype of study of glacial retreat in a warmer ocean. Berry has also one of the longest IGZ in\nAntarctica, as first noted in Mohajerani et al. (2022). There are others: Barnett Glacier,\nVictoria Land; Vanderfjord Glacier, Wilkes Land, Lambert/Mellor/Fisher, Jutulstraumen,\netc. In the revision, we discussed the broader implications of this study at Lines 226-240.\n2. Treatment of uncertainties: There is an insufficient and inconsistent treatment of errors and\nuncertainties throughout the analysis, including in the main text, methods and attached\ndataset. I strongly recommend that the authors include a more comprehensive discussion\nof the uncertainties and errors in each part of the analysis and how these propagate through\nthe calculations. I recognize that quantifying some of these uncertainties may be\nchallenging; nevertheless, an open discussion of these limitations is essential to avoid\npotentially misleading readers or overinterpreting the results.\nResponse: We documented the uncertainty of the grounding line retreat, the time series of\nice velocity, surface height, and inferred bed topography, which led us to find that some\nMCoRDS data were not correctly interpreted. We double checked the discussion of the\nuncertainty of the melt rates.\n3. Robustness of Conclusions: I have some concerns about the validity of some of the data\ninterpretation that form the basis of the paper’s main conclusions. I would like to see a\nresponse from the authors providing further clarification or justification of these.\n1) Long-term GL retreat 1995-2021\nThe authors report that the length of the Berry IGZ increased from 0.6 km in 1995/96 to\n18±0.9 km in 2020-21. The 1995/96 IGZ length seems to be based on data from just two\nERS1/2 interferograms, compared to >15 interferograms for 2020-21. I appreciate that\nthere is limited data availability from 1995/96, but I wonder whether the reported shorter\nIGZ length in the 1990s may be influenced by the tidal sampling resolution. If the two\nERS1/2 interferograms only captured the GL when it was in its Stage 1 position, it’s\npossible that any inland migration of the GL during other tidal phases was missed. This\ncould lead to an underestimation of the IGZ length during this period, and therefore an\noverestimation of the long-term change by 2020-21. I recommend that the authors discuss\nthis potential bias in the interpretation and clarify how tidal sampling might impact the\nresults (1-2 sentences would suffice). Providing the modelled differential tide and\nmaximum sampled tide for each of the ERS1/2 SAR scenes in both Supplementary Figure\n6 and in the data spreadsheet (as for the Sentinel-1 and COSMO-SkyMed data) would\nprovide valuable context.\nResponse: Yes, because we have only two interferograms, the width of the IGZ in 1996 is\nnot as well determined as in later years. Yet, there is also several lines of evidence\nindicating that the GL was at state 1 in 1996. First, we see no residual signal upstream of\nthe state 1 IGZ in the two 1996 interferograms. In the more recent interferograms, there is\nalways a residual signal upstream, between state 1 and 3, that indicates that during some\npart of the tidal cycle, there is residual vertical motion in that region (Fig. 1). It is unlikely\nthat seawater intruded farther than state 1 in 1996. Furthermore, the glacier thinned\nconsiderably since 1996 (11±1 m/yr). Ice deviated from flotation by a few meters at state\n2, which the grounding line reached in 2006. See Lines 277-280. The 1996 IGZ could not\nhave extended to state 2. As requested, in the revision, we also added information about the\nmodeled differential tide for ERS1/2 SAR in Supplementary Fig. 7.\n2) Calculations of ice thickness using HE\nThe authors compare the GL retreat at Berry to observed changes in velocity and ice\nelevation from 1996-2022 and estimate how this translates to changes in ice thickness,\ndischarge and total mass balance. I am concerned about the validity of using hydrostatic\nequilibrium (HE) to convert ice elevation change observations to thickness changes.\nAdditional justification of this approach is needed, along with a more open discussion of\nthe associated limitations and uncertainties in the main text.\nWhile using HE for this calculating ice thickness is appropriate for fully floating ice, the\nInSAR data indicate that the ice in the Berry IGZ is grounded for a significant portion of\nthe tidal cycle, with an average depth of seawater intrusions of ~0.6 m. This implies the ice\nwithin the IGZ cannot be in hydrostatic equilibrium, casting doubt on the validity of ice\nthickness values derived from this method, and, by extension, the estimates of thinning\nrates, ice discharge, basal melt rates and bathymetry. For example, the authors’\ninterpretation that a 24 m surface elevation decrease in the IGZ translates to 222 m of\nthinning (L101-102) is likely to be a significant overestimation.\nIt is well-documented that satellite-derived estimates of ice thickness and basal melt rate\nwithin GZs are subject to large uncertainties due to the breakdown of the hydrostatic\nassumption (e.g., Chuter & Bamber, 2015; Griggs & Bamber, 2011; Kim et al., 2024),\nwhich often leads to these regions being masked out (e.g., Griggs & Bamber, 2011; Paolo\net al., 2015; Wilson et al., 2017) or interpolated across (e.g., Fretwell et al., 2013;\nMorlighem et al., 2020). While I note that some attempt has been made by the authors to\naddress this (e.g. L150-152; 296-298), the discussion remains too vague and does not\nadequately address the impact on the key findings.\nResponse: As ice is afloat during part of the tidal cycle and the tidal amplitude is only +/-\n0.6 m, ice must be within 1-2 meter of flotation, hence the error of using HE during the\ntidal cycle is less than 10-20 m. Following the 24 m surface lowering (Line 108), the\ngrounding line reached state 2 in 2006, so the assumption of HE is valid. Finally, in Figure\n3g, we match the bed depth from MCoRDS with the ice draft derived from the 2019 DEM,\nwhich gives another line of evidence that this calculation is reliable. This is now stated\nmore clearly in the manuscript.\n3) Refinement of bed topography\nThe authors propose that the bed topography within the Berry IGZ is up to 1,300 m deeper\nthan measurements from MCoRDS airborne radar and in BedMachine v3.7 (which\ninterpolates between sparse radar measurements). However, I do not think that the evidence\npresented here is strong enough to support this conclusion; I suggest the authors either\nreconsider this interpretation or provide a more robust justification, clearly communicating\nany limitations. Specific concerns include:\na) Methods description: The description of methods used to reconstruct bed topography\nlacks sufficient detail, both in the main text (L125-127) and methods (L278-279). I\nrecommend expanding the description to clarify the approach, including details on the\ngeoid and specifying the areas or flowlines over which bed topography was calculated.