399 research outputs found
A gradient-boosted tree framework to model the ice thickness of the world's glaciers (IceBoost v1.1)
No full textKnowledge of glacier ice volumes is crucial for constraining future sea level potential, evaluating freshwater resources, and assessing impacts on societies, from regional to global. Motivated by the disparity in existing ice volume estimates, we present IceBoost, a global machine learning framework trained to predict ice thickness at arbitrary coordinates, thereby enabling the generation of spatially distributed thickness maps for individual glaciers. IceBoost is an ensemble of two gradient-boosted trees trained with 3.7 million globally available ice thickness measurements and an array of 39 numerical features. The model error is similar to those of existing models outside polar regions and is up to 30 %-40 % lower at high latitudes. Providing supervision by exposing the model to available glacier thickness measurements reduces the error by a factor of up to 2 to 3. A feature-ranking analysis reveals that geodetic data are the most informative variables, while ice velocity can improve the model performance by 6 % at high latitudes. A major feature of IceBoost is a capability to generalize outside the training domain, i.e. producing meaningful ice thickness maps in all regions of the world, including on the ice sheet peripheries
Glacier catchments/basins for the Greenland Ice Sheet
No full textWe divide Greenland, including its peripheral glaciers and ice caps, into 260 basins grouped in seven regions: southwest (SW), central west (CW), (iii) northwest (NW), north (NO), northeast (NE), central east (CE), and southeast (SE). These regions are selected based on ice flow regimes, climate, and the need to partition the ice sheet into zones comparable in size (200,000 km2 to 400,000 km2) and ice production (50 Gt/y to 100 Gt/y, or billion tons per year). Out of the 260 surveyed glaciers, 217 are marine-terminating, i.e., calving into icebergs and melting in contact with ocean waters, and 43 are land-terminating.The actual number of land-terminating glaciers is far larger than 43, but we lump them into larger units for simplification.
Each glacier catchment is defined using a combination of ice flow direction and surface slope. In areas of fast flow (> 100 m), we use a composite velocity mosaic (Mouginot et al. 2017). In slowmoving areas, we use surface slope using the GIMP DEM (https://nsidc.org/data/nsidc- 0715/versions/1) (Howat et al. 2014) smoothed over 10 ice thicknesses to remove shortwavelength undulations.
References:
Mouginot J, Rignot E, Scheuchl B, Millan R (2017) Comprehensive annual ice sheet velocity mapping using landsat-8, sentinel-1, and radarsat-2 data. Remote Sensing 9(4).
Howat IM, Negrete A, Smith BE (2014) The greenland ice mapping project (gimp) land classification and surface elevation data sets. The Cryosphere 8(4):1509–1518
Warm ocean is eroding West Antarctic Ice Sheet
No full textSatellite radar measurements show that ice shelves in Pine Island Bay have thinned by up to 5.5 m yr^{-1} over the past decade. The pattern of shelf thinning mirrors that of their grounded tributaries-the Pine Island, Thwaites and Smith glaciers- and ocean currents on average 0.5degreesC warmer than freezing appear to be the source. The synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini, and the drawdown of grounded ice shows that Antarctica is more sensitive to changing climates than was previously considered
Multi-year mosaics of Antarctic ice motion from satellite radar interferometry: 1995 to 2022
No full textIce motion and ice boundary are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent-wide, maps of Antarctic ice motion for time period 1995-2022. The results reveal no change in the interior regions, block rotation and rift propagation along ice shelf fronts, and widespread glacier speedup that propagates from 10 km's to 100 km's inland. Speedup affects the entire drainage of the Amundsen Sea Embayment (ASE) sector, the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf, the east coast down to Larsen C Ice Shelf, the Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis and Mertz glaciers, glaciers in Porpoise Bay and Vincennes Bay, Denman Glacier in Wilkes Land, and Robert, Wilmaand Rayner glaciers in Enderby Land, East Antarctica, which we attribute to faster melting by warmer ocean waters.Funding provided by: National Aeronautics and Space AdministrationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000104Award Number: 80NSSC18M0083The SAR data employed in this study have been acquired under the umbrella of the Polar Space Task Group (PSTG), which was established under the auspices of the World Meteorological Organization (WMO) Executive Council Panel of Experts on Polar Observations Research and Services. The group mandate was to provide coordination across International Space Agencies to facilitate acquisition and distribution of fundamental satellite datasets in support of polar science. Independently, the Landsat Science team independently provided the Landsat project at the United States Geological Survey with specific recommendations for ice sheet wide acquisitions for Landsat-8.
