Cities near volcanoes are increasingly threatened by volcanic hazards

Urban populations are increasingly moving into hazardous areas, which leads to widespread and frequent impacts from hazard events. Although these exposure trends have been quantified for other hazards, global exposure analyses for volcanic hazards remain limited. Here we quantify global and regional changes in city exposure to volcanic hazards through time. With GHS-UCDB and GHS-POP datasets, we use spatio-temporal metrics to track urban expansion within 100 km of volcanoes active in the Holocene from 1975 to 2020 and project trends to 2030. The number of cities within 100 km of volcanoes is projected to more than double and the population living within these cities to increase by 155% between 1975 and 2030. The proportion of people within 100 km of volcanoes who live in cities, compared to outside of cities, increases from 44% (~ 186 million) in 1975 to 50% (~ 430 million) in 2020 and 52% in 2030 (~ 473 million). Globally, exposed city populations concentrate within 20–30 km from volcanoes, and average city population density generally decreases towards volcanoes. Exposure growth is highest in Southeast Asia and East Africa. Among the cities spanning less than 10 to more than 30 km from a volcano, 58% grow proportionally faster towards volcanoes, within 10 km, with the majority in Southeast Asia. Across all regions, most cities expand and densify over time. For key cities, we compare patterns of urban expansion with the directionality of volcanic hazards. These analyses reveal that Jakarta has had the world’s largest population living within 100 km of volcanoes since 2005, and major cities, including Jakarta, Manila, San Salvador, Naples, and Quito, are spreading into areas likely to be affected by volcanic flows or tephra fall. In some cases cities are extending along roads. These trends indicate that urban expansion is amplifying volcanic risk and highlight the need to integrate hazard data into urban planning.