Spatiotemporal patterns of urbanization during the last four decades in Switzerland and their impacts on urban heat islands

Urbanization is nowadays a global phenomenon which is increasingly concentrating the world's population in cities. In Switzerland, recent decades have seen an unprecedented loss of arable land due to urbanization, which has triggered amendments in the spatial planning laws with the aim of promoting urban densification. Nevertheless, despite remarkable efforts, the environmental impacts of distinctive urban patterns such as compact cities and urban sprawl remain poorly understood. One of the most remarkable environmental impacts of urbanization is the urban heat island effect, a phenomenon by which urban temperatures are warmer than in its rural surroundings. Central Europe, and therefore Switzerland, is among the regions in the world where temperatures are rising faster and the urban heat island effect is most prominent, which represents a central challenge for spatial planning. Most studies suggest that the urban heat island effect can be aggravated in compact cities, especially when considering the larger share of urban dwellers that are exposed to the highest temperatures. At the same time, the literature on the subject has seen a growing development of mitigation strategies, which suggest that the urban heat island effect can be significantly alleviated by an adequate planning of the building materials and urban green spaces.

This doctoral dissertation intends to address the issues expressed above by performing a quantitative evaluation of the spatiotemporal patterns of urbanization in Switzerland and their impact on the urban heat island effect. To that end, the thesis adopts a landscape ecology perspective to quantify urban patterns and to spatially simulate the biophysical processes that underpin the urban heat island effect. The first article presents PyLandStats, an open- source library to compute landscape metrics in a repeatable and reproducible manner. In the second article, such a library is used to evaluate the spatiotemporal patterns of urbanization observed in the urban agglomerations of Bern, Lausanne and Zurich from 1980 to 2016. The results reveal that the outer zones of Bern and Lausanne are still undergoing diffusive urban expansion, whereas infill development is the dominant growth mode in both the inner and outer zones of Zurich. The thesis follows with the development of a spatially-explicit method to simulate urban heat mitigation using a recent model of urban cooling based on three biophysical mechanisms, namely tree shade, evapotranspiration and albedo. The study introduces an automated procedure to calibrate the parameters of the model, and shows that the proposed approach can outperform regression models based on remote sensing features. Then, in the fourth article, such an approach is applied to Lausanne in order to evaluate heat mitigation in a variety of urban greening scenarios which modify both the abundance and spatial configuration of the tree canopy cover. The simulations suggest a potential alleviation of the maximum nighttime temperatures of 2°C, which represents a major reduction of the human exposure to the urban heat island effect.

Finally, a concluding chapter summarizes the main contributions of the dissertation and reviews key implications for urban planning in Switzerland. Overall, rather than prescribing urban densification as the customary strategy for spatial development, land use regulations and local plans should incorporate spatially-explicit evaluations of the ecosystem services.