Detection of 85 new active subglacial lakes in Antarctica from a decade of CryoSat-2 data

Domain: earth_science
Published: 2025-09-19
Editorial outcome: minor_revision

Abstract
Subglacial lake activity influences ice sheet flow, grounding line discharge and ice shelf basal melting. Although 146 active subglacial lakes have been detected in Antarctica via ice surface elevation change associated with their activity, only 36 fill-drain cycles have been observed worldwide, and knowledge of these mechanisms is limited. Here, we use a decade of CryoSat-2 radar altimetry to detect 85 active subglacial lakes in Antarctica, documenting 37 and 34 complete draining and filling events respectively. We delineate time-varying boundaries of subglacial lake activity and investigate their variability over time. Our observations increase the number of known active subglacial lakes in Antarctica by 58%, with six of these located within 8 km of the grounding zone. We observe five subglacial lake networks, with concurrent upstream drainage and downstream filling, and 25 clusters of lakes, improving our knowledge of interconnected subglacial hydrological pathways.

Introduction
Lakes beneath glaciers and ice streams form in subglacial valleys1, or cavities that evolve as ice slides over bedrock2,3. They were first identified beneath the Antarctic Ice Sheet using airborne radio-echo sounding (RES) surveys from the 1960s1,4. In Greenland, subglacial lakes can form when seasonal surface melt percolates down from the ice surface to bedrock via moulins, crevasses5 and hydrofractures6,7. Beyond the Antarctic Peninsula8, there is limited evidence that surface water reaches the bed in this way in Antarctica. Antarctic subglacial water is primarily produced by geothermal heat melting the underside of the ice sheet9 from pressure-induced melting or through frictional heating of basal ice as it flows downstream10. Our understanding of subglacial melt rate magnitudes and how they might vary over time is incredibly limited due to the paucity of measurements in this inaccessible region of the ice base, and as such, subglacial lake activity offers a rare opportunity to measure the timing of change and to quantify episodic water flux at the ice sheet base11. Filling and draining events channel water through hydrological pathways between subglacial lakes and have the potential to rapidly influence ice dynamics12 by modulating basal friction and pressure conditions. If subglacial water reaches the grounding line it can produce buoyant, cold freshwater plumes that increase basal melt rates locally on ice shelves13, influence the pattern and strength of ocean circulation14, and alter glacial landforms and habitats15. These effects are especially pronounced in Antarctica where subglacial lakes often occur close to the ice sheet margins16.

Of the 779 subglacial lakes identified globally, 681 are located in Antarctica, 20% (146) of which have exhibited surface elevation changes suggestive of lake draining and filling cycles16,17. An observed cycle refers to a subglacial lake filling event along with its subsequent drainage event. “Active” subglacial lakes, which exhibit these cycles, were first observed in Antarctica in 1997, on the Kamb and Bindschadler Ice Streams, using interferometric synthetic aperture radar (InSAR) data from the Canadian RADARSAT satellite18. Clusters of active subglacial lakes have been observed along subglacial hydrological pathways which enable the transfer of water within connected lake networks12,19–21. Smith and others10 used the NASA ICESat laser altimetry satellite mission to identify and delineate the first inventory of 124 Antarctic subglacial lake boundaries based on filling and draining episodes observed between 2003 and 2008. Although laser altimeters have a small footprint size (70 m) and high along-track spatial resolution (~170 m), the spatial coverage is limited by data gaps between repeat tracks which reduces the likelihood that data will be acquired over the relatively small subglacial lake features. Further data gaps are caused by the intermittent observational phases that ICESat was operated in and because data collected in the visible part of the electromagnetic spectrum cannot penetrate through clouds. A range of other complimentary airborne and satellite-derived instruments and methods have been used to make measurements of subglacial lake activity. This includes radar interferometry (InSAR)17,18 which has a high spatial resolution but is limited by the coverage of synthetic aperture radar (SAR) data acquisitions and the methods’ reliance on coherence being maintained between SAR image pairs. Stereo-image DEM differencing provides accurate 3D information about ice surface changes. However, this is limited by high data acquisition costs and the impact of noise due to snow and ice reflectance, sky clarity and surface roughness variability22. The vertical component of ice surface motion can also be detected using ice velocity measurements from SAR feature tracking15. However, these measurements are computationally expensive to produce and have lower vertical accuracy than interferometric techniques. Repeat-pass radar altimetry data has been successfully used to detect active subglacial lakes in isolated cases along the Siple Coast23–25, at Thwaites Glacier26–28 on the West Antarctic Ice Sheet (WAIS), and at Slessor29, Academy25 and Cook15 Glaciers on the East Antarctic Ice Sheet (EAIS). This technique provides a consistently processed, long-term record of surface elevation change over subglacial lake features.

Despite efforts to characterise this understudied component of ice sheet mechanics, further work is required to identify the location and extent of subglacial lakes and to characterise the timing and frequency of subglacial lake activity. Currently, the lack of a continent-wide subglacial lake boundary dataset inhibits investigations into their influence on Antarctic Ice Sheet dynamics, in addition to basal hydrology which is currently not represented in continent-scale ice sheet models30. Triggers of lake drainage events as well as filling and drainage mechanisms themselves are currently unresolved, and the variability of lake draining and filling cycles over individual lakes is neither well observed nor understood. Over short-time periods drainage events have been associated with ice speed increases of up to 10%12,27,31 for up to 14 months, with subsequent reductions in ice flow observed. However, the overall influence of subglacial lake activity on ice speed in Antarctica is yet to be determined.

In this study, we use 10 years of swath-processed CryoSat-2 satellite radar altimetry data to detect ice sheet surface elevation change caused by subglacial lake activity. We identify and delineate the boundaries of 85 active Antarctic subglacial lakes and document modes of their variability. We produce an Antarctic-wide observational record featuring multiple cycles of often multi-year episodes of hydrological activity, providing a spatially comprehensive study of subglacial lakes and their evolution over time. We validate our findings by evaluating our method over known subglacial lakes and we compare our results with studies using both data from CyroSat-2 and independent satellites.

Results
We identify 85 active Antarctic subglacial lakes (Fig. 1), increasing the number of known active subglacial lakes by 58%, from 146 to 231, and the total number of subglacial lakes in Antarctica to 766. We find that 73 of these subglacial lakes are located in East Antarctica, with 12 identified in West Antarctica. Consistent with the majority of active subglacial lakes globally, which are typically found close to ice margins16, we observe 6 active subglacial lakes located within 8 km of the grounding line and 81% occur under fast flowing ice streams where ice flows >50 m yr−1, (Supplementary Table 1). Our surface elevation change maps show that the active subglacial lake shapes tend to be quasi-circular or elongated (Fig. 2). However, where the subglacial lakes are elongated, we find no correlation between lake orientation and ice flow direction. Although we expect lakes to be aligned with the direction of subglacial trenches, the impression of subglacial lakes observed at the surface are influenced by attenuation of the height change signal through thick ice, in addition to ice speed, all of which may alter the shape of these height change signals at the ice surface.

Our observations show that 35 active subglacial lakes in this study exhibit monotonic activity during the 10-year CryoSat-2 study period, with 17 subglacial lakes demonstrating just filling and 18 only draining behaviour. Of the 50 subglacial lakes showing both draining and filling, 10 subglacial lakes experience at least one fill-drain cycle over our observational period, two of which, subglacial lakes Whillans_180 and Scott_12, feature two cycles (Fig. 2l, k). During our study period, 37 subglacial lake drainage events and 34 filling events are captured in their entirety (Fig. 2; and Table 1, Supplementary Fig. 2). The median time-period for a complete subglacial lake drainage event in this study is 2.2 years, with the median time for lake recharge being 3.5 years. We define complete events as a subglacial lake fill or drainage episode which is bracketed by the opposite active mode, or a dormant period. We observe the largest drainage event at Cook West_67 subglacial lake, located beneath an ice stream that flows into the Cook Ice Shelf in East Antarctica. This lake drained ~1.3 km3 between early-2012 and early-2016 (Fig. 2f, and Table 1), an order of magnitude larger than the previously reported median Antarctic subglacial lake drainage volume of 0.12 km316. The Lambert_84 subglacial lake experiences the largest filling episode by volume, filling by ~2.5 km3 from early-2015 to mid-2017, with 8 m of surface uplift observed (Supplementary Fig. 2; and Supplementary Table 1).

Subglacial lake drainage and recharge behaviour is highly variable, both between lakes and for individual lakes themselves. For example, the ice surface over subglacial lake Whillans_180 underwent 5 meters of uplift as the lake filled between 2011 and 2013, followed by a period of surface subsidence from 2013 to 2014 where we observe 5 meters of lowering as the subglacial lake drained to its previous level (Fig. 2l). After filling to approximately the same level for a second time over our study period (1 year duration), this subglacial lake then only partially drains, with 2.8 m of surface subsidence observed before refilling commences (Fig. 2l, and Table 1). In contrast, a number of subglacial lakes also fill and drain to the same level, for example, Totten_52 which was associated with 8 m of both subsidence and subsequent uplift (Fig. 2d). Additional variability is observed in “stepped” filling events (subglacial lakes Totten_52, Byrd_10, and Support Force_3, Fig. 2d, i, and n) and drainage events (subglacial lakes King Baudoin East_3 and Mellor_112, Fig. 2b, c) where we observe quiescent periods during filling or draining, and quiescent periods between draining and filling episodes. Other active subglacial lakes continually cycle through draining and filling. The Moscow University_44 subglacial lake (Fig. 2e) experienced a small drainage event over 6 months in 2017 whilst undergoing longer-term filling. Fill levels vary within individual subglacial lakes, as demonstrated by the time series of surface elevation change over Whillans_180 and Scott_12 subglacial lakes (Fig. 2l, k). Given the length scale of active subglacial lake filling and draining episodes characterised in this study, future studies will require longer-term multi-decadal observational records to improve assessments of Antarctic active subglacial lake behaviour.

Our results indicate that at least 60% of our active subglacial lakes are permanent features rather than transient hydrological signals, as they undergo either a full fill-drain cycle or the opposing mode following a fill or drain event in the same location over the decade long study period. The remaining lakes are likely also permanent features. However, as the study period was not long enough to capture a repeat cycle in these cases, further monitoring will be required to confirm this. Our surface elevation change maps showed that the active region does not always remain constant through time. We delineate time evolving boundaries for each subglacial lake, with a separate boundary for each filling and draining episode (Fig. 2; and Supplementary Fig. 2). The resulting time series of subglacial lake outlines shows that the area of subglacial lake activity-induced surface elevation change can evolve by up to 50%, in the case of two filling events at Scott_12 subglacial lake (Fig. 2k, and Table 1, Supplementary Table 1) where the area of 59.4 km2 for the 2014–2016 filling event decreases to 29.8 km2 for the later 2018–2020 filling event. The minimum amount of area change we observe between cycles is 9%, for 2 drainage events at Lambert_84 subglacial lake (Supplementary Table 1, Supplementary Fig. 2), indicating large variability on individual lakes. This result highlights the dynamic nature of the subglacial hydrological system in Antarctica and indicates that future studies must assume that this system responds to changes in conditions that vary through time.

In addition to identifying these subglacial lakes, we extended the record of activity over previously identified features in the Antarctic subglacial lake inventories10,16,32. We identify 19 lakes in the inventory of active Antarctic subglacial lakes10,26 which are active during our 2010 to 2020 study period and are also located within the CryoSat-2 SARIn mode mask study region (Fig. 1). We assessed the accuracy of the methods used in this study by comparing the timing and magnitude of all documented active subglacial lake episodes during any overlap period with previous studies (Supplementary Table 2). Overall, our results agree with the timing and magnitude of subglacial lake drainage and filling events documented in previous studies. For example, at the Mercer Ice Stream in East Antarctica we observe Subglacial Lake Mercer (SLM) draining rapidly in late 2012 with 7 m of ice surface subsidence, followed by 6 m uplift from a refilling event between early 2015 and early 2018 to almost the same level as before the previous drainage, before 4 m of rapid drainage over 1 year from early 2018 to early 2019 (Supplementary Fig. 3). These results show good correlation with a previous study on SLM activity with CryoSat-2 and ICESat-2 data25 in both the timing and magnitude of the dominant filling and drainage events and in shorter-term, small amplitude variability. Beneath the adjacent Whillans Ice Stream, our results show that subglacial Lake Conway fills from late 2010 onwards, followed by a rapid drainage episode in 2019 (Supplementary Fig. 3), a pattern also observed in Siegfried and Fricker’s study25.

