Ocean warming drives rapid dynamic activation of marine-terminating glacier on the west Antarctic Peninsula

Domain: earth_science
Published: 2023-11-28
Editorial outcome: major_revision

Abstract
Ice dynamic change is the primary cause of mass loss from the Antarctic Ice Sheet, thus it is important to understand the processes driving ice-ocean interactions and the timescale on which major change can occur. Here we use satellite observations to measure a rapid increase in speed and collapse of the ice shelf fronting Cadman Glacier in the absence of surface meltwater ponding. Between November 2018 and December 2019 ice speed increased by 94 ± 4% (1.47 ± 0.6 km/yr), ice discharge increased by 0.52 ± 0.21 Gt/yr, and the calving front retreated by 8 km with dynamic thinning on grounded ice of 20.1 ± 2.6 m/yr. This change was concurrent with a positive temperature anomaly in the upper ocean, where a 400 m deep channel allowed warm water to reach Cadman Glacier driving the dynamic activation, while neighbouring Funk and Lever Glaciers were protected by bathymetric sills across their fjords. Our results show that forcing by warm ocean water can cause the rapid onset of dynamic imbalance and increased ice discharge from glaciers on the Antarctic Peninsula, highlighting the region’s sensitivity to future climate variability.

Introduction
Research on the Antarctic Peninsula (AP) has shown major changes in the region’s glaciers and ice shelves throughout the 20th and 21st centuries. The AP has been particularly exposed to the impact of climate change, with near surface atmospheric temperatures on the northern Peninsula warming by +0.54 °C per decade from 1951 to 20111. Similarly, surface waters in the Southern Ocean and Bellingshausen Sea on the west coast of the AP warmed by more than 1 °C from 1955 to 19942. Calving front position measurements show that ice shelves on the AP lost an area of 28,000 km2 from 1947 to 2008, with major retreat starting in the 1970s and 1980s3. More recent records show that this trend has accelerated, with ice shelves losing an area of 20,500 km2 between 1997 and 20214, with 90% of the Peninsula’s 860 tidewater glaciers retreating in the period up to 20105. The associated loss of buttressing from ice shelves and retreat of glaciers has contributed to a substantial increase in ice mass loss from the Antarctic Peninsula Ice Sheet (APIS) 6–9. Satellite observations show that ice loss from the region increased by 400% between the periods 1992 to 1997 (7 ± 13 Gt/yr) and 2007 to 2012 (35 ± 17 Gt/yr), peaking in the decade after the partial collapse of the Larsen-B ice shelf in 200210. Overall, an average of 20 ± 15 Gt/yr of ice mass was lost from the APIS in the 25-years between 1992 and 201710, and ice loss is projected to increase in the future with 7 to 16 mm of sea-level equivalent by 2100 and 10 to 25 mm by 220011.

Both atmospheric and oceanic forcing have driven change on the AP. In 1996, the retreat and collapse of ice shelves was linked to atmospheric warming, specifically the southerly migration of the −5 °C isotherm12. Since then, studies have attributed the further retreat and collapse of the Larsen-A and -B Ice Shelves on the eastern AP to thinning, foehn winds and increased summertime melt that led to hydrofracture of crevasses 13–20. On the west AP, a pattern of increased calving front retreat on glaciers in the south has been attributed to the influence of different water masses, with cooler Bransfield Strait Water in the north and warmer Circumpolar Deep Water (CDW) in the south which is transported by the Antarctic Circumpolar Current and intrudes onto the continental shelf from the open ocean21. High temporal resolution ice speed measurements from synthetic aperture radar (SAR) satellite imagery have shown pronounced seasonal fluctuations in ice speed of up to 22.3% on west AP tidewater glaciers22 and glaciers feeding into the George VI Ice Shelf23. On the west AP, these speed fluctuations are linked with seasonal ocean warming and surface melt22,23, showing the sensitivity of this region to short-term variability on sub-annual timescales.

The east and west sides of the AP have significantly different atmospheric and oceanographic conditions. The west AP has high accumulation due to orographically driven precipitation and strong westerly winds24,25. There is significant surface melt in the summertime, but less runoff compared with the east AP due to a thick, porous firn layer which takes time to saturate, and which can support perennial firn aquifers26–28. South of the Bransfield Strait, CDW flows onto and across the continental shelf, becoming modified CDW (mCDW) through mixing and vertical heat loss to the overlying Antarctic Surface Water layer29,30. This contrasts with the much colder waters of the Bransfield Strait and Weddell Sea on the east of the Peninsula31. Sea-ice on the west AP follows a seasonal pattern of summertime retreat and wintertime advance, with the wintertime sea-ice cover decreasing strongly in duration since the 1980s32,33. In the western Weddell Sea, a large area of sea-ice regularly persists during austral summer, becoming thicker multi-year ice. Between 1978 and 2015 sea-ice extent (SIE) in the Weddell Sea grew in extent with a small positive trend superimposed upon large interannual variability34,35. However, from 2016 onwards this trend reversed, with satellite data showing a reduction in SIE, reaching a record low in 2019 and contributing to record low total Antarctic SIE in 202234,36,37.

The future evolution of glaciers on the AP has great significance for broad-scale oceanographic and glaciological changes. Model results show that increased freshwater input to the ocean from precipitation, snow, and glacier melt on the AP can strengthen the Antarctic Coastal Current, leading to enhanced basal melt rates beneath ice shelves in West Antarctica, including Pine Island and Thwaites38. The waters of the west AP shelf support highly productive and diverse ecosystems which are impacted by changes in sea-ice and meltwater flux39,40, and glacier retreat exposes new seabed for colonisation by benthic species and enhanced carbon sequestration41. The combination of substantial long-term glaciological change, short-term seasonal variability in ice dynamics, and atmospheric and ocean forcing, have a broad impact on Antarctica’s ice and oceans and makes the ongoing study and monitoring of the AP a priority.

Cadman Glacier is located on the west coast of the AP south of Anvers Island at 65.6 °S and flows north-west into Beascochea Bay (Fig. 1a). The glacier drains an area of 322 km2 from the Bruce Plateau at 2080 m elevation through a steep sided valley, which is 2 km wide at its narrowest point5,42. The Cadman Glacier system has four major zones inland from the fjord entrance: i) the Cadman Ice Shelf, ii) the lower glacier which we define as the fastest flowing grounded ice up to 300 m elevation, iii) the Cadman Icefall which is the steepest part of the glacier between 300 and 500 m elevation, and iv) the upper glacier which includes slower flowing ice above 500 m elevation. Satellite image records show that the calving front of Cadman Glacier was remarkably stable throughout the second half of the 20th century, with the ice-front deviating by less than 1 km from a position at the end of its fjord throughout the observational record5,21. The glacier catchment has a high rate of surface mass accumulation, up to 4000 mm water equivalent per year, with substantial snow melt and runoff in the austral summer due to its location on the west coast of the AP22,24,27. Due to the snow surface conditions and the speed of ice flow, it is challenging to measure the grounding line location on this glacier using differential interferometry for which coherent Synthetic Aperture Radar (SAR) image pairs are required. Instead, the grounding line position has been measured using altimetry and photogrammetric methods43. These observations show that between 1999 and 2003 Cadman Glacier had an ice shelf confined to the lower fjord, where the grounding line was located 6 km inland of the mouth of the fjord.

In this study we measure ice dynamic change on Cadman Glacier over a 31-year period between 1991 and 2022. We use satellite data to measure the calving front position, ice velocity and surface elevation change, which showed that between 2018 and 2019 the glacier accelerated by 94 ± 4% (1.47 ± 0.6 km/yr), with over 8 km of calving front retreat observed by 2021. We use direct oceanographic measurements and ocean reanalysis data to determine the environmental drivers of this major change, and contextualise it alongside the longer-term evolution of the AP.

Results
Change in calving front position
The earliest recorded position of the Cadman Glacier calving front is 196844, when the glacier terminated at the entrance to the Cadman Fjord. Prior to 2006, the calving front position did not change significantly, with variability limited to 500 m of retreat from 1985 to 1997, before it re-advanced to the 1984 position by 2001. From 2006 onwards, the glacier underwent a significant retreat of the calving front, moving inland from the fjord entrance by 1.8 km between 2005 and 2009. The glacier calving front remained in this retreated position from 2010 to 2014 (Figs. 1c, 2).

Between 2014 and 2022, 6 to 12-day repeat Sentinel-1 SAR imagery resolves change in the glacier’s calving front position with much greater temporal detail than the period from 1968 to 2014. These observations show that the glacier readvanced from May 2015 to February 2018, when it reached its maximum observed extent relative to the full observational record from 1968, extending 700 m beyond the mouth of its fjord into Beascochea Bay. Although advanced, during this period the seaward 3 km of glacier tongue shows a substantial increase in damage and crevassing on the ice shelf surface and a widening of the shear margins (Fig. S1), suggesting that the structural integrity of the ice was decreasing. Notably in the 17th January 2020 image (Fig. S1d) there is a distinctive along-flow damage feature on the central ice shelf, which may indicate channelised sub-shelf melting. The glacier calved and retreated by 2.8 km coincident with the start of its period of most rapid acceleration which was sustained throughout 2019. From March 2019 to March 2021, the glacier underwent a cycle of calving and readvance in this retreated position, while flowing approximately 1 km/yr (44 %) faster than pre-2019. Throughout this period of faster flow, the floating ice shelf became increasingly damaged and crevassed, until eventually breaking up into large tabular blocks. Notably, although summer melt is widespread in the region, we observe no surface meltwater ponds visible on the ice surface in any optical satellite image for our period of study (1984–2021), which we attribute to the north-west AP’s thick firn layer and high firn air content45. Cadman Ice Shelf completely collapsed in March 2021, with the calving front location retreating by a further 5.2 km to a position 2.4 km inland of the ASAID (1999–2003) grounding line location, and it remained in this position for the remainder of the study period to July 2022.

Ice velocity from 1991 to 2022
Our ice velocity measurements show the rate of ice flow on Cadman Glacier at high spatial and, since 2015, temporal resolution (Fig. 1b). The grounded ice flows at speeds above 1 km/yr 10 km inland of the 1999–2003 grounding line position and the highest speeds of more than 4 km/yr are observed on the Cadman Glacier icefall and lower glacier in 2021/22. This makes Cadman Glacier the fastest flowing glacier on the west AP, where average flow speeds are approximately 1 km/yr, and comparable to the largest glaciers on the AP such as Fleming Glacier which flows at 2.5 km/yr22.