\nb) Use of HE/flotation: If the new bathymetry estimates in the IGZ are based on ice\nthickness calculated using HE/flotation, they are subject to the same uncertainties outlined\nabove. Prior studies have demonstrated that ice thicknesses calculated in this way close to\nthe GL tend to overestimate values compared to radar measurements due to the breakdown\nof HE (e.g. Chartrand & Howat, 2023).\nc) Radargram interpretation: The authors propose that the bed depths from the MCoRDS\nradar in the Berry IGZ have been misinterpreted as too shallow (L132-134). However, upon\nreviewing Supp. Figure 2 (noting that I am not a radar specialist), I find this interpretation\nunconvincing. While weak bed echoes at the glacier center may well indicate higher\nuncertainty in bed depths, the picked bed depths (red dashed lines) in all four radargrams\nlook reasonable and there is little indication of any deeper echoes at the depths suggested by\nthe authors. Radar is generally considered most reliable way to measure ice thickness/bed\ndepth in these regions, and it seems more plausible that the HE-based calculations in this\nstudy might overestimate thickness/depth compared to the radar. Labelling specific features\nin the radargrams that support the deeper bed interpretation would be helpful, as well as\nproviding a comparison to data from the gravity inversion mentioned in L136.\nd) Clarification of the 1300m depth difference: The origin of the 1,300 m value is unclear. In\nFigure 3(g-h), the difference between the BedMachine bed elevation and the author’ new\ncalculations appear to max out at around 1,000 m. Since this 1,300 m value is presented as\na key result in the abstract, its basis should be clarified in the text.\nResponse: 1. As stated in Lines 348-349, the EIGEN-6C4 geoid model from BedMachine\nv3.7 is used to geoid-correct the DEMs. The spatial extent over which the bed topography\nis reconstructed is clarified in Lines 137-139 and 340-341; 2. As noted earlier, the\nagreement between the ice draft inferred from HE and the basal interface detected by\nMCoRDS radar supports HE in this region; 3. The MCoRDS data document the ice–bed\ninterface; therefore, in the IGZ, the bed elevation inferred from MCoRDS cannot be below\nthe HE-inferred ice draft. Along profile A–B, the radar signal over the central IGZ region\n(above the red dashed line) suggests that the bed was misidentified, prompting us to shade\nthe area between the MCoRDS-derived bed and the ice draft to highlight this uncertainty.\nAlong profiles C–D and C–F, radar wave reflections from the steep valley sidewalls (off-\nnadir returns) obscure the basal interface in the valley center. For profile E–F, discrepancies\nare observed between the 2011 OIB Level 2 dataset and the corresponding radargrams:\nwhile the radargrams have the correct bed picks, the Level 2 data contained erroneous\nvalues, leading to the substantial mismatch between our reconstructed bed topography and\nBedMachine. Unfortunately, users have used the Level 2 values instead of the Level 1\nvalues. We have updated Supplementary Fig. 2d to show the erroneous data for clarity; 4.\nIn the revision, we note that the maximum depth discrepancy of 1300 m is located on the\neastern side of the IGZ, but it does not overlap with any radar profile examined in this study\n(See Lines 140-143), so it is a maximum offset with BedMachine which is interpolated data\nat this location.\n4) Long-term melt rates / basal melt regime\nThe text in this section (L146-192) needs refining; it is currently difficult to follow the\nspecific areas and time periods for which melt rates have been calculated. Clear distinctions\nare needed between discussions of: (a) Melt rates within the Berry IGZ (2019-2022); (b)\nMelt rates within a zone 10 km seaward of the IGZ, compared to neighboring glaciers and\nprevious measurements; and (c) Predicted melt rates within the Berry IGZ due to tidal\nseawater intrusions Again, my concerns about the use of HE to calculate thinning rates\nwithin the IGZ alongside the lack of uncertainty quantification make me skeptical about\nthe validity of the derived melt rates. The authors briefly mention using 4.5 km spatial\nsmoothing to mitigate deviations from hydrostatic equilibrium (L150-152), but further\ndetail is needed to explain what was done and how this reduces uncertainty. As I\nunderstand, the authors have interpreted the formation of a >100m thick cavity beneath the\nBerry IGZ over the past 25 years (L156), reaching depths up to 300 m near State 2. This\nimplies the whole region is afloat, which starkly contradicts the InSAR GL results which\nshow the whole IGZ is ephemerally grounded with depths of seawater intrusions averaging\njust 0.6 m. Clarification of this discrepancy is important.\nResponse: 1. The distinction of areas and periods has been updated at Lines 162-163 and\n190-193; 2. We used different window sizes for the filtering until the melt rates do not\nchange at a detectable level and heterogeneities are eliminated. The window size is 4.5 km,\nwhich roughly corresponds to the size of the bending zone discussed by Chartran et al.\n2024; 3. We attribute the formation of the subglacial cavity at state 2 to basal melting.\nThe\nDInSAR fringe patterns observed in this region reflect the vertical displacement of the ice\nsurface driven by subglacial water motion, rather than the total water thickness. In other\nwords, the cavity has areas of partial grounding, and the water column must be thicker in\nbetween these pinning points, perhaps 10’s m, but we do not have data to quantify it.\n5) Tidal seawater intrusions\nThe estimates of melt rates due to tidal seawater intrusions (L181-192) appear to rely\nheavily on assumptions and values that are either poorly constrained or lack clarity on their\nuncertainties. This undermines confidence in the results and may mislead readers,\nespecially if the values are subsequently used for in models. It is recommended that the authors clearly outline all assumptions, cite where the values are taken from (note specific comments below about the CTD data), and acknowledge the scale of the limitations.\nResponse: Yes, the melt rates quoted in this section are order-of-magnitude melt numbers.\nIn the revision, we made this clearer (Lines 205-225).\n4. Diversity, Equity and Inclusion: I would like to draw attention to the lack of female\nrepresentation among the nine co-authors, which might be something the authors or the\njournal could consider addressing in future to foster more diversity in scientific\ncollaborations.\nResponse: This comment is not appropriate in a scientific review.\n5. Specific Comments\n1) Title – The title doesn’t really reflect the key findings of the paper; I suggest this could\nbe updated to include something on seawater intrusions which I think are arguably the\nmost interesting result. Also ‘and other data’ is a bit weak.