The extended observational record shows new modes of activity over two previously documented subglacial lakes: Rutford 1 and Institute E1, both located in East Antarctica, along with a continuation of the previously identified activity at four locations including the existing Kamb Trunk 1, L78, Cook W1 and Cook W2 subglacial lakes on the Siple Coast (Supplementary Fig. 3, Supplementary Table 2). The latest episode of activity on the Rutford 1 subglacial lake shows a period of draining between late-2010 and mid-2014 where the ice surface lowered by 6 m, followed by 4 m of surface uplift as the lake refilled through to early 2017. Rutford 1 then drained by 3.5 m from early 2017 to late 2018, at which point it refilled through to the end of our time series in 2020. Rough topography over the Rutford Ice Stream introduced large uncertainties into previously assessed CryoSat-2 radar altimetry data33. Therefore, elevation change had not been observed over the Rutford_1 subglacial lake since an assessment of the 2003 to 2008 ICESat laser altimetry record10. Use of swath processed CryoSat-2 radar altimetry data in this study enabled the Rutford 1 subglacial lake activity to be characterised throughout the decade-long study period by retrieving an entire swath of surface height measurements at high spatial resolution, where previously only one aggregated satellite footprint had been observed using the point of closest approach method. Further activity is also observed at the Institute E1 subglacial lake located in Queen Elizabeth Land. Previously recorded filling at Institute E133 was briefly halted by a small drainage episode in 2017, after which the lake continued to fill until it drained rapidly from mid-2018 to early 2019, lowering the ice sheet surface by 3.5 m (Supplementary Fig. 3, and Supplementary Table 2). Our results show that long, annual to multi-decadal timescales are required for subglacial lake filling and drainage cycles to be fully captured, and the as-yet unpredictable nature of these events means that updates to this record will be required in the future.

We generated updated lake boundaries for 15 of the previously documented Antarctic subglacial lakes and multiple time-varying boundaries for 12 (Supplementary Fig. 3). Updated outlines are particularly beneficial for subglacial lakes first detected over a decade ago in the period 2003–2009 by ICESat laser altimetry data10 as the ground track locations and adjacent track spacing provide only partial lake coverage, leading to a more generalised circular boundary delineation. For example, in the David Glacier catchment in East Antarctica, the David 2 subglacial lake boundary was inferred from a single ICESat track (Fig. 4a)10. The improved coverage from the swath-processed CryoSat-2 data allows a more elongated subglacial lake area to be measured along the north-south axis of the original location, in addition to indicating that the feature is centred 6 km west of the previously delineated boundary (Fig. 4a). An area of surface uplift between two areas of subsidence on David 2 has been identified here via aero-geophysical surveys of the area by Lindzey and others34, a pattern consistent with our results (Fig. 3a). This study suggests that the observed subsidence is a feature reflecting water flowing in and out of the lake, which is plausible given that both the hydraulic potential gradient and ice flow direction is approximately west-to-east. While it is possible that the lake area that drained in the historical ICESat period is not precisely the same as the area that drained in the more recent 2012 to late-2019 event, it is likely that the higher spatial resolution elevation change data with good spatial coverage did also aid a more accurate delineation of the subglacial lake boundary.

Previous studies showed that the Institute E1 subglacial lake (Fig. 3b) has an area of 460 km2 when delineated from five ICESat satellite tracks, with one documented filling episode observed between late 2003 to early 200810. Our results show that a subsequent drainage episode occurred on this lake between 2018 and 2020, with the lake area confined to a smaller, more elongated region which extends downstream (Fig. 3b). We observed an earlier period of filling from 2014 to 2018 which occurred with two main centres of uplift within this elongated area (solid black lines, Fig. 3b). Our surface elevation change results indicate that there is a second centre of activity adjacent to, but distinct from, the main Institute E1 lake area, which we prescribe as a separate lake called Institute 142. Overall, the David 2 and Institute E1 and 142 areas of dynamic subglacial hydrology show the small-scale variability of surface elevation change caused by subglacial lake drainage and filling episodes (Fig. 3), demonstrating the need for high spatial resolution satellite observations to accurately monitor these events. Due to both this and continual improvement to satellite-derived surface elevation datasets, subglacial lake boundaries should be updated over time to improve their accuracy, to document the time-variable regions of change and to avoid under-estimation of surface elevation change magnitudes. Consequently, differences between historical subglacial lake boundaries and those presented in this study (Supplementary Fig. 3) are largely attributable to differences in study time period and satellite data product resolution from which they were produced.

Methods
Ice sheet elevation change
We use ~15×109 individual swath-processed55 elevation measurements over Antarctica from ESA’s CryoSat-2 radar altimeter satellite mission to investigate ice surface height changes for the period October 2010 to July 2020 (Supplementary Fig. 1). This enables us to identify areas of spatially consistent height change indicative of subglacial lake activity. With a latitudinal limit of 88° and a monthly sub-cycle, operating in Synthetic Aperture Interferometric (SARIn) mode over the ice sheet margins56, the CryoSat-2 radar altimeter instrument acquires surface height measurements at ~400 m horizontal spatial resolution over Antarctica. Swath-processing utilises phase information to unwrap the data across the track, providing a swath of elevation data rather than a single measurement at the point of closest approach (POCA)55. This yields a substantial increase in the spatial density of the altimetry data points, which in turn enables the subsequent surface elevation change processing steps to be run on grids at a 10-times finer spatial resolution than possible using traditional approaches. This makes swath processed CryoSat-2 data ideal for detecting relatively small subglacial lake signals around the Antarctic Ice Sheet margins.

We separate elevation fluctuations evolving over time from those resulting from local topography and temporal variations in radar backscatter within 500×500 m grid cells using a model fit method57–59. Firstly, we remove unreliable swath elevation measurements where the coherence value is <0.8, the scaled power is <10,000 and the difference with respect to a reference DEM60 is >25 m. We model the elevation z as a function of local surface terrain (x,y), time (t), satellite heading (h) and the mean height within each grid cell (Δz):

z(x,y,t,h) = Δz + a1x + a2y + a3x2 + a4y2 + a5xy + a6h + a7t (1)

The satellite heading (h) is a binary term set to 0 or 1, based on whether the satellite track is ascending or descending, respectively. We solve Eq. 1 within each grid cell using an iterative least-squares fit to minimise the impact of outliers and discard poor-ly constrained solutions based upon the following set of statistical thresholds: a minimum of 10 time points, a time series length of at least 2 years, a maximum root mean squared difference of elevation residuals from the model of 12 m, a maximum elevation rate magnitude of 10 m yr−1, and a maximum surface slope of 5 °58. We compute the maximum surface slope from static BedMap2 ice surface elevation data60.

We account for spurious variations in range associated with changes in radar echo shape by applying an empirical correction based upon correlated changes in elevation and backscattered power, consistent with existing CryoSat-2 SARIn altimetry studies58,59. To do this, we compute the correlation gradient in elevation as a function of power, dz/dp, using a linear fit in each grid cell. The change in backscattered power dp is multiplied by this correlation gradient and then subtracted from the model elevation anomalies to produce power-corrected anomalies59:

dz corrected = dz - (dz/dp) dp (2)

The elevation anomalies are averaged over 90-day epochs to produce mean elevation change time series within subglacial lake boundaries and their surrounding area. We estimate the uncertainty in our regional elevation time series at each epoch by computing the average of the standard error (σ = σ/√n) of the model elevation anomalies within all contributing grid cells57,58. We assume this component is temporally uncorrelated, and for each epoch we then accumulate all preceding uncertainties in quadrature57.

Subglacial lake boundaries and elevation change time series
We use our 10-year elevation change map (Oct 2010–July 2020) to identify potential subglacial lake points of interest (POI). To maximise the likelihood of ice surface height change reflecting real subglacial lake activity we pinpoint areas of spatially consistent surface height change ≥5 km that are distinct from measurement noise. Time series are first extracted from a 5 km box centred around each active subglacial lake POI, and a surrounding box 20 km in either direction which excludes the subglacial lake area, which we class as the non-subglacial lake region (Supplementary Fig. 1a). Each mean elevation data point is normalised relative to the first timestep’s elevation to yield a final elevation change time series. We remove any large scale regional surface elevation change signals not linked to subglacial lake activity, such as from ice dynamic thinning57, by calculating the 10-year linear elevation trend calculated from the 20 km non-subglacial lake box region and subtracting this from the subglacial lake region. These time series are then compared to determine locations featuring distinct elevation change above the background non-subglacial lake elevation change (Supplementary Fig. 1b), which we interpret to be periods of significant lake activity.

We note that processes such as the viscous deformation of ice or the impact of ice flow may alter the surface impression of behaviour at the ice bed61. The use of “subglacial lake boundaries” in this manuscript refer to the surface impression of elevation change observed due to subglacial lake filling and draining, rather than a direct delineation of the physical subglacial water edge. In order to measure the maximum extent of ice elevation change over each subglacial lake, we produce a new surface elevation change map for the discrete period of surface uplift or subsidence as determined from the elevation change time series. Surface elevation change maps produced from only part of a filling or drainage event cycle will result in a partial elevation change signal in the gridded maps. In the event that the map is produced over the full period of the cycle, there will be zero elevation change visible in the gridded data making it impossible to accurately delineate the subglacial lake boundary. We use the maximum elevation change maps to manually delineate boundaries for each subglacial lake, producing a separate lake boundary for every filling and draining event (Supplementary Fig. 1c). We draw the lake contour at the ± 0.3 m yr-1 elevation change threshold, based on noise sensitivity testing, and we adjust this depending on the signal magnitude, altimetry data coverage, and elevation patterns compared to the surrounding ice. Time series of ice surface elevation change are then re-extracted from within each subglacial lake boundary, including pixels whose centres fall within the boundary. We adjust the non-subglacial lake box size to equal the maximum subglacial lake diameter plus 5 km, before also re-extracting time series of surface elevation change for each non-subglacial lake region. All time series are once again detrended using a newly calculated 10-year long linear trend and the subglacial lake active periods are updated based on this second processing iteration with the more accurate lake boundary information. We perform a final processing iteration by smoothing each subglacial lake elevation change map using a median filter with a kernel size 10 % of the lake polygon area or 2 km2, whichever is smaller. We compare the raw and smoothed data to ensure they agree, and we update the lake boundaries with the smoothed data in regions where noise prevented a continuous outline being drawn from the raw data exclusively. Following this final iteration, we re-extract the elevation change time series from each active subglacial lake. We assess uncertainty in our subglacial lake boundary delineation technique by drawing boundaries for three subglacial lakes, five times each62, to represent variability with subglacial lake size: King Baudouin West_51 (19.74 km2), Mill_165 (35.8 km2) and Borchgrevink_198 (78.49 km2) (quartile lakes by area). The pooled standard deviation of the subglacial lake area measurements is 0.453 km2, which reflects the 500×500 m spatial resolution of the swath-processed altimetry dataset. Therefore, we assume this standard deviation to be the uncertainty on our subglacial lake area measurements.

Volume change estimation
To estimate the magnitude of water movement beneath the ice sheet from this subglacial lake activity, we calculated the volume of surface ice displaced for each filling and drainage event. We note that equating these two volumes varies in accuracy as a function of ice thickness, subglacial lake drainage rate and basal drag coefficient61. We therefore perform a simple volume calculation (Eq. 3) and suggest coupled ice-hydrology models for the subglacial lake networks presented in this study for future work, in order to assess variability in the surface-bed relationship. We do not provide volume change estimates for 79 cases (53%) where the subglacial lake boundary was not delineated for a discrete period of activity. This usually corresponds to more rapid (less than 2 years) filling or drainage events where poorer altimetry data coverage leads to higher noise levels in the surface elevation change maps, making it hard to delineate the lake boundary. Volume change, dV, is estimated using lake area, A, and mean surface elevation change, dz, from each time period of lake activity, t, as follows:

