Extreme heat reduces and reshapes urban mobility.

Extreme heat is a problem in Southern European countries and cities, aggravated by rising temperatures and aging populations. Research on mobility during extreme heat remains limited to small samples and geographic contexts, leaving gaps in our systematic understanding of how populations adjust their day-to-day mobility patterns and how these adaptations vary across social groups. Here, we use mobile phone data from passive and active mobile network connections covering 13 million individuals in Spain (27% of the population) to examine extreme heat's impact on mobility at scale. We stratify by age, gender, economic class, and type of activity. Our findings show mobility falls by as much as 10% on hot days generally and 20% on hot afternoons specifically, when temperatures peak. Further differences emerge on hot days. Older adults cut travel to work and other activities, while those earning less are less able to avoid work; social mixing declines and spatial structure changes as activity falls in city centers. These disruptions have implications for urban economies, as curbed activity and interaction could threaten the dynamism of cities as hubs of social and economic exchange.