Investigating the potential for catastrophic collapse of Greenland's 'land'-terminating glacier margins

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The Greenland Ice Sheet (GrIS) has been losing mass over the past three decades and is now a significant contributor to global sea-level rise. In recent decades, the ice sheet's rate of mass (or ice) loss has accelerated, driven by a warming climate and substantial increases both in: 1) the flow speed and retreat rate of many large glaciers that drain the ice sheet and terminate in the ocean; and 2) the surface melt rates and area of the ice sheet experiencing summer melting. However, a critical area of future potential dynamic change and ice-mass loss, which is unaccounted for in our current model projections of the Greenland Ice Sheet's future evolution, concerns the influence of ice-marginal (or proglacial) lake formation on the dynamic stability of outlet glaciers.It is well known from numerous observations elsewhere, that glaciers which terminate in proglacial lakes typically flow much faster than similar sized glaciers that terminate on land. It is now also clear that the number and size of proglacial lakes around the margins of the GrIS are increasing and that trend will continue in to the future. There is therefore the clear potential for the development of more lake-terminating glaciers affecting the ice-sheets' ice-dynamics and long-term stability with the possibility of a dramatic (or 'catastrophic') acceleration in ice-mass loss from these hitherto slowly changing ice-margins.Greenland's land-terminating ice-sheet margins currently flow rather slowly (~100 m/yr) and their mass loss is controlled almost entirely by surface-melt processes. Since the climate is warming, these land-terminating glaciers are thinning and retreating slowly. However, in numerous glaciated regions around the globe, glacier termini are accelerating (by a factor of 2 or more) where glaciers terminate in lakes as opposed to adjacent land-terminating glaciers. This occurs because when a glacier terminates in a lake, it experiences processes which lead to glacier calving, thinning and acceleration. These processes lead to enhanced ice mass loss from the terminus calving and retreat but also through the glacier acceleration which brings ice more rapidly from higher to lower elevations on the ice-sheet thereby exposing the ice to warmer temperatures that promote increased surface melt. As such, a rather simple change in glacier terminus morphology can have a dramatic impact on the glaciers' ice dynamics and mass loss. This project will determine the extent to which these developing proglacial lakes will impact future ice-sheet mass loss, and thus contribute to sea-level rise, over the coming century.We have already undertaken a proof-of-concept study revealing contrasting behaviour at two adjacent lake- and land-terminating glaciers in SW Greenland. Using satellite data to derive glacier velocities, our study shows that ice-motion at the lake-terminating margin more than doubled between 2017-2021 (to ~200 m/yr); by contrast, the neighbouring land-terminating glacier decelerated over the same time-period. We now aim to determine the extent to which these observations of recent acceleration are typical at Greenland's numerous lake terminating margins and more importantly, investigate how important ice-marginal lake terminating glacier dynamics will become in the future for ice-sheet mass loss.In order to achieve this broad aim, the project will use a range of satellite data in conjunction with surface mass balance and ice-sheet modelling to determine: i) how glacier terminus position, motion and surface elevation have changed, both at the ice-margin and inland, in recent decades in response to glacier termination in proglacial lakes; ii) what processes are driving these observed changes in terminus behaviour; and iii) the impact of proglacial lake-induced ice-margin acceleration, thinning and retreat, on the Greenland Ice Sheet's sea level rise contributions, under projected climate warming over the next century.