\nResponse: The title has been changed to “Rapid retreat of Berry Glacier, West Antarctica,\nlinked to seawater intrusions revealed by satellite radar interferometry data”.\n2) Abstract – should be updated to reflect any changes made to the manuscript during\nrevision.\nResponse: The changes in the manuscript are updated in the abstract.\n3) L42 – typo; ‘affected’\nResponse: Fixed at Line 43.\n4) L47-49 – These sentences are quite vague. Are you referring to the effect of short-term\nGL retreat and advance periods (i.e. with the tides), or longer-term retreat/advance\nphases? Please clarify and consider adding references for support.\nResponse: If we refer to \"GL retreat reduces bottom friction and speeds up the glacier, GL\nadvance increases drag and slows down the glacier\", then it is the same effect on short and\nlong time scales.\n5) L51 – This paper doesn’t directly discuss CDW; you might consider citing more\nrelevant oceanography papers, such as Dinniman et al. (2012)\nhttps://doi.org/10.1175/JCLI-D-11-00307.1, Wåhlin et al. (2010)\nhttps://doi.org/10.1175/2010JPO4431.1; Schmidt et al. (2023)\nhttps://doi.org/10.1038/s41586-022-05691-0; Thoma et al. (2008)\nhttps://doi.org/10.1029/2008GL034939.\nResponse: We added these references at Line 72: Thoma et al. 2008, Dinniman et al. 2012,\nand Schmidt et al. 2023.\n6) L66 – I recommend using the full name ‘Berry Glacier’ here, since this is its first\nmention in the main text.\nResponse: Added at Line 64.\n7) L67 – ‘locus’ suggests a single point; consider using ‘area/region’.\nResponse: “locus” changed to “region” at Line 65.\n8) L75 – The dash between 0.6 and km can be removed here, and similarly in other places\nwhere it is not used as an adjective.\nResponse: The dash has been removed at Line 75, and other places.\n9) L79 – It is not clear where this observation is shown on Figure 2; I suggest adding\nlines/labels\nResponse: The GL from HE has been added to Figure 2a.\n10) L87-98 – This analysis would be strengthened by discussing potential reasons for the\nlack of correlation between tidal forcing and short-term GL migration, as well as\nconsidering other controls that may drive this short-term variability. Comparing to\nstudies that have documented different patterns of short-term GL migration elsewhere\nin Antarctica (e.g. Rignot et al., 2024; Chen et al., 2023; Freer et al., 2023) would be\nvaluable. For example, could this interpretation be affected by calculating modelled\ntides 50-100 km seaward of the IGZ, where tidal forcing may be out of phase with the\nresulting tidal flexure within the IGZ itself? Exploring whether there any correlations\nwith the longer-period spring-neap tide cycle, phase lags between rising and falling\ntides, or interactions with upstream subglacial hydrology—as observed in the\nThwaites IGZ (e.g. Chartrand et al., 2024; Rignot et al., 2024)—would enhance the\nanalysis. I note that Figure 5b shows several subglacial channels upstream of the IGZ,\nbut this has not been addressed in the text.\nResponse: The spreadsheet in the Dryad archive includes tidal height computed at two\noffshore locations, 50 km and 100 km seaward of the IGZ. The record shows negligible\ndifference in tidal amplitude and phase between the two sites. To further investigate the\ntidal influence on GL migration, we analyzed the spring–neap tidal cycle as well as the ebb\nand flow phases for each GL state, as now shown in Supplementary Fig. 1. Our results\nshow no discernible correlation between grounding line retreat and sea surface height, even\nwhen accounting for variations caused by the spring–neap cycle and tidal phase.\n11) L87-90 – It would also be interesting to see a comparison of the modelled Δtide (at\nthis same position 50-100 km seaward of the GL) compared to the measured InSAR\nΔSSH within the IGZ. This may provide some insight into the non-linear relationship\nbetween tidal forcing further from the ice front and the ice shelf flexural response/ GL\nmigration within the IGZ.\nResponse: This was shown in Figure 2c, which indicates that tide model and DInSAR agree.\n12) L97 – I suggest it is challenging to confidently state that the GL was \"predominantly\"\nin State 1 in 1996 based on only two observations. Perhaps rephrasing to reflect this\nuncertainty would be provide more context.\nResponse: Yes, we added a note of caution and indicated no evidence for signal upstream\nof that position in 1996, in contrast with recent interferograms (See Lines 76-80).\n13) L101-115 – This section is somewhat challenging to follow as currently structured.\nConsider reorganizing and re-wording to improve clarity. It appears to mix different\ntime periods (2006-2022 and 1996-2022), various regions (e.g., \"between states 1 and\n2,\" \"west flank,\" \"between the 1996 and 2008 GLs,\" \"from the 2008 GL to 28 km\nbeyond the 2021 IGZ upper limit,\" \"along profiles G-H and C-D,\" \"glacier on\naverage\"), and different values (ice elevation, thickness, thinning, speed, and speed-up).\nThis makes the text difficult to follow. Additionally, when Δh, ΔH and ΔV are\naveraged over specific areas, these areas should be clearly defined or located on a map.\nResponse: Agreed. We revised the paragraph for improved clarity and logical flow (Lines\n106-123). We clarified our description of the western flank (Lines 110-111), now explicitly\ndefining it as the region where the IGZ expanded laterally. Figure 3b highlights the area of\nice flow acceleration, extending up to 28 km beyond the 2021 inland limit of the IGZ.\n14) L104 – Clarification is needed regarding how glacier speed changes were calculated;\nnot enough detail is given here or in the methods section. Are these glacier speed\nchanges calculated as an average along flowlines, at specific points, or averaged across\nspecific areas? Including this information in the text or figure would provide useful\ncontext.\nResponse: The reported speed changes represent average values calculate over specified\nregions or along specific profiles. We clarified each case in the revised text.\n15) L105 – The sentence on glacier speed feels out of place between other sentences\ndiscussing glacier speed-up. Consider moving it to a more logical position or removing\naltogether. It is important here to be specific about the time stamp of the velocity value.\nResponse: The purpose of referencing the maximum speed here, along with the subsequent\nstatement, was to highlight that the location of peak ice speed does not coincide with the\nregion exhibiting the largest speed change. The sentence has been removed for clarity.\n16) L106 – The 1996 and 2008 GLs are not currently shown on Supplementary Figure 1,\nbut should be added.\nResponse: Based on the comment above, we removed the sentence related to the maximum\nspeed, the only place where Supplementary Fig. 1 was cited, therefore we removed the\nSupplementary Fig. 1 as well.