Rethinking Antarctic Sea Level Projections (RASP)

全球变暖的一个主要后果是海平面上升，威胁到世界各地的沿海社区、生态系统和工业。自工业革命开始以来，全球海平面上升了约20厘米，主要是通过四个因素：变暖的海水扩张、冰川融化以及南极和格陵兰冰盖变薄。在国际气候变化专门委员会最近发布的2021年报告中总结了对地球系统的物理理解，报告强调，在上述四个组成部分中，南极冰盖未来的冰川融化是最不确定的。计算机模拟表明，到2100年，南极冰盖可能会略微降低海平面，或者更有可能会使海平面上升50厘米以上。南极洲与英国特别相关，因为北半球海平面对南极洲冰川融化的反应是成比例的，这是由于当地引力的减少。此外，英国议会的一份研究简报(POSTnote 555)讨论了当地海平面上升50厘米将使英国约200公里的沿海防洪设施容易失效。进一步委托进行的研究表明，到2080年，较高端海平面估计值在英国造成的年损失将比较低端估计值高出5.4亿GB。显然，迫切需要减少英国沿海规划中海平面预测的不确定性。在“重新思考南极海平面预测”(RASP)项目中，我们提出了一种新的方法来更好地理解和约束南极部分的不确定性。南极冰盖未来演变如此不确定的原因之一是对周围南大洋如何在未来融化南极冰盖的科学理解上的差距，从而在计算机模型中表示。更温暖的海水位于南极大陆近海更深、更开阔的海洋中。在一些地区，它们已经进入大陆架并到达冰层，导致高度融化。例如，阿蒙森海就是一个例子，那里观测到了南极洲的大部分冰川融化。其他地区，如韦德尔海，目前受到较冷水域的保护。这些温暖的水团是否以及如何进入大陆架取决于风、降水和气温等区域气候驱动因素的复杂相互作用。但这些不同的气候驱动因素对未来南极不同地区的冰雪融化有多重要，目前尚不清楚。我们在此建议使用一个代表有关南大洋过程的数值模式来回答这个问题。重要的是，它将包括大陆架和南极洲冰层与海洋接触的冰架洞穴。这给了我们一个独特的机会来进行一系列有针对性的数值实验，以确定这些区域气候驱动因素中最相关的因素，以及它们是如何相互作用的。使用这种新的理解，我们就可以在远场、开阔的海洋和冰盖附近之间架起一座桥梁。我们将使用数值冰流模型来预测南极的未来。这个模型代表了大气变化和海洋驱动的融化如何影响南极洲的冰流，从而导致海平面上升。我们将进行一系列实验，测试未来大气和海洋变化中的不确定性，以及模型物理中的不确定性。特别是，我们可以绘制出每个气候驱动因素在多大程度上导致南极海平面预测的不确定性。因此，我们可以填补在南大洋和南极冰盖之间的物理联系方面的知识空白，以及在未来几十年到几个世纪里，每个联系将在多大程度上促进海平面上升。

One major consequence of global warming is the rising of sea levels that threaten coastal communities, ecosystems and industries worldwide. Since the beginning of the industrial revolution, global sea-levels have risen by about 20cm, largely through four components: the expansion of warming ocean waters, the melting of glaciers and the thinning of the Antarctic and Greenland ice sheets. In the recent 2021 report from the International Panel on Climate Change summarising the physical understanding of the Earth System, it is emphasised that future ice loss of the Antarctic Ice Sheet is the most uncertain of the four components above. Computer simulations suggest that the Antarctic Ice Sheet could slightly lower sea level or, more likely, raise sea level by more than 50 cm by 2100. Antarctica is particularly relevant for the UK, since northern hemisphere sea level responds over proportionately to ice loss in Antarctica, due to a reduction of the local gravitational pull. Furthermore, a research briefing for UK parliament (POSTnote 555) discusses that 50 cm of local sea level rise would make about 200 km of UK coastal flood defences vulnerable to failure. Further commissioned research suggests that by 2080, higher end sea-level estimates cause yearly damages in the UK that are £540m higher than those expected for the lower end estimates. Clearly, there is an urgent need to reduce uncertainty in sea-level projections for UK coastal planning. In the project "Rethinking Antarctic Sea-level Projections" (RASP), we propose a new approach to better understand and constrain the uncertainty for the Antarctic component.One reason why the future evolution of the Antarctic Ice Sheet is so uncertain is a gap in the scientific understanding, and thus representation in computer models, of how the surrounding Southern Ocean melts the Antarctic Ice Sheet in the future. Warmer ocean waters are found offshore of the Antarctic continent in the deeper, open ocean. In some regions, they already access the continental shelf and reach the ice, causing high melting. This is for example the case in the Amundsen Sea, where the bulk of Antarctica's ice loss is observed. Other regions, such as the Weddell Sea, are currently protected by colder waters. If, and how, those warm water masses access the continental shelf is dependent on a complex interaction of regional climate drivers such as winds, precipitation and air temperatures. But how important these different climate drivers will be for ice loss in the different Antarctic regions in the future, is unclear. We here propose to answer this question using a numerical model that represents the relevant Southern Ocean processes. Importantly, it will include the continental shelf and the ice-shelf cavities where the ice is in contact with the ocean in Antarctica. This gives us the unique opportunity to make a suite of targeted numerical experiments to identify the most relevant of those regional climate drivers, and how they interact. Using this novel understanding, we can then bridge the gap between the far-field, open ocean and the vicinity of the ice sheet. We will use a numerical ice flow model to make Antarctic future projections. This model represents how the changes in the atmosphere and ocean-driven melting affect the ice flow in Antarctica, and thereby lead to sea-level rise. We will run a range of experiments testing for uncertainties in future changes in the atmosphere and ocean, as well as uncertainties in the model physics. In particular, we can map out how much each climate driver contributes to the uncertainty in Antarctic sea-level projections. Thereby, we can fill the knowledge gap in the physical links between the Southern Ocean and the Antarctic Ice Sheet, and how much each link will contribute to sea-level rise over the coming decades to centuries.