Summertime climatic effects of urbanization and their impacts on human thermal comfort in Xiangjiang watershed, South-Central China

Abstract

Urbanization has been a substantial force to change the natural environment, and several urbanization-related climatic issues, such as urban heat island (UHI) effect, would pose a great threat to public health. However, the climatic effects of urbanization and their possible links with human thermal comfort remain poorly understood. In this study, we examined the impact of urbanization on summertime air temperature, extreme hot events and human-perceived heat stress in Xiangjiang watershed, a typical inland highly urbanized area of China. Results showed that in the context of global warming Xiangjiang watershed experienced extensive summertime warming, especially nighttime warming in the past nearly half-century, and urbanization aggravated the warming effect in urban areas by increasing nighttime air temperature as well, creating the UHI effect. Meanwhile, all three types of extreme hot events, i.e. independent hot days (IHD), independent hot nights (IHN), and compound hot events (CHE) showed increasing trends. Urbanization significantly increased the occurrence, duration and intensity of CHE, which involved both the daytime and nighttime extreme hot temperatures. Urban expansion contributed 51.39%, 30.91% and 25.69% of the increases in occurrence, duration and intensity of CHE, respectively. Besides, increasing air temperature and extreme hot events would inevitably enhance the heat stress on people, especially in the nighttime, and urbanization exacerbated the nighttime discomfort on people with contribution to 26.92% of increases in nighttime heat stress index (HI). Nearly one quarter of summer nights in urban areas have reached very warm level of severe heat situation for human beings in the late stage (1996–2019). Urbanization-induced increases in CHE and nighttime HI caused an aggravation of human heat discomfort since people failed to relief the excess heat stress of daytime at night. This study provided new insights to understand urban climatic effect and its relation to human heat-related comfort.