The Impact of Delays and Interruptions on Optimal Sea-Level Rise Adaptation under Uncertainty

Global warming leads to sea level rise (SLR), and coastal zones will have to adapt to avoid extensive impacts on people and capital. Possible adaptation strategies can be broadly categorized into no (only autonomous) adaptation, adaptation via retreat from the coast, and protection construction or other forms of accommodation to rising sea levels. Cost-benefit analysis often suggests retreat as the “optimal” strategy for the majority of the (rural) coastline, whereas protection is typically suggested for coastal zones with relatively high population or capital densities.Here, we use the new FRISIA modelling tool to explore the effect that delays in adaptation and interruptions in flood protection investments can have on what the optimal SLR adaptation strategy is. We thereby define optimality not just by a single metric that combines several quantities, but look at monetary costs, people affected and flood fatalities separately, thereby offering more insights and avoiding the difficult weighting of people and capital.Sensitivity experiments indicate that delaying the start year of adaptation via retreat or protection reduces the likelihood that protection is the optimal strategy in favour of retreat, especially when considering people rather than monetary impacts. This is mostly because protection construction takes longer and might be imperfect due to limitations in money availability in regions with low population and capital density.Accounting for interruptions in flood protection investments reduces the likelihood that protection remains the optimal adaptation strategy, particularly in coastal zones that are close to the affordability threshold for building protection. We demonstrate that a reinforcing feedback loop, whereby increasing SLR-induced damage depresses economic growth and thereby places further constraints on protection investments, can be triggered in regions with low population and capital density. Our results further indicate a heightened risk of escalating damages in regions with intermediate population and capital density. In these areas, conventional cost–benefit analysis may still identify protection as the preferred strategy, yet this outcome is highly sensitive to interruptions or constraints in investment, rendering these regions especially vulnerable to adverse development pathways.