A synthesis of the thermal state of the bed of the Greenland Ice Sheet

Thursday, 17 December 2015: 11:20
3007 (Moscone West)
Joseph A MacGregor1, Mark A Fahnestock2, Ginny A Catania3, Andy Aschwanden4, Gary D Clow5, William T Colgan6, Prasad S Gogineni7, Mathieu Morlighem8, John Drysdale Paden7, Stephen F Price9 and Helene L Seroussi10, (1)University of Texas, Institute for Geophysics, Austin, TX, United States, (2)University of Alaska Fairbanks, Fairbanks, AK, United States, (3)University of Texas at Austin, Austin, TX, United States, (4)University of Alaska Fairbanks, Arctic Region Supercomputing Center, Fairbanks, AK, United States, (5)INSTAAR, Boulder, CO, United States, (6)York University, Toronto, ON, Canada, (7)University of Kansas, Lawrence, KS, United States, (8)University of California Irvine, Irvine, CA, United States, (9)Los Alamos National Laboratory, Los Alamos, NM, United States, (10)NASA Jet Propulsion Laboratory, Pasadena, CA, United States
The thermal state of an ice sheet’s bed (frozen or thawed) is an important control upon its history, present dynamics and future vulnerability to external forcings. However, this thermal state can only be either observed directly within sparse boreholes or inferred from the presence of subglacial lakes. Hence, indirect inferences of this state are required for the vast majority of an ice sheet. Here we synthesize new and existing spatially extensive inferences of the present thermal state of the bed of the Greenland Ice Sheet to better constrain this state, identify the most poorly constrained regions and inform ice-flow modeling. New remote inferences of this basal thermal state are derived from modeling of this ice sheet’s radiostratigraphy, analysis of surface imagery and interpretation of surface-velocity patterns. Existing inferences include outputs from thermomechanical ice-flow models with varied spin-ups, forcings and underlying assumptions. Between similarly forced models, there is general agreement regarding the thermal state of the bed, except where this state transitions from frozen to thawed. Both observations and modeling generally agree that the Northeast Greenland Ice Stream (NEGIS) and significant portions of the southwestern ice-drainage basins are thawed at the bed, while the vicinities of central ice divides, particularly their west-facing slopes, are frozen. There is poor agreement regarding the basal thermal state within the NEGIS drainage basin outside of NEGIS itself, above ~2000 m surface elevation in the southern drainage basins and along the eastern ice-sheet margin. Further, remote inferences and modeling rarely represent the observed basal thermal state accurately at NorthGRIP, DYE-3 and near northeastern subglacial lakes. This synthesis identifies large regions of the Greenland Ice Sheet where additional observations could significantly improve assessment of its basal thermal state, and where the ice sheet is likely most sensitive to millennial-scale external forcings. Finally, we discuss the possible sources of disagreement between the observed and modeled basal thermal state.