Seasonal Evolution of Supraglacial Lakes on Baltoro Glacier From 2016 to 2020

The existence of supraglacial lakes influences debris-covered glaciers in two ways. The absorption of solar radiation in the water leads to a higher ice ablation, and water draining through the glacier to its bed leads to a higher velocity. Rising air temperatures and changes in precipitation patterns provoke an increase in the supraglacial lakes in number and total area. However, the seasonal evolution of supraglacial lakes and thus their potential for influencing mass balance and ice dynamics have not yet been sufficiently analyzed. We present a summertime series of supraglacial lake evolution on Baltoro Glacier in the Karakoram from 2016 to 2020. The dense time series is enabled by a multi-sensor and multi-temporal approach based on optical (Sentinel-2 and PlanetScope) and Synthetic Aperture Radar (SAR; Sentinel-1 and TerraSAR-X) remote sensing data. The mapping of the seasonal lake evolution uses a semi-automatic approach, which includes a random forest classifier applied separately to each sensor. A combination of linear regression and the Hausdorff distance is used to harmonize between SAR- and optical-derived lake areas, producing consistent and internally robust time series dynamics. Seasonal variations in the lake area are linked with the Standardized Precipitation Index (SPI) and Standardized Temperature Index (STI) based on air temperature and precipitation data derived from the climate reanalysis dataset ERA5-Land. The largest aggregated lake area was found in 2018 with 5.783 km2, followed by 2019 with 4.703 km2, and 2020 with 4.606 km2. The years 2016 and 2017 showed the smallest areas with 3.606 and 3.653 km2, respectively. Our data suggest that warmer spring seasons (April–May) with higher precipitation rates lead to increased formation of supraglacial lakes. The time series decomposition shows a linear increase in the lake area of 11.12 ± 9.57% per year. Although the five-year observation period is too short to derive a significant trend, the tendency for a possible increase in the supraglacial lake area is in line with the pronounced positive anomalies of the SPI and STI during the observation period.

Distribution and Evolution of Supraglacial Lakes in Greenland during the 2016–2018 Melt Seasons

In recent decades, the melting of the Greenland Ice Sheet (GrIS) has become one of the major causes of global sea-level rise. Supraglacial lakes (SGLs) are typical hydrological features produced on the surface of the GrIS during the melt seasons. The existence and evolution of SGLs play an important role in the melting process of the ice sheet surface. To understand the distribution and recent changes of SGLs in Greenland, this study developed a random forest (RF) algorithm incorporating the texture and morphological features to automatically identify SGLs based on the Google Earth Engine (GEE) platform. Sentinel-2 imagery was used to map the SGLs inventory in Greenland during the 2016–2018 melt seasons and to explore the spatial and temporal variability characteristics of SGLs. Our results show changes in SGLs from 2016 to 2018, with the total area decreasing by ~1152.22 km2 and the number increasing by 1134; SGLs are mainly distributed in western Greenland (SW, CW, NW) and northeastern Greenland (NE), where the NE region has the largest number of observed SGLs and the largest SGL was with the surface area of 16.60 km2 (2016). SGLs were found to be most active in the area with the elevation of 800–1600 m and the slope of 0–5°, and showed a phenomenon of retreating to lower elevation areas and developing to steeper slope areas. Our work provided a method for rapid inventory of SGLs. This study will help monitor the mass balance of the GrIS and predict future rapid ice loss from Greenland.

Seasonal evolution of Antarctic supraglacial lakes in 2015–2021 and links to environmental controls

Supraglacial meltwater accumulation on ice shelves may have important implications for future sea level rise. Despite recent progress in the understanding of Antarctic surface hydrology, potential influences on ice shelf stability as well as links to environmental drivers remain poorly constrained. In this study, we employ state-of-the-art machine learning on Sentinel-1 synthetic aperture radar (SAR) and optical Sentinel-2 satellite imagery to provide new insight into the inter-annual and intra-annual evolution of surface hydrological features across six major Antarctic Peninsula and East Antarctic ice shelves. For the first time, we produce a high-resolution record of supraglacial lake extent dynamics for the period 2015–2021 at unprecedented 10 m spatial resolution and bi-weekly temporal scale. Through synergetic use of optical and SAR data, we obtain a more complete mapping record also enabling the delineation of buried lakes. Our results for Antarctic Peninsula ice shelves reveal below-average meltwater ponding during most of melting seasons 2015–2018 and above-average meltwater ponding throughout summer 2019–2020 and early 2020–2021 considering years 2015–2021 as a reference period. Meltwater ponding on investigated East Antarctic ice shelves was far more variable, with above-average lake extents during most 2016–2019 melting seasons and below-average lake extents during 2020–2021, considering the reference interval 2016–2021. This study is the first to investigate relationships with climate drivers both spatially and temporally including time lag analysis. The results indicate that supraglacial lake formation in 2015–2021 is coupled to the complex interplay of local, regional and large-scale environmental drivers with similar driving factors over both ice sheet regions. In particular, varying air temperature, solar radiation and wind conditions influenced supraglacial lake formation over all six ice shelves despite strong local to regional discrepancies, as revealed through pixel-based correlation analysis. Furthermore, regional climatic conditions were shown to be influenced by Southern Hemisphere atmospheric modes showing large-scale impacts on the spatio-temporal evolution of supraglacial lakes as well as on above- or below-average meltwater ponding with respect to the period 2015–2021. Finally, the local glaciological setting, including melt–albedo feedbacks and the firn air content, was revealed to strongly influence supraglacial lake distribution. Recent increases in Antarctic Peninsula surface ponding point towards a further reduction in the firn air content, implying an increased risk for ponding and hydrofracture. In addition, lateral meltwater transport was observed over both Antarctic regions with similar implications for future ice shelf stability.

