NSFGEO-NERC:Collaborative Research: A New Mechanistic Framework for Modeling Rift Processes in Antarctic Ice Shelves Validated through Improved Strain-rate and Seismic Observations

板状冰山的崩解占南极冰盖冰质量损失的很大一部分。除了这种直接的质量损失外，冰裂还可能通过减少冰架（冰原的浮动延伸部分）提供的阻力应力，进一步加速接地冰向海的流动。板状冰山比它们的厚度长得多，宽得多，当全层裂缝（即裂缝）与冰架边缘相交时形成。冰架裂陷的过程和驱动因素在冰流模型中既没有得到很好的理解，也没有得到准确的描述。因此，目前还不可能预测板状冰山何时会形成，它们会有多大，以及随着气候变化，冰解会如何演变。这种预测能力的缺乏有两个重要的后果。首先，不可能有把握地预测由于浮冰架崩解而造成的质量损失率。其次，我们不可能自信地预测板状冰山崩解将如何影响地面冰盖的质量损失。第二个后果是在人类时间尺度上预测海平面上升的不确定性的主要来源。在这个项目中，该团队的目标是通过详细的观察和一套日益复杂的模型来研究冰架断裂的过程和驱动因素，提高对板状冰山崩解的理解。这项工作将集中在一个自然实验室：由布伦特冰架和南极洲东部的斯坦库姆-威尔斯冰川舌组成的冰架系统，该系统拥有南极洲最长、最详细的观测记录之一。截至2020年初，两个活跃的裂谷系统正在布伦特冰架上传播，其中一个很快就会形成一个巨大的平板冰山。因此，有一个难得的机会来观察多个活动裂缝。该项目将利用这种情况，利用从各种星载仪器收集的遥感观测资料，对冰架上的速度和应变率场进行详细的随时间变化的测量，特别是在活动裂谷尖端附近。在这些尖端，由于裂谷的存在，应力预计会加剧。这些观测结果将为一系列冰流和断裂模型提供信息，这些模型将被开发并用于更好地理解裂缝是如何扩展的，以及如何在大尺度冰流模型中最好地表示裂缝的扩展。建模目标遵循一个开发路径，旨在产生一个能够模拟裂缝传播的社区冰流模型，并已根据观测结果进行了测试。在活动裂谷附近已经收集的地震数据将提供裂谷过程的详细知识，并将补充遥感观测并为建模工作提供信息。这种多方面观测和模式的结合旨在阐明冰架裂谷的基本过程，从而为可靠地预测冰盖演变和海平面上升提供必要的知识。本项目由美国国家科学基金会地球科学理事会（NSF/GEO）和英国国家环境研究委员会（UKRI/NERC）通过NSF/GEO-NERC牵头机构协议共同资助。该协议允许美国/英国提交一份联合提案，并由其调查员拥有最大比例预算的机构进行同行评审。在成功地共同确定一项奖励后，每个机构为预算的比例和与自己的调查人员有关的调查人员和工作的组成部分提供资金。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Calving of tabular icebergs accounts for a significant fraction of ice mass loss from the Antarctic Ice Sheet. In addition to this direct mass loss, calving may further accelerate the seaward flow of grounded ice by reducing resistive stresses provided by ice shelves, the floating extensions of the ice sheet. Tabular icebergs are much longer and wider than they are thick and form when full-thickness fractures known as rifts intersect the edges of an ice shelf. The processes and drivers of ice-shelf rifting are neither well understood nor accurately represented in ice-flow models. As a result, it is not currently possible to predict when tabular icebergs may form, how large they will be, and how calving may evolve as the climate changes. This lack of predictive capability has two important consequences. First, it is not possible to confidently project rates of mass loss due to calving from floating shelves. Second, it is not possible to confidently project how tabular iceberg calving will influence the mass loss from the grounded ice sheet. This second consequence is a major source of uncertainty in projections of sea-level rise over human timescales.In this project, the team aims to improve understanding of tabular iceberg calving using a combination of detailed observations and a suite of increasingly sophisticated models to study the processes and drivers of rifting in ice shelves. The work will focus on a natural laboratory: the ice-shelf system formed by the Brunt Ice Shelf and Stancomb-Wills Glacier Tongue, East Antarctica, which has one of the longest and most detailed observational records in Antarctica. As of early 2020, two active rift systems are propagating across the Brunt Ice Shelf, one of which should soon form a large tabular iceberg. Thus, there is a rare opportunity to observe multiple active rifts. The project will take advantage of this situation by employing remote-sensing observations collected from a variety of spaceborne instruments to make detailed time-dependent measurements of velocity and strain-rate fields across the ice shelf and, notably, in the vicinity of the active rift tips. At these tips, stresses are expected to intensify due to the presence of the rift. The observations will inform a suite of ice-flow-and-fracture models that will be developed and used to better understand how rifts propagate and how best to represent rift propagation in large scale ice-flow models. The modeling objective follows a development path that aims to yield a community ice-flow model capable of simulating rift propagation, and that has been tested against observations. Seismic data already collected in the vicinity of an active rift will provide detailed knowledge of rifting processes and will complement the remote sensing observations and inform the modeling efforts. This combination of multi-faceted observations and models aims to illuminate the fundamental processes of ice-shelf rifting, thereby contributing to the knowledge necessary to make reliable projections of ice-sheet evolution and sea-level rise.This project is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (UKRI/NERC) of the United Kingdom (UK) through the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own ivestigators and component of the work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.