05 September 2017

Urbanization is Key Contributor to Increased Intensity of Extreme Heat Events in the Yangtze River Delta

Researchers analyzed the influence of urban heat island effects on temperature extremes in a heavily populated region of eastern China.


Brisk population and industrial growth in cities generates an urban heat island effect by drastically modifying the land processes that influence the surface energy balance. Much less is known about how urbanization influences changes in extreme heat events. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory contributed to a study using nighttime lights data and a homogenized temperature record to analyze previously unexplored changes in temperature extremes in the highly urbanized Yangtze River Delta region of eastern China during the past four decades.


Despite rapid urbanization in China over the past 30 years, the extent to which urban warming has affected extreme temperature trends in the country has been uncertain. In this study, researchers found that urbanization increased the risk of heat extremes in the Yangtze River Delta, which is home to about 11 percent of China’s population and contributes almost a quarter of the country’s gross domestic product. Results showed urbanization contributed to more than one-third of the increased intensity of extreme heat events in the region. This study also demonstrated that future risk assessments for extreme heat in urbanized areas should consider both large-scale warming trends and the increasing urban heat island effect.


This study explored the urbanization effect in China’s Yangtze River Delta region using homogenized daily maximum and minimum temperature data, and by classifying stations as rural or urban sites based on nighttime lights data. Researchers quantified changes in extreme heat events (EHEs) using summer extreme indices, with a heat wave defined as a period of three or more consecutive days in the summer when the daily maximum temperature exceeded the long-term daily 90th percentile. Analyses showed that urbanization considerably exacerbated the summer EHEs in the Yangtze River Delta and that urban heat island effects contributed more than one-third to the increase of EHE intensity between 1971-2013. Scientists found that rapid urbanization resulted in a prominent contrast in the distribution of EHE frequency and intensity between the urban and rural areas. Researchers also saw more notable shifts in the probability distribution toward increasing temperature and summer extreme indices in urban stations compared to rural stations. The study showed that the urbanization in the Yangtze River Delta resulted in large increases in the risk of heat extremes.

L. Ruby Leung
Pacific Northwest National Laboratory
Yang, X., Leung, L., Zhao, N., et al. "Contribution of Urbanization to the Increase of Extreme Heat Events in an Urban Agglomeration in East China." Geophysical Research Letters 44, 6940-6950 (2017). [10.1002/2017GL074084].