Constraining Antarctica's contribution to sea-level change: development of a new glacial isostatic adjustment model for Antarctica

南极冰盖目前正在融化并经历快速的动态变化。然而，目前对它融化的速度以及它对海平面上升的贡献知之甚少。我的研究金的目的是得出一个更好的估计目前整个南极洲的冰量变化。这种估计可以从卫星重力数据中得出，卫星重力数据用于根据地球重力场的测量结果推断冰量的变化。然而，这些数据必须根据冰后期反弹（也称为冰川均衡调整; GIA）的持续影响进行校正。卫星重力测量无法区分冰质量的变化和固体地球的质量运动。在最后一次冰期，南极冰盖要大得多，额外的质量导致冰下的土地下沉。当冰盖缩小到现在的大小时，陆地开始反弹。由于地幔的粘性，这个过程- GIA -今天仍在继续。事实上，由卫星重力数据记录的GIA信号可以与当前冰量变化引起的信号具有相同的幅度。因此，尽可能精确地限制GIA的模式以确定冰量变化的模式是至关重要的。我将解决目前用于模拟南极洲GIA的方法中的两个基本问题，从而大大减少南极洲海平面贡献的不确定性。在我的研究的第一部分中，我将模拟南极洲下方的3D地球结构。目前的南极GIA模型没有考虑到地壳厚度或地幔粘度的变化。然而，这种变化是已知的大，因为东南极洲和西南极洲有非常不同的地质历史：东南极洲下面的地壳是旧的和“硬”，因此不会在负载下容易变形。相比之下，南极洲西部下方的地壳年轻且“脆弱”，因此在负载下更容易变形。为了真实地确定回弹信号，必须考虑地球结构的空间变化，以便计算对载荷的正确响应;不这样做可能会改变回弹信号高达30%。最近获得的有关南极地球结构的新数据和GIA模型的发展，使我的建议这方面非常及时。南极洲西部是一个以海洋为基础的冰盖，这意味着它的海床低于目前的海平面。为了模拟冰范围随时间的变化，从而确定GIA信号，有必要考虑海平面变化和冰动态之间的反馈。海平面直接控制着冰盖开始漂浮的位置，从而控制着冰流失到海洋中的位置。然而，由于GIA，海平面并不随时间均匀变化：在冰盖生长的地区，由于冰盖的引力增加，陆地被额外的负荷向下推，海面被向上拉，导致海平面上升。当冰消退时，情况正好相反。在这两种情况下，反馈影响冰盖的动力学行为。这种反馈在当前的冰盖模型中没有考虑，尽管它们有可能增加冰盖的稳定性：它们的加入可能从根本上改变未来海平面上升情景下冰盖行为的预测。因此，在我的项目的第二部分，我将开发一个耦合GIA冰盖模型。这两个研究领域都将从改进的模型能力中受益，这些模型改进将用于对整个南极洲的GIA进行更切合实际的估计。该模型将进行校准，以适应现场的限制有关以前的冰的范围，过去的海平面，和现在的反弹。然后，它将被用来纠正卫星数据的影响，GIA，从而确定一个更准确的地图，目前南极冰量的变化。

The Antarctic Ice Sheet is currently melting and undergoing rapid dynamic change. However, the rate at which it is melting, and hence its contribution to sea-level rise, is currently poorly known. The aim of my Fellowship is to derive a better estimate of current ice-mass change across Antarctica. Such an estimate can be derived from satellite gravity data, which are used to infer ice-mass changes from measurements of the Earth's gravity field. However, these data must be corrected for the ongoing effects of postglacial rebound (also known as glacial isostatic adjustment; GIA).Satellite gravity measurements cannot distinguish between changes in ice mass and mass movement of the solid Earth. During the last glaciation the Antarctic Ice Sheet was much larger and the extra mass caused the land beneath the ice to subside. As the ice sheet shrank towards its present size, the land began to rebound. This process - GIA - continues today due to the viscous nature of Earth's mantle. In fact, the GIA signal, as recorded by satellite gravity data, can be of the same magnitude as the signal due to current ice-mass change. Therefore, it is vital to constrain the pattern of GIA as accurately as possible in order to determine the pattern of ice-mass change.I will address two fundamental problems with the methods currently used to model GIA in Antarctica, thus significantly reducing the uncertainty on Antarctica's sea-level contribution.In the first part of my Fellowship I will model the 3D Earth structure beneath Antarctica. Current Antarctic GIA models do not account for variations in crustal thickness or mantle viscosity. However, such variations are known to be large because East and West Antarctica have very different geological histories: the crust beneath East Antarctica is old and 'stiff' and thus will not deform easily under loading. In contrast, the crust beneath West Antarctica is young and 'weak' and so deforms more easily under loading. In order to realistically determine the rebound signal, spatial variations in Earth structure must be accounted for so that the correct response to loading is calculated; not doing so can alter the rebound signal by up to 30%. The recent acquisition of new data relating to Antarctic Earth structure, and GIA model developments, make this aspect of my proposal very timely.The second problem I will address is related to sea-level change. West Antarctica is a marine-based ice sheet meaning that its bed is grounded below present sea level. In order to model changes in ice extent over time, and hence determine the GIA signal, it is necessary to consider feedbacks between sea-level change and ice dynamics. Sea level directly controls the position where an ice sheet begins to float and thus where ice is lost to the ocean. However, due to GIA, sea level does not vary uniformly over time: In areas where the ice sheet grows the land is pushed down by the extra load and the sea surface is drawn upwards due to the increased gravitational attraction of the ice sheet, resulting in an increase in sea level. The opposite happens when ice retreats. In both cases, feedbacks influence the dynamical behaviour of the ice sheet. Such feedbacks are not accounted for in current ice-sheet models, despite their potential to increase the stability of an ice sheet: Their inclusion could fundamentally alter predictions of ice-sheet behaviour under future sea-level rise scenarios. In the second component of my project I will therefore develop a coupled GIA-ice-sheet model. Both fields of research will benefit from improved model capabilities.These model improvements will be used to derive a more realistic estimate of GIA across Antarctica. The model will be calibrated to fit field constraints relating to former ice extent, past sea-levels, and present-day rebound. It will then be used to correct satellite data for the effects of GIA, and hence determine a more accurate map of current Antarctic ice-mass change.