Impact of urbanization and climate change on underground temperatures: a modelling study in Milan (Italy)

This study investigates the long-term evolution of the subsurface urban heat island (SUHI) effect in Milan, integrating historical records, present observations and future climate projections through a coupled fluid-flow and heat-transport numerical model. A N-S cross-section through the city serves as the domain for this study and boundary conditions were derived from historical maps starting in 1884, long-term air temperature time series starting in 1700, and distributed land surface temperatures from Landsat 8 satellite remote sensing. The research quantifies the temperature variations in the shallow subsurface over the past 150 years (1875-2025), calibrating the model against groundwater temperature measurements, and predicts trends up to 2100. Current estimates indicate urban temperature anomalies up to +5°C at the water table depth, and an expansion of the SUHI along the two-dimensional cross-section from 3 km in 1884 to 9 km in 2025. The findings highlight the heterogeneous distribution of subsurface temperature anomalies, influenced by variations in the groundwater depth, flow patterns, land cover and urban and infrastructure expansion. Future projections suggest a further increase in subsurface temperatures, particularly in areas with shallow groundwater. These results underscore the need to incorporate mitigation strategies into urban planning and policies, such as sustainable urban cooling measures and optimized geothermal energy utilization. This article is part of the theme issue 'Urban heat spreading above and below ground'.