Greenland’s Jakobshavn Glacier (locally known as Sermeq Kujalleq) is one of the fastest-moving glaciers in the world and a major contributor to sea-level rise. We have a new study looking at the ice-sheet area, or catchment, that Jakobshavn drains. One of the approaches for assessing the mass balance, or health, of Jakobshavn is the input-output method. This method differences iceberg discharge into the ocean across the grounding line from net snow accumulation within its upstream catchment. This means you need a pretty good idea of Jakobshavn’s catchment area. But, today’s currently available delineations of Jakobshavn’s catchment area vary by ±12%. This uncertainty in catchment area translates into an uncertainty in area-integrated net snow accumulation.
Figure 1 – Four widely used delineations of Jakobshavn Glacier’s ice-sheet catchment vary by ±12%, or approximately ±10,000 km2. Although we want to understand how Jakobshavn’s catchment will evolve over the coming century, it is challenging to simply agree on its delineation today.
Glacier catchments are not constant through time. For this study, we looked at how Jakobshavn’s catchment area might evolve in the future. We used an ensemble of future ice flow simulations created for the Ice Sheet Model Inter-comparison Project (ISMIP6) to delineate Jakobshavn’s catchment under different climate scenarios to the year 2100. The ensemble suggests that Jakobshavn’s catchment could expand by 3–9%, depending on the intensity of ocean and atmospheric warming of a given climate scenario. These changes in Jakobshavn’s catchment appear to trigger a phenomenon called “dynamic piracy,” whereby Jakobshavn is essentially stealing ice from its neighboring glaciers, redirecting it into its own flow toward the ocean.
Figure 2 – The Jakobshavn Glacier catchment area delineated in 2015 and 2100 in thirteen ISMIP6 ensemble members. There is a diversity of model opinion on how Jakobshavn’s catchment looks, both today and tomorrow, but the ensemble generally agrees that catchment area will expand over the next century.
Generally, however, the ensemble of models has some challenges reproducing recently observed reorientations in inland ice flow. The models are generally less sensitive to climate change, producing less acceleration than actually observed. All but one of the ensemble members fail to reproduce recent accelerations in ice flow observed about 100 km inland from Jakobshavn’s terminus. We interpret this as suggesting that the current ensemble of models likely underestimates future reorientations in deep inland ice flow. Simply put, they may not fully capture how rapidly the ice sheet’s catchments are reorganizing themselves under future climate change.
Figure 3 – Comparison of modelled ice acceleration and rotation with the mean observed at ten GPS stations clustered at approximately 100 km inland from Jakobshavn’s terminus.The ensemble of models has difficulty reproducing this recently observed reorganization of inland ice flow.
Our analysis of the ISMIP ensemble reminds us that big outlet glaciers are not just passive responders to climate change; they actively reshape their catchments in ways that ripple through the ice sheet. So, if we want accurate glacier-scale input-output assessments, then we need to have accurate glacier-scale catchments, both today and in the future. This highlights the importance of improving our delineation of ice-sheet catchments using both observational methods and ice flow models. This also means continually improving the ice flow models used to predict the future form and flow of Earth’s ice sheets.
Løkkegaard A., W. Colgan, A. Aschwanden and S.A. Khan. 2024. Recent and future variability of the ice-sheet catchment of Sermeq Kujalleq (Jakobshavn Isbræ), Greenland. Journal of Glaciology. 1-15. https://doi.org/10.1017/jog.2024.73
We have a new open-access study out in the current volume of The Cryospherethat looks at geothermal heat flow beneath the Greenland Ice Sheet. Geothermal heat flow is an important boundary condition for ice flow models because it influences the temperature of the ice-bed interface, which in turn influences how easily ice can deform and flow. Right now, there are about seven widely used estimates of geothermal heat flow beneath the Greenland Ice Sheet (Figure 1). These different heat flow maps come from different research groups, using different methods. It hasn’t been entirely clear what the influence of choice of heat flow map has on modeled ice flow. For example, the ensemble of Greenland ice flow projections within the Ice-Sheet Model Inter-comparison Project for CMIP6 (ISMIP6) used differing heat flow maps. Our new study simply spins up a Greenland ice flow model with all seven heat flow maps and tries to understand the resulting differences in ice thickness and velocity.
Figure 1 – The differences in the magnitude and spatial distribution of geothermal heat flow, relative to ensemble mean, in the seven Greenland geothermal heat flow maps we assessed in this study.
Although the average geothermal heat flow only varies by about ±10 mW/m2 across the seven heat flow maps, there are pronounced variations in the spatial distributions of this heat flow. This means that there can be local heat flow differences of up to ±100 mW/m2 between individual heat flow maps. When the ice sheet model is spun up in a fully transient mode with these different heat flow maps, there are lots of areas where the difference in ice thickness from ensemble mean exceeds ±150 meters (Figure 2). This is due to spatial differences in basal ice temperatures, which strongly influence the viscosity and deformation of ice. Across the seven transient spin ups, the discharge of icebergs into the ocean varies by about ±10 gigatonnes per year, which is equivalent to about ±2.5% of the total ice-sheet iceberg discharge.
Figure 2 – The differences in ice thickness after a fully transient 10,000 year spin up, relative to ensemble mean, associated with the seven Greenland geothermal heat flow maps we assessed in this study.
We also spun up the ice flow model with the seven different heat flow maps under a so-called “nudged” spin up that was a key part of ISMIP6. Unlike a fully transient spin up, a “nudged” spin up constrains modeled ice thicknesses with observed ice thicknesses; it generally ensures a very realistic geometry for the simulated ice sheet. Although the ice geometry is more-or-less constant across these “nudged” simulations, there are still pronounced differences in the magnitude and spatial distribution of ice velocity. This largely results from large differences in the extents of the frozen and thawed ice-bed areas. Depending on choice of heat flow map, between 22 and 54% of the ice-bed area is simulated as thawed (Figure 3). This has strong implications for the proportion of the ice sheet beneath which water-dependent processes, like basal sliding, can occur.
Figure 3 – The differences in temperature at the ice-bed interface after a nudged ISMIP6-style spin up, relative to pressure melting point, associated with the seven Greenland geothermal heat flow maps we assessed in this study.
So, what is the way forward after showing that the choice of heat flow map has a non-trivial impact on Greenland ice sheet simulations? Well, deciding which heat flow map is “the best” is one approach. We highlight a small, but growing, database of in situ temperature measurements against which simulated ice temperatures can be compared. There are also qualitative hints as to which heat flow map might be most appropriate. For example, does it preserve the widespread frozen basal conditions that we find in North Greenland? In terms of community ice-sheet projections, we recommend that it may be prudent to limit the direct inter-comparison of ice-sheet simulations to those using a common heat-flow map. In terms of ISMIP specifically, we suggest that future ensemble should perhaps use a range of basal geothermal forcing scenarios, similar to how it employs a range climate forcing scenarios.
Zhang, T., Colgan, W., Wansing, A., Løkkegaard, A., Leguy, G., Lipscomb, W. H., and Xiao, C.: Evaluating different geothermal heat-flow maps as basal boundary conditions during spin-up of the Greenland ice sheet, The Cryosphere, 18, 387–402, https://doi.org/10.5194/tc-18-387-2024, 2024.
