Abstract
AbstractProglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide due to continued climate warming and glacier retreat, and such lakes have important consequences for the dynamics and future stability of these glaciers. In light of this, we quantified decadal changes in glacier velocity since 1991 using satellite remote sensing for Breiðamerkurjökull, a large lake‐terminating glacier in Iceland. We investigated its frontal retreat, lake area change and ice surface elevation change, combined with bed topography data, to understand its recent rapid retreat and future stability. We observed highly spatially variable velocity change from 1991 to 2015, with a substantial increase in peak velocity observed at the terminus of the lake‐terminating eastern arm from ~1.00 ± 0.36 m day−1 in 1991 to 3.50 ± 0.25 m day−1 in 2015, with mean velocities remaining elevated from 2008 onwards. This is in stark comparison to the predominately land‐terminating arms, which saw no discernible change in their velocity over the same period. We also observed a substantial increase in the area of the main proglacial lake (Jökulsárlón) since 1982 of ~20 km2, equating to an annual growth rate of 0.55 km2 year−1. Over the same period, the eastern arm retreated by ~3.50 km, which is significantly greater than the other arms. Such discrepancies between the different arms are due to the growth and, importantly, depth increase of Jökulsárlón, as the eastern arm has retreated into its ~300 m‐deep reverse‐sloping subglacial trough. We suggest that this growth in lake area, forced initially by rising air temperatures, combined with the increase in lake depth, triggered an increase in flow acceleration, leading to further rapid retreat and the initiation of a positive feedback mechanism. These findings may have important implications for how increased melt and calving forced by climate change will affect the future stability of large soft‐bedded, reverse‐sloped, subaqueous‐terminating glaciers elsewhere. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
Continued and more intensive global climate warming, over the last decade, is driving patterns of glacier recession and dynamics, and it is widely established that almost all glaciers worldwide are undergoing widespread retreat (Björnsson et al, 2013; Zemp et al, 2015; Glasser et al, 2016)
The mean velocities for the years 2001–2018, calculated using the glacier flowlines, indicate that the greatest change in mean velocities over the period is observed for the eastern arm (Figure 4; full dataset shown in Figures S2–S5 of the online Supporting Information)
Our data illustrate a clear increase in surface velocities for the eastern arm since 1991, with heightened velocity increases occurring from ~2008 up to 2015, where values of up to 3.50 ±0.25m dayÀ1 were recorded in the near-terminus region
Summary
Continued and more intensive global climate warming, over the last decade, is driving patterns of glacier recession and dynamics, and it is widely established that almost all glaciers worldwide are undergoing widespread retreat (Björnsson et al, 2013; Zemp et al, 2015; Glasser et al, 2016). This has important consequences for their meltwater contribution to global sea level rise (s.l.r.) (Huss and Hock, 2015; Cazenave and WCRP Global Sea Level Budget Group, 2018; Rossini et al, 2018; Zemp et al, 2019), as well as for regional hydrology due to the strong control glacier meltwater has on modulating down-glacier streamflow, which in turn affects freshwater availability, hydropower operations and sediment transport (Immerzeel et al, 2014; Huss and Hock, 2018; Shannon et al, 2019).
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