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Ice motion and boundaries are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent‐wide, maps of Antarctic‐wide ice motion and boundaries for the time period 1995–2022. The results reveal no change in the interior region of East Antarctica, iceberg detachments at ice shelf fronts, and widespread glacier speedup that propagates 100 km's inland in West Antarctica and the Antarctic Peninsula. Speedup affects the entire drainage of the Amundsen Sea Embayment sector; the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf; the east coast down to Larsen C Ice Shelf; Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis, Mertz and Denman glaciers, glaciers in Porpoise and Vincennes Bay; and Robert, Wilma and Rayner glaciers in Enderby Land, in East Antarctica. We attribute the observed glacier changes to increased melting by warmer ocean waters. Plain Language Summary: Ice velocity is a fundamental variable of glacier evolution that governs the transfer of mass from the ice sheets to the ocean. It is best measured from satellite platforms but long time series require combination of multiple satellite data of various performance levels and incomplete spatial coverage. We assembled the first set of continent‐wide mosaics using 2–5 years of data in each period to maximize spatial coverage and detect changes in ice dynamics over the entire continent. The results reveal no signal over most of the interior but important changes at the coast concentrated along channels occupied by outlet glaciers. In the Amundsen Sea Embayment sector of West Antarctica, changes are pervasive over the entire drainage, which indicates that the coastal forcing from the ocean affects the entire ice sheet. The products reveal precise areas of rapid change in the West and East coasts of the Antarctica Peninsula, and in West and East Antarctica. These observations will help constrain ice sheet models in charge of reconstructing ice sheet past changes and projecting its evolution in a changing climate. Key Points: We present historical, continent‐wide, mosaics of Antarctic ice motion using a suite of satellite sensors spanning 28 yearsWe report areas of accelerated flow stretching over 100 km's from the coast and the entire drainage of the Amundsen Sea Embayment sectorThe observed, ongoing, detailed glacier changes will help constrain ice sheet numerical models in charge of reconstruction and projection [ABSTRACT FROM AUTHOR]

Humboldt Gletscher is a 100‐km wide, slow‐moving glacier in north Greenland which holds a 19‐cm global sea level equivalent. Humboldt has been the fourth largest contributor to sea level rise since 1972 but the cause of its mass loss has not been elucidated. Multi‐beam echo sounding data collected in 2019 indicate a seabed 200 m deeper than previously known. Conductivity temperature depth data reveal the presence of warm water of Atlantic origin at 0°C at the glacier front and a warming of the ocean waters by 0.9 ± 0.1°C since 1962. Using an ocean model, we reconstruct grounded ice undercutting by the ocean, combine it with calculated retreat caused by ice thinning to floatation, and are able to fully explain the observed retreat. Two thirds of the retreat are caused by undercutting of grounded ice, which is a physical process not included in most ice sheet models. Plain Language Summary: Humboldt Gletscher is the widest glacier in Greenland, slow moving, and terminating in shallow waters in its southern half, but grounded 200 m deeper than previously known in its northern half, with a submarine trough extending more than 100 km inland. The glacier has been retreating at 0.6 km/year and contributing significantly to sea level rise. We attribute the retreat to undercutting of grounded ice by warmer ocean waters combined with a retreat caused by ice thinning to floatation sooner due to glacier speed up. The glacier, which hosts an ice volume equivalent to a 19‐cm global sea level, will remain a major contributor to sea level rise this Century. Key Points: The 100‐km wide Humboldt Gletscher holds a 19‐cm sea level rise equivalent, lost 161 billion tons of mass, and retreated 13 km since 1972Warm waters at 0°C flood a 350–400 m deep trough on its northern flank that remains below sea level more than 100 km inlandWe explain the glacier retreat as 70% from ocean‐induced undercutting and 30% from thinning‐induced retreat [ABSTRACT FROM AUTHOR]

Landsat imagery has long been used to measure glacier and ice sheet surface velocity, and this application has increased with increased length and accessibility of the archive. The radiometric characteristics of Landsat sensors, however, have limited these measurements generally to only fast-flowing glaciers with high levels of surface texture and imagery with high sun angles and cloud-free conditions, preventing wide-area velocity mapping at the scale achievable with synthetic aperture radar (SAR). The Operational Land Imager (OLI) aboard the newly launched Landsat 8 features substantially improves radiometric performance compared to preceding sensors: enhancing performance of automated Repeat-Image Feature Tracking (RIFT) for mapping ice flow speed. In order to assess this improvement, we conduct a comparative study of OLI and the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) performance for measuring glacier velocity in a range of surface and atmospheric conditions. To isolate the effects of radiometric quantization and noise level, we construct a model for simulating ETM+ imagery from OLI and compare RIFT results derived from each. We find that a nonlinearity in the relationship between ETM+ and OLI radiances at higher brightness levels results in a particularly large improvement in RIFT performance over the low-textured interior of the ice sheets, as well as improved performance in adverse conditions such as low sun-angles and thin clouds. Additionally, the reduced noise level in OLI imagery results in fewer spurious motion vectors and improved RIFT performance in all conditions and surfaces. We conclude that OLI imagery should enable large-area ice sheet and glacier mapping so that its coverage is comparable to SAR, with a remaining limitation being image geolocation. [ABSTRACT FROM AUTHOR]