Dylan McNamara , Eli Lazarus 

There are nearly 300 barrier islands between Maine and Texas, and of these, at least 70 are intensively developed. Mean population density along the U.S. Atlantic and Gulf coasts are the highest in the country. Such concentrated development exists and continues despite the fact that barrier islands are transient landscapes, not only over geologic time scales of millennia but also within human and economic time scales of centuries to decades. Populated barrier islands are inherently vulnerable to natural hazards such as sea-level rise, cumulative erosion, and storm events; this vulnerability drives humans to actively modify barrier geometry and environments. The most common manipulations are beach nourishment, to mitigate shoreline erosion, and increases to dune height or seawall construction to prevent flooding and damage from overwash during storm events. Over time scales of years to decades, hazard-mitigation actions impact natural, spatio-temporal barrier processes such as washover deposition and planform transgression, which in turn affect future efforts to manage, control, or prevent barrier change. Through their maintenance and persistence, interventions against coastal hazards represent a significant dynamical component of developed barrier-island system evolution, such that, within the past century, human actions and natural barrier-island processes have become dynamically coupled. This coupling leads to steady-state barrier island behaviors that are fundamentally new. The only way to understand how developed barrier islands will respond to climate change over decadal time scales is to treat these settings as strongly coupled human–natural systems. Over time scales longer than centuries, human interventions may be coupled only weakly to long-term barrier dynamics. Short of major technological advancements or sweeping decisions to transform these environments into comprehensively geoengineered terrains, high-density development on U.S. barrier islands will likely cease to exist in its current configuration.