Researchers at The University of Hong Kong (HKU) have uncovered a critical, underrecognized trade-off in global environmental policies: while essential for improving public health, large-scale air pollution reductions are unintentionally intensifying warm-season surface urban heat islands (UHI) in humid climates. Published in the prestigious journal Nature Cities, the findings highlight the urgent need for integrated policies that address both air quality and urban heat adaptation, particularly in rapidly urbanising, humid regions. Many countries face compounded risks from intensified urban heat islands and air pollution. Over the past decades, stringent emission controls have successfully improved background air quality by reducing aerosols - tiny particles in the atmosphere that can reflect sunlight. However, the cascading effects of these clean air initiatives on UHI have remained largely underexplored. Using large-ensemble global climate simulations, a research team led by Professor Yuyu Zhou, Department of Geography, Faculty of Social Sciences, and Professor Peng Gong, Vice-President and Pro-Vice-Chancellor (Academic Development), Chair Professor of Global Sustainability at the Department of Geography and the Department of Earth Sciences at HKU, has demonstrated that UHI responses to background aerosol mitigation diverge significantly depending on the local hydroclimate. When aerosols are reduced, more downward solar radiation reaches the Earth, warming both urban and rural surfaces. In humid climates, such as East Asia and Indonesia, this creates a severe imbalance. Urban areas, dominated by impervious surfaces like concrete and asphalt, have limited capacity for evaporative cooling. In contrast, surrounding rural areas benefit from the increased sunlight, rainfall, and vegetation growth, which enhance their natural cooling processes. Consequently, urban warming significantly outpaces rural warming, exacerbating the UHI effect. Conversely, in arid regions, rural areas lack the moisture and vegetation to cool down, causing them to warm faster than cities and thereby weakening the UHI effect. The urgency of this issue is most