Deep uncertainty describes situations when there is either ignorance or disagreement over (1) models used to describe key system processes and (2) probability distributions used to characterize the uncertainty of key variables and parameters. Future projections of Antarctic ice sheet (AIS) mass loss remain characterized by deep uncertainty. This complicates decisions on long‐lived coastal protection projects when determining what margin of safety to implement. If the chosen margin of safety does not properly account for uncertainties in sea level rise, the effectiveness of flood protection could decrease over time, potentially putting lives and properties at a greater risk. To address this issue, we develop a flood damage allowance framework for calculating the height of a flood protection strategy needed to ensure that a given level of financial risk is maintained. The damage allowance framework considers decision maker preferences such as planning horizons, protection strategies, and subjective views of AIS stability. We use Manhattan—with the population and built environment fixed in time—to illustrate how our framework could be used to calculate a range of damage allowances based on multiple plausible scenarios of AIS melt. Under high greenhouse gas emissions, we find that results are sensitive to the selection of the upper limit of AIS contributions to sea level rise. Design metrics that specify financial risk targets, such as expected flood damage, allow for the calculation of avoided flood damages (i.e., benefits) that can be combined with estimates of construction cost and then integrated into existing financial decision‐making approaches (e.g., benefit‐cost analysis). Plain Language Summary Analyses of how Antarctica will respond to a warming planet embody large uncertainties that increase with time. As such, future projections of sea level rise are strongly dependent upon expert judgment about how much and how quickly Antarctica will melt. This uncertainty complicates any decisions regarding how high to build flood protections in order to reduce coastal flood damages from rising sea levels. Generally, the higher the structure, the greater the margin of safety afforded. If the prescribed margin of safety does not properly account for sea level rise and its uncertainties, the effectiveness of the flood protection will decrease over time. This could potentially waste money and put lives and properties at a greater risk. We create a decision‐making approach for flood protection designers that allows them to calculate the height of various flood protection strategies based on their risk tolerance with respect to future Antarctic melt. This framework is compatible with existing decision making protocols such as those used by the U.S. Army Corps of Engineers. We find that projections of flood frequencies and the design of strategies to reduce damages are sensitive to views of how much and how quickly Antarctica will melt.
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