Long-Term Changes in Summer Extreme Wet Bulb Globe Temperature over China

The wet bulb globe temperature (WBGT) has important implication for human health. Previous studies widely use the monthly data but rarely investigate the extreme WBGT because of data limitation. In this study, we use 6-h station data to analyze the changes in the WBGT and three categories (intensity, absolute threshold, and frequency indices) of extreme WBGT indices in summers of 1961–2017. It is found that the spatial distributions of long-term trends in summer mean WBGT are consistent with those in the mean temperature. The trend value of WBGT is smaller than the mean temperature, because of the decrease of relative humidity. For the extreme indices, the intensity and frequency of WBGT and fixed threshold indices have changed. The increase of intensity indices and warm WBGT days and nights, and decrease of cold WBGT days and nights have been observed in most China, especially over northwestern China. The number of days with daily maximum WBGT exceeding 31.4°C (WXge31) and minimum WBGT exceeding 27.9°C (WNge27) over southeastern China have increased since 1961. The spread of probability distributions of WXge31, WNge27, and warm WBGT days and nights is becoming wider, reflecting increased variability of extreme indices. In addition, urbanization effects on the WBGT are investigated. The impacts of urbanization on most of extreme WBGT indices are not detected, except for absolute thresholds indices. This may be due to the decrease of relative humidity in urban stations, which are almost two times larger than that in rural stations. However, we also note that the homogenization issue of humidity data may affect the conclusions.