What plausible urban coastal futures may look like? Spatially explicit urbanization projections for 10 Mediterranean countries.

The urban extent in the Low Elevation Coastal Zone (LECZ) is increasing faster than in the surrounding regions, which will lead to increased exposure to climate change-related hazards. Societies’ risk to these hazards will, therefore, depend on the rate and pattern of urban expansion and in what ways decision-makers will drive future urban development. One opportunity to investigate how urban development influences potential future coastal flood risk is to combine impact assessments with spatiotemporal urban land cover analysis and spatially explicit future urban projection. In this study, we have developed spatially explicit urban extent scenarios for 10 countries in the Mediterranean. The urban extent scenarios are quantitatively and qualitatively consistent with the assumptions of the global Shared Socioeconomic Pathways (SSPs). We employ a machine learning approach, namely Artificial Neural Networks (Multi-Layer Perceptron - MLP), to develop an Urban Change Model. The MLP model employs simple inputs as proxies for processes that drive urban development on a regional scale and estimates the likelihood of urban transformation for every grid cell between 2000 and 2012. In a next step, we calculate, for each SSP, the future urban land demand in 5-year time steps until 2100 and classify the ANN model outputs accordingly. These projections are then employed for calculating future exposure to coastal flooding.

The urban change models are able to reproduce the observed patterns of urban development with an overall accuracy of approximately 99% in all countries. The future projections indicate that accounting for the spatial patterns of coastal development can lead to significant differences in exposure. The increase in urban extent in the extended LECZ (below 20m) until 2100 varies, for instance,  by 67% (2075km²) for Italy, 104% (2331km²) for France (Mediterranean coast only) and 86% (691km²) in Greece depending on the urban development scenario chosen. This highlights that accounting for urban development in long-term adaptation planning, e.g. in the form of land-use planning is a very effective measure for reducing future coastal flood risk on a regional scale.