For the 1995-2001 map, we employ ERS-1/2 and RADARSAT-1 SAR data. For the 2007-2009 map, we employed ERS-2 SAR, Envisat ASAR, ALOS PALSAR and RADARSAT-2. For the 2014-2016 and 2020-2022 maps, we employed S1-a/b, RADARSAT-2, and Landsat-8 data.
The processing algorithms is described in
"Mouginot, J., Rignot, E., Scheuchl, B., & Millan, R. (2017). Comprehensive annual ice sheet velocity mapping using Landsat-8, Sentinel-1, and RADARSAT-2 data. Remote Sensing, 9(4), 364."
The gridded velocity data is posted at 450 m. These data are accompanied by time-dependent shape files of the ice front and grounding line positions derived from optical and SAR data, error maps for the velocity products, and amplitude imagery. The error is a weighted average of the nominal error in speed from each sensor. The products are distributed in NetCDF format in Polar Stereographic (ESPG 3031) projection with true scale at 71 degree South
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Ice grounding zone processes in Antarctica from satellite SAR interferometry and other data
No full textThis dissertation presents a comprehensive study on the dynamics of ice grounding zones in Antarctica’s ice shelves, with a particular focus on the Amery Ice Shelf and Berry Glacier. Using satellite radar interferometry and other data, this research examines the intricate processes of grounding line migration, ice dynamics, and basal melting. The findings highlight the significant impact of bed topography, subglacial hydrology, and seawater intrusions on ice grounding zone dynamics, challenging traditional models that assume static grounding line positions. In the Amery Ice Shelf region, the study investigates the Lambert, Mellor, and Fisher glaciers, where the analysis of Sentinel-1 data reveals extensive tide-induced grounding line migrations, with movements far exceeding previous estimates. Moreover, high speed of the seawater intrusions brings impressive basal melting in the ice grounding zone. These results underscore the importance of including detailed ice grounding zone processes in ice sheet models to better predict the glaciers’ responses to ocean warming. For Berry Glacier in West Antarctica, the study documents a rapid ice grounding zone retreat over a 25-year period, driven by increased basal melting from warm Circumpolar Deep Water incursions. This retreat has resulted in a marked increase in ice discharge and surface elevation changes, demonstrating the glacier’s accelerated response to oceanic forces. The research presented here contributes to a deeper understanding of the ice grounding zone processes in Antarctica and emphasizes the need for refined modeling approaches that account for the complex interactions between ice, ocean, and subglacial environments. These insights are crucial for improving sea level rise projections and assessing the broader impacts of climate change on polar ice sheets
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Bed topography of Greenland glaciers from high-resolution gravity data
No full textThe mass balance of glaciers is influenced by their bed elevation below sea level, surface melt, and ocean-induced ice melt at calving fronts. It is essential to know the glacier thickness, bed elevation and fjord bathymetry to interpret the glacier evolution in the ongoing warm climate. Traditional methods for mapping ice thickness from radar sounding fail in the terminal valleys occupied by glaciers and near glacier calving fronts because of challenging conditions: side returns, rough surface, warm ice, water inclusions. We had to explore new ways to infer ice thickness. The advent of modern airborne gravimeters capable of sub milligal precision made it possible to explore the usage of gravity. Such instrument had been used widely for oil and mineral surveys and we applied it to glacier ice.In this dissertation, we use airborne gravity data collected in August 2012 with a 0.5 mGal precision at a spatial resolution 750 m, combined with measurements of the fjord bathymetry and mass conservation reconstruction solution to obtain novel mapping of bed topography for several glaciers in Greenland. We use both 2D and 3D modeling to interpret the gravity data in these areas. The models are heavily constrained by geological information as much as we got, such as ocean bathymetry data, rock density information from Geological Survey of Denmark and Greenland with the selection of more optimized initial solutions. We use the gravity misfit to quantify the uncertainty of the inversion. The inversion significantly reduces the gravity misfit from the initial bed, as expected. The gravity misfit ranges from -3 to +3 mGal, the nominal precision of our bed mapping is about 60 m. Our study demonstrated the practical use of high-resolution airborne gravity to fill critical gaps in bed elevation in Greenland, especially in deep fjords that cannot be surveyed with deep radar sounders. The results provide more definite view of the bed topography of these major glaciers system than available previously, meanwhile, at a spatial resolution of 750 m along the trough and with an average precision of about 60 m. However, more precise rock information or supplementary data e.g., magnetic data is needed because of the difficulty in associating with space-varying geology/density.This study provides simple guidelines for utilizing gravity data to obtain glaciers bed topography and then to understand glaciers' evolution in ongoing warm climate for both ocean and atmosphere
