Outlet glaciers are the main arteries of the Greenland Ice Sheet transporting ice from the interior towards the ocean. Understanding their dynamics is crucial in the context of accelerating Arctic warming. While glacier flow dynamics is monitored with satellites, important rapid processes remain undetected at intervals of days between acquisitions. The short-term dynamics of fast flowing glaciers therefore remains poorly understood and insufficiently constrained in numerical models.
Here, we use high-rate field observations on Greenland's fastest ice stream, Sermeq Kujalleq in Kangia (SKK), to characterize its response to the drainage of two surface lakes. The ensuing subglacial flood caused a surface speedup pulse that efficiently propagated towards the terminus where it triggered large calving events. This full process chain has so far not been documented. Using this large and abrupt disturbance as a full-scale natural experiment, we investigate SKK's dynamic sensitivity in unprecedented detail. The undamped and fast-pace nature of the speedup pulse, together with the instantaneous response of the surrounding ice margins, indicate strong coupling within the ice stream system. Our results highlight how inland areas can accommodate large disturbances and efficiently propagate them downstream to trigger irreversible changes at dynamic glacier termini potentially prone to destabilization.