\n17) L107-109 – It is important to clearly define the region over which the average speed-\nup of 134 m/yr and a 59 m elevation decrease is calculated.\nResponse: Agreed. As noted at Line 115-117, the region refers to the area extending from\nthe state 2 to inland by 28 km beyond its upper limit of 2021 IGZ.\n18) L110-111 – The results in L100-109 discussed change in V and h over the period 2006-\n2022, but it now seems for the flowline results you switch here a longer period 1996-\n2022. Please explain this discrepancy in time periods, and ensure the text explicitly\nmentions these timeframes to avoid confusion.\nResponse: Agreed. In the revision, we separate the discussion of the change in 2006-2022,\nfor which we have complete spatial maps for the basin, from the discussion for 1996-2022\nfor which the 1996 have data gaps, therefore we only analyze profiles.\n19) L110 – Consider rephrasing for clarity: ‘… the surface elevation decreased by up to\n59 m and ice sped up by 48% from 448 m/yr in [YEAR] to 665 m/yr in [YEAR]’.\nResponse: The sentence has been updated at Lines 118-119.\n20) L113 – is ‘on average’ referring to the entire glacier basin?\nResponse: The statement refers to the 2021 IGZ. See update at Line 121.\n21) L113 – As explained in the general comments above, I suspect these thinning rates are\nsignificantly overestimated given the region is not in hydrostatic equilibrium\n(especially if these values are referring to the entire glacier basin, the majority of which\nis fully grounded).\nResponse: We apply the hydrostatic equilibrium (HE) assumption only to regions where\nseawater intrusions are inferred at a specific time period. For the period 1996–2005,\nchanges in ice thickness were equated directly with surface elevation changes, as the GL\nreached to state 2 around 2006. Between 2006 and 2011, the HE assumption was applied\nwithin the region from state 1 to state 2. From 2012 to 2022, HE was applied across the\nentire 2021 IGZ extent.\n22) L114-15 – Is this referring to the spatial pattern of thinning across the whole basin?\nI suggest you are more explicit about what is being cited here from Selley et al. (2021);\nis this an observation they also made?\nResponse: This was a mistake. Text has been corrected.\n23) L116-123 – I recommend the authors clarify the time period and locations for ice\ndischarge calculations, and how this relates to the two time periods of observed h and\nV change discussed above (2006-2022 vs 1996-2022). It is currently difficult to\nascertain how this was done.\nResponse: The time period and locations of flux gates for ice discharge are indicated in Fig.\n1a and mentioned more clearly at Lines 316-320. The relationship between ice discharge\nand changes of h/V is discussed at Lines 327-329. We initiate the ice discharge calculation\nin 1996. For the time periods 1996-2005, 2007, and 2013, a linear interpolation is used to\nfill gaps in ice discharge as mentioned in the caption of Fig. 4. This is now clarified at Lines\n124-126 and 316-331. The time period and locations for the ice discharge calculations are\nrelated to the timing when the GL reaches each state.\n24) L116 – Against which baseline are these discharge anomalies measured? Is it the same\n1979-2004 reference period cited for the SMB anomalies in the following sentence?\nIf so, please clarify.\nResponse: The reference SMB is 1979-2004 is used as the balance discharge. See Lines\n126-130 and 334-337.\n25) L119 – Consider rewording for clarity: ‘Between 1996 and 2022, annual anomalies in\nmass balance, M, increased by a factor of 6.6, from X Gt/yr to Y Gt/yr’.\nResponse: The sentence has been re-written at Lines 131-133.\n26) L122 – It would be more logical to introduce the total increase in ice discharge at the\nstart of this paragraph, and then transition from discharge > SMB > mass balance.\nResponse: The sentence has been rewritten at Lines 133-135.\n27) L127 – ‘jack up’ is a somewhat colloquial phrase which can have negative\nconnotations; a different term like ‘lift up’ would be more suitable here.\nResponse: Agreed. The sentence has been rewritten at Lines 137-139.\n28) L131 – The interchangeable use of \"OIB\" and \"MCoRDS\" in the text can be confusing.\nConsider specifying \"MCoRDS\" when referring to radar-derived thickness data and\n\"ATM\" for altimetry data from OIB and updating this consistently throughout.\nResponse: Agreed. “OIB” has been replaced by “MCoRDS” when referring to radar-\nderived thickness.\n29) L141 – It would help to clarify here that you are comparing results to a modeling study,\nnot direct observations. Consider rewording to something like: \"... gravity gradients,\nas modeled in [20].\".\nResponse: The sentence has been rewritten at Line 156.\n30) L144 – A number of previous studies have also observed this; you may wish to cite\nworks such as Chen et al. (2023), Freer et al (2023), Rignot et al. (2024).\nResponse: These citations have been added at Line 160.\n31) L146 – Please specify the time period over which basal melt rates were calculated—\nwas this solely for 2019-2022, as indicated in Figure 5?\nResponse: The time information has been added at Line 163.\n32) L154 – It would be helpful to clarify which area is being referred to here and to mark\nthis region on Figure 5.\nResponse: This has been clarified at Line 168. The region is the IGZ.\n33) L156 – you should use “meters” instead of “m’s”.\nResponse: “m’s” has been changed to “meters” at Line 172.\n34) L171-173 - There appears to be a contradiction between these sentences. The text\ninitially states that the Berry IGZ was unknown during previous studies, but then\nreferences comparisons of melt rates \"in the IGZ of Berry\" between 2003-2008 and\n2019-2022. Please clarify this.\nResponse: Yes, we needed to clarify that we compare the melt rates within 10 km of the\nlower limit of the IGZ as stated at Line 190 where we have coincident measurements.\n35) L172 - The “area of coincident measurements\" for comparing the three time periods\nof basal melt rates is unclear. I suggest the authors label this area in Supplementary\nFigure 5. This applies to the areas used for the melt rate calculations at the other\nglaciers, such as De Vicq, as well.\nResponse: We added white dashed polygons in Fig. 5c–d and Supplementary Fig. 5 to\ndelineate the region of coincident measurements.\n36) L176-177 – The statement \"the melt rates in the IGZ of Berry are also the largest for\nthe Getz Ice Shelf\" appears in a section discussing melt rates in the 10 km downstream\nof the IGZ, rather than the IGZ itself. Consider moving this sentence to the previous\nsection or clarifying that it refers to the downstream zone.\nResponse: “the downstream” has been added at Line 201.\n37) L177-180 – I suggest adding a reference to Supp. Fig 5d here, as that is the only place\nthis result is shown.\nResponse: The reference has been added at Line 204.\n38) L182 – There should be a unit given in ‘… from 0 m at state 3 to…’.\nResponse: The sentence has been re-phased at Lines 205-206.