Examination and Projection of Urbanization Effect on Summertime Hot Extremes in China

Extreme hot events have profound impacts on human health. Especially, consecutive hot days without heat relief during nighttime can significantly increase the rates of mortality and morbidity. Using an urbanized global earth system model and treating urban and rural land as subgrid units, the urbanization effect on long-term changes in three types of summertime hot extremes (i.e., independent hot days, independent hot nights, and compound events) is simulated under the present-day climate and two future scenarios in China. The model is first evaluated using a homogenized observational dataset drawn from over 2,000 meteorological stations. The results show that the model can well capture urban and rural temperature changes during the historical period from 1961 to 2000. The urban and rural temperatures are both increased under two future scenarios, but the urban heat island intensities (i.e., urban minus rural) do not show significant changes. However, urbanization has a significant impact on the frequencies of summertime hot extremes under two future scenarios. The increasing frequencies of independent hot days and independent hot nights in urban areas are less than that in rural areas, but more importantly, the increasing frequency of compound events in urban areas is larger than that in rural areas. This suggests that urbanization will aggravate the trend of the extreme hot events changing from independent hot days and independent hot nights to compound events, and seriously increase the health risks of urban residents in the future.