Extreme melting at Greenland's largest floating ice tongue

Abstract

Abstract. The 79° North Glacier (Nioghalvfjerdsbrae, 79NG) is one of three remaining glaciers with a floating tongue in Greenland. Although the glacier has been considered exceptionally stable in the past, earlier studies have shown that the ice tongue has thinned in recent decades. By conducting high-resolution ground-based and airborne radar measurements in conjunction with satellite remote-sensing observations, we find significant changes in the geometry of 79NG. In the vicinity of the grounding line, a 500 m high subglacial channel has grown since ∼ 2010 and has caused surface lowering of up to 7.6 m a−1. Our results show extreme basal melt rates exceeding 150 m a−1 over a period of 17 d within a distance of 5 km from the grounding line, where the ice has thinned by 32 % since 1998. We find a heterogeneous distribution of melt rates, likely due to variability in water column thickness and channelization of the ice base. Time series of melt rates show a decrease in basal melting since 2018, indicating an inflow of colder water into the cavity below 79NG. We discuss the processes that have led to the changes in geometry and conclude that the inflow of warm ocean currents has led to the extensive thinning of 79NG's floating ice tongue near the grounding line over the last 2 decades. In contrast, we hypothesize that the growth of the channel results from increased subglacial discharge due to a considerably enlarged area of summer surface melt due to the warming of the atmosphere.

Supraglacial lake drainage through gullies and fractures

“…In the mid-1990's the weather station Danmarkshavn (76.8°N,18.7°W) recorded an increase in 2 m temperature of more than 1 °C (Zhang et al, 2022). Although the weather station is located roughly 300 km south of 79NG and ZI, the observations of changes in these glaciers correspond to this change in atmospheric forcing (AF), as discussed in Khan et al (2014), Humbert et al (2023b) and Zeising et al (2024). In this study, we will address how the formation of a massive meltwater lake is linked to changes in surface temperatures.…”

“…The main question as of now is if the subglacial system is due to the frequent drainage events in transition into a new state, or if it is (still) getting back to a normal winter state despite such extreme water inputs. As further downstream from this lake, massive changes of the system originate still on the grounded ice sheet with forming channels at the underside of the ice (Zeising et al, 2024), the imminent question is, if both developments are linked -and how this system will evolve in future.…”

“…Since then, only three floating tongues are remaining, Petermann Glacier, Ryder Glacier and Nioghalvfjerdsbrae (79 • N Glacier, 79NG), with the latter showing first signs of destabilisation (Humbert et al, 2023b). For floating tongue glaciers, oceanic heat is supposed to be a main driver for thinning and stability (Straneo and Heimbach, 2013;Schaffer et al, 2020), but subglacial freshwater discharge is also playing a role in this system (Reinert et al, 2023;Zeising et al, 2024). The mass loss of GrIS is attributed to two contributions:…”