\n39) L185 – Does the 0.6 m value represent a calculated average of the observed InSAR\ndisplacements? Or is this a prediction? More clarity (+associated uncertainties) is\nneeded.\nResponse: “0.6” is half of the maximum vertical amplitude at state 1, 1.2 m, which comes\nfrom the tidal model. “Observed vertical displacements at the entry of the IGZ of 1, 4 and\n4.7 CSK fringes translate into a minimum change in water column of 1.4, 5.5 and 6.5 cm,\nrespectively” has been deleted to avoid confusion.\n40) L186 – the stated water flux of 6.9 x 107 m3 in 6 hours would assume that the GL\nmigrates all the way to state 3 each tide cycle. However, your results indicate there is\nno observed correlation between tide forcing and GL migration distance (Fig 2) – so\ndo you think this is still a reliable estimate of water flux? Please clarify and/or be more\nopen about the associated limitations of this assumption.\nResponse: This calculation is a maximum melt rate if water intrusions flush the entire cavity\nand all available heat is used to melt the ice.\n41) L186 – You use an area of 114 km2 for the IGZ here, but earlier a 100 km2 IGZ area\nwas used to calculate melt rates (L154)? Clarifying why these values differ would be\nhelpful.\nResponse: Agreed. “100 km2” has been changed to “114 km2” at Line 168.\n42) L187 – This is the first and only time you mention a 2018 CTD, but there is not\nassociated citation. Earlier, you referenced a 0.7C ocean temperature at 1,000m depth\nin front of Berry measured from a 2007 CTD (Jacobs et al. 2013). The ‘2018 CTD’\ndata gives ocean thermal forcing at 1,000m depth of +3.6C. Is this a mistake in the text\nwith the date, or are you referencing two different datasets here? If this is the case,\nplease clarify the date and location, alongside the appropriate citation.\nResponse: We originally cited Selley et al. 2021. Upon careful examination of the CTD\ndataset from KOPRI—used in the 2021 study—we found that the measurements extend\nonly as far west as Dean Island but do not reach Berry Glacier. Consequently, to estimate\npost-2007 ocean temperature changes at the ice front of Berry Glacier, we analyzed CTD\nprofiles collected at distant locations in west Getz Ice Shelf after 2007 instead\n(Supplementary Fig. 6). The CTD records indicate a warming trend since 2000, which\npersisted beyond 2007. If we assume that the ocean in front of Berry Glacier experienced\na comparable degree of warming, we find a thermal forcing of +3.6°C at 1,000 m depth in\n2018. This discussion has been added at Lines 208-216.\n43) L188-189 – I suggest more detail is needed regarding the calculation of ocean heat and\nfreshwater fluxes, including any associated uncertainties or assumptions.\nResponse: The primary sources of uncertainty in the ocean heat estimate stem from two\ncomponents: the thermal forcing and the volume of intruding seawater. We revised the\ncalculation using the CTD from 2018 (Supplementary Fig. 6). For the intrusion water\nvolume, we assume an average cavity thickness of 0.6 m. We estimate a low efficient of\nthe melt process at 30% and obtain an average melt rate of 6 m/year which indicates that\nwater thickness in the IGZ must be higher than 0.6 m as noted at Lines 221-225.\n44) L190 – the statement that ‘only 30% of the heat will be used to melt ice’ is based on\nmodelling at Petermann Glacier in Greenland. How confident are you that this can also\nbe applied in this setting? Without providing uncertainty estimates or being open about\nthese limitations, this quantification could be misleading.\nResponse: The citation of the 30 ± 10 % is from a simulation of ocean circulation using\nthe MITgcm ocean model in a grounding zone. There is no fundamental reason for the\nphysics to be different in Greenland vs Antarctica. We revised the calculation, fixing the\nerror in water height, and used a more recent value of ocean temperature from 2018 at Lines\n205-221.\n45) L191 – Please clarify the basis for the statement that the seawater intrusions may be\n\"thicker by a factor of two\"; has this been quantified or just speculative?\nResponse: Sentence has been removed.\n46) L206 – In general, the methods lack sufficient detail to reproduce the analysis,\nalthough perhaps this has been limited by the word count of the article. Citations\nshould be provided for all datasets.\nResponse: Reference has been added.\n47) L218 – I suggest this should cite a paper describing the Sentinel-1 instrument, rather\nthan an ice sheet study that has also used Sentinel-1.\nResponse: The citation of the Sentinel-1 instrument has been added at Line 264.\n48) L222-228 – This section is missing a description of how the differential SSH was\ncalculated from the interferograms, and whether the GLs were delineated manually or\nusing an automated approach. Also, how do you define a ‘high quality’ interferogram?\nResponse: The descriptions of how the differential SSH is calculated and the method for\ndelineating the GLs have been added at Lines 283-283 and 273, respectively. The “high\nquality” means we have a high-precision, low-noise determination of the GL position.\n49) L226 – should be ‘dense’ instead of ‘densely’.\nResponse: Changed at Line 272.\n50) L226 - The attached data spreadsheet includes a column for GL migration distance\n('GL migration_Berry') as well as columns of 'positive error' and 'negative error',\nhowever there is no description of these errors in the text. I suggest the authors include\na robust description of these error terms and the calculations used to derive them here.\nResponse: When the fringes are noisy or merge or split, we estimate the positive and\nnegative errors based on the fringe pattern to show the potential minimum and maximum\nposition of the GL. See Line 274-276.\n51) L249-258 – The temporal coverage of each of the elevation change products used here\nshould be provided:\n• ICESat-2 ATL14 DEM is timestamped to Jan 1 2020\n• ICESat-2 ATL15 elevation change products – between 2019 and 2024\n• Add date ranges for other satellite missions used in the MEaSUREs elevation change\nproducts.\nPlease clarify whether you use the ATL15 ICESat-2 elevation products (1km) as well\nas the MEaSUREs product using ICESat-2 (2 km)? Or just the former? And this\nsection is missing information and citation for the WorldView-derived DEM and ATM\ndata (e.g. used in Fig. 3, and in calculation of ice bottom and bed topography below).\nResponse: The information of the products above has been added in Lines 304-313. We\nonly use the MEaSUREs product for the period of 1996-2018. We do not use ICESat-2 at\n2 km resolution. The citation and information for WV and ATM have been added in Lines\n343-345 and 321-325.\n52) L262-3 – this seems to be a new result that hasn’t been presented in the main text?\nPlease clarify.