Navigating the unknown: Nature-based solutions for coastal climate adaptation under deep uncertainty

Decision-making on long-term coastal climate change adaptation faces deep uncertainty. Compared to traditional hard-engineered coastal defences, nature-based solutions (NbS) are promising multifunctional adaptation strategies under uncertainty. Yet, their adoption remains limited. This research identified how deep uncertainty affects planning and management of NbS for coastal adaptation. We performed unstructured and semi-structured interviews with Dutch experts on coastal NbS, adaptation and decision-making under deep uncertainty. We identified which topics of deep uncertainty affect decision-making, their interactions with NbS, and the implications for adopting NbS. These topics were categorised into three areas: conceptual system understanding, probabilities of outcomes and drivers, and outcome prioritisation. Decision-making primarily faces uncertainties regarding the rate of sea-level rise and political uncertainties on flood risk acceptance. Under these conditions, NbS are promising short-term interventions due to their adaptivity and multifunctionality. However, this potential is limited in the longer term, depending on increasingly uncertain factors. These include climate change impacts, NbS performance, conflicting functions, risk acceptance and the desired state of nature. Our results imply that the multifunctionality and adaptivity intrinsic to NbS are two sides of a coin. They enable robust short-term strategies but increase uncertainty and the risk of lock-in and sunk investments in the longer term. Navigating this trade-off in coastal adaptation requires the integration of political and technical scenario planning. Balancing uncertain multifunctionality against more predictable flood prevention and vulnerability reduction early on is essential for developing resilient long-term coastal management strategies.