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

格陵兰冰盖（GrIS）在过去的三十年中一直在失去质量，现在是全球海平面上升的一个重要因素。近几十年来，由于气候变暖和以下两方面的大幅增加，冰盖的质量（或冰）损失率加快了：1)许多大冰川的流动速度和退缩速度加快，这些冰川耗尽了冰盖，最终流入海洋；2)经历夏季融化的冰盖表面融化速率和面积。然而，未来潜在动力变化和冰质量损失的一个关键区域涉及冰边缘（或前冰期）湖泊形成对出口冰川动力稳定性的影响，这在我们目前对格陵兰冰盖未来演变的模型预测中没有考虑到。从其他地方的大量观察中我们都知道，终止于原冰川湖的冰川通常比终止于陆地的类似大小的冰川流动得快得多。现在也很清楚，在GrIS边缘的前冰期湖泊的数量和规模都在增加，而且这种趋势将持续到未来。因此，显然有可能出现更多的湖泊末端冰川，影响冰盖的冰动力和长期稳定性，这些迄今为止缓慢变化的冰缘可能会急剧（或“灾难性”）加速冰质量损失。目前，格陵兰岛与陆地接壤的冰盖边缘流动相当缓慢（约100米/年），它们的质量损失几乎完全由地表融化过程控制。由于气候变暖，这些陆地冰川正在变薄并缓慢退缩。然而，在全球许多冰川地区，冰川终结的速度正在加快（以2倍或更多的速度），冰川终结于湖泊，而不是邻近的陆地终结冰川。这是因为当冰川终止于湖泊时，它会经历导致冰川崩解、变薄和加速的过程。这些过程导致冰的质量损失增加，从冰裂和退缩的终点，但也通过冰川加速，使冰更快地从冰盖的高海拔到低海拔，从而使冰暴露在更高的温度下，促进表面融化的增加。因此，冰川末端形态的一个相当简单的变化就可以对冰川的冰动力和质量损失产生巨大的影响。该项目将确定这些发展中的前冰期湖泊对未来冰盖质量损失的影响程度，从而在未来一个世纪导致海平面上升。我们已经进行了一项概念验证研究，揭示了格陵兰岛西南部两个相邻的湖泊和陆地冰川的对比行为。利用卫星数据得出冰川速度，我们的研究表明，在2017-2021年期间，湖端边缘的冰运动增加了一倍多（达到200米/年）；相比之下，邻近的陆地冰川在同一时期减速。我们现在的目标是确定这些最近加速的观测结果在格陵兰岛众多湖泊终止边缘的典型程度，更重要的是，调查冰边缘湖泊终止冰川动力学在未来对冰盖质量损失的重要性。为了实现这一广泛的目标，该项目将使用一系列卫星数据，结合地表质量平衡和冰盖模型来确定：i)近几十年来，冰川末端的位置、运动和地表高度如何在冰缘和内陆变化，以响应冰川在原冰期湖泊的终止；Ii)哪些过程在驱动这些观察到的终端行为变化；以及iii)在预估的下个世纪气候变暖的情况下，原冰期湖泊引起的冰缘加速、变薄和退缩对格陵兰冰盖海平面上升贡献的影响。

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.