ALBERT

Description: Thermokarst is expected to drive major changes in ice-rich permafrost regions, but its current and future extent and rates of change remain only partially understood; in part due to limited broad-scale observations. Here we show that time-lapse digital elevation models from single-pass interferometry can provide important synoptic observations of thermokarst-induced terrain changes and novel insight into the drivers and controls of thermokarst. We focus on retrogressive thaw slumps, an important and dynamic form of thermokarst. On sub-seasonal time scales, sparse measurements indicate that mass wasting at active slumps is often limited by the energy available for melting ground ice, but other factors such as rainfall or the formation of an insulating veneer are also thought important. To study the sub-seasonal drivers, we use TanDEM-X observations (12 m resolution) acquired during the Science Phase in summer 2015 over two study regions. The high vertical precision (30 cm), frequent observations (11 days) and large coverage (5000 km2) allow us to track volume losses as drivers (e.g. available energy) vary through time. We find that thaw slumps in the Tuktoyaktuk coastlands, Canada, are not energy limited in June, as they undergo limited mass wasting (height loss of around 0 cm/day) despite the ample available energy, indicating the widespread presence of an insulating snow or debris veneer. Later in summer, height losses generally increase (around 3 cm/day), but they do so in distinct ways. For many slumps, mass wasting tracks the available energy, a temporal pattern that is also observed at coastal yedoma cliffs on the Bykovsky Peninsula, Russia. However, the other two common temporal trajectories are asynchronous with the available energy, as they track strong precipitation events or show a sudden speed-up in late August, respectively. The contrasting temporal behaviour of nearby thaw slumps highlights the importance of complex local and temporally varying controls on mass wasting. This complexity reinforces the need for circum-arctic monitoring efforts, for which remote sensing approaches such as single-pass interferometry are indispensable.