Rising Waters: Tradeoffs in Climate Migration and Adaptation Under Future Flood Risk

Flooding is among the most disruptive natural hazards to society, resulting in damage to property and infrastructure, displacement, and loss of life. Climate change is expected to increase flood hazards in many parts of the world. Riverine flood hazards are expected to increase due to more intense extreme rainfall events, while sea-level rise (SLR) will increase coastal storm surge heights. Simultaneously, urbanization processes and socioeconomic development in low-lying areas will further increase floods in the future. However, the impacts of increasing flood risk on exposed communities will largely depend on their capacity to adapt. The dissertation first examines urbanization and socioeconomic developments. Chapter 2 addresses this uncertainty by projecting future urban development as a combination of urban expansion and densification in mainland Southeast Asia. Without considering climate change and GDP development, the results indicate that the expected annual flood damage (or simply “risk”) will increase in all countries and both urbanization scenarios and that this increase in flood risk is likely to be underestimated when urban land is represented by a single land-use class. We furthermore demonstrate that the preferable urbanization trajectory resulting in the least future flood risk is context-dependent. The dissertation then investigates how different socioeconomic and behavioral factors influence the adaptation and migration decisions of households residing in the floodplain. Chapter 3 describes the development of a novel agent-based model (ABM) for France that simulates adaptation and migration decisions of individual households under increasing flood risk based on projections using the flood risk model presented in Chapter 2. This modeling framework is extended to also consider coastal erosion in Chapter 4. The results of the combined model for France in Chapters 3 and 4 illustrate how flood risk and coastal erosion will drive further household adaptation and migration in response to SLR. Chapter 5 assesses how and where governmental and household protection could offset or postpone global SLR-induced migration. For this, the modeling framework presented in Chapters 3–4 is upscaled to the global scale using a synthetic population dataset and extended with a government agent investing in flood protection infrastructure. The results presented in this chapter show that with no additional adaptation measures, SLR could induce the migration of 27 million coastal inhabitants globally by 2080 under a worst-case climate change scenario. However, household-level adaptations alone could reduce this projection of SLR-induced migration by 21%. When combined with government-funded flood defenses, the net impact of SLR on coastal migration could be nearly neutralized. Local adaptation to flood risk by households could only delay the onset of SLR migration by several decades in some areas. Thereby, local adaptation provides time to develop policy responses, such as managed retreat through buyout programs. Synthesis Chapter 6 illustrates how the research presented in this dissertation could help policymakers manage future flood risk, considering climate change and socioeconomic development. Adaptation gaps—driven by financial, institutional, and behavioral constraints—limit the implementation of government and household flood protection measures, resulting in migration when adaptation limits are reached. Policies to reduce the adaptation gap, for example, through subsidizing property-level floodproofing measures, could postpone or avoid reaching these adaptation limits. Such policies would be better informed by improvements in exposure and hazard data, refined building-level vulnerability modeling, and expanding behavioral calibration in large-scale ABMs with global survey and migration flow data. Finally, future research should integrate broader migration theories into models, enhancing their relevance to developing equitable and effective climate adaptation and migration policies.