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Marine-terminating outlet glacier terminus traces, mapped from satellite and aerial imagery, have been used extensively in understanding how outlet glaciers adjust to climate change variability over a range of timescales. Numerous studies have digitized termini manually, but this process is labor intensive, and no consistent approach exists. A lack of coordination leads to duplication of efforts, particularly for Greenland, which is a major scientific research focus. At the same time, machine learning techniques are rapidly making progress in their ability to automate accurate extraction of glacier termini, with promising developments across a number of optical and synthetic aperture radar (SAR) satellite sensors. These techniques rely on high-quality, manually digitized terminus traces to be used as training data for robust automatic traces. Here we present a database of manually digitized terminus traces for machine learning and scientific applications. These data have been collected, cleaned, assigned with appropriate metadata including image scenes, and compiled so they can be easily accessed by scientists. The TermPicks data set includes 39 060 individual terminus traces for 278 glaciers with a mean of 136 ± 190 and median of 93 of traces per glacier. Across all glaciers, 32 567 dates have been digitized, of which 4467 have traces from more than one author, and there is a duplication rate of 17 %. We find a median error of ∼ 100 m among manually traced termini. Most traces are obtained after 1999, when Landsat 7 was launched. We also provide an overview of an updated version of the Google Earth Engine Digitization Tool (GEEDiT), which has been developed specifically for future manual picking of the Greenland Ice Sheet. [ABSTRACT FROM AUTHOR]

Helheim Glacier, one of the largest marine‐terminating outlet glaciers draining the Greenland Ice Sheet, underwent significant retreat and acceleration in the early 2000s, accounting for an appreciable proportion of the ice sheet's mass loss during that period. Using a range of remotely sensed datasets, we show that despite a subsequent readvance, the glacier has continued to lose mass and thin, and has retreated inland of the retracted position occupied in 2005. Critically, the near‐terminus is up to 100 m thinner than during 2005, and the front 5 km is within 25–50 m of flotation, with retrograde bed slopes extending several kilometers inland of the terminus. The neighboring Fenris and Midgard Glaciers have both undergone recent large‐scale and rapid retreat once their near‐terminus regions began to float, suggesting that under projected climate warming and associated glacier thinning, Helheim Glacier is poised to pass a threshold whereby the near‐terminus region will retreat rapidly. Plain Language Summary: A significant proportion of the Greenland Ice Sheet's contribution to global sea‐level rise is as a result of mass loss from its marine‐terminating glaciers. Helheim Glacier, located in southeast Greenland, is one of the largest and fastest‐flowing glaciers draining the Greenland Ice Sheet. During the early 2000s, Helheim Glacier underwent dramatic retreat and acceleration, but subsequently readvanced during relatively cooler conditions in 2006. However, persistently high ocean and atmospheric temperatures have meant that Helheim Glacier has in fact been continuously losing mass since 2003, despite this readvance. Here, we use a range of remotely sensed data to show that since 2014, Helheim Glacier has accelerated and retreated to a greater extent than occurred in 2003–2005. More importantly, as the glacier has been losing mass over the past two decades, it is currently much thinner than during its peak retreat in 2005. As the glacier continues to lose mass, it will pass a threshold whereby the ice will float and rapidly disintegrate. We observe that this process has already occurred at the glaciers neighboring Helheim Glacier, which are subject to the same climate forcing and thus provide an analogue for the future response of Helheim Glacier to continued warming. Key Points: Helheim Glacier is more retreated and the near‐terminus region is up to 100 m thinner than during its much‐reported dramatic retreat in 2005Helheim, one of Greenland's largest ice dischargers, is now more vulnerable than at any point since the Little Ice AgeHelheim's new configuration offers potential for sustained dynamic instability and a major contribution to global sea‐level rise [ABSTRACT FROM AUTHOR]

Frontal ablation from tidewater glaciers is a major component of the total mass loss from the Greenland ice sheet. It remains unclear, however, how changes in atmospheric and oceanic temperatures translate into changes in frontal ablation, in part due to sparse observations at sufficiently high spatial and temporal resolution. We present high-frequency time-lapse imagery (photos every 30 min) of iceberg calving and meltwater plumes at Kangiata Nunaata Sermia (KNS), southwest Greenland, during June–October 2017, alongside satellite-derived ice velocities and modelled subglacial discharge. Early in the melt season, we infer a subglacial hydrological network with multiple outlets that would theoretically distribute discharge and enhance undercutting by submarine melt, an inference supported by our observations of terminus-wide calving during this period. During the melt season, we infer hydraulic evolution to a relatively more channelised subglacial drainage configuration, based on meltwater plume visibility indicating focused emergence of subglacial water; these observations coincide with a reduction in terminus-wide calving and transition to an incised planform terminus geometry. We suggest that temporal variations in subglacial discharge and near-terminus subglacial hydraulic efficiency exert considerable influence on calving and frontal ablation at KNS. [ABSTRACT FROM AUTHOR]

The successful prediction of the response of the Greenland Ice Sheet (GrIS) to climate warming relies in part on the accurate estimation of future ice loss from tidewater glaciers. Despite being a major component of ice loss from the GrIS, the mechanisms controlling iceberg calving remain poorly understood. Understanding calving processes requires observations at sufficiently high spatial and temporal resolutions to isolate the critical and potentially ubiquitous controls on calving that will enable more accurate modelling of both past and future glacier dynamics. Here we present high-resolution (photos every 30 minutes) time-lapse image analysis of calving activity and meltwater plume presence at Kangiata Nunaata Sermia (KNS), southwest Greenland during the 2017 summer melt season (May-Oct) which we analyse in conjunction with satellite-derived ice velocity estimates and modeled estimates of subglacial discharge. Our results suggest that iceberg calving at KNS is likely dominated by variations in runoff and, by extension, the subglacial hydrological system. Early in the melt season, we infer a distributed subglacial hydrological network that promotes widespread submarine melting and terminus undercutting, which subsequently enhances terminus-wide calving and ice loss. Later in the melt season, we suggest an evolution to a channelized subglacial system, due to the observation of meltwater plumes at the fjord surface signalling more focussed emergence of subglacial water. This switch coincides with a reduction in iceberg calving and a transition to a more complex terminus geometry with embayments coincident with the location of meltwater plumes. These observations are indicative of localised preferential melting of the terminus but most importantly, a reduced impact of runoff on terminus-wide calving activity. We suggest that seasonal variations in both runoff volume and subglacial hydrology therefore exert considerable influence on calving activity at KNS. Further work is needed to constrain the influence of runoff and subglacial hydrology on ice loss from tidewater glaciers at an ice-sheet wide scale. [ABSTRACT FROM AUTHOR]