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严重低估。已经确定了可能导致全球平均海平面到2100年上升2米(6英尺)或更多的物理上可信的机制。这一数字大约是政府间气候变化专门委员会最新报告(2013年)所评估的“可能”海平面上升的两倍。这种规模的海平面上升将很快改变许多沿海城市和社区发生极端洪水风险的可能性，有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大气和海洋变暖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核实海平面可能出现如此极端的上升速度。该项目将使用最先进的冰川学理论、模型和对过去和现在冰盖行为的观测，以更好地表征这种由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产生的不确定性。它将对格陵兰和南极冰盖对一系列看似合理的未来温室气体排放情景的反应进行新的预测。将使用先进的统计技术，将新的冰盖预测与导致全球和当地海平面变化以及相关沿海洪水的其他因素结合起来，以便产生海平面预测和全球有人居住的海岸线上随时间演变的水位概率。该项目将为国家和地方政策制定者和利益攸关方提供：1)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(任何一年的概率)，3)这些频率和风暴潮高度在未来可能如何演变的评估，以及4)预测不确定性的量化测量。将通过与气候中心密切合作，开发易于解释和普遍使用的网络工具，广泛传播成果，气候中心是一个将气候科学和公共传播联系在一起的组织。其目标是在海岸复原力和备灾能力方面为明智的决策提供尽可能最佳的工具包。预测极地冰盖的未来仍然是地球科学建模中跨学科的重大挑战之一。以前被低估的冰川学过程(冰架水力破裂和冰崖崩塌)最近被纳入冰盖模型，但还需要进一步的工作来量化和校准这些机制，确定结构和参数不确定性的范围，并确定能够引发冰盖剧烈和可能不可逆转的冰盖消退的气候阈值，特别是在格陵兰和南极洲的海洋区域。该项目的技术方面包括用新的过程扩展数值冰盖-陆架模型(水增强的破裂、降雪对冰上和冰川水文和水文破裂的影响、冰崖坍塌、混杂影响)、冰、海洋和大气模型组成部分之间更直接的联系，以及与固体地球-重力-海平面模型的双向耦合。将使用大集合方法来确定格陵兰和南极冰盖中气候驱动的不稳定阈值和包络，这些集合将与全球和当地其他相对海平面的贡献者在统计上结合起来，包括非气候过程(冰川均衡调整、重力/旋转效应、下沉/压实、构造、陆地水储存)和气候过程(山脉冰川丧失、海洋热膨胀、海洋动力学、陆地水储存)，以“缩小”极地冰盖导致的全球现有验潮地点网络。将单个验潮仪时间序列的极值统计与我们新的地方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重现期随时间演变的变化的概率评估。

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.