Reducing the uncertainty in estimates of the sea level contribution from the westernmost part of the East Antarctic Ice Sheet

这项提议旨在改善对南极洲对海平面的贡献的估计。海平面目前正以大约3毫米/年的速度上升。如果我们想要了解海平面为什么在上升，以及未来海平面上升将如何持续--也许会加速--并导致广泛的社会影响，那么我们就需要了解对海平面的不同贡献。其中一些最大的贡献来自南极洲和格陵兰岛的巨大冰盖，但南极洲流失的冰量尤其难以确定。有三种主要的方法来测量从南极洲失去或获得的冰量--它的“质量平衡”。它们是(I)卫星测高(非常精确地测量冰盖表面随时间的升降)；(Ii)投入-产出法(计算对南极洲降雪量的估计与海岸有多少冰断裂或因融化而消失之间的差额)；(Iii)卫星重力测量(测量南极洲冰随时间的消失或增加引起的地球重力场的微小变化)。理想情况下，这三种技术将提供类似的答案，但目前并非如此。所有的技术都有问题或缺点，都是正在进行的研究的主题。在本提案中，我们重点介绍卫星重力测量方法。重力测量的质量平衡计算起来特别困难，因为重力场的变化不仅受冰的损失/增加的影响，而且还受地壳下的质量运动的影响。在上一个冰河时代结束时，南极洲的一大块冰融化了，地球深处的岩石在1000年后仍然对这种变化做出反应。这种反应的结果--科学家们称之为冰川均衡调整或“GIA”--是卫星测量必须进行大量修正，以考虑岩石物质的移动，从而提供冰块质量损失/增加的“真实”数字。正确地进行这种修正是非常有问题的，因为它要求我们了解一万多年来冰盖的历史(包括过去的积雪)，还需要知道南极洲下面的地球结构。最近的项目，包括我们团队之前由NERC资助的一个项目，在确定这种修正方面取得了实质性的改进，但我们最近发表的工作非常清楚地表明，我们仍然缺乏足够严格地确定东南极部分地区GIA修正的数据。换句话说，东南极洲仍然存在不可接受的不确定性，这直接导致了海平面贡献的不确定性。在这项提案中，我们确定了东南极洲一个名为Coats Land的地区，该地区是GIA校正中剩余的最大不确定性，但我们已经设法确定了合适的地点，在那里我们可以获得必要的冰史信息、新的地壳结构地震测量和地壳隆起的GPS测量(测试GIA模型的关键部分)。通过访问这些地点，并利用我们的现场数据和现有积雪数据的综合，进行一些世界领先的模拟，我们将为南极洲提供一个新的和大大改进的GIA校正。虽然我们的数据收集重点将放在Coats Land上，但我们随后的建模工作将涵盖整个南极洲。这些数据将被用来开发南极GIA的改进模型，以修正GRACE数据集。我们保守地估计，通过我们提议进行的测量和建模，我们至少可以将南极物质平衡的卫星重力测量的总不确定度减半，而且可能会比这好得多。这一提议提高了对南极对当今全球海平面上升的贡献做出更好估计的可能性。

This proposal aims to improve estimates of Antarctica's contribution to sea level. Sea level is currently rising at approximately 3mm/yr. If we are to understand why it is rising and how future sea-level rise will continue - perhaps accelerate - and lead to a wide range of societal impacts then we need to understand the different contributions to sea level. Some of the largest contributions come from the great ice sheets in Antarctica and Greenland but the amount of ice being lost from Antarctica is particularly difficult to establish. There are three main ways to measure the amount of ice being lost or gained from Antarctica - its 'mass balance'. These are (i) satellite altimetry (measuring very precisely how the ice sheet surface is going up or down through time); (ii) the input-output method (calculating the difference between estimates of how much snow falls on Antarctica, and how much ice breaks off at the coast or is lost by melting); (iii) satellite gravimetry (measuring minute changes in Earth's gravitational field caused by loss or gain of ice in Antarctica through time). Ideally, these three techniques would provide similar answers but they currently do not. All the techniques have problems or drawbacks and all are the subject of ongoing research. In this proposal we focus on the satellite gravimetry method. Mass balance from gravimetry is particularly tricky to calculate because the changes to the gravitational field are not only affected by ice loss/gain but also by mass moving around beneath the Earth's crust. At the end of the last ice age, a large thickness of ice in Antarctica melted and the rocks deep within the Earth are still responding to this change 1000s of years later. The consequence of this response - which scientists call glacial-isostatic adjustment or 'GIA' - is that the satellite measurements have to be corrected by a very large amount that accounts for movements of the rocky material and thus to provide the 'real' figure for ice mass loss/gain. It is getting this correction right that has been so problematic because it requires us to know the history of the ice sheet (including past snow accumulation) for over 10,000 years and also to know the structure of the Earth underneath Antarctica. Recent projects including a previous one by our group that was funded by NERC have made substantial improvements in determining this correction but our recently published work has shown very clearly that we still lack data to pin down the GIA correction tightly enough in parts of East Antarctica. In other words there is still an unacceptable level of uncertainty in East Antarctica, which leads directly to uncertainty in sea-level contribution. In this proposal we have identified a region called Coats Land, in East Antarctica, which accounts for the greatest remaining uncertainty in the GIA correction but where we have managed to identify suitable sites where we can obtain the necessary ice history information, new seismic measurements of crustal structure, and GPS measurements of crustal uplift (a key part of testing GIA models). By visiting these sites and undertaking some world-leading modelling using our field data and a synthesis of existing snow accumulation data we will provide a new and much improved GIA correction for Antarctica. Whilst our data collection focus will be on Coats Land our subsequent modelling effort will encompass all of Antarctica. The data will be used to develop an improved model of GIA in Antarctica in order to correct the GRACE dataset. We conservatively estimate that with the measurements and modelling that we propose to carry out then we can at least halve the total uncertainty in satellite gravimetry measurements of Antarctic mass balance, and probably do substantially better than this. This proposal raises the prospect of getting an improved estimate of the Antarctic contribution to present-day global sea level rise.