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: 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.