Our 30-year time series of ice velocity (Figs. 2, 3a, sample areas defined Figs. 1b, 3b) on Cadman Glacier shows substantial changes in flow speed throughout the study period. The grounded glacier accelerated from 1.67 ± 0.27 km/yr in February 1991 to a peak of 3.20 ± 0.05 km/yr in May 2022, a speed increase of 91 ± 17 % (1.53 ± 0.29 km/yr). The floating ice shelf experienced a greater acceleration, from 1.33 ± 0.24 km/yr in 1991 to 3.87 ± 0.05 km/yr in May 2022, an acceleration of 191 ± 19 % (2.54 ± 0.25 km/yr). The majority of this speed increase (1.19 km/yr on the grounded ice and 2.01 km/yr on the shelf) occurred in the period from December 2014 to June 2022 (Fig. 1d). This included a major acceleration event from November 2018 to December 2019 where flow speeds increased by 44 ± 3 % (0.96 ± 0.6 km/yr) on the grounded glacier and 94 ± 4% (1.47 ± 0.6 km/yr) on the floating ice shelf. In contrast, the neighbouring glaciers in Beascochea Bay, Lever and Funk Glaciers, have not accelerated over the same time period (2014 to 2021). Instead, they have maintained a constant ice speed with small seasonal fluctuations. Lever Glacier flowed at a mean speed 0.84 ± 0.04 km/yr and Funk at 1.22 ± 0.03 km/yr (Fig. S2), suggesting that the drivers of Cadman Glacier’s 2018/19 acceleration are localised and specific to this glacier.

In addition to this overall acceleration, the spatial pattern of ice velocity on Cadman Glacier changed during the study period. In 1991, there was a local speed minimum of 0.96 km/yr in the lower glacier section 700 m inland of the grounding line location (Fig. 3a). From 2010 to 2022, this local speed minimum is absent, with ice speed constant throughout this section instead. By 2010, another local speed minimum of 1.91 km/yr is observed 2.5 km upstream of the 1999–2003 grounding line. Although it was not possible to measure ice speed on this section of the glacier in 1991, this local speed minimum is also observed in December 2014 at the start of our Sentinel-1 velocity time-series. This ice speed minimum persists until the glacier’s rapid acceleration in 2018/2019, when by mid-2019 it has transitioned from a distinct local minimum to a zone where ice speed transitions to flow at a constant 3.5 km/yr, which we interpret as being indicative of the downstream floating ice shelf. In March 2021, the Cadman Ice Shelf completely collapsed, with the glacier calving front retreating to a position 2.4 km inland of the 1999–2003 grounding line. Over the 30-year study period, the observed change in ice dynamics indicates that Cadman Glacier’s grounding zone configuration has undergone substantial change. Specifically, the loss of the most seaward local ice speed minima between 1991 to 2010 and 2018 to 2019 (Fig. 3a) suggests that there has been a loss of buttressing resistive force which would have been local to both these speed features.

Surface elevation change
Surface elevation measurements from CryoSat-2 were used to assess thinning on the Cadman Ice Shelf from 21st November 2010 to 10th March 2019. Between November 2010 and March 2019 prior to the ice speed acceleration, we observed a total of −16.2 ± 4.3 m surface elevation change in a sample region covering the floating ice shelf (Figs. 1b, 4a), an average annual rate of −1.95 ± 0.53 m/yr (Fig. 4b). Assuming the ice shelf is in hydrostatic equilibrium, has an average ice density of 917 kg/m3, and that mass losses are entirely driven by basal melting rather than changes in the firn layer, implies a total ice shelf thinning rate of 18.5 ± 5.0 m/yr46,47. This result compares well with a previous study that estimated surface lowering at a rate of 2.23 m/yr between 2001 and 2010 using laser altimetry and stereo image DEMs, albeit this study included all floating and grounded ice below 1 km elevation48. Nonetheless, these results show a consistent trend in long-term thinning of Cadman Glacier’s floating shelf from 2001 until its collapse in 2021.

Comparison of two co-registered, but temporally separated DEMs (Cook AP DEM 2000–2009 and REMA DEM 2015) shows the spatial pattern of surface elevation change on Cadman Glacier prior to the 2018/19 ice speed acceleration event. The elevation profile (Fig. 3b) shows that a surface bump is visible in the Cook DEM 500 m inland of the grounding line location. This bump (referred to as point (iii) in Fig. 3b, henceforth 1990s bump), located 2 km inland of the ASAID grounding line, is coincident with a local ice speed minimum and region of compressive stress in the 1991 ice speed profile. This can be interpreted as either indicative of the slope boundary layer, where the transition from vertical shear stress dominated flow to extensional stress dominated flow creates the distinctive grounding zone bump through viscous beam behaviour, or more simply grounding on a bed slope ridge49. By 2010, the local ice speed minimum was replaced by a region of constant ice speed and the 2015 REMA DEM shows that the point (iii) surface bump has disappeared. Additionally, taking the break in surface slope in the REMA DEM as an approximation for the location of Cadman Glacier’s grounding line in 2015, we find that in 2015 the grounding line is 1.25 km inland of the ASAID 1999-2003 position (Fig. 3b). These changes between the Cook DEM (2000-2009) and the REMA DEM (2015) further support our interpretation from ice velocity measurements that the grounding line of the glacier retreated between 1991 and 2010.

Further inland, the DEMs show another surface slope inflection point 2.5 km up-glacier of the ASAID grounding line, called point (ii) (Fig. 3b, henceforth 2015 bump). This inflection point deepens significantly in the 6-year period between the two DEMs (approximately 2009 to 2015). Again, we interpret this to be a feature of the slope boundary layer or a surface manifestation of a bed ridge. The presence of the point (ii) surface bump in 2015 following the point (iii) bump present in the 1990s, and given that the 2015 bump is several coupling lengths up-glacier of the 1999–2003 grounding line position, we favour the interpretation that this feature is the surface manifestation of a bed ridge. This corresponds to a bed ridge, point (i), at the same location in the modelled bed topography (Fig. 3a) and is supported by our ice velocity measurements which show an ice speed minimum immediately inland of this surface bump from 2010 to 2018.

Since Cadman Glacier’s rapid acceleration in 2018/19, there have also been substantial changes in the surface elevation on the inland grounded section of the glacier. Repeat track laser altimetry data from ICESat-2 between 21st April 2019 and 17th August 2021, acquired in the period immediately following the 2018/19 ice speed acceleration event show a maximum surface elevation change rate of −26.4 ± 5.1 m/yr at the base of the Cadman Icefall (Fig. 4d), and a mean rate of −20.1 ± 2.6 m/yr across the 2.2 km glacier width. The mean ice thickness of this transect is 313 m, which shows that the mean percentage thinning rate is 6.4%/yr, with the maximum percentage thickness change rate of 8.6%/yr observed in the glacier margins where ice is thinnest.

To determine the spatial extent of glacier thinning, we complemented our ICESat-2 measurements with estimates of surface elevation change derived from repeat 2 meter resolution REMA Digital Elevation Models that were acquired in November 2016 and February 202050. These measurements show that the main trunk of the glacier thinned by a maximum of 67.1 ± 1.5 m during this 3.3-year period, with the majority of ice volume loss occurring within a region extending 8 km upstream of the grounding line (Fig. 4a). Over the lower glacier grounded ice sampling region (Fig. 1b) the mean rate of surface elevation change was −12.8 ± 0.5 m/yr, with a maximum rate of −17.1 ± 0.5 m/yr. These data also support our measurements of thinning of Cadman Ice Shelf, where between November 2016 and February 2020 we measure a surface elevation change of −1.61 ± 0.46 m/yr, in agreement with the rate of −1.95 ± 0.53 m/yr from 2010 to 2019 measured with CryoSat-2.

Together, these elevation change results show that there has been an exceptionally high rate of ice thinning across Cadman Glacier’s drainage basin, in stark contrast to the relative stability and absence of any observed change in ice thickness on neighbouring Funk and Lever Glaciers which also did not change speed during this period (Fig. 4a, d, Fig. S2a, b). The coincident timing of these thinning observations with the ice speed change measurements demonstrates that this is a dynamically induced thinning caused by the 2018/19 ice acceleration, initiated by the loss of basal traction over the 2015 bed ridge.

Ice discharge
We used the ice speed and surface elevation change observations to calculate change in ice discharge from Cadman Glacier over the 30-year study period. Our results show that overall, ice discharge increased significantly by 49.6%, from 1.34 ± 0.11 Gt/yr in 1991 to 2.00 ± 0.16 Gt/yr in December 2021 (Fig. 5). Following an initial increase from 1991 to 2010 (1.57 ± 0.13 Gt/yr), ice discharge was stable between 2010 and December 2014 as a consequence of there being no speed change and no evidence of ice thickness increase in this period. By January to March 2015, mean ice discharge remained at 1.57 ± 0.11 Gt/yr, despite being a period where seasonal speed fluctuations would be at their maximum. Throughout 2015 ice discharge increased until it reached a mean of 1.85 ± 0.13 Gt/yr between 2016 and 2018 with no positive or negative trend. Ice discharge increased rapidly by 28.1% from December 2019 as ice speeds accelerated, reaching a maximum discharge rate of 2.37 ± 0.17 Gt/yr by January 2020. While ice speed remained high throughout 2020 to 2022, ice discharge decreased over this period due to the extremely high rate of surface elevation change. By December 2021 ice discharge was 2.00 ± 0.16 Gt/yr, with surface lowering (measured from ICESat-2) accounting for a reduction in the discharge rate of 0.25 ± 0.23 Gt/yr (Fig. 5). The importance of accounting for ice surface lowering term is demonstrated by fitting a linear trend to the post acceleration time series of ice discharge for two years from 1st January 2020 to 31st December 2021. When ice thickness change is accounted for, our results show a trend in the ice discharge of −0.09 ± 0.02 Gt/yr2 as the glacier evolves toward a state of dynamic balance, in contrast to calculations of discharge without the surface lowering term applied (Fig. 5).

Ocean Temperature 1993–2021
We use global ocean reanalysis output and direct oceanographic station measurements to investigate the role of thermal forcing from warm ocean water at Cadman Glacier. We extracted ocean temperature data from a region up to 80 km offshore from Cadman Glacier (Fig. 1a), to produce a time-series of ocean potential temperature anomaly from the reanalysis (Fig. 2). In this time-series, the top 50 m of the water column exhibits seasonal ocean temperature fluctuations driven by atmospheric conditions. In contrast, longer term fluctuations in temperature are apparent below this depth, with positive potential temperature anomalies of up to 2 °C present between 1993–95, 2004–2009 and 2016–2021. This follows an approximately cyclical pattern on decadal timescales, with the strongest anomalies in 1993–95 and 2016–2021. We see significant warm temperature anomalies from 50 to 500 m in the water column from 2017 onwards, which coincide with the period of ice shelf retreat and ice speed acceleration observed on Cadman Glacier. In-situ observations from the Palmer Long-Term Ecological Research (PAL-LTER) programme show that the highest January temperatures for the 27-year period between 1993 and 2020 occurred in 2019 for the Anvers Hugo Trough (AHT) and Palmer Deep (PD). The observations also show a significant positive salinity anomaly in the AHT in 2019 (Fig. 6c, d), which indicates a shoaling of the pycnocline in January 2019, where warm CDW was able to flow over a topographic boundary into PD and likely further onwards to Cadman Glacier. Without direct measurements in Beascochea Bay, it is difficult from these data alone to ascertain unambiguously the duration of this shoaling or the temperature anomaly of the water that impinged ultimately on Cadman Glacier. However, they demonstrate that deep water conditions in the shelf region upstream of Cadman Glacier exhibited variability with the timing and magnitude consistent with strong oceanographic forcing of the glacier system.

