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Recent satellite-remote sensing studies have documented the multi-decadal acceleration of the Antarctic Ice Sheet in response to rapid rates of ice-sheet retreat and thinning. Unlike the Greenland Ice Sheet, where historical, high-temporal-resolution satellite and in situ observations have revealed distinct changes in land-ice flow within intra-annual timescales, observations of similar seasonal signals are limited in Antarctica. Here, we use high-spatial- and high-temporal-resolution Copernicus Sentinel-1A/B synthetic aperture radar observations acquired between 2014 and 2020 to provide the first evidence for seasonal flow variability of the land ice feeding George VI Ice Shelf (GVIIS), Antarctic Peninsula. Our observations reveal a distinct austral summertime (December–February) speed-up of ∼0.06±0.005 m d -1 (∼ 22±1.8 m yr -1) at, and immediately inland of, the grounding line of the glaciers nourishing the ice shelf, which constitutes a mean acceleration of ∼15 % relative to baseline (time-series-averaged) rates of flow. These findings are corroborated by independent, optically derived velocity observations obtained from Landsat 8 imagery. Both surface and oceanic forcing mechanisms are outlined as potential controls on this seasonality. Ultimately, our findings imply that similar surface and/or ocean forcing mechanisms may be driving seasonal accelerations at the grounding lines of other vulnerable outlet glaciers around Antarctica. Assessing the degree of seasonal ice-flow variability at such locations is important for quantifying accurately Antarctica's future contribution to global sea-level rise. [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]

The Greenland and Antarctic ice sheets are losing ice mass at an accelerating rate in response to atmospheric, oceanic and internal dynamic forcing. Detailed knowledge on ice flow dynamics and processes leading to the depletion of glacier mass is essential for future scenarios on contributions of polar ice masses to sea level rise. Satellite Earth observation data from Copernicus Sentinel-1, Sentinel-2 and TerraSAR-X/TanDEM-X play a key role hereby in providing spatially and temporally detailed climate data records on ice flow and surface elevation change. A recent and major advancement, emerging from the ESA CCI and Austrian ASAP/FFG programs, is the development and implementation of an automatic system for generation of ice velocity maps from repeat pass Sentinel-1 SAR data. In combination with a dedicated polar acquisition strategy, this marked a transition from traditional campaign-style observation to true operational monitoring of ice flow in polar regions.We generated time series of ice velocity from weekly repeat pass Sentinel-1 Interferometric Wide Swath and TerraSAR-X/TanDEM-X Stripmap data by applying customized iterative offset tracking tools. The ice velocity maps, in combination with high-resolution digital elevation models, derived from the bistatic interferometric radar mission TanDEM-X, and available ice thickness from radio echo sounding, form the basis for studying glacier dynamics, ice discharge and mass balance of Greenland and Antarctic outlet glaciers.Including the latest observational data, we present detailed ice sheet wide velocity maps of the Greenland and Antarctic mapping campaigns. The continuous 6 to 12-day repeat acquisitions of the ice sheet margins by Sentinel-1 are used to produce monthly-averaged ice velocity maps that, supplemented with ice thickness data, allow for calculating sub-annual discharge variations in order to distinguish short-term fluctuations from trends. Time series of ice velocity are combined with volumetric changes calculated from DEM differencing to analyse the evolution of outlet glaciers along the Weddell Coast of the Antarctic Peninsula. Our results reveal a complex and versatile pattern of mass balance and ice flow.This presentation highlights some of the capabilities of the satellite missions and emphasizes the importance of long-term comprehensive monitoring of polar ice masses, which is essential for understanding and predicting their response to climatic changes. [ABSTRACT FROM AUTHOR]