Hysteresis of the Antarctic Ice Sheet

南极冰盖是地球上最大的淡水蓄水池，其冰量相当于全球海平面上升约58米。因此，即使是冰盖质量平衡的微小变化也会产生巨大的全球影响，这使得冰盖未来的演变问题与社会高度相关。目前南极洲的冰净流失正在加速，有可能成为21世纪海平面上升的主要(如果不是主要的)贡献者。一些积极的反馈，包括海洋冰盖不稳定、海洋冰崖不稳定以及冰的反照率和融化高度的反馈，可能导致南极洲周围某些区域的突然和可能不可逆转的衰退。在这个项目中，我们计划使用平行冰盖模式(PISM)来探索南极冰盖的稳定地貌。中心重点将包括与可能驱动南极不稳定的快速冰动力学和地表质量平衡反馈有关的机制，特别是引入一个地表模块，使太平洋空间科学研究所能够捕获融化-海拔和冰-反照率反馈。此外，我们还将实施水压致裂和冰崖失稳的参数化。基于PISM的最新物理学，我们计划为整个南极冰盖创建一个可信的现代状态集合。对于最适合的初始状态，我们将通过气候边界条件的缓慢变化来创建滞后图。这里变化的时间尺度需要比冰盖的动态时间尺度慢，这样冰盖才能保持准平衡，并将瞬变效应降至最低。因此，迟滞模拟将在一百万年的时间尺度上运行。我们希望找到一组不同冰盖盆地在不同变暖水平下的阈值行为，并旨在量化阈值的不确定范围。因此，拟议的项目将首次对当今南极冰盖的稳定性进行全面分析，并对(突然)冰川消失背后的驱动力解体产生新的见解。这种对临界温度阈值和相关不确定性的了解，是目前关于遵守雄心勃勃的气候目标的讨论和谈判的关键，将有助于澄清我们对长期气候变化后果的责任。

The Antarctic Ice Sheet is the largest reservoir of freshwater on the planet with an ice volume that is equivalent to approximately 58 m global sea level rise. Even small changes in the mass balance of the ice sheet thus have enormous global impacts, which makes the question of its future evolution highly relevant for society. The current net ice loss from Antarctica is accelerating, bearing the risk to become a major, if not the dominant contributor to sea level rise in the 21st century. Several positive feedbacks, including the marine ice-sheet instability, marine ice-cliff instability as well as the ice-albedo and melt-elevation feedbacks, could lead to abrupt and potentially irreversible decay of certain regions around Antarctica. In this project we propose to explore the stability landscape of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM). Central focus will be given to include the mechanisms relevant for rapid ice dynamics and surface mass balance feedbacks that can drive Antarctic instability, in particular introducing a surface module which will enable PISM to capture the melt-elevation and ice-albedo feedbacks. Furthermore, we will implement hydrofracturing and ice-cliff instability parametrizations. Based on the updated physics in PISM, we plan to create an ensemble of plausible present-day states for the entire Antarctic Ice Sheet. For the best-fit initial state, we will then create a hysteresis diagram through the slow change of the climatic boundary conditions. The time scale of change herein needs to be slower than the dynamic time scale of the ice sheet, so that the ice sheet remains in quasi-equilibrium and transient effects are minimized. The hysteresis simulations will thus be run on a million-year timescale. We expect to find threshold behavior for a set of different ice sheet basins at different levels of warming and aim at quantifying the uncertainty range of thresholds. The proposed project will thus provide a first comprehensive analysis of the stability of the present-day Antarctic Ice Sheet and yield new insights concerning the disentanglement of driving forces behind (abrupt) ice loss. This knowledge of critical temperature thresholds and related uncertainties are key in the current discussions and negotiations on the compliance with ambitious climate targets and will help to clarify our responsibility for the consequences of long-term climate change.