The pathway for warm ocean water to circulate across the west AP shelf and reach Cadman Glacier is complex. Bathymetric data show that Beascochea Bay is up to 760 m deep, with a historic grounding zone wedge with a minimum depth of 360 m crossing the bay 7 km seaward of the Cadman Fjord entrance (Fig. 6a). From the bay to the continental shelf, there is a bathymetric high with a minimum depth of 300–350 m separating the deep water of Beascochea Bay from the west AP shelf. There is only a minor bathymetric sill at the entrance to Cadman Fjord, with a minimum depth of 460 m, therefore, any warm water masses entering Beascochea Bay can continue relatively unimpeded to access Cadman Glacier’s shelf cavity and grounding zone. In contrast, neighbouring Funk and Lever Glaciers are shielded from deeper water by seaward bathymetric sills at depths of 230 m and 200 m respectively, meaning they are substantially more protected from incursions of warm ocean water in comparison to Cadman Glacier.

Methods
Ice velocity and discharge
We measured ice velocity on Cadman Glacier (Fig. 1a) by using the full time-series of Sentinel-1a and -1b synthetic aperture radar (SAR) Interferometric Wide (IW) swath mode single look complex (SLC) image pairs acquired every 6 to 12-days between December 2014 and July 2022. The images were co-registered using the REMA 200 m Antarctic digital elevation model (DEM)64 and precise orbit determination (POD) data from the European Space Agency. We tracked the displacement of surface features using the frequency domain intensity cross-correlation technique65,66 and converted this to ice velocity using the REMA 200 m DEM assuming surface parallel flow. The results were posted on a 100 m grid, and a spatially variable ice velocity error estimate was calculated by multiplying the signal to noise ratio of the cross-correlation with the ice speed66,67.

To assess change in Cadman Glacier’s ice speed (Fig. 1b), we extracted a time series of velocity measurements and associated errors along the glacier’s central flow line and averaged in the ice shelf and lower glacier grounded ice sample boxes defined in Fig. 1b. This data is post-processed using SNR filtering at a threshold of 5.868 and a Bayesian recursive smoother22 to compute the maximum likelihood estimate for ice speed and associated uncertainty at daily time steps for the profile. In addition, we processed historical ice speed measurements using the same feature tracking technique applied to a pair of Landsat-5 images from 9th February 1991 and 25th February 1991, and a triplet of TerraSAR-X images from 10th November 2010 to 2nd December 2010. Optical feature tracking of Landsat images was performed with the ImGRAFT toolbox69.

We used the ice velocity measurements to calculate ice discharge on Cadman Glacier by integrating ice velocity across a flux gate defined 2.2 km inland from the post-collapse calving front position in January 2022 (Fig. 1b where coverage across all velocity datasets is good and where the ice surface elevation change can be accurately measured using repeat tracks of the ICESat-2 laser altimeter10,70. Flow directions were taken from the MEaSUREs ice speed mosaic dataset71 and ice thickness was calculated from the difference between the bed elevation72 and the ice surface elevation using the REMA 100 m DEM64. We apply a surface height correction to the ice thickness data using the mean thinning rate calculated across the flux gate between 21st April 2019 and 17th August 2021, to account for the observed surface lowering during the study period (Fig. 5). A time varying firn air content correction is also applied to the ice thickness data using the IMAU firn densification model73.

Calving front location
We measured the change in calving front location on Cadman Glacier by manually digitizing the calving front position in Landsat-5, Landsat-7, Landsat-8 and Sentinel-2 optical imagery, and Sentinel-1 SAR imagery using the GEEDiT digitization tool74, where the availability of optical images in limited by cloud cover and a lack of daylight in the Antarctic winter. This produced a time-series of calving front positions at variable intervals between 1984 and 2014, and a continuous time-series from December 2014 to July 2022 at a maximum of 12-day intervals during the period of operational Sentinel-1 SAR coverage. The uncertainty of these digitisations is defined to be the resolution of the Sentinel-1 Ground Range Detected (GRD) product (±10 m). Change in calving front position was calculated using a curvilinear box with a 1 km width using the MaQiT tool74.

Grounding line position
Measuring the grounding line position of Cadman Glacier in the period immediately prior to its acceleration and retreat is difficult because Sentinel-1 interferometric synthetic aperture radar (InSAR) is not coherent on the glacier, for example an automated InSAR processing and grounding line delineation campaign for 2018 was not able to resolve any grounding lines in this location75. Other remote sensing methods which rely on detecting tidal motion such as repeat track laser altimetry76,77 and differential range offset tracking78 do not adequately resolve Cadman Glacier. For example ICESat-2 tracks are widely spaced at this latitude and transverse to Cadman Glacier’s flow direction, so do not cover the grounding zone. To locate the grounding line position in 2015, we measure the break in surface slope of the REMA 100 m Antarctic Peninsula DEM64 between the ice shelf and grounded glacier79. We resample the REMA DEM to 10 m resolution using a cubic spline and delineate the grounding line at the most seaward local peak in surface slope gradient along the glacier’s central flow line and extend this perpendicular to flow to the shear margins.

Surface elevation change
We used altimetry data from the ICESat-2 laser altimeter, CryoSat-2 synthetic aperture radar interferometric (SARIn) altimeter, and stereo image DEMs to evaluate change in surface height and hence thinning of Cadman Ice Shelf and grounded inland ice. For ICESat-2, we use the ATL06 grounded ice elevation product to measure thinning of grounded ice from 2018 to 202280 using four repeats of track 361 from 21st April 2019, 19th Jan 2020, 17th April 2021, 17th August 2021. We differenced repeat measurements along the satellite track to calculate total surface elevation change and used a linear fit with all tracks to calculate thinning rates.

For CryoSat-2, we use Cryo-TEMPO radar altimetry data retrieved from the European Space Agency CryoSat-2 Science Server portal to evaluate the rate of surface lowering on Cadman Glacier’s floating ice shelf81,82. There is insufficient swath-mode data to form a time-series, and so we use the Cryo-TEMPO Baseline B point of closest approach land ice (POCA_LI) product, selecting retrievals which are inside a box which covers the floating ice shelf (Figs. 1b, 4a). The altimetry data points are not consistently distributed within our sampling area between years, as the precessing orbit of CryoSat-2 means that exact repeat tracks only occur every 369-days, and POCA locations are rarely the same. To compute elevation changes from these non co-located measurements we correct for spatial variations in elevation of the glacier by referencing the CryoSat-2 data to the REMA DEM and measuring the difference between the CryoSat-2 elevation and the REMA elevation. We then calculated a surface elevation change rate for our whole sample area using a linear fit to these topographically corrected elevation differences.

We also performed a direct comparison between digital elevation models from different time periods to study changing surface elevation, using the Cook et al. (2012) 100 m Antarctic Peninsula DEM (Cook DEM)83 and the REMA 100 m Antarctic Peninsula DEM64. The Cook DEM was produced as a corrected version of the ASTER Global Digital Elevation Map (A-GDEM) optimised for the AP. The A-GDEM is produced from ASTER stereo image tiles spanning 2000–2009. The REMA DEM is mosaiced from a large number of DEMs derived from several stereo imaging satellites with a mean acquisition date of 9th May 2015 and a standard deviation of 432 days. We co-registered the Cook and REMA DEMs using the demcoreg.py package84,85.

We used repeat 2 m resolution version 4.1 Reference Elevation Model of Antarctica (REMA) strips and the version 2 REMA composite Mosaic50,64,86 to calculate ice thinning across the Cadman Glacier between November 2016 and February 2020. Specifically, the stereo imagery used to determine the DEM strips were acquired on 10/11/2016 and 25/02/2020. These were registered to the Mosaic over relatively stable surfaces, using the demcoreg.py package84,85, with masks applied to remove surface water, clouds, blunders (using the binary mask provided with the REMA product); rapidly (>75 m/yr) flowing ice71, open ocean and floating ice87. By co-registering both DEMs to reference elevations over slow-flowing ice, and then differencing the two DEMs, we were thus able to estimate the anomalous dynamical thinning over the Cadman glacier relative to any background climatological signal.

Ocean temperature and bathymetry
We used the GLORYS12V1 European Commission (EC) Copernicus Marine Service global ocean eddy-resolving reanalysis model (1/12° horizontal resolution, 50 vertical levels), to evaluate ocean temperature variability in the Bellingshausen Sea, between 1993 and 202088. We define a 0.75° x 1.5° (approx. 70 km x 85 km) region of interest to evaluate changes in the properties of water masses which may access Cadman Glacier (Fig. 1a) and extract monthly potential temperature values at all model levels. To evaluate the temporal pattern of temperature anomalies, we calculate a monthly climatology for the region by averaging potential temperature for the period January 1993 to January 2010 at every grid point, and we then calculated a temperature anomaly by subtracting the climatology from potential temperature time-series. While ice thickness at the grounding line is highly uncertain on the AP due to the complex terrain and a paucity of bed elevation measurements, we aim to assess the temperature variability of ocean water in contact with Cadman Glacier’s shelf and grounding zone.

We complement this ocean potential temperature reanalysis data by using direct ocean temperature and salinity measurements from CTD’s from the Palmer Long-Term Ecological Research programme (LTER) campaign cruises on the west AP shelf. This dataset provides austral summer measurements of ocean physical properties through the water column at a regular grid of hydrographic stations on the west AP shelf and in the shelf-adjacent ocean during cruises typically conducted in January and February cruises from 1993 to 2021. Stations sampled for this dataset are not fully consistent between years, therefore, given the highly localised nature of west AP shelf conditions we ensure fair comparison between years by restricting the dataset to 16 locations sampled for at least 20 of the 28 years. There are few direct oceanographic measurements in the LTER dataset close to Cadman Glacier, the nearest points that meet our sampling threshold are the Anvers-Hugo Trough (AHT) and Palmer Deep (PD), however, these locations are suitable for studying occurrences of warm water intrusion onto the shelf and onwards to Cadman Glacier. The flow of warm CDW onto the shelf is bathymetrically constrained and observations have shown evidence of preferential cross shelf transport in large bathymetric troughs30,89. Consequently, the AHT is an effective pathway for shelf-ocean exchange and likely for delivering heat to Cadman Glacier. The bathymetry of the west AP continental shelf is a product of past glaciations, with glacially carved throughs and over-deepened basins separated by shallow moraine sills90–93. These features control the flow of deep, warm ocean water toward the coast and are important for the modification of water masses through mixing94. For this reason, comparing temperatures between AHT and PD is useful to inform on the shoreward propagation of warm water masses, however, this is limited by a lack of measurements in the immediate vicinity of Beascochea Bay and Cadman Glacier.

