Abstract
The Greenland Ice Sheet is now the largest land ice contributor to global sea level rise, largely driven by increased surface meltwater runoff from the ablation zone, i.e., areas of the ice sheet where annual mass losses exceed gains. This small but critically important area of the ice sheet has expanded in size by ~50% since the early 1960s, and satellite remote sensing is a powerful tool for monitoring the physical processes that influence its surface mass balance. This review synthesizes key remote sensing methods and scientific findings from satellite remote sensing of the Greenland Ice Sheet ablation zone, covering progress in (1) radar altimetry, (2) laser (lidar) altimetry, (3) gravimetry, (4) multispectral optical imagery, and (5) microwave and thermal imagery. Physical characteristics and quantities examined include surface elevation change, gravimetric mass balance, reflectance, albedo, and mapping of surface melt extent and glaciological facies and zones. The review concludes that future progress will benefit most from methods that combine multi-sensor, multi-wavelength, and cross-platform datasets designed to discriminate the widely varying surface processes in the ablation zone. Specific examples include fusing laser altimetry, radar altimetry, and optical stereophotogrammetry to enhance spatial measurement density, cross-validate surface elevation change, and diagnose radar elevation bias; employing dual-frequency radar, microwave scatterometry, or combining radar and laser altimetry to map seasonal snow depth; fusing optical imagery, radar imagery, and microwave scatterometry to discriminate between snow, liquid water, refrozen meltwater, and bare ice near the equilibrium line altitude; combining optical reflectance with laser altimetry to map supraglacial lake, stream, and crevasse bathymetry; and monitoring the inland migration of snowlines, surface melt extent, and supraglacial hydrologic features.
Highlights
The Greenland Ice Sheet (GrIS) is the second-largest ice mass on earth
Multi-sensor methods, for example, optical imagery combined with synthetic aperture radar (SAR) imagery or scatterometry, shows promise for detecting dynamic regions characterized by changing snow, firn, and ice surface types, including supraglacial lakes and slush fields obscured by snow or clouds, and may reduce detection bias caused by cloud cover [362,363,364]
Satellite remote sensing data show an ablation zone expanded in size, its albedo and surface elevation lower in response to enhanced melting and ice discharge, and an ice sheet transition from steady state to negative mass balance that represents the largest land ice contributor to global sea level rise
Summary
The Greenland Ice Sheet (GrIS) is the second-largest ice mass on earth. If the entire ice sheet were to melt, global sea level would rise by about seven meters [1]. Surface processes in the ablation zone, those that control the exposure and melting of bare ice, will play an enhanced role in determining the long-term GrIS mass balance [1,14,26,30,32]. Previous reviews of satellite remote sensing of glaciers and ice sheets exist [e.g., 46], including recent reviews focused on regional climate modeling of the GrIS surface mass balance [21,60]. To the authors’ knowledge, no review has focused on satellite remote sensing the GrIS ablation zone, a small but critically important area of the ice sheet with unique physical processes and strong potential to expand in the coming years. Min the GrIS ablation zone [26,29,30]
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