Surging glaciers in Svalbard: Observing their distribution, characteristics and evolution

Glacier surges are episodes of significantly increased ice flow due to ice-dynamical feedbacks, and are often repeated in a quasi-periodical manner. Ice mass is redistributed during a surge, which leads to surface lowering at high elevation as ice is transferred down-glacier and thickening nearer the terminus. In this paper, we review different approaches for monitoring and detecting glacier surges in Svalbard, one of the most prominent global clusters of surge-type glaciers. Current surge detection is mainly based upon tracking the speed of glaciers over time, measuring elevation and frontal changes, and more recently automatically detecting surface changes such as increased crevassing. Thermal and hydrological changes near the glacier bed drive surge dynamics and can be measured using geophysical sensors such as ground-penetrating radar (GPR) and seismometers. When glaciers surge, they often produce diagnostic landforms in subglacial and proglacial environments, allowing historical surging to be identified even if surges have not been directly observed. Through this review, we have compiled an updated database of surge-type glaciers in Svalbard and find that 36% of glaciers display surge-type behaviour, rising to 52% when removing glaciers smaller than 1 km2. Only 9% of all glaciers have been directly observed to surge. Svalbard surge-type glaciers have gentler slopes, are generally longer, and extend across a larger elevation range. We find that the behaviour of surge-type glaciers is variable and more closely resembles a continuum from glaciers that do not surge to those which redistribute mass in a single event of strongly enhanced ice flux. We can describe the variability in surge behaviour using the concept of enthalpy and a six-stage surge model that characterises the build-up of energy at the glacier bed driven initially by thermal change and then ice acceleration which is prompted by changes in subglacial hydrology. Observations of glacier surges have improved significantly with routine mapping from satellites such as Sentinel-1, Sentinel-2 and the Landsat satellite series. Furthermore, an increasing number of geophysical measurements is enabling an improved understanding of subglacial processes before, during and after a surge, which is crucial for improving models of surge behaviour. As our observations of surges continue to improve, we expect to uncover new elements and details of surge behaviour which suggests we need to rethink the binary classification of glaciers as either ‘surge-type’ or ‘not surge-type’ in Svalbard and across the world.