The GLORYS reanalysis product assimilates the Coriolis Ocean Dataset for Reanalysis (CORA), which includes the World Ocean Database and at least part of the Palmer LTER dataset, so these data are not independent, and thus a formal validation is challenging. However, we compared the GLORYS reanalysis potential temperature data and measured potential temperature from the Palmer LTER profiles to confirm the reanalysis output reflects the variability of the in situ data (Fig. S3). We spatially resampled the monthly reanalysis data using linear interpolation onto the full set of sample locations for every LTER CTD in our dataset producing 1.3 × 106 sample points of measured and reanalysis potential temperature, comparing all points, we find a Pearson’s correlation coefficient of 0.823 (Fig. S3). Our results show that the correlation between the two datasets is high and significant (p < 0.05) throughout the water column, with the magnitude varying with depth. We find that correlation is weakest in the 0–100 m and 100–200 m ranges, 0.672 and 0.593 respectively, with a positive bias in the reanalysis potential temperature in the 100–200 m interval, below the correlation improves significantly to 0.930 for depths below 200 m and 0.968 below 300 m (Fig. S3a–c); all correlation coefficients are significant (p < 0.05). Lower correlations higher in the water column may be expected because the reanalysis output is a monthly mean, while LTER CTDs are snapshots in time, so it is likely that higher frequency variability in surface waters is not well captured in the reanalysis product. Below 200 m, however, the reanalysis and measurements agree well; these depths are where the warmest waters are found and can cause enhanced glacier melt near the grounding line, therefore, we consider the reanalysis output of sufficient representativeness and appropriate for our investigation.

