Urban heat increasingly governed by humidity in a warming world

Urbanization and climate change are intensifying heat stress in cities worldwide, yet the role of humidity remains poorly understood. While temperature has historically been the focus of urban climatic studies, humidity is gaining increasing recognition as a key driver of heat stress, as high moisture levels limit both urban evaporative cooling and human body perspiration, thus increasing health risks. Here we present a physics-based theoretical framework that enables a unified attribution of humid heat to temperature and humidity across humidity metrics. Applying this framework to global urban climate simulations, we show that conflicting conclusions in the existing literature on urbanization-induced drying or humidifying stem largely from humidity metric choice. True moisture changes are evenly split worldwide, with 51% and 49% of urban areas drying and humidifying, respectively. The influence of these changes on urban humid heat is substantial. In 39% of urban grid cells, humidity contributes more to urbanization-induced increases in heat stress than temperature does. Under climate change, this dominance is projected to increase by almost twofold. Together, these findings highlight humidity as a previously underappreciated and increasingly important dimension of climatic change in cities. Our study underscores the critical necessity to move beyond temperature-centric approaches toward humidity-sensitive strategies for effective urban heat adaptation and improved livability in cities.