Abstract
Abstract. By the end of 2018 Kangerlussuaq Glacier in southeast Greenland had retreated further inland than at any time in the past 80 years and its terminus was approaching a region of retrograde bed slope from where further rapid retreat would have been inevitable. Here we show that the retreat occurred because the glacier failed to advance during the winters of 2016/17 and 2017/18 owing to a weakened proglacial mélange. This mixture of sea ice and icebergs is normally rigid enough to inhibit calving in winter, but for 2 consecutive years it repeatedly collapsed, allowing Kangerlussuaq Glacier to continue to calve all year round. The mélange break-ups followed the establishment of anomalously warm surface water on the continental shelf during 2016, which likely penetrated the fjord. As calving continued uninterrupted from summer 2016 to the end of 2018 the glacier accelerated by 35 % and thinned by 35 m. These observations demonstrate the importance of near-surface ocean temperatures in tidewater glacier stability and show that it is not only deep-ocean warming that can lead to glacier retreat. During winter 2019 a persistent mélange reformed and the glacier readvanced by 3.5 km.
Highlights
Since the early 1990s the Greenland Ice Sheet (GrIS) has been a major contributor to sea level rise, losing a total of 2700±930 Gt of ice between 1992 and 2011 (Shepherd et al, 2012)
We conclude that the retreat of Kangerlussuaq Glacier (KG) in 2017 was caused by weakened winter ice mélange that allowed sustained calving when the glacier would normally be advancing
Continued retreat in 2018 was facilitated by warm air temperatures hindering the formation of sea ice to bind a rigid mélange
Summary
Since the early 1990s the Greenland Ice Sheet (GrIS) has been a major contributor to sea level rise, losing a total of 2700±930 Gt of ice between 1992 and 2011 (Shepherd et al, 2012). About 40 % of the 0.47 ± 0.23 mm yr−1 mean 1991– 2015 sea level rise originating from Greenland was caused by increases in the rate at which glaciers calve ice into the oceans and the remainder by increases in surface melt and runoff (van den Broeke et al, 2016). Kangerlussuaq Glacier (KG) is a large tidewater-terminating glacier in southeast Greenland (Fig. 1), which delivers around 24 Gt yr−1 of ice to the ocean, equivalent to about 5 % of GrIS total discharge (Enderlin et al, 2014). The rapid retreat of KG was accompanied by a similar pattern of change in many other southeast Greenland outlet glaciers and accounted for ∼ 16 % of the total 2000–2005 net mass loss of the GrIS (Rignot and Kanagaratnam, 2006). After 2006, KG slowed down, speeds remained at least 20 % greater than pre-retreat values, and the ice front maintained a steady mean annual position, with seasonal advances and retreats of up to 6 km (Kehrl et al, 2017)
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