We extracted sea floor topography in the Hugo-Anvers Trough and Cadman Glacier fjord using direct bathymetric measurements from British Antarctic Survey vessels and data from the International Bathymetric Chart of the Southern Ocean (IBCSO v2)95,96, supplemented by a previous study which mapped the geomorphology of Beascochea Bay from data collected in 200790.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Wallis et al, Nature Comms\nThe authors marshal an impressive array of data sets to describe the recent evolution of Cadman Glacier, a major outlet of the northern section of the western side of the Antarctic Peninsula, The study focusses on the rapid thinning and acceleration that has occurred since about 2015, culminating in the loss of a floating ice tongue / ice shelf in 2021. The authors attribute the loss to a strong increase in basal melting beginning in 2017 or so as evidenced by detailed oceanographic data collected under the Palmer LTER program.\nThe study is well done and could be published after minor revisions and addressing a few notes and questions in the review. I will say that similar work has been reported numerous times for Greenland outlets by a host of researchers (Howat, Rignot, Joughin, Hamilton, Catania, Enderlin, Moon, Willis….) and this study details a very similar course of evolution for a fjord-bounded outlet glacier. However, this study is important in that the data sets are fairly comprehensive, span a significant period of time, and are temporally and spatially dense during the period of rapid evolution. Moreover, the breakup style of the relatively extensive ice tongue (relative to most Greenland outlets) appears to be unique - -and more might be made of this fact: while hydrofracture processes and damage-related processes have dominated most literature on ice shelf break-up, the observation of a breakup with a significant component from basal melting (augmented by increased damage) may be new (not sure).\nSeveral detailed comments are set as notes within the submitted .pdf file\nThere are valuable Landsat 8 images of the Cadman showing details of the loss of the ice shelf in Dec-Jan 2020 and 2021 that might illustrate the break-up mechanism, and could be added to your S1 figure. This style of break-up could be unique in the area (possible exception of the Wordie) in that melt ponding or hydrofracture does not appear to be involved. This strengthens your case that the ice shelf broke apart by a combination of increased damage due to acceleration and loss of contact with bedrock ridges near the evolving grounding line. Check 17 Jan 2020 and 26 Jan 2021 (and there are likely more) – to me it looks as though sub-ice shelf channels weakened the shelf by creating ~polygonal blocks that then concentrated strain and led to rapid calving along the channel lines. More Could Be Made Of This in your paper as an example of a style of ice shelf break-up that has not been widely discussed to date. Make note that your study does not show a presence of surface ponding (if that’s the case; I think it is), a harbinger of hydrofracturing processes on a shelf.", "reviewer_2": "This paper investigates Cadman Glacier in the west Antarctic Peninsula (AP), focusing on its unique characteristics and recent changes. Unlike other glaciers in the region, Cadman Glacier velocity increased between 2018 and 2019 and experienced persistent thinning of its ice shelf. This thinning eventually led to retreat and rapid acceleration of the glacier. Warm ocean temperatures and bed geometry played significant roles in these changes. The study emphasizes the vulnerability of glaciers in the region to warm ocean waters and the need for further research to understand their future evolution.\nThe paper provides a comprehensive analysis of Cadman Glacier in the west Antarctic Peninsula, focusing on its unique characteristics and recent changes. The authors effectively present the background information and clearly articulate the research objectives. The methodology employed in the study, including the use of CryoSat-2 data and previous studies, appears to be partially appropriate for the research questions at hand.\nThe analysis of surface elevation, elevation change, and ice velocity provides valuable insights into the glacier's behavior, but grounding zone measurements are missing which could hinder the interpretation of the results. Additionally, I was surprised the authors did not provide both an eulerian and lagrangian melt rate analysis.\nOverall, the findings of the study are significant and contribute to the understanding of glacier dynamics in the region. The observed persistent thinning of Cadman Glacier's ice shelf and its subsequent retreat and acceleration are compelling evidence of the glacier's vulnerability to warm ocean waters.\nThe paper effectively discusses the role of bed geometry and bathymetric features in influencing the glacier's response to environmental forcing. The inclusion of comparisons with neighboring glaciers adds depth to the analysis and supports the conclusions drawn. The implications of the study are well-discussed, particularly in terms of the impact on ocean circulation, productivity, and the ecosystem. The mention of potential regional-scale consequences further strengthens the significance of the research.\nThe paper is well-written, with a clear and concise presentation of the research findings. The academic tone and language throughout the manuscript contribute to its overall quality.\nHowever, one area that requires improvement is the discussion of limitations and future research directions. While the authors briefly mention the need for further studies and fieldwork measurements, more specific recommendations or suggestions for future investigations would enhance the paper's completeness specifically focused on the need of frequent grounding line measurements and melt rate measurements see (Shean et al 2019 Milillo et al 2022)\nAdditionally, it would be beneficial to provide a clearer connection between the findings of this study and their broader implications for climate change and sea-level rise. This would help contextualize the significance of the research within the larger scientific discourse.\nOverall, this paper presents valuable research on Cadman Glacier and its response to environmental changes. With minor revisions and additions to the discussion section, this study has the potential to make a significant contribution to the field of glaciology and climate science.\nFew more comments follows:\nUnless a reference is missing, lines 73-75\nseem a bit out of context for the introduction section and could be more appropriate in the conclusions\n79-81 when referencing the 4 areas of the Cadman glacier a reference to a figure should guide the reader and enable the spatial identification of these areas on a map.\nFig1 missing reference to the DEM used for shading.\n152-153 Which grounding line has been used to measure these proportions ?\nShean, D. E., Joughin, I. R., Dutrieux, P., Smith, B. E., & Berthier, E. (2019). Ice shelf basal melt rates from a high-resolution digital elevation model (DEM) record for Pine Island Glacier, Antarctica. The Cryosphere, 13(10), 2633-2656.\nMilillo, P., Rignot, E., Rizzoli, P., Scheuchl, B., Mouginot, J., Bueso-Bello, J. L., ... & Dini, L. (2022). Rapid glacier retreat rates observed in West Antarctica. Nature Geoscience, 15(1), 48-53.", "reviewer_3": "This manuscript presents a detailed set of observations of Cadman Glacier, a marine-terminating glacier on the west coast of the Antarctic Peninsular, spanning a period of rapid change in the glacier’s ice shelf. The observations, which include newly derived data on ice thickness, velocity and terminus position, clearly document the thinning, ungrounding, acceleration, retreat and ultimate collapse of the ~5 km ice shelf over the period ~2010-2022. Through comparison with existing hydrographic and bathymetric data, the authors present a convincing case that this reflects the influence of the ocean, with gradual weakening of the shelf due to long term thinning followed by a final collapse triggered by the presence of particularly warm ocean waters on the shelf. Altogether, this makes a nice case study of a moderately sized marine terminating glacier experiencing ice shelf thinning and collapse in response to warming ocean waters. The detailed observations document this process well, and I believe the methodology, analysis and interpretation are sound.\nWhile I am confident that the work is worthy of publication in a relevant journal, I am less confident that it represents the kind of major, high-impact advance that would typically be associated with Nature Communications. The finding that the retreat of glaciers on the western Antarctic Peninsular is predominantly ocean-driven was presented on a much larger scale by Cook et al (2016) – the current manuscript presents a detailed case study of one such example of this, but does not to me really advance our broader understanding of this topic beyond this particular glacier (which is not in itself of particular importance as a source of sea level rise etc). The sequence of processes observed - ice shelf thinning, acceleration and break up, grounding line retreat, dynamic thinning upstream – are quite familiar from the loss of other ice shelves and tongues in Antarctica and Greenland (e.g. Motyka et al (2011), to give just one example). The attribution to oceanic forcing supports the findings of Cook et al (2016), but the study lacks the kind of novel observations (such as in situ hydrographic measurements close to the glacier, or high resolution / spatially resolved measurements of shelf thinning) that might really advance our understanding of the underlying processes.\nPutting this aspect aside, I do not have any major concerns with the methods, analysis or interpretation as currently presented. I found the manuscript to be largely well written, if perhaps overly descriptive at times – it might be possible to make better use of the figures and reduce the amount of descriptive text a little. A have a few more minor comments, outlined below.\nL28. The ‘cryosphere’ is generally used to refer to ice and snow on a global scale – better to refer to the glaciers of this region or similar\nL37. Ambiguous phrasing – suggest something like ‘increased by 400 % between the periods 1992-1997 and 2007-2012’\nL161. A figure reference would be useful\nL175-185. It would be useful to add a reminder of the years of the ASAID grounding line and DEMs\nL244-5. It’s not easy to discern the grey dots in the figure, perhaps a different colour would work better\nL264-275. It wasn’t clear to me why the addition of surface lowering merited such attention – is this making a methodological point (that other researchers need to make sure they factor this in), or that surface lowering has an important impact on the dynamics of the glacier?\nL303-306. Yes these glaciers have shallower sills, but its notable in Figures 6c-d that there is still substantial warming at depths shallower than 200-230 m, so they should still have experienced significantly warmer temperatures at this time.\nL311-312. What year is the ocean temperature anomaly shown?\nL326-328. It would be helpful to include a figure reference\nL328-329. What is the postulated driver of this long term thinning? Ocean warming?\nL371-373. Again it would be helpful to include a figure reference\nReferences\nCook, A., Holland, P., Meredith, M., Murray, T., Luckman, A., & Vaughan, D. (2016). Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353(6296), 283-286.\nMotyka, R. J., Truffer, M., Fahnestock, M., Mortensen, J., Rysgaard, S., & Howat, I. (2011). Submarine melting of the 1985 Jakobshavn Isbrae floating tongue and the triggering of the current retreat. Journal of Geophysical Research-Earth Surface, 116, Article F01007. https://doi.org/10.1029/2009jf001632"}, "author_response": "Response to reviewers: Ocean warming drives rapid dynamic activation of\na marine-terminating glacier on the west Antarctic Peninsula\nRef: NCOMMS-23-19496\nReviewer 1:\nRef. Original Comment Revised Response\nLine Line\n1.1 The authors marshal an impressive We thank the reviewer for their comments\narray of data sets to describe the recent regarding the quality and importance of the\nevolution of Cadman Glacier, a major study, and for their insightful, informative and\noutlet of the northern section of the productive comments as a whole. In particular\nwestern side of the Antarctic we thank the reviewer for taking the time to\nPeninsula, The study focusses on the independently study satellite images of Cadman\nrapid thinning and acceleration that Glacier and provide helpful suggestions based on\nhas occurred since about 2015, these.\nculminating in the loss of a floating\nice tongue / ice shelf in 2021. The We have replied to each comment in turn and\nauthors attribute the loss to a strong made changes to the manuscript to address them.\nincrease in basal melting beginning in A major suggestion by the reviewer was to\n2017 or so as evidenced by detailed include more optical satellite images of Cadman\noceanographic data collected under the Glacier in figure S1. We have done this and\nPalmer LTER program. expanded the discussion of the failure\nmechanism for the Cadman Ice Shelf. We agree\nThe study is well done and could be with the reviewer that the importance of this\npublished after minor revisions and wasn’t highlighted clearly enough in the\naddressing a few notes and questions previous version.\nin the review. I will say that similar\nwork has been reported numerous We believe that changes and clarifications\ntimes for Greenland outlets by a host presented in the revised manuscript have\nof researchers (Howat, Rignot, addressed the reviewer’s concerns and have\nJoughin, Hamilton, Catania, Enderlin, enriched the manuscript. We thank the reviewer\nMoon, Willis….) and this study details for their time and effort.\na very similar course of evolution for a\nfjord-bounded outlet glacier.\nHowever, this study is important in\nthat the data sets are fairly\ncomprehensive, span a significant\nperiod of time, and are temporally and\nspatially dense during the period of\nrapid evolution. Moreover, the\nbreakup style of the relatively\nextensive ice tongue (relative to most\nGreenland outlets) appears to be\nunique - -and more might be made of\nthis fact: while hydrofracture\nprocesses and damage-related\nprocesses have dominated most\nliterature on ice shelf break-up, the\nobservation of a breakup with a\nsignificant component from basal\nmelting (augmented by increased\ndamage) may be new (not sure).