Spatially and Temporally Resolved Monitoring of Glacial Lake Changes in Alps During the Recent Two Decades

Climate warming is intensifying the melting of glaciers and the growth of glacial lakes in the Alps, which has a profound impact on the management of water resources and high-mountain hydropower in this region. However, the research on the spatial distribution and temporal evolution of the Alps glacial lakes of various types still lacks a holistic view. In this study, we developed an inventory of Alps glacial lakes of different types and then obtained the annual areas of these lakes from 2000 to 2019 using JRC Global Surface Water and Global Land Analysis and Discovery data at a resolution of 30 m. A total of 498 glacial lakes (>0.01 km2) with the net area of 33.77 ± 6.94 km2 were identified in the Alps in 2019 and are mainly distributed in the western and central Alps. These Alps glacial lakes, with the area ranging 0.01–1.59 km2, are generally dominated by small-sized ones. The comparison of lakes of different types indicated that ice-uncontacted lakes are dominant in number and area, accounting for 59.4 and 58.4%, respectively. In terms of the elevation distribution, almost half of the lakes are concentrated at the altitude of 2,250–2,750 m (a.s.l.). Meanwhile, the mean altitude of small glacial lakes is higher than that of large lakes. The distribution of ice-contacted lakes and supraglacial lakes were more concentrated, and the mean altitude was higher. During the study period, the number, area, and water volume of glacial lakes were increasing, but the expansion varied between different periods. The changing trends of the glacial lake area and volume were consistent and presents in three stages, as the glacial lake expanded rapidly in the first 5 years and in the last 7 years and remained relatively stable between 2005 and 2012. The number and area of glacier-fed lakes increased rapidly, while the non-glacier-fed lakes were relatively stable. The area change rate of supraglacial lakes was the largest (+47%). This study provides a spatially-complete and temporally-consecutive picture of glacial lake changes in the Alps and can be greatly helpful for future research on climate-glacier-lake interactions, glacial lake outburst floods, and freshwater resources in this region.

An inventory of supraglacial lakes and channels across the West Antarctic Ice Sheet

Quantifying the extent and distribution of supraglacial hydrology, i.e. lakes and streams, is important for understanding the mass balance of the Antarctic ice sheet, and its consequent contribution to global sea level rise. The existence of meltwater on the ice surface has the potential to affect ice shelf stability and grounded ice flow, through hydrofracturing and the associated delivery of meltwater to the bed. In this study, we systematically map all observable supraglacial lakes and streams in West Antarctica, by applying a semi-automated Dual-NDWI (Normalised Difference Water Index) approach to > 2000 images acquired by the Sentinel-2 and Landsat-8 satellites during January 2017. We use a K-Means clustering method to partition water into lakes and streams, which is important for understanding the dynamics and inter-connectivity of the hydrological system. When compared to a manually-delineated reference dataset on three Antarctic test sites, our approach achieves average values for sensitivity (85.3 % and 77.6 %), specificity (99.1 % and 99.7 %) and accuracy (98.7 % and 98.3 %) for Sentinel-2 and Landsat-8 acquisitions, respectively. In total, we identified 10,478 supraglacial features (10,223 lakes and 255 channels) on the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula (AP), with a combined area of 119.4 km2 (114.7 km2 lakes, 4.7 km2 channels). 27.3 % of feature area was found on grounded ice, 17.8 % of feature area comprised lakes which crossed the grounding line, while 54.9 % of feature area was found on floating ice shelves. New continental-scale inventories such as these, the first produced for WAIS and AP, are made possible by the recent expansion in satellite data provision. The inventories provide a baseline for future studies and a benchmark to monitor the development of Antarctica’s surface hydrology in a warming world, and thus enhance our capability to predict the collapse of ice shelves in the future. The dataset is available at https://doi.org/10.5281/zenodo.5109856 (Corr et al., 2021).

The distribution and evolution of supraglacial lakes on 79° N Glacier (north-eastern Greenland) and interannual climatic controls

The Nioghalvfjerdsfjorden glacier (also known as the 79∘ North Glacier) drains approximately 8 % of the Greenland Ice Sheet. Supraglacial lakes (SGLs), or surface melt ponds, are a persistent summertime feature and are thought to drain rapidly to the base of the glacier and influence seasonal ice velocity. However, seasonal development and spatial distribution of SGLs in the north-east of Greenland are poorly understood, leaving a substantial error in the estimate of meltwater and its impacts on ice velocity. Using results from an automated detection of melt ponds, atmospheric and surface mass balance modelling, and reanalysis products, we investigate the role of specific climatic conditions in melt onset, extent, and duration from 2016 to 2019. The summers of 2016 and 2019 were characterised by above-average air temperatures, particularly in June, as well as a number of rainfall events, which led to extensive melt ponds to elevations up to 1600 m. Conversely, 2018 was particularly cold, with a large accumulated snowpack, which limited the development of lakes to altitudes less than 800 m. There is evidence of inland expansion and increases in the total area of lakes compared to the early 2000s, as projected by future global warming scenarios.

Seasonal evolution of Antarctic supraglacial lakes in 2015–2021 and links to environmental controls

Supraglacial meltwater accumulation on ice shelves may have important implications for future sea-level-rise. Despite recent progress in the understanding of Antarctic surface hydrology, potential influences on ice shelf stability as well as links to environmental drivers remain poorly constrained. In this study, we employ state-of-the-art machine learning on Sentinel-1 Synthetic Aperture Radar (SAR) and optical Sentinel-2 satellite imagery to provide new insight into the inter-annual and intra-annual evolution of surface hydrological features across six major Antarctic Peninsula and East Antarctic ice shelves. For the first time, we produce a record of supraglacial lake extent dynamics for the period 2015–2021 at unprecedented 10 m spatial resolution and bi-weekly temporal scale. Through synergetic use of optical and SAR data, we obtain a more complete mapping record enabling the delineation of also buried lakes. Our results for Antarctic Peninsula ice shelves reveal below average meltwater ponding during most of melting seasons 2015–2018 and above average meltwater ponding throughout summer 2019–2020 and early 2020–2021. Meltwater ponding on investigated East Antarctic ice shelves was far more variable with above average lake extents during most of melting seasons 2016–2019 and below average lake extents during 2020–2021. This study is the first to investigate relationships with climate drivers both, spatially and temporally including time lag analysis. The results indicate that supraglacial lake formation in 2015–2021 is coupled to the complex interplay of varying air temperature, solar radiation, snowmelt, wind and precipitation, each at different time lags and directions and with strong local to regional discrepancies, as revealed through pixel-based correlation analysis. Southern Hemisphere atmospheric modes as well as the local glaciological setting including melt-albedo feedbacks and the firn air content were revealed to strongly influence the spatio-temporal evolution of supraglacial lakes as well as below or above average meltwater ponding despite variations in the strength of forcing. Recent increases of Antarctic Peninsula surface ponding point towards a further reduction of the firn air content implying an increased risk for ponding and hydrofracture. In addition, lateral meltwater transport was observed over both Antarctic regions with similar implications for future ice shelf stability.

Simultaneous drainage events from supraglacial lakes on the southern Inylchek Glacier, Central Asia

To understand the mechanism of simultaneous drainage event related to supraglacial lakes on a debris-covered glacier, we investigated water-level variations of supraglacial lakes on the southern Inylchek Glacier in Kyrgyzstan. To examine these variations, we used daily aerial images for 2017–2019 from an uncrewed aerial vehicle that were converted to 15 cm-digital surface models and ortho-images. Our main results are as follows: (1) When one lake drained, the water levels of other lakes simultaneously increased, indicating that drainage water is shared with several lakes through a main englacial conduit. In one drainage event, a branched off englacial conduit clearly connected to a main englacial conduit. (2) Sometimes several lakes discharged simultaneously, indicating that several lakes had connected to a main englacial conduit that had opened. Such cases can cause larger-scale drainage than that from the opening of a branched off englacial conduit. (3) Simultaneous drainage occurred twice in the same year, each time through a different conduit, indicating that the main englacial conduit can be abandoned and reused. (4) In some lakes, the water level on the hydraulic gradient line increased gradually with nearly the same increase rate just before drainage. Such an increase may be an indicator of a possible simultaneous drainage event.