\nResponse: Agreed. The elevation change (∂h/∂t) along flowlines, derived from ICESat-2\nATL15 and MEaSUREs products, was not intended to be a new, independent result. Rather,\nit serves as supporting evidence to corroborate the GL migration established based on\nDInSAR-derived GL positions. These indicators help understand the GL migration and the\nplacement of flux gates at different times.\n53) L278 – The current description lacks sufficient detail. In the main text it says the bed\ntopography is reconstructed ‘based on the minimum depth of ice that ensures flotation\nof ice over the zone of flexing, i.e. where a meter of change in SSH is sufficient to\njack up the ice surface’. Please elaborate here on this calculation, specifying e.g. the\ngeoid used and whether the reconstruction was performed over the whole IGZ region,\nor along individual flowlines?\nResponse: Agreed. We elaborated on the bed reconstruction method in the revised text.\nSpecifically, the reconstruction was performed across the entire IGZ, see Lines 340-341.\nThe method involves calculating the minimum ice draft required to ensure flotation based\non the surface elevation data (using the BedMachine v3.7 geoid; Lines 348-349) and firn\nair content. After corrections, we calculate the minimum depth of the ice bottom (Lines\n340-353).\n54) L281 - Given that FAC values are known to be uncertain over GZs, it would be\nimportant to acknowledge this uncertainty here (even if it cannot be precisely\nquantified). Additionally, consider citing the firn model used in BM3.7 to provide\nmore context.\nResponse: Agreed. The citation about the firn model have been added at Line 349-350.\nAlthough the uncertainty in firn air content (FAC) over grounding zones is not quantified\nin BedMachine v3.7, Ligtenberg et al. (2011)—the basis of the firn densification model—\nhighlight that firn properties exhibit substantial spatial variability across Antarctica, and\nthat firn correction values may vary up to 7–9% in coastal and shelf regions. The authors\nacknowledge that their model is semi-empirical and does not fully capture the physical\ncomplexity of firn processes in dynamically evolving zones such as the IGZ where complex\nprocesses such as surface melt, tidal flexure, and dynamic thinning may deviate from the\nmodeled steady-state assumptions. We added a statement to the revised text at Line 350-\n353.\n55) L283-286 – see previous comment on this interpretation; I would suggest it’s more a\ncase that your calculations are inconsistent with the radar measurements (rather than\nthe other way round). A stronger justification is required, beyond simply that than the\nL2 data is ‘not as robust’.\nResponse: The GL positions derived from DInSAR suggest that portions of profiles A–B,\nC–D, and C–F are partially afloat. Under these conditions, the bed elevation inferred from\nMCoRDS radar sounding cannot be deeper than the ice draft derived from HE using ATM\nsurface elevation data. In our previous response, we discussed in detail the limitations of\nthe MCoRDS-derived bed along these profiles, including misidentification of the basal\nreflector due to off-nadir returns or weak radar backscatter in the valley center. These issues\nare well documented in the radargrams and explain why the radar-derived bed is\ninconsistent with flotation-based estimates. It is not the first time erroneous bed picks have\nbeen reported from MCoRDS.\n56) L289 – It would be useful to see a justification of applying a Eulerian vs Lagrangrian\napproach here, or acknowledgement of the associated uncertainties when applied to\nfloating ice.\nResponse: Agreed. We clarified in the revised text that the use of a Eulerian approach is\nrequired for ICESat-2 ATL15 data. Specifically, ATL15 provides surface elevation\nchanges relative to a fixed DEM, rather than absolute surface elevations at discrete time\nsteps referenced to moving ice parcels. We acknowledge that applying the Eulerian method\nto floating ice introduces additional uncertainties, particularly in regions experiencing\nadvection or basal geometry changes. To mitigate these effects, we have applied a spatial\nfilter based on the length of the bending stresses and the size of heterogeneities. We\nincluded this discussion in the revised Methods section (Lines 372-378).\n57) L293-4 – Please provide more detail on this, as per previous comment.\nResponse: The information regarding uncertainties has been provided at Lines 383-385.\n58) L295-299 - It is good that the potential overestimation of melt rates is acknowledged\nhere. However, this point should be emphasized more strongly within the main text,\naccompanied by a more detailed explanation of the uncertainty estimation process.\nSpecifically, it is unclear how you arrived at the stated uncertainty of \"about 36%\". It\nwould be helpful to clarify this methodology, and any reported uncertainty values\nshould be presented alongside the corresponding results in the main text for\ntransparency and clarity.\nResponse: We evaluate the uncertainty at 21%, see methods section, where we explain the\ndetails.\n6. Figures\n1) In general, the figures and captions (in main text and supplementary) deserve some\nattention to bring up to publication standard. Most text is too small to read\ncomfortably, there are a mix of font types and sizes, and overlapping data and sub-\npanels obscures some important information. I recommend using a consistent structure\nfor figure captions throughout, i.e. Fig X - main title. (a) subpanel caption; (b) subpanel\ncaption; (c) etc. I have made some suggestions here for a number of (relatively minor)\nedits that could be made to the main text figures that I believe would significantly\nimprove their quality. The same principles should be applied to the supplementary\nfigures.\nResponse: Thank you. We increased the size of letters and made the styles of the figures\nmore consistent.\n2) Figure 1: This figure contains a lot of valuable and important data, but its impact is\ncurrently limited by the way it is presented. I suggest that the authors should consider\nre-formatting to a portrait layout, which would allow it to be displayed as a full-page\nfigure. Alternatively, consider separating panel (a) as its own figure, and organizing\npanels (b-g) into a separate portrait figure (e.g. a 2x3 grid). Some suggested\nimprovements include:\n• Panel (a) –\no Avoid use of < > symbols in the velocity legend for clarity.\no The colors of the flux gates get a bit lost; perhaps keeping these in bold black\nwhile reducing the opacity of the rest of the OIB lines would improve clarity.\no Add a box to the map to show the area in panels (b-d).\no The elevation contour values could be removed; they are somewhat redundant\nalongside the blue elevation color scale. This would simplify things and avoid\ncontour values overlapping the main data you are presenting (i.e. the GL positions).\n• Panels (b-d)\no These panels are well-presented, but the caption should provide more\ninformation (i.e. explain these are interferograms composed from images sampled\nfrom the 3-4 tidal states shown in panels (e-g); and describe what the color bar\nrepresents).