A 75,000-y-old Scandinavian Arctic cave deposit reveals past faunal diversity and paleoenvironment
No full textDuring the last glacial period (~118 to 11.7 ka), the Arctic has been characterized by a major redistribution of flora and fauna as a consequence of extreme climatic fluctuations, with associated glacial advances and retreats, sea-level changes, and shifting sea ice extent. In the high-latitude regions of Northern Europe that are currently subject to rapid climate warming, we lack a comprehensive understanding of faunal biodiversity in the last glacial period due to the extreme rarity of preserved organic remains. Here, we present a stratified sediment deposit with a diverse faunal composition preserved in a bone-bearing layer in Arne Qvamgrotta, part of the Storsteinhola cave system (68.10° N 16.38° E) in Northern Norway. Chronological analyses of sediments and bones including radiocarbon, optically stimulated luminescence, uranium–thorium, and phylogenetic dating place the faunal assemblage in Marine Isotope Stage 5a (MIS 5a, Odderade interstadial, ~85 to 71 ka). Combining comparative osteology and bulk-bone metabarcoding, we identify 46 taxa, including mammals, birds, and fish, with several species not previously found in Fennoscandia. The fauna implies a nonanalogous cold-adapted coastal community, with close proximity to sea ice and nearby freshwater bodies. Mitogenome analyses of a subset of taxa identify extinct lineages which attest to a lack of habitat tracking and the absence of a local refugium during the subsequent fully glaciated periods. This faunal record demonstrates long-term faunal dynamics and coastal environmental conditions during MIS 5a in the European Arctic
Annual Ice Velocity of the Greenland Ice Sheet (2001-2010)
No full textWe derive surface ice velocity using data from 16 satellite sensors deployed by 6 different space agencies. The list of sensors and the year that they were used are listed in the following (Table S1). The SAR data are processed from raw to single look complex using the GAMMA processor (www.gamma-rs.ch). All measurements rely on consecutive images where the ice displacement is estimated from tracking or interferometry (Joughin et al. 1998, Michel and Rignot 1999, Mouginot et al. 2012). Surface ice motion is detected using a speckle tracking algorithm for SAR instruments and feature tracking for Landsat. The cross-correlation program for both SAR and optical images is ampcor from the JPL/Caltech repeat orbit interferometry package (ROI_PAC). We assembled a composite ice velocity mosaic at 150 m posting using our entire speed database as described in Mouginot et al. 2017 (Fig. 1A). The ice velocity maps are also mosaicked in annual maps at 150 m posting, covering July, 1st to June, 30th of the following year, i.e. centered on January, 1st (12) because a majority of historic data were acquired in winter season, hence spanning two calendar years.
We use Landsat-1&2/MSS images between 1972 and 1976 and combine image pairs up to 1 year apart to measure the displacement of surface features between images as described in Dehecq et al., 2015 or Mouginot et al. 2017. We use the 1978 2-m orthorectified aerial images to correct the geolocation of Landsat-1 and -2 images (Korsgaard et al., 2016). Between 1984 and 1991, we processed Landsat-4&5/TM image pairs acquired up to 1-year apart. Only few Landsat-4 and -5 images (~3%) needed geocoding refinement using the same 1978 reference as used previously. Between 1991 and 1998, we process radar images from the European ERS-1/2, with a repeat cycle varying from 3 to 36 days depending on the mission phase. Between 1999 and 2013, we use Landsat-7, ASTER, RADARSAT-1/2, ALOS/PALSAR, ENVISAT/ASAR to determine surface velocity (Joughin et al., 2010; Howat, I. 2017; Rignot & Mouginot, 2012). After 2013, we use Landsat-8, Sentinel-1a/b and RADARSAT-2 (Mouginot et al., 2017). All synthetic aperture radar (SAR) datasets are processed assuming surface parallel flow using the digital elevation model (DEM) from the Greenland Mapping Project (GIMP; Howat et al., 2014) and calibrated as described in Mouginot et al., 2012, 2017.
Data were provided by the European Space Agency (ESA) the EU Copernicus program (through ESA), the Canadian Space Agency (CSA), the Japan Aerospace Exploration Agency (JAXA), the Agenzia Spaziale Italiana (ASI), the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) and the National Aeronautics and Space Administration (NASA). SAR data acquisition were coordinated by the Polar Space Task Group (PSTG).
References:
Dehecq, A, Gourmelen, N, Trouve, E (2015). Deriving large-scale glacier velocities from a complete satellite archive: Application to the Pamir-Karakoram-Himalaya. Remote Sensing of Environment, 162, 55–66.
