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Surface topography across the marginal zone of the Greenland Ice Sheet is constantly evolving in response to changes in weather, season, climate and ice dynamics. Yet current Digital Elevation Models (DEMs) for the ice sheet are usually based on data from a multi-year period, thus obscuring these changes over time. We here present four 500-meter resolution annual (2019-2022) summer DEMs of the Greenland ice sheet marginal zone (PRODEMs). The PRODEMs cover 50km wide band from the ice edge, and they capture all outlet glaciers of the Greenland ice sheet. Each PRODEM is based on data fusion of CryoSat-2 radar altimetry and ICESat-2 laser altimetry using a regionally-varying Kriging method. They are validated using leave-one-out cross-validation, showcasing their ability to correctly represent surface elevations within the associated spatially varying prediction uncertainties, which have a median value of 1.4m. We observe a general lowering of surface elevations compared to ArcticDEM, but the spatial pattern of change is highly complex and with annual changes superimposed. The PRODEMs will enable studies of the marginal ice sheet elevation changes in great detail, temporally as well as spatially. With their high spatio-temporal resolution, the PRODEMs will be of value to a wide range of researchers and users studying ice sheet dynamics and monitoring how the ice sheet responds to changing environmental conditions. Incorporating the PRODEM surface elevations in estimates of the solid ice discharge from Greenland outlet glaciers will e.g. improve our assessment of the mass balance of the Greenland ice sheet and its interannual variability. PRODEMs from summers 2019 through 2022 are available at https://doi.org/10.22008/FK2/52WWHG (Winstrup, 2023), and we plan to annually update the product henceforth. [ABSTRACT FROM AUTHOR]

The mass of the Greenland ice sheet is declining as mass gain from snow accumulation is exceeded by mass loss from surface meltwater runoff, marine-terminating glacier calving and submarine melting, and basal melting. Here we use the input–output (IO) method to estimate mass change from 1840 through next week. Surface mass balance (SMB) gains and losses come from a semi-empirical SMB model from 1840 through 1985 and three regional climate models (RCMs; HIRHAM/HARMONIE, Modèle Atmosphérique Régional – MAR, and RACMO – Regional Atmospheric Climate MOdel) from 1986 through next week. Additional non-SMB losses come from a marine-terminating glacier ice discharge product and a basal mass balance model. From these products we provide an annual estimate of Greenland ice sheet mass balance from 1840 through 1985 and a daily estimate at sector and region scale from 1986 through next week. This product updates daily and is the first IO product to include the basal mass balance which is a source of an additional ∼24 Gtyr-1 of mass loss. Our results demonstrate an accelerating ice-sheet-scale mass loss and general agreement (coefficient of determination, r2 , ranges from 0.62 to 0.94) among six other products, including gravitational, volume, and other IO mass balance estimates. Results from this study are available at 10.22008/FK2/OHI23Z. [ABSTRACT FROM AUTHOR]

