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贡献的估计。海平面目前正以每年约3毫米的速度上升。如果我们要了解海平面上升的原因，以及未来海平面上升将如何继续——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。一些最大的贡献来自南极洲和格陵兰岛的大冰原，但南极洲失去的冰的数量特别难以确定。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：(1)卫星测高（非常精确地测量冰盖表面如何随时间上升或下降）；（ii）投入产出法（计算在南极洲降雪量与在海岸断裂或因融化而损失的冰的估计之间的差值）；（三）卫星重力法（测量由于南极洲冰的损失或增加而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，但它们目前没有。所有的技术都有问题或缺点，都是正在进行的研究的主题。在这个建议中，我们重点讨论卫星重力测量方法。重力学的质量平衡计算尤其棘手，因为重力场的变化不仅受到冰的损失/增加的影响，还受到地壳下质量移动的影响。在最后一个冰河时代结束时，南极洲冰层的厚度很大，几千年后，地球深处的岩石仍在对这种变化做出反应。这种反应的结果——科学家称之为冰川均衡调整（GIA）——是卫星测量必须进行大量的校正，以解释岩石物质的运动，从而提供冰块损失/增加的“真实”数字。正确的修正是有问题的，因为它要求我们了解冰盖（包括过去的积雪）超过1万年的历史，还需要了解南极洲下面的地球结构。最近的项目，包括我们小组之前由NERC资助的一个项目，在确定这一校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洲部分地区GIA校正的数据。换句话说，东南极洲仍然存在着不可接受的不确定性，这直接导致海平面贡献的不确定性。在这个建议中，我们已经确定了一个名为Coats Land的区域，位于南极洲东部，它解释了GIA校正中最大的不确定性，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历史信息，地壳结构的新地震测量，以及地壳隆起的GPS测量（测试GIA模型的关键部分）。通过访问这些站点，并利用我们的实地数据和现有积雪数据的综合进行一些世界领先的建模，我们将为南极洲提供一个新的和改进的GIA校正。虽然我们的数据收集重点将放在科茨陆地上，但我们随后的建模工作将涵盖整个南极洲。这些数据将用于开发一个改进的南极洲GIA模型，以纠正GRACE数据集。我们保守地估计，通过我们建议进行的测量和建模，我们至少可以将卫星重力测量南极物质平衡的总不确定性减少一半，并且可能做得比这好得多。这一提议提高了对南极对当今全球海平面上升的贡献的更好估计的前景。

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.