\nSeveral detailed comments are set as\nnotes within the submitted .pdf file\n1.2 There are valuable Landsat 8 images 120-123 We thank the reviewer for this helpful\nof the Cadman showing details of the 128-130 suggestion, which we agree would add a further\nloss of the ice shelf in Dec-Jan 2020 361-365 interesting perspective to the manuscript.\nand 2021 that might illustrate the\nbreak-up mechanism, and could be\nadded to your S1 figure. This style of Done – we have expanded Figure S1 to include\nbreak-up could be unique in the area the 2020/01/17 Landsat-8 image that the\n(possible exception of the Wordie) in reviewer suggested and an additional image of\nthat melt ponding or hydrofracture the Cadman fjord after the ice shelf collapse.\ndoes not appear to be involved. This\nstrengthens your case that the ice shelf We have changed the description of damage on\nbroke apart by a combination of the shelf to describe this:\nincreased damage due to acceleration\nand loss of contact with bedrock 120: ‘Although advanced, during this period the\nridges near the evolving grounding seaward 3 km of glacier tongue shows a\nline. Check 17 Jan 2020 and 26 Jan substantial increase in damage and crevassing\n2021 (and there are likely more) – to on the ice shelf surface and a widening of the\nme it looks as though sub-ice shelf shear margins (Fig. S1), suggesting that the\nchannels weakened the shelf by structural integrity of the ice was decreasing.\ncreating ~polygonal blocks that then Notably in the 17th January 2020 image (Fig.\nconcentrated strain and led to rapid S1d) there is a distinctive along-flow damage\ncalving along the channel lines. More feature on the central ice shelf, which may\nCould Be Made Of This in your paper indicate channelised sub-shelf melting.’\nas an example of a style of ice shelf\nbreak-up that has not been widely And we have expanded the discussion of the lack\ndiscussed to date. Make note that your of surface melt ponds throughout the manuscript:\nstudy does not show a presence of\nsurface ponding (if that’s the case; I 128: ‘Notably, although summer melt is\nthink it is), a harbinger of widespread in the region, we observe no surface\nhydrofracturing processes on a shelf. meltwater ponds visible on the ice surface in any\noptical satellite image for our period of study\n(1984 -2021), which we attribute to the north-\nwest AP’s thick firn layer and high firn air\ncontent.’\n361: ‘The Cadman Ice Shelf fully collapsed in\nMarch 2021 due to this thinning, possibly\nintensified through channelised melting, and the\nloss of pinning on the 2015 ridge. The absence of\nsurface melt ponding on the Cadman Ice Shelf\nprior to its collapse demonstrates that surface\nponding is not a necessary precondition for ice\nshelf collapse, which can occur through ocean-\ndriven thinning and unpinning alone.’\n1.3 15 please re-write this sentence! These 14 Done – changed to:\ntypes of statements are meaningless,\nand a bit disrespectful. Don't we have 14: ‘Ice dynamic change is the primary cause of\na very good idea of the processes mass loss from the Antarctic Ice Sheet, thus it is\ninvolved? Don't you cite that work as important to understand the processes driving\nthe basis for your work? ice-ocean interactions and the timescale on\nwhich major change can occur.’\nSomething more positive perhaps -\ninstead of the 'however' phrase, maybe Apologies if there was a perception of disrespect\n' and detailed studies of areas with which was certainly not our intention.\nsignificant ongoing change inform and\nimprove models for better forecasting.'\n1.4 16 remove 'on' -- 'fronting' or 'at the 16 Done – changed to ‘fronting’\nterminus of'\n1.5 47 remove 'from the deeper layers of' 47 Done – changed to:\na bit confusing... the continental shelf 47: ‘…transported by the Antarctic Circumpolar\nintrusion of warm watter comes from Current and intrudes onto the continental shelf\nteh -upper- layer of the ACC, but its from the open ocean’\ndeeper than the cooler Bransfield\nStrait water (I believe so).\n1.6 47 insert: 47 Done – changed to ‘continental shelf’, we\nAntarctic continental shelf believe Antarctic is clear from context.\n1.7 54 change to -- 54 Done – changed to:\nThe western flank of the AP has very\nhigh accumulation, due to 54: ‘The west AP has high accumulation, due to\norpgraphically driven precipitation orographically driven precipitation and the\nand the strong westerly winds strong westerly winds’\n1.8 55 change 'air pocketed' to 'porous'. 56 Done – much better wording, thank you\n1.9 56 please add reference: 56 Done – reference added\nMontgomery, L., Miège, C., Miller, J.,\nScambos, T.A., Wallin, B., Miller, O.,\nSolomon, D.K., Forster, R. and\nKoenig, L., 2020. Hydrologic\nproperties of a highly permeable firn\naquifer in the Wilkins Ice Shelf,\nAntarctica. Geophysical Research\nLetters, 47(22), p.e2020GL089552.\nThis is the discovery paper for firn\naquifers.\n1.10 58 I don't think heat loss to the 57 We agree this wording should be refined.\natmosphere directly affects the\nmCDW layer - mixing with the polar Done – rephrased to\nwater layer may be said to be induced 57: ‘…becoming modified CDW (mCDW)\nby heat loss to the atmosphere, but this through mixing and vertical heat loss to the\nshould be rephrased. overlying Antarctic Surface Water layer’ this\nmatches the description used by Moffat &\nMeredith, 2018.\n1.11 58 add: ' ,both shallow and deep, ' 59 Refers to: This contrasts with the much colder\nwaters of the Bransfield Strait and Weddell\nSea\nComment – We feel this wording would make\nthis sentence less clear, so have left it\nunchanged.\n1.12 61 ... ,a large area of sea ice persists 61 Done – changed to:\nduring austral summers. becoming....\n61: ‘…a large area of sea-ice regularly persists\nduring austral summer’\n1.13 62 This should not be characterized as a 62 Done – we have rephrased this as:\nmonotonic increase spanning 1979\nthrough 2015; there was a strong 62: ‘Between 1978 and 2015 sea-ice extent in the\nreduction in the NW Weddell from the Weddell Sea grew with a small positive trend\nlate 1980s through the early 2000s, but superimposed upon large interannual\nan erratic increasing trend on either variability’\nside of that time-span\nWe have also added a reference to Turner et al.,\n2020, which describes the Weddell Sea sea-ice\nconditions in detail.\n1.14 64 this was surpassed in 2022 and again 63 For Weddell Sea specific sea ice extent this may\nin 2023. not be true, however a negative anomaly did\ncontribute to the total Antarctic record low in\n2022.\nDone – rephrased and added reference to (Turner\net al., 2022)\n63: ‘…from 2016 onwards this trend reversed\nwith satellite data showing a reduction in SIE,\nreaching a record low in 2019 and contributing\nto record low total Antarctic SIE in 2022’\n1.15 66 please add comma -- 67 Done\n...precipitation, snow, and glacier\nmelt...\n1.16 80 this does not read like a 'definition' of 79 Done – changed to ‘we define’ as this is a\nthe lower glacier naming we have chosen. We chose to call this\npart lower as it is below the Cadman icefall\nwhich separates the upper slow flowing basin\nfrom this lower section.\n1.17 94 please spell this out a bit more -- 93 Comment – This short paragraph is just to\naccelerated by 94%, a speed-up of 1.5 introduce the reader to the acceleration event\nkm/yr, reaching (something like) 2.9 being discussed, so that the following paragraphs\nkm/yr by xx date. are not out of context. We discuss the ice speed\nchanges in detail in the results section so we\nwould prefer not to duplicate such detail in the\nintroduction.\n1.18 129 There are valuable images of the loss 120-129 Done – see response above to point 1.2\nof the ice shelf in Dec-Jan 2020 and\n2021 that appear to illustrate the\nbreak-up mechanism\n1.19 Fig 3 add a note or symbol here that this Fig 3 Done – changed to:\nreference is the grounding line in the\nfigure -- saves time in understanding ‘given with respect to the ASAID grounding line\nwhat is what here. location (illustrated in Fig 1b)’\n1.20 217 I think you should consider the 217 Comment – This is a really interesting\npossibiltiy that this feature, the area suggestion and something we will consider in\nnear the peak of elelvation loss, is future work, but we feel it would be too\nrelated to a sub-glacial lake behind the speculative to include here.\nridge that was induced to drain by\nchanges in teh glacier and therefore\nchanges in the hydrostatic gradient at\nthe ice base. Papers by Gary Clarke\ndiscuss these sotrs of possibilities.\n1.21 239 I would add 'and loss of basal stress 244 Done – changed to:\nover the ridge region'\n244: ‘The coincident timing of these thinning\nobservations with the ice speed change\nmeasurements demonstrates that this is a\ndynamically induced thinning caused by the\n2018/19 ice acceleration, which was in turn\ncaused by the loss of basal traction over the\n2015 bed ridge.’\n1.22 282 Split the sentence here 287 Done\n1.23 331 its not really 'damage' in the sense that 338 Comment – We use ‘damage’ as an inclusive\nthat is usually discussed - it is a term to describe fracture and weakening.\nweakeniong and incising by basal\nmelting (I think.... more images are\navailabe of the events)\n1.24 409 change 'chance' -- more like: 432 Done – changed to:\n'...represents a uniquiely detailed\nassemblage of data, allowing us to 432: ‘This study represents a uniquely detailed\ncharacterize the transition of an assembly of data allowing us to observe and\nAntarctic...' characterise the forced transition of an Antarctic\nglacier’\n1.25 431 just saying: have a llook at ITS_LIVE Comment - The ITS_LIVE online data tool\ndata set and the other MEaSURES timeseries begins in 2014 for points sampled at\ntools Cadman glacier and includes only a small\nnumber of additional optically tracking results\nduring this period, so it is broadly comparable to\nour own dataset. However, it was a useful\nexercise to confirm that ITS_LIVE measures the\nsame acceleration of Cadman as we observe in\nthis study.\nReviewer 2\nRef. Original Comment Revised Response\nLine Line\n2.1 This paper investigates Cadman We thank the reviewer for their comments and\nGlacier in the west Antarctic constructive feedback. In particular we were\nPeninsula (AP), focusing on its unique pleased to hear the reviewer found the study had\ncharacteristics and recent changes. the potential to make a significant contribution to\nUnlike other glaciers in the region, the field of glaciology.\nCadman Glacier velocity increased\nbetween 2018 and 2019 and We have replied to the reviewers comments in\nexperienced persistent thinning of its turn and made changes to the manuscript to\nice shelf. This thinning eventually led address them. Specifically, we have added an\nto retreat and rapid acceleration of the updated grounding line measurement, including\nglacier. Warm ocean temperatures and details of the method used. We justify why we\nbed geometry played significant roles do not feel it is appropriate to include a\nin these changes. The study Lagrangian basal melt rate analysis in the\nemphasizes the vulnerability of manuscript, through comparison to other studies\nglaciers in the region to warm ocean and considerations relating to the other datasets\nwaters and the need for further required for this calculation. Finally, we have\nresearch to understand their future significantly expanded the discussion section of\nevolution. the manuscript to provide concrete\nrecommendations for future work and to place\nthe results in the greater context of the Antarctic\nPeninsula region’s contribution to rising sea-\nlevels.\nOverall, we believe that addressing the\nreviewer’s comments has substantially enhanced\nthe manuscript. We thank the reviewer for their\ntime and consideration.\n2.2 The paper provides a comprehensive 211-212 Grounding line:\nanalysis of Cadman Glacier in the 345-351 Directly measuring the grounding line position\nwest Antarctic Peninsula, focusing on 491-501 of Cadman Glacier is difficult because there is\nits unique characteristics and recent Fig. 1b no InSAR coherence on the glacier in available\nchanges. The authors effectively Fig. 3b datasets. For example, a study which processed\npresent the background information all available Sentinel-1 data in Antarctica for\nand clearly articulate the research 2018 and automatically delineated grounding\nobjectives. The methodology lines did not produce any measurements for\nemployed in the study, including the Cadman Glacier (Mohajerani et al., 2021). We\nuse of CryoSat-2 data and previous investigated measuring grounding line position\nstudies, appears to be partially using repeat track ICESat-2 laser altimetry\nappropriate for the research questions (Fricker and Padman, 2006; Brunt et al., 2010),\nat hand. but found insufficient data coverage for the\nThe analysis of surface elevation, Cadman grounding zone, due to the large\nelevation change, and ice velocity spacing between tracks at lower latitudes and an\nprovides valuable insights into the unfavorable across-glacier track orientation.\nglacier’s behavior, but grounding zone\nmeasurements are missing which In the original manuscript, we discuss that the\ncould hinder the interpretation of the grounding line of Cadman glacier likely retreated\nresults. from the ASAID dataset (1999-2003) position to\na position around 2km inland based on changes\nin ice flow speed and DEMs:\nOriginal MS L340-344: ‘We interpret these\nchanges to be indicators of a loss of basal\ntraction and therefore the retreat of the\ngrounding line, from the ASAID grounding line\nlocation to a position 2 km further inland which\nis co-located near the point (ii) 2015 surface\nbump and associated speed minimum (Fig. 3).’\nWhile a precise InSAR grounding line\nmeasurement is not possible, we have formalised\nour analysis of the pre-acceleration grounding\nline position by measuring the break in surface\nslope of the 2015 REMA DEM (Howat et al.,\n2019). We define this as the most seaward local\npeak in surface slope gradient (Hogg et al., 2018;\nFriedl et al., 2020) and find the grounding line\nposition to be 1.25 km inland of the ASAID\n(1999 – 2003) position.\nDone – we have amended the text in several\nplaces to include this measurement.\nIn the methods section:\n491: ‘Measuring the grounding line position of\nCadman Glacier in the period immediately prior\nto its acceleration and retreat is difficult because\nSentinel-1 interferometric synthetic aperture\nradar (InSAR) is not coherent on the glacier, for\nexample an automated InSAR processing and\ngrounding line delineation campaign for 2018\nwas not able to resolve any grounding lines in\nthis location. Other remote sensing methods\nwhich rely on detecting tidal motion such as\nrepeat track laser altimetry and differential\nrange offset tracking do not adequately resolve\nCadman Glacier. For example ICESat-2 tracks\nare widely spaced at this latitude and transverse\nto Cadman Glacier’s flow direction, so do not\ncover the grounding zone. To locate the\ngrounding line position in 2015, we measure the\nbreak in surface slope of the REMA 100 m\nAntarctic Peninsula DEM between the ice shelf\nand grounded glacier. We resample the REMA\nDEM to 10 m resolution using a cubic spline and\ndelineate the grounding line at the most seaward\nlocal peak in surface slope gradient along the\nglacier’s central flowline and extend this\nperpendicular to flow to the shear margins.’