\no Adding labels for the maximum tide height and/or differential SSH of each\ninterferogram could assist readers in understanding the data presented.\no Clarify whether the inclusion of three interferograms comprised of scenes all\nsampled at low tides is a deliberate choice; if so, consider briefly explaining this in the\ncaption as it is interesting for the interpretation!\n• Panels (e-g)\no Consider reducing the number of values on each axis label (e.g. intervals of 12h\non x-axis) to allow for an increase in text size.\no The grey lines pointing to sections of the longer tide time series clutter the\nfigure. Consider removing or ensuring they do not cross over one other.\no Consider repositioning the inset panels to prevent them from covering\nimportant data on the main panel, as this is an issue across all three panels (e-g) where\nsome of the inset panels cover the red dots.\nResponse: Figure 1 has been updated based on the comments, along with the caption. The\ninterferograms are not chosen deliberately for the low tides. We choose them to show the\nbest example of each state.\n3) Figure 2:\n• Panel (a)\no This density plot is a very effective way to illustrate the three GL states.\no Some clarification is needed for the values presented in the DInSAR/HE box.\nConsider adding details to the caption, including an explanation of HE as hydrostatic\nequilibrium, and incorporating lines on the map to show the locations of the HE GLs\nfor better reference.\n• Panel (b)\no Is there a particular reason for using a different color map and phase difference\nscale compared to Figure 1(b-d)? I’d say the color scale in Figure 1(b-d) is clearer.\no From your caption, I would expect the downstream seawater intrusions to be\nmarked by a region of uplift, and upstream extrusion of seawater trapped in a prior\ncycle marked by a region of subsidence – but it appears that these labels are reversed\non the map? Or is the region of extrusion further upstream (between states 2 and 3)?\nThis may be my own misinterpretation, but it would be good to clarify this. Adding\nlabels to show the regions of intrusion vs extrusion would be beneficial.\no Including arrows above/below the legend to illustrate the meaning of the\ndiverging fringe colors could greatly enhance comprehension of this figure for those\nless familiar with DInSAR maps (i.e. pink>yellow>blue = uplift; and\nblue>yellow>pink = subsidence).\n• Panel (d)\no This is an effective presentation of these results. A few minor clarifications\nwould further aid interpretation:\nIndicate what the error bars represent (both for modelled max tide and GL\n■\nmigration distance).\nConfirm that h refers to the maximum modelled tide and clarify in the\n■ max\ncaption.\nResponse: Figure 2 has been updated based on the comments. The caption related to\nintrusion/extrusion is corrected. 1. The different color map and phase difference scale for\nthe interferograms are because the CSK and Sentinel-1 are generated with different\nscaling; 2. For panel (d), the horizontal error comes from tides model and mean sea level\npressure, and the vertical error shows the range that the farthest and closest distance that\nthe GL may reach because when the fringe boundary is unclear or fringes merge or split.\n4) Figure 3:\n• The caption currently lacks clarity, specifically,\no L2 – Clarify that (a) and (b) represent change in h / change in V\no L3-4 - Remove ‘Year 2006 is the reference for (a-b)’; this info is already\nconveyed in the preceding text and figure.\no L4-5 – Maintain consistent phrasing for panels (c,d) and (e,f), i.e. ‘V(/h) along\nprofiles G-H and C-D for each year 1996-2022’\no L6 – specify that the elevation data is from the WorldView (WV) DEM (i.e.\nnot just from WV imagery)\no L6-12 – separate captions for panels (g) and (h); currently confusing as different\ndatasets are used in each for the same thing.\no L6-12 – Clarify descriptions of the different blue lines (‘blue line’/’blue dash\nline’/’blue solid line’/’dark blue line’). It is confusing where the same line style is used\nin panels (g) and (h) to show different data (e.g. in (g) the solid light blue line is from\nthe 2019 WV DEM, but in (g) the same line style shows the ‘2012 DEM’). Consider\nchanging the line styles or providing a more explicit legend.\no L7 – please clarify which DEM is used for ice draft in (h)? It is unclear what\ninstrument is used for the ‘2012 DEM’ – it says ICESat-2 in the caption, but this cannot\nbe correct as ICESat-2 only launched in 2018.\no L8 – please clarify if by ‘flotation from ICESat-2’ you mean: flotation depth\nderived from the 2020 ICESat-2 DEM?\no L10-12 – ensure consistency in the description of the OIB data points.\n• GL positions/IGZ zones\no On panels c, e and g, it is confusing having both the horizontal and vertical bars\nshowing GL locations, positioned both above and below the main data. I suggest\nkeeping just the horizontal bars for 2019-21 to show the short-term range of GL\npositions in each of the 'states’ and labelling the three states in each panel (as has been\ndone in g). Then you can use a vertical bar/dot for 1996 and 2008/9 GL positions where\nyou don’t have sampling of the IGZ length.\no For panels d, f and h, it is fine to use vertical lines, as this is showing the total\nwidth of the glacier/shelf.\no I suggest positioning all GL positions at the bottom of each panel (including in\ng) for consistency, rather than having some above and below the main data.\no Reflect any changes made in the caption.\n• Panels g-h – These are currently quite confusing to interpret.\no Consider moving both the x-axis and IGZ labels to the bottom of each figure\n(rather X than in the middle of the glacier).\no Clarify if elevation measurements are relative to mean sea level\no Legend and caption currently missing info on the ?s, striped areas and blue\nshaded areas.\no It is currently unclear what the areas labelled as ice/bed/water are using data\nfrom? BedMachine? Or the bed you’ve deduced from flotation? In (h) looks like the\n‘bed’ is from the OIB data, but in (g) it’s from a mixture of BedMachine and your\nWV-DEMderived bed.\no In panel (g), the blue dots representing the OIB data are not easily visible. The\nlegend entry for OIB reads like a date (i.e. 2011/12/16 could be interpreted as 16\nDecember 2011). I suggest since there are only four OIB points on this panel, each\ncould be individually labelled with their measurement year.\no There doesn’t seem to be a consistency in the results at the expected crossover\npoint between profiles G-H in panel (g) and C-D in panel (h); perhaps this is the result\nof using the same colors in both panels for different data?\nResponse: Figure 3 and its caption have been revised in response to the reviewer’s\ncomments. 