Description: 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.

Description: In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion.

Description: Radar and laser satellite altimeters have been routinely monitoring the changes in surface elevation of the Greenland Ice Sheet since the early 1990s and 2000s, respectively. Radar and laser signals interact with the surface and sub-surface of the Greenland Ice Sheet in different ways depending on their wavelength and scattering properties of the illuminated firnpack, which vary both spatially and temporally. Here, we compare elevation changes observed by CryoSat-2’s radar and ICESat-2’s laser altimeters across the entire Greenland Ice Sheet between October 2018 and March 2022 to understand their similarities and differences. Averaged over the whole ice sheet, observed elevation changes from both CryoSat-2 and ICESat-2 altimeters agree very closely, with thinning rates of 15.3 ± 0.7 cm/yr and 12.7 ± 0.9 cm/yr, respectively. We perform an epoch to epoch comparison on the monthly elevation changes observed by both CryoSat-2 and ICESat-2 and find an average difference of -7.2 cm and a high correlation (R-value = 0.95) between them, demonstrating that both capture the same inter-annual variability in elevation changes over Greenland. This correlation is highest in the ablation zone of the ice sheet (R-value = 0.98), where the seasonal changes are most pronounced, with both altimeters capturing thinning and thickening due to surface melting in the summer and snowfall accumulation in the winter, respectively. Through this work, we aim to examine the suitability of producing an improved and merged record of Greenland elevation changes from radar and laser altimetry by improving the understanding of and resolving their residual differences.

Description: Totten Glacier is the principal source of ice loss from the East Antarctic Ice Sheet. Although the East Antarctic Ice Sheet as a whole has remained approximately in balance, the response of Totten Glacier to climate forcing remains a key source of uncertainty in predicting its future contribution to sea level rise. Here, we compare and combine estimates of the mass change of Totten Glacier and it's surrounding region from satellite measurements of changes in its volume, ice speed and gravitational potential acquired over the past two decades between 2002 and 2022. Ice losses from the Totten Glacier catchment and two surrounding areas – the Vincennes Bay region and the Moscow University catchment – have doubled since 2002 from 8.5 ± 0.7 Gt/yr to 20 ± 1.5 Gt/yr. We find the largest disagreement in Vincennes Bay, which remains a challenging region in which to monitor mass changes - likely a combination of a paucity in observations of ice thickness, and the regions’ small mass imbalance compared to local SMB fluctuations. Using a regional climate model, we show that only Totten Glacier is losing ice due to it flowing faster than it’s equilibrium state, although the rate of its dynamic ice loss has slowed by 60 %. In total, the region has lost 285 ± 19 Gt of ice and raised the global sea level by 0.8 ± 0.1 mm, with the majority (62 %) of this loss originating from Totten Glacier itself.

Description: Ice-sheet mass balance is commonly estimated by the input output method, the altimetry method, the gravimetry method, or their combination. All of them are subject to uncertainties, e.g. due to limitations in sampling and resolution, residual calibration issues, and corrections from geophysical models involved. Error budgets have been built for each of them. Intercomparison exercises have been conducted to analyze residual uncertainties, to learn about the implications this may have on our understanding of physical processes controlling the mass balance, and to generate compound estimates. Here we review approaches and discussions on uncertainty characterization related to the Ice-sheet Mass Balance Intercomparison Exercise (IMBIE). We discuss the following problems: First, the target mass balance estimates embrace different temporal scales from monthly to multi-decadal, and embrace different mass balance representations, either as rates of mass change or as mass anomalies w.r.t. a reference state or as volume change converted into mass change. The intercomparison needs to accommodate these representations in a consistent uncertainty characterization framework, and the organizers need to consider what compromises are needed and acceptable to simplify and unify the uncertainty characterization tasks posed to the contributors to the intercomparison. Second, compound estimates need to weight the different contributions. The intercomparison needs to decide how assessed uncertainties should affect the definition of weights and hence of the compound estimates. Discussions on these and other aspects are ongoing within the IMBIE Working Group on Error Budget. Soliciting feedback from the wider community is one aim of this presentation.

Description: The Greenland Ice Sheet is a major contributor to global mean sea level rise, contributing about 20% to the global mean sea level rise since 1993. In recent years, ice losses from Greenland have accelerated, rising from 34 Gt yr〈sup〉-1〈/sup〉 in the 1990s to 244 Gt yr〈sup〉-1〈/sup〉 between 2016-2020. About a third of Greenland’s total ice losses come from the Northwest sector, which has experienced widespread retreat and thinning of its outlet glaciers. Here, we study the imbalance of the Northwest Greenland Ice Sheet with the aid of satellite altimetry, satellite gravimetry and the input-output method, from 1972 to 2022. Over the input-output record (1972-2019), the Northwest sector lost 1,200.7 ± 19.4 Gt of ice, contributing 3.3 ± 0.1 mm to sea level rise. While the rate of mass loss was of only 16.0 ± 0.9 Gt yr〈sup〉-1〈/sup〉 in 1972-2000, the pace of mass loss was more than twice as high rising to 36.8.5 ± 0.8 Gt yr〈sup〉-1〈/sup〉 in 2000-2010, before increasing further to 50.1 ± 1.0 Gt yr〈sup〉-1〈/sup〉 in 2010-2019 due to the combined effect of reduced surface mass balance and increased ice discharge. This acceleration in ice losses post 2000s is confirmed by the satellite gravimetry record (2002-2021), with a rate of mass loss of 55.1 ± 2.0 Gt yr〈sup〉-1〈/sup〉. Finally, thanks to the high spatial and temporal resolution of the CryoSat-2 altimetry record (2010-2022), we find that in 22 glacier basins, high thinning rates spread to more than 10 % of the area of these basins.