We present the first 1992–2020 record of Greenland Ice Sheet (GrIS) mass balance derived from multisatellite Ku‐band altimetry. We employ an empirical approach as an alternative detailed to radar‐propagation modeling, and instead convert elevation changes observed by radar altimetry into mass changes using spatiotemporal calibration fields. This calibration field is derived from a machine learning approach that optimizes the prediction of a previously published mass balance field as a function of ice sheet variables. Our mass balance record shows a GrIS contribution of 12.1 ± 2.3 mm sea‐level equivalent since 1992, with more than 80% of this contribution occurring after 2003. Our record also suggests that the 2017 hydrological year is the first year in the 21st century which, within uncertainties, the GrIS was in balance. Overall, the 28‐year radar‐derived mass balance record we present highlights the potential of the method to provide operational mass balance estimates derived from multisatellite Ku‐band altimetry. Plain Language Summary: Earth's land ice masses are shrinking due to climate change. The Greenland Ice Sheet (GrIS) has been a major contributor to sea‐level rise in recent years. Earth‐observing satellites have enabled us to monitor ice‐sheet changes, but common problems for these estimates of ice‐sheet mass balance are mission gaps and changing instrumentation. Since 1991, the European Space Agency has used the same instrumentation for satellite radar altimeters in orbits applicable for monitoring the GrIS. Additionally, before retiring one satellite altimeter, a replacement has been commissioned with a sufficient timespan of overlapping measurements to estimate offsets between satellites. The resulting satellite altimetry record can be processed to provide a lengthy, and consistent, record of ice‐sheet elevation and volume changes. Here, we apply machine learning to convert radar‐derived volume changes to radar‐derived mass changes. This novel mass balance record shows a contribution from the GrIS to global sea level rise of 12.1 ± 2.3 mm since 1992, with more than 80% of this contribution originating after 2003. The method we present is an alternative to modeling both radar propagation and firn compaction, and provides a path toward operational mass‐balance estimates from satellite altimetry. Key Points: Novel estimate of Greenland Ice Sheet mass balance from calibrated multisatellite Ku‐band altimetryTotal eustatic sea‐level rise contribution of 12.1 ± 2.3 mm from the ice sheet since 1992Atmospheric circulation can shift ice‐sheet mass balance by 3σ within 5 years [ABSTRACT FROM AUTHOR]

Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain surface meltwater, influencing the ice sheet mass balance and contribution to sea-level rise. Multilayer firn models are traditionally used to simulate firn processes and estimate meltwater retention. We present, intercompare and evaluate outputs from nine firn models at four sites that represent the ice sheet's dry snow, percolation, ice slab and firn aquifer areas. The models are forced by mass and energy fluxes derived from automatic weather stations and compared to firn density, temperature and meltwater percolation depth observations. Models agree relatively well at the dry-snow site while elsewhere their meltwater infiltration schemes lead to marked differences in simulated firn characteristics. Models accounting for deep meltwater percolation overestimate percolation depth and firn temperature at the percolation and ice slab sites but accurately simulate recharge of the firn aquifer. Models using Darcy's law and bucket schemes compare favorably to observed firn temperature and meltwater percolation depth at the percolation site, but only the Darcy models accurately simulate firn temperature and percolation at the ice slab site. Despite good performance at certain locations, no single model currently simulates meltwater infiltration adequately at all sites. The model spread in estimated meltwater retention and runoff increases with increasing meltwater input. The highest runoff was calculated at the KAN_U site in 2012, when average total runoff across models (±2σ) was 353±610 mm w.e. (water equivalent), about 27±48 % of the surface meltwater input. We identify potential causes for the model spread and the mismatch with observations and provide recommendations for future model development and firn investigation. [ABSTRACT FROM AUTHOR]

Greenland Ice Sheet surface melting has increased since the 1990s, affecting the rheology and scattering properties of the near‐surface firn. We combine firn cores and modeled firn densities with 7 years of CryoVEx airborne Ku‐band (13.5 GHz) radar profiles to quantify the impact of melting on microwave radar penetration in West Central Greenland. Although annual layers are present in the Ku‐band radar profiles to depths up to 15 m below the ice sheet surface, fluctuations in summer melting strongly affect the degree of radar penetration. The extreme melting in 2012, for example, caused an abrupt 6.2 ± 2.4 m decrease in Ku‐band radar penetration. Nevertheless, retracking the radar echoes mitigates this effect, producing surface heights that agree to within 13.9 cm of coincident airborne laser measurements. We also examine 2 years of Ka‐band (34.5 GHz) airborne radar data and show that the degree of penetration is half that of coincident Ku‐band. Plain Language Summary: Radar waves emitted by satellites can be used to measure changes in surface elevation of the Greenland Ice Sheet. However, they do not reflect off the ice sheet surface itself but penetrate into the snow to a depth of about 15 m for radar wavelengths of 2.3 cm. When the snow melts, meltwater can percolate into the snow or refreeze at the surface. Layers of refrozen ice sharply reduce the degree of radar penetration and may be mistaken for an elevation increase in radar measurements. Here, we combine firn cores and modeled firn densities with 7 years of airborne radar data collected during field campaigns in West Central Greenland to quantify this effect. We identify internal layers corresponding to annual stratigraphy within the snowpack, and we show that more melt means less radar penetration into the firn. The unprecedented surface melting which occurred across Greenland in 2012 caused a sharp reduction in the degree of radar penetration, from 11.5 to 5.3 m. However, if the effects of penetration are corrected for, radar altimeters can accurately measure the surface elevation of the ice sheet. Key Points: We use airborne radar, firn cores, and firn models to confirm that summer melting is the principal source of radar penetration fluctuationsThe largest fluctuations are recorded after the extreme melt event of 2012, which caused a 6.2 ± 2.4 m reduction in radar penetrationEcho retracking compensates for radar penetration fluctuations, leading to surface heights that agree with laser data to within 13.9 cm [ABSTRACT FROM AUTHOR]