dV t = dz t × A t (3)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of manuscript “Detection and variability of 85 new active Antarctic subglacial lakes with CryoSat-2 radar altimetry, 2010-2020” by Sally F. Wilson, Anna E. Hogg, Richard Rigby, Noel Gourmelen, Thomas Slater, and Isabel Nias. Submitted to Nature Communications.\n\nOverall, I found the manuscript to be well-written and well-structured, and I enjoyed reading it. The results are important and advances current knowledge on subglacial hydrology in Antarctica. There are somethings elements that are less well explained, and some methods and arguments that should be either revised or justified better. These are described below.\n\nKey results:\nThe study uses CryoSat-2 swath-processed radar altimetry data to monitor surface elevation changes over Antarctica between 2010 and 2020, identifying 85 new active subglacial lakes. A key contribution of the study is the creation of a time-evolving dataset of subglacial lake boundaries, capturing large variation in active lake areas. The study findings provide important insights into the subglacial water flux beneath the Antartica Ice Sheet.\n\nValidity:\nWhile the detection of new lakes is very timely and interesting and the identification of activity patterns is promising, I have a few concerns regarding the interpretation of some of the results.\nSpecifically, the delineation of subglacial lake perimeter outlines from surface change signals is potentially overinterpreted. The assumption that surface expressions reflect the true subglacial lake boundaries is not adequately justified in the current version of the manuscript. Previous work (e.g., Stubblefield et al., 2021) has shown that surface depressions may not reliably indicate subglacial lake extents due to the influence of viscous ice flow, and this should be addressed more specifically. Maybe it is simply a question of the terminology, so that you do not claim to delineate subglacial lake perimeters but rather subglacial lake expressions or signals?\nAdditionally, the volume estimates are derived from a simple equation (Eq. 3) that lacks detailed justification, and it is unclear if this method meets current standards in the field (Stubblefield et al., 2021).\n\nSignificance\nThe study presents a significant expansion of the known inventory of active subglacial lakes in Antarctica and it offers a rare, decade-long dataset of lake activity. This contributes to our understanding of Antarctic subglacial hydrology, which is a topic that is important and timely.\nHowever, the significance of the volume estimates is less clear, especially given uncertainties in methodology. The most impactful findings relate to the detection, variability, and spatial distribution of the lakes rather than their calculated volumes. Therefore, I would recommend that the authors reconsider if the lake volume estimation actually adds anything crucial – and if not, that they take that part of the study.\n\nImpact: I would like to see the authors elaborate on the impact of their study. How exactly will our understanding be improved by the detection of these new active subglacial lakes? I see one obvious one, being the fact that basal melt is currently neglected in mass balance studies of the AIS. And as you state in L 30-31 the SGL variability can inform us on the water availability at the base.\n\nData and Methodology\nThe use of CryoSat-2 radar altimetry to track surface elevation changes is appropriate and well established. The spatial and temporal resolution is very suitable for the research objectives.\nHowever, I find that parts of the methodology needs improved explanation.\nThe approach to assessing uncertainty in subglacial lake delineations (manual drawing of a few lake perimeters is simplistic and I believe it does not fully address an important source of uncertainty: the mismatch between surface signal and actual lake geometry. This needs to be made clearer throughout the manuscript.\nFurther with respect to the delineation; outlines are provided less frequent than the elevation change results. How come? I would think that the outlines would change continuously and that more regular outlines (and hence areas) are needed for the volume calculation. Here, I am especially thinking of the outline change caused by ice deformation.\nWhen removing outliers (L. 88) a threshold is applied on how far from a reference DEM the altimetry data can be to be included. The threshold is 25 m. Does this mean that you will not be able to detect surface depressions deeper than 25m? Does this not potentially filter out some subglacial lake variability signals?\nCan you be more specific about what the standard error of the height change is? How is that derived? (L. 101).\n\nAnalytical Approach\nThe analytical approach is generally sound in terms of identifying elevation change events and characterising their timing and extent. However, there are a few things that could be improved:\nPlease elaborate on the correlation described in Eq. 2. It is not clear to me why you would expect such a relationship in swath-processed altimetry data. The Davis et al., (2004) reference is for a study using conventional RA from ERS-2, using a retracking algorithm that can be sensitive to correlations between the waveform and estimated height.\nAs mentioned above, the volume estimation via Eq. 3 requires additional justification and comparison with more advanced techniques. Reference to recent work such as Stubblefield et al. (2021) is warranted and should be included to contextualize the limitations of the current approach.\n\nFurther suggested (mostly technical) improvements:\n- The current title, \"Detection and variability of 85 new active Antarctic subglacial lakes with CryoSat-2 radar altimetry, 2010–2020\", appears somewhat awkward in phrasing to me. Particularly the construction \"variability ... with\" may suggest that variability is being performed using the instrument, which is conceptually unclear. I recommend rephrasing for clarity and grammatical correctness. For example: \"Detection of variability of 85 new active Antarctic subglacial lakes from CryoSat-2 radar altimetry (2010–2020)\".\n- L 14: what is meant by worldwide here? You are only discussing subglacial lakes in Antarctica.\n- L 16 do you mean complete cycle or complete drainage?\n- L 17 at this stage it is not clear what a cluster or a network refers to\n- L 22 I would change valleys to depressions.\n- L 37: provide exact number instead of percentages.\n- Equation 1: I believe that you have not explained all the terms in the Eq.\n- L 174-175 I would suggest to not provide the average time periods as I do not see how these numbers provide any useful insight, since these lakes are located in different regions and have different driving processes.\n- Table 1: Please include uncertainties\n- Fig 1. The scale number for Mill 165 is partly hidden behind the figure.\n\nReferences:\nDavis, C. H. & Ferguson, A. C. Elevation change of the Antarctic ice sheet, 1995-2000, from ERS-2 satellite radar 573 altimetry. IEEE Trans. Geosci. Remote Sensing 42, 2437–2445 (2004).\nStubblefield, A. G., Creyts, T. T., Kingslake, J., Siegfried, M. R., & Spiegelman, M. (2021). Surface expression and apparent timing of subglacial lake oscillations controlled by viscous ice flow. Geophysical Research Letters, 48(17), e2021GL094658.", "reviewer_2": "General comments:\nThis manuscript uses CryoSat-2 radar altimetry swath-processed surface elevation data to develop a new dataset of active subglacial lake boundaries and time series of their filling and draining over a decade (2010-2020). The authors document complete filling and draining cycles and identify a number of interconnected subglacial lake networks. They found a total of 85 active subglacial lakes and five interconnected lake networks and conclude that individual lake areas can change by up to 280% over multiple periods of filling and draining.\n\nHydrologically active lakes are an important, yet poorly constrained component of the ice sheet subglacial hydrological system. Active lakes can transport excess meltwater episodically towards the grounding line, where it is discharged into sub-ice shelf cavities and can enhance ice shelf basal melting. Subglacial hydrology beneath the Antarctic Ice Sheet remains poorly understood and challenging to represent in ice-sheet models. Observing active lakes using repeat satellite data is therefore crucial to understanding the impact of subglacial hydrology on ice dynamics and the ice sheet-ocean system. Few studies have produced ice sheet-scale inventories of active subglacial lakes, and studies using different satellite sensors each have their own limitations, e.g. limited coverage.\n\nIt is my view that this study is of broad interest to the cryospheric community as it builds upon previous work focused on subglacial lake detection beneath the Antarctic Ice Sheet, especially in the context of subglacial hydrology and ice-sheet dynamics, by extending the record of active subglacial lake activity and their time-evolving filling and draining behaviour. I think the ice sheet modelling community would be very interested to use this dataset as validation at an ice-sheet scale in future. Overall, this is a well-written manuscript which could be clearer some places, detailed by my specific comments.\nOnce the authors address these, I can therefore recommend that this manuscript is suitable for publication in Nature Communications.\n\nI have a few general points, which I outline below:\n• In the abstract and several other places in the manuscript, 5 new subglacial lake networks and 25 clusters are mentioned. However, it is unclear to me what these 25 clusters refer to – are these lakes that intersect with modelled subglacial water flux (LeBrocq et al., 2013)? From Figure 1, it is unclear which of the 85 new lakes constitute these 25 clusters, and it is difficult to see in any detail the relationship between the authors’ new reported active lakes and modelled subglacial water flux. This latter point is not really elaborated on in the manuscript.\n• The manuscript does a great job of validating lake outlines using previously reported lakes derived from other sensors. I would like to suggest that the results of our recent study (Arthur et al., 2025) would be a valuable addition to this comparison, as it provides one of a few records of active subglacial lakes near the grounding line with several lakes coincident or located near those identified in this study. Specifically, locations such as Lazarevisen (coincident with our Lake L1), Vigridisen (46 km upstream of our lake V1), Muninisen (170 km upstream of our lakes M1 and M2) and Roi Baudouin (78 km downstream of our Lake R2) could provide useful points of comparison. I have included two of these examples below. In particular, relevant discussion could be added to the intercomparison of subglacial lake inventories (Section 4.1) and in Supplementary Figure 3.\n• In the Discussion (Section 4.3), I suggest mentioning the lake cluster upstream of Jutulstraumen (Jutlustraumen_69, _71, _74), which you detect filling and draining between 2013–2015 and 2017–2020. This cluster is located 81 km upstream of the active lake cluster reported by Neckel et al. (2021) using Sentinel-1 DInSAR, validated with ICESat-2, and observed to fill and drain between 2017–2020 (see right-hand map). Given their proximity and temporal activity, these lakes may form part of a larger interconnected network and could be associated with cascading drainage.\n• One of the interesting findings from this new inventory is the detection of active lakes near the grounding line in certain locations (e.g. King Baudouin East_3, less than 4 km from the GL). I think this deserves further emphasis (for example, an additional figure highlighting some of these new examples) due to its important implications for subglacial water routing and the impact on local ice dynamics at the transition between grounded and floating ice in these regions.\n• As a general point, some of the discussion about future work suggestions could be more suited to Section 4.5 (Future Outlook), for example the last few sentences of Section 4.1 and 4.2.\n• It is good to see details of the subglacial lakes, drainage/filling periods and associated area and volume changes provided in the Supplementary Data. I would encourage the authors to consider making their code available to reproduce the results presented in this manuscript, in line with Nature Communication guidelines and to promote reproducibility.", "reviewer_3": "The authors present a new inventory of Antarctic subglacial lakes derived from CryoSat-2 altimetry data extending current inventories by 85 (+ 58%) newly detected lakes. These newly detected lakes were mapped in higher spatial resolution than previously detected lakes by laser altimeters and the presented time series on lake variability allow detailed analysis on lake dynamics. For every newly detected lake, the authors present time series on lake drainage and fill events between the period 2010-2020.\n\nEven though, I am not an altimetry expert the methodology employed appears to be both robust and comprehensively delineated. Especially making use of the POCA processing approach allows a much finer spatial resolution for lake mapping. Therefore, this study presents a much more detailed and accurate dataset compared to the existing inventory as can be seen in Figure 3. It is positive to note that the shapefiles of the newly detected lakes have been published open access. The text is clear and accessible, with results presented in a well-structured manner.\n\nThe significance of the work is given in any case due to the importance of Antarctica’s subglacial hydrology for ice loss (Fricker et al. 2025). Nevertheless, for the scope of Nature Communications I would have expected more insights regarding the process understanding of subglacial lake activity and distribution. The authors define lake variability and behavior into seven categories (L 351 ff.) and provide insights into connected subglacial networks (L 367ff.). But the data produced within this study would facilitate a greater level of insight with minimal effort. The following ideas could be pursued:\n- The supplementary includes all time series of the 85 analysed lakes. There seems to be a strong seasonality in the drainage and filling events. Did you have a look into the seasonality (e.g. detrending and preforming a seasonality analysis on time series with sufficient measurement points). Why is there strong variability in some years but not in other?\n- Subglacial lakes seem to be clustered in some specific areas (Figure 1). Can you explain why?\n- The lakes are categorized into seven categories. How is their spatial distribution? Have you tried to create a map based on these categories and where you able to identify any spatial patterns?\n\nSpecific comments:\nFigure1: Add the CryoSat data acquisition limit at the South Pole to the map to show where potential active lakes might have been detected.\nTable 1 and L 139 ff. Can you provide any uncertainties for the volume change estimate? In L 157 you mention that the height change signal at the surface is attenuated by thick ice and hence the change in lake level is very likely different form the surface elevation change and might introduce large errors.