Collaborative Research: PREEVENTS Track 2: Thresholds and envelopes of rapid ice-sheet retreat and sea-level rise: reducing uncertainty in coastal flood hazards

基于对格陵兰岛和南极洲极地冰盖的观测和数值模拟的新兴科学表明，目前对未来海平面上升的预测可能被大大低估了。物理上合理的机制已经确定，到2100年，全球平均海平面可能上升2米（6英尺）或更多。这一数字大约是政府间气候变化专门委员会（ipcc）最近（2013年）报告评估的“可能”海平面上升幅度的两倍。如此规模的海平面上升将很快改变许多沿海城市和社区面临极端洪水风险的可能性，可能造成毁灭性的经济后果，并对战略基础设施造成严重影响。虽然最近在模拟极地冰盖对变暖的大气和海洋的未来反应方面取得了进展，但仍然存在很大的不确定性，需要做更多的工作来验证这种极端海平面上升速度的可能性。该项目将使用最先进的冰川学理论、模型以及对过去和现在冰盖行为的观测，以更好地表征这种不确定性，这种不确定性源于复杂的冰盖物理以及冰盖、海洋、大气和底层固体地球之间的相互作用。它将对格陵兰岛和南极冰盖对一系列可能的未来温室气体排放情景的反应做出新的预测。将使用先进的统计技术，将新的冰盖预测与导致全球和地方海平面变化及相关沿海洪水的其他因素结合起来，以便在全球有人居住的海岸线上产生海平面预测和随时间变化的水位概率。该项目将为国家和地方的政策制定者和利益相关者提供：1)对未来海平面可能上升水平的评估；2)特定洪水高度被超过的频率（在任何给定年份的概率）；3)对这些频率和风暴潮高度在未来可能如何演变的评估；4)对预测不确定性的量化测量。研究结果将与气候中心（一个将气候科学与公众传播联系起来的组织）密切合作，通过开发易于解释和普遍可访问的网络工具广泛传播。目标是在沿海恢复力和准备方面为知情决策提供最好的工具包。预测极地冰盖的未来仍然是地球科学建模中重大的跨学科挑战之一。以前未得到充分重视的冰川过程（冰架水力破裂和冰崖崩塌）最近已被纳入冰盖模型，但需要进一步的工作来量化和校准这些机制，建立结构和参数不确定性的范围，并确定能够引发冰盖急剧和可能不可逆转的退缩的气候阈值，特别是在格陵兰岛和南极洲的海洋部分。该项目的技术方面包括用新的过程（水增强的裂缝作用、对冰上和冰上水文和水力压裂的强烈影响、冰崖崩塌、莫姆姆兰格影响）扩展数值冰原-冰架模型，在冰、海洋和大气模式组成部分之间建立更直接的联系，以及与固体地球引力-海平面模式的双向耦合。大集合方法将用于确定格陵兰和南极冰盖的气候驱动的不稳定阈值和封套，并将这些集合与其他全球和当地相对海平面贡献因素进行统计整合，包括非气候过程（冰川均衡调整、重力/旋转效应、沉降/压实、构造、陆地蓄水）和气候过程(山地冰川损失、海洋热膨胀、海洋动力学，陆地蓄水)将极地冰盖的结果“缩小”到现有的全球潮汐测量站网络。将个别潮汐计时间序列的极值统计数据与我们新的本地相对海平面预测相结合，将提供全球海岸线上风暴洪水频率和回归期随时间变化的概率评估。

Emerging science based on observations and numerical modeling of the polar ice sheets on Greenland and Antarctica suggests that current projections of future sea-level rise could be significantly underestimated. Physically plausible mechanisms have been identified that could produce a rise in global mean sea level of 2 meters ( 6 feet) or more by 2100. This amount is roughly twice the "likely" sea-level rise assessed by the most recent (2013) report of the Intergovernmental Panel on Climate Change. Sea-level rise of this magnitude would soon transform the potential for extreme flood risk in many coastal cities and communities, with the potential for devastating economic consequences and severe impacts on strategic infrastructure. While progress has recently been made in modeling the future response of the polar ice sheets to a warming atmosphere and ocean, substantial uncertainty remains and more work is needed to verify the potential for such extreme rates of sea-level rise. This project will use state-of-the-art glaciological theory, modeling, and observations of past and present ice sheet behavior to better characterize this uncertainty stemming from complex ice-sheet physics and interactions among the ice sheets, ocean, atmosphere, and the underlying solid Earth. It will produce new projections of the Greenland and Antarctic ice sheets' response to a range of plausible future greenhouse gas emissions scenarios. Advanced statistical techniques will be used to combine the new ice-sheet projections with other factors contributing to global and local sea-level change and associated coastal flooding, in order to produce both sea-level projections and time-evolving water-level probabilities along inhabited coastlines around the globe. The project will provide national and local policy makers and stakeholders with: 1) an assessment of possible levels of future sea-level rise, 2) the frequency (probability in any given year) of specific flood heights being exceeded, 3) an assessment of how those frequencies and storm-surge heights might evolve in the future, and 4) quantified measures of the uncertainty in the projections. The results will be disseminated widely through the development of easily interpretable and universally accessible web-based tools, in close cooperation with Climate Central, an established organization linking climate science and public communication. The goal is to provide the best possible toolkit for informed decision making in terms of coastal resilience and preparedness.Predicting the future of the polar ice sheets remains one of the grand interdisciplinary challenges in geoscientific modeling. Previously underappreciated glaciological processes (hydrofracturing of ice shelves and ice-cliff collapse) have recently been incorporated into ice-sheet models, but further work is needed to quantify and calibrate these mechanisms, establish ranges of structural and parametric uncertainty, and identify climatic thresholds capable of triggering drastic and possibly irreversible ice-sheet retreat, particularly in the marine-based sectors of Greenland and Antarctica. Technical aspects of this project include extending a numerical ice sheet-shelf model with new processes (water enhanced crevassing, firn influence on supraglacial and englacial hydrology and hydrofracturing, ice-cliff collapse, mélange influence), more direct linkages among ice, ocean, and atmospheric model components, and two-way coupling with solid Earth-gravitational-sea-level models. Large-ensemble methods will be used to identify climatically driven instability thresholds and envelopes in the Greenland and Antarctic ice sheets, and the ensembles will be statistically integrated with other global and local relative sea-level contributors including both non-climatic processes (glacio-isostatic adjustment, gravitational/rotational effects, subsidence/compaction, tectonics, land water storage) and climatic processes (mountain glacier loss, ocean thermal expansion, ocean dynamics, land water storage) to "downscale" the polar ice sheet results to the global network of existing tide gauge locations. Blending extreme value statistics of individual tide gauge time series with our new local relative sea level projections will provide a probabilistic assessment of time-evolving changes in storm-flood frequencies and return periods along global coastlines.