The Greenland ice sheet is a major contributor to sea level rise, adding on average 0.47 ± 0.23 mm year − 1 to global mean sea level between 1991 and 2015. The cryosphere as a whole has contributed around 45% of observed global sea level rise since 1993. Understanding the present-day state of the Greenland ice sheet is therefore vital for understanding the processes controlling the modern-day rates of sea level change and for making projections of sea level rise into the future. Here, we provide an overview of the current state of the mass budget of Greenland based on a diverse range of remote sensing observations to produce the essential climate variables (ECVs) of ice velocity, surface elevation change, grounding line location, calving front location, and gravimetric mass balance as well as numerical modelling that together build a consistent picture of a shrinking ice sheet. We also combine these observations with output from a regional climate model and from an ice sheet model to gain insight into existing biases in ice sheet dynamics and surface mass balance processes. Observations show surface lowering across virtually all regions of the ice sheet and at some locations up to −2.65 m year − 1 between 1995 and 2017 based on radar altimetry analysis. In addition, calving fronts at 28 study sites, representing a sample of typical glaciers, have retreated all around Greenland since the 1990s and in only two out of 28 study locations have they remained stable. During the same period, two of five floating ice shelves have collapsed while the locations of grounding lines at the remaining three floating ice shelves have remained stable over the observation period. In a detailed case study with a fracture model at Petermann glacier, we demonstrate the potential sensitivity of these floating ice shelves to future warming. GRACE gravimetrically-derived mass balance (GMB) data shows that overall Greenland has lost 255 ± 15 Gt year − 1 of ice over the period 2003 to 2016, consistent with that shown by IMBIE and a marked increase compared to a rate of loss of 83 ± 63 Gt year − 1 in the 1993–2003 period. Regional climate model and ice sheet model simulations show that surface mass processes dominate the Greenland ice sheet mass budget over most of the interior. However, in areas of high ice velocity there is a significant contribution to mass loss by ice dynamical processes. Marked differences between models and observations indicate that not all processes are captured accurately within models, indicating areas for future research. [ABSTRACT FROM AUTHOR]

Hagen Glacier is a major marine-terminating outlet-glacier that drains 6 % of the Northern part of the Greenland ice sheet. This part of the part of the ice sheet is critically understudied, and here we show that the dynamics of the glaciers in the region can be highly variable, potentially impacting the total mass budget of the whole northern sector. We present 35 years of ice velocity measurements derived from both synthetic aperture radar (Sentinel-1) and optical satellite data (Landsat). In conjunction with satellite observations of glacier area change and surface elevation change, the data document the surge of Hagen Glacier starting in 2002 in high detail, and reveal both prior and subsequent dramatic changes in flow regime and geometry. During the past 35 years, Hagen Glacier has experienced three distinct flow regimes: i) a long period of 'stagnant' flow causing a general thickening, ii) a period of fast flow with speed-up occurring over winter and iii) a deceleration with constant flow velocities during the winter and a marked summer speed-up. We investigate the possible drivers and controls of the observed changes in the context of atmospheric and ocean forcings, and discuss the future of glacier. [ABSTRACT FROM AUTHOR]