\nTable 1: What is the reason for your choice of these 13 lakes in particular?\nL174: Also consider providing the median and quantiles for drainage events. Just the mean average could be miss leading in case of strongly varying time spans.\nL 328: You mention that subglacial lakes can exist closer to the grounding line (GL) than previously thought. A recent study by Arthur et al. 2025 also mentions subglacial lakes close to the grounding line. Consider discussing these findings with yours.\nL332: You used the MEaSUREs grounding line. Did you compare the distance between the GL and your detected lakes with a newer grounding line product?\nFigure 4: Here you present the drainage of Francais_46 and subsequent filling of Francais_36. Did you do any statistical analysis on the sub-annual fluctuations and the connection between drainage and filling? For example, in 2015 F_46 re-fills and drains half a year later. Do you see in return a filling signal in F_36 with a temporal lag?\nSupplementary: There seem to be some lakes that experienced a surface lowering but lake area expansion (e.g. Jutulstraument_71, Fisher_168). Is this a common pattern and what is the explanation behind?\n\nReferences\nArthur, J.F., Shackleton, C., Moholdt, G., Matsuoka, K., van Oostveen, J., 2025. Evidence of active subglacial lakes under a slowly moving coastal region of the Antarctic Ice Sheet. The Cryosphere 19, 375–392. https://doi.org/10.5194/tc-19-375-2025\nFricker, H.A., Galton-Fenzi, B.K., Walker, C.C., Freer, B.I.D., Padman, L., DeConto, R., 2025. Antarctica in 2025: Drivers of deep uncertainty in projected ice loss. Science 387, 601–609. https://doi.org/10.1126/science.adt9619"}, "author_response": "Response to referees: Detection and variability of 85 new active Antarctic\nsubglacial lakes with CryoSat-2 radar altimetry, 2010–2020\nRef: NCOMMS-25-13112\nWe thank each reviewer for their positive and constructive comments. Please see a response to each\ncomment addressed in the table below.\nReviewer 1:\nID Comment Line Response\n1.1 Overall, I found the manuscript to be well-written We thank reviewer 1 for their time, positive\nand well-structured, and I enjoyed reading it. The comments and helpful feedback.\nresults are important and advances current\nknowledge on subglacial hydrology in Antarctica.\nThere are somethings elements that are less well\nexplained, and some methods and arguments\nthat should be either revised or justified better.\n1.2 The delineation of subglacial lake perimeter 128 Done. The subglacial lake “perimeter” / “boundary”\noutlines from surface change signals is potentially terminology used in the manuscript is used to define\noverinterpreted. The assumption that surface the surface expression of this feature, so we thank\nexpressions reflect the true subglacial lake the reviewer for highlighting the potential for\nboundaries is not adequately justified in the misunderstanding here. We agree that there are\ncurrent version of the manuscript. Previous work factors, including viscous ice flow (Stubblefield et al.,\n(e.g., Stubblefield et al., 2021) has shown that 2021) that might cause the surface expression of the\nsurface depressions may not reliably indicate subglacial Lake (SGL) to not fully reflect water\nsubglacial lake extents due to the influence of movement at the ice base.\nviscous ice flow, and this should be addressed\nmore specifically. Maybe it is simply a question of To address the comment we have moved the\nthe terminology, so that you do not claim to following sentence, from section 2.3, to the first\ndelineate subglacial lake perimeters but rather mention of delineating lake boundaries in section\nsubglacial lake expressions or signals? 2.2. We have acknowledged the point also in the\ndiscussion. We hope this clarifies that our ‘boundary’\nterminology relates to the surface expression of a\nfeature at the ice base that we do not directly\nobserve. We have also added in the reviewers\nrecommended citation, to highlight an example of\nthe processes that might prevent this assumption\nfrom holding:\n“We note that processes such as the viscous\ndeformation of ice or the impact of ice flow may\nalter the surface impression of behaviour at the ice\nbed (Stubblefield et al., 2021).”\n<line 156>\nWe add the following here, for clarity:\n“The use of “subglacial lake boundaries” in this\nmanuscript refers to the surface impression of\nelevation change observed due to subglacial lake\nfilling and draining, rather than a direct delineation\nof the physical subglacial water edge. In order to\nmeasure the maximum extent of ice elevation\nchange over each subglacial lake, (…)”\n<line 157>\nWe also add to the discussion:\n“Our subglacial lake boundaries and volume\nestimates also provide new case studies for\ninvestigating the correspondence between basal and\nsurface dynamics and the effect of ice flow on\nsubglacial signals observed from the surface.”\n<line 904>\n1.3 Additionally, the volume estimates are derived Done. We note reviewer 1’s concerns with the\nfrom a simple equation (Eq. 3) that lacks detailed volume estimate calculation. However, reviewer 2\njustification, and it is unclear if this method has conversely stated that they value having these\nmeets current standards in the field (Stubblefield numbers presented in the manuscript. Given the\net al., 2021). difference in opinion we have decided to retain the\nvolume estimate calculations as we do think they will\nbe of interest to the community. We have also\nensured that Equation 3 clearly states the method\nused to calculate this number.\nWe also now include the following two sentences in\nsection 2.3:\n“We note that equating these two volumes varies in\naccuracy as a function of ice thickness, subglacial\nlake drainage rate and basal drag coefficient\n(Stubblefield et al., 2021). We therefore perform a\nsimple volume calculation (Equation 3) and suggest\ncoupled ice-hydrology models for the new subglacial\nlake networks presented in this study for future\nwork, in order to assess variability in the surface-bed\nrelationship.”\n<line 195>\nTo reflect the reviewer’s concern, we have taken care\nto not state the volume estimate numbers in the\nabstract or conclusions, therefore giving them\nslightly lower prominence in the manuscript. We\nhave also only included volume estimates, both in\nTable 1 and Supplementary Table 1, where we have\nlake boundaries delineated from surface elevation\nchange maps produced from a 2-year period of\nsurface elevation change measurements. Where\nboundary delineation does not allow us to calculate\nan area within this 2-year period, due to data\ncoverage or the short time frame for some\ndrainage/filling events, we mark this with a “-“ in\nTable 1, and a blank space in the Supplementary\nTable.\n1.4 The study presents a significant expansion of the Thank you for your comments on the significance\nknown inventory of active subglacial lakes in and timeliness of our study.\nAntarctica and it offers a rare, decade-long\ndataset of lake activity. This contributes to our\nunderstanding of Antarctic subglacial hydrology,\nwhich is a topic that is important and timely.\n1.5 However, the significance of the volume Done. We have addressed reviewer 1’s concerns\nestimates is less clear, especially given about the volume change numbers in answer to\nuncertainties in methodology. The most comment 1.3 above. Please see this comment.\nimpactful findings relate to the detection,\nvariability, and spatial distribution of the lakes\nrather than their calculated volumes. Therefore, I\nwould recommend that the authors reconsider if\nthe lake volume estimation actually adds\nanything crucial – and if not, that they take that\npart of the study.\n1.6 Impact: I would like to see the authors elaborate Done. Thank you, this is a great point, and we have\non the impact of their study. How exactly will our provided more detail in discussion section 4.1 of the\nunderstanding be improved by the detection of paper:\nthese new active subglacial lakes? I see one\nobvious one, being the fact that basal melt is “Combining our new subglacial lakes with those\ncurrently neglected in mass balance studies of documented in existing inventories provides a more\nthe AIS. And as you state in L 30-31 the SGL comprehensive picture of subglacial lake distribution\nvariability can inform us on the water availability in Antarctica (Fig. 1). This is important as subglacial\nat the base. lake dynamics are currently not accounted for in ice\nmass loss projections, yet subglacial discharge can\nhave significant impacts on ocean melting of ice\nshelves, ice stream thinning and acceleration\n(Gourmelen et al., 2025). Furthermore, subglacial\nlake variability detected in this study can inform us\non\non water availability at the ice base, projections of\nwhich have thus far been limited due to a scarcity of\nobservations.”\n<line 581>\n1.7 The use of CryoSat-2 radar altimetry to track We thank reviewer 1 for this positive assessment of\nsurface elevation changes is appropriate and well the dataset chosen for our research objectives.\nestablished. The spatial and temporal resolution\nis very suitable for the research objectives.\nHowever, I find that parts of the methodology Done. We have addressed the reviewer’s concerns\nneeds improved explanation.\nregarding surface signal vs. subglacial water\nThe approach to assessing uncertainty in geometry in 1.3 above. Please see this comment.\nsubglacial lake delineations (manual drawing of a\nfew lake perimeters is simplistic and I believe it\ndoes not fully address an important source of\nuncertainty: the mismatch between surface\nsignal and actual lake geometry. This needs to be\nmade clearer throughout the manuscript.\nFurther with respect to the delineation; outlines\nare provided less frequent than the elevation Comment. The outlines provided are less frequent\nchange results. How come? I would think that the because, as stated in the methods, “We do not\noutlines would change continuously and that provide volume change estimates for 79 cases (53%)\nmore regular outlines (and hence areas) are where the subglacial lake boundary was not\nneeded for the volume calculation. Here, I am delineated for a discrete period of activity. This\nespecially thinking of the outline change caused usually corresponds to more rapid (less than 2 years)\nby ice deformation.\nfilling or drainage events where poorer altimetry\ndata coverage leads to higher noise levels in the\nsurface elevation change maps, making it hard to\ndelineate the lake boundary.”\nAs stated by reviewer 1, subglacial water areas do\nchange continuously. However, indirect signals of this\nat the ice surface are only able to be delineated over\nlonger time periods due to measurement noise in\nswath data for periods shorter than 2 years. For this\nreason, we implemented a minimum threshold of 2\nyears of data in the methods section (line 123): “We\nsolve Eq. 1 within each grid cell using an iterative\nleast-squares fit (…) based upon the following set of\nstatistical thresholds: (…), a time series length of at\nleast 2 years…”\nFurthermore, we investigate the magnitude of lake\nsignal detectable with our processed CryoSat-2 data\nin response to reviewer 2’s comment below. Please\nsee comment 2.14 for more details.\n1.8 When removing outliers (L. 88) a threshold is 88 Comment. This threshold is increased from that used\napplied on how far from a reference DEM the in previous published work (Slater et al. (2021) use a\naltimetry data can be to be included. The 10 m/yr threshold), so is in line with methods best\nthreshold is 25 m. Does this mean that you will practice for surface elevation change studies. We\nnot be able to detect surface depressions deeper increase this threshold from 10 m/yr to 25 m/yr to\nthan 25m? Does this not potentially filter out allow our method to capture “fast” subglacial lake\nsome subglacial lake variability signals?\nsurface elevation change signals, determined by\nreviewing existing literature.\nThe threshold we have applied means that the\nelevation change rate would need to be more than\n25 m/yr to be removed as an outlier, which would be\nmuch larger than any previously observed SGL\ndrainage rate. To the best of our knowledge, the\nlargest subglacial lake elevation change signal that\nhas been observed is 20 m subsidence over\nThwaites_124 subglacial lake, over an 18-month\nperiod, 2011-2013 (Smith et al., 2017).\nIn our paper, we have not measured elevation\nchange larger than 10 m on any individual lake, and\nthis change happened over 5 years (on\nMoscowUniversity_19 subglacial lake).\n1.9 Can you be more specific about what the 101 Done. We have edited the text to be more specific\nstandard error of the height change is? How is regarding the standard error (ie. Stdev/sqrt(n)) of\nthat derived? (L. 101).\nthe modelled elevation anomalies in each grid\ncell.\n“The elevation anomalies are averaged over 90-day\nepochs to produce mean elevation change time\nseries within subglacial lake boundaries and their\nsurrounding area. We estimate the uncertainty in our\nregional elevation time series at each epoch by\ncomputing the average of the standard error\n(sigma/sqrt(n)) of the model elevation anomalies\nwithin all contributing grid cells (Shepherd et al.,\n2019, Slater et al., 2021).”\n<lines 139>\n1.10 The analytical approach is generally sound in Done. Power corrections are now routinely applied\nterms of identifying elevation change events and to all radar altimetry surface elevation change\ncharacterising their timing and extent. However, studies (McMillan et al., 2014; Gourmelen et al.,\nthere are a few things that could be improved: 2018), so we have applied this correction to stay in\nPlease elaborate on the correlation described in line with that. The purpose of this study was to apply\nEq. 2. It is not clear to me why you would expect existing SEC methods to a new time period and\nsuch a relationship in swath-processed altimetry region to detect new SGL activity, so it was out of\ndata. The Davis et al., (2004) reference is for a scope for us to change the surface elevation\nstudy using conventional RA from ERS-2, using a processing chain substantially here. We have\nretracking algorithm that can be sensitive to updated the text and citation for clarity to address\ncorrelations between the waveform and this comment.\nestimated height.\n“We account for spurious fluctuations in range\nassociated with changes in radar echo shape by\napplying an empirical correction based upon\ncorrelated changes in elevation and backscattered\npower, consistent with existing CryoSat-2 SARIn\naltimetry studies (McMillan et al., 2014; Slater et al.,\n2021). To do this, we compute the correlation\ngradient in elevation as a function of power, dz/dp,\nusing a linear fit in each grid cell. The change in\nbackscattered power dp is multiplied by this\ncorrelation gradient and then subtracted from the\nmodel elevation anomalies to produce power-\ncorrected anomalies (McMillan et al., 2014):\n*Equation 2*\n.”