\nIn the surface elevation change results section:\n211: ‘…the break in surface slope in REMA\nDEM as an approximation for location of\nCadman Glacier’s grounding line in 2015, we\nfind that in 2015 the grounding line is 1.25 km\ninland of the ASAID 1999-2003 position (Fig\n3b).’\nIn the discussion:\n345: ‘Between 1991 and 2010 there was a\nchange from a local ice speed minimum feature\nto an area of spatially constant ice speed\nimmediately behind the ASAID grounding line\nlocation, which was matched by the loss of an ice\nsurface bump between the Cook AP (2000-2009)\nand the REMA DEMs (2015), together with the\ninland migration of the break in surface slope.\nWe interpret these changes to be indicators of a\nloss of basal traction and the retreat of the\ngrounding line, from the ASAID grounding line\nlocation to a position 1.25km further inland\nlocated near the point (ii) 2015 surface bump\nand associated speed minimum (Fig. 3). Such a\nloss of basal traction could also explain the\nincrease in ice speed of 0.25 km/yr (15 %)\nbetween 1991 and 2010.’\nWe have also updated Figure 1b and Figure 3b to\nshow the 2015 grounding line position.\n2.3 Additionally, I was surprised the Basal melt rates:\nauthors did not provide both an The reviewer raises a question about why both\neulerian and lagrangian melt rate Eulerian and Lagrangian basal melt rate\nanalysis. calculations were not included. Note that we did\nnot calculate a Eulerian or Langrangian basal\nmelt rate value for the Cadman Ice Shelf in the\noriginal manuscript, but instead calculated a total\nice shelf thinning rate for the shelf area defined\nin Fig 1b and Fig 4a, which has an area of 9.3\nkm2.\nThis was a deliberate choice we made because\nwe did not feel that the Cadman Ice Shelf was\nadequately resolved in the additional datasets\nrequired to calculate a basal melt rate.\nSpecifically, for surface mass balance (SMB),\nthe highest spatial resolution regional climate\nmodel available to us is the 5.5 km RACMO\n2.3p2 Antarctic Peninsula model (van Wessem et\nal., 2016) and for firn air content (FAC),\nexisting\nproducts, such as those produced by the IMAU\nfirn densification model have a resolution of 27\nkm. The width of the Cadman ice shelf is\napproximately 3 km, therefore it would not be\nwell resolved in either of these datasets. The\nshelf is surrounded by steep topography up to 2\nkm high, meaning the values for SMB and FAC\nin these models for the single grid cell containing\nthe Cadman ice shelf will likely not be\nrepresentative of conditions on the shelf.\nFurthermore, for any Lagrangian calculation\nindividual altimetry results must be advected\nwith ice flow and an elevation change rate\ncalculated via a plane fit to an appropriate grid\npostings resolution. Grid resolutions for\naltimetry based basal melt data in the literature\nvary from 500 m (Gourmelen et al., 2017) to 30\nkm (Adusumilli et al., 2018), with the finer grid\nresolution calculations using high resolution\nswath mode CryoSat-2 altimetry which was not\navailable on Cadman Ice Shelf. Additionally, the\nice shelves to which Lagrangian calculations\nhave been applied are many orders of magnitude\nlarger than the Cadman ice shelf.\nOverall, we believe these methodological\nlimitations prevent the accurate calculation of a\nuseful and robust basal melt rate value, which\njustifies our choice to use a Eulerian average ice\nshelf thinning rate which we can report with\nconfidence.\n2.4 Overall, the findings of the study are No change required, thank you for the positive\nsignificant and contribute to the assessment.\nunderstanding of glacier dynamics in\nthe region. The observed persistent\nthinning of Cadman Glacier’s ice shelf\nand its subsequent retreat and\nacceleration are compelling evidence\nof the glacier’s vulnerability to warm\nocean waters.\n2.5 The paper effectively discusses the No change required\nrole of bed geometry and bathymetric\nfeatures in influencing the glacier’s\nresponse to environmental forcing.\nThe inclusion of comparisons with\nneighboring glaciers adds depth to the\nanalysis and supports the conclusions\ndrawn. The implications of the study\nare well-discussed, particularly in\nterms of the impact on ocean\ncirculation, productivity, and the\necosystem. The mention of potential\nregional-scale consequences further\nstrengthens the significance of the\nresearch.\nThe paper is well-written, with a clear\nand concise presentation of the\nresearch findings. The academic tone\nand language throughout the\nmanuscript contribute to its overall\nquality.\n2.6 However, one area that requires 433-451 Done – Added the flowing to the discussion\nimprovement is the discussion of\nlimitations and future research 433: ‘This study represents a uniquely detailed\ndirections. While the authors briefly assembly of data allowing us to observe and\nmention the need for further studies characterise the forced transition of an Antarctic\nand fieldwork measurements, more glacier from one with a small floating shelf to a\nspecific recommendations or tidewater glacier. It highlights the impact of\nsuggestions for future investigations continuous, high-resolution multi-decadal\nwould enhance the paper’s climate observations and remote sensing\ncompleteness specifically focused on campaigns which have contributed to our\nthe need of frequent grounding line understanding of the complex interaction of\nmeasurements and melt rate Earth-system processes and how Antarctica will\nmeasurements see (Shean et al 2019 respond to a warmer climate. This work also\nMilillo et al 2022) shows the importance of continuing and\nimproving these observations. Dense time-series\nof ice velocity measurements, such as those from\nSentinel-1, could be further enhanced by\ndedicated campaigns of in-situ validation to\nquantify the impact of surface processes on\nvelocity measurement error, allowing greater\nsensitivity to detect short-term ice dynamic\nchanges. Substantial further insights would also\nbe gained from frequent grounding line position\nmeasurements, particularly in regions such as\nthe AP where low SAR coherence currently\nmakes these measurements difficult. Improving\nthe temporal and spatial resolution of altimetry\nmeasurements will improve the monitoring of ice\nshelf thinning across the Antarctic continent.\nExpanded airborne ground penetrating radar\ncampaigns to measure ice thickness and bed\ntopography near the grounding zone would\nreduce uncertainty in ice discharge and better\ncharacterise which glaciers are vulnerable to\ndynamic tipping points like the one observed in\nthis study. Furthermore, Cadman Glacier and\nthe Beascochea Bay system including Funk and\nLever Glaciers are strong candidates for further\nstudies and fieldwork measurements. New and\nsustained oceanographic measurements of the\nbay and individual glacier fjords behind the\nbathymetric sills would be valuable to\nunderstand heat transport across the west AP\nshelf and to compare circulation between\nGreenlandic fjords and the AP helping to better\nunderstand the future evolution of the Antarctic\nPeninsula’s glaciers.’\n2.7 Additionally, it would be beneficial to 361-365 Done – we have extended the discussion of the\nprovide a clearer connection between 415-425 broader implications of this study and we have\nthe findings of this study and their expanded a number of points in the discussion.\nbroader implications for climate\nchange and sea-level rise. This would On how this study presents an example of ice\nhelp contextualize the significance of shelf collapse without surface ponding (also see\nthe research within the larger scientific comments to reviewer 1, comment 1.3):\ndiscourse.\n361: ‘. The Cadman Ice Shelf fully collapsed in\nMarch 2021 due to this thinning, possibly\nintensified through channelised melting, and the\nloss of pinning on the 2015 ridge. The absence of\nsurface melt ponding on the Cadman Ice Shelf\nprior to its collapse demonstrates that surface\nponding is not a necessary precondition for ice\nshelf collapse, which can occur through ocean-\ndriven thinning and unpinning alone.’\nDescribing the broader implications for sea-level\nrise and climate:\n414: ‘Our results demonstrate that such tipping\npoints can increase ice discharge extremely\nrapidly, in Cadman Glacier’s case by 28.1% in\n13 months. Other glaciers on the Antarctic\nPeninsula may be vulnerable to similar sudden\nacceleration and retreat, where bed conditions\nand bathymetry allow, similar to heterogeneous\npatterns of tidewater glacier retreat controlled\nby bed topography observed in Greenland.\nFor example, the largest glacier on the west coast,\nFleming Glacier, has a floating tongue and is\ngrounded on a retrograde bed slope, similar to\nCadman Glacier. Despite the relatively small\nsize of the Antarctic Peninsula Ice Sheet\ncompared to the West and East Antarctic Ice\nSheets, the stability of the AP’s glaciers is\nimportant to global sea-level. The region has\ncontributed 1.4 ± 0.37 mm to global mean sea-\nlevel from 1992 to 2017, 19% of the Antarctic\ntotal. Additionally, increased freshwater input\nfrom ice discharge and newly exposed seabed\ndue to glacier retreat would have regional-scale\nconsequences for ocean circulation, productivity,\ncarbon sequestration, and the ecosystem.’\n2.8 Overall, this paper presents valuable Done – Thank you for the comment. No change\nresearch on Cadman Glacier and its required.\nresponse to environmental changes.\nWith minor revisions and additions to\nthe discussion section, this study has\nthe potential to make a significant\ncontribution to the field of glaciology\nand climate science.\n2.9 73-75 Unless a reference is missing, lines 431 Done – Removed this from here and made point\n73-75 in discussion where it has better context, see\nseem a bit out of context for the response to 2.6 above.\nintroduction section and could be\nmore appropriate in the conclusions\n2.10 79-81, when referencing the 4 areas of the 77-79 Comment – This description is intended to\nFig. 1 Cadman glacier a reference to a figure familiarize the reader with the glacier and the\nshould guide the reader and enable the definitions here are only used qualitatively for\nspatial identification of these areas on description. Quantitative geometric definitions\na map. and those used for calculations are defined on a\nmap in Figure 1b. We feel adding more labels to\nFigure 1b would reduce clarity, so have left it\nunchanged.\n2.11 Fig. 1 Fig1 missing reference to the DEM Fig. 1 No action required – Figure 1b,c,d all use the\nused for shading. Landsat-8 image referenced in the caption as a\nbackground, no DEM was used.\n2.12 152-153 Which grounding line has been used to 152 This relates to the sample areas defined in Figure\nmeasure these proportions ? 1b and Figure 3b.\nDone - Added detail to the introductory\nsentence:\n152: ‘Our 30-year timeseries of ice velocity (Fig.\n2, Fig. 3a, sample areas defined Fig 1b, 3b)’\nReviewer 3\nRef. Original Comment Revised Response\nLine Line\n3.1 This manuscript presents a detailed set Comment - We thank the reviewer for their\nof observations of Cadman Glacier, a consideration of the manuscript and helpful\nmarine-terminating glacier on the west comments. We are please that the reviewer found\ncoast of the Antarctic Peninsular, the work to be robust and well written. We thank\nspanning a period of rapid change in the reviewer for their time.\nthe glacier’s ice shelf. The\nobservations, which include newly Done - We have addressed the reviewers\nderived data on ice thickness, velocity comment point by point below. Specifically, the\nand terminus position, clearly reviewer’s comment about sill depth prompted\ndocument the thinning, ungrounding, us to expand the discussion of how fjord sills\nacceleration, retreat and ultimate influence circulation and basal melt rates, which\ncollapse of the ~5 km ice shelf over we feel notably improved this section of the\nthe period ~2010-2022. Through manuscript.\ncomparison with existing\nhydrographic and bathymetric data,\nthe authors present a convincing case\nthat this reflects the influence of the\nocean, with gradual weakening of the\nshelf due to long term thinning\nfollowed by a final collapse triggered\nby the presence of particularly warm\nocean waters on the shelf. Altogether,\nthis makes a nice case study of a\nmoderately sized marine terminating\nglacier experiencing ice shelf thinning\nand collapse in response to warming\nocean waters. The detailed\nobservations document this process\nwell, and I believe the methodology,\nanalysis and interpretation are sound.\n3.2 While I am confident that the work is Comment: We feel the results presented in this\nworthy of publication in a relevant study will be of great interest to scientists from a\njournal, I am less confident that it broad range of disciplines, which is why we\nrepresents the kind of major, high- chose to submit to Nature Communications. Our\nimpact advance that would typically work combines high resolution remote sensing\nbe associated with Nature data with direct oceanographic measurements\nCommunications. The finding that the and reanalysis to significantly develop the\nretreat of glaciers on the western process-based link between ice-ocean\nAntarctic Peninsular is predominantly interactions in Antarctica, which is at the\nocean-driven was presented on a much forefront of modern climate science and is\nlarger scale by Cook et al (2016) – the essential for accurately projecting future sea-\ncurrent manuscript presents a detailed level rise.\ncase study of one such example of\nthis, but does not to me really advance In response to the comments of Reviewer 1, we\nour broader understanding of this topic have provided more detail about the novelty of\nbeyond this particular glacier (which the break-up of Cadman Ice Shelf without melt\nis not in itself of particular importance ponding and hydrofracture, a point which was\nas a source of sea level rise etc). The not emphasized sufficiently in the original\nsequence of processes observed - ice manuscript. See response to point 1.2.