Two decades of urban heat intensification and exposure across 1400 cities

Urban heat is a growing challenge, compounded by climate change and urban development. Yet, we lack a global understanding of heat extremes and where they coincide with population growth. Here, we explore satellite-based land surface temperature and population exposure across 1400 cities worldwide. Over two decades, urban surface temperature rose by 0.03-0.2 °C/year during daytime and 0.01-0.15 °C/year at night. The fastest increases were observed in cold-climate cities and in winter, particularly in Eastern Europe and Western Asia. Extreme temperatures have also become more frequent, particularly hot nights, which now occur 47% more often in arid cities. Compounded by population growth, this translates to a 51% increase in global exposure to extreme daytime heat. Climate warming amplified the exposure increase by 82% relative to population growth alone. Relative importance of population and climate change varies by climate zone, with climate dominating daytime exposure in continental regions (71%), and population growth in arid cities (69%).