1. Elevation measurements are referenced to mean sea level using the EIGEN-\n6C4 geoid model from BedMachine v3.7 for geoid correction; 2. Descriptions of the\nshaded areas have been added to the figure caption and the main text (Lines 359-365);\n3. We clarified the priority order for bed topography data sources: where available, we adopt\nthe bed from MCoRDS; within the IGZ, we use flotation depths derived from geoid-\ncorrected surface DEMs when MCoRDS is not available, as BedMachine performs poorly\nover Berry Glacier; outside the IGZ, BedMachine is used; 4. The crossover point between\nprofiles G–H and C–D in panel (h) does not correspond to the deepest part of the bed.\nWe have added the crossover locations to the X-axes in the relevant panels for clarity.\n5) Figure 5:\n• The sizing of this figure is much better\n• On panel (b) red GLs get lost in the red melt areas; consider using a different color\nscale?\nResponse: A thin white outline has been added to the red GLs to enhance their visual\ncontrast against the background in Figure 5.\n7. Datasets\nThe authors have provided easily accessible datasets of derived GL positions, flux gates\netc. that were straightforward to open in GIS software. I recommend that all processed\ninterferograms should also be included in the dataset. The excel spreadsheet ‘GL analysis,\ntidal heights, speed & surface height.xlsx’ contains a lot of useful data, but some\nclarification and reorganization is required to improve its usability. Notably:\n• Columns F and G (‘positive error’ and ‘negative error’) appear to indicate the number\nof days between the first and last SAR image date and the midpoint between these dates.\nPlease clarify how this constitutes an error, and—if so—how this error is dealt with in the\nsubsequent processing.\n• Columns P and Q are also labelled ‘positive error’ and ‘negative error’, but these\nappear to indicate some error in the derived GL position. Please provide an explanation or\nthis error and its calculation, assumptions etc.\n• This dataset contains columns for modelled tides/IBE at 50 km and 100 km. Please\nclarify what these distances refer to – i.e. is it the distance between the downstream IGZ\nlimit and the position used to calculate the tide? If so, which of these locations (50 or 100\nkm) did you use? If only one location is used for the tide calculation, only these values\nshould be included in the dataset to avoid confusion.\n• It is confusing that the SAR image acquisition dates are repeated three times in the\nsame Excel sheet: in columns A-D, S-V and AP-AS. There is also repeated GL migration\ndata in columns O (‘GL migration_Berry’) and AM/AN (‘GL migration_Sentinel-1’/’GL\nmigration_COSMO-SkyMed’). Consider consolidating these columns to remove\nunnecessary duplication.\nResponse: 1. The interferograms generated from Sentinel-1 and CSK cannot be uploaded\ndue to copyright; 2. The purpose of the \"positive/negative error\" of the dates is to show the\ntemporal range of the interferograms. We removed it; 3. The positive/negative error of the\nGL position is explained above; 4. The value is the distance between the downstream IGZ\nlimit and the position used to calculate the tide. The location at 100 km is used. The reason\nof including the SSH at 50 km is to show the SSH does not significantly change from 100\nkm to 50 km; 5. The duplication has been removed. We have revised the figure to improve\nclarity.\n8. Code availability\n• The Zenodo link points to code used to compute basal melt rates at Thwaites,\noriginally from a different paper (Gadi, 2024). I am not sure of the specific journal\nguidelines on this, but I suggest this should either be appropriately cited alongside the\ndataset, or specific code used for this work at Berry should be provided.\n• No code is provided for the other calculations described in the methods, including ice\nbottom/bed topography, ice mass balance, GZ length or tides. This should be included to\nensure full transparency.\nResponse: 1. Ratnakar Gadi is a co-author of this manuscript. The code is identical; 2. The\ncalculations are done using the built-in tools and the Python console in QGIS."}, {"round": 3, "reviewer_comments": {"reviewer_1": "Response to Revisions (Round 2)\nThank you to the authors for their thoughtful responses to my comments and for the efforts made to address these in the revised manuscript. I am satisfied with their responses and with all changes made. The additions to both the text and figures have greatly strengthened the manuscript. Congratulations to the authors.\nI have two very minor residual comments that the authors may wish to consider before publication:\n• L140 - Missing unit on “13 ± 15” for the SMB.\n• Figure 3 – Thank you for clarifying these points in your response and the revised manuscript. To avoid any potential confusion, I suggest using a different color for the blue striped regions (locations where vertical motion from seawater intrusion is detected) and the blue shaded regions (ice-bottom uncertainty bounds). At present, the two blues are difficult to distinguish, particularly where they overlap.\n(Remarks on code availability)\nI did not re-review the code, as I do not believe any changes have been made since the first round of reviews.", "reviewer_2": "I am satified with the author's response to my comments and the resulting edits in the manuscript. I hereby recommend publication of the manuscript.\n(Remarks on code availability)", "reviewer_3": "Thank you for addressing my comments and revising the paper. I only have some minor suggestions below.\nLine 71: Please consider rephrasing the sentence into ‘are in contact with warm waters coming from Circumpolar Deep Water that …’\nLine 72: The mention of ice shelf meltwater feels out of place here. I suggest adding a statement on the implications of ice shelf basal melting.\nFigure 1a: It’s difficult to discern the black arrow associated with Berry Glacier from the background ice velocity map. I suggest changing to a different color. The same issue applies to the star marker at the tide location. Also, the star marker is very small.\nLine 78: ‘but there is no residual signal upstream of state 1 in 1996’, please add a reference to Figure 7.\nLine 83-86: Please specify which bed topography product you are referring to.\nLine 172: Please rephrase ‘several 100 meters’\nLine 194 - 201: I can’t tell changes in basal melt rates in different time periods or different locations mentioned in this paragraph directly from Figure 5, as the current colorbar of basal melt rate has a limit of [-6,6] m/yr. Please revise the figure to improve the illustration of basal melt rate variations.\nLine 204: Consider removing the repetitive ‘that is’ in this sentence\n(Remarks on code availability)"}, "author_response": "Dear Editor, Reviewers,\nWe are grateful for the reviewers’ thoughtful, constructive comments in the whole revision\nprocess, which have substantially improved the manuscript and brought it into compliance with\npublication standards.\nOn behalf of the co-authors – Hanning Chen."}]

Decision letter