\n<line 134>\n1.11 As mentioned above, the volume estimation via Done. Please see our response to Reviewer 1\nEq. 3 requires additional justification and comment 1.3 for a comprehensive discussion of the\ncomparison with more advanced techniques. volume change estimations.\nReference to recent work such as Stubblefield et\nal. (2021) is warranted and should be included to\ncontextualize the limitations of the current\napproach.\n1.12 The current title, \"Detection and variability of 85 1 Done. We thank reviewer 1 for highlighting the\nnew active Antarctic subglacial lakes with grammar in our title. In response, we have edited the\nCryoSat-2 radar altimetry, 2010–2020\", appears title and removed punctuation to align with the\nsomewhat awkward in phrasing to me. Nature Communications guidelines:\nParticularly the construction \"variability ... with\" “Detection of 85 new active subglacial lakes in\nmay suggest that variability is being performed Antarctica from a decade of CryoSat-2 radar\nusing the instrument, which is conceptually altimetry data”\nunclear. I recommend rephrasing for clarity and\ngrammatical correctness. For example:\n\"Detection of variability of 85 new active\nAntarctic subglacial lakes from CryoSat-2 radar\naltimetry (2010–2020)\".\n1.13 What is meant by worldwide here? You are only 14 Comment. Worldwide means across the world.\ndiscussing subglacial lakes in Antarctica. Active subglacial lakes have been found under the\nGreenland Ice sheet (Bowling et al., 2019) and under\nsome glaciers and ice caps (Björnsson, 2003). As the\nstatistic doesn’t relate to only Antarctica, we have\nstated the larger relevant geographical region.\n1.14 Do you mean complete cycle or complete 16 Done. We define a complete cycle as trough to\ndrainage? trough or peak to peak, i.e. both filling and draining\ncombined. A complete drain, or complete fill, would\nbe only half a cycle. We define these terms in line 63\nof the paper.\n“Of the 779 subglacial lakes identified globally, 681\nare located in Antarctica, 20 % (146) of which have\nexhibited surface elevation changes suggestive of\nlake draining and filling cycles 17,18. An observed cycle\nrefers to a subglacial lake filling event along with its\nsubsequent drainage event. “Active” subglacial lakes,\nwhich exhibit these cycles, were first observed (…)”\nIt was not possible to include these definitions in the\nabstract of this paper for length constraint reasons,\nso we hope it is helpful to clarify that the terms are\nfully defined early in the paper.\n1.15 At this stage it is not clear what a cluster or a 17 Done. We have updated the text where we define\nnetwork refers to clusters and networks.\n“ … five new subglacial lake networks, with\nconcurrent upstream lake draining and downstream\nfilling, and 25 lake cluster located within a small\ngeographical region, improving our knowledge of…”\n<line 18>\n1.16 I would change valleys to depressions. 22 Comment. We use the word “subsidence”\nconsistently in the manuscript to describe surface\nelevation change due to subglacial lake drainage.\nTherefore, we have decided to keep this wording as\n“subglacial valleys” here, to avoid confusion for the\nreader between small surface changes vs. relatively\nlarge magnitude variability in bedrock topography.\n1.17 Provide exact number instead of percentages. 37 Done. We have added the absolute numbers in\nbrackets next to the percentage as requested, so\nreaders can read the metric they prefer.\n“Of the 779 subglacial lakes identified globally, 681\nare located in Antarctica, 20 % (146) of which have\nexhibited surface elevation changes suggestive of\nlake draining and filling cycles.”\n<line 61>\n1.18 Equation 1: I believe that you have not explained E1 Done. We have updated this text to ensure all terms\nall the terms in the Eq. are described.\n“We model the elevation 𝑧 as a function of local\nsurface terrain (𝑥,𝑦), time (𝑡), satellite heading (ℎ)\nand the mean height within each grid cell (𝑧̅):\n𝑧(𝑥,𝑦,𝑡,ℎ)=𝑧̅+𝑎 𝑥+𝑎 𝑦+𝑎 𝑥2 (1)\n0 1 2\n+𝑎 𝑦2+𝑎 𝑥𝑦\n3 4\n+𝑎 ℎ+𝑎 𝑡\n5 6\nThe satellite heading (ℎ) is a binary term set to 0 or\n1, based on whether the satellite track is ascending\nor descending, respectively. We solve Eq. 1 within\n(…)”\n<Line 118>\n1.19 I would suggest to not provide the average time 174- Done. The time-period of subglacial lake drainage\nperiods as I do not see how these numbers 175 events changes their effect on ice dynamics. When\nprovide any useful insight, since these lakes are this water reaches the grounding line, subglacial lake\nlocated in different regions and have different drainages sustained over a long time-period (several\ndriving processes. years) can increase ice shelf basal melt rates and\ncontribute to ice stream thinning and acceleration to\na much greater extent than transient pulses of\nsubglacial water from faster lake drainages which\ntake months (Gourmelen et al., 2025).\n“The median time-period for a complete subglacial\nlake drainage event in this study is 2.2 years, with the\nmedian time for lake recharge being 3.5 years.”\n<line 265>\nWe thank the reviewer for their comment, as it has\nprompted us to validate the inclusion of these\nstatistics by adding the following point to our\ndiscussion:\n“Subglacial lake drainage events observed in this\nstudy sustained over relatively long time periods (a\nmedian subglacial lake drainage time of 2.2 years and\nan upper quartile of 4.4 years), which suggests that a\nlarge percentage of these subglacial lakes have\npotential to affect ice dynamics (Gourmelen et al.\n2025).”\n<line 578>\n1.20 Table 1: Please include uncertainties T1 Done. We have added uncertainties to Table 1.\n1.21 Fig 2. The scale number for Mill 165 is partly F1 Done.\nhidden behind the figure.\nReviewer 2:\nID Comment Line Response\n2.1 It is my view that this study is of broad interest to We thank the reviewer for their positive comments,\nthe cryospheric community as it builds upon time, attention to detail and constructive feedback.\nprevious work focused on subglacial lake\ndetection beneath the Antarctic Ice Sheet,\nespecially in the context of subglacial hydrology\nand ice-sheet dynamics, by extending the record\nof active subglacial lake activity and their time-\nevolving filling and draining behaviour. I think the\nice sheet modelling community would be very\ninterested to use this dataset as validation at an\nice-sheet scale in future. Overall, this is a well\nwritten manuscript which could be clearer some\nplaces, detailed by my specific comments.\nOnce the authors address these, I can therefore\nrecommend that this manuscript is suitable for\npublication in Nature Communications.\n2.2 In the abstract and several other places in the Done. Please see reviewer comment 1.15 for\nmanuscript, 5 new subglacial lake networks and updates on subglacial lake network and cluster\n25 clusters are mentioned. However, it is unclear definitions.\nto me what these 25 clusters refer to – are these\nlakes that intersect with modelled subglacial We have also updated the ocean area in Fig. 1 so\nwater flux (LeBrocq et al., 2013)? From Figure 1, that the ice sheet is surrounded by a Southern Ocean\nit is unclear which of the 85 new lakes constitute bathymetry dataset, in similar colours to that of the\nthese 25 clusters, and it is difficult to see in any subglacial hydrological pathways. We hope that this,\ndetail the relationship between the authors’ new along with increasing the opacity of the subglacial\nreported active lakes and modelled subglacial pathways in the figure, will help the reader identify\nwater flux. This latter point is not really and follow these pathways between subglacial lakes\nelaborated on in the manuscript. more easily. Inclusion of this dataset helps to\ndemonstrate that active lakes often occur on\nmodelled pathways, which we elaborate on in Figure\n4.\n2.3 The manuscript does a great job of validating Comment. We thank reviewer 2 for their positive\nlake outlines using previously reported lake assessments of our method validation. Whilst we\nderived from other sensors. I would like to agree that incorporating recent results from Arthur\nsuggest that the results of our recent study et al., 2025 would be a great addition to this work,\n(Arthur et al., 2025) would be a valuable addition we did not see significant surface elevation change\nto this comparison, as it provides one of a few signals at these locations in our 10-year elevation\nrecords of active subglacial lakes near the change map from which we selected our points of\ngrounding line with several lakes coincident or interest for this study. We recognize that Arthur et al.\nlocated near those identified in this study. use predominantly ICESat (2003–2009) and ICESat-2\nSpecifically, locations such as Lazarevisen (2019–2023) data for their study, which may explain\n(coincident with our Lake L1), Vigridisen (46 km why we only observed one of the same lakes in our\nupstream of our lake V1), Muninisen (170 km 10-year elevation change map, as our study periods\nupstream of our lakes M1 and M2) and Roi only overlapped from late-2019 to mid-2020.\nBaudouin (78 km downstream of our Lake R2) Reprocessing our data for this area would be suitable\ncould provide useful points of comparison. I have for a more in-depth case study of this region,\nincluded two of these examples below. In possibly amalgamating CryoSat-2 observations with\nparticular, relevant discussion could be added to ICESat missions, to fill in the 2009–2019 gap at a\nthe intercomparison of subglacial lake higher time resolution than possible with the REMA\ninventories (Section 4.1) and in Supplementary DEM strips used in Arthur et al. We think this would\nFigure 3. be really exciting future work. Extending the\nvalidation to cover more lakes is unfortunately\noutside of the scope of this manuscript.\nWe have updated our paper to cite the reviewer\nrecommended manuscript in the following places:\n“When subglacial lake drainage events occur and\nwater discharges into the ocean, the flux of water\ncan drive high, localised regions of basal melt on ice\nshelves through turbulent mixing from ocean plumes\n14,54. Basal meltwater discharge downstream from\nsubglacial lakes across the grounding line, resulting\nin these processes, is potentially more likely to occur\nat grounding line proximal subglacial lakes, such as\nthat observed 15 km upstream of Muninisen Ice\nShelf in Dronning Maud Land (Arthur et al., 2015).\nGiven that 11 of our new subglacial lakes are closer\nthan 15 km to the grounding line, these processing\nmay be influencing ice melt even more than the\nlimited recent observations.”\n<line 613>\n2.4 In the Discussion (Section 4.3), I suggest Done. We thank the reviewer for this suggestion.\nmentioning the lake cluster upstream of Discussing the proximity of the Jutulstraumen cluster\nJutulstraumen (Jutlustraumen_69, _71, _74), we have observed here (Jutlustraumen_69, _71, _74)\nwhich you detect filling and draining between to the network upstream reported by Neckel et al.\n2013–2015 and 2017–2020. This cluster is (2021) highlights a key finding in the paper. We have\nlocated 81 km upstream of the active lake cluster added this to the discussion as suggested.\nreported by Neckel et al. (2021) using Sentinel-1\nDInSAR, validated with ICESat-2, and observed to “Furthermore, the aforementioned cluster at\nfill and drain between 2017–2020 (see right- Jutulstraumen Glacier is 81 km downstream of an\nhand map). Given their proximity and temporal active subglacial lake cluster observed to fill and\nactivity, these lakes may form part of a larger drain between 2017–2020 18. Given their proximity\ninterconnected network and could be associated and temporal activity, these lakes may form part of a\nwith cascading drainage. larger interconnected network and could be\nassociated with cascading drainage.”\n<line 684>\n2.5 One of the interesting findings from this new Comment. This is a brilliant idea, but we felt this was\ninventory is the detection of active lakes near a whole additional block of work which would be\nthe grounding line in certain locations (e.g. King better served by a more in-depth additional paper.\nBaudouin East_3, less than 4 km from the GL). I This would allow us to look at localized impact of\nthink this deserves further emphasis (for example, lake drainage on short term grounding line migration\nan additional figure highlighting some of these and basal melt rates where there are ice shelves. Our\nnew examples) due to its important implications assessment was that this was a huge amount of extra\nfor subglacial water routing and the impact on content that would be best presented in a stand-\nlocal ice dynamics at the transition between alone paper. As we are freely publishing our lake\ngrounded and floating ice in these regions. datasets with this paper, it will enable the scientific\ncommunity to do this work if we are not quick\nenough ourselves.\n2.6 As a general point, some of the discussion about Done.\nfuture work suggestions could be more suited to\nSection 4.5 (Future Outlook), for example the We moved:\nlast few sentences of Section 4.1 and 4.2. “While the likely routing of subglacial hydrological\npathways has been modelled 15,55, very little is\nknown about the temporal variability of this water\nflux. Subglacial lake activity provides one of the few\nmechanisms for measuring subglacial hydrology\nvariability with any frequency across the whole ice\nsheet, therefore, we hope these results will be used\ninform future work in this area, for example, as\nvalidation dataset for high resolution subglacial\nwater routing models.” From 4.1 to the future\noutlook section.\n<Line 892>\nWe also edited and moved:\n“Future satellite missions and studies should\ntherefore extend the record of subglacial lake activity\non all documented lakes, in addition to exploring\nlocations where additional subglacial lake networks\nmay exist, by assessing both short and long-term ice\nsheet surface elevation change signals.” From 4.2 to\nthe future outlook section.\n<line 832>\n2.7 It is good to see details of the subglacial lakes, Done. The altimetry processing chain is taken from\ndrainage/filling periods and associated area and existing studies, therefore we do not provide the\nvolume changes provided in the Supplementary code here as it is available elsewhere (Slater et al.\nData. I would encourage the authors to consider 2021). We have added elevation time series data and\nmaking their code available to reproduce the code to reproduce these figures to the data\nresults presented in this manuscript, in line with repository in response to this comment, to make the\nNature Communication guidelines and to article of further use to the community.\npromote reproducibility.\n“The CryoSat-2 satellite altimetry data are freely\navailable from the European Space Agency\n(https://cs2eo.org/), and we provide time series of\nelevation change over all 85 new subglacial lakes in\nour data repository.\nCode availability. The above-referenced repository\ncontains code to reproduce elevation change time\nseries over each new subglacial lake presented in our\nstudy. The altimetry processing chain is available\nfrom Slater and others (2021) 36.”\n<line 940>\n2.8 The Abstract refers to 25 clusters, and it is nice Done. We have produced a new figure 4 that\nthat you show individual lakes in Supplementary illustrates all 5 networks.