Impact of extreme melt events on the future mass balance of the Greenland Ice Sheet

在过去十年中，格陵兰经历了几次极端融化事件，对地表物质平衡和冰流产生了出人意料的强烈影响，特别是在2010年、2012年和2015年。虽然其中一些融化事件是相当局部的(如2015年的融化事件)，但另一些融化事件几乎覆盖了整个冰盖表面(如2010年的情况)。随着气候变化的加剧，预计极端融化事件将更频繁地发生，并变得更加严重和持久。然而，到目前为止，格陵兰岛冰融化的较长期预测通常依赖于只考虑气候相当渐进变化的情景，例如基于IPCC上一份报告中常用的代表性集中路径(RCP)。到目前为止，极端融化事件在预测中通常被低估了--它们对未来气候和海平面上升的影响仍然是一个悬而未决的问题。在拟议的项目中，我们计划调查极端融化事件对格陵兰冰盖未来演变的影响。特别是，我们的目标是量化可能对表面物质平衡和冰动力学产生的直接和长期影响，从而对海平面上升作出贡献。这一研究项目的主要目的是评估极端融化事件对冰盖质量平衡的潜在影响，并确定极端事件的频率、强度或持续时间的临界阈值，一旦极端事件被破坏，就会引发格陵兰冰盖的大规模动态冰流失。为此，我们将调查格陵兰冰盖对一系列不同气候情景的动态响应，包括不同开始、持续时间和强度的极端融化事件。为了评估间接影响，例如通过增强的海底融化，我们将结合公认的平行冰盖模型(PISM)和最先进的线羽模型(LPM)。LPM通过考虑海洋温度和冰下流量的变化来计算海底融化。它的计算效率非常高，因此耦合的PISM-LPM允许在高分辨率上进行系综运行。因此，在未来的项目中可以考虑广泛的模式参数和气候输入不确定性。通过交互耦合模式PISM-LPM，我们将给出到2100年格陵兰对海平面上升的贡献的不确定范围，考虑到区域降水、大气和海洋温度的变化，以及重要的是极端事件的影响。作为我们的主要成果之一，我们将创建一张风险地图，指示格陵兰最容易受到未来极端融化情景影响的地区。

Over the past decade, Greenland has experienced several extreme melt events, with unexpectedly strong impacts on the surface mass balance and ice flow, particularly in the years 2010, 2012 and 2015. While some of these melt events were rather localized (like the one in 2015), others covered almost the entire ice-sheet surface (as was the case in 2010).With progressing climate change, extreme melt events can be expected to occur more frequently and become more severe and persistent. So far however, longer-term projections of ice loss from Greenland typically rely on scenarios which only take rather gradual changes in the climate into account, as for instance based on the Representative Concentration Pathways (RCPs) commonly used in the last IPCC report. So far, extreme melt events have generally been underestimated in projections – and their effect on the future climate and sea-level rise remains an open question. In the proposed project, we plan to investigate the impact of extreme melt events on the future evolution of the Greenland Ice Sheet. In particular, we aim at quantifying the possible immediate and long-lasting effects on the surface mass balance and ice dynamics and thus contributions to sea-level rise. These include direct effects from mass loss through surface melting as well as indirect effects, e.g., through submarine melting or basal lubrication that may alter glacier velocities.The main objective of this research project is to assess the potential impacts of extreme melt events on the ice-sheet’s mass balance, and to identify critical thresholds in the frequency, intensity or duration of extreme events which, once transgressed, trigger large-scale dynamic ice loss from the Greenland Ice Sheet.To this end, we will investigate the dynamic response of the Greenland Ice Sheet to a suite of different climate scenarios including extreme melt events with varying onset, duration and magnitude. To evaluate indirect effects via, e.g., enhanced submarine melting, we will couple the well-established Parallel Ice Sheet Model (PISM) with the state-of-the-art Line Plume Model (LPM). The LPM calculates submarine melting by accounting for changes in ocean temperature and subglacial discharge. It is computationally very efficient, such that the coupled PISM-LPM allows for ensemble runs on high resolution. Consequently, a wide range of model parameters and climate input uncertainties can be considered in future projections.With the interactively coupled model PISM-LPM we will give an uncertainty range of Greenland’s contribution to sea level rise by the year 2100 considering regional changes in precipitation, atmosphere and ocean temperatures and, importantly, the impacts of extreme events. As one of our main results, we will create a risk map indicating the most vulnerable regions of Greenland to future extreme melt scenarios.