Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus

The recent marked retreat, thinning and acceleration of most of Greenland’s outlet glaciers south of 70 N has increased concerns over Greenland’s contribution to future sea level rise1–5. These dynamic changes seem to be parallel to the warming trend in Greenland, but the mechanisms that link climate and ice dynamics are poorly understood, and current numerical models of ice sheets do not simulate these changes realistically6–8. Uncertainties in the predictions of mass loss from the Greenland ice sheet have therefore been highlighted as one of the main limitations in forecasting future sea levels9. Here we present a numerical ice-flow model that reproduces the observed marked changes in Helheim Glacier, one of Greenland’s largest outlet glaciers. Our simulation shows that the ice acceleration, thinning and retreat begin at the calving terminus and then propagate upstream through dynamic coupling along the glacier. We find that these changes are unlikely to be caused by basal lubrication through surface melt propagating to the glacier bed. We conclude that tidewater outlet glaciers adjust extremely rapidly to changing boundary conditions at the calving terminus. Our results imply that the recent rates of mass loss in Greenland’s outlet glaciers are transient and should not be extrapolated into the future. Two main hypotheses have been advanced to explain the rapid dynamic changes of Greenland’s outlet glaciers. The first postulates that the dynamical changes result from processes that act at the terminus and trigger a retreat and reduce along-flow resistive stresses (backstress)2,3,10. This leads then to faster ice flow and thinning that propagates rapidly upstream and leads to further retreat. Several climate-related processes may initiate these nearterminus changes, such as surface-melt induced thinning and increased calving due to enhanced hydro-fracturing of water-filled crevasses from increased surface melt11. For Helheim Glacier, the sensitivity to such processes may be further enhanced by a basal overdeepening in the fjord12, as has been suggested for tidewater glaciers13–15. The second hypothesis is that warmer air temperatures increase the amount of surfacemeltwater reaching the glacier bed, increasing basal lubrication and the rate at which ice slides over its bed, leading to glacier acceleration, thinning and retreat16,17. To better understand the processes driving rapid outlet glacier change and assess their potential future impact, we developed a numerical flow model for Helheim Glacier that includes horizontal (along-flow and lateral) stress transfer and a dynamically determined adjustment of the grounded calving front (see the Methods section and Supplementary Information,Model).