\nFigure 2, but it is also difficult to see how these <line 730>\nrelate to each other. Might be nice to have an\nadditional figure highlighting the clusters and the\nrelationships between the lakes.\n2.9 I would specific ‘lake clusters’ here. 17 Done.\n2.10 I’m not sure that we can definitively say that ice 26 Done. Thank you, we have updated the text.\nthicknesses prevent surface meltwater from\nreaching the bed, given that direct observations “In Greenland, subglacial lakes can form when\nof this are currently lacking here. I think you also seasonal surface melt percolates down from the ice\nmeant for this sentence to join with the next surface to bedrock via moulins, crevasses 5 and\none? hydrofractures 6,7. Beyond the Antarctic Peninsula 9,\nthere is limited evidence that surface water reaches\nthe bed in this way in Antarctica. Antarctic subglacial\nwater is primarily produced (...)”\n<line 24>\n2.11 I suggest rewording this to ‘Our understanding of 29 Done.\nsubglacial melt rate magnitudes’ rather than\nwhat they are.\n2.12 Which inaccessible region are you referring to 30 Done.\nhere – Antarctica as a whole? “inaccessible region of the ice base”\n<line 29>\n2.13 There is some inconsistent use of the terms 69 Done.\n‘boundaries’ vs ‘perimeters’ when referring to All instances of “perimeters” changed to\nlake outlines, so please choose one and amend “boundaries”.\naccordingly.\n2.14 Can you specify the magnitude of lake signal 85 Done. In principle radar altimeters can measure\ndetectable with processed CryoSat-2 data (how surface elevation change with centimeter precision\nsmall)? (Shepherd et al., 2019, Slater et al., 2021). We\ncalculated an error estimate on the surface elevation\nchange measured in this study and show that it is on\naverage +/- 0.1 m (Table 1), which is in line with\nprevious studies. This suggests that the methods we\nhave applied to satellite radar altimeter data would\nnot enable us to confidently detect SGL activity\nbelow this level. Other methods of detecting SGL\nactivity, including laser altimetry and in situ GPS data\ndo have higher vertical resolution on their\nmeasurement accuracy, but these datasets don’t\nalways have the spatial and temporal coverage\nrequired to monitor unpredictable SGL filling and\ndraining continuously. Hence, long term continuous\nmonitoring provided by CryoSat-2 was required in\norder for the satellite to be collecting data at the\ntime of the activity.\nIn addition, we implemented the following\nthresholds in our processing, which are detailed in\nthe manuscript text:\n“To maximise the likelihood of ice surface height\nchange reflecting real subglacial lake activity we\npinpoint areas of spatially consistent surface height\nchange ≥ 5 km that are distinct from measurement\nnoise.”\n<line 146>\n“We draw the lake contour at the ± 0.3 m yr-1\nelevation change threshold and we adjust this\ndepending on the signal magnitude, altimetry data\ncoverage, and elevation patterns compared to the\nsurrounding ice.”\n<line 176>\n2.15 I would repeat the observation period here to 105 Done.\nhelp the reader: ‘We use our 10-year elevation\nchange map (Oct 2010 – July 2020) to identify\n(…)’.\n2.16 I suggest ‘with a major axis’ is slightly confusing 107 Done.\nand not needed – consider changing the\nsentence to ‘areas of spatially consistent surface\nheight change ≥ 5 km that were distinct from\nmeasurement noise’.\n2.17 Was the ±0.3 m yr-1 elevation change threshold 122 Done. We sensitivity tested different thresholds for\nselected based on previous approaches, or based delineating lakes and found that 0.3 m yr-1 was the\non judgement after trying different thresholds or minimum level where we could generally see a lake\nfollowing a different approach? signal distinct from noise. We found that previous\nstudies often didn’t report this type of information in\nthe methods sections, so hopefully its helpful for\nfuture studies that we have reported our methods\nwith this level of detail.\n2.18 I suggest rewording caption to ‘Existing F1 Done.\nsubglacial lake inventory is also shown’. I also\nthink it would help the reader by labelling key\nregions as mentioned in the text, e.g. Wilkes\nLand, Victoria Land, Adelie and Bakutis Coasts,\nas well as major ice streams. Subglacial water\nflux should really be included in the Figure legend.\n2.19 I think it would be useful to refer to Figure 2 172 Done. Sentence should have referenced figures 2l\nearlier in this paragraph, for example here. and 2k instead of 3l and 3k. Rectified.\n2.20 Specify that the median lake drainage volume is 178 Done.\npreviously-reported.\n“(…)an order of magnitude larger than the previously\nreported median Antarctic subglacial lake drainage\nvolume of 0.12 km3 39.”\n<line 269>\n2.21 Should Panel e (Moscow University 44) include a F2 Done. Yes, thank you for spotting this. Dash added to\ndashed line to indicate when it starts to fill? Also Panel e for when the subglacial lake starts to fill\ncaption should read (…) compared to elevation (2012).\nchange from non-subglacial lake regions (…)’.\n“compared to elevation change from non-subglacial\nlake regions (light green lines)”\n<line 288>\n2.22 Refer to Fig. 2l here. 189 Done.\n2.23 The wording here is slightly confusing, I suggest 196 Done.\nchanging to: ‘where we observe quiescent\nperiods during filling or draining’.\n2.24 Unclear to me why some lake volume change T1 Done. We thank the reviewer for pointing this out.\nestimates are starred, please explain in caption. Starred volume estimates did represent volumes\nestimated from a lake boundary at least +/- 2 years\ndifferent to the period of filling or draining the\nestimate was calculated for. As these numbers were\nhere for illustrative purposes and not included in\nSupplementary Table 1, due to reduced accuracy, we\nhave redacted them from Table 1 and explained this\nin the caption.\n“Blank volume estimates could not be accurately\nestimated, due to lake boundaries at least 2 years\ndifferent to the period of filling or draining.”\n<line 411>\n2.25 The meaning of this sentence is unclear, given 229 Done. We thank reviewer 2 for their comment on\nthe previous sentence. this sentence. Upon further reflection, we have\nrevised this section of work to make the results\nclearer.\n“The resulting time series of subglacial lake outlines\nshows that the area of subglacial lake activity-\ninduced surface elevation change can evolve by up to\n50 %, in the case of two filling events at Scott_12\nsubglacial lake (Fig. 2k, Table 1, Supplementary Table\n1) where the area of 59.4 km2 for the 2014–2016\nfilling event decreases to 29.8 km2 for the later\n2018–2020 filling event. The minimum amount of\narea change we observe between cycles is 9 % , for 2\ndrainage events at Lambert_84 subglacial lake\n(Supplementary Table 1, Supplementary Fig. 2),\nindicating large variability on individual lakes.”\n<line 420>\nSentence deleted:\n“Moreover, our results show that only three\nsubglacial lakes, Cook West_67, Institute_14 and\nMill_161, fill and drain over the same area constantly\n(within < 1 km2 change between active periods)\n(Supplementary Table 1).”\n2.26 Suggest specifying how much area change. 235 Done. Please see our response to reviewer comment\n2.25 above.\n2.27 Suggest adding small insets to each panel to help F3 Done.\nsituate lake locations in Antarctica.\n2.28 Suggest it could be clearer to refer to ‘activity 350 Done.\nrate patterns’ as modes of activity, here and on +\nL352. 352\n2.29 It seems unclear to me how you arrived at this 432 Done. We thank the reviewer for this comment and\npoint, as I can’t find any mention of this or any think that on reflection we need to soften our\ncorrelation analysis with these variables. I see language here. We did a preliminary assessment of\nthat ice speed and ice thickness for each lake are the statistical correlations between the lake activity\nreported as part the supplementary dataset, on our new lakes and a range of geophysical\nalthough mean drainage flux is not. Could you parameters and didn’t find a clear correlation\nelaborate further? between them, hence our statement in the paper.\nHowever, in reality, this is a topic that needs further\nwork, running the correlations over all known SGL’s\nrather than only our new lakes, and investigation into\nthe implications of any findings in more detail. We\nhave changed the text to reflect this.\n“Large variability in both the magnitude and\nfrequency of subglacial lake activity reflects the\nheterogeneity of their geographical and geometric\nsettings in Antarctica, including factors such as ice\nthickness and flow speed local to the subglacial lake.\nDespite evidence that basal hydrology can be highly\nsensitive to these geophysical parameters 56, an\ninitial assessment suggests that there is no evidence\nof a correlation with either the four distinct\nbehaviours listed above or mean lake activity, but in\nthe future, further work is required to assess this\nover all Antarctic subglacial lakes.”\n<line 623>\n‘…, but in the future, further work is required to\nassess this over all Antarctic subglacial lakes.’\n<line 654>\n2.30 Very long sentence, suggest splitting. 360 Done.\n“As complete subglacial lake fill-drain cycles typically\nlast about a decade in Antarctica, it is possible that\nour 20-year record of ice sheet wide surface\nobservations is currently too short to assess whether\nsubglacial lakes change their modes of behaviour\nover time.”\n<line 654>\n2.31 This is mentioned as a general point above, but it 368 Done. Please see reviewer comment 1.15 for\nis unclear what the 25 clusters are that are updates on subglacial lake network and cluster\nreferred to, and what you mean by ‘the same’ definitions.\nmodelled subglacial hydrological pathways.\nSentence altered to the following for clarity:\n“Of 25 lake clusters situated along the same\nmodelled subglacial hydrological pathways 15,\nindicating hydrological connectivity, our results show\nthat five study areas exhibit upstream drainage\nevents concurrent with downstream filling.”\n<line 662>\n2.32 I suggest rewording the term ‘length-scale’ to, 383 Done.\nfor example: ‘indicates decadal variability in\nconnected subglacial system’. “Eight years of potential interconnectivity observed\nat the Français Glacier subglacial lakes, (…), indicates\ndecadal variability in connected subglacial systems.”\n<line 656>\n2.33 It would be relevant to cite Neckel et al. (2021) 412 Done.\nhere.\n2.34 By bed properties, are you referring to sediment 434 Done. We have updated the text to clarify this.\nsaturation or other parameters?\n“bed properties, such as geometry, sediment\nsaturation and temperature, or the volume of\nsubglacial water available.”\n<line 811>\n2.35 cycles and activity to me refer to similar things, 444 Done.\nso I would reword this to ‘subglacial lake filling\nand draining’. “filling and draining patterns”\n<line 820>\n2.36 Suggest changing ‘pooling’ to ‘ponding’. 462 Done.\nSimilarly, suggest changing ‘pools’ to ‘lakes’ on +\nL466. 466\n2.37 Suggest changing ‘activity variability’ to ‘filling 464 Done.\nand draining activity’ for clarity.\n2.38 no need to hyphenate ‘subglacial’. 17 Done.\n2.39 Add comma after Antarctica. 25 Done.\n2.40 The methods’ reliance (add apostrophe). 50 Done.\n2.41 New sentence: ‘However, this is limited by (…)’. 51 Done.\n“However, this is limited by”\n<line 76>\n2.42 New sentence: ‘However, these measurements 53 Done.\n(…)’.\n2.43 No need to hyphenate 10-years. 68 Done.\n2.44 No need to capitalise ice sheet. 79 Done.\n2.45 New sentence: ‘In the event that the map is 119 Done.\nproduced over the full period of the cycle, (…)’.\n2.46 Add ‘measurements’ after ‘subglacial lake area’. 136 Done.\n2.47 New sentence: ‘Therefore, we assume (…)’. 137 Done.\n2.48 New sentence: ‘However, we note that (…)’. This 142 Done.\nsentence would also benefit from a citation. Change to new paragraph, so “however” redacted.\nCitation added as suggested.\n“We note that (…) (Stubblefield et al., 2021).”\n<line 156>\n2.49 For brevity, I suggest rewording to: ‘We identify 150 Done.\n85 new active Antarctic subglacial lakes (Fig. 1)..’.\n2.50 37, not thirty-seven. Also suggest bracketing 172 Done.\n(subglacial lakes Whillans_180 and Scott_12).\n“During our study period, 37 subglacial lake drainage\nevents and 34 filling events are captured in their\nentirety”\n<line 263>\n2.51 New sentence: ‘This lake drained approximately 177 Done.\n1.3 km3 (…)’.\n2.52 Suggest slight rewording for clarity: ‘for example 194 Done.\nTotten_52, which was associated with 8 m of\nuplift’. “which was associated with 8 m of both subsidence\nand subsequent uplift”\n<line 300>\n2.53 Specify ‘subglacial lake filling and draining 201 Done.\nepisodes’ here.\n2.54 Missing full stop. New sentence following line: 260 Done.\n‘Therefore, elevation change (…)’.\n2.55 Hyphenate ‘decade long’. 263 Done.\n2.56 ‘Adjacent to, but distinct from, the main Institute 301 Done. Done. (Sentence deleted).\nE1 lake area’. I think the following sentence is\nalso unnecessary and could be deleted.\n2.57 Given the point made in the previous sentence, I 307 Done. (Sentence deleted, see above, comment 2.56).\nsuggest this sentence is unnecessary.\n2.58 Suggest this sentence can be more concise: 314 Done.\n‘Previous subglacial lake inventories document\nthe existence of active subglacial lakes beneath “Previous subglacial lake inventories document the\nfast ice streams alongside less active lakes existence of active subglacial lakes beneath fast ice\ntowards the interior of the continent, often streams alongside less active lakes towards the\nbeneath slower flowing ice1, 11, 40, 41, 45, 47, interior of the continent, often beneath slower\n48’. flowing ice1, 11, 39, 40, 45, 47, 48”\n<line 567>\n2.59 Suggest ‘subglacial lake clusters in East 377 Done.\nAntarctica.’ (…) Secondly, our results show (…)’.\n2.60 Change ‘cascade’ to cascading. 403 Done.\n+\n412\n+\n416\n2.61 New sentence: ‘Ice dynamics dominate mass loss 429 Done.\nin Antarctica, so it is important to quantify any\nimpact from subglacial hydrology’.\n2.62 New sentence: ‘Therefore, subglacial lake 439 Done.\nsystems (…)’.\n2.63 New sentence: ‘However, further observations 451 Done.\nare required so we can more accurately quantify\nthe impact of drainage events on ice dynamic\nprocesses (…)’.\n2.64 Comma, not semicolon. 454 Done.\n2.65 New sentence: ‘We provide the first time- 474 Done.\nevolving (…)’.\nReviewer 3:\nID Comment Line Response\n3.1 Even though, I am not an altimetry expert the We thank the reviewer for their time and positive\nmethodology employed appears to be both assessment.\nrobust and comprehensively delineated.\nEspecially making use of the POCA processing\napproach allows a much finer spatial resolution\nfor lake mapping. Therefore, this study presents\na much more detailed and accurate dataset\ncompared to the existing inventory as can be\nseen in Figure 3. It is positive to note that the\nshapefiles of the newly detected lakes have been\npublished open access. The text is clear and\naccessible, with results presented in a well-\nstructured manner.