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International audience; Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9 mm to global mean sea level, with the rate of mass loss rising from 105 Gt yr−1 between 1992 and 1996 to 372 Gt yr−1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9 Gt yr−1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86 Gt yr−1 in 2017 to 444 Gt yr−1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9 Gt yr−1) and, to a lesser extent, from the Antarctic Peninsula (13±5 Gt yr−1). East Antarctica remains close to a state of balance, with a small gain of 3±15 Gt yr−1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at https://doi.org/10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021).

Retreating and thinning glaciers are icons of climate change and impact the local hazard situation, regional runoff as well as global sea level. For past reports of the Intergovernmental Panel on Climate Change (IPCC), regional glacier change assessments were challenged by the small number and heterogeneous spatio-temporal distribution of in situ measurement series and uncertain representativeness for the respective mountain range as well as by spatial and temporal limitations and technical challenges of geodetic methods. Towards IPCC SROCC and AR6, there have been considerable improvements with respect to available geodetic datasets. Geodetic volume change assessments for entire mountain ranges have become possible thanks to recently available and comparably accurate digital elevation models (e.g., from ASTER or TanDEM-X). At the same time, new spaceborne altimetry (CryoSat-2, IceSat-2) and gravimetry (GRACE-FO) missions are in orbit and about to release data products to the science community. This opens new opportunities for regional evaluations of results from different methods as well as for truly global assessments of glacier mass changes and related contributions to sea-level rise. At the same time, the glacier research and monitoring community is facing new challenges related to the spread of different results as well as new questions with regard to best practises for data processing chains and for related uncertainty assessments.In this presentation, we introduce the Glacier Mass Balance Intercomparison Exercise (GlaMBIE) project of the European Space Agency, which is building on existing activities and the network of the International Association of Cryospheric Sciences (IACS) working group on Regional Assessments of Glacier Mass Change (RAGMAC) to tackle these challenges in a community effort. We will present our approach to develop a common framework for regional-scale glacier mass-change estimates towards a new data-driven consensus estimate of regional and global mass changes from glaciological, DEM-differencing, altimetric, and gravimetric methods.

Most of the ice in the Antarctic ice sheet drains from the continent to the ocean through fast-flowing ice streams and glaciers. The high velocity of these features is thought to be maintained by water at the ice sheet's base, which reduces friction. Subglacial water moving has been linked to transient glacier flow acceleration and enhanced melt at the grounding line. Therefore, the presence, location, and movement of water at the ice sheet's base are likely significant controls on the mass balance of Antarctica.The transport of subglacial water from the interior of Antarctica to the grounding line was once thought to be a steady-state process. It is now known that subglacial water collects in hydrological sinks, which store and release water in episodic events. These features can be detected and quantified by satellite altimetry. This behaviour is interpreted as water moving in and out of 'active' subglacial lakes.Detecting active subglacial lakes with satellite altimetry commonly involves searching for localized regions of surface elevation change over short temporal time frames. In practice, this can be incredibly cumbersome due to the large amounts of data that need to be processed and a high degree of guesswork regarding where potential lakes might be located.Here we present a semi-automatic active subglacial lake approach for detecting and classifying drainage and filling events across Antarctica from an entire archive of satellite altimetry. We first use CryoSat-2 altimetry to produce time-dependent rate of elevation change maps for the whole of the Antarctic continent. From these maps, we search for localized regions of elevation change indicative of subglacial lake activity. We then extract time series for these features and perform change point analysis to automatically detect subglacial lake activity and extract important parameters such as discharge volumes and recharge rates. This approach reveals several new lakes previously undetected.For example, five new lakes are found over the Thwaites glacier in addition to the four previously recorded. Here we present the approach and the resulting updated inventory of subglacial lake activity across Antarctica.

