Structural evolution triggers a dynamic reduction in active glacier length during rapid retreat: Evidence from Falljökull, SE Iceland

Over the past two decades Iceland's glaciers have been undergoing a phase of accelerated retreat set against a backdrop of warmer summers and milder winters. This paper demonstrates how the dynamics of a steep outlet glacier in maritime SE Iceland have changed as it adjusts to recent significant changes in mass balance. Geomorphological evidence from Falljökull, a high‐mass turnover temperate glacier, clearly shows that between 1990 and 2004 the ice front was undergoing active retreat resulting in seasonal oscillations of its margin. However, in 2004–2006 this glacier crossed an important dynamic threshold and effectively reduced its active length by abandoning its lower reaches to passive retreat processes. A combination of ice surface structural measurements with radar, lidar, and differential Global Navigation Satellite Systems data are used to show that the upper active section of Falljökull is still flowing forward but has become detached from and is being thrust over its stagnant lower section. The reduction in the active length of Falljökull over the last several years has allowed it to rapidly reequilibrate to regional snowline rise in SE Iceland over the past two decades. It is possible that other steep, mountain glaciers around the world may respond in a similar way to significant changes in their mass balance, rapidly adjusting their active length in response to recent atmospheric warming.