\n3.2 The significance of the work is given in any case Done. We agree with the reviewer that the Fricker et\ndue to the importance of Antarctica’s subglacial al (2025) review highlights the importance of SGL’s in\nhydrology for ice loss (Fricker et al. 2025). the Antarctic systems. We believe that the size of our\nNevertheless, for the scope of Nature new dataset, which discovers 85 new active\nCommunications I would have expected more subglacial lakes and documents a previously\ninsights regarding the process understanding of unreported decade of filling and drainage events, will\nsubglacial lake activity and distribution. The be of wide interest to the science community and\nauthors define lake variability and behaviour into the Nature Communications readership. We note\nseven categories (L 351 ff.) and provide insights that other reviewers commented on the suitability of\ninto connected subglacial networks (L 367ff.). But this manuscript for this journal.\nthe data produced within this study would\nfacilitate a greater level of insight with minimal\neffort. The following ideas could be pursued:\nPlease see our response to reviewer comment 3.11\nbelow regarding the potential seasonality of lake\n- The supplementary includes all time series of draining and filling events.\nthe 85 analysed lakes. There seems to be a We are excited that future studies will do exactly as\nstrong seasonality in the drainage and filling Reviewer 3 suggests, rigorously investigating small\nevents. Did you have a look into the seasonality amplitude signals and connecting filling and drainage\n(e.g. detrending and preforming a seasonality events with physical processes, which we still know\nanalysis on time series with sufficient remarkably little about. It was our judgement that\nmeasurement points). Why is there strong this work was substantial and would therefore need\nvariability in some years but not in other? to be extended over all active Antarctic subglacial\nlakes, or all active lakes within a small geographical\nregion, to be the topic of hopefully many future\npapers. We are making our lake boundaries and time\nseries freely available to the community for this work\nto be done by both them and us.\nWe have added in the reviewers excellent\nsuggestions into the future work section (4.5) of the\npaper.\n“The significance of short-term variability on each\nsubglacial lake elevation time series could be\nassessed by comparing magnitudes to localised\nsnowfall variability.”\n<line 895>\n3.3 - Subglacial lakes seem to be clustered in some Please see reviewer comment 2.8.\nspecific areas (Figure 1). Can you explain why?\n3.4 - The lakes are categorized into seven categories. Done. We used categorisations determined from\nHow is their spatial distribution? Have you tried previous studies (Livingstone et al., 2019) in our\nto create a map based on these categories and analysis here, adding two categories which we\nwhere you able to identify any spatial patterns? observed in our dataset. We did not produce a map\nof these categorisations for this this study it was out\nof scope, however we have added in a comment to\nour future work section suggesting that this would\nbe an excellent next step for a future paper. We\nsuggest that this work could be done in a more\ncomprehensive way by including our new lakes along\nwith all the 146 previously identified active AIS\nsubglacial lakes too, allowing spatial patterns to be\nidentified.\n“By combining our observations of new Antarctic\nsubglacial lakes with the existing inventory, a\ncomprehensive analysis of the subglacial behaviour\npatterns expanded on in our study can be used to\ninvestigate spatial patterns of variability and the\nphysical processes causing them.”\n<line 893>\n3.5 Add the CryoSat data acquisition limit at the F1 Done. We have added this shaded area to the figure\nSouth Pole to the map to show where potential legend and altered the formatting of this to make it\nactive lakes might have been detected. stand out.\n3.6 Can you provide any uncertainties for the volume 139 Done. We have added in error estimates for the\nchange estimate? In L 157 you mention that the + T1 elevation change data and included this new\nheight change signal at the surface is attenuated information into Table 1 for each lake.\nby thick ice and hence the change in lake level is\nvery likely different form the surface elevation We have added a few sentences to the manuscript\nchange and might introduce large errors. (please see reviewer comment 1.2) to reiterate that\nour data and methods only measure the surface\nexpression of any lake filling and drainage event, we\ndon’t have any method of measuring how that\ncorresponds to lake cavity area change beneath the\nice. As ice thickness is up to 4km in some parts of\nAntarctica, we wanted to make the point that there\nis a possibility that the surface expression might not\nreflect the entire subglacial cavity area change.\nHowever, there is no data or method that we are\naware of that has ever been able to investigate this\npoint directly. Hopefully the readers will value having\nreasonable methodological limitations stated clearly.\nWe have clarified the statement around this by\nadding in new detail, so hopefully this makes our\nstatement even clearer.\n3.7 What is the reason for your choice of these 13 T1 Done. These are the statistics for the lakes we have\nlakes in particular? shown elevation change maps and time series for in\nFig. 2. We chose to highlight lakes that had either\nparticularly large drainage events, that had\ninteresting repeat modes of filling and draining\nwithin our time period, or that had interesting other\nbehaviour that would be of interest to the reader.\nWe aimed to highlight the full range of behaviour\nthat we observed, and we discuss all of these\nexamples in more detail in the text. We note that our\nsupplementary figures provide the same information\nfor all lakes in the study, so if a reader had particular\ninterest in a different lake to the ones we have\nhighlighted, they would be able to look at this figure\nto get that information.\n“Table 1. Log of ice surface elevation change over\nsubglacial lakes in Fig. 2.”\n3.8 Also consider providing the median and quantiles 174 Done. With our study providing information on 85\nfor drainage events. Just the mean average could be new lakes, and data on 19 existing ones that were\nmiss leading in case of strongly varying time active during our time period, we have a large\nspans. number of measurements and statistics to report. As\nsuch, in this paper, we had to be selective about the\nstatistics we provided, so felt that the median was\nmost useful.\n“The median time-period for a complete subglacial\nlake drainage event in this study is 2.2 years, with the\nmedian time for lake recharge being 3.5 years.”\n<line 265>\nWe have provided the median and quartiles for the\nreviewer (see below) but felt that it would reduce\nreadability of the sentence to provide this in this part\nof the main manuscript. We have now added some\nof this information to our discussion; please see\nreviewer comment 1.19.\nSubglacial lake drainage episode lengths\nMean: 3.1 years\nLower quartile: 1.2 years\nMedian: 2.2 years\nUpper quartile: 4.4 years\nSubglacial lake filling episode lengths\nMean: 4.1 years\nLower quartile: 2.5 years\nMedian: 3.5 years\nUpper quartile: 5.5 years\n3.9 You mention that subglacial lakes can exist closer 328 Done.\nto the grounding line (GL) than previously\nthought. A recent study by Arthur et al. 2025 also “When subglacial lake drainage events occur and\nmentions subglacial lakes close to the grounding water discharges into the ocean, the flux of water\nline. Consider discussing these findings with can drive high, localised regions of basal melt on ice\nyours. shelves through turbulent mixing from ocean plumes\n13,53,56. Basal meltwater discharge downstream from\nsubglacial lakes across the grounding line, resulting\nin these processes, is potentially more likely to occur\nat grounding line proximal subglacial lakes, such as\nthat observed 15 km upstream of Muninisen Ice\nShelf in Dronning Maud Land (Arthur et al., 2015).\nGiven that 11 of our new subglacial lakes are closer\nthan 15 km to the grounding line, these processing\nmay be influencing ice melt even more than the\nlimited recent observations. Considering the\nenhanced ice melt and glacier retreat observed in\nthe 21st century (...)”\n<line 617>\n3.10 You used the MEaSUREs grounding line. Did you 332 Comment. No, we used the MEaSUREs grounding\ncompare the distance between the GL and your line throughout because we wanted to provide\ndetected lakes with a newer grounding line numbers from a widely used consistent product that\nproduct? would be easy to interpret. The timestamp on this\ndataset also maps on well to the period of our study.\nWe think it could be interesting future work to\ninvestigate the ‘near-grounding line’ SGL’s in more\ndetail, to learn more about the timing and impact of\nSGL water exit to the ocean, and the impact this\nmight have on the glacier dynamics.\n3.11 Here you present the drainage of Francais_46 F4 Done. We did not do a full statistical analysis on the\nand subsequent filling of Francais_36. Did you do shorter-term variations in these time series. We did a\nany statistical analysis on the sub-annual preliminary investigation to see if they might be\nfluctuations and the connection between seasonal, and we don’t think they are, but as we did\ndrainage and filling? For example, in 2015 F_46 not have the capacity to do an in-depth case study\nre-fills and drains half a year later. Do you see in on every lake our results, we have not investigated\nreturn a filling signal in F_36 with a temporal lag? this further. The reviewer makes a really interesting\npoint about whether the shorter-term variability\nmight be interlinked in addition to the long term\n(decadal) variability. We hadn’t considered this, but\nwe think it would be an excellent topic for a future\nstudy, so have added it into our future work\nrecommendations.\nWe have included the below text in our future work\nsection to highlight this comment:\n“The significance of short-term subglacial lake\nvariability could also be assessed by comparing sub-\nannual elevation fluctuations on each subglacial lake\nto key weather variables, such as localised snowfall\nvariability.”\n<line 900>\n3.12 There seem to be some lakes that experienced a Sup. Done. Again, this is a fantastic question that we don’t\nsurface lowering but lake area expansion (e.g. think we can fully answer in this study. This paper\nJutulstraument_71, Fisher_168). Is this a reports for the first time that there are substantial\ncommon pattern and what is the explanation lake area changes with subsequent filling and\nbehind? drainage events, and there are no previous studies\neven documenting that area change routinely occurs.\nOur study shows comprehensively that area change\ndoes happen to subglacial lakes, with 19 out of our\n85 lakes having multiple drainage boundaries. We\nthink that investigating this in detail on one lake will\nrequire a dedicated case study, or if we were to\ninvestigate this in a more systematic way over every\nlake, then our dataset would be a good start, but we\nmay require a longer time series. Even our long\ndataset has only one filling and drainage event on\nmost of the lakes (79 out of 85) so we would need to\nobserve the same event multiple times in order to\nmake more generalisable statements of this kind.\nAdditional edits made:\n All figure fonts changed to Helvetica to fit the Nature Communications requirements.\n Title added to all figure captions with bold/regular formatting updates where required.\n Abstract updated from 180 words to 149 in line with Nature Communications requirements.\n We have removed numbers from headings and subheadings.\n Typo updated: “The historical subglacial lake inventories suggest that a larger number of active lakes are located\nin West Antarctica compared to the East Antarctic Ice Sheet” <line 601>.\n Reference to Figure 1 made in Future outlook <line 834>\n “Competing interests. The authors declare no competing interests.” <line 959>"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Dear authors,\nThank you very much for your response and the revisions you have made to the manuscript.\nI have still a few outstanding issues that I would you to address. I refer to the numbering used in your rebuttal.\n# 1.2: Thanks for the revisions. I acknowledge your explanation which is fair – but I would suggest that you rephrase the statement in the abstract to make it clear there as well.\n#1.8: I think that the threshold applied by Slater et al., (2021) is very appropriate given the aim of that study. But in this study the aim is different, and I do not understand why it is needed to have such a threshold. In McMillan et al., (2013) it is stated about the Cook_E2 lake that “Between November 2006 and March 2008, the ice surface fell by ~ 45 m” so isn´t that an example of a signal that you would miss?\n# 1.10: I have read now the three mentioned papers, and I apologize if I have misunderstood something, but I believe that neither Slater et al., (2021) or McMillan et al. (2014) used swath-processed data as you do here. Further, if I understood correctly, the power is indeed used in Gourmelen et al., (2018) but not for surface elevation change correction but for weighing individual elevation points from the swath, which is not what I understand that you are doing here. Please elaborate to justify this approach.", "reviewer_2": "I have reviewed the revised manuscript and feel that the authors have addressed the comments from myself and the two other reviewers effectively. I’m pleased with the improved clarifications of terminology, and extended descriptions where requested.\nI only have a couple of minor comments:\n• I think it could be helpful for the reader to have a key in Figure 3 (as in Figure 4).\n• In the Abstract, I appreciate the authors have tried to clarify their definition of what constitutes individual lake clusters, but this could be interpreted as 25 lake clusters located within (the same) small geographical region. I would therefore remove this from the abstract and instead add this to in the Discussion (‘Connected Subglacial Lake Networks’).\n• I appreciate the authors’ explanation of their selection of the ±0.3 m yr-1 elevation change threshold in their response to my comments, and I think it would be useful to include this in the main manuscript (L129).\n• It may be worth adding a caveat that, while you observe that 25 lake clusters appear aligned along the same modelled subglacial hydrological pathways (LeBrocq et al., 2013), and this is used to infer hydrological connectivity, the underlying water routing methodology is based on the Bedmap2 grid. This grid can be significantly smoothed between radar profiles, meaning that flow routing is primarily influenced by topographic features near the closest radar tracks. As a result, for lakes located farther from these profiles, the inferred locations of subglacial streams may be overly confident or less reliable. For more info see Shackleton et al. (2023, https://doi.org/10.1029/2023JF007269).", "reviewer_3": "I appreciate the authors for their thorough responses and clarifications to all reviewer comments. It's creditable to see the manuscript enhanced in various aspects, particularly with the addition of Figure 4, which significantly contributes to the comprehension of lake clusters and their interconnections. I recognize that some of my questions related to lake dynamics necessitate further analysis that fall outside the scope of this paper. Therefore, I look forward to the authors' future publications that will likely explore the dynamics of subglacial lakes, including the potential seasonality in filling and drainage events."