The Greenland Ice Sheet (GrIS) is losing mass as the climate warms through both increased meltwater runoff and ice discharge at marine terminating sectors. At the ice sheet surface, meltwater runoff forms a dynamic supraglacial hydrological system which includes stream/river networks and large supraglacial lakes (SGLs). Streams/rivers can route water into crevasses, or into supraglacial lakes with crevasses underneath, both of which can then hydrofracture to the ice sheet base, providing a mechanism for the surface meltwater to access the bed. Understanding where, when and how much meltwater is transferred to the bed is important because variability in meltwater supply to the bed can increase ice flow speeds, potentially impacting the hypsometry of the ice sheet in grounded sectors, and iceberg discharge to the ocean. Here we present a new, physically-based, supraglacial hydrology model for the GrIS that is able to simulate a) surface meltwater routing and SGL filling, b) rapid meltwater drainage to the ice-sheet bed via the hydrofracture of surface crevasses both in, and outside of, SGLs, c) slow SGL drainage via overflow in supraglacial meltwater channels and, by offline coupling with a second model, d) the freezing and unfreezing of SGLs from autumn to spring. We call the model Supraglacial Hydrology Evolution and Drainage (or SHED). We apply the model to three study regions in South West Greenland between 2015 and 2019 inclusive and evaluate its performance with respect to observed supraglacial lake extents, and proglacial discharge measurements. We show that the model reproduces 80 % of observed lake locations, and provides good agreement with observations in terms of the temporal evolution of lake extent. Modelled moulin density values are in keeping with those previously published and seasonal and inter-annual variability in proglacial discharge agrees well with that observed, though the observations lag the model by a few days since they include transit time through the subglacial system and the model does not. Our simulations suggest that lake drainage behaviours may be more complex than traditional models suggest, with lakes in our model draining through a combination of both overflow and hydrofracture, and some lakes draining only partially and then refreezing. This suggests that in order to simulate the evolution of Greenland’s surface hydrological system with fidelity, then a model that includes all of these processes needs to be used. In future work we will couple our model to a subglacial model and an ice flow model, and thus use our estimates of where, when and how much meltwater gets to the bed to understand the consequences for ice flow.

Arctic glaciers and ice caps are currently major contributors to global sea level rise. The monitoring of smaller land-ice masses is challenging due to the high temporal and spatial resolution required to constrain their response to climate forcing. This dynamic response of land-ice to climate forcing constitutes the main uncertainty in global sea level projections for the next century. The relative significance of these forcings is currently unknown with most recent categorisations focusing on separating loss caused by internal dynamics versus surface mass balance changes, with only initial investigations into processes instigating these changes. This leaves the specific roles of processes in the atmosphere, ocean and sea ice unconstrained. This knowledge is key to improving our projections of how these smaller land-ice masses will respond to future climate forcing and by extension their contribution to future sea level rise.This study uses CryoSAT-2 swath interferometric radar altimetry to provide high spatial and temporal observations to produce elevation timeseries for the land-ice masses in Svalbard Archipelago. It also utilises the regional atmospheric model (MAR) to gain timeseries of surface mass balance. These are combined with climate datasets, and by separating land-ice mass into land versus marine terminating, are used to quantify the effects of different processes. Additionally, in order to observe the relative impact of atmospheric versus oceanic forcing, an ocean thermal forcing model, previously used to study Greenland’s outlet glaciers, has been initialised.The aim of this case study is to develop a framework that will quantify the connections and processes linking loss of land-ice to processes in the ice and, atmosphere and sea ice across the Arctic region.

Subglacial lakes may form beneath ice sheets and ice caps, given the availability of water and appropriate bedrock and surface topography to capture the water. On a regional scale, these lakes can modulate the freshwater output to the ocean by acting as reservoirs that may periodically drain and recharge. Several such active subglacial lakes have been documented under the Antarctic ice sheet, while only a few are observed under the Greenland ice sheet. The small size of the hydrologically active subglacial lakes in Greenland compared to those in Antarctica, puts additional demands on our mapping capabilities to resolve in great detail the evolving surface topography over these lakes to document their temporal behavior. Here, we explore the potential of combining CryoSat-2 swath data and high resolution DEMs generated from TanDEM-X scenes and ArcticDEM strips to improve our knowledge of the evolution of four active subglacial lake sites previously documented in the literature. We find that the DEM data complement each other well in terms of time and resolution and thus provide new information about the subglacial lake activity, though the small size of the collapse basins is challenging for CS2, and we are only able to derive useful CS2 data for the two largest of the four investigated lakes. Based on these data sets we can e.g. conclude that the collapse basin at Flade Isblink was actually as deep as 95 m when it formed, which is 30 m deeper than previously documented. We also present evidence of a new active subglacial lake in Southwest Greenland.

