Hysteresis of the Antarctic Ice Sheet

南极冰盖是地球上最大的淡水水库，其冰量相当于全球海平面上升约58米。因此，即使冰盖质量平衡的微小变化也会产生巨大的全球影响，这使得其未来演变的问题与社会高度相关。目前南极洲的净冰损失正在加速，有可能成为世纪海平面上升的主要因素，如果不是主要因素的话。若干正反馈，包括海洋冰盖不稳定性、海洋冰崖不稳定性以及冰融化和融化高度反馈，可能导致南极洲周围某些地区突然和可能不可逆转的衰退。在这个项目中，我们提出了探索南极冰盖的稳定性景观与平行冰盖模式（PISM）。中心重点将包括快速冰动力学和表面质量平衡反馈相关的机制，可以驱动南极不稳定，特别是引入一个表面模块，这将使PISM捕捉融化海拔和冰的反馈。此外，我们将实施水力压裂和冰崖不稳定性参数化。基于PISM中最新的物理学，我们计划为整个南极冰盖创建一个合理的现今状态的集合。对于最佳拟合的初始状态，我们将通过气候边界条件的缓慢变化创建滞后图。在此，变化的时间尺度需要比冰盖的动态时间尺度慢，以便冰盖保持准平衡，并使瞬态效应最小化。因此，滞后模拟将在百万年的时间尺度上运行。我们希望找到一组不同的冰盖盆地在不同程度的变暖阈值行为，并旨在量化阈值的不确定性范围。因此，拟议的项目将首次全面分析现今南极冰盖的稳定性，并就（突然）冰损背后的驱动力的解开提出新的见解。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，并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.