HKU Research Reveals Unintended Climate Trade-off: Clean Air Policies Intensify Urban Heat Island in Humid Cities
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 acute in rapidly urbanising humid zones, including parts of Asia, Africa, and South America. Observational evidence from China's Air Pollution Prevention and Control Action Plan (APPCAP) corroborates these findings, revealing that a period of stringent air pollution controls was accompanied by a marked increase in UHI intensity.
Key implications of the research:
- Divergent UHI responses: Background aerosol reductions amplify warm-season surface urban heat islands in humid climates but weaken them in arid regions.
- Mechanism in humid climates: Urban warming exceeds rural warming because impervious surfaces restrict evaporative cooling, while rural areas benefit from enhanced evapotranspiration driven by increased solar radiation, rainfall, and vegetation.
- A call for integrated policy design: The findings do not undermine the necessity of emission controls for public health; rather, they highlight the pressing need to pair air pollution reduction with proactive, targeted urban heat adaptation strategies.
The study was published in the leading journal Nature Cities.
Link to the paper: https://doi.org/10.1038/s44284-026-00463-7
About the research team:
This research was led by Professor Yuyu Zhou and Professor Peng Gong. Professor Yuyu Zhou: AAAS Fellow, Professor of the Department of Geography and the Institute for Climate and Carbon Neutrality, HKU, Director of Sustainable Cities Lab (SCL). Professor Peng Gong: Chair Professor of Global Sustainability & Vice-President and Pro-Vice-Chancellor (Academic Development), HKU. The collaborative team includes Dr Shu Liu, Postdoctoral Researcher, Department of Geography, HKU; Professor Lei Zhao, University of Illinois at Urbana‐Champaign; Professor Gerald Mills, University College Dublin; Dr Xinchang "Cathy" Li, Pacific Northwest National Laboratory; Professor Hongsheng Zhang, Department of Geography, HKU; Professor Karen C. Seto, Yale University.
This study is supported by the University of Hong Kong HKU-100 Scholars Fund, the HKU Social Sciences Internal Seed Grant Scheme, and Research Grants Council-Strategic Topics Grant STG2/P-705/24-R.
About the SCL:
SUSTAINABLE CITIES LAB (SCL) aims to leverage geospatial and remote sensing big data and advanced interdisciplinary approaches including remote sensing, geographic information system, regional and global climate modelling, and integrated assessment modelling to understanding the problems of environmental change and their potential solutions for a sustainable environment. SCL lab's research focuses on six primary directions: (1) Urbanisation mapping & modelling; (2) Urban environmental changes; (3) Urban energy use and emissions; (4) Urban sustainability; (5) Water-energy-food nexus; (6) Climate change impacts, adaption, and vulnerability.
To promote knowledge exchange, more recent research activities can be viewed through https://www.scl.hku.hk/