}, "author_response": "Response to referees: Detection and variability of 85 new active Antarctic\nsubglacial lakes with CryoSat-2 radar altimetry, 2010–2020\nRef: NCOMMS-25-13112\nReviewer 1:\nID Comment Line Response\n1.2 The delineation of subglacial lake perimeter 128 Done. The subglacial lake “perimeter” / “boundary”\noutlines from surface change signals is potentially terminology used in the manuscript is used to define\noverinterpreted. The assumption that surface the surface expression of this feature, so we thank\nexpressions reflect the true subglacial lake the reviewer for highlighting the potential for\nboundaries is not adequately justified in the misunderstanding here. We agree that there are\ncurrent version of the manuscript. Previous work factors, including viscous ice flow (Stubblefield et al.,\n(e.g., Stubblefield et al., 2021) has shown that 2021) that might cause the surface expression of the\nsurface depressions may not reliably indicate subglacial Lake (SGL) to not fully reflect water\nsubglacial lake extents due to the influence of movement at the ice base.\nviscous ice flow, and this should be addressed\nmore specifically. Maybe it is simply a question of To address the comment we have moved the\nthe terminology, so that you do not claim to following sentence, from section 2.3, to the first\ndelineate subglacial lake perimeters but rather mention of delineating lake boundaries in section\nsubglacial lake expressions or signals? 2.2. We have acknowledged the point also in the\ndiscussion. We hope this clarifies that our ‘boundary’\nterminology relates to the surface expression of a\nfeature at the ice base that we do not directly\nobserve. We have also added in the reviewers\nrecommended citation, to highlight an example of\nthe processes that might prevent this assumption\nfrom holding:\n“We note that processes such as the viscous\ndeformation of ice or the impact of ice flow may\nalter the surface impression of behaviour at the ice\nbed (Stubblefield et al., 2021).”\n<line 156>\nWe add the following here, for clarity:\n“The use of “subglacial lake boundaries” in this\nmanuscript refers to the surface impression of\nelevation change observed due to subglacial lake\nfilling and draining, rather than a direct delineation\nof the physical subglacial water edge. In order to\nmeasure the maximum extent of ice elevation\nchange over each subglacial lake, (…)”\n<line 157>\nWe also add to the discussion:\n“Our subglacial lake boundaries and volume\nestimates also provide new case studies for\ninvestigating the correspondence between basal and\nsurface dynamics and the effect of ice flow on\nsubglacial signals observed from the surface.”\n<line 904>\nThanks for the revisions. I acknowledge your Done.\nexplanation which is fair – but I would “We delineate time-varying boundaries of subglacial\nsuggest that you rephrase the statement in lake activity and investigate their variability over\nthe abstract to make it clear there as well. time.”\n<line 16>\n1.8 When removing outliers (L. 88) a threshold is 88 Comment. This threshold is increased from that used\napplied on how far from a reference DEM the in previous published work (Slater et al. (2021) use a\naltimetry data can be to be included. The 10 m/yr threshold), so is in line with methods best\nthreshold is 25 m. Does this mean that you will practice for surface elevation change studies. We\nnot be able to detect surface depressions deeper increase this threshold from 10 m/yr to 25 m/yr to\nthan 25m? Does this not potentially filter out allow our method to capture “fast” subglacial lake\nsome subglacial lake variability signals?\nsurface elevation change signals, determined by\nreviewing existing literature.\nThe threshold we have applied means that the\nelevation change rate would need to be more than\n25 m/yr to be removed as an outlier, which would be\nmuch larger than any previously observed SGL\ndrainage rate. To the best of our knowledge, the\nlargest subglacial lake elevation change signal that\nhas been observed is 20 m subsidence over\nThwaites_124 subglacial lake, over an 18-month\nperiod, 2011-2013 (Smith et al., 2017).\nIn our paper, we have not measured elevation\nchange larger than 10 m on any individual lake, and\nthis change happened over 5 years (on\nMoscowUniversity_19 subglacial lake).\nI think that the threshold applied by Slater et Comment. Given the characteristics of the input\nal., (2021) is very appropriate given the aim radar altimetry data, we had to choose a threshold\nof that study. But in this study the aim is that provided a balance between noise reduction\ndifferent, and I do not understand why it is and subglacial lake signal identification. It is possible\nneeded to have such a threshold. In McMillan\nthat our chosen threshold, while over double the\net al., (2013) it is stated about the Cook_E2\nSlater et al. threshold, is too small to capture the\nlake that “Between November 2006 and\nmost extreme active subglacial lake signals, such as\nMarch 2008, the ice surface fell by ~ 45 m” so\nthe example indicated on Cook. However, as the\nisn´t that an example of a signal that you\nfocus of this study was to provide a continent-wide\nwould miss?\nassessment across the whole of Antarctica, our view\nwas that a more conservative estimate would be\nappropriate for that context.\nIn our initial look at the 10-year long surface\nelevation change map, we visually examined the raw\nand the thresholded data. This initial investigation,\nwhile not systematic, led us to conclude that our\ncurrent method was appropriate for the study, as all\nthe signals we wished to investigate from the 10-year\nelevation change map were visible with these\nthresholds applied. This initial investigation led us to\nconclude that our current method was appropriate\nfor the study, as all the signals we wished to\ninvestigate from the 10-year elevation change map\nwere visible with these thresholds applied. While our\nstudy is as complete and comprehensive as we can\nmake it, it is of course possible we have missed an\nimportant SGL event that others will subsequently\nfind.\n1.10 The analytical approach is generally sound in Done. Power corrections are now routinely applied\nterms of identifying elevation change events and to all radar altimetry surface elevation change\ncharacterising their timing and extent. However, studies (McMillan et al., 2014; Gourmelen et al.,\nthere are a few things that could be improved: 2018), so we have applied this correction to stay in\nPlease elaborate on the correlation described in line with that. The purpose of this study was to apply\nEq. 2. It is not clear to me why you would expect existing SEC methods to a new time period and\nsuch a relationship in swath-processed altimetry region to detect new SGL activity, so it was out of\ndata. The Davis et al., (2004) reference is for a scope for us to change the surface elevation\nstudy using conventional RA from ERS-2, using a processing chain substantially here. We have\nretracking algorithm that can be sensitive to updated the text and citation for clarity to address\ncorrelations between the waveform and this comment.\nestimated height.\n“We account for spurious fluctuations in range\nassociated with changes in radar echo shape by\napplying an empirical correction based upon\ncorrelated changes in elevation and backscattered\npower, consistent with existing CryoSat-2 SARIn\naltimetry studies (McMillan et al., 2014; Slater et al.,\n2021). To do this, we compute the correlation\ngradient in elevation as a function of power, dz/dp,\nusing a linear fit in each grid cell. The change in\nbackscattered power dp is multiplied by this\ncorrelation gradient and then subtracted from the\nmodel elevation anomalies to produce power-\ncorrected anomalies (McMillan et al., 2014):\n*Equation 2*\n.”\n<line 134>\nI have read now the three mentioned papers, Comment. Its correct that Slater and McMillan use\nand I apologize if I have misunderstood standard POCA radar altimetry data, but the\nsomething, but I believe that neither Slater et Gourmelen et al., 2018 reference is a methods paper\nal., (2021) or McMillan et al. (2014) used for processing swath mode altimetry data. All papers\nswath-processed data as you do here.\nuse radar altimetry data from the CryoSat-2 satellite,\nFurther, if I understood correctly, the power is\ntherefore use data at the same radar frequency\nindeed used in Gourmelen et al., (2018) but\nwhich means penetration depth and broader\nnot for surface elevation change correction\ninteractions with the snowpack are the same.\nbut for weighing individual elevation points\nfrom the swath, which is not what I\nOverall, application of power corrections is standard\nunderstand that you are doing here. Please\npractice, but as the reviewer points out there could\nelaborate to justify this approach.\nbe slight differences in the impact of the correction if\napplied at different stages of the data processing\nchain. The McMillan et al. power correction applied\nto retracked data, would likely not work as intended\nfor swath processed data because the backscatter\nwithin a specific waveform has a wide amplitude\nwith respect to range gate position, much more than\nthe power variation due to scattering variability with\ntime.\nAt an earlier stage in the study we did run the\nanalysis without the power correction to sensitivity\ntest how much of an effect it made, but the impact\nappeared negligible. At an earlier stage in the study,\nwe did run the analysis without the power correction\nto sensitivity test how much of an effect it made, but\nthe impact appeared negligible. We retained the\ncorrection, because in addition to following best\npractice, we thought there were some cases where it\nseemed to moderately reduce the time series noise.\nBased on previous studies this is the type of benefit\nyou would expect to see. As the focus of this study\nwas to scientifically exploit mature methods and\ndatasets for a new application, we didn’t spend more\ntime adjusting the core altimetry processing chain.\nThis would be a good topic for future altimetry data\nintercomparisons, so we will feed that suggestion\nback into the relevant teams.\nReviewer 2:\nID Comment Line Response\n2.2 In the abstract and several other places in the Done. Please see reviewer comment 1.15 for\nmanuscript, 5 new subglacial lake networks and updates on subglacial lake network and cluster\n25 clusters are mentioned. However, it is unclear definitions.\nto me what these 25 clusters refer to – are these\nlakes that intersect with modelled subglacial We have also updated the ocean area in Fig. 1 so\nwater flux (LeBrocq et al., 2013)? From Figure 1, that the ice sheet is surrounded by a Southern Ocean\nit is unclear which of the 85 new lakes constitute bathymetry dataset, in similar colours to that of the\nthese 25 clusters, and it is difficult to see in any subglacial hydrological pathways. We hope that this,\ndetail the relationship between the authors’ new along with increasing the opacity of the subglacial\nreported active lakes and modelled subglacial pathways in the figure, will help the reader identify\nwater flux. This latter point is not really and follow these pathways between subglacial lakes\nelaborated on in the manuscript. more easily. Inclusion of this dataset helps to\ndemonstrate that active lakes often occur on\nmodelled pathways, which we elaborate on in Figure\n4.\nIn the Abstract, I appreciate the authors have Done. Thank you for this comment on the readability\ntried to clarify their definition of what of this point. We have removed this definition from\nconstitutes individual lake clusters, but this the abstract, as we believe “cluster” is accurate\ncould be interpreted as 25 lake clusters enough to describe lakes which are clustered\nlocated within (the same) small geographical\ntogether. We leave the more detailed definition in\nregion. I would therefore remove this from\nthe main manuscript for readers to clarify if needed.\nthe abstract and instead add this to in the\nWe have also changed the wording from “25 lake\nDiscussion (‘Connected Subglacial Lake\nclusters” to “25 clusters of lakes” to make the\nNetworks’).\ndescription more clear.\nIt may be worth adding a caveat that, while Comment. We thank reviewer 2 for this comment\nyou observe that 25 lake clusters appear and we fully agree with their concerns about\naligned along the same modelled subglacial gridding and over smoothing of the bed topography\nhydrological pathways (LeBrocq et al., 2013),\ndata based on measurement proximity. It’s clear that\nand this is used to infer hydrological\nthe topography (well represented or not) is the\nconnectivity, the underlying water routing\nprimary driver of where water is expected to pond.\nmethodology is based on the Bedmap2 grid.\nWe used the modelled subglacial hydrological\nThis grid can be significantly smoothed\npathway information only as a secondary dataset to\nbetween radar profiles, meaning that flow\ninfer possible connectivity between lake clusters,\nrouting is primarily influenced by topographic\ntherefore, we think that including this caveat about\nfeatures near the closest radar tracks. As a\nthe quality of the bed data would be slightly outside\nresult, for lakes located farther from these\nprofiles, the inferred locations of subglacial of the scope of the paper.\nstreams may be overly confident or less\nreliable. For more info see Shackleton et al.\n(2023, https://doi.org/10.1029/2023JF007269\n).\n2.17 Was the ±0.3 m yr-1 elevation change threshold 122 Done. We sensitivity tested different thresholds for\nselected based on previous approaches, or based delineating lakes and found that 0.3 m yr-1 was the\non judgement after trying different thresholds or minimum level where we could generally see a lake\nfollowing a different approach? signal distinct from noise. We found that previous\nstudies often didn’t report this type of information in\nthe methods sections, so hopefully its helpful for\nfuture studies that we have reported our methods\nwith this level of detail.\nI appreciate the authors’ explanation of their Done.\nselection of the ±0.3 m yr-1 elevation change “We draw the lake contour at the ± 0.3 m yr-1\nthreshold in their response to my comments, elevation change threshold, based on noise\nand I think it would be useful to include this sensitivity testing, and we adjust this depending on\nin the main manuscript (L129).\nthe signal magnitude, altimetry data coverage, and\nelevation patterns compared to the surrounding ice.”\n<line 129>\n2.66 I think it could be helpful for the reader to Done.\nhave a key in Figure 3 (as in Figure 4)."}]

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