The increasing threat of sea-level rise (SLR) and coastal flooding necessitates adaptation strategies, including the planned relocation of entire communities. Relocation, while a drastic measure, offers the potential to eliminate flood exposure, unlike incremental strategies that may leave residual risk. However, relocations can be maladaptive if they unintentionally create new risks at the destination site. This study addresses a critical gap in the literature by quantitatively assessing whether planned relocations achieve their primary goal of reducing future coastal hazard exposure. Existing research has primarily focused on qualitative assessments of socio-economic outcomes or on limited case studies, neglecting the systematic analysis of future risk across a global scale and multiple climate scenarios. This research aims to fill this gap by evaluating the effectiveness of completed and ongoing planned relocation cases at reducing communities’ exposure to projected coastal hazards (SLR and flooding) throughout the 21st century, considering different emission scenarios and geographical contexts. The importance of this study stems from the need to optimize scarce resources for climate change adaptation. Prioritizing relocation efforts where risk reduction is demonstrably achieved is crucial for maximizing the effectiveness of this costly and disruptive strategy and ensures that relocation leads to improved, sustainable livelihood trajectories for the affected communities. The study will provide crucial evidence to inform policy and practice related to planned relocation as a climate change adaptation strategy, particularly in vulnerable coastal regions worldwide.

Existing qualitative research has examined the socio-economic impacts of planned relocation, but quantitative assessments of its primary goal – risk reduction – are scarce. One study found significant flood damage cost reduction after riverine relocation in the Midwest US. Another examined coastal storm surge exposure reduction in the Philippines. A hypothetical study explored future SLR exposure reduction potential in French Polynesia. However, a systematic, global-scale quantitative assessment of future coastal hazard exposure reduction after planned relocations has been lacking. This dearth of evidence is common in climate adaptation research; a recent review highlighted the limited evaluation of enacted or planned strategies to lower exposure. This type of evaluation is challenging because it requires counterfactuals (what would happen without intervention) and consideration of multiple time horizons and climate change scenarios.

This study quantitatively evaluated the effectiveness of 17 coastal planned relocation cases from a global database. These cases met specific criteria: initiation due to one or more coastal hazards, origin site within 1 km of a coastline, whole-community movement from a single origin to a single destination, available geolocated site data, and comparable elevation models. Combining maps of origin and destination sites with a high-resolution coastal digital elevation model, SLR projections from the IPCC Sixth Assessment Report (considering low, middle, and high emissions scenarios: SSP1-2.6, SSP2-4.5, and SSP5-8.5), and flood risk statistics, the researchers assessed inundation exposure in 2030, 2050, and 2100. The measure of inundation was the fraction of the site area projected to be exposed to SLR and a once-per-year flooding event. Exposure reduction was calculated as the difference in inundated area between the origin and destination sites. The study also examined relocation assessments conducted before relocation to understand the level of future risk considered in decision-making. A thorough search was undertaken to identify available assessments for each of the 17 cases (Supplementary Table 5). In this search, only two assessments could be found.

The 17 cases spanned the Pacific, South and Southeast Asia, and North, South, and Central America. Most were in island nations or the USA. Four illustrative cases demonstrated the range of future risk reduction outcomes: one showed complete exposure reduction (Gardi Sugdub, Panama), two showed substantial reduction (Vunidogoloa and Taholah), and one showed modest reduction (Kandholhudhoo, Maldives). Analysis across all 17 cases revealed that origin sites showed projected exposure to SLR and flooding in all cases, with increasing exposure under higher emissions scenarios. Nine destination sites also had projected inundation under some scenarios. Exposure reduction varied over time and across emissions scenarios. By 2100 under a mid-emissions scenario, average exposure decreased by 47%, ranging from complete reduction to minimal reduction. The type of relocation (small island-to-small island, small island-to-mainland, mainland-to-mainland) influenced exposure reduction. Small island-to-mainland relocations had the highest exposure reduction potential, whereas small island-to-small island moves showed the least, especially under high emissions. A striking finding was the absence of long-term site suitability assessments for 15 of the 17 cases, indicating limited consideration of future SLR projections in relocation decisions. Further, existing assessments often prioritized factors beyond environmental hazards, including socio-economic and political considerations. Several cases illustrated the influence of multiple factors beyond just coastal hazards (i.e. political incentives, social issues, economic conditions) on the decision making for relocation sites. The study identifies several factors that complicate risk reduction and adaptation including limited land availability in small island developing states (SIDS) for relocation and the complexity of considering multiple co-occurring hazards.

This study demonstrates that planned relocation can reduce coastal hazard exposure, but its effectiveness is dynamic and dependent on emissions pathways and geographic context. The results highlight the importance of land availability. SIDS, particularly atoll nations, often lack safe locations free from coastal hazards, limiting the potential of small island-to-small island relocations. Mainland-to-mainland moves, while providing some exposure reduction, often involve higher initial exposure. The limited use of systematic assessments of future risk in relocation planning is a critical concern. Future relocations should incorporate rigorous assessments that consider multiple time horizons, emissions scenarios, and hazard types, alongside socio-economic and cultural factors. The diversity of motivations for relocation, encompassing environmental, political, economic, demographic, and social factors, underscores the need for a holistic approach to relocation planning. The study's findings support the need for more anticipatory planning, community engagement, and international support for vulnerable communities lacking internal relocation alternatives. This is particularly critical in cases where relocation may not completely solve exposure issues even in low emissions scenarios.

This study provides quantitative evidence on the effectiveness of planned relocation in reducing future coastal hazard exposure. While relocation can be effective, its success depends on emissions scenarios and geographic context, highlighting the need for careful site selection and comprehensive assessments of future risks. The lack of long-term risk assessments in many cases underscores the need for improved planning processes that integrate future climate projections, socioeconomic considerations, and community participation. Future research should expand upon these findings by incorporating a wider range of hazard types, socioeconomic indicators, and methodological approaches to offer even more robust evaluation of planned relocation’s effectiveness.

The study employed several simplifications: the use of median SLR projections, the assumption of linear inundation patterns, uncertainties in elevation data and flood modeling, and the exclusion of some hazards (e.g., storms, landslides). The analysis focused on exposure reduction, not considering all aspects of relocation success (e.g., socioeconomic outcomes, community wellbeing). The number of cases studied was also limited. Despite these limitations, the findings provide valuable insights for informing policies and practices related to planned relocation.