\nshelf thinning, acceleration and break\nup, grounding line retreat, dynamic Furthermore, through the responses to this point\nthinning upstream – are quite familiar and those raised by other reviewers, we have\nfrom the loss of other ice shelves and substantially expanded the discussion to stress\ntongues in Antarctica and Greenland how the results of this study relate to broader\n(e.g. Motyka et al (2011), to give just research questions regarding ice-ocean\none example). The attribution to interactions in Antarctica. See response to point\noceanic forcing supports the findings 2.7. We also expanded our discussion of\nof Cook et al (2016), but the study recommendations for future work and highlight\nlacks the kind of novel observations (such as in situ hydrographic measurements\nclose to the glacier, or high resolution / spatially resolved measurements of shelf thinning) that might really advance our understanding of the underlying processes.\nPutting this aspect aside, I do not have any major concerns with the methods, analysis or interpretation as currently presented. I found the manuscript to be largely well written, if perhaps overly descriptive at times – it might be possible to make better use of the figures and reduce the amount of descriptive text a little. A have a few more minor comments, outlined below.\nL28. The ‘cryosphere’ is generally used to refer to ice and snow on a global scale – better to refer to the glaciers of this region or similar\nL37. Ambiguous phrasing – suggest something like ‘increased by 400 % between the periods 1992-1997 and 2007-2012’\nL161. A figure reference would be useful\nL175-185. It would be useful to add a reminder of the years of the ASAID grounding line and DEMs\nL244-5. It’s not easy to discern the grey dots in the figure, perhaps a different colour would work better\nL264-275. It wasn’t clear to me why the addition of surface lowering merited such attention – is this making a methodological point (that other researchers need to make sure they factor this in), or that surface lowering has an important impact on the dynamics of the glacier?\nL303-306. Yes these glaciers have shallower sills, but its notable in Figures 6c-d that there is still substantial warming at depths shallower than 200-230 m, so they should still have experienced significantly warmer temperatures at this time.\nL311-312. What year is the ocean temperature anomaly shown?\nL326-328. It would be helpful to include a figure reference\nL328-329. What is the postulated driver of this long term thinning? Ocean warming?\nL371-373. Again it would be helpful to include a figure reference\nReferences\nCook, A., Holland, P., Meredith, M., Murray, T., Luckman, A., & Vaughan, D. (2016). Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353(6296), 283-286.\nMotyka, R. J., Truffer, M., Fahnestock, M., Mortensen, J., Rysgaard, S., & Howat, I. (2011). Submarine melting of the 1985 Jakobshavn Isbrae floating tongue and the triggering of the current retreat. Journal of Geophysical Research-Earth Surface, 116, Article F01007. https://doi.org/10.1029/2009jf001632"}, {"round": 2, "reviewer_comments": {"reviewer_1": "This is a re-review.\nAs I noted in my initial review, this is a well-done paper that assembles several data sets for a detailed analysis of a relatively important western Antarctic Peninsula glacier.\nIn my view, all of the reviewer comments were well-addressed. In particular, I appreciated the discussion of Reverwer2's suggestion of Eulerian and Langrangian melt tracking -- without very dense altimetry data (in both time and space) this would not be easily possible, nor would such a result materially change the resuts presented. Reviewer 3 has only a few substantive comments. All three reviewers expressed appreciation of the work.\nReviewer 3 pointed out that this paper might not be apropos for Nature Communications, and there is some merit to that -- Nature Geoscience might have been a better choice,, with essentially the same overall impact on the physical science community. However, similar works have appeared in Nature Communications in the past, and are higihly cited and I think appreciated by the wider earth and climate research community.", "reviewer_2": "The Authors have addressed all my comments, I encourage the editor to accept this paper for publication.", "reviewer_3": "Thank you to the authors for carefully and clearly responding to the comments I raised on the previous version of this manuscript. These were largely relatively minor queries and suggestions, which the authors have addressed satisfactorily. I am also pleased to see some increased discussion of the wider significance of the findings.\nI have only a few comments on this revised manuscript, along with a number of suggestions relating to the clarity of the writing, which I will attach as an annotated PDF.\nComments:\nL266-7. The discharge here is described as being constant between 2016 and 2018, when Figure 5 shows it varies quite substantially in this time period. I think what is meant is that there is no trend of increasing ice discharge over this time period – if so, the phrasing should be amended accordingly.\nL363-5. This indicates that one would assume that surface ponding was a necessary condition for ice shelf collapse, but this seems to be a bit of an Antarctic Peninsula–centric perspective. The laterally constrained ice shelf at Cadman may have more in common with the Greenland’s relatively small ice shelves and tongues, where surface melt ponds are not generally deemed to have such an important part to play, a comparison that should probably be made here.\nL396-425. A missing link here seems to be to make the connection between the divergent behaviour of Cadman, Lever and Funk Glaciers and the discussion around tipping points. Yes Lever and Funk Glaciers may be less sensitive to ocean conditions than Cadman due to the shallower sills, but this may be only part of the story. As discussed in this section, the rapid changes at Cadman Glacier may reflect the culmination of a lengthy period of mass imbalance in the terminal region, which is finally sufficient to trigger a dramatic dynamic response. Lever and Funk may also be slowly responding to ocean warming, but not presently in a position whereby this is sufficient to trigger a major response. A comparison can be made to Cadman Glacier around 1995 – if you only had the data for that period in Figure 2, you might argue that Cadman Glacier was insensitive to ocean warming, but then an ocean warming event of similar magnitude 25 years later caused the tongue to collapse. Similarly, Lever and Funk may seem insensitive now to ocean warming now, but this could dramatically change at some point down the line. It would be interesting to see if there were any more gradual changes in ice thickness apparent on the tongues of these glaciers if they fall within the area covered by your data. At the least though, I think this is a point that merits mention in the Discussion.\nFigure 4. The new markers in part a are better, but they are still the same colour as the topographic shading. Why not use a different colour, like green or pink?"}, "author_response": "Response to second round of reviewers’ comments: Ocean warming drives\nrapid dynamic activation of a marine-terminating glacier on the west\nAntarctic Peninsula\nRef: NCOMMS-23-19496A\nReviewer 1:\nRef. Original Comment Revised Response\nLine Line\n1.1 n/a This is a re-review. N/A We thank the reviewer for their time reviewing\nthis manuscript and for their kind and\nAs I noted in my initial review, this is constructive comments.\na well-done paper that assembles\nseveral data sets for a detailed analysis No further changes are required to address these\nof a relatively important western comments.\nAntarctic Peninsula glacier.\nIn my view, all of the reviewer\ncomments were well-addressed. In\nparticular, I appreciated the discussion\nof Reverwer2's suggestion of Eulerian\nand Langrangian melt tracking --\nwithout very dense altimetry data (in\nboth time and space) this would not be\neasily possible, nor would such a\nresult materially change the resuts\npresented. Reviewer 3 has only a few\nsubstantive comments. All three\nreviewers expressed appreciation of\nthe work.\nReviewer 3 pointed out that this paper\nmight not be apropos for Nature\nCommunications, and there is some\nmerit to that -- Nature Geoscience\nmight have been a better choice,, with\nessentially the same overall impact on\nthe physical science community.\nHowever, similar works have\nappeared in Nature Communications\nin the past, and are higihly cited and I\nthink appreciated by the wider earth\nand climate research community.\nReviewer 2:\nRef. Original Comment Revised Response\nLine Line\n2.1 N/A The Authors have addressed all my N/A We thank the reviewer for their time reviewing\ncomments, I encourage the editor to this paper and their helpful comments.\naccept this paper for publication.\nNo further changes are required to address these\ncomments.\nReviewer 3:\nRef. Original Comment Revised Response\nLine Line\n3.1 N/A Thank you to the authors for carefully N/A\nand clearly responding to the\ncomments I raised on the previous\nversion of this manuscript. These were\nlargely relatively minor queries and\nsuggestions, which the authors have\naddressed satisfactorily. I am also\npleased to see some increased\ndiscussion of the wider significance of\nthe findings.\nI have only a few comments on this\nrevised manuscript, along with a\nnumber of suggestions relating to the\nclarity of the writing, which I will\nattach as an annotated PDF.\n3.2 266-267 L266-7. The discharge here is 266 Thank you for this comment, we agree that\ndescribed as being constant between changing the phrasing to be more precise would\n2016 and 2018, when Figure 5 shows be better.\nit varies quite substantially in this time\nperiod. I think what is meant is that Changed to:‘Throughout 2015 ice discharge\nthere is no trend of increasing ice increased until it reached a mean of 1.85 ± 0.13\ndischarge over this time period – if so, Gt/yr between 2016 and 2018 with no positive or\nthe phrasing should be amended negative trend’\naccordingly.\n3.3 363-365 L363-5. This indicates that one would 371 Thank you, we agree that the precise meaning\nassume that surface ponding was a could be made clearer.\nnecessary condition for ice shelf\ncollapse, but this seems to be a bit of Changed to:\nan Antarctic Peninsula–centric\nperspective. The laterally constrained 370: ‘The absence of surface melt ponding on the\nice shelf at Cadman may have more in Cadman Ice Shelf prior to its collapse\ncommon with the Greenland’s demonstrates that surface ponding is not a\nrelatively small ice shelves and necessary precondition for ice shelf collapse in\ntongues, where surface melt ponds are the Antarctic Peninsula, which can occur\nnot generally deemed to have such an through ocean-driven thinning and unpinning\nimportant part to play, a comparison alone, similar to small ice shelf failures observed\nthat should probably be made here. in Greenland’\n3.4 396-425 L396-425. A missing link here seems Changed to:\nto be to make the connection between\nthe divergent behaviour of Cadman, 410: ‘Our results suggest differences in\nLever and Funk Glaciers and the bathymetric structure played a strong role in the\ndiscussion around tipping points. Yes different response of Cadman and neighbouring\nLever and Funk Glaciers may be less glaciers to warming shelf water, however in\nsensitive to ocean conditions than future Lever and Funk Glaciers may become\nCadman due to the shallower sills, but substantially more sensitive to ocean forcing if\nthis may be only part of the story. As temperature anomalies are greater and they\ndiscussed in this section, the rapid progressively thin in an evolution which is\nchanges at Cadman Glacier may similar to Cadman Glacier’s but delayed.’\nreflect the culmination of a lengthy\nperiod of mass imbalance in the\nterminal region, which is finally\nsufficient to trigger a dramatic\ndynamic response. Lever and Funk\nmay also be slowly responding to\nocean warming, but not presently in a\nposition whereby this is sufficient to\ntrigger a major response. A\ncomparison can be made to Cadman\nGlacier around 1995 – if you only had\nthe data for that period in Figure 2,\nyou might argue that Cadman Glacier\nwas insensitive to ocean warming, but\nthen an ocean warming event of\nsimilar magnitude 25 years later\ncaused the tongue to collapse.\nSimilarly, Lever and Funk may seem\ninsensitive now to ocean warming\nnow, but this could dramatically\nchange at some point down the line. It\nwould be interesting to see if there\nwere any more gradual changes in ice\nthickness apparent on the tongues of\nthese glaciers if they fall within the\narea covered by your data. At the least\nthough, I think this is a point that\nmerits mention in the Discussion.\n3.5 Figure 4. The new markers in part a Done: Figure 4a markers changed to gold with\nare better, but they are still the same black outline and made larger\ncolour as the topographic shading.\nWhy not use a different colour, like\ngreen or pink?\n3.6 16 suggest 'the rapid acceleration and We chose to use ‘increase in speed’ because this\ncollapse' period of net acceleration stopped after\nDecember 2019 and features some brief periods\nof slowing speed superimposed on the positive\ntrend. Given there is not a constant ongoing\nacceleration, we have chosen to use ‘increase in\nspeed’.\n3.7 19 Of? We have chosen to keep the original phrasing.\n3.8 19 By? We have chosen to keep the original phrasing.\n3.9 20 Awkward sentence We have chosen to keep the original phrasing.\n3.10 23 Unclear terminology – imbalance of 22 Done: Added ‘dynamic imbalance’\nwhat?\n3.11 23 grammar - 'increased ice discharge 23 Done: Changed to the suggested wording.\nfrom glaciers on the Antarctic\nPeninsula' would be preferable.\n3.12 27 Awkward (and unecessary) to start 27 Done: changed to: ‘Research on the Antarctic\nand finish with reference to time Peninsula (AP) has shown major changes in the\nframes - suggest just one or the other region’s glaciers and ice shelves throughout the\n20th and 21st centuries.’\n3.13 31 Off We have chosen to keep the original phrasing.\n3.13 40 Predicted Unchanged, as we have already said projected:\n‘and ice loss is projected to increase in the\nfuture…’\n3.14 44 Complex sentence We have chosen to keep the original phrasing.\n3.15 54 Delete ‘the’ 54 Done\n3.16 78 Delete ‘these are’ 78 Done\n3.17 124 ‘ice speed’ –unnecessary? 124 Done\n3.18 203 ‘ice speed’ – unnecessary? Maintained here for clarity as we refer to surface\nelevation change which could also be\naccelerating.\n3.19 212 Comma 212 Added\n3.20 219 Check grammar and punctuation for\nclarity\n3.21 260 Change ‘on’ to ‘from’ 264 Done\n3.22 260 Add: ‘the’ We have chosen to keep the original phrasing.\n3.23 264 Meaning unclear – do you mean by We feel this sentence is adequately clear\nthe time of some observation that\nspans this time period?"}]

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