Monitoring glacier fade out in Austrian Eastern Alps

<p>In the Austrian Alps, recent rapid glacier melt affected the glaciers up to the summits. Glacier disintegration, debris flows, rock falls and increased melt rates are challenging glacier monitoring. Apart from the technical and safety issues arising, the scientific question arises how long a glacier fade out can and should be observed.</p> <p>From a technical perspective, direct mass balance monitoring is hampered by destruction of stakes by rock fall and snow pressure mounted on the increasingly steep glacier surfaces. Glacier changes require a frequent repositioning of stakes. Deep parts of the glaciers where stakes were not mounted in the past because the area was heavily crevassed remain longer that the thinner parts where stakes were operated in past decades, for example on Jamtalferner/Silvretta.</p> <p>Area loss meanwhile has a significant impact on glacier wide specific mass balance, but an annual resurvey of glacier area by terrestrial photogrammetry, airborne LiDAR or UAV rises the monitoring costs and effort considerably, while the uncertainty of mapping glacier area from remote sensing images is hampered by resolution and/or debris cover.  </p> <p>Subglacial melt in large cavities which developed during the last years so far is not quantified, but estimated from thickness loss data suggest that the contribution of basal mass loss to total mass loss can be as large as surface mass balance.</p> <p>Common definitions of a glacier imply ice formation from snow and firn, ice dynamics and a runoff system. For several Austrian glaciers, snow and firn cover is entirely gone. There, no new ice can form during the next decades even in case accumulation of snow would take place in future. In addition to that, flow velocities drop to <5 m/year, which is a similar magnitude than that measured for rock glaciers. From a glaciological perspective, those (former?) glaciers rather meet the definition of dead ice, especially when covered by debris.</p> <p>From a hydrological perspective, monitoring of those glacier remnants still makes sense, as they are still part of the local hydrological system. From the perspective of hazard monitoring, glacier remnants are also worth monitoring as they are relevant for generation of debris flows and englacial/thermokarst lakes.</p> <p>To continue local glacier monitoring and contribute to a global database, a joint global monitoring strategy and methodology would be beneficial. For example, a definition of ‘transient glaciers’ in fade out with a specification of monitoring methods would be helpful to prolongate mass balance time series when direct measurements become impossible for the total area. That could be single stakes, geodetic methods when glacier margins are still clear, or hydrological methods.   </p>