Timing and volume of glacial lake outburst floods in Greenland

Around Greenland more than 3300 lakes intersect with the ice sheet margin and act as natural buffer for freshwater runoff from the Greenland Ice Sheet (GrIS). More than 300 of those lakes have been identified as having drained in the past or to drain periodically. These glacial lake outburst floods (GLOFs) strongly affect the fjord ecosystem due to the large amount of freshwater abruptly entering the saline ocean environment. By draining underneath glaciers, these events affect glacier dynamics. Specifically, they can increase glacier flow velocities due to hydrostatic pressure and reduced friction at the glacier bed, permanently change the subglacial drainage system and trigger calving events due to the formation of subglacial plumes.

While the existence of these GLOF type lakes has been reported, a Greenland wide assessment of the timing, periodicity and lake volume is lacking. Here we present such an assessment conducted by leveraging open-source data, specifically Sentinel-1 radar images and the ArcticDEM strip elevation data. In Sentinel-1 radar data, water is distinguishable from other surfaces due to its characteristically low backscatter intensity. Therefore, any change in lake extent will be reflected in a change of the backscatter signal. For the 10-year period 2016-2025 and for each GLOF type lake, we create a time-series of the mean backscatter intensity over the approximate maximum lake outline. Subsequently, we detect drainages where the backscatter signal abruptly increases and stays elevated. To quantify the volume of each lake drainage, we first outline pre and post lake extents. Then, we extract the lake level by intersecting these outlines with a DEM of the lake at an empty stage. Finally, we convert lake level change to lake drainage volume by filling the DEM to the respective elevations.

We find that many lakes follow a pattern of lake drainage in the summer followed by refilling through ice sheet runoff in the following year(s). The duration of drainage cycles varies between yearly to decadal. A longer filling period is usually also associated with a larger GLOF volume. The GLOF volumes are among the largest ever reported with a magnitude of several gigatons. Furthermore, we show that the combined GLOF volumes modulate the runoff pattern of the GrIS. The runoff is buffered by filling these lakes and eventually released at a later point in time. The changing of the seasonality of freshwater reaching the ocean potentially has widespread impacts on fjord and ocean circulation, which are not captured in regional climate and ocean models.