Ice losses from the Greenland and Antarctic Ice Sheets have accelerated since the 1990s, accounting for a significant increase in global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume and in Earth’s gravity field. Between 1992 and 2020, the ice sheets contributed 21.0 ± 1.9 mm to global mean sea-level, with the rate of mass loss rising from 105 Gt yr-1 between 1992 and 1996 to 372 Gt yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169 ± 9 Gt yr-1 between 1992 and 2020 but there are large inter-annual variations in mass balance with mass loss ranging from 86 Gt yr-1 in 2017 to 444 Gt yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (-82 ± 9 Gt yr-1) and to a lesser extent from the Antarctic Peninsula (-13 ± 5 Gt yr-1). East Antarctica remains close to a state of balance (3 ± 15 Gt yr-1), but is the most uncertain component of Antarctica’s mass balance.

We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (58 %) of the ice loss was from the Northern Hemisphere, and the remainder (42 %) was from the Southern Hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 34.6 ± 3.1 mm. The majority of all ice losses were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, these elements of the cryosphere have taken up 3.2 % of the global energy imbalance.

Subglacial water at the base of the ice sheet has been shown to impact ice flow and its discharge into the ocean – which modifies ocean properties and melt rates of fringing ice shelves. Subglacial water, generated from frictional heating and geothermal heat flux, collects in hydrological sinks which store and release water in episodic events. These events, interpreted as water moving in and out of active lakes, have a signature at the surface of the ice sheet which can be detected and quantified by satellite altimetry. By quantifying the volume of water involved in these active events, it is possible to obtain insight into processes otherwise hidden from view. In particular, observing the gain in volume following a drainage event provides constraints on hydrological recharge rates, subglacial water fluxes, and processes leading to basal melt water production. Such a method was applied to the inventory of Antarctic active subglacial lakes using CryoSat’s radar altimeter observations over the 2010-2020 period. Out of 136 known lakes, 13 lakes show clear signs of recharge during the observation period. From these we can extract recharge rates, which act as a lower bound on subglacial melt production. These rates are compared against rates of recharge derived by routing modelled subglacial melt production across the Antarctic Ice Sheet. We demonstrate that, for connected subglacial lake networks, satellite observations can provide a robust validation dataset for sub-glacial melt models. Our results show that, at a local level, estimates of melt water flux is highly dependent on the routing method chosen. We also highlight subglacial lakes where observed rates of recharge are too high to be explained by our current knowledge of the bed and of processes responsible for subglacial melting rates.

Meltwater forms at the base of the Antarctic Ice Sheet due to geothermal heat flux (GHF) and basal frictional dissipation. Despite the relatively small volume, this meltwater has a profound effect on ice-sheet stability, controlling the dynamics of the ice sheet and the interaction of the ice sheet with the ocean. However, observations of subglacial melting and hydrology in Antarctica are limited. Here we use numerical modelling to assess subglacial melt rates and hydrology beneath the Antarctic Ice Sheet. Our case study, focused on the Amery Ice Shelf catchment, shows that total subglacial melting in the catchment is 6.5 Gt yr-1, over 50% larger than previous estimates. Uncertainty in estimates of GHF leads to a variation in total melt of ±7%. The meltwater provides an extra 8% flux of freshwater to the ocean in addition to contributions from iceberg calving and melting of the ice shelf. GHF and basal dissipation contribute equally to the total melt rate, but basal dissipation is an order of magnitude larger beneath ice streams. Remote-sensing observations, from CryoSat-2, indicating active subglacial lakes and ice-shelf basal melting constrain subglacial hydrology modelling. We observe a network of subglacial channels that link subglacial lakes and trigger isolated areas of sub-ice-shelf melting close to the grounding line. Building upon this Amery case study, we expand our analysis to quantify subglacial melt rates and hydrology beneath the entire Antarctic Ice Sheet.

