| Description |
Understanding long-term ice dynamic response to climate change remains of the utmost importance with respect to constraining sea level rise (SLR) projections for 2100. SLR contributions from Alaska approximate those from Greenland and may be dominated by mass losses from changes in flow dynamics. But due to a lack of data on flow dynamics, projections for future mass change in Alaska only consider surface mass balance. Here we present the first regionally extensive dataset of mountain glacier flow velocities in Alaska-covering 28,022 km2 of ice. This dataset reveals that more than 50% of the mass flux in Alaska comes from only eleven key glacier systems that have high mass fluxes due to high balance velocities and are not necessarily linked to tidewater glacier retreat. In south central Alaska, we find that the rate of mass loss from tidewater calving is equivalent to 75% of the total net mass loss annually; thus surface mass balance alone is inadequate to project future statewide mass losses. Our dataset also enables a close examination of a surge (periodic acceleration) event on Bering Glacier, the largest surging glacier in the world. There, velocities exceed quiescent speeds by 18 times over two periods lasting a total of 3 years. Results suggest that downstream propagation of the surge is closely linked to the evolution of the driving stress during the surge because driving stress appears to be tied to the spatial variability of resistive stress provided by the bed. Finally, we are able to examine regional changes in wintertime flow velocities and find that wintertime flow speed is inversely correlated with summertime positive degree days. We propose that this relationship is the result of a negative feedback mechanism whereby increased meltwater production enlarges subglacial conduit systems that are more effective at discharging water from subglacial cavities. As cavities close during the fall, less remaining water reduces bed separation during winter and thus engenders slower sliding velocities. We find this mechanism exerts a secondary control on glacier surge triggering, encouraging/discouraging initiation after cold/warm summers. This mechanism could have important ice dynamic implications when forced by a changing climate. Increases in summertime temperatures could result in a gradual slowing of land terminating ice, thus providing a negative feedback (self correcting) mechanism that could slightly slow projected mass losses from land terminating glaciers. |
