Towards a comprehensive assessment of exposure to sea-level rise at continental scale

Coastal areas are increasingly at risk from coastal hazards due to sea-level rise (SLR), which will accelerate in the 21st century driven by anthropogenic climate change. Future coastal risks are not only driven by the amount of SLR, but also by a high concentration of population and assets in locations potentially exposed to SLR-related hazards. The uncertainty related to future SLR and socioeconomic development can be explored with the SSP-RCP scenario framework, which consists of physical scenarios (RCPs – Representative Concentration Pathways) and socioeconomic scenarios (SSPs – Shared Socioeconomic Pathways), and is increasingly used in coastal risk assessments at continental to global scales. Thus far, such assessments have primarily focused on characterizing future changes in SLR-related hazards in a spatially explicit manner, while research on the spatial representation of variables to characterize exposure (e.g. population, assets, infrastructure) to those hazards has been limited, in particular with regard to exploring plausible future changes under the SSPs. Previous work that has characterized exposure spatially has used global-scale data, modeling approaches, and scenario assumptions when assessing coastal risks at continental scale. Therefore, this thesis advances the spatial representation of exposure to SLR-related hazards at continental scale to facilitate assessment of future coastal risks in an integrated manner. It focuses on two exposure variables, i.e. population and cultural assets, using the Mediterranean region as a study area. First, the global-scale SSP narratives are extended to the Mediterranean region, by including regional drivers of socioeconomic development, additionally differentiating northern versus southern and eastern countries of the region. The extended narratives are interpreted to develop spatial population projections for each SSP until 2100, accounting for the uncertainty related to future rural-urban and inland-coastal migration. Results show that the population potentially exposed to SLR-related hazards ranges from 34.1 million (SSP1) to 96.2 million (SSP3) in 2100, with marked differences across the Mediterranean. Comparison of these results with results based on the global SSPs shows a deviation of as much as 15% in the exposed population (SSP1), thereby spanning a wider range of uncertainty regarding population exposure. As this approach does not account for urban sprawl, a gravity-based modeling approach is developed, which allows for modeling urban sprawl as well as rural-urban and inland-coastal migration. The spatial population projections produced with this approach result in 51.3 million (SSP4) to 107.8 million (SSP3) people potentially exposed to SLR-related hazards in 2100. The results of the two approaches differ substantially, thereby stressing the need to consider the strengths and weaknesses of both approaches in future work. For more comprehensive coastal risk assessments, additional exposure variables need to be considered. Therefore, a spatial database of cultural assets, an exposure variable not commonly analyzed due to a lack of high-resolution spatial data, is assembled by producing spatial representations (i.e. polygons) of 49 UNESCO World Heritage Sites (WHS) located in low-lying coastal areas of the Mediterranean. A first application of the database in a continental-scale assessment shows that, already under current conditions, 75% and 85% of the WHS are at risk from coastal flooding and erosion, respectively. Both risks will increase until 2100, depending on the SLR scenario considered, with considerable differences between WHS and across the Mediterranean basin due to spatially varying characteristics of both risks. The results show that awareness regarding SLR-related risks posed to WHS is low and that adaptation is urgently needed to preserve WHS in the future. This thesis offers an important contribution to characterizing exposure to SLR-related hazards at continental scale, thereby facilitating the integrated assessment of coastal risks, accounting for future uncertainties in physical as well as socioeconomic processes. The results of this thesis stress the importance to explore different migration processes in spatial population projections; characterize additional exposure variables not commonly analyzed; and account for regional characteristics when assessing coastal risks at continental scale. Future work can extend the developed data and modeling approaches and can contribute to harmonizing data and modeling approaches that have gradually increased in recent years, to further advance coastal risk assessments at continental scale.

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