Ice shelves play a crucial role in controlling rates of ice discharge across Antarctica’s grounding lines. Mass loss from ice shelves, predominately due to basal melting and calving, can reduce the buttressing force provided by ice shelves, leading to increased grounded ice discharge. Despite the importance of ice shelves, existing estimates of calving and freshwater fluxes from ice shelves have utilised disparate datasets valid for inconsistent time periods or have relied on simplifying assumptions, resulting in a limited account of the health of many ice shelves and little indication of processes driving ice shelf mass imbalance.Here, we quantify calving and basal melt fluxes at annual temporal resolution during 2010 to 2019. Our annual measurements account for annual variations in ice velocity and basal melt rate for 183 ice shelves, and annual variations calving front position for 34 major ice shelves (accounting for ~90% of the ice shelf area). On average during the study period, a calving flux of 1283±109 Gt yr-1 is roughly equal to a melt flux of 1247±149 Gt yr-1. Inter-annual variations in the fluxes of both basal meltwater and calving mean that the melt contribution to ice shelf mass loss varies between 35% and 62%, with the lowest contributions in years with large calving events. These large (>100 Gt) calving events are rare (8 events during 2010-2019), yet account for 35% of the total ice shelf calving flux, highlighting the importance of large calving events for ice shelf mass balance over short time scales. Eighty percent of ice shelves, including many in East Antarctica, are melting at or faster than their balance rates, indicating that ocean-driven erosion of ice shelf grounding lines is widespread around Antarctica. Furthermore, we find a significant and strong positive correlation (R=0.68) between basal melt flux and grounding line discharge, implying that ocean-driven melt may pace grounded ice loss from Antarctica.

Flood hazard maps used to inform and build resilience in remote communities in the Terai region of southern Nepal are based on outdated and static digital elevation models (DEMs), which do not reflect dynamic river configuration or hydrology. Episodic changes in river course, sediment dynamics, and the distribution of flow down large bifurcation nodes can modify the extent of flooding in this region, but these processes are rarely considered in flood hazard assessment. Here, we develop a 2D hydrodynamic flood model of the Karnali River in the Terai region of west Nepal. A number of scenarios are tested examining different DEMs, variable bed elevations to simulate bed aggradation and incision, and updating bed elevations at a large bifurcation node to reflect field observations. By changing the age of the DEM used in the model, a 9.5% increase in inundation extent was observed for a 20‐year flood discharge. Reducing horizontal DEM resolution alone resulted in a

Surface melt over the Antarctic ice shelves is one of the largest uncertainties related to sea level rise over the 21st century. However, current climate models still struggle to accurately represent it, limiting our comprehension of processes driving melt spatial and temporal variability and its consequences on the stability of the Antarctic ice sheet. Recent advances in Earth monitoring thanks to satellites have enabled new estimations of Antarctic melt extent. They can detect if and where melt occurs, while the amount of meltwater produced can only be deduced from model simulations. In order to combine advantages of both tools, we present new melt estimates based on a regional climate model assimilating the satellite-derived melt extent. This improves the comparison between model and satellite estimates paving the way for a re-estimation of the amount of melt produced each year on the surface of the entire Antarctic ice sheet.

Swath mode processing of CryoSat-2 Synthetic Aperture Radar Interferometric (SARIn) mode has been used to monitor elevation of areas with complex topography such as over ice sheet and ice cap margins. Swath processing relies on an accurate measure of the angle of arrival of the measured echo and, therefore, requires custom strategies in order to resolve the ambiguous phase measurement. In mountainous regions of complex terrain, it may be necessary to apply different phase ambiguities across a waveform record when returns come from different scatters distributed perpendicularly to the CryoSat-2 ground tracks. In this letter, we present modifications to the conventional swath processing method whereby a multisurface retracker is first applied to the record in order to identify potential different scattering surfaces. Phase ambiguity is then independently resolved for each of these subsurfaces. The improvements with this new method over the Karakoram glaciers are a 10% increase in the number of measurements with improvements of almost 50% for individual glaciers and a reduction in the median absolute deviation of the elevations from 20.18 to 14.69 m.

