Estimating changes in the frequency or height of extreme sea levels (ESLs; e.g., the 100-yr event) is a popular approach used by climate and sea level scientists to communicate future coastal flood risk to policy makers and the public under various climate change scenarios. However, physical ESL metrics and associated thresholds only account for water levels (i.e., the hazard). They do not consider societal outcomes (e.g., loss of life, property damage). As a result, policy makers may inadvertently disseminate misleading estimates of future coastal flood risk under different climate scenarios. This has critical implications for risk communication and adaptation decision-making. Here, we illustrate how some measures of societal exposure can lead to sizable differences in estimates of future coastal flood risk, relative to when only considering physical impacts using 1) projected ESLs under +2 degree C and +5 degree C temperature stabilization scenarios and 2) the current population exposure of 414 cities around the world. For some locations with a modest projected increase in the height of an ESL event, the corresponding change in local population exposure is substantial. This suggests that physical ESL metrics may be poor surrogates for capturing some societal impacts. While population exposure is just one measure, communicating a variety of human system, natural resource, and ecosystem-based outcomes may provide a more complete snapshot of either exposure or coastal flood risk under a wide array of climate scenarios.

Coastal climate adaptation public works, such as storm surge barriers and levees, are central elements of many strategies to limit damages from coastal storms and sea-level rise. Academic analysis of such public works projects is dominated by technocratic and engineering-driven frameworks. However, social conflict and politics have been crucial in the conception, design, and implementation of other public infrastructure and natural hazard preparedness projects. In this review, we highlight the role of interest mobilization, political motivations, siting opposition, and flexible/adaptive decision-making in both creating and overcoming political obstacles. Better understanding the social and political factors that enable or hinder the implementation of adaptation works could encourage strategies and policies that are less likely to result in deadlocks, delays, or failure, thus saving valuable time and planning resources.

Storm surge barriers, levees, and other coastal flood defense megaprojects are currently being proposed as strategies to protect several U.S. cities against coastal storms and rising sea levels. However, social conflict and other political factors add a layer of complexity that casts doubt on their status as practical climate adaptation options. The specific mechanisms for why some projects do not progress beyond initial planning stages has remained unclear. Here we study the outcome of two U.S. Army Corps of Engineer (USACE) storm surge barrier proposals to explore the political reasons why some coastal flood protection megaprojects break ground in the U.S., while others do not. Using original archive research, we conclude that storm surge barriers are politically challenging climate adaptation options because of 1) modern environmental laws that provide avenues for expression of oppositional views within the decision process and 2) the allure of alternative options that are more aesthetically pleasing and cheaper and faster to implement. To better allocate public resources and the expertise of the USACE, future flood protection megaprojects should first achieve broad support from the public, NGOs, and elected officials before beginning serious planning. This support could be achieved through new innovative designs that simultaneously address adverse environmental impacts and provide co-benefits (e.g., recreation). New designs should be studied to better understand the level of protection offered and associated reliability so that the USACE has confidence in their use.