GBaTSv2: A revised synthesis of the likely basal thermal state of the Greenland Ice Sheet

Abstract

Abstract. The basal thermal state (frozen or thawed) of the Greenland Ice Sheet is under-constrained due to few direct measurements, yet knowledge of this state is becoming increasingly important to interpret modern changes in ice flow. The first synthesis of this state relied on inferences from widespread airborne and satellite observations and numerical models, for which most of the underlying datasets have since been updated. Further, new and independent constraints on the basal thermal state have been developed from analysis of basal and englacial reflections observed by airborne radar sounding. Here we synthesize constraints on the Greenland Ice Sheet’s basal thermal state from boreholes, thermomechanical ice-flow models that participated in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), BedMachine v4 bed topography, Making Earth Science Data Records for Use in Research Environments (MEaSUREs) multi-year surface velocity mosaic v1, and multiple inferences of a thawed bed from airborne radar sounding. Most constraints can only identify where the bed is likely thawed rather than where it is frozen. This revised synthesis of the Greenland likely Basal Thermal State version 2 (GBaTSv2) indicates that 32 % of the ice sheet’s bed is likely thawed, 39 % is likely frozen, and the remainder (29 %) is too uncertain to specify. Although the spatial pattern of GBaTSv2 is broadly similar to the previous synthesis, including a scalloped frozen core and thawed outlet-glacier systems, the likely basal thermal state of nearly half (48 %) of the ice sheet has changed designation. This revised synthesis suggests that more of northern Greenland is likely thawed at its bed, and conversely that more of southern Greenland is likely frozen, both of which influence interpretation of the ice sheet’s present subglacial hydrology and models of its future evolution. The GBaTSv2 dataset, including both code that performed the analysis and the resulting raster products, is freely available at https://doi.org/10.5281/zenodo.5714527.

Geologic Provinces Beneath the Greenland Ice Sheet Constrained by Geophysical Data Synthesis

“…This method is motivated by the observation and supporting theory that an ice mass flowing over variable bed topography transfers a signature of that subglacial topography to its subaerial surface (Gudmundsson, 2003;Ng et al, 2018), a phenomenon that is not presently considered to infer bed topography in the ice-sheet interior (Morlighem et al, 2022). To calculate this hillshade, we first filter the measured flow direction across the GrIS interior (Joughin et al, 2016(Joughin et al, , 2017 toward the direction of the surfaceelevation gradient instead, as measured by the Greenland Ice Mapping Project (GrIMP; Howat et al, 2022) and following MacGregor et al (2022). We illuminate the GrIMP DEM using a standard hillshade algorithm (Greene et al, 2017), modified so that the illumination azimuth may vary for each pixel, which we select as 90°counterclockwise to the filtered flow direction.…”

Subglacial lake activity beneath the ablation zone of the Greenland Ice Sheet

“…This distribution is consistent with that predicted in Bowling et al (2019), with hydrologically active lakes located near or below the Equilibrium Line Altitude (ELA). There is a general paucity of active lakes in the southeastern sector of Greenland where high accumulation rates and thick firn limit the amount of surface-derived water that reaches the ice bed, and inland sectors of Greenland, where the bed is thought to be largely frozen (MacGregor et al, 2022). In contrast, stable subglacial lakes tend to be located in northern and eastern regions above the ELA (Bowling et al, 2019).…”