Global trends of city exposure to volcanic hazards

Cities near volcanoes expose dense concentrations of people, buildings, and infrastructure to volcanic hazards. Identifying urban centres exposed to volcanic hazards at a global scale supports local risk assessments, better land-use planning, and hazard mitigation. Previous approaches dominantly relied on city centroids to assess population exposure and proximity to volcanoes, overlooking the spatial variability of population distribution within city margins. In this research, firstly, we propose a novel framework to rank 1,106 cities globally in terms of volcanic hazard exposure using population count, distances to 596 Holocene volcanoes, and the number of nearby volcanoes. Notably, 50% of people living within 100 km of a volcano reside in cities. Bandung, Indonesia, ranks highest overall, with over 8 million people exposed within 30 km of up to 12 volcanoes. Regional rankings highlight Jakarta (~38 million), Tokyo (~30 million), and Manila (~24 million) having the largest populations within 100 km of a volcano. Finally, we show average trends in city population expansion towards volcanoes since 1975 and projected to 2070. We use case studies to show directions of expansions towards or away from hazardous areas, to emphasise how potential local drivers may influence hazard exposure. For some countries, such as El Salvador, Japan, or the Philippines, where >70% of land in each country is exposed to volcanic hazards, there are limits on the availability of safer areas for expansion. By understanding how urban environments are expanding towards volcanoes, we can better inform adaptive strategies to volcanic risks.