Howat IM, Negrete A, Smith BE (2014) The greenland ice mapping project (gimp) land classification and surface elevation data sets. The Cryosphere 8(4):1509–1518.
Howat, I (2017). MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from Optical Images, Version 2. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center.
Joughin, I., B. Smith, I. Howat, T. Scambos, and T. Moon. (2010). Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping, J. of Glac.. 56. 415-430.
Joughin IR, Kwok R, Fahnestock MA (1998) Interferometric estimation of three dimensional ice-flow using ascending and descending passes. IEEE Trans. Geosci. Remote Sens. 36(1):25–37.
Joughin, I, Smith S, Howat I, and Scambos T (2015). MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data, Version 2. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center.
Michel R, Rignot E (1999) Flow of Glaciar Moreno, Argentina, from repeat-pass Shuttle Imaging Radar images: comparison of the phase correlation method with radar interferometry. J. Glaciol. 45(149):93–100.
Mouginot J, Scheuchl B, Rignot E (2012) Mapping of ice motion in Antarctica using synthetic-aperture radar data. Remote Sens. 4(12):2753–2767.
Mouginot J, Rignot E, Scheuchl B, Millan R (2017) Comprehensive annual ice sheet velocity mapping using landsat-8, sentinel-1, and radarsat-2 data. Remote Sensing 9(4).
Rignot E, Mouginot J (2012) Ice flow in Greenland for the International Polar Year 2008-2009. Geophys. Res. Lett. 39, L11501:1–7
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Short-Timescale Dynamics of Marine-terminating Glaciers in Western Greenland
No full textIceberg calving is a major component of glacier mass ablation that is not well understood due to a lack of detailed temporal and spatial observations. For better understanding, it is critical to examine processes occurring on the time scale of calving processes, sub-daily to sub-hourly. Current satellites are not able to observe the same location at time scales small enough to measure sub-daily phenomena. This research aims to increase the temporal resolution of ice speed and elevation measurements during the calving season to allow for analysis of short-term variations that are otherwise unobserved. We measure glacier speed and surface elevation at 3-minute intervals using a portable radar interferometer at three marine-terminating glaciers in West Greenland over two summer field campaigns. We detect diurnal variations in glacier speed caused by tidal height changes that propagate far inland, the effect of which varies by glacier but are consistent with simple models where basal stress is tidally modulated. We find no speed up from ice shedding off the calving face or the detachment of floating ice blocks, as expected. We detect a 30% speedup within a few hundred meters of the ice front that persists for days when calving removes full thickness grounded ice blocks. Within one ice thickness from the calving front, we detect strain rates 2 to 3 times larger than observable from satellite data, which has implications for studying iceberg calving as a fracturing process, in particular to select an appropriate value of the threshold tensile stress necessary for ice cliff failure
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Ocean-induced Melting of Greenland Ice Shelves
No full textThe Greenland glaciers have been experiencing ongoing acceleration and significant calving events during the last two decades. Ocean-induced melt is a potential trigger for destabilizing the glaciers and ice shelves, and consequently contributing to global sea level rise. However, its mechanism is still uncertain.In this dissertation, we employ observational and numerical methods to improve our under- standings of ocean-induced melt under major Greenland glaciers. Using improved remote sensing data, we calculate melt rates with an improved accuracy. We then employ the Mas- sachusetts Institute of Technology general circulation model (MITgcm) to study ice-ocean interactions beneath an ice shelf in a 2-D configuration at a high resolution. We include ther- mal forcing from the ocean, cavity shape, and for the first time subglacial water discharge at the grounding line. We optimize the heat and salt transfer coefficients to match observed results. The model replicates the general pattern of melting: high near the grounding zone, decreasing rapidly downstream. Melt increases below linear with subglacial discharge and above linear with thermal forcing from the ocean. Next, we investigate the role of the slope of the ice shelf draft in controlling ice shelf melt. The simulations indicate that the melt rate is sensitive to the slope, hence is larger for steeper ice shelves; and the location of the region of high melt migrates toward the grounding line as the slope becomes steeper. In the limit case of a vertical wall, no ice shelf, we know that the locus of ice melt undercuts the glacier.This study provides major new insights on the sensitivity of ice shelf melt to (1) subglacial water discharge: a direct product of ice sheet surface melt (2) thermal forcing from the ocean: a direct product of changes in ocean circulation as a result of wind forcing, and (3) a time-evolving cavity which affects the melt regimes: shallow, nearly flat cavities do not favor high melt; deep, steep cavities favor high melt. These results are important to interpret recent changes on the ice shelves and to inform ice sheet numerical models how to parameterize ice shelf melt in a changing climate
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