The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics, and response to external forcings. However, this state can only be observed directly at sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the Sea Level Response to Ice Sheet Evolution (SeaRISE) effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity, and Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west facing slopes, is frozen. Elsewhere, there is poorer agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area), where additional observations would most improve knowledge of its overall basal thermal state (MacGregor et al. 2016).

The present basal thermal state of the GrIS was evaluated using four independent methods: 3-D thermomechanical modeling and basal motion inferred from radiostratigraphy, surface velocity, and surface texture, respectively.

There are four data sources:

• Basal temperature fields from SeaRISE control runs of 8 ice-sheet models from Nowicki et al. 2013
• Gridded basal melt rate, shear layer thickness and shape factor inferred from 1-D modeling of the radiostratigraphy presented by MacGregor et al. 2015
• Modeled ratio of surface speed (Joughin et al. 2010) to modeled maximum deformation speed
• Traced onset of surface undulations in the MODIS Mosaic of Greenland (Haran et al. 2013)

Data Collection Methods:

• 3-D thermomechanical numerical modeling
• 1-D steady-state modeling of dated Holocene radiostratigraphy
• Modeling of the maximum deformation speed of a temperate ice column and comparison with observed surface speed
• Visual analysis of MOG surface texture

Each method is included without weighting so as to generate an unbiased assessment of the agreement between these methods and the likely basal thermal state. See MacGregor et al. 2016 for additional details regarding the algorithm applied. 

We synthesize these multiple methods of constraining the basal thermal state across Greenland described above by simply assessing where each of the above independent methods produces a clear signal regarding this state. We initialize a 5-km gridded ice-sheet “agreement” mask to zero. For each method at each grid point, if a signal exists for a frozen (thawed) bed, then –1 (+1) is added to this mask.

If a given method does not yield an unambiguous signal regarding the basal thermal state, then this agreement is not adjusted there based on that method. We do not weight any of the methods with respect to each other. Prior to but following the same procedure as for the main mask, a separate mask is generated using the 3-D thermomechanical model outputs, each weighted equally. In this manner, each independent method contributes to an unbiased synthesis of the basal thermal state.

Based on confidence bounds or uncertainty estimates for each of the four methods described above and their discriminating characteristics, two additional instances of the agreement mask are generated: a cold-bias instance and a warm-bias instance. We then generate a new likely basal thermal state mask that synthesizes the agreement between the different methods and represents the likely thermal state of the bed. This new mask is also initialized to zero and then assigned –1 (+1), representing a frozen (thawed) bed where at least two of the three instances of the agreement mask agree on the basal thermal state (sign, regardless of their degree of agreement in this state (magnitude). If only two instances agree, then the assignment is made only if the other instance does not suggest the opposite basal thermal state.

We assume that regions of the bed where the likely basal thermal state is uncertain that are surrounded by likely thawed or frozen bed are more likely than not to possess the same basal thermal state as their surroundings. Following this reasoning, we reassign uncertain “holes” less than 10 grid cells in size (≤250 km2) with their surrounding basal thermal state. Similarly, in regions where this mask is uncertain, we reassign likely frozen or thawed “holes” of the same limited size to the surrounding basal thermal state.

Trajectory and Attitude Data

Processing Steps

See Derivation Techniques and Algorithms.

Version History

First release, Version 1.

Error Sources

Potential error sources are numerous and are considered explicitly in this dataset. Confidence intervals or existing uncertainties for the datasets used are incorporated into the cold-bias and warm-bias agreement masks, which ultimately inform the likely basal thermal state mask.