Standardized heat islands and persistence drive modeled urban heat events

Urban environments are usually hotter than their rural surroundings, a phenomenon known as the urban heat island (UHI) effect. The mean UHI effect implies that urban environments would experience more heat events if the same temperature threshold is used to identify heat events in both urban and rural environments. However, the role of higher-order temperature statistics, such as temperature variance and persistence, in determining urban–rural differences of heat event occurrence remains elusive. Here, using numerical simulations from two global models, we demonstrate that up to 94% of urban–rural differences in hot day occurrence are driven by the mean UHI effects normalized by temperature variance, that is, the standardized mean UHI effects. For multi-day heat events, temperature persistence further plays an important role. These findings reveal how the temperature mean, variance and persistence interact to determine the urban–rural difference in heat event occurrence. Cities with more pronounced standardized mean UHI effects and enhanced temperature persistence should place greater emphasis on mitigating the adverse impacts caused by extreme heat. Heat waves are increasing, and cities seem especially prone. Using global climate models, this study finds that cities with stronger standardized heat islands and more-persistent heat experience more heat waves.