Helheim Glacier, one of the largest marine-terminating outlet glaciers draining the Greenland Ice Sheet, underwent significant retreat and acceleration in the early 2000s, accounting for an appreciable proportion of the ice sheet's mass loss during that period. Using a range of remotely sensed datasets, we show that despite a subsequent readvance, the glacier has continued to lose mass and thin, and has retreated inland of the retracted position occupied in 2005. Critically, the near-terminus is up to 100 m thinner than during 2005, and the front 5 km is within 25–50 m of flotation, with retrograde bed slopes extending several kilometers inland of the terminus. The neighboring Fenris and Midgard Glaciers have both undergone recent large-scale and rapid retreat once their near-terminus regions began to float, suggesting that under projected climate warming and associated glacier thinning, Helheim Glacier is poised to pass a threshold whereby the near-terminus region will retreat rapidly.

Mass loss from glaciers and ice caps represents the largest terrestrial component of current sea level rise. However, our understanding of how the processes governing mass loss will respond to climate warming remains incomplete. This study explores the relationship between surface elevation changes (dh/dt), glacier velocity changes (du/dt), and bedrock topography at the Trinity-Wykeham Glacier system (TWG), Canadian High Arctic, using a range of satellite and airborne datasets. We use measurements of dh/dt from ICESat (2003–2009) and CryoSat-2 (2010–2016) repeat observations to show that rates of surface lowering increased from 4 m yr−1 to 6 m yr−1 across the lowermost 10 km of the TWG. We show that surface flow rates at both Trinity Glacier and Wykeham Glacier doubled over 16 years, during which time the ice front retreated 4.45 km. The combination of thinning, acceleration and retreat of the TWG suggests that a dynamic thinning mechanism is responsible for the observed changes, and we suggest that both glaciers have transitioned from fully grounded to partially floating. Furthermore, by comparing the separate glacier troughs we suggest that the dynamic changes are modulated by both lateral friction from the valley sides and the complex geometry of the bed. Further, the presence of bedrock ridges induces crevassing on the surface and provides a direct link for surface meltwater to reach the bed. We observe supraglacial lakes that drain at the end of summer and are concurrent with a reduction in glacier velocity, suggesting hydrological connections between the surface and the bed significantly impact ice flow. The bedrock topography thus has a primary influence on the nature of the changes in ice dynamics observed over the last decade.

ANM is supported by a University of Edinburgh Principal’s Career Development PhD Scholarship. We acknowledge NERC grants NE/K015249/1 (to PWN) and NE/K014609/1 (to AJS) and DLR projects XTI_GLAC0296 and LAN1534 (to NG). Time-lapse imagery was acquired under NSF grant PLR-0909552 (to MT and MF). PROMICE and DMI data are available at http://www.promice.dk and http://www.dmi.dk, respectively. ArcticDEM was created from DigitalGlobe Inc. imagery, funded under NSF awards 1043681, 1559691, and 1542736. Submarine melting of tidewater glaciers is proposed as a trigger for their recent thinning, acceleration and retreat. We estimate spring submarine melt rates (SMRs) of Kangiata Nunaata Sermia in southwest Greenland, from 2012 to 2014, by examining changes in along-fjord freeboard and velocity of the seasonal floating ice tongue. Estimated SMRs vary spatially and temporally near the grounding line, with mean rates of 1.3 ± 0.6, 0.8 ± 0.3 and 1.0 ± 0.4 m d−1 across the tongue in 2012, 2013 and 2014, respectively. Higher melt rates correspond with locations of emerging subglacial plumes and terminus calving activity observed during the melt season using time-lapse camera imagery. Modelling of subglacial flow paths suggests a dynamic system capable of rapid re-routing of subglacial discharge both within and between melt seasons. Our results provide an empirically-derived link between the presence of subglacial discharge plumes and areas of high spring submarine melting and calving along glacier termini. Publisher PDF

Assessment of ocean-forced ice sheet loss requires that ocean models be able to represent sub-ice shelf melt rates. However, spatial accuracy of modelled melt is not well investigated, and neither is the level of accuracy required to assess ice sheet loss. Focusing on a fast-thinning region of West Antarctica, we calculate spatially resolved ice-shelf melt from satellite altimetry, and compare against results from an ocean model with varying representations of cavity geometry and ocean physics. Then, we use an ice-flow model to assess the impact of the results on grounded ice. We find that a number of factors influence model-data agreement of melt rates, with bathymetry being the leading factor; but this agreement is only important in isolated regions under the ice shelves, such asshear margins and grounding lines. To improve ice sheet forecasts, both modelling and observations of ice-ocean interactions must be improved in these critical regions.

Greenland's future contribution to sea-level rise is strongly dependent on the extent to which dynamic perturbations, originating at the margin, can drive increased ice flow within the ice-sheet interior. However, reported observations of ice dynamical change at distances >~50 km from the margin have a very low spatial and temporal resolution. Consequently, the likely response of the ice-sheet's interior to future oceanic and atmospheric warming is poorly constrained. Through combining GPS and satellite-image-derived ice velocity measurements, we measure multi-decadal (1993–1997 to 2014–2018) velocity change at 45 inland sites, encompassing all regions of the ice sheet. We observe an almost ubiquitous acceleration inland of tidewater glaciers in west Greenland, consistent with acceleration and retreat at glacier termini, suggesting that terminus perturbations have propagated considerable distances (>100 km) inland. In contrast, outside of Kangerlussuaq, we observe no acceleration inland of tidewater glaciers in east Greenland despite terminus retreat and near-terminus acceleration, and suggest propagation may be limited by the influence of basal topography and ice geometry. This pattern of inland dynamical change indicates that Greenland's future contribution to sea-level will be spatially complex and will depend on the capacity for dynamic changes at individual outlet glacier termini to propagate inland.

The dynamics of the Greenland Ice Sheet (GrIS) is greatly affected by surface meltwater that is routed from the surface to the bed, for example when a supraglacial lake (SGL) drains. The South-West Greenland Ice Sheet (SWGrIS) has an abundance of such lakes that form and decay over every hydrological year. In case a crevasse is opened up underneath an SGL, the lake water is likely to drain via the crevasse into the ice-sheet’s bed. This in turn influences the ice sheet motion by increasing the lubrication at the ice-sheet’s base. SGLs may also either drain laterally via a supra-glacial meltwater channel or the water they contain can stay put throughout the hydrological year, refreezing in the winter. These processes may affect the ice rheology in addition to influencing ice flow. While simulating the future evolution of the GrIS, it is thus important to account for processes associated with the evolution of SGLs. Until now, however, none of the existing ice sheet models have fully accounted for these processes, in part because no hydrological model yet includes them all. Here we propose a new process-based hydrological model for the SWGrIS which fully accounts for the evolution of SGLs. The model consists of four units. The first is a surface water routing unit where the daily-generated surface meltwater is routed assuming steepest decent into the surface depressions forming SGLs. The second unit uses principles of Linear Elastic Fracture Mechanics (LEFM) to deal with the scenario where an SGL drains into the bed through an underlying crevasse. The third deals with the SGL drainage event that occurs when a surface meltwater channel gets incised though the ice sheet’s surface due to erosion from the SGL’s overflowing meltwater i.e. channel incision. Finally, the fourth unit simulates the freezing/unfreezing of SGLs by calculating the energy balance at the SGL’s surface. Using this model forced by Modèle Atmosphérique Régionale (MAR) derived daily surface melt-water values we quantify a) the amount and location of surface meltwater injection to the ice-sheet’s bed via moulins or crevasses and ,b) the meltwater that is either retained in SGL or drained overland via meltwater channels and stored elsewhere over the period 2011-2020, in the Leverett glacier catchment. In the future, we plan to integrate this hydrological model with the sophisticated state-of-the-art BISICLES ice sheet model.

Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and their impact on ice sheet mass balance. We use high-resolution altimetry measurements over the Thwaites Glacier to show that a lake system underwent a second episode of drainage activity in 2017, only four years after another substantial drainage event. Our observations suggest significant modifications of the drainage system between the two events, with 2017 experiencing greater upstream discharge, faster lake-to-lake connectivity, and the transfer of water within a closed system. Measured rates of lake recharge during the inter-drainage period are significantly larger than modelled estimates, suggesting processes which drive subglacial melt production are currently underestimated. Our study highlights new methods of exploring subglacial environments through the application of altimetry, with potential applications for studying subglacial lakes across Antarctica

Glaciers are currently experiencing the largest land-ice imbalance and are the largest contributor to sea level rise after ocean thermal expansion, contributing ~30% to sea level budget. Global monitoring of these regions remains a challenging task since global estimates rely on a variety of observations and models to achieve the required spatial and temporal coverage, and significant differences remain between current estimates. Here we report, for the first time, the application of radar altimetry to retrieve spatially and temporally resolved elevation and mass changes of glaciers on a global scale. We apply interferometric swath altimetry to CryoSat-2 data acquired between 2010 and 2020 over all large mountain glacier regions and provide monthly and annual time series of glacier mass loss for each region, together with linear mass losses. We report ubiquitous and sustained ice loss ranging from 82.3 ± 6.3 Gt yr−1 in Alaska, to 3.4 ± 2.5 Gt yr−1 for the Antarctica Periphery. While there is a considerable spatial and temporal variability in imbalance, reflecting the complexity of regional atmospheric and oceanic forcing and of glacier forcing, the global glacier trend is remarkably sustained over this period. Globally, glaciers have lost a combined mean of 275 ± 15 Gt yr−1 between 2010 and 2020 contributing 0.76 ± 0.5 mm yr−1 to global Sea Level Rise.

Satellite observations are the best method for tracking ice loss, because the cryosphere is vast and remote. Using these, and some numerical models, we show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (58 %) of the ice loss was from the northern hemisphere, and the remainder (42 %) was from the southern hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland, and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 34.6 ± 3.1 mm. The majority of all ice losses were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, these elements of the cryosphere have taken up 3.2 % of the global energy imbalance.

International audience; CryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance.

We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.2 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (60 %) of the ice loss was from the northern hemisphere, and the remainder (40 %) was from the southern hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland, and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 35.0 ± 3.2 mm. The majority of all ice losses from were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, the cryosphere has taken up 3.2 % of the global energy imbalance.

Glaciers and ice caps are currently the largest non-steric contributor to sea level rise, contributing ~30 % to sea level budget. Global monitoring of these regions remains a challenging task since global estimates rely on a variety of observations and models to achieve the required spatial and temporal coverage, and significant differences remain between current estimates. Here we report the first application of a novel approach to retrieve spatially-resolved elevation and mass change from Radar Altimetry over entire mountain glaciers areas. We apply interferometric swath altimetry to CryoSat-2 data acquired between 2010 and 2019 over High Mountain Asia (HMA) and in the Gulf of Alaska (GoA). In addition, we extract monthly time series of elevation change, exploiting CryoSat's high temporal repeat, to reveal seasonal and multiannual variation in rates of glaciers' thinning at unprecedented spatial detail. We find that during this period, HMA and GoA have lost an average of −27.9 ± 2.4 Gt yr−1 (−0.29 ± 0.03 m w.e. yr−1) and −76.3 ± 5.6 Gt yr−1 (−0.89 ± 0.07 m w.e. yr−1) respectively, corresponding to a contribution to sea level rise of 0.048 ± 0.004 mm yr−1 and 0.217 ± 0.015 mm yr−1. Glacier thinning is ubiquitous except for the Karakoram-Kunlun region experiencing stable or slightly positive mass balance. In the GoA region the intensity of thinning varies spatially and temporally and correlates with the strength of the Pacific Decadal Oscillation. In HMA we observe sustained multiannual trends until 2015-6, and decreased loss or even mass gain from 2016-17 onwards.