In recent years, Hong Kong has experienced repeated episodes of intense rainfall that left significant impacts on social operations, including the "once‑in‑a‑century" rainstorm in September 2023, the exceptionally heavy rain in May 2024, and a series of Black Rainstorm events in early August 2025. These extreme weather events have not only disrupted residents' daily travel and public infrastructure but have also highlighted the challenges faced by high‑density coastal cities in coping with extreme weather.
A research team led by Prof. GAN Jianping, Chair Professor and Head of the Department of Ocean Science at The Hong Kong University of Science and Technology (HKUST), and Director of the Centre for Ocean Research in Hong Kong and Macao has found that such intense rainfall events are not solely driven by large-scale weather systems. As one of the key cities in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), Hong Kong's weather is increasingly influenced by rapid urban expansion and anthropogenic heat emissions within the region. The study shows that urbanization alters surface properties and the regional energy balance, amplifying summertime warming and rainfall by strengthening local atmospheric circulation and moisture transport, thereby increasing the risk of extreme heat and heavy rainfall events.
The study, titled "Warming and Wetting Induced by Urbanization and Anthropogenic Heat over a Fast-Developing Large River Delta," has been published in the Journal of Applied Meteorology and Climatology (JAMC) and was featured in the American Meteorological Society (AMS) Science Preview in March 2026. The study focuses on the GBA cities on the mainland, a region characterized by high population density, complex terrain, and strong coastal influences. Facing multiple climate challenges, the region also exerts a direct impact on weather conditions in Hong Kong.
The Pearl River Delta, where the nine major cities of the GBA are located, is one of the world's fastest-urbanizing regions, with a population exceeding 60 million. Over the past two decades, large areas of cropland and natural vegetation have been converted into impervious urban surfaces. Between 2001 and 2019, cropland, water bodies, barren land, and forest/grassland decreased by 44.03%, 8.05%, 50.22%, and 0.43%, respectively, while construction land increased by 20.05%. These dramatic land-use changes have fundamentally altered the regional energy and water balance, intensified the urban heat island effect, and reshaped the local precipitation patterns.
The research shows that urbanization has significantly increased both regional temperature and rainfall. Air temperatures in urban areas increased by more than 1°C compared to surrounding rural regions, while anthropogenic heat further elevated surface temperatures by up to 0.5°C. At the same time, rainfall intensifies over urban centers and downwind areas, driven by enhanced moisture convergence and strengthened land-sea circulation.
Prof. Gan said, "Urbanization and anthropogenic heat are amplifying regional climate effects. These local factors not only increase temperatures but also alter atmospheric circulation, leading to more frequent and intense rainfall events."
The study highlights the important role of the GBA's coastal setting in amplifying the impacts of human activity and climate change. Interactions between land, ocean, and atmosphere create a feedback loop: urban surfaces store heat, rising warm air draws in moisture from the South China Sea, and the enhanced convection leads to heavier rainfall.
Dr. LAI Wenfeng, HKUST Postdoctoral Fellow and the paper's first author, said, "Urban expansion strengthens regional atmospheric vertical motion and transports moisture to higher levels of the atmosphere. This enhances rainfall, even as surface evaporation is reduced by impervious urban land."
The findings suggest that continued urbanization will further intensify heatwaves and increase flood risks during the summer monsoon season. By isolating the effects of land-use change and anthropogenic heat, the study provides new insights into how megacity clusters can influence regional climate systems. The research team emphasizes the need to better represent urban processes in climate models for improving weather prediction and supporting climate-resilient urban planning.
