Urban heat above and below ground: towards improved understanding, modelling, mitigation and adaptation

Heat islands have been recognied at the surface of urban areas since the nineteenth century, while their subsurface counterparts were only identified in the late twentieth century. Since then, surface and subsurface urban heat islands (UHIs and SUHIs), respectively have drawn increasing scientific attention, along with technologies and policies designed to limit their impacts on public comfort and health, infrastructure resilience, the environment and energy efficiency. Yet they have been traditionally studied in isolation. This Theme Issue seeks to bridge that gap by presenting recent advances on Urban Heat Above and Below Ground. As the introductory piece of this compilation of works, this article provides an overview of the drivers and impacts of UHIs and SUHIs and offers a perspective on the need to transition towards integrated studies that explicitly account for the thermal interactions between the urban surface and subsurface. Three priority research directions are outlined to address overlooked aspects of urban heat propagation and improve the fidelity of analyses, with a focus on computational simulations. The article concludes by summarizing the contributions in this Theme Issue, which expand knowledge of urban heat dynamics and lay the foundation for capturing the full three-dimensional thermal complexity of cities-above and below ground